Linking Expansion Behaviour of Extruded Potato Starch/Rapeseed Press Cake Blends to Rheological and Technofunctional Properties

polymers

Article Linking Expansion Behaviour of Extruded Potato Starch/Rapeseed Press Cake Blends to Rheological and Technofunctional Properties

Anna Martin 1,* , Raffael Osen 1, Heike Petra Karbstein 2 and M. Azad Emin 2

1 Department of Food Process Development, Fraunhofer Institute for Process Engineering and Packaging IVV, 85354 Freising, Germany; [email protected] 2 Food Process Engineering, Institute of Process Engineering in Life Sciences, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany; [email protected] (H.P.K.); [email protected] (M.A.E.) * Correspondence: [email protected]; Tel.: +49-8161-491-457

Abstract: In order to valorise food by-products into healthy and sustainable products, extrusion technology can be used. Thereby, a high expansion rate is often a targeted product property. Rapeseed press cake (RPC) is a protein- and ﬁbre-rich side product of oil pressing. Although there is detailed knowledge about the expansion mechanism of starch, only a few studies describe the inﬂuence of press cake addition on the expansion and the physical quality of the extruded products. This study assessed the effect of RPC inclusion on the physical and technofunctional properties of starch- containing directly expanded products. The effect of starch type (native and waxy), RPC level (10, 40, 70 g/100 g), extrusion moisture content (24, 29 g/100 g) and barrel temperature (20–140 ◦C) on expansion, hardness, water absorption, and solubility of the extrudates and extruder response was evaluated. At temperatures above 120 ◦C, 70 g/100 g of RPC increased the sectional and volumetric

expansion of extrudates, irrespective of starch type. Since expansion correlates with the rheological properties of the melt, RPC and RPC/starch blends were investigated pre- and postextrusion in a

Citation: Martin, A.; Osen, R.; closed cavity rheometer at extrusion-like conditions. It was shown that with increasing RPC level Karbstein, H.P.; Emin, M.A. Linking the complex viscosity |η*| of extruded starch/RPC blends increased, which could be linked to Expansion Behaviour of Extruded expansion behaviour. Potato Starch/Rapeseed Press Cake Blends to Rheological and Keywords: extrusion; plant proteins; canola; expansion; rheology Technofunctional Properties. Polymers 2021, 13, 215. https://doi.org/ 10.3390/polym13020215 1. Introduction Received: 23 December 2020 The cultivation and processing of rapeseed—primarily for the production of rapeseed Accepted: 6 January 2021 oil—increased worldwide from 50.6 mt in 2007 to 76.2 mt in 2017 [1]. About 50 mt of Published: 9 January 2021 rapeseed press cake (RPC) accumulates as a residue every year. RPC comprises the solid

Publisher’s Note: MDPI stays neu- residues that remain after the extraction of oil from the oil containing plant parts such as tral with regard to jurisdictional clai- the seeds. Depending on the extraction method, the protein, lipid and fibre content of RPC ms in published maps and institutio- can vary largely and lies in the range of 19–40 g/100 g, 5–18 g/100 and 11–14 g/100 g [2–7]. nal affiliations. The amino acid profile of RPC has been shown to be comparable to soybean press cake but is richer in sulphur-containing amino acids, e.g., methionine and cysteine [8]. In addition, RPC has a higher fat content than soybean press cake and is a good source of calcium, iron, manganese, selenium and many B vitamins. It is also one of the richest sources Copyright: © 2021 by the authors. Li- of nonphytate phosphorus. Due to its all-year availability, competitive price and high censee MDPI, Basel, Switzerland. protein content, RPC has received attention as a possible feed component for livestock and This article is an open access article aquaculture [9,10]. distributed under the terms and con- Up until now there has only been limited use of RPC. This has mostly been for animal ditions of the Creative Commons At- and not human nutrition due to the fact that RPC contains high contents of antinutritional tribution (CC BY) license (https:// factors (ANFs) such as phenolic compounds, phytic acid and glucosinolates [11–13]. How- creativecommons.org/licenses/by/ ever, various processing methods have been studied that successfully reduce the ANF 4.0/).

Polymers 2021, 13, 215. https://doi.org/10.3390/polym13020215 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 215 2 of 19

content of RPC (i.e., dehulling, steaming, toasting, wet heating, water washing, chemical treatment, microbial degradation [14], seed germination [15,16], enzymatic treatment and fermentation [6,17]). This opens up the possibility to valorise RPC into, i.e., directly expanded snacks in order to produce sustainable new food products with an enhanced nutritional value. Low moisture extrusion is one technology used for texturising starch-based and protein-based biopolymers into food products having an expanded porous structure. There is detailed knowledge about the expansion mechanism of several starch types, but only two studies describe the influence of oilseed press cake addition on the expansion and the physical quality of the extruded products. To our knowledge, there are no studies investigating the impact of RPC on food products. For instance, it has been reported recently that the addition of RPC to starch-based multicomponent products (i.e., aquafeed) resulted in inferior physical quality of the extrudates [18,19]. In particular, the sectional expansion decreased and longitudinal expansion increased due to the addition of RPC. When fruit press cakes were added to a starch matrix, lower sectional expansion of extrudates was found in previous studies [20–23]. A sharp decrease in sectional expansion was also reported by Wang and Kowalski [23] when 15 g/100 g cherry pomace (a by-product of cherry juice processing) was added to a corn starch matrix in twin-screw extrusion and by Day and Swanson [24] when whey protein, gluten or soy protein were added to extruded starch-based products. Accompanied by a decreased sectional expansion, some studies report a decreased hardness when the protein content was increased in starch-based extrudates [25,26]. Decreased sectional expansion and hardness of extrudates was attributed to change in rheological properties in the starch melt due to the addition of proteins or other particles (i.e., fibres) in the added component [27,28]. Moreover, in these studies, a decrease in sectional expansion due to the addition of high-protein components was accompanied by an increase in longitudinal expansion and was attributed to a decrease in viscous properties of the melt [23]. It can be assumed that with the addition of a high-protein and high-fibre ingredient such as RPC to starch, the rheological properties of the melt are highly influenced due to the reaction behaviour of RPC components at extrusion temperatures. When the rheological properties change due to RPC addition to starch, an alteration of the expansion mechanism (nucleation, bubble growth and bubble shrinkage) is expected compared to starch expansion. Additionally, the starch type might have an impact on rheological properties and therefore on expansion due to starch type-related physicochemical and functional properties. Rheological material properties (i.e., viscous and elastic properties) influence the time of water vapor pressure underrun in the die and determine the direction of expansion (sectional or longitudinal). Expansion in turn governs physical product characteristics like specific hardness, wherefore it can be assumed that with different expansion mechanisms, the physical product properties of starch or starch/RPC blends are affected by RPC inclusion. Furthermore, it can be assumed that the technofunctional product properties of starch and starch/RPC blends like water absorption and solubility depend on the addition of RPC and extrusion conditions and indicate underlaying physicochemical reactions of starch and RPC during extrusion. To enable the design of food products including RPC and starch, a systematic evaluation of the impact of RPC on expansion, rheological and technofunctional properties is required. In this study, we have evaluated the impact of the RPC content, starch type and extrusion temperature on the expansion, physical quality and extruder response of extruded starch/RPC blends. In order to draw a correlation between expansion mechanisms and rheological properties, the effect of RPC addition on the reaction behaviour and the complex viscosity of starch/RPC blends was investigated at extrusion-like conditions in a novel closed cavity rheometer. With the investigation of technofunctional properties of starch and starch/RPC blends, indications regarding physicochemical changes in starch and RPC during extrusion processing were collected. A list of abbreviations is provided in Table1. Polymers 2021, 13, 215 3 of 19

Table 1. List of abbreviations.

DM Dry matter (g/100 g)

Hspec Speciﬁc hardness LEI Longitudinal expansion index n.a. Not analysed NFE Nitrogen free extract (g/100 g) PS Potato starch RO Rapeseed oil RP Rapeseed peel RPC Rapeseed press cake SEI Sectional expansion index SME Speciﬁc mechanical energy (Wh/kg) T Temperature (◦C) ◦ TB Barrel temperature ( C) ◦ TM Temperature of measurement ( C) ◦ TPre Temperature of Pre-treatment ( C) ◦ TT Treatment temperature ( C) VEI Volumetric expansion index WAI Water absorption index WPS Waxy potato starch WSI Water solubility index

2. Materials and Methods 2.1. Materials and Composite Flour Preparation Cold-pressed 00 type RPC, rapeseed peel (RP) and rapeseed oil (RO) were kindly provided by Teutoburger Ölmühle (Ibbenbüren, Germany). The rapeseed used for RPC production was dehulled before deoiling. For all experiments, RPC and RP were milled to a particle size of < 0.5 mm as described by Martin and Osen [18]. Potato starch (PS) and waxy potato starch (WPS ElianeTM 100) were kindly provided by Avebe (Veendam, The Netherlands). The amylose/amylopectin ratio of the starches were 25/75 for PS and 1/99 for WPS. PS and WPS were mixed with RPC, RP and RO in proportions of 70/10/10/10, 50/40/5/5 and 30/70/0/0 (w/w), respectively, to achieve rapeseed protein contents of 5, 16 and 27 g/100 g in the ﬁnal product. RP and RO were added to the mixtures containing 50 and 70 g/100 g starch to achieve constant ﬁbre and lipid contents in all the mixtures. The dry ingredients were mixed in a Spiral-Mixer SP 12 (DIOSNA Dierks & Söhne GmbH, Osnabrück, Germany) for 60 min. The mixtures were incubated at 20 ◦C for at least 8 h. Before extrusion, the dry matter in the mixtures was analysed in duplicate as laid down by legislation [29].

2.2. Extrusion The starch/RPC mixtures were extruded as laid down in [18] with the temperature settings reported in Table2. The moisture content was set to 24 g/100 g or 29 g/100 g (dry matter basis), the mass flow rate was kept constant at 10 kg/h, the screw speed was set to 300 rpm and a 2 mm × 4.5 mm circular die was used. The screw was configured according to Tyapkova and Osen [19]. After extrusion, the extrudates were dried in an oven (Thermo Scientific Heraeus UT 6760, Thermo Electron LED GmbH, Langenselbold, Germany) at 40 ◦C for 24 h. The samples were stored in vacuum-sealed bags at 20 ◦C until further analyses. Polymers 2021, 13, 215 4 of 19

Table 2. Extrusion barrel temperature settings for extruded starches and starch/rapeseed press cake (RPC) mixtures.

Temperature (◦C) Barrel Segment 1 2 3 4 5 6 Sample 20 ◦C – 20 20 20 20 20 Sample 80 ◦C – 60 80 80 80 80 Sample 100 ◦C – 60 80 100 100 100 Sample 120 ◦C – 60 80 100 120 120 Sample 140 ◦C – 60 100 120 140 140

2.3. Determination of Extruder Response As soon as steady-state operating conditions were reached, samples were taken. The average die pressure (bar) and product temperature in front of the die (◦C) during the sample-taking period were recorded using CSpro medium software (Coperion GmbH, Stuttgart, Germany). The speciﬁc mechanical energy input (SME) was calculated as described in a number of recent studies [30,31].

2.4. Characterisation of the Raw Material and Extrudates 2.4.1. Chemical Composition The chemical compositions of the starches, RPC, RP, starch/RPC mixtures and all extruded samples were analysed. Regarding the latter, the extrudates were ground after drying to < 0.5 mm particle size using a knife mill (Grindomix GM 200, Retsch GmbH, Haan, Germany). The dry matter (DM) was measured as laid down in the German Food Act [29]. The protein content was determined using the Dumas method as laid down in the German Food Act [29] using a TruMac N Protein Analyzer (LECO Corporation, St. Joseph, MI, USA) with a conversion factor of 6.25. The lipid content was determined in accordance with Caviezel, DGF K-I 2c (00) [32]. The ash content (at 950 ◦C) was determined thermogravimetrically in accordance with AOAC International [33]. The crude ﬁbre content was analysed with a Fibretherm (Gerhardt GmbH & Co. KG, Königswinter, Germany) in accordance with AOAC International [34]. Soluble and insoluble ﬁbre contents of RPC were analysed by enzymatic-gravimetric analysis according to AOAC 991.43. The starch content was determined using the hexokinase method [35]. All analyses were at least carried out in duplicate.

2.4.2. Particle Size Distribution The particle size distribution of the RPC, RP and starches was determined using a Malvern laser diffraction particle size analyser (Mastersizer S Long Bed Version 2.15, Malvern Instruments Ltd, Malvern, UK) as described by Osen and Toelstede [36]. Analyses were carried out in duplicate.

2.5. Physical Properties of Extrudates 2.5.1. Expansion Sectional and longitudinal expansion indices (SEI and LEI) were calculated according to Alvarez-Martinez and Kondury [37] and Carvalho and Takeiti [38]. The volume expansion index (VEI) is the product of LEI and SEI.

2.5.2. Speciﬁc Hardness For texture analyses, 10 randomly chosen extrudate strands were measured with a digital calliper to determine their length lE and diameter dE before being placed on the measuring platform of a texture analyser (TA.XTplus, Stable Micro Systems, Surrey, UK) equipped with a 50 kg measuring cell. A cylindrical punch (SMS P/25L) compressed the Polymers 2021, 13, 215 5 of 19

samples to 30% deformation at a test speed of 8 mm s−1 until breakage. The applied force 2 (Fmax) on the lateral surface of the extrudate strands is the speciﬁc hardness (N/mm ).

2 Hspec [N⁄mm ] = Fmax/(dE × lE × π). (1)

2.6. Water Absorption and Water Solubility The water absorption was determined following the method of 56-20.01 with slight modiﬁcation [39]. For this, 2 g of powder (mSample) was placed in a 50 mL centrifuge tube (mCentr) and mixed with 40 mL demineralised water. After 10 min stirring with a reagent shaker (VF 2, Janke & Kunkel, IKA-Labortechnik GmbH, Staufen, Germany), the samples were centrifuged for 10 min at 1000 relative centrifugal force (RCF) and 20 ◦C using a 6K15 centrifuge (Sigma Laborzentrifugen GmbH, Osterode am Harz, Germany). The supernatant was decanted into a tared aluminium dish (mAlu), dried in an oven at 105 ◦C for 18 h (Heraeus UT 6060, Thermo Electron LED GmbH, Langenselbold, Germany) and then weighed after drying (mAlu + Solids). The residual sediment in the centrifuge tube was also weighed (mCentr + Sed). The water absorption index (WAI) was calculated using Equation (2). WAI [g/g]= (m(Centr + Sed) − mCentr)/msample (2) The water solubility index (WSI) was calculated according to Equation (3) and described the amount of dry solids in the supernatant relative to the original sample mass [40]. The samples were analysed in duplicate.

WSI [%]= (m(Alu + Solids) − mAlu)/mSample × 100 (3)

2.7. Rheological Properties The impact of the thermal treatment on structure formation in the starches, RPC and starch/RPC mixtures were investigated using a closed cavity rheometer (RPA ﬂex, TA Instruments, New Castle, Delaware, USA) as described by Quevedo and Jandt [41]. Before the rheological analyses, the dry powders were mixed with demineralised water to obtain moisture contents of 24 or 29 g/100 g (corresponding to the moisture conditions in the extruder). After mixing, the samples were stored at 4 ◦C for at least 8 h to allow adequate moisture distribution and brought to room temperature before analysis. For the oscillatory tests, 6 g of sample were placed between two grooved and thermoregulated cones. The lower cone oscillated and a transducer in the upper cone received the torque response of the sample, allowing rheological properties such as the complex modulus |G*|, storage modulus G’, loss modulus G” and complex viscosity |η*| to be calculated. The complex modulus |G*| describes the stress–strain relationship of a linear viscoelastic material. The ratio of |G*| to the frequency gives the complex viscosity of a sample. Experiments were conducted in duplicate and on the LVE region (data not shown). The impact of thermal treatment on the reaction onset temperature was evaluated by . temperature sweep analyses in a range from 30 to 140 ◦C (heating rate 5 K/min, γ = 0.1 s−1). To evaluate the impact of extrusion on the rheological properties of the samples, . extruded samples were treated 10 s at a shear rate of γ = 31 s−1, before they were kept . at a constant low shear rate of γ = 0.1 s−1 for 8 min. The pretreatment and measurement ◦ temperatures TPre and TM were 100, 120 or 140 C.

2.8. Statistical Analysis All experiments were at least performed in duplicate and were given as mean values with standard errors. The effects of starch/RPC ratio and process conditions were determined by one-way analysis of variance (ANOVA) with a signiﬁcance threshold of p < 0.05. Where appropriate, mean values were compared using Tukey’s honest signiﬁcance test using Origin Lab Software (OriginLab, Northampton, MA, USA). Polymers 2021, 13, 215 6 of 19

3. Results 3.1. Chemical Composition The composition of the RPC, RP, PS, WPS and RPC/starch mixtures is summarised in Table3. Table4 reports the dry matter content (g/100 g) of the extruded and dried samples.

Table 3. Chemical composition and particle size of rapeseed press cake (RPC), rapeseed peel (RP), potato starch (PS), waxy potato starch (WPS) and the mixtures of starch with 10, 40 or 70 g/100 g rapeseed press cake (RPC).

DM Protein × 6.25 Lipid Raw Fibre Ash Starch Particle Size Raw Material (g/100 g) (g/100 g) (%DM) (%DM) (%DM) (%DM) (%DM) d50.3 (µm) RPC 95.1± 0.03 38.2 ± 0.30 23.4 ± 0.90 4.7 ± 0.03 7.3 ± 0.02 3.00 ± 0.02 261.1 ± 4.5 RP 93.6 ± 0.18 15.7 ± 0.08 25.5 ± 1.12 29.4 ± 0.31 4.1 ± 0.03 4.39 ± 0.03 418.9 ± 15.9 PS 81.9 ± 0.02 n.a. n.a. n.a. n.a. 97.6 ± 0.40 40.3 ± 1.2 WPS 80.5 ± 0.07 n.a. n.a. n.a. n.a. 91.7 ± 0.25 38.1 ± 0.9 Calculated chemical composition (g/100 g) 30PS/70RPC 91.1 26.7 16.4 3.3 n.a. 32.4 n.a. 50PS/40RPC 88.7 16.0 15.6 3.4 n.a. 50.2 n.a. 70PS/10RPC 86.2 5.4 14.9 3.4 n.a. 69.0 n.a. 30WPS/70RPC 90.7 26.7 16.4 3.3 n.a. 29.6 n.a. 50WPS/40RPC 88.0 16.0 15.6 3.4 n.a. 47.3 n.a. 70WPS/10RPC 85.2 5.4 14.9 3.4 n.a. 64.9 n.a.

Table 4. Dry matter content of potato starch (PS) and waxy potato starch (WPS) mixtures with 10, 40 or 70 g/100 g rapeseed press cake (RPC) after extrusion processing at 24 or 29 g/100 g moisture content and a barrel temperature of 20, 80, 100, 120 or 140 ◦C and drying (40 ◦C, 24 h).

Barrel Temperature (◦C) 24 g/100 g moisture Not extruded 20 80 100 120 140 PS 90.4 ± 0.02 90.5 ± 0.04 92.1 ± 0.12 87.9 ± 0.11 88.6 ± 0.03 86.2 ± 0.04 70PS/10RPC 91.2 ± 0.01 91.5 ± 0.02 92.3 ± 0.02 88.3 ± 0.04 89.8 ± 0.12 88.5 ± 0.05 50PS/40RPC 86.9 ± 0.02 92.5 ± 0.03 92.3 ± 0.07 92.5 ± 0.05 89.9 ± 0.04 91.3 ± 0.15 30PS/70RPC 91.9 ± 0.04 91.7 ± 0.11 95.5 ± 0.03 93.2 ± 0.06 91.9 ± 0.18 95.8 ± 0.14 WPS 92.5 ± 0.03 92.1 ± 0.09 91.6 ± 0.04 90.7 ± 0.05 – – 70WPS/10RPC 89.9 ± 0.03 90.8 ± 0.05 88.5 ± 0.05 88.6 ± 0.11 89.9 ± 0.15 90.1 ± 0.05 50WPS/40RPC 90.0 ± 0.02 92.3 ± 0.02 90.0 ± 0.02 88.6 ± 0.09 90.0 ± 0.04 89.8 ± 0.06 30WPS/70RPC 91.1 ± 0.02 92.2 ± 0.04 93.0 ± 013 91.8 ± 0.11 92.3 ± 0.11 91.2 ± 0.02 29 g/100 g moisture Not extruded 20 80 100 120 140 PS 91.1 ± 0.07 90.6 ± 0.02 92.3 ± 0.06 88.7 ± 0.18 89.6 ± 0.12 90.1 ± 0.08 70PS/10RPC 92.0 ± 0.06 92.1 ± 0.09 92.8 ± 0.12 89.0 ± 0.08 90.4 ± 0.06 89.8 ± 0.09 50PS/40RPC 87.1 ± 0.08 93.2 ± 0.20 93.0 ± 0.07 93.0 ± 0.14 90.0 ± 0.05 91.3 ± 0.14 30PS/70RPC 92.1 ± 0.08 92.7 ± 0.01 97.8 ± 0.04 94.3 ± 0.01 92.6 ± 0.04 96.6 ± 0.11 WPS 92.5 ± 0.02 92.1 ± 0.15 91.6 ± 0.14 90.7 ± 0.12 – – 70WPS/10RPC 89.9 ± 0.04 90.8 ± 0.03 88.5 ± 0.02 88.6 ± 0.01 89.9 ± 0.04 90.1 ± 0.12 50WPS/40RPC 90.0 ± 0.01 92.3 ± 0.09 90.0 ± 0.06 88.6 ± 0.06 90.0 ± 0.20 89.8 ± 0.08 30WPS/70RPC 91.1 ± 0.11 92.2 ± 0.15 93.0 ± 0.07 91.8 ± 0.06 92.3 ± 0.16 91.2 ± 0.13

The data show that RPC had a protein content of 38.2 g/100 g. Previous studies reported lower RPC protein contents (18.6–30.0 g/100 g) [2,5,7]. The comparatively high protein content of the RPC used in this study might be due to the protein-rich Brassica napus L. variety our supplier processes. Furthermore, peeling the rapeseed before deoiling results in a comparatively low ﬁbre content, increasing the RPC protein content. However, the protein content of the RPC that was used for this work lies within the range of that of rapeseed meals previously studied (34.4–40.6 g/100 g) [4]. Polymers 2021, 13, 215 7 of 19

The RP obtained from the dehulling of the rapeseed had a lower protein content and higher lipid content than RPC (15.7 g/100 g protein and 25.5 g/100 g lipid). The fibre content of RPC (4.7 g/100 g) was much lower than the fibre content of the RP (29.4 g/100 g). In the RPC/starch mixtures, the protein content increases from 5.4 to 26.7 g/100 g with increasing RPC content, whereas the starch content decreases from 69.0 to 29.6 g/100 g. The lipid and fibre contents of the RPC/starch mixtures were kept constant by adding small amounts of RO and RP. PS and WPS had the smaller particle sizes than RPC and RP.

3.2. Physical Properties of the Extrudates 3.2.1. Expansion As expansion phenomena is linked to flash evaporation at > 100 ◦C barrel temperature, the expansion indices of samples extruded at such temperatures are discussed here. Figure1a shows that the addition of RPC to potato starch at 40 g/100 g increased the SEI, but decreased the LEI and VEI, significantly. A further increase in the RPC content to 70 g/100 g resulted in an even higher SEI, whereas LEI was not further influenced. This resulted in a product with higher VEI compared to the sample without RPC. The results PolymersPolymersPolymers 2021 20212021,, , 13 1313,, , x xx FOR FORFOR PEER PEERPEER REVIEW REVIEWREVIEW 888 ofofof 212121 with WPS depicted in Figure1b show a different response to the RPC addition. Here, the Polymers 2021, 13, x FOR PEER REVIEWaddition of 40 g/100 g RPC leads to lower SEI and VEI but higher LEI compared to samples8 of 21

with only 10 g/100 g RPC. Samples containing 70 g/100 g RPC exhibited a larger SEI and VEI than samples containing 40 g/100 g RPC, whereas the LEI only slightly increased. 555 a)a)a) 555 b)b)b) 444 5 a) 444 5 b) 333 4 333 4 3 SEI [-] SEI SEI [-] SEI SEI [-] SEI 222 SEI [-] SEI SEI [-] SEI SEI [-] SEI 222 3

SEI [-] SEI 2

111 [-] SEI 111 2 1 0.80.80.8 040700407004070 0.8 1 0.80.8 101010 40 40 40 70 70 70 0.60.60.6 0.8 04070 0.60.60.6 0.8 10 40 70 0.40.40.4 0.6 0.40.40.4 0.6 LEI [-] LEI LEI [-] LEI LEI [-] LEI 0.4 LEI [-] LEI LEI [-] LEI 0.20.20.2 [-] LEI 0.20.20.2 0.4 LEI [-] LEI

0.00.00.0 0.2 [-] LEI 0.00.00.0 0.2 1.01.0 040700407004070 1.0 0.0 1.01.01.0 10 40 70 0.01010 40 40 70 70 1.0 04070 1.0 10 40 70 0.50.50.5 0.50.50.5 VEI [-] VEI VEI [-] VEI VEI [-] VEI VEI [-] VEI VEI [-] VEI 0.5 [-] VEI 0.5 VEI [-] VEI 0.00.0 [-] VEI 0.0 0.00.00.0 040700407004070 10 40 70 0.0 0.01010 40 40 70 70 RPCRPCRPC contentcontentcontent [[[%%%]]] RPCRPC contentcontent [%][%] 04070 RPC10 content [%] 40 70 RPC content [%] RPC content [%]

FigureFigureFigureFigure 1. 1.1. Sectional, Sectional,Sectional, 1. Sectional, longitudinal longitudinallongitudinal longitudinal and andand volumetric volumetricvolumetric and volumetric expansion expansionexpansion expansion indices indicesindices indices (SEI, (SEI,(SEI, LEI, LEI,(SEI,LEI, and andand LEI, VEI) VEI)VEI) and of VEI)ofof a aa))) potato potato ofpotato (a) potato starch starchstarch starch (PS) (PS)(PS) and andand (PS) b bb) and) ) waxy waxywaxy (b) waxy potatopotatopotatopotato starchstarchstarch starch (WPS)(WPS)(WPS) (WPS) andandand their andtheirtheir their mixturesmixturesmixtures mixtures withwithwith with 10,10,10, 10, 404040 40 andandand and 707070 70 g/100g/100g/100 g/100 ggg grapeseedrapeseedrapeseed rapeseed presspresspress press cakecakecake (RPC) (RPC)(RPC) extrudedextrudedextruded at atatat a aaa moisture moisturemoisturemoisture content Figure 1. 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RPC.RPC. g LEI LEI RPC.LEI and andand LEIhas VEI VEIVEI andmainly were werewere VEI ledlowest werelowestlowest to lowestan for forfor increase 40 4040 for g/100 g/100g/100 40 in g/100 g gg SEI, RPC RPCRPC gLEI samples samplessamples RPC and samples VEI in PS extrudedextrudedextruded samples at atat 100 100100 °C. °C.°C. containing In InIn WPS WPSWPS samples, samples,samples, 70 g/100 a a ag barrel barrel barrelRPC. temperature temperatureLEItemperature and VEI of wereofof 100 100100 lowest °C °C°C resulted resultedresulted for 40 in in ing/100 higher higherhigher g RPC SEI SEISEI samples andandand VEI VEIVEI regardless regardlessextrudedregardless at of ofof 100 RPC RPCRPC °C. content, content,content, In WPS but but butsamples, did diddid not notnot a have barrelhavehave a a a temperature large largelarge impact impactimpact of on onon 100 LEI. LEI.LEI. °C resulted in higher SEI ItItIt isisis and expectedexpectedexpected VEI regardless thatthatthat thethethe additionadditionaddition of RPC content, ofofof proteiproteiprotei butn-n-n- andand anddid fibre-richfibre-richnotfibre-rich have a biopolymersbiopolymersbiopolymers large impact suchsuch suchon LEI. as asas presspress press cakescakescakes to toto starch starchstarchIt influences influences influencesis expected the thethe that rheological rheologicalrheological the addition prop propprop ofertiesertieserties protei of ofof n- the thethe and melt meltmelt fibre-rich [42], [42],[42], which whichwhich biopolymers further furtherfurther affects affectsaffects such as press thethethe expansionexpansionexpansioncakes behaviourtobehaviourbehaviour starch influences ofofof thethethe matrix.matrix.matrix. the rheological AAA numbnumbnumbererer prop ofofof experimentalexperimentalertiesexperimental of the melt studiesstudiesstudies [42], describe describedescribewhich further thethethe affects closecloseclose correlationcorrelationcorrelationthe expansion ofofof thethethe rheologicalrheologicalbehaviourrheological of propertipropertiproperti the matrix.eseses ofofof biopolymersbiopolymersbiopolymersA number of andandand experimental the thethe correspondingcorrespondingcorresponding studies describe ex-ex-ex- the pansionpansionpansion behaviour behaviourclosebehaviour correlation in inin extrusion extrusionextrusion of the [42–46]. [42–46]. [42–46].rheological It ItIt was waswas properti found foundfound es that thatthat of the thebiopolymersthe viscosity viscosityviscosity of ofofand the thethe the melt meltmelt corresponding is isis asso- asso-asso- ex- ciatedciatedciated with withwithpansion axial axialaxial expansion expansionbehaviourexpansion and inandand extrusion LEI, LEI,LEI, whereas whereaswhereas [42–46]. melt meltmelt It was elasticity elasticityelasticity found is is isthat correlated correlatedcorrelated the viscosity with withwith radial ofradialradial the meltex- ex-ex- is asso- pansionpansionpansion and andandciated therefore thereforetherefore with axial the thethe SEI SEIexpansionSEI [37]. [37].[37]. In InIn andour ourour stud LEI,studstud y,y,whereasy, we wewe did diddid meltnot notnot differentiate differentiate differentiateelasticity is betweencorrelated betweenbetween viscous viscousviscous with radial ex- andandand elastic elasticelasticpansion properties propertiesproperties and thereforein inin rheo rheorheologicallogicallogical the SEI analyses, analyses,analyses, [37]. In but butourbut showed showedstudshowedy, we th thth atatdidat the thethe not starch/RPC starch/RPCstarch/RPC differentiate blends blendsblends between ex- ex-ex- viscous hibitedhibitedhibited lower lowerlowerand elastic |G*| |G*||G*| with withwithproperties increa increaincrea singinsingsing rheo RPC RPCRPClogical content contentcontent analyses, and andand temperature. temperature.temperature. but showed An AnAnthat alteration alterationalteration the starch/RPC of ofof rheo- rheo-rheo- blends ex- logicallogicallogical properties propertiespropertieshibited lower due duedue to toto|G*| RPC RPCRPC with addition additionaddition increa associated associatedassociatedsing RPC contentwith withwith an anan and increase increaseincrease temperature. in inin LEI LEILEI and andand An a aa alteration decrease decreasedecrease of rheo- ininin SEISEISEI was,was,was,logical i.e.,i.e.,i.e., propertiesshownshownshown ininin WPS-basedWPS-baseddueWPS-based to RPC samplessamplesadditionsamples with withwithassociated 404040 ororor 70with7070 g/100g/100g/100 an increase ggg RPC,RPC,RPC, respectively.inrespectively.respectively. LEI and a decrease 30WPS/70RPC30WPS/70RPC30WPS/70RPCin SEI showed showedshowedwas, i.e., the thethe shown lowest lowestlowest in |G*| |G*||G*| WPS-based in inin rheolo rheolorheolo samplesgicalgicalgical analyses analysesanalyses with 40 (discussed (discussed(discussed or 70 g/100 later, later,later, g RPC, Figure FigureFigure respectively. 4) 4)4) andandand the thethe highest highesthighest30WPS/70RPC LEI LEILEI at atat 29 2929 showed g/100 g/100g/100 g gg the moisture moisturemoisture lowest content. content. content.|G*| in rheological analyses (discussed later, Figure 4) TheTheThe observedand observedobserved the highest decrease decreasedecrease LEI in in inat SEI SEI SEI29 g/100due duedue to toto g 40 4040moisture g/ g/g/100100100 g gg content. RPC RPCRPC addition additionaddition in inin WPS WPSWPS samples samplessamples in inin our ourour studystudystudy is isis in inin accordance accordanceaccordanceThe observed to toto previous previousprevious decrease studies studiesstudies in SEI that that thatdue found found foundto 40 g/lower lowerlower100 g sectional sectional sectionalRPC addition expansion expansionexpansion in WPS of ofof extru- extru-samplesextru- in our datesdatesdates when whenwhenstudy fruit fruitfruit is presspressinpress accordance cakes cakescakes or oror other tootherother previous by-produ by-produby-produ studiesctsctscts were werewere that added foundaddedadded lowerto toto a aa starch starchstarch sectional matrix matrixmatrix expansion [20–23]. [20–23].[20–23]. of extru- TheTheThe authors authorsauthorsdates of ofof whenstudies studiesstudies fruit on onon bilberry, pressbilberry,bilberry, cakes apple appleapple or other or oror bl blblackcurrant ackcurrantackcurrantby-products press presspress were cake cakecake added attributed attributedattributed to a starch this thisthis matrixto toto the thethe [20–23]. presencepresencepresence Theof ofof dietary dietarydietary authors fibre fibrefibre of studies in inin the thethe press press presson bilberry, cakes. cakes.cakes. Es Es Esapplepeciallypeciallypecially or bl the thetheackcurrant presence presencepresence press of ofof inert inertinert cake insoluble insolubleinsoluble attributed fibre fibrefibre this to the presence of dietary fibre in the press cakes. Especially the presence of inert insoluble fibre

Polymers 2021, 13, 215 8 of 19

extruded at 100 ◦C. In WPS samples, a barrel temperature of 100 ◦C resulted in higher SEI and VEI regardless of RPC content, but did not have a large impact on LEI. It is expected that the addition of protein- and fibre-rich biopolymers such as press cakes to starch influences the rheological properties of the melt [42], which further affects the expansion behaviour of the matrix. A number of experimental studies describe the close correlation of the rheological properties of biopolymers and the corresponding expansion behaviour in extrusion [42–46]. It was found that the viscosity of the melt is associated with axial expansion and LEI, whereas melt elasticity is correlated with radial expansion and therefore the SEI [37]. In our study, we did not differentiate between viscous and elastic properties in rheological analyses, but showed that the starch/RPC blends exhibited lower |G*| with increasing RPC content and temperature. An alteration of rheological properties due to RPC addition associated with an increase in LEI and a decrease in SEI was, i.e., shown in WPS-based samples with 40 or 70 g/100 g RPC, respectively. 30WPS/70RPC showed the lowest |G*| in rheological analyses (discussed later, Figure 4) and the highest LEI at 29 g/100 g moisture content. The observed decrease in SEI due to 40 g/100 g RPC addition in WPS samples in our study is in accordance to previous studies that found lower sectional expansion of extrudates when fruit press cakes or other by-products were added to a starch matrix [20–23]. The authors of studies on bilberry, apple or blackcurrant press cake attributed this to the presence of dietary fibre in the press cakes. Especially the presence of inert insoluble fibre particles was associated with a strong physicochemical incompatibility with starch, a decrease in viscosity and a reduced SME that overall resulted in a decreased SEI [22,23,47,48]. Furthermore, insoluble fibre particles tend to rupture the cell walls before the gas bubbles extend to their full size [49]. The RPC used in this study contains 35.67 ± 5.19 g/100 g total dietary fibre, whereof 88.3% are insoluble and 11.7% are soluble. The impact of extrusion on the dietary fibre content of RPC will be part of future studies. Since with RPC addition, the amount of insoluble fibre in the blends, which act mainly as filler particles, increases markedly, the decrease in SEI in sample WPS40/RPC50 might be explained. However, the high SEI and VEI of samples with 70 g/100 g do not agree with this hypothesis. It can be assumed that high SEI and VEI in RPC-rich samples are driven by swelling expansion mechanisms more than by sudden water evaporation and may be associated with the presence of soluble fibres and an overall high water binding and holding capacity of RPC compared to starch. Soluble fibre fractions have been shown to exhibit large expansion volumes due to their greater water-binding capacity, which enables more nucleation sites for water vapor to develop at die exit [48,50]. Many authors have described that the high shear force in the extruder barrel might mechanically break parts of the insoluble fibre fractions, which increases the fraction of soluble dietary fibre and increases the water binding of extruded materials compared to unextruded materials [51]. We assume that parts of the insoluble fibre fractions in RPC are solubilized during extrusion processing and enhance swelling expansion, accompanied by an increase in WAI of extruded samples. Results in Section 3.2.3 confirm that RPC-rich samples show increased WAI due to extrusion treatment. Unfortunately, the extrusion of 70PS/10RPC resulted in severe extruder clogging, wherefore no coherent extrudate strains were generated and the direct measurement of expansion indices was not possible. This means that the impact of starch type on the expansion of samples containing 10 g/100 g RPC cannot be compared here. However, the increase in RPC from 40 to 70 g/100 g increased the SEI and VEI of both PS and WPS based samples. At 140 ◦C barrel temperature, PS samples containing 70 g/100 g RPC showed a higher SEI compared to WPS-based samples. This effect cannot be explained with our results, but might be due to a low gelatinisation temperature, large granule size and favourable amylose content of PS compared to WPS that allowed bubble growth and extrudate fixation and enhanced the sectional expansion [52]. Polymers 2021, 13,Polymers x FOR PEER 2021 ,REVIEW 13, x FOR PEER REVIEW 8 of 21 8 of 21 Polymers 2021, 13, 215 9 of 19

3.2.2.5 Specific Hardness5 5 5 The specifica) hardnessa) of the samples is shown in Figure2. Forb) WPS samples,b) the 4 4 specific hardness of the extrudates decreased with increasing RPC4 content. Similar4 results Polymers 2021, 13, x FOR PEER REVIEW were3 observed with3 PS samples regardless of the barrel temperature and moisture10 of content. 21 3 3 In WPS as single component, a higher moisture content significantly increased the hardness, SEI [-] SEI SEI [-] SEI 2 2 SEI [-] SEI but in samples containing RPC, this effect was not observed. [-] SEI 2 2 1 1 1 1 0.8 040700.8 04070 0.8 0.8 10 4010 40 70 70 0.6 0.6 0.6 0.6 0.4 0.4 0.4 0.4 LEI [-] LEI LEI [-] LEI LEI [-] LEI 0.2 0.2 [-] LEI 0.2 0.2 0.0 0.0 0.0 0.0 1.0 040701.0 04070 1.0 101.0 4010 40 70 70

0.5 0.5 0.5 0.5 VEI [-] VEI VEI [-] VEI VEI [-] VEI VEI [-] VEI

0.0 0.0 0.0 0.0 04070 04070 10 4010 40 70 70 RPC contentRPC [%] content [%] RPC contentRPC [%] content [%]

Figure 1. Sectional,Figure longitudinal 1. Sectional, and longitudinal volumetric andexpansion volumetric indices expansion (SEI, LEI, indices and VEI) (SEI, of LEI, a) potato and VEI) starch of a(PS)) potato and starchb) waxy (PS) and b) waxy FigureFigure 2.potato Specific 2. Specific starch hardness (WPS) hardnesspotato of and waxystarch of their waxy potato (WPS) mixtures potato starch and starch withtheir (WPS (WPS)10,mixtures) and 40 and potato with potato70 starchg/100 10, starch40 (PS)g andrapeseed (PS)and 70 their andg/100 press theirmixtures g rapeseedcake mixtures (RPC) press extruded cake (RPC) at a moisture extruded at a moisture withwith 0, content10, 0, 40 10, and 40of 2970 and g/100g/100 70content g/100 gg andrapeseed of gbarrel29 rapeseed g/100 press temperatures g andcake press barrel (RPC) cake of temperatures 100 (RPC)extruded °C extruded( at ) ,1 24 of20 ( 100°C at ) ( 24°Cand ) ( and 29 ) (,1 and14020 ) g/100°C 29 ( ( g )). and g/100Values 140 g are°C (shown ). Values as mean are shownvalues as mean values moisturemoisturewith content standard content and andbarreldeviationswith barrel standardtemperatures temperatures based deviations on of10 100,duplicates. of based 100, 120 120and on and 14010 duplicates. 140°C. ◦ValuesC. Values are areshown shown as mean as mean val- values ues withwith standardstandard deviationsdeviations basedbased onon 1010 duplicates.duplicates. The increase inThe temperature increase in has temperature mainly led has to mainlyan increase led toin anSEI, increase LEI and in VEI SEI, in LEI PS and VEI in PS Furthermore,Furthermore, although although extrudedsamples extruded PS containing was PSsamples was soft softerer 70 than containingg/100 than WPS, g WPS,RPC. no70 LEIg/100difference no differenceand g RPC.VEI in were LEIspecific inspecific andlowest VEI for were 40 g/100lowest g forRPC 40 samples g/100 g RPC samples hardnesshardness was was seen seen between between theextruded thetwo two starch starchat 100typesextruded types°C. when In whenWPS at RPC 100 samples, RPC °C.was wasIn added, WPS a added,barrel samples, which temperature which was a wasbarrel also also of temperature 100 °C resulted of 100 in °Chigher resulted SEI in higher SEI seenseen for forthe thecorrelation correlation between betweenand starch VEI starch typeregardless type andand and expansion. VEI of expansion. RPCregardless content, The The hardestof butRPC hardest did content,extrudates not extrudates have but werea did large were not impact have a on large LEI. impact on LEI. ◦ foundfound for forPS PSand and WPS WPS extruded extruded at Ita at ismoisture a expected moisture content Itthat content is expectedthe of addition29 of 29g/100 that g/100 of gthe proteiand gaddition and 120n- 120andor of100 or fibre-richprotei 100 °C n-C andbiopolymers fibre-rich suchbiopolymers as press such as press barrelbarrel temperature, temperature, respectively. respectively.cakes to starchcakes influences to starch the influencesrheological the prop rheologicalerties of theprop melterties [42], of whichthe melt further [42], whichaffects further affects RPCRPC contains contains a number a number of componentsthe of componentsexpansion thatthe behaviour that mightexpansion might have of have thebehavioura large amatrix. large impact impactof A the numbon thematrix. oner thespecific ofspecific Aexperimental numb er of experimentalstudies describe studies the describe the hardnesshardness of the of theextrudates, extrudates, such suchclose as pr ascorrelationoteins, proteins, fibres,close fibres,of correlation thesuga sugarsrheologicalrs and andof lipids. the lipids.properti rheological es of biopolymersproperties of andbiopolymers the corresponding and the corresponding ex- ex- TheThe results results suggest suggest that that onepansion one reason reason behaviour for forapansion decrease a decrease in extrusion behaviour in hardness in hardness [42–46]. in extrusion with It with wasincreasing increasing[42–46]. found thatRPC It was RPC the found viscosity that of the the viscosity melt is asso-of the melt is asso- contentcontent in our in study our study might might be theciated be relatively the with relatively axialhiciatedgh proteinexpansion high with protein content axial and expansion content LEI,of RPC. whereas of The and RPC. work meltLEI, The ofwhereaselasticity workZhu of melt is correlated elasticity with is correlated radial ex- with radial ex- andZhu Shukri and [26] Shukri showed [26] that showed thepansion addition that theand of addition thereforepansion 20 g/100 of andthe g 20 soySEI therefore g/100 [37].protein gIn the soy ourconcentrate SEI proteinstud [37].y, weIn concentrate to ourdid corn studnot differentiatey, we did not between differentiate viscous between viscous starchto cornextrudates starch resulted extrudates in a resulted decreasedand elastic in hardness a properties decreasedand elastic of inthe hardness rheoproperties product,logical of which theinanalyses, rheo product, islogical in butaccordance which showedanalyses, is th in butat the showed starch/RPC that the blends starch/RPC ex- blends ex- to ouraccordance results. The to oursame results. trend was Thehibited found same lower by trend [25], hibited|G*| was where with foundlower 25increa g/100 |G*| bysing [25 g with ],whey RPC where increa proteincontent 25sing g/100 inand RPC starch- gtemperature. content whey and Antemperature. alteration Anof rheo- alteration of rheo- basedprotein extrudates in starch-based resulted in extrudates a decreaselogical resulted properties in hardness.logical ina due decrease However,properties to RPC in addition hardness.a due decrease to RPCassociated However, in additionhardness with adecrease associatedwas an increase with in anLEI increase and a decrease in LEI and a decrease oftenin accompanied hardness was oftenby a decreased accompaniedin SEI SEI was, byin a thesei.e., decreasedin shownSEI studies. was, SEI in Ini.e.,WPS-based in our these shown work, studies. in samples RPC-rich WPS-based In our with samples work, samples40 RPC-or 70 withg/100 40 g orRPC, 70 g/100respectively. g RPC, respectively. exhibitedrich samples a high exhibited SEI and aVEI high but30WPS/70RPC SEI low and specific VEI but30WPS/70RPC showed hardness. low specific the Therefore,lowest showed hardness. |G*| thewe Therefore,in lowestassume rheolo |G*|gical that we assume inanalysesthe rheolo gical(discussed analyses later, (discussed Figure 4) later, Figure 4) swellingthat the expansion swelling mechanisms expansionand mechanisms described the highest befoand described LEIre the generated at highest29 before g/100 LEIa g generatedporous moisture at 29 g/100structure acontent. porous g moisture that structure did content. not thatrequire did large not require breaking large forces breaking to Thebe forcesmech observedanically to be Thedecrease mechanically disrupted. observed in SEI Furthermore,decrease disrupted. due to 40in Furthermore,g/SEI 100the due gsoluble RPC to 40 addition theg/100 g RPCin WPS addition samples in WPSin our samples in our β fibresoluble fractions fibre in fractions RPC, mostly in RPC, pectins,study mostly is hemicelluloses, in pectins, accordancestudy hemicelluloses, is in tomixed accordanceprevious β-glucans, mixed studies to previous gums-glucans, that foundand studies gums muci- lower that and sectional found lower expansion sectional of extru-expansion of extru- mucilages [53,54], might have softened the structure of the expanded RPC-rich extrudates, lages [53,54], might have softeneddates the when structur fruitdatese ofpress thewhen expandedcakes fruit or press other RPC-rich cakes by-produ orextrudates, othercts were by-produ es- addedcts to were a starch added matrix to a [20–23].starch matrix [20–23]. especially under the assumption that parts of the insoluble fibre fractions have been pecially under the assumption Thethat authorsparts of of theThe studies insoluble authors on bilberry, offibre studies fractions apple on bilberry, orhave blackcurrant been apple solu- or pressblackcurrant cake attributed press cake this attributed to the this to the solubilised due to extrusion processing. This in in alignment with previous studies who bilised due to extrusion processing.presence This of in dietary inpresence alignment fibre of in dietary withthe press previous fibre cakes. in the studies Es presspecially who cakes. the re- Espresencepecially of the inert presence insoluble of inert fibre insoluble fibre ported that pectin and other soluble fibres have been shown to decrease specific hardness of directly expanded snack products [55].

3.2.3. Water Absorption Index and Water Solubility Index Table 5 summarises the WAI of PS and WPS mixtures for different extrusion moisture contents and temperatures > 100 °C. In both PS/RPC and WPS/RPC mixtures, the WAI increased with increasing barrel temperature. The opposite was found for PS and WPS as

Polymers 2021, 13, 215 10 of 19

reported that pectin and other soluble ﬁbres have been shown to decrease speciﬁc hardness of directly expanded snack products [55].

3.2.3. Water Absorption Index and Water Solubility Index Table5 summarises the WAI of PS and WPS mixtures for different extrusion moisture contents and temperatures > 100 ◦C. In both PS/RPC and WPS/RPC mixtures, the WAI increased with increasing barrel temperature. The opposite was found for PS and WPS as single ingredients. Untreated mixtures with 70 g/100 g RPC had higher WAIs than starch-rich mixtures.

Table 5. Water absorption index (WAI) of potato starch (PS) and waxy potato starch (WPS) and their mixtures with 10, 40 and 70 g/100 g rapeseed press cake (RPC) extruded at 24 and 29 g/100 g moisture content and barrel temperatures of 20, 80, 100, 120 and 140 ◦C. Mean values with different superscript letters (a, b, c, d) within one column (starch type and moisture content) indicate signiﬁcant differences (p > 0.05) based on a one-way analysis of variance (ANOVA). Where appropriate, the mean values were compared using Tukey’s honest signiﬁcance test.

Moisture Content Barrel Temperature Mixture Untreated (g/100 g) 20 ◦C 80 ◦C 100 ◦C 120 ◦C 140 ◦C PS 1.67 ± 0.01 a 24 – 1.78 ± 0.25 a 0.59 ± 0.04 a 0.58 ± 0.17 a 0.86 ± 0.13 a 70PS/10RPC 1.78 ± 0.02 a 24 2.84 ± 0.04 a 2.69 ± 0.01 b 4.63 ± 0.02 b 4.63 ± 0.01 b 8.16 ± 0.10 b 50PS/40RPC 2.29 ± 0.01 b 24 2.60 ± 0.01 b 2.70 ± 0.02 b 3.14 ± 0.02 c 3.45 ± 0.02 c 5.99 ± 0.10 c 30PS/70RPC 2.88 ± 0.08 c 24 3.11 ± 0.01 c 3.17 ± 0.02 b 4.17 ± 0.02 d 4.83 ± 0.35 b 5.53 ± 0.01 d PS 1.67 ± 0.01 a 29 – 1.05 ± 0.27 a 1.77 ± 0.04 a 1.57 ± 0.08 a 0.96 ± 0.24 a 70PS/10RPC 1.78 ± 0.02 a 29 2.78 ± 0.03 a 3.71 ± 0.02 b 4.41 ± 0.00 b 5.05 ± 0.04 b 7.73 ± 0.05 b 50PS/40RPC 2.29 ± 0.01 b 29 2.47 ± 0.04 b 3.12 ± 0.02 c 3.88 ± 0.07 c 4.47 ± 0.13 c 6.07 ± 0.33 c 30PS/70RPC 2.88 ± 0.08 c 29 2.93 ± 0.01 c 3.01 ± 0.06 c 3.63 ± 0.02 d 4.46 ± 0.03 c 4.48 ± 0.07 d Moisture content Barrel temperature Mixture untreated (g/100 g) 20 ◦C 80 ◦C 100 ◦C 120 ◦C 140 ◦C WPS 1.88 ± 0.05 a 24 – 0.51 ± 0.05 a ––– 70WPS/10RPC 1.89 ± 0.15 a 24 2.64 ± 0.02 a 3.22 ± 0.02 b 5.55 ± 0.01 a 6.00 ± 0.02 a 8.89 ± 0.99 a 50WPS/40RPC 2.30 ± 0.04 b 24 2.41 ± 0.01 b 2.54 ± 0.00 c 3.55 ± 0.01 b 4.41 ± 0.01 b 6.72 ± 0.60 ab 30WPS/70RPC 2.79 ± 0.00 c 24 3.05 ± 0.01 c 2.96 ± 0.12 a 3.65 ± 0.01 c 4.28 ± 0.00 c 4.79 ± 0.01 b WPS 1.88 ± 0.05 a 29 – 1.42 ± 0.05 a 1.12 ± 0.09 a –– 70WPS/10RPC 1.89 ± 0.15 a 29 2.64 ± 0.00 a 3.65 ± 0.02 b 3.56 ± 0.07 b 4.16 ± 0.07 a 7.91 ± 0.21 a 50WPS/40RPC 2.30 ± 0.04 b 29 2.44 ± 0.01 b 3.00 ± 0.02 c 4.33 ± 0.02 c 4.45 ± 0.13 a 6.49 ± 0.47 b 30WPS/70RPC 2.79 ± 0.00 c 29 3.19 ± 0.02 c 2.93 ± 0.01 d 4.13 ± 0.03 c 4.13 ± 0.02 a 4.67 ± 0.00 c

In starch-rich samples, the temperature-dependent WAI increase was greater than in RPC-rich samples. In PS and WPS samples extruded at 140 ◦C, the WAI decreased with increasing RPC content, whereas extruded PS and WPS as single materials exhibited low WAI. Water absorption is dependent on the availability of hydrophilic groups that are able to bind water, on capillary forces and on the porosity of the matrix [27]. The high WAI of unextruded RPC-rich samples may be attributed to the water binding ability of fibre components in the RPC, as has been investigated in earlier studies [56]. Furthermore, the increase in WAI due to increased barrel temperature in samples containing RPC can be attributed to a modification of the morphology (i.e., particle size and shape) of fibres present in RPC due to heat and shear treatment during extrusion. Arrigoni and Caprez [57] and Zhang and Bai [58] found that the swelling capacity of extruded depectinised apple pomace or oat bran was increased after extrusion. The decrease in WAI in extruded starch can be attributed to heat and shear during extrusion. Heat in the extruder barrel causes the starch to lose its crystalline structure and shear leads to molecular fragmentation, both factors leading to a homogenous “melt” and a decrease in the WAI. Polymers 2021, 13, 215 11 of 19

The WSI of the PS and WPS mixtures increased with increasing RPC content at all barrel temperatures and at both moisture contents (Table6). The WSI of PS/RPC and WPS/RPC mixtures decreased as the barrel temperature was increased from 20 to 140 ◦C, hence this effect was stronger for RPC-rich samples compared to starch-rich mixtures.

Table 6. Water solubility index (WSI) of potato starch (PS) and waxy potato starch (WPS) mixtures extruded at a moisture content of 24 and 29 g/100 g and barrel temperatures of 20, 80, 100, 120 and 140 ◦C. Mean values with different superscript letters (a, b, c, d) within one column (starch type and moisture content) indicate signiﬁcant differences (p > 0.05) based on a one-way analysis of variance (ANOVA). Where appropriate, mean values were compared using Tukey’s honest signiﬁcance test.

Moisture Content Barrel Temperature Mixture Untreated (g/100 g) 20 ◦C 80 ◦C 100 ◦C 120 ◦C 140 ◦C PS 0.50 ± 0.00 a 24 – 36.75 ± 12.37 a 87.75 ± 0.35 a 87.50 ± 4.24 a 81.75 ± 3.18 a 70PS/10RPC 8.13 ± 0.74 b 24 9.20 ± 0.39 a 8.28 ± 0.39 b 9.16 ± 0.05 b 7.76 ± 0.44 b 8,11 ± 0.24 b 50PS/40RPC 11.57 ± 0.08 c 24 12.61 ± 0.06 b 11.50 ± 0.00 b 12.00 ± 0.00 c 11.50 ± 0.00 b 9.10 ± 0.33 b 30PS/70RPC 17.39 ± 0.28 d 24 20.49 ± 0.00 c 19.03 ± 0.37 a 18.92 ± 0.43 d 14.51 ± 0.01 b 14.33 ± 0.27 b PS 0.50 ± 0.00 a 29 – 72.50 ± 12.73 a 52.00 ± 2.12 a 64.21 ± 2.54 a 76.75 ± 11.67 a 70PS/10RPC 8.13 ± 0.74 b 29 8.06 ± 0.31 a 7.33 ± 0.36 b 7.15 ± 0.24 b 6.59 ± 0.07 b 8.15 ± 0.17 b 50PS/40RPC 11.57 ± 0.08 c 29 12.68 ± 0.82 b 12.50 ± 0.71 b 11.00 ± 0.00 bc 8.75 ± 0.35 bc 8.61 ± 0.21 b 30PS/70RPC 17.39 ± 0.28 d 29 19.04 ± 0.37 c 18.42 ± 1.53 b 12.83 ± 0.90 c 12.66 ± 0.40 c 12.52 ± 0.12 b Moisture content Barrel temperature Mixture Untreated (g/100 g) 20 ◦C 80 ◦C 100 ◦C 120 ◦C 140 ◦C WPS 0.75 ± 0.35 a 24 – 82.00 ± 2.83 a ––– 70WPS/10RPC 6.93 ± 0.15 b 24 10.08 ± 0.04 a 8.07 ± 0.43 b 8.48 ± 0.66 a 8.06 ± 0.13 a 4.78 ± 1.76 a 50WPS/40RPC 11.25 ± 0.06 c 24 14.19 ± 0.07 b 13.08 ±0.10 b 11.50 ± 0.00 b 10.50 ± 0.00 b 10.00 ±0.71 b 30WPS/70RPC 17.95 ± 0.13 d 24 18.97 ± 1.34 c 20.39 ± 0.55 c 17.08 ± 0.12 c 14.54 ± 0.07 c 12.73 ± 0.06 b WPS 0.75 ± 0.35 a 29 – 42.50 ± 4.24 a 57.50 ± 6.36 a –– 70WPS/10RPC 6.93 ± 0.15 b 29 7.28 ± 0.24 a 3.43 ± 0.04 b 4.24 ± 0.02 b 5.25 ± 2.22 a 5.81 ± 0.56 a 50WPS/40RPC 11.25 ± 0.06 c 29 14.20 ± 0.03 b 11.50 ± 0.00 bc 7.50 ± 0.00 b 8.00 ± 0.00 a 7.50 ± 0.00 b 30WPS/70RPC 17.95 ± 0.13 d 29 18.68 ± 0.22 c 20.11 ± 0.05 c 13.09 ± 0.03 b 11.91 ± 0.05 a 12.31 ± 0.11 c

Although PS and WPS were not soluble at room temperature (WSI of PS = 0.5 and WSI of WPS = 0.75), the extrusion process markedly increased the WSI of the samples. The starch type and moisture content during the extrusion process did not have a large impact on the WSI of samples. For starch-based matrices, the water solubility (WSI) is often correlated to the extent of degradation of the molecular components of the starch. In multicomponent systems, the WSI indicates the amount of components (polysaccharides, proteins and ﬁbres) that can be solubilised after mechanical breakdown resulting from, e.g., high shear forces in the extruder [59,60]. A higher RPC content in the formulations leads to an increase in rapeseed protein from 5 g/100 g in the 70PS/10RPC and 70WPS/10RPC mixtures to 26.7 g/100 g in the 30PS/70RPC and 30WPS/70RPC mixtures. The main protein fractions of rapeseed—cruciferin and napin—are soluble at room temperature at neutral pH [61,62]. This explains the higher WSI of RPC-rich samples compared to starch-rich samples. An increased WSI with increasing protein content of starch/protein formulations has also been reported by Zhu and Shukri [26]. The marked WSI increase in PS and WPS as single ingredients may be attributed to a greater extent of macromolecular degradation due to higher barrel and melt temperatures, respectively, as described in previous research [63]. In general, the WAI and WSI correlated negatively at high barrel temperatures in this study. This trend agrees with other literature [27,64,65].

3.3. Rheological Properties 3.3.1. Reaction Behaviour of RPC and RPC/Starch Blends before Extrusion The complex modulus |G*| of the starches, RPC and their mixtures were measured as samples were heated in the CCR rheometer (Figures3 and4). The values of |G*| indicate Polymers 2021, 13, 215 12 of 19

that structural changes occur in the biopolymers as a function of temperature [41]. Of all the samples investigated, RPC had the lowest |G*| and the value decreased as the temperature increased from 30 to 50 ◦C. Upwards of 50 ◦C, there was a slight increase in |G*| followed by a steep increase as the temperature reached 100 ◦C (Figure3). In contrast, the PS/RPC and WPS/RPC mixtures showed no increase in |G*| in that temperature region (Figure4a,b). It can be assumed that the slight increase in |G*| for the RPC starting at 70 ◦C corresponds to the onset of rapeseed protein denaturation. The most abundant protein types in rapeseed (Brassica napus canola) are the 11S globulin cruciferin and the 1.7–2S albumin napin, representing, respectively, 60% and 20% of the total accumulated protein [66]. It has been reported that at temperatures of 65–70 ◦C, the hexameric quaternary structure of cruciferin unfolds, weakening the matrix structure as indicated by a decrease in |G*| [66]. Due to the resulting increase in the number of exposed reactive binding sites, new linkages occur that lead to larger molecular size or mass. However, these structure- forming processes (i.e., aggregation) lead to higher viscosity and increase |G*| accordingly, as seen by the small increase above 70 ◦C and large increase above 100 ◦C[41,67–69]. Polymers 2021, 13, x FORPrevious PEER REVIEW studies have reported thermal transition peaks at 91 and 110 ◦C for puriﬁed 14 of 21

cruciferin and napin, respectively. Studies on rapeseed protein isolate found reaction onset at 84 ◦C for cruciferin and 102 ◦C for napin [11]. Although the purification method, protein concentrationand other and the protein method components of thermal affect analysis the therma has al large stability impact of rapeseed on the measured materials [70]. This thermal transition,is in line these with findingsour results, indicate as RPC that (comparabl the presencee ofto nonproteinrapeseed flour) and other contains protein a number of componentsnonprotein affect the components thermal stability (i.e., fibre of rapeseedand lipid) materials that may reduce [70]. This the isthermal in line stability with of rape- our results,seed as proteins RPC (comparable and therefore to rapeseedaccelerateflour) the reaction contains onset a number temperature. of nonprotein components (i.e., fibre and lipid) that may reduce the thermal stability of rapeseed proteins and therefore accelerate the reaction onset temperature.

1000 Rapeseed press cake 900

800

700

600

500

400

300

200 Complex modulus |G*| [kPa]

100

0 30 40 50 60 70 80 90 100 110 120 130 140

Treatment temperature TT [°C]

Figure 3. Complex modulus |G*| as a function of temperature (30–140 ◦C; heating rate 5 K/min) at . Figure 3. Complexa shear modulus rate of|G*|γ = as 0.1 a sfunction−1 for rapeseed of temperature press cake (30–140 (RPC) °C; at aheating moisture rate content 5 K/min) of 24at g/100a shear g. rate of 𝛾 = 0.1 s−1 for rapeseed press cake (RPC) at a moisture content of 24 g/100 g.

Between 100 and 120 °C, the complex modulus |G*| of RPC increases indicating structure formation induced by aggregation reactions. At temperatures above 120 °C, the complex modulus |G*| decreases. There are two possible explanations for this. The aggregation reactions might be complete and |G*| may decrease as a consequence of higher molecular mobility [41]. Alternatively, the newly formed structure of RPC no longer re- mains intact with disintegration and deaggregation reactions destabilising the matrix [11]. We assume that |G*| decreases as a consequence of increased temperature, as indicated by an increasing |ƞ*| in RPC-rich extruded samples (see Section 3.3.2).

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Polymers 2021, 13, 215 13 of 19

4 4 3.5x10 3000 3.5x10 3000 100PS/0RPC b) 100WPS/0RPC a) 2500 70PS/10RPC 2500 70WPS/10RPC 4 4 3.0x10 50PS/40RPC 3.0x10 50WPS/40RPC 2000 2000 30PS/70RPC 30WPS/70RPC 4 4 2.5x10 1500 2.5x10 1500

1000 1000 2.0x104 2.0x104

500 500 1.5x104 1.5x104 0 0 100 110 120 130 140 100 110 120 130 140

1.0x104 1.0x104

Complex modulus |G*| [kPa] 5.0x103 Complex modulus |G*| [kPa] 5.0x103

0.0 0.0 30 40 50 60 70 80 90 100 110 120 130 140 30 40 50 60 70 80 90 100 110 120 130 140

Treatment temperature TT [°C] Treatment temperature TT [°C]

. Figure 4. Complex modulus |G*| as a function of temperature (30–140 ◦C; heating rate 5 K/min) at a shear rate of γ = 0.1 s−1 forFigure (a) potato 4. Complex starch (PS) modulus and (|G*|b) (WPS) as a function mixed with of temperature rapeseed press(30–140 cake °C; (RPC) heating at rate a moisture 5 K/min) content at a shear of rate 24 g/100 of 𝛾= g.0.1 s−1 for a) potato starch (PS) and b) (WPS) mixed with rapeseed press cake (RPC) at a moisture content of 24 g/100 g. Between 100 and 120 ◦C, the complex modulus |G*| of RPC increases indicating ◦ structureThe PS formation and WPS inducedas single ingredients by aggregation both showed reactions. a steep At and temperatures constant decrease above in 120 C, the|G*| complex as the temperature modulus |G*|increa decreases.sed from 30 Thereto 70 °C. are At two > 70 possible°C, |G*| decreased explanations less steeply for this. The aggregationand at > 105 °C reactions abruptly. might In contrast be complete to WPS with and constant |G*| may |G*| decrease in the 120–140 as a consequence°C region, of higherPS showed molecular a marked mobility increase [41 ].in Alternatively,|G*| starting at the 120 newly °C. Overall, formed the structure complex of modulus RPC no longer remainsthroughout intact temperatures with disintegration between and30 and deaggregation 110 °C was higher reactions for destabilisingPS compared theto WPS. matrix [11]. WeThis assume can be thatattributed |G*| to decreases the higher as amylose a consequence content ofof increasedPS (25 g/100 temperature, g amylose) compared as indicated by anto increasingWPS (1 g/100 |η g*| amylose). in RPC-rich A high extruded amylose samples content can (see lead Section to increased 3.3.2). entanglements betweenThe the PS andlinear WPS amylose as single chains ingredients when starchy both melts showed are heated a steep resulting and constant in a higher decrease inviscosity |G*| as[71]. the temperature increased from 30 to 70 ◦C. At > 70 ◦C, |G*| decreased less steeply and at > 105 ◦C abruptly. In contrast to WPS with constant |G*| in the 120–140 ◦C 3.3.2. Complex Viscosity of Extruded RPC/Starch Blends region, PS showed a marked increase in |G*| starting at 120 ◦C. Overall, the complex ƞ modulusFigure throughout 5 illustrates temperatures the complex viscosity between | 30*| of and extruded 110 ◦C PS/RPC was higher blends for as PS a func- compared tion of time and temperature. At 100 °C, TPre and TM, PS/RPC70 is extruded at 100 °C ex- to WPS. This can be attributed to the higher amylose content of PS (25 g/100 g amylose) hibited the highest |ƞ*|, followed by PS/RPC70 extruded at 140 °C and PS/RPC40 ex- compared to WPS (1 g/100 g amylose). A high amylose content can lead to increased truded at 120 °C. PS extruded at 120 °C exhibited the lowest |ƞ*|. The |ƞ*| of PS/RPC70 entanglements between the linear amylose chains when starchy melts are heated resulting extruded at 100 °C increased over time, when TPre and TM was 100 °C, which indicates that inaggregation a higher viscosity reactions [ 71in]. RPC that were induced by extrusion are not completed. PS/RPC70 extruded at 140 °C only shows a slight curve slope, wherefore we assume that 3.3.2. Complex Viscosity of Extruded RPC/Starch Blends a higher TB during extrusion results in a higher degree of aggregation reactions. The com- parativelyFigure low5 illustrates |ƞ*| of PS theextruded complex at 120 viscosity °C and th |e smallη*| of impact extruded an increase PS/RPC in TPre blends and as a ◦ ◦ functionTM (100 or of 120 time °C) and indicates temperature. that the At starch 100 wasC, T gelatinizedPre and TM during, PS/RPC70 extrusion is extruded and consti- at 100 C exhibitedtutes an amorphous the highest melt |η during*|, followed rheological by PS/RPC70analysis. extruded at 140 ◦C and PS/RPC40 extruded at 120 ◦C. PS extruded at 120 ◦C exhibited the lowest |η*|. The |η*| of PS/RPC70 ◦ ◦ extruded at 100 C increased over time, when TPre and TM was 100 C, which indicates that aggregation reactions in RPC that were induced by extrusion are not completed. PS/RPC70 extruded at 140 ◦C only shows a slight curve slope, wherefore we assume that a higher TB during extrusion results in a higher degree of aggregation reactions. The ◦ comparatively low |η*| of PS extruded at 120 C and the small impact an increase in TPre ◦ and TM (100 or 120 C) indicates that the starch was gelatinized during extrusion and constitutes an amorphous melt during rheological analysis.

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55 55 Polymers 2021, 13, x FOR PEER REVIEW a)a) b)b) 16 of 21 5 Polymers 2021, 13, 215 14 of 19 5 44 a) b) 44 4 4 33 33 3 SEI [-] SEI 3 [-] SEI 22 { SEI [-] SEI PS/RPC70 extr. 100°C [-] SEI 22

SEI [-] SEI 2 PS extr. 120°C T and T = 100°C SEI [-] SEI 2 11 Pre M 120000 PS/RPC70 extr. 140°C 11 1 PS/RPC40 extr. 120°C 1 0.80.8 { PS/RPC40 extr. 120°C0407004070 0.80.8 TPre and TM = 120°C 1010 40 40 70 70 0.8 PS extr. 120°C { 04070 0.8 0.60.6 10000010 PS/RPC70 40 extr. 140°C 70TPre and TM = 140°C 0.60.6 0.6 0.6 0.40.4 0.40.4 LEI [-] LEI 0.4 [-] LEI LEI [-] LEI LEI [-] LEI 0.4 * [Pas] 80000 0.20.2 LEI [-] LEI η 0.20.2

0.2 [-] LEI 0.2 0.00.0 0.00.0 0.0 1.01.0 0407004070 0.0 60000 1.01.0 1010 40 40 70 70 1.0 04070 1.0 10 40 70

40000 0.50.5 0.50.5 VEI [-] VEI 0.5 [-] VEI VEI [-] VEI 0.5 [-] VEI VEI [-] VEI VEI [-] VEI Complex viscosity 0.0 20000 0.0 0.00.0 0407004070 0.0 0.0 1010 40 40 70 70 04070 10 40 70RPCRPC contentcontent [[%%]] RPCRPC contentcontent [%][%] RPC content [%] 0RPC content [%] 02468 FigureFigure 1. 1. Sectional, Sectional,Time longitudinal longitudinal [min] and and volumetric volumetric expansion expansion indices indices (SEI, (SEI, LEI, LEI, and and VEI) VEI) of of a a)) potato potato starch starch (PS) (PS) and and b b)) waxy waxy potatopotato starchstarch (WPS)(WPS) andand theirtheir mixturesmixtures withwith 10,10, 4040 andand 7070 g/100g/100 gg rapeseedrapeseed presspress cakecake (RPC)(RPC) extrudedextruded atat aa moisturemoisture Figure 1. Sectional, longitudinal and volumetric expansion indices (SEI, LEI, and VEI) of a) potato starch (PS) and b) waxy potato starch (WPS) and their mixtures with 10, 40 and 70 g/100 g rapeseedFigure 5.pressComplex cake (RPC) viscosity extrudedcontentcontent |η*| asof ofat a29 29a function g/100 moistureg/100 g g and ofand time barrel barrel and temperatures temperatures temperature of of (100 100 100 ◦°C °CC( ( ( )), ),1 ,1 1202020 °C ◦°CC( ( ( ) )and and 140 140 °C °C ( ( ). ). Values Values are are shown shown as as mean mean values values content of 29 g/100 g and barrel temperatures of 100 °C ( )Figure ,120 °C 5. ( Complex ) and 140 viscosity ◦°CC( ()) ). |Values ofƞ*| potato as aare function starch withshownwith standard (PS)standard ofas time singularlymean anddeviations deviations values temperature and inbased based blends (100on on with10 10 °C duplicates. duplicates. ( 40 ),or 120 70 g/100°C ( ) g and rapeseed 140 °C press ( )) of potato starch (PS) singularly and in blends with 40 or 70 g/100 g rapeseed press cake (RPC). Samples extruded and analysed at with standard deviations based on 10 duplicates. cake (RPC). Samples extruded and analysed at 29 g/100 g moisture content. Pretreatment 10 s at 29 g/100 g moisture. content. Pretreatment 10 s at. 𝛾= 31 s−1; measurement 8 min at 𝛾 = 0.1 s−1. γ = 31 s−1; measurement 8 min at γ = 0.1 s−1. TheThe increaseincrease inin temperaturetemperature hashas mainlymainly ledled toto anan increaseincrease inin SEI,SEI, LEILEI andand VEIVEI inin PSPS The increase in temperature has mainly led to an increase in SEI, LEI and VEI in PSsamples samples containing containing 70 70 g/100 g/100 g g RPC. RPC. LEI LEI and and VEI VEI were were lowest lowest for for 40 40 g/100 g/100 g g RPC RPC samples samples To mimic the rheological properties of the melt at the moment of die exit, TPre and TM samples containing 70 g/100 g RPC. LEI and ToVEI mimic were thelowest rheological for 40 g/100 properties g RPC of samples the meltextruded extruded at the moment at at 100 100 °C. °C. of dieIn In WPS exit,WPS Tsamples,samples,Pre and Ta aM barrel barrel temperature temperature of of 100 100 °C °C resulted resulted in in higher higher SEI SEI were set to the corresponding TB to which the samples were applied to during extrusion. extruded at 100 °C. In WPS samples, a barrelwere settemperature to the corresponding of 100 °C resulted TB to whichin higher the SEI samplesand and VEI VEI were regardless regardless applied of toof RPC duringRPC content, content, extrusion. but but did did not not have have a a large large impact impact on on LEI. LEI. ƞ ◦ ◦ and VEI regardless of RPC content, but |didη*| not decreased have |a large in*| thedecreased impact order PS/RPC70on in LEI.the order extruded PS/RPC70 at 140 extrudedItIt C,isis expected PS/RPC40expected at 140 that that°C, extruded thePS/RPC40the additionaddition at 120 extruded ofofC proteiprotei atn-n- 120 andand °C fibre-richfibre-rich biopolymersbiopolymers suchsuch asas presspress ◦ It is expected that the addition of proteiand PSn- extruded and fibre-richand at PS 120 extruded biopolymersC. A high at viscosity120 such °C. asA during presshigh cakes extrusioncakesviscosity to to starch starch canduring be influences influences associated extrusion the the with can rheological rheological abe larger associated prop prop ertieswitherties aof of the the melt melt [42], [42], which which further further affects affects cakes to starch influences the rheologicalSEI, prop sinceerties high oflarger viscositythe melt SEI, [42], resultssince which high in high furtherviscosity pressure affects results atthe the the in expansionexpansion die, high wherefore pressure behaviourbehaviour theat the vapour of ofdie, thethe pressurewherefore matrix.matrix. AA the numbnumb vapourerer ofof experimentalexperimental studiesstudies describedescribe thethe the expansion behaviour of the matrix.is A undergoing number ofpressure laterexperimental during is undergoing expansion studies describelater and during the the matrixclose closeexpansio correlation cancorrelationn facilitate and the ofof its thematrixthe structure rheologicalrheological can facilitate during propertiproperti its esstructurees ofof biopolymersbiopolymers andand thethe correspondingcorresponding ex-ex- close correlation of the rheological propertibubblees of growth. biopolymersduring A low bubble viscosityand the growth. corresponding can in A contrast low viscosity beex-pansion associatedpansion can behaviour behaviourin with contrast a more in in be extrusion extrusion pronounced associated [42–46]. [42–46]. with LEI. It Ita was wasmore found found pro- that that the the viscosity viscosity of of the the melt melt is is asso- asso- pansion behaviour in extrusion [42–46].In It ourwas study,found fornouncedthat PS the samples, viscosity LEI. In the our of SEI the study, increasesmelt for is asso-PS and samples,ciatedciated LEI decreased withwith the axialSEIaxial withincreases expansionexpansion an increasing and andand LEI LEI,LEI, decreased RPC whereaswhereas with meltmelt an elasticityelasticity isis correlatedcorrelated withwith radialradial ex-ex- ciated with axial expansion and LEI, whereascontent. melt These elasticityincreasing observations is correlatedRPC can content. be linkedwith These radial to |observationsη *|ex-pansion inpansion rheological and canand betherefore therefore analyses linked the to andthe | SEI ƞSEI indicate*| [37]. in[37]. rheological In In that our our stud stud analysesy,y, we we did did not not differentiate differentiate between between viscous viscous pansion and therefore the SEI [37]. In oura closedstudy, cavitywe didand rheometer not indicate differentiate can that be a usedbetweenclosed as acavity toolviscous torheometer ﬁndand and indications elasticelastic ca n propertiesproperties be regardingused as inin a rheorheo thetoollogical expansionlogical to find analyses,analyses, indications butbut showedre-showed ththatat thethe starch/RPCstarch/RPC blendsblends ex-ex- and elastic properties in rheological analyses,properties but ofshowed biopolymers.garding that the the expansion starch/RPC properties blends ex- ofhibited hibitedbiopolymers. lowerlower |G*||G*| withwith increaincreasingsing RPCRPC contentcontent andand temperature.temperature. AnAn alterationalteration ofof rheo-rheo- hibited lower |G*| with increasing RPC content and temperature. An alteration of rheo-logicallogical propertiesproperties duedue toto RPCRPC additionaddition associatedassociated withwith anan increaseincrease inin LEILEI andand aa decreasedecrease 3.4. Extruder Response3.4. Extruder Response logical properties due to RPC addition associated with an increase in LEI and a decreasein in SEISEI was,was, i.e.,i.e., shownshown inin WPS-basedWPS-based samplessamples withwith 4040 oror 7070 g/100g/100 gg RPC,RPC, respectively.respectively. ◦ in SEI was, i.e., shown in WPS-based samplesThe with extruder 40 orThe response 70 extruderg/100is g shownRPC, response respectively. in Tables is shown7 and30WPS/70RPC 30WPS/70RPC in8 Table. At a 7 barrel and showed showedTable temperature 8. the theAt lowest alowest barrel of 20 |G*| |G*|temperatureC, in in rheolo rheolo ofgicalgical 20 analyses analyses (discussed (discussed later, later, Figure Figure 4) 4) ◦ ◦ starch-rich samples°C, starch-rich resulted insamples ca. 10 resultedC higher in product ca.and 10 the °C temperatures highest higher LEIproduct at (62.7 29 temperaturesg/100C) than g moisture RPC- (62.7 content. °C) than 30WPS/70RPC showed the lowest |G*| in rheological analyses (discussed◦ later, Figure 4) and the highest LEI at 29 g/100 g moisture content. rich mixturesRPC-rich (52.2–55.0 mixturesC). Hence, (52.2–55.0 when the°C). barrelHence, temperature when the barrel was settemperature to 80, 100 andwas set to 80, 100 and the highest LEI at 29 g/100 g moisture content.◦ TheThe observed observed decrease decrease in in SEI SEI due due to to 40 40 g/ g/100100 g g RPC RPC addition addition in in WPS WPS samples samples in in our our 120 C, mixturesand with120 °C, 70 g/100mixtures g RPC with generated 70 g/100 g the RPC highest generated product the temperaturehighest product of all temperature of The observed decrease in SEI due to 40 g/100 g RPC addition in WPS samples◦ in ourstudy study is is in in accordance accordance toto previousprevious studiesstudies that that foundfound lower lower sectionalsectional expansion expansion of of extru-extru- study is in accordance to previous studiesthe that mixtures. foundall lower When the mixtures.sectional extruded expansionWhen at 140 extrudedC, ofno extru- formulation-related at dates140dates °C, whenwhen no formulation-related fruitfruit press differencepress cakescakes in oror product otherdifferenceother by-produby-produ in productctscts werewere addedadded toto aa starchstarch matrixmatrix [20–23].[20–23]. dates when fruit press cakes or other by-produtemperaturects were wastemperature added observed. to awas starch Almost observed. matrix no difference [20–23].AlmostThe Theno in differenceauthorsSMEauthors was ofof observed studiesinstudies SME ononwas between bilberry,bilberry, observed PS appleapple or between oror blblackcurrantackcurrant PS or presspress cakecake attributedattributed thisthis toto thethe WPS based samples having different moisture and RPC contents. In mixtures containing The authors of studies on bilberry, apple or blackcurrantWPS press based cake samples attributed having this different to thepresence presence moisture of of and dietary dietary RPC fibre fibre contents. in in the the Inpress press mixtures cakes. cakes. containing Es Especiallypecially the the presence presence of of inert inert insoluble insoluble fibre fibre WPS, the pressure decreased with increasing barrel temperature regardless of the RPC presence of dietary fibre in the press cakes. EspeciallyWPS, the presence the pressure of inert decreased insoluble with fibre increasing barrel temperature regardless of the RPC content, whereascontent, WPS-rich whereas samples WPS-rich generated samples a higher generated pressure a higher at the pressure die (40.51 at the bar die at a (40.51 bar at a moisture contentmoisture of 24 g/100content g) of than 24 g/100 RPC-rich g) than samples RPC-rich (12.90 samples bar at a (12.90 moisture bar contentat a moisture of content of 24 g/100 g). The24 g/100 lowest g). pressure The lowest was pressure found for was the found 50WPS/40RPC for the 50WPS/40RPC sample. sample.

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Table 7. Product temperatures measured for waxy potato starch (WPS) samples with 10, 40 and 70 g/100 g rapeseed press cake (RPC) extruded at a barrel temperature of 20, 80, 100, 120 and 140 ◦C and a moisture content of 29 g/100 g. Mean values with different superscript letters (a, b, c) within one row indicate signiﬁcant differences (p > 0.05) based on a one-way analysis of variance (ANOVA). Mean values were compared using Tukey’s honest signiﬁcance test.

Product Temperature (◦C) Barrel Temperature (◦C) 30WPS/70RPC 50WPS/40RPC 70WPS/10RPC 20 52.2 ± 0.5 a 55.0 ± 0.3 b 62.7 ± 1.6 c 80 90.2 ± 0.4 a 82.7 ± 1.1 b 88.2 ± 1.3 c 100 100.0 ± 0.2 a 96.7 ± 0.1 b 95.3 ± 0.8 c 120 108.8 ± 0.5 a 107.5 ± 0.6 b 104.8 ± 0.4 c 140 123.4 ± 0.4 a 125.7 ± 1.0 b 124.2 ± 1.6 c

Table 8. Extruder system parameters used for starch based extrudates containing waxy potato starch (WPS) mixed with rapeseed press cake (RPC) extruded at a barrel temperature of 20, 80, 100, 120 and 140 ◦C. SME = specific mechanical energy. Mean values with different superscript letters (a, b, c, d, e) within one column (mixture and moisture content) indicate significant differences (p > 0.05) based on a one-way analysis of variance (ANOVA). Mean values were compared using Tukey’s honest significance test.

PS WPS Moisture Content Barrel Temperature Mixture SME SME (g/100 g) (◦C) Pressure (bar) Pressure (bar) (kWh/kg) (kWh/kg) 30starch/70RPC 20 0.11 ± 0.01 a 6.94 ± 0.73 a 0.12 ± 0.00 a 12.90 ± 1.87 a 80 0.09 ± 0.00 a 10.63 ± 0.24 b 0.11 ± 0.00 a 6.49 ± 0.41 b 24 100 0.08 ± 0.00 a 5.42 ± 1.48 a 0.09 ± 0.00 a 5.98 ± 0.50 b 120 0.08 ± 0.00 a 2.39 ± 1.35 c 0.08 ± 0.00 a 5.04 ± 0.31 b 140 0.07 ± 0.00 a 1.74 ± 1.03 c 0.08 ± 0.00 a 3.51 ± 0.46 c 20 0.10 ± 0.01 a 6.12 ± 1.26 a 0.10 ± 0.00 a 13.75 ± 0.58 a 80 0.08 ± 0.00 a 9.49 ± 0.82 b 0.08 ± 0.00 a 6.18 ± 0.67 b 29 100 0.07 ± 0.00 a 6.47 ± 1.14 a 0.08 ± 0.00 a 4.53 ± 0.43 c 120 0.07 ± 0.00 a 4.13 ± 0.48 c 0.07 ± 0.00 a 2.83 ± 0.71 d 140 0.07 ± 0.00 a 0.62 ± 0.91 d 0.07 ± 0.00 a 0.60 ± 0.07 e 50starch/40RPC 20 0.10 ± 0.01 a 13.43 ± 2.39 a 0.10 ± 0.01 a 8.11 ± 1.24 a 80 0.08 ± 0.00 a 6.69 ± 0.78 b 0.09 ± 0.01 a 4.84 ± 2.57 b 24 100 0.08 ± 0.00 a 2.78 ± 0.56 c 0.08 ± 0.01 a 4.87 ± 2.22 b 120 0.07 ± 0.00 a 0.68 ± 1.38 d 0.08 ± 0.00 a 2.63 ± 1.90 c 140 0.07 ± 0.00 a 0.25 ± 0.94 d 0.07 ± 0.00 a 0.26 ± 0.04 d 20 0.09 ± 0.00 a 10.92 ± 1.10 a 0.09 ± 0.00 a 10.57 ± 0.40 a 80 0.07 ± 0.00 a 7.08 ± 0.46 b 0.07 ± 0.00 a 8.82 ± 1.64 b 29 100 0.07 ± 0.00 a 2.95 ± 0.70 c 0.07 ± 0.00 a 3.67 ± 0.35 c 120 0.06 ± 0.00 a 1.28 ± 0.45 d 0.07 ± 0.00 a 0.66 ± 0.14 d 140 0.06 ± 0.00 a 1.08 ± 0.56 d 0.06 ± 0.00 a 0.70 ± 0.17 d 70starch/10RPC 24 20 0.11 ± 0.00 a 16.86 ± 1.72 a 0.12 ± 0.00 a 40.51 ± 5.28 a 80 0.09 ± 0.00 a 8.67 ± 1.98 b 0.10 ± 0.00 ab 32.10 ± 6.25 b 100 0.08 ± 0.00 a 4.53 ± 0.23 c 0.09 ± 0.01 ab 25.08 ± 5.48 c 120 0.07 ± 0.00 a 4.12 ± 5.21 c 0.08 ± 0.01 ab 14.04 ± 3.91 d 140 0.07 ± 0.00 a 7.34 ± 4.75 b 0.07 ± 0.00 b 11.28 ± 2.71 d 29 20 0.07 ± 0.01 a 10.81 ± 2.76 a 0.11 ± 0.00 a 31.42 ± 6.99 a 80 0.09 ± 0.00 a 14.41 ± 5.51 b 0.09 ± 0.00 a 16.33 ± 5.85 b 100 0.08 ± 0.00 a 8.14 ± 4.67 c 0.07 ± 0.00 a 8.52 ± 1.53 c 120 0.07 ± 0.00 a 4.91 ± 3.80 d 0.07 ± 0.00 a 6.68 ± 1.35 c 140 0.07 ± 0.00 a 5.84 ± 3.48 d 0.07 ± 0.00 a 2.11 ± 1.37 d

Starch might cause higher viscous dissipation and therefore increase the product temperature due to its high shear viscosity at low temperatures as described, i.e., by Ye and Hu [72]. The pressure at the die is a result of the melt viscosity and is, in turn, inﬂuenced by shear and the thermal input in the extruder. Rheological investigations showed, e.g., that Polymers 2021, 13, 215 16 of 19

the 70WPS/10RPC sample had a dynamic viscosity of 179.5 kPas when treated at 80 ◦C for 1 min, whereas the 30WPS/70RPC sample had a dynamic viscosity of 54.5 kPas (data not shown). The higher viscosity of the extruded 70WPS/10RPC melt leads to higher die pressure.

4. Conclusions The aim of this study was to investigate the impact of rapeseed press cake (RPC) concentration on expansion, specific hardness, water binding and solubility, rheological properties and extruder response using a suitable model system. To correlate expansion behaviour with the rheological properties of the melt, a specific rheometer simulating extrusion conditions was used to evaluate the rheological properties of the extruded blends. Starch was chosen as the basis ingredient and the press cake content was varied far beyond those concentrations used in previous studies. Results gained in a specific rheometer show that the addition of RPC had a large effect on the complex viscosity of starch/RPC blends when treated at extrusion like conditions. The RPC increased the complex viscosity of the starch/RPC blends, which could be linked to a more pronounced sectional and decreased longitudinal expansion of the samples. It was possible to extrude starch/RPC mixtures containing up to 70 g/100 g RPC at barrel temperatures of 100–140 ◦C and moisture contents of 24 or 29 g/100 g. The physical properties of the extrudates were highly dependent on barrel temperature and RPC content, whereas starch type and moisture content did not have a large impact on expansion or texture in the range investigated. At temperatures above 120 ◦C, 70 g/100 g of RPC increased the sectional and volumetric expansion of extrudates, irrespective of starch type. At the same temperatures and RPC inclusion level, the longitudinal expansion decreased when RPC was added to potato starch and slightly increased when blends contained RPC and waxy potato starch. Moreover, 40 and 70 g/100 g RPC decreased the hardness and water absorption of the extrudates and increased the water solubility. Especially the extrusion of native potato starch/RPC at a high RPC content and high barrel temperatures resulted in a high expansion. In this study, we were able to correlate expansion behaviour and rheological properties using a closed cavity rheometer. However, expansion properties were only physically investigated > 10 s after die exit, whereas it is known from previous studies, that expansion comprises several stages of initial growth and shrinkage. To give a more detailed causal link between material properties and expansion behaviour during all expansion phases, digital imaging at the die exit should be used and the rheological investigations should be extended to the viscous and elastic modulus of the blends under thermomechanical treatment.

Author Contributions: A.M.: data curation, writing—original draft preparation, formal analysis and visualisation. R.O.: supervision and writing—review and editing. H.P.K.: writing—review and editing. M.A.E.: conceptualisation, methodology and writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to thank Sigrid Gruppe, Elfriede Bischof and Evi Muller for the chemical analyses and Michael Schott for the particle size analysis. The authors are grateful to Caroline Kofler for the extrusion processing and analysis of the extrudates. Conflicts of Interest: The authors declare no conflict of interest. Polymers 2021, 13, 215 17 of 19

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