Gelling Properties of Egg White Produced Using a Conventional and a Low-shear Reverse Osmosis Process M.M. OULD ELEYA AND S.

Transcription

1 JFS: Gelling Properties of Egg White Produced Using a Conventional and a Low-shear Reverse Osmosis Process M.M. OULD ELEYA AND S. GUNASEKARAN ABSTRACT: Gelling properties of heat-induced egg white (EW) powders obtained using a conventional (control) and a reverse osmosis (RO) process were investigated under different phs (3-9) and protein concentrations (5-20%) by dynamic and uniaxial compression rheological tests. At ph 9, temperature sweep tests indicated that gelation behavior of RO and control samples were similar. Both gelation temperature and G were concentration-dependent; the increase in G with concentration followed a power-law relationship with an exponent of Failure stress and strain of cured gels were concentration- and ph-dependent but were not significantly (p = 0.05) different for the two powders. Results suggest that gelling properties of EW powders are not significantly affected by the RO treatment. Keywords: egg white, reverse osmosis, gels, rheology Introduction EGG WHITE, IN ITS DRIED FORM, IS AN IMPORTANT INGREDIENT THAT is widely used in many food preparations such as bakery products, cookies, meat products, and meringues (Mine 1995). It is normally produced from liquid egg albumen by a process consisting of 2 main unit operations: fermentation (for glucose removal) and spray drying. Previous studies (Lowe and others 1969; Froning and others 1987; Conrad and others 1993) have shown that liquid egg white, which contains naturally high amounts of water (about 88% w/w), could be concentrated by ultrafiltration (UF) and reverse osmosis (RO). The use of membrane filtration technique offers an efficient means to reduce the energy cost of the process. Nonetheless, some modifications in composition and functional properties of egg white could occur (Froning and others 1987; Conrad and others 1993). The effects of UF and RO processes on composition and functional properties of egg white have been investigated by Froning and others (1987). Both UF and R O treatments of liquid egg white increased solids from 11% to 23% without any protein loss into the permeate. In contrast to RO, UF treatment significantly reduced glucose, sodium, and potassium levels in the albumen retentate. Neither UF nor RO seemed to have a significant effect on the quality of egg white gels as determined by true strain and stress measurements. Foaming properties of egg white, as assessed by angel cake volume and foaming stability studies, were not different after UF or RO treatment. These results however do not agree with those reported by Conrad and others (1993), who showed that RO severely decreased the foaming properties of egg white. According to these authors, the decrease in foaming properties was due to the shearing action of the pump, caused by the high pressure (3 MPa) needed to process the samples by RO (Conrad and others 1993). The higher pressure associated with RO may indeed cause shearing action, and possibly induce protein denaturation during the process (Froning and others 1987). Recently, the United States Department of Agriculture (USDA) approved the addition of a low-shear RO system for concentrating liquid egg white prior to fermentation and spray drying operations. This process enables removal of 50% of water from egg albumen without any loss of protein, sugar, or vitamins. In addition, it has many advantages over the conventional process including an increase in dryer capacity by 230%, reduction in drying cost by 60%, reduction in fermentation volume and hence storage tank capacity by 50% (Singh 1998). However, the effects of such an RO treatment on functional properties of spray-dried egg white products, particularly their gelation, is still unknown. The ability of egg white to form heat-induced gels is one of their most important functional properties. The thermal gelation of egg white proteins is generally considered a 4-step process, characterized first by conformational changes in the protein structure, with exposition of buried hydrophobic groups, aggregation, formation of a primary gel network, and a reinforcement of this network upon cooling (Mine 1995). This process is regulated by different types of intermolecular interactions such as hydrogen bonding, electrostatic and hydrophobic interactions, van der Waals forces, and covalent bonds (disulfide cross-links) (Ziegler and Foegeding 1990, Mine 1995). Heat-induced gelation of egg white has been previously studied by several rheological techniques such as small amplitude oscillatory shear (SAOS) (Hsieh and others 1993), uniaxial compression (Hsieh and Regenstein 1993; Lee and others 1997), double compression (Handa and others 1998), penetration (Handa and others 1998) and torsion failure (Li and others 1999). These studies show that egg white gelation is strongly affected by heating conditions, protein origin and concentration, ph, and ionic strength. In the present study, the gelling properties of two egg white powders, produced using both the conventional process (without membrane technique) and a process incorporating RO, were investigated by SAOS and uniaxial compression tests. Gelation and mechanical properties of the resulting gels were studied at various protein concentrations and ph values. The objective was to determine whether or not the concentration of liquid egg white by reverse osmosis affects the gelling properties of spraydried egg white Institute of Food Technologists Vol. 67, Nr. 2, 2002 JOURNAL OF FOOD SCIENCE 725

2 Materials and Methods Egg white Two pasteurized spray-dried egg white powders, produced with and without membrane concentration, were obtained from Oskaloosa Food Products Corp. (Oskaloosa, Iowa, U.S.A.). In the following sections, the egg white powder produced using conventional process is referred as control, whereas, that processed using reverse-osmosis is denoted RO. Preparation of protein solution Protein solutions were prepared in distilled water from the 2 types of egg white powders. Spray-dried egg white powder at 5, 10, 15, and 20% w/v were hydrated by first mixing a small amount of water with solids to form a smooth paste, followed by adding more water and stirring for 1 h. The protein suspensions were then held overnight at 4 C for complete hydration, and their ph was adjusted, at room temperature, using 1 N of HCl or NaOH. Dynamic rheological measurements SAOS measurements were performed on a controlled-stress dynamic rheometer (Bohlin CVO, Bohlin Rheologics, Inc., Cranbury, N.J., U.S.A.) using the Bohlin C14 couette geometry. The protein suspensions, prepared at ph 9 and at various concentrations (5 to 20%), were poured directly into the measuring system of the rheometer and equilibrated to 25 C. The sample was covered with a thin layer of paraffin oil to prevent water evaporation during the experiment. The protein samples were heated to 90 C at a rate of 3 C/min, and then cooled to 25 C at the same thermal rate. During the entire thermal treatment, the storage modulus (G ), loss modulus (G ), and phase angle ( ) of the samples were measured at a frequency of 1 Hz and a maximum target strain of Gel preparation and testing Egg white protein gels were prepared, at constant ph of 9 and at various protein concentrations (5 to 20%) and/or at constant concentration of 10% and under various ph conditions (3, 5, 7), according to the procedure of Lee and others (1997) with slight modifications. Chlorinated polyvinyl chloride tubes (18-mm i.d., 68-mm long), coated with mineral oil, to prevent sticking, were filled with the protein solutions and closed with rubber stoppers. Tubes were placed vertically in a holding rack, then placed in a water bath and heated at 90 ± 0.5 C for 15 min. After completion of heating, tubes were removed and cooled first in a water bath at 10 C ± 0.5 C for 20 min and then left at 4 C overnight. Prior to compression tests, gel-containing tubes were equilibrated to room temperature for 2 h. The gels were carefully removed from the tubes and cut into cylindrical specimens (18-mm diameter 11-mm long), corresponding to a height-to-diameter ratio of 0.6. Uniaxial compression tests were performed using a universal testing machine (Synergie 200, MTS System Corporation, Cary, N.C., U.S.A.) equipped with a 50-N load cell. The following tests were performed on both control and RO egg white gels: 1) at ph 9 and three protein concentrations (10, 15, and 20%) and 2) at 10% protein concentration and four ph values (3, 5, 7, and 9). The gel specimens were deformed to 80% of their initial height at a constant crosshead speed of 1 mm.s -1, and data were collected at a speed of 10 points per s. The compressive force at failure and the corresponding relative deformation were determined from the force-deformation compression profiles. Hencky s or true strain ( T ) and true compressive stress ( T ) at failure were calculated as follows. T = ln[h/(h h)] and T = F[1 ( h/h)]/ r 2, where F = compressive force at failure (N); h = specimen height (m); h = deformation at failure (m); r = initial sample radius (m). Cauchy s or engineering strain ( E ) and stress ( E ) were also determined using the following expressions: E = h /h; E = F/ r 2. Statistical Analysis The experiments were carried out in triplicates for SAOS measurements, and at least 6 specimens of each gel were analyzed per sample for uniaxial compression tests. The mean and standard deviation were determined, and 1-way ANOVA (analysis of variance) was used for the significance of the differences at p = 0.05, employing a statistical Minitab software (version 11.2, Minitab Inc., State College, Pa., U.S.A.). Results and Discussion FIGURE 1 SHOWS THE DEVELOPMENT OF STORAGE MODULUS (G ) AS function of temperature (T) for egg white samples, prepared from the 2 egg powders (control and RO) at three protein concentrations (10, 15, and 20% w/w) and at ph 9. This ph was chosen because it corresponded to that of fresh egg white (Li and others 1999). For each protein concentration, the gelation profiles (G vs. T.) of the 2 egg white samples were almost perfectly superimposed. Upon heating from 25 to 90 C, G remained low until gelation temperature and then increased rapidly, indicating a typical transition from liquid-like to solid (gel)-like state. The gelation temperature, temperature at which the onset of gelation occurs, was determined as illustrated in the inset in Figure 1. This temperature corresponds to that where G increases and becomes greater than the background noise. This is one of the common methods of gel point detection (Ross-Murphy 1995; Gunasekaran and Ak, 2000). For both RO and control egg white samples, the gelation temperature decreased as protein concentration increased, which is in agreement with previous results on egg white gels (Hsieh and others 1993). The gelation temperatures for samples containing 10, 15, and 20% (w/w) of EW powders were about 78, 75, and 70 C, respectively. On subsequent cooling from 90 to 25 C, G of RO and control Figure 1 Temperature sweep (from 25 to 90 ºC) data for 2 egg white powders at ph 9 and at 3 protein concentrations (10, 15, 20%). Egg white powders were produced by a process with (filled symbols) and without (open symbols) a low-shear reverse osmosis system. (test conditions: frequency = 1 Hz; strain = 1%.) 726 JOURNAL OF FOOD SCIENCE Vol. 67, Nr. 2, 2002

3 Table 1 Effects of protein concentration (10, 15, and 20%) on compressive failure force, strain, and stress for egg white gels prepared at ph 9 from 2 egg white protein powders. The egg white powders were produced by a process with (RO) and without (Control) a low-shear reverse osmosis system. Compressive force True strain True stress at failure (N) at failure at failure (kpa) Egg white Concentration (%) Concentration (%) Concentration (%) type Control 1.60 aa 5.73 ab ac 0.67 aa 0.88 ab 1.07 ac 3.21 aa 9.31 ab ac RO 1.38 aa 5.41 ab ac 0.64 aa 0.76 ab 0.97 ac 2.76 aa 9.92 ab bc a Different letters indicate significant differences, lower case letters between products and upper case letters between concentrations (p = 0.05). adifferent letters indicate significant differences, lower case letters between products and upper case letters between concentrations (p = 0.05). egg white gels at 10, 15, and 20% increased further (Figure 2). The increase in G of protein gels with decreasing temperature has been previously observed for different protein systems (Cooney and others 1993; Aguilera 1995; Ould Eleya and Turgeon 2000). This increase is generally attributed to a consolidation of attractive forces such as van der Waals and hydrogen bonding between the protein particles within the gel network (Ould Eleya and Turgeon 2000). Figure 2 also shows that, at the same protein concentration, the G values of RO egg white gels and those of the control were almost the same. Moreover, as protein concentration increased, G of egg white gels increased. This can be clearly seen in Figure 3, where G values of egg white gels at 25 C are plotted against protein concentration. The following power-law relationship was used to relate the G for the gels at 25 C and the protein concentration (C, %): G = kc n where, n = power-law exponent; k = constant. The relationship fit well (r 2 = 0.99) with n = This result is in agreement with literature which reports protein gels as exhibiting a power law behavior with n ranging from 2.2 to 2.8 (Zoon and others 1988; Paulsson and others 1990; Hagiwara and others 1998). Results of the uniaxial compression tests on both control and RO egg white gels at ph 9 and 3 protein concentrations (10, 15, and 20%) are summarized in Table 1. For a given protein concentration, the values of the compressive force, failure strain, and stress at failure for RO samples were not significantly different (p = 0.05) from those for the control. In agreement with previous results (Froning and others 1987; Conrad and others 1993; Hsieh and Regenstein 1993; Lee and others 1997), as protein concentration increased, the compressive force (or stress) at failure significantly increased. The stress at failure, generally considered as a measure of gel strength, is indeed greatly affected by the solid content of the sample. The compressive true strain at failure, an indication of the gel deformability, increased slightly but significantly with protein concentration (Table 1). This result is similar to that previously reported by Froning and others (1987) on egg white gel, but differed from those reported by Conrad and others (1993) and Hsieh and Regenstein (1993). The latter authors have shown that strain at failure of egg white gels was independent of protein concentration. More recently, Lee and others (1997) showed, in an extensive study of compression failure of protein gels, that the response patterns of strain values to protein concentration were inconsistent, and therefore have concluded that strain was not as dependable a measurement as Figure 2 Development of elastic modulus (G ) during subsequent cooling to 25 ºC, for 2 egg white gels at ph 9 and at 3 protein concentrations (10, 15, 20%). Egg white powders were produced by a process with (filled symbols) and without (open symbols) a low-shear reverse osmosis system. (test conditions: frequency = 1 Hz; strain = 1%). Figure 3 Concentration dependence of storage modulus (G ) of egg white gels at 25 ºC. Egg white powders were produced by a process with (filled symbols) and without (open symbols) a low-shear reverse osmosis system. (test conditions: frequency = 1 Hz; strain = 1%). Vol. 67, Nr. 2, 2002 JOURNAL OF FOOD SCIENCE 727

4 Table 2 Effects of ph (3, 5, 7, and 9) on compressive true stress at 20% deformation and true strain and stress at failure for 10% (w/v) egg white gels prepared from 2 protein powders. The egg white powders were produced by a process with (RO) and without (Control) a low-shear reverse osmosis system. True stress at (20%) True strain True stress Deformation (kpa) at failure at failure (kpa) Egg white ph ph ph type Control 2.90 aa 0.95 ab 1.79 ac 0.89 ad 0.34 aa --- * 0.87 ab 0.67 ac 4.02 aa --- * 2.79 ab 3.12 ab RO 3.24 aa 1.14 ab 1.63 ac 0.79 ad 0.31 aa --- * 0.84 ab 0.67 ac 4.35 aa --- * 3.22 ab 2.76 ab a Different letters indicate significant differences, lower case letters between products and capital letters between ph (p = 0.05). * No failure behavior was observed. force (or stress) for comparison of gelation properties. In our case, the change in strain with protein content was not as pronounced as change in force (or stress), but it was significantly important to be taken into account. The results of uniaxial compression tests performed on RO and control egg white gels, at constant concentration of 10% and at different ph values (3, 5, 7, and 9), are reported in Table 2. The mechanical properties of RO and control egg gels were compared under various ph conditions because the strength of egg white gels has been reported to be dependent on ph (Handa and others 1998; Li and others 1999). For a given ph, the value of stress at 20% deformation ( 20 ) for RO egg white gels was not significantly (p = 0.05) different from that for the control. Similarly, the values of true stress or strain at failure for RO sample at ph 3, 7, or 9 were not significantly (p = 0.05) different from those for the control at similar ph values (Table 2). However, true stress at 20% deformation ( 20 ), true strain and stress at failure significantly differed with ph of the medium, which is in agreement with previous results on egg white gels (Handa and others 1998). Failure stress and strain could not be determined at ph 5 (Table 2) because at this ph, egg gel did not fracture even at 80% strain. This is illustrated in Figure 4, where stress-strain curves for a 10% egg white gel at 4 ph values are presented. At ph 3, 7, and 9, egg white gels showed a typical failure profile stress increasing with strain, and reaching a peak. This peak is usually considered as the failure or yield point (Hsieh and Regenstein 1993). Above the failure point, there is a significant decrease in stress with an increasing strain (Figure 4). In contrast, at ph 5 as strain increased the stress increased continuously without any characteristic failure point (Atkins 1987). The egg white gels formed at this ph were milky white and syneresed heavily. This is in agreement with previous studies on egg gels (Handa and others 1998), which showed that the water-holding capacity of egg white gels was the lowest at ph 5. This is because, at ph 5, which is around the isoelectric point (pi) of egg white protein, protein-protein interactions are stronger than water-protein interactions. In our case, we observed that during uniaxial compression of egg white gel at ph 5, large amounts of water was expelled from the gel specimen. At the end of compression, the gel was a sponge-like mass. Conclusions GELLING PROPERTIES OF TWO EGG WHITE SAMPLES, PRODUCED USing a process with (RO) and without (control) a low-shear reverse osmosis system were compared. Based on the SAOS temperature sweep data, the gelation pattern of these 2 egg white powders was the same. For both samples at ph 9, as protein concentration increased gelation temperature decreased but G increased. The increase in G with protein concentration at 25 C, was according to a power law relationship with an exponent of For a given protein concentration, the values of compressive fracture force, true stress, and strain at failure of the RO egg white gels at ph 9 were not significantly (p = 0.05) different from those of the control. However, compressive force and true stress at failure strongly increased while strain at failure slightly increased with protein concentration. Results of compression tests at other ph values (3, 5, and 7) showed that the values of stress at 20% deformation ( 20 ), failure stress and strain for a 10% RO egg white gels were also not significantly (p = 0.05) different from those of the control at similar ph. These values varied significantly, however, with ph. Our results suggest that gelling properties of egg white are not significantly affected by the use of a low-shear RO unit for concentrating liquid egg white. Figure 4 Engineering stress-strain curves of 10% egg white gels at various ph values. The failure point was not observed at ph 5. References Aguilera JM. Gelation of whey protein Food Technol 10: Atkins,AG The basic principles of mechanical failure in bilogical systems. In Food Structure and Behavior. Blanschard KMV, Lillford P.,editors,.London, Academic Press LTD., p Conrad KM, Mast MG, Ball RB, Froning G, Mac Neil JH Concentration of liquid egg white by vacuum evaporation and reverse osmosis. J Food Sci 58: Cooney MJ, Rosenberg M, Shoemaker CF Rheological properties of whey protein concentrate gels. J Text Stud 24: Froning GW, Wehling RL, Ball HR, Hill R M Effects of ultrafiltration and reverse osmosis on the composition and functional properties of egg white. Poultry Sci 66: Gunasekaran S, Ak M M Dynamic oscillatory shear testing of foods selected applications. Trends Food Sci Technol 11(3): JOURNAL OF FOOD SCIENCE Vol. 67, Nr. 2, 2002

5 Hagiwara T, Kumagai H, Nakamura K Fractal analysis of aggregates in heat-induced BSA gels. Food Hydrocoll 12: Handa A, Takahashi K, Kuroda N, Froning GL Heat-induced egg white gels as affected by ph. J Food Sci 63: Hsieh YL, Regenstein JM Failure deformation and stress relaxation of heated egg white gels. J Food Sci 58: Hsieh YL, Regenstein JM, Rao MA Gel point of whey and egg proteins using dynamic rheological data. J Food Sci 58: Lee CM, Filipi I, Xiong Y, Smith D, Regenstein JM, Damodaran S, Ma C-Y, Haque ZU Standardized failure compression test of protein gels from a collaborative study. J Food Sci 62: Li H, Errington AD, Foegeding EA Iso-strength comparison of large-strain (fracture) rheological properties of egg white and whey protein gels. J Food Sci 64: Lowe E, Durkee ELL, Merson RL, Ijichi K, Cimino L Egg white concentrated by reverse osomosis. Food Technol 23: Mine Y Recent advances in the understanding of egg white protein functionality. Trends Food Sci Technol 6: Ould Eleya MM, Turgeon SL The effects of ph on the rheology of -lactoglobulin/ -carrageenan mixed gels. Food Hydrocoll 14: Paulsson, M, Dejmek P, van Vliet T Rheological properties of heat-induced -lactoglobulin gels. J Dairy Sci 73: Ross-Murphy SB Rheological characterization of gels. J Text Stud 26: Singh N Low shear RO concentrates egg albumen. Membrane Technol. 98(6):6. Ziegler GR, Foegeding EA Gelation of proteins. Adv. Food Nutr. Res. 34: Zoon P, van Vliet T, Walstra P Rheological properties of rennet-induced skim milk gels. 1. Introduction. Neth Milk Dairy J 42: MS Submitted 2/8/01, Accepted 4/13/01, Received 5/1/01 We thank Dr. Navpreet Singh of Central Soya Co., formerly with FES International, for providing details of the membrane processing technique. We also thank Oskaloosa Food Products Corp. for providing egg white samples for the study. Authors are with the Food and Bioprocess Engineering Laboratory, Biological Systems Engineering Dept., Univ. of Wisconsin Madison, 460 Henry Mall, Madison, WI Direct inquiries to author Gunasekaran ( guna@facstaff.wisc.edu). Vol. 67, Nr. 2, 2002 JOURNAL OF FOOD SCIENCE 729