SMALL DEFORMATION RHEOLOGY for CHARACTERIZATION of ANHYDROUS MILK FAT/RAPESEED OIL SAMPLES STINE RØNHOLT1,3*, KELL MORTENSEN2 and JES C

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Journal of Texture Studies ISSN 1745-4603

SMALL DEFORMATION RHEOLOGY FOR CHARACTERIZATION OF ANHYDROUS MILK FAT/RAPESEED OIL SAMPLES STINE RØNHOLT1,3*, KELL MORTENSEN2 and JES C. KNUDSEN1

1Department of Food Science, University of Copenhagen, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark 2Niels Bohr Institute, University of Copenhagen, Copenhagen Ø, Denmark

KEYWORDS ABSTRACT Method optimization, milk fat, physical properties, rapeseed oil, rheology, structural Samples of anhydrous milk fat and rapeseed oil were characterized by small analysis, texture evaluation amplitude oscillatory shear rheology using nine different instrumental geometrical combinations to monitor elastic modulus (G′) and relative deformation 3 + Corresponding author. TEL: ( 45)-2398-3044; (strain) at fracture. First, G′ was continuously recorded during crystallization in a FAX: (+45)-3533-3190; EMAIL: ﬂuted cup at 5C. Second, crystallization of the blends occurred for 24 h, at 5C, in [email protected] *Present Address: Department of Pharmacy, external containers. Samples were gently cut into disks or ﬁlled in the rheometer University of Copenhagen, Universitetsparken prior to analysis. Among the geometries tested, corrugated parallel plates with top 2, 2100 Copenhagen Ø, Denmark. and bottom temperature control are most suitable due to reproducibility and dependence on shear and strain. Similar levels for G′ were obtained for samples Received for Publication May 14, 2013 measured with parallel plate setup and identical samples crystallized in situ in the Accepted for Publication August 5, 2013 geometry. Samples measured with other geometries have G′ orders of magnitude lower than identical samples crystallized in situ. This emphasizes the importance doi:10.1111/jtxs.12048 of gentle sample pre handling, temperature control and preventing slip.

PRACTICAL APPLICATIONS Small deformation rheology is widely used to study and evaluate textural behavior of fat-based systems. As different research groups use different geometries, a systematic evaluation of data gained using different geometrical combinations is needed. By conducting a methodic evaluation, this work provides an increased knowledge of how to characterize fat-based systems using small deformation rheology and evaluates the relations between geometries. Because of the rigid nature of fat-based systems, wall slip is likely to occur. Moreover, when characterizing such systems during storage and/or produced at industrial scale, in situ crystallization is not possible. In such cases, the sample must be loaded at the geometry prior to analysis. Such loading will affect the fat crystal network to some extent and consequently the data obtained. However, as the physical conditions differ between the geometries available, they affect both the fat crystal network and the tendency of wall slip to occur differently.

INTRODUCTION presence of irreversible (primary) bonds and reversible (secondary) bonds (Haighton 1965). The reversible bonds In order to characterize the textural behavior of food prod- are due to van der Waals attraction forces between the ucts, it is essential to understand the underlying structure crystals, while the stronger irreversible bonds exist where and interactions between structural elements. Within a fat the crystals are mechanically interlinked, as occurring crystal network, the interactions can be characterized by the during crystal growth (van den Tempel 1958). One way to

20 Journal of Texture Studies 45 (2014) 20–29 © 2013 Wiley Periodicals, Inc. S. RØNHOLT, K. MORTENSEN and J.C. KNUDSEN SMALL DEFORMATION RHEOLOGY FOR AMF/RO BLENDS characterize the bulk properties of the fat crystal network is 2012a,b; Kaufmann et al. 2012; Pothiraj et al. 2012; Buldo rheological measurements, as described by Rønholt et al. and Wiking 2012). (2013). As fat-based systems are rigid systems, wall slip is likely Previous studies have compared different rheological to occur (Kalyon 2005a). Consequently, choosing the right techniques to characterize fat-based systems. van den geometry is essential to obtain a proper characterization of Tempel (1958) used a model system of glycerol tristearate the system. In the present study, we will therefore make a crystals in paraffin oil to compare creep experiments at low methodical evaluation by comparing the elastic modulus stress values with concentric cylinder viscometer tests at (G′) and strain at fracture obtained using nine different very low shear rates. The tests were run at 0C. The study geometries available for small deformation rheology. Strain concludes that while the concentric cylinder primarily at fracture is defined as the strain at 50% decrease in G′. involves rupture of irreversible bonds, the creep experiment We use a model system consisting of anhydrous milk fat provides more detailed information about the interplay (AMF) and rapeseed oil (RO) as an increasing number between reversible and irreversible bonds. Information can of spreads based on milk fat blended with vegetable be gained by following changes in the slope of the creep oils are introduced on the market (Kaufmann et al. 2012; curves (van den Tempel 1958). A similar conclusion was Marangoni et al. 2012), hence emphasizing the need for a drawn by Davis in 1973 after comparing cone penetrometry, suitable method for rheological characterization of such Ferranti-Shirley cone and plate viscometer, concentric cyl- blends. inder creep, creep by spherical indentor, and oscillatory testing of lard and shortening. Davis (1973) concludes that MATERIALS AND METHODS small deformation tests provide the best characterization of the molecular structure of a fat crystal network as it only Materials induces minimal structure breakdown during analysis. Sone (1961) combined three techniques for small deformation AMF was received from ARLA Foods, Götene, Sweden, and rheology to study the behavior of butter during manufac- RO from DLG FOOD Oil, Dronninglund, Denmark. turing. A vibration-plate viscometer was used to test the dynamic and rigidity modulus during working, a cone- Preparation of Blends plate viscometer to static measurements of viscosity after working, while a parallel plate plastometer was used for Figure 1 shows a schematic presentation of the experimen- viscosity measurements during working. However, the tal setup for both preliminary tests (steps 1, 2 and 3a) and study does not make a direct comparison of the different method evaluation (steps 1, 2, 3b and 4). As a preliminary techniques used. test, blends containing 50, 60, 70, 80, 90 and 100% AMF Later, Dixon (1974) studied the spreadability of butter at (weight/weight), the rest being RO, were prepared. The 13C using extruder, compression tester, disk penetrometer, blends were prepared by heating AMF and RO for 15 min in cone penetrometer and sectility tester, focusing on precision a water bath set to 65C. This was done to erase all crystal and convenience of operation. Dixon concludes that memory. After heating, the desired amount of RO was because the methods used involve different physical test added to AMF and the two phases were mixed for 10 s in a conditions, unequivocal conversion between the methods is kitchen machine, set to maximum speed (CombiMax 600, not possible. Nevertheless, the extrusion method has best Braun, Kronberg, Germany). The kitchen machine was precision while disk penetrometer is more convenient due equipped with a 2.0 L work bowl and a universal chopping to easier sample preparation (Dixon 1974). blade. Finally, the blends were transferred to a temperature- Penetrometry is still one of the most commonly used controlled fluted cup, mounted at the rheometer and set techniques to characterize the rheological properties of to 65C. In this setup, a large gap vane was used as upper milk fat-based products, due to its correlation with geometry. sensorial data (Dixon 1974), its simplicity and low cost The blends prepared for methodical evaluation were pre- (Wright et al. 2001). Moreover, penetrometry serves as pared as described above, containing respectively 50, 60 and the official method according to the American Oil 70% AMF (weight/weight). After blending, the blends were Chemists’ Society (1980; Deman and Beers 1987). transferred to plastic containers and incubated at 5C for However, as equipment for small deformation rheology 24 h before analysis. measurements become more accessible, the number of studies using small deformation rheology to characterize Small Deformation Rheology fat-based systems is continuously increasing (Segura et al. 1990; Borwankar et al. 1992; Herrera and Hartel 2000; An AR G2 Rheometer (TA Instruments, West Sussex, U.K.) Litwinenko et al. 2004; Wiking et al. 2009; Rønholt et al. was used for all small deformation measurements. For the

FIG. 1. SCHEMATIC ILLUSTRATION OF THE EXPERIMENTAL SETUP FOR BOTH PRELIMINARY TESTS (STEPS 1, 2 AND 3A) AND METHOD EVALUATION (STEPS 1, 2, 3B AND 4). FAST COOLING WAS 5.0C/MIN AND SLOW COOLING WAS 0.05C/MIN. THE RATIOS OF ANHYDROUS MILK FAT TO RAPESEED OIL WERE 50:50, 60:40, 70:30, 80:20, 90:10 AND 100:0 FOR THE PRELIMINARY TESTS AND 50:50, 60:40 AND 70:30 FOR THE METHOD EVALUATION preliminary tests only, a temperature-controlled ﬂuted cup Moreover, the corrugated parallel plate geometry is seen in and a large gap vane (dimensions according to Table 1 and Fig. 3. The samples, all incubated at 24 h at 5C, were gently Fig. 2.5) were used. Crystallization was followed during loaded at the different geometries. Because of the different fast (5C/min) and slow (0.05C/min) cooling from 65 to physical conditions of the geometries tested, the fat crystal 5C (strain 0.001% and an angular frequency of 1.000 rad/s network was affected differently upon loading. This is within the linear viscoelastic region). Moreover, the crys- exactly one of the key issues when studying the rheological tallization was monitored during subsequent isothermal properties of products and not model systems: loading does storage 5C in the rheometer using a time sweep (strain inﬂuence the rheological properties, but differently. In the 0.002% and an angular frequency of 1.000 rad/s within the following, both the effect of sample treatment during linear viscoelastic region). The time sweep duration was loading in relation to rheological behavior and the ability of 4 h for the slow-cooled sample and 2 h for the fast-cooled the different geometrical combinations to follow the rheo- sample. There was no wait between cooling and measur- logical changes as a function of RO will be discussed. As all ing. The heat transfer properties from the rheometer to the measurements were aimed to be made within the linear vis- sample were expected to be the same for all samples. coelastic region, settings for frequency and stress sweep For the methodical evaluation, nine different geometry were changed according to the geometry used (Table 1). The combinations were tested according to Table 1 and Fig. 2. temperature was held constant at 5C during analysis using

TABLE 1. DIMENSIONS AND SETTINGS FOR THE GEOMETRIES USED (h = HEIGHT, t = TRUNCATION, c = CONE ANGLE, d = DIAMETER)

Upper geometry Lower geometry Frequency sweep Stress sweep

Angular Angular Temperature Temperature frequency Oscillatory Oscillatory frequency Geometry Dimension controlled Dimension controlled (rad/s) stress (Pa) stress (Pa) (rad/s)

Cone plate d: 60 mm, t: 56 μm No Yes 500–0.05 500 1–3,618 1 c: 1°, 59 min 53 s Corrugated parallel plates d: 25 mm Yes Yes 500–0.05 500 1–3,618 1 Smooth parallel plates d: 40 mm Yes Yes 500–0.05 500 1–3,618 1 Bob cup h: 30 mm, d: 28 mm No Yes 500–0.05 10 1–3,000 1 Vane-ﬂuted cup d: 28 mm, h: 42 mm No d: 30 mm Yes 500–0.05 10 1–3,000 1 Large gap vane-ﬂuted cup d: 15 mm, h: 38 mm No d: 30 mm Yes 500–0.05 10 1–3,000 1 Vane cup d: 28 mm, h: 42 mm No d: 30 mm Yes 500–0.05 10 1–3,000 1 Large gap vane cup d: 15 mm, h: 38 mm No d: 30 mm Yes 500–0.05 10 1–3,000 1 Starch pasting rotor cell d: 32 mm No d: 37 mm Yes 500–0.05 1 0.001–2,160 1

blends at a random location. The cylinder was then carefully removed from the metal cylinder by using the back of a plastic piston from a 20-mL disposable syringe. The disks were cut to a height of 4 mm using a wire cutter.

Statistical Analysis The data were analyzed using a one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison tests using GraphPad Prism (Version 5.02, GraphPad Software, Inc., La Jolla, CA). ANOVA was applied on the repeated measurements (at least nine replicates) of all blends using the elastic modulus (G′) and % strain at 50% decrease in G′ as variables. Furthermore, the variation within each blend was analyzed using one-way ANOVA. To improve clarity,

FIG. 2. GEOMETRIES USED FOR SMALL DEFORMATION RHEOLOGY only statistically significant results are reported. In all (h = HEIGHT, c = CONE ANGLE, d = DIAMETER): (1) CONE PLATE, figures, the data are represented as an average of all repli- (2) CORRUGATED PARALLEL PLATE GEOMETRY, (3) SMOOTH cates and the error bars represent the standard deviation. PARALLEL PLATE GEOMETRY, (4) VANE- FLUTED CUP, (5) LARGE GAP VANE-FLUTED CUP, (6) BOB CUP, (7) VANE CUP, (8) LARGE GAP VANE-FLUTED CUP, (9) STARCH PASTING ROTOR CELL. THE RESULTS AND DISCUSSION DRAWINGS ARE NOT TO SCALE Elastic modulus (G′) during crystallization of blends was characterized with a vane and cup geometry. Prior to the methodical evaluation, the effect of cooling rate and crystal- the Peltier element attached to the rheometer. Details about lization time on the rheological behavior was tested (Fig. 4) temperature control are listed in Table 1. using a fluted cup and large gap vane geometry according to For both the smooth and the corrugated parallel plates, Table 1 and Figs. 1 and 2.5. This was done in order to the samples were gently cut into disks by punching a pre- exclude the effect of post-crystallization on the results cooled metal cylinder (diameter 25 mm for corrugated par- obtained. allel plates and 40 mm for smooth parallel plates) into the Figure 4 suggests that the rates of shear-induced crystallization are very different between the two cooling rates studied, resulting in different elasticity between the blends. A similar temperature-dependent and shear-induced crystallization behavior has previously been reported for poly- mers (Mago et al. 2009). Upon changes in the crystallization behavior, the microstructure and consequently material properties are likely to be affected (Mago et al. 2009). For milk fat-based products, fast cooling is known to induce formation of many small homogenous crystals forming a strong fat crystal network due to a high number of crystal- crystal interactions. Contrary, slow cooling induces formation of fewer, but lager crystals, and consequently a fat crystal network with a lower G′ when compared to G′ for fast-cooled milk fat (Wiking et al. 2009; Kaufmann et al. 2012; Rønholt et al. 2012a). While the blends in Fig. 4 are subjected to a low shear during cooling, the blends shown in Fig. 5 are cooled under static conditions. For the slow-cooled blends, increasing the amount of RO delays the crystallization process as evident from the fact that the time it takes to reach a stable G′ is increased (Fig. 4b). Contrary to the fast-cooled blends, only the crys- FIG. 3. PHOTOGRAPH OF THE CORRUGATED PARALLEL PLATE tallization process for the blend containing 50% AMF is GEOMETRY USED IN THE PRESENT WORK affected (Fig. 4a). At the point where a stable G′ is reached,

of RO is known to decrease the hardness of milk fat-based products (Kaufmann et al. 2012). These effects are, however, beyond the scope of the present work and will not be discussed further. As the aim of this work was to conduct a methodical evaluation of rheological measurements of AMF/RO blends including the ability of the different geometries to distin- guish between different samples, a modiﬁed procedure for slow cooling was used in the continuing work. The method is further described in Preparation of Blends section. The blends used for further analysis were the 50–70% AMF. The preliminary studies showed that for blends containing 80–100% AMF, the sinusoidal stress phase deviated from the sinusoidal response. Likely, this behavior is a consequence of a high solid fat content in AMF with a very rigid structure, causing slip to occur between the geometry and the sample. Therefore, the samples containing 80–100% were excluded from further analysis.

Elastic Modulus of Blends Characterized with Different Geometries Figure 5 shows G′ as a function of angular frequency for each blend ratio using different geometries. The modulus for all blends seems in general to be rather constant as a function of frequency. The only exception being the smooth parallel plates for the 60% AMF blend, where an increase in G′ was observed as a function of frequency. Such shear thickening behavior has, for concentrated colloidal suspensions, been shown to occur as the hydrodynamic lubrication forces cause formation of so-called hydroclusters (Catherall et al. 2000). A behavior, which later has been related to slip ′ FIG. 4. ELASTIC MODULUS (G ) FOLLOWED DURING AND AFTER FAST in the shear thickening state (Lee and Wagner 2003). (A) AND SLOW (B) COOLING FROM 65 TO 5C USING A However, in an overall perspective, the blends show typical TEMPERATURE-CONTROLLED FLUTED CUP AND LARGE VANE GEOMETRY. THE COMPOSITION OF THE BLENDS WAS ANHYDROUS solid-like behavior. To ease the comparison of the difference MILK FAT (AMF) WITH 0–50% RAPESEED OIL (RO) in G′ versus the geometrical combination used, Fig. 6 shows G′ at 5 rad/s as a function of AMF content for the nine tested geometries. RO addition has a more pronounced effect on G′ for the The results show that G′ depends significantly on the slow-cooled samples (Fig. 4b). A recent study compares the applied geometry. This behavior is closely related to signifi- effect of cooling rate and RO addition on the rheological cant changes in the structure of the blends occurring upon properties and microstructure of AMF (Kaufmann et al. sample loading. When comparing the same blend character- 2012). Kaufmann et al. (2012) found the cooling rate to ized using different geometries (Figs. 5 and 6), cone-plate have a major impact on the crystallization kinetics, hence and corrugated parallel plates have about one to two orders changing the solubilization properties of the AMF in RO. of magnitude higher G′ compared to all other geometries. Consequently, the decrease observed in G′ for the slow- The same goes for the smooth parallel plates when testing cooled blends compared to fast cooled (Fig. 4) most likely the 70% AMF blend. Furthermore, when comparing the corresponds to a decreased solid fat content. A decrease in rheological properties between blends as in Fig. 5, the solid fat content was caused by the solubilization effect of outcome depends on the geometry used. For cone-plate RO (Kaufmann et al. 2012) combined with fewer, but larger geometry, there is no difference in G′ between the three crystals as a result of a slow cooling rate (Wiking et al. 2009; blends (Fig. 6). However, when using parallel plates, bob Kaufmann et al. 2012; Rønholt et al. 2012a). The decrease in cup, vane-fluted cup or vane cup, the measured G′ value G′ observed upon addition of RO is as expected, as addition decreases upon increasing the amount of RO. For the large

FIG. 5. ELASTIC MODULUS (G′)ASA FUNCTION OF THE ANGULAR FREQUENCY OBTAINED USING DIFFERENT GEOMETRICAL COMBINATIONS. THE BLENDS CONTAINED ANHYDROUS MILK FAT (AMF) AND RAPESEED OIL (RO) IN THE RATIO OF (A) 50:50 (WITH THE EXCEPTION OF SMOOTH PARALLEL PLATES), (B) 60:40 OR (C) 70:30 % (WEIGHT/WEIGHT). THE MEASUREMENTS WERE CONDUCTED AT 5C gap vane-fluted cup and the large gap vane-cup combina- Each rheological instrument affects the yield and flow tions, G′ is higher for the 70% AMF blend compared to the differently, as the physical conditions change with the rhe- 50% AMF blend. For the starch pasting rotor cell, G′ ometer. So is the case when changing the geometry. Conse- increases from 50 to 60% AMF but decreases again from 60 quently, the response may differ due to the different physical to 70% AMF. Likely, the force needed to lower the upper properties the sample may exhibit (Haighton 1959; Dixon geometry to the specified gap disturbs the fat crystal 1974). The cone-plate and corrugated parallel plate geom- network. For all blends, the modulus obtained with corru- etries show continuously the highest G′ among the geom- gated parallel plates is similar to the modulus measured for etries tested. The idea of the truncated cone is that shear an identical sample crystallized in situ in the geometry rate remains constant throughout the sample. However, to (Figs. 4 and 5). maintain a constant shear rate throughout the full sample

when using parallel plate geometry is, however, to ensure full contact between sample and plate without affecting the sample. One possibility is lowering the upper geometry until full contact is visually confirmed and afterward lowering the geometry slightly more until a defined normal force is reached, as done in the present work. In the remaining combinations, all including a fluted or nonfluted cup, G′ is lower compared to the results obtained using the plate combinations. With respect to difference between emulsions, only combinations with the large gap vane did not have significant difference in G′ between all blends (i.e., 50 versus 60 versus 70%). During sample FIG. 6. THE ELASTIC MODULUS (G′) AT 5 RAD/S AS A FUNCTION OF mounting with the cup and cell combinations, the sample is ANHYDROUS MILK FAT CONTENT (THE REMAINING FAT PHASE BEING subjected to a significant shear deformation and conse- RAPESEED OIL). THE ANALYSIS WAS RUN AT 5C USING NINE quently destruction of the fat crystal network not only when DIFFERENT GEOMETRIES FOR SMALL DEFORMATION RHEOLOGY. placing the sample in the cup/cell, but also when lowering SMOOTH PARALLEL PLATES ARE ONLY SHOWN FOR 60 AND 70% the upper geometry to the set gap. A fraction of the crystal RAPESEED OIL bonds are likely broken, resulting in a decreased G′ compared to the plate combinations, which allow more gentle volume, a sufficiently small cone angle and hence a small sample mounting. An advantage, when using the parallel gap between the cone and the plate is required (Mewis and plate geometry, is that the fat crystal network can be kept Wagner 2012). Consequently, when lowering the cone to the unruptured simply by ensuring that the normal force required gap, the sample is squeezed in such a way that a reached when mounting the sample is below the fracture significant fraction of the liquid fat migrates out of the force for the crystal bonds. Interestingly, the large gap vane blend, resulting in a higher percentage of solid fat within shows less difference between the emulsions studied. The the sample, hence an increase in G′. As the liquid fat has gap between the large gap vane and both cups is 7.5 mm migrated out of the fat matrix, no difference is observed in (Table 1). A large gap results in less contact with the sample G′ between the blends when using a cone-plate geometry. as compared to a small gap. Consequently, the resulting flow This migration of the binder from a solid matrix during field is likely to be less uniform. This might explain why press-induced flow is well known for other materials, such there is not a significant difference between G′ for all blends as highly filled polymer suspensions (Yaras et al. 1994). It studied when using the large gap vane. has been shown that migration is enhanced by a relatively small sample size, large particle size, rough sample surface and high temperatures (Yaras et al. 1994). Additionally, the Strain at Fracture in Blends Characterized present work shows that also the choice of geometry with Different Geometries strongly affects the tendency of a binder to migrate out from a solid phase due to the press induced by the defined Fat-based systems are not only characterized by hardness, gap. However, not only the flow inside the geometry and the but also brittleness. To evaluate the brittleness of the tendency of phase separation to occur affects G′, but also samples and to which extent the choice of geometry affects the ratio between crystal size and gap dimensions is impor- the measured brittleness, the strain applied to obtain a 50% tant to consider. A small gap combined with large crystal decrease in G′ was recorded, as earlier described by Rønholt results will rupture the crystals, hence affect the fat crystal et al. (2012a). In Fig. 7, the % strain obtained by different network and consequently G′. geometries is plotted as a function of percentage AMF. For the parallel plate geometries, the shear is higher at the The data show that changing the geometry can have a sig- edge of the plate compared to the center of the geometry, nificant impact on the strain at fracture measured (Fig. 7). which requires careful data treatment. A huge advantage for Among blends, there is no systematic relationship between fat-based samples sensitive to deformation during handling the amount of AMF added and percentage strain at 50% is, however, that the gap height can be changed as needed. decrease in G′. Kaufmann et al. (2012) measured the frac- In this way, rupture of the reversible and irreversible crystal ture stress of AMF/RO blends subjected to different cooling bonds before analysis is avoided. Consequently, if slip is not rates and found no significant increase in fracture stress occurring, a proper characterization of the system is pos- between blends of 60 and 70% AMF (the rest being RO) for sible (Fig. 6) and an increase in G′ can be monitored subse- the samples cooled at 0.05C/min, which is in accordance quently to a decreasing amount of RO. A practical challenge with our findings. However, when the amount of AMF was

result of partial melting of the fat crystal network. It must be kept in mind, however, that the temperature gradient is affected by sample amount, hence gap between the plates. The degree of handling of the sample prior to analysis is also an important parameter, as it might result in partially melting of the sample or eventually formation of cracks. This is essential as the measurements otherwise cannot be related to the crystallization process (van den Tempel 1958). The more brittle a sample is, the more likely it is to obtain cracks during handling, and consequently the variation between samples will increase. This phenomenon was observed in previous studies of butter-like samples during ageing (Rønholt et al. 2012b; Pothiraj et al. 2012). The geometries tested require different degrees of sample han- FIG. 7. STRAIN AT 50% DECREASE IN G′ OBTAINED DURING STRESS dling prior to analysis. For the cup and cell combinations, SWEEP. THE BLENDS CONTAINED ANHYDROUS MILK FAT (AMF) AND the cup or cell must be densely packed without breaking the RAPESEED OIL (RO) IN THE RATIO OF 50:50, 60:40 OR 70:30 % fat crystal network. This can be challenging, depending on (WEIGHT/WEIGHT). SMOOTH PARALLEL PLATES ARE ONLY SHOWN the consistency of the sample tested. In the case of the bob, FOR 60 AND 70% RAPESEED OIL. THE MEASUREMENTS WERE CONDUCTED AT 5C the sample is squeezed when placing the bob at the set gap. For the cone plate, a specific sample amount is needed in order to use a fixed gap, as earlier described. Consequently, increased to 90 and 100%, the fracture stress increased sig- the fat crystal network is potentially affected both during nificantly (Kaufmann et al. 2012). mounting of the sample and specifically when setting the gap. Last are the parallel plate geometries. A huge advantage is, as earlier discussed, that the method is not dependent on General Discussion a specific gap. Consequently, the sample is only slightly When characterizing the behavior of fat-based samples affected when mounting it on the plate. using rheology, there are several parameters that must be The last parameter mentioned is wall slip. When working considered before choosing geometry. Parameters such as with suspensions of rigid particles such as fat-based ability to control temperature, the dimensions of the dis- samples, wall slip might occur as an adhesive failure. Within persed phase within the sample, gap dimensions, degree of such systems, the particles are not physically able to occupy handling and hence deformation of the sample prior to the the space close to the wall as efficiently as further away from analysis and also wall slip are all likely to affect the outcome the wall (Kalyon 2005a). Consequently, a thin layer, referred of the measurements. to as the apparent slip layer, is formed on the wall, between Triacylglycerides in milk fat have a broad thermal range the shearing surface and the sample (Vand 1948). The ten- of melting points ranging from −40 to 40C (Deffense 1993). dency of wall slip to occur depends on the geometry used The temperature at measurements can therefore affect the and the physical properties of the sample (Yilmazer and crystalline state of the fat crystal network, and consequently Kalyon 1989; Aral and Kalyon 1994; Kalyon 2005a). the rheological properties (Rønholt et al. 2013). deMan Attempts have been made to mathematically correct data (1962) studied the rheological behavior of margarine at 10, for the effect of slip when measuring incompressible and 15, 20, 25 and 30C using cone penetrometry. deMan (1962) viscoplastic fluids (Kalyon 2005a). However, as this proce- found a significant increase in penetration value when dure is for idealized model systems, it cannot be directly increasing the temperature, corresponding to a decrease in applied for fat-based samples due to their very dense nature solid fat content, which was later confirmed by Vithanage (Mewis and Wagner 2012). For rheological characterization et al. (2009). In the present study, the lower geometry was of fat-based samples, the best approach is to prevent slip by temperature controlled in all combinations, while only both choosing the right geometry. One possibility is to use the smooth and the corrugated parallel plates allowed roughened geometries such as a fluted cup and corrugated temperature control of both upper and lower geometries plates (Fig. 2) (Aral and Kalyon 1994), or to glue sandpaper (Table 1 and Fig. 2). Because of this double temperature to the geometry (Citerne et al. 2001). However, a disadvan- control, the parallel plate geometries are expected to have tage is that the gap is not well defined due to the roughened the lowest temperature gradient through the sample. Conse- surface and it might result in fracture of the sample and quently, those experimental setups are therefore least pos- consequently a large variation between samples (Aral and sible to change the crystalline state within the samples as a Kalyon 1994). Corrugated parallel plate geometry allows a

Journal of Texture Studies 45 (2014) 20–29 © 2013 Wiley Periodicals, Inc. 27 SMALL DEFORMATION RHEOLOGY FOR AMF/RO BLENDS S. RØNHOLT, K. MORTENSEN and J.C. KNUDSEN small loading force (within the linear region) compared to temperature gradient through the sample, and the rough the fracture force. Based on the data shown in Fig. 6, only a surface minimizes the degree of slip when compared to the minor variation is observed between the samples measured smooth parallel plates. using the roughened parallel plate geometry, indicating that the roughened surfaces do not cause fractures in the ACKNOWLEDGMENTS samples analyzed. If a smooth surface geometry is used, the results must be compared with data collected by using Thanks to the Danish Dairy Research Foundation and the roughened surfaces and analyzed for the effect of wall slip Danish Food Industry Agency for financial support. Thanks (Lee and Wagner 2003; Kalyon 2005b). In the present work, to ARLA Foods Denmark for kindly supplying anhydrous data collected with the smooth parallel plates largely differ milk fat and rapeseed oil. TA Instruments is thanked for from those obtained with the corrugated parallel plates kindly lending us their starch pasting rotor cell. for the 50 and 60% AMF blends. For the 50% AMF blend, the effect of slip hindered collection of reproducible REFERENCES data (data not shown), whereas a shear thickening behavior is observed for the 60% blend, likely occurring as a conse- AMERICAN OIL CHEMISTS’ SOCIETY. 1980. Official and quence of slip (Fig. 5). For the 70% AMF blend, G′ was Tentative Methods of the American Oil Chemists’ Society. Methods Cc 16-60, AOCS Press, Chicago, IL. independent of frequency. When comparing G′ for 70% ARAL, B. and KALYON, D.M. 1994. 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