PROCESSING WHEY PROTEIN ISOLATE AND CORN STARCH USING A TORQUE RHEOMETER.

Transcription

1 PROCESSING WHEY PROTEIN ISOLATE AND CORN STARCH USING A TORQUE RHEOMETER. C. W. P. Carvalho 1 *, C. I. Onwulata 2, P. Tomasula 2 1 * Embrapa Agroindústria de Alimentos, Av. das Américas, 29501, Guaratiba, Rio de Janeiro - RJ cwpiler@ctaa.embrapa.br; 2 Dairy Processing and Products Research ERRC/ARS/USDA charles.onwulata@ars.usda.gov, peggy.tomasula@ars.usda.gov Abstract - Processing a combination of starch and protein mixtures for food or nonfood applications involves shearing and heating to produce desired texture, appearance and thermo-mechanical properties. In this study, corn starch and whey protein isolate (WPI) were processed in a torque rheometer at varying rotational speeds (32 to 88 rpm), and corn starch/wpi ratio (0 to 100%), using two mixer blade types (sigma or roller), at constant moisture (40%). The mixing temperature profile was: hold at 50 C for 10 min, heat at the rate of 2 C/min to 90 C; hold for 10 min, then cool at 2 C/min to 50 C, and hold for 10 min. Mixing torque using the roller blades was generally 5% higher than that from the sigma blades and ranged from to Nm. Torque for WPI were the highest (58.77 Nm) particularly during mixing. Fine structures seen with microscopy showed that starch granules were present in all starch mixtures after mixing, and that proteins tended to agglomerate into round shaped structures upon denaturation. Increased shearing or higher mixing energy could have enhanced net-working of starch and protein. Introduction World industrialization has generated substantial quantities of petroleum materials, which are not biodegradable or compostable. Total or partially degradable materials have been found to be an alternative, to help reduce the impact of petro plastics. The replacement of petro plastics by materials from agriculture, biopolymers, such as starch (1) and protein (2) is increasing. The demand for these materials for short-life use such as in garbage bags and packaging is growing. For biopolymers, starch is the largest in the nature and combines two interesting characteristics: abundance and low cost. The functionality of starch however can be enhanced by combining it with certain protein sources such as whey protein. One natural property of proteins and starch is their high hydrophilicity. During mixing, heat and mechanical shear the physical structures changes and impacts on its rheological properties. For protein, induced structural changes form insoluble cross-linked protein aggregates (4), and for starch breakage of its crystalline structure, reducing molecular weight and exposes the hydrophilic sites, increasing solubility. In this work, the application of controlled heat and shear on mixtures of corn starch and whey protein in an internal mixer, and its impact on rheological properties and starch conversion rates was studied. Experimental Material Whey protein isolate (WPI) (Provon 190) and native corn starch were purchased from Glanbia Foods (Richfield, ID) and Staley (Decatur, IL), respectively. WPI and corn starch were pre-mixed in a bench top mixer with water at the final water content of 40%. Time and pre-mixer speed were adjusted to obtain a homogeneous mixture with no clumps that was then processed in an Intelli-Torque Plasti-Corder Rheometer (C.W. Brabender, South Hackensack, NJ) fitted with a pair of roller blades. A central rotational experimental design was used to combine varied rotational speed of the blades and corn starch-wpi ratio as independent variables. The levels of both variables are shown in Table 1. Both experimental design and statistical analysis were performed by using Statistica Software version 5.0 StatSoft (Tulsa, OK). y = Bo + B 1 X 1 + B 2 X 2 + B 11 X 12 + B 22 X 22 + B 12 X 1 X 2 + ε (1) where y is response; X 1 refers to rotational speed of the blades, X 2 refers to starch concentration, Bo is the intercept; B 1 and B 2 represent linear effects; B 11 and B 22 represent quadratic effects; and B 12 represents interaction effects. Analyses

2 of variance (ANOVA) were generated for the response functions. The significance of the individual terms in the equation 1 was determined statistically by judging them at the probability level of at least 5%. For each response, surface three-dimensional plots were produced from regression equations by considering the two variables. The temperature regime used to process the mixture in the torque rheometer is described as follow: the pre-mixer was preheated at 50 C for 10 min, heating up to 90 C at ratio of 2 C/min, holding for 10 min and cooling down to 50 C at 2 C/min ratio when kept for 10 min, totalizing 60 min each run. The torque rheometer curves were analyzed by using the flow behavior analysis available in the Brabender Software WinMix version (Duisburg, Germany). In order to illustrate the data extracted from the torque rheometer curve, a graph model is shown in Figure 1, where the distance in time (min), called fusion time (T) was defined as the distance from point A to point X. In addition, the integration of the area under the curves delimited by the points A, B, C and D, was defined as following regions E1, E2, E3, E4, which represented the energy input; where E1 was defined as the area from the first minute (A) until next torque drop (B), E2 from B to the maximum peak (C), E3 from C to the end of the run (D) and E4 defined as the total energy represented by the sum of E1, E2 and E3. The temperature where the maximum torque was observed was termed Max Temp, in C. RVA Measurement After each experiment, a sample was removed from the mixer, placed in hermetic plastic bag and stored at 5 C overnight when was cut into small pieces by using a pair of scissors and then placed on aluminum trails to facilitate drying and milling. The samples were dried in a fan oven Corbett Industries Inc. model 40B-350 (Waldwick, NJ, USA) at 60 C overnight, milled in a disk mill Perten Instruments 3600 (Huddinge, Switzerland) and then manually sieved and the particles between 105 and 250 µm sieves were used for paste viscosity analyses. A Rapid Visco Analyser (RVA, Newport Scientific, Warriewood, Australia) was used to measure the apparent paste viscosity of samples as a function of temperature. The paste viscosity in cp was recorded along to the accompanying software Themoclyne for Windows Version 2.2. Sample (~3 g, adjusted to 10% of solid concentration) was added to ~25 g of distilled water. The time-temperature profile included initially holding the sample with the paddles rotating at 160 rpm, at 25 C for 4 min, to investigate the cold-swelling starch peak (1) and heated to 95 C at a constant rate of 14 C/min, held at that temperature for 3 min, and then cooled to 25 C in 5 min at the same rate. Each analysis took 21 min and was done in triplicate. The readings of the paste curve were maximum viscosity peak at 95 C (MV95), minimum viscosity after heating (MV), breakdown (MV-MV25), maximum final viscosity (FV) and set back (FV-MV). Microscopy Confocal Fluorescence and Reflection Microscopy were carried out on clean fractured pieces of fresh collected samples that were immersed in 20 ml aliquots of 2.5% glutaraldehyde-0.1 M imidazole solution (ph 7.0), to crosslink and amplify autofluorescence in protein components, during storage in sealed vials for more than two hours. For imaging, suspensions of the powders were transferred to microwell dishes (MatTek Corp., Ashland, MA) and mounted on the stage of a model IRBE optical microscope system equipped with a 20 phase contrast lens and coupled to a model TCS NT/SP confocal scanner head (Leica Microsystems, Exton, PA). Samples were illuminated with the 488 nm line of an Argon laser and reflection or phase-contrast transmission was collected in one channel, green autofluorescence and in some cases red fluorescence from Nile Red staining was collected in a second and third channel. Overlays of the two or three channels were visualized in extended focus images to resolve the arrangement of components in the corn starch, WPI and corn starch/wpi (50:50) samples. Table 1. Coded levels for the central composite design Independent variables Rotational speed (rpm), x 1 b Starch (%), x 2 c Levels -α a α a α = b Rotational speed of the blades in rpm. c Starch (%, db) represents the ratio corn starch and WPI in the mixture, in other words, 0.00 means that no starch was mixed with 100% WPI.

3 Torque (Nm) Temperature ( C) A T B C Time (min) Figure 1. Representation of a torque curve of WPI showing the points: A, B, C, D and T used to calculate the energy of the process: E1, E2, E3 and E4. Results and Discussion The coefficients of equation 1 (Table 1 and 2) were obtained by fitting the response data applying the backward elimination procedure using StatSoft (Tulsa, OK). Some of the conclusions are illustrated in the response surface graphs shown in Figures 1-3. Table 1 - Significant coefficients of regression equations (eqn 1) for dependent variables of torque curves Coefficients Fusion time, t (min) Max. Temp. ( C) E3 (kj) E4 (kj) B 0 2.4E01*** 3.3E03* B 1 B 2 2.2E01*** 2.7E01** -1.4E03*** -1.6E03*** B 11 B E01*** 1.2E03*** 1.6E03*** B E-01* -2.5E-01* R *** Significant at the 0.1% level; **significant at the 1% level; *significant at the 5% level. Where x 1 =rotational blade speed of the blades (rpm) and x 2 = starch content (ratio of corn starch and WPI in the mixture). E1 and E2 had R 2 lower than 70%, therefore were not considered in this Table. Table 2 - Significant coefficients of regression equations (eqn 1) for dependents variables of RVA curves Coefficients Max. viscosity, (cp) Breakdown (cp) Setback (cp) B 0 B 1 B 2-1.4E04** -7.1E03* -2.3E04** B 11 B E04*** 2.8E04*** B E01* 5.5E03** R *** significant at the 0.1% level; **significant at the 1% level; *significant at the 5% level. 1where x 1 = rotational blade speed of the blades (rpm) and x 2 = starch content (ratio of corn starch and WPI in the mixture). E1 and E2 had R 2 lower than 70%, therefore were not considered in this Table. In Table 1, fusion time was significantly affected by the positive linear, negative quadratic effects of starch ratio and by the negative effect of starch ratio and blade speed which is represented by the surface graph in Figure 1. Fusion time is an important variable in the process because it defines when the material melts at known energy input to the system (6) D 0

4 As starch increased in the mixture, temperature and time shifted to higher values. This can be attributed to the lower thermal transition of whey proteins which usually start to denature at temperature around 60 C (7) at lower starch concentration. Figure 1 Temperature (left) and time (right) of fusion as a function of starch ratio and blade speed (rpm). Figure 2 Energy input E3 (left) and E4 (right) as function of starch ratio and blade speed (rpm). Regarding the calculated energy from the torque curve, there were no significant differences in E1 and E2 values, which may not have contributed to the total energy input E4 (Table 1). On the other hand, most of the energy spent in the process was due to E3, which represented the energy from the maximum fusion till the end of an hour process. Although, it was seen that whey protein curves showed an initial increase in torque (Figure 1), but similar results were not seen as starch ratio in the mixture was increased. By replacing whey protein with corn starch, the linear effect on the response variables, fusion time and temperature of maximum torque, increased (P 0.05) whereas the energy input E3 and E4 reduced (Table 2). The quadratic effect of fusion time was negative indicating that addition of starch presented a gradual increase in the time the material melted. Maximum torque temperature was reduced at high starch concentration, increasing blade speed; but, rpm did not affect energy input. The positive quadratic effect of the response variables E3 and E4 was significant (P < 0.05). From Figure 1, the quadratic effect can be interpreted as the plasticizing effect of the mixture starch and protein, since low values of E3 and E4 were observed in 50:50 mixtures. The paste viscosity readings and gelatinization peaks, breakdown and setback, were significant (P < 0.05). Maximum viscosity was also affected by the negative linear effect of starch/wpi ratio, and the negative interaction of starch/wpi ratio with blade speed. High gelatinization peak was found at high starch ratio, and increased at higher blade speeds. As starch was mechanically sheared, maximum viscosity increased particularly in the presence of starch. Similar behavior was observed for the response variables, breakdown and setback, as these variables are dependent on starch content.

5 Figure 3- Starch conversion through RVA readings of dried and milled blends of corn starch and whey protein isolate as a function of starch content and blade speed. Maximum temperature or peak viscosity (top); Breakdown (bottom left) and setback (bottom right). The micrographs of pure corn starch, pure whey protein and a 50:50 mixture are presented in Figure 4(A, B, C). In Figure 4A, the round clear red structures represent remnants of starch granules which were not completely broken and incorporated into the matrix. In the Figure 4B, a dominant fluorescent matrix shows round protein aggregates of varied sizes. The combination of reflectance and fluorescence is clearly seen in Figure 4C, where small fluorescence structures are spread along the matrix. A B C Figure 4 Confocal laser scanning images of (A) corn starch matrix with clear evidence of starch remnants (circular red shapes), (B) WPI sample showing autofluorescence of the protein matrix, and (C) corn starch/wpi showing sparse agglomeration protein represented by green round shape structures and some red granular starch structures.

6 Acknowledgement The help of Dr. Peter Cooke with Confocal Laser Scanning Microscopy is appreciated. Conclusion Increasing protein concentration increased energy input reaching maximum torque when 100% of whey protein was processed. Energy input was reduced by addition of starch which lowered the torque and may have helped plasticize the protein matrix. Processibility of starch and protein blends was greatly enhanced by varying starch/wpi ratio. References 1. X. You; L. Li, J. Gao, J. Yu, Z. Zhao Journal of Applied Polymer Science. 2003, 88, S. Domenek; P. Feuilloley; J. Gratraud; M. H. Morel; S. Guilbert Chemosphere. 2004, 54, R. M. S. M. Thiré; R. A. Simão; P. J. G. Araújo; C. A. Achete; C. T. Andrade Polímeros. 2004, 14, C. I. Onwulata; S. Isobe; P. M. Tomasula; P. H. Cooke. J. dairy Sci. 2006, 89, P. J. Whalen; M. L. Bason; R. I. Booth; C. E. Walker; P. J. Williams Cereal Foods World. 1997, 42, L. Meynie; F. Fenouillot; J. P. Pascault Polymer. 2004, 45, J. N. De Wit Journal of Dairy Science, 1990, 73, 3602.