bs_bs_banner A journal to advance the fundamental understanding of food texture and sensory perception 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 geometri- cal 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 sys- tematic 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 rhe- ology 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 crystalliza- tion 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 Journal of Texture Studies 45 (2014) 20–29 © 2013 Wiley Periodicals, Inc. 21 SMALL DEFORMATION RHEOLOGY FOR AMF/RO BLENDS S. RØNHOLT, K. MORTENSEN and J.C. KNUDSEN 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