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Functional and gel characteristics of liquid whole egg as affected by pH alteration
Journal of Food Engineering 45 (2000) 237±241
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Functional and gel characteristics of liquid whole egg as aected by pH alteration Y.I. Chang, T.C. Chen * Poultry Science Department, Mississippi State University, Mississippi State, MS 39762, USA Received 28 October 1999; accepted 20 March 2000
Abstract Studies were conducted to evaluate the eect of pH alteration of liquid whole egg (LWE) from newly laid shell eggs on their functional and gel characteristics. The pH of LWE was altered either with 1 N NaOH or 1 N HCl to 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0 and tested for their foaming capacities (FCs), foam stabilities (FSs), and baked volumes (BVs). Gels were also prepared and tested for ®rmness (FM), strain at failure (NF), and stress at failure (SF), as well as Hunter color values. The FC and FS of LWE were altered by pH alterations. As the pH increased, the FM, NF and SF of gels increased and the structure became more dense. Hunter L (lightness) and b (yellowness) values of the gel decreased and Hunter )a (greenness) values increased as the pH of LWE increased. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Liquid whole egg; Functional; Gel; pH; Color
1. Introduction Eggs provide many desirable attributes as a food ingredient because they are considered polyfunctional. Three-dimensional egg protein gels are stabilized by intermolecular linkages such as disul®de cross-link, hydrogen bonds, and hydrophobic interaction with heating for a desired texture in many food systems. Egg proteins are responsible for many functional properties such as foaming, whipping, and viscosity building (Hsieh & Resenstein, 1989). Eggs are used in food related capacities for coagulation, foaming, emulsi®cation, color and ¯avor. Hickson et al. (1982) reported that the pH of egg albumen was the main factor in governing the rheological properties of gels formed during the heat treatment of albumen at 80°C. Textural properties of protein gel were greatly aected by pH. Hermansson (1982) reported that a ®ne, uniform gel matrix was formed at high pH with a high gel strength. Beveridge, Arnt®eld, Ko and Chung (1980) reported that ®rmness of albumen gel increased with increasing pH due to sulfhydryl-disul®de exchange, which was accelerated up to pH 9.0. Su and Lin (1993) reported that the transparency of duck egg gel could be increased through appropriate alkal*
Corresponding author. Tel.: +1-601-325-3382. E-mail address: [email protected] (T.C. Chen).
ization from pH 8.0±8.5 (fresh egg) to 11.5±12.8. Arnt®eld and Bernatsky (1993) reported that gel network formation of the ovalbumen±lysozyme mixture was not aected at pH 5.5. However, the mixture gel network increased at pH 7.0 and 8.5. Texture is one of the major quality attributes of foods. Compression tests are most commonly used to determine the functional properties of gelling protein. Color is the ®rst visual impression of the acceptability of food products. Color from egg yolk aects the acceptability of many food products, especially baked products (Bornstein & Bartov, 1966). The pH of the shell egg and egg products is an important factor in quality retention, processing, and performance. The pH is one of the most useful indicators for shell egg during storage due to the pH increase of albumen (Silversides, Twizeyimana & Villeneuve, 1993). The objective of this study was to investigate the eect of adjusted pH with naturally existing ranges on the functional properties and gel characteristics of liquid whole egg.
2. Materials and methods 2.1. Liquid whole egg Liquid whole egg (LWE) was prepared from newly laid shell eggs. The pooled LWE was homogenized and
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 6 6 - 2
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Y.I. Chang, T.C. Chen / Journal of Food Engineering 45 (2000) 237±241
adjusted to pH 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0 with 1 N HCl or 1 N NaOH. A Sargent±Welch pH meter (Model LS) was used to measure pH. To minimize the possible dilution eect, the acid or alkali added to the LWE was less than 2% (V/V). 2.2. Gel preparation Gel samples of LWE at various pH levels were prepared by pouring the liquid phase of the protein into cellulose casings (Nojax Casing, size 22-diameter 20 0:5 mm), sealing and cooking in boiling water for 30 min. The prepared gels were cooled to room temperature (24 1°C) and cut into 2 cm length for the gel quality measurement. 2.3. Measurement The foaming capacity (FC) and foam stability (FS) of the LWE, baked volume (BV) of sponge cake, ®rmness (FM), stress at failure (SF), strain at failure (NF) and Hunter L, a, and b values of the prepared gels were measured. 2.4. Foaming capacity and foam stability Twenty milliliters of LWE at 24 1°C were placed in 100 ml stoppered graduated cylinders and shaken twice for 30 s at a rate of two strokes per second. The volume of foam and liquid phase were measured 30 s after the second shaking. Drainage was measured after standing for 60 min. The following formulae were used to determine FC and FS (Janssen, 1971; Germ, 1994). FC % FV=ILV 100; FS % ILV ÿ DV=ILV 100; Drainage ml LVM ÿ LVS; where FV is the volume of foam (ml), ILV the volume of initial liquid phase (ml), DV the volume of drainage (ml), LVM the volume of liquid phase after 60 min (ml), and LVS is the volume of liquid phase after 30 s (ml). 2.5. Baked volume of sponge cake Sponge cakes were prepared (Norris & Cotterill, 1971). Sixty four grams of LWE were mixed with 44.7 g of sugar, 25.7 g of cake ¯our and 50 g of the butter was poured into rectangular loaf pans and baked in a preheated oven at 177°C for 21 min. After cooling, the cake volume was measured by using a glass bead solid (3 mm, Fisher Scienti®c, Pittsburgh, PA) displacement method.
2.6. Firmness, stress and strain of gel The prepared gel samples were placed on the ¯at hold plate of an Instron Universal Testing Machine Model 1130 with compression accessories and compression was carried out at room temperature (24 1°C). Gel samples were cut straight with a uniform geometry (2 cm height 2 cm diameter). Gel properties were measured using the Instron set at the following conditions: (1) calibration, 10 kg load cell, (2) cross head speed, 5 cm/min, (3) chart speed, 10 cm/min, (4) range, 0±10, and (5) deformation, 70%. Firmness (KN) was expressed as the compressive force at 70% deformation. The stress at failure (SF, KN/cm2 ), and the strain at failure (NF) of gels were calculated by using the following equation (Hsieh & Resenstein, 1993; Kucukoner, 1995). SF fF t1 ÿ dh=hg=pr2 ; where SF is the stress at failure, F t the compressive force at failure point (kN), dh the deformation height of sample, h the initial sample height, dh/h the strain at failure, and r is the radius of initial cross-section. 2.7. Color The Hunter color values of the prepared gel samples were measured using a Hunter color and color dierence meter (Model D-25: Hunter Associate Laboratory) with reference plate L 92:4;
a ÿ0:7;
b ÿ0:9:
2.8. Data analysis Analysis of the data was performed using the general linear model (GLM) procedure of SAS software (SAS, 1995). Means, where signi®cant dierences occurred (P < 0:05), were separated using the least square dierence (LSD) test. Data were then treated using the PROC COR procedure of SAS software (SAS, 1995) to determine the correlation coecient between all parameters and adjusted pH. 3. Results and discussion 3.1. Functional properties of liquid whole egg The FC and FS of LWE altered slightly as pH changed and the trend was nonlinear. The BV of LWE increased (P < 0:05) from pH 7.0 to 8.0; however, there were no BV changes between pH 6.5 and 7.0, and among pH 8.0, 8.5, and 9.0, respectively (Table 1). Eggs are good foaming agents because they develop a large foam volume which is relatively stable for cooking, and
Y.I. Chang, T.C. Chen / Journal of Food Engineering 45 (2000) 237±241
239
Table 1 Eects of liquid whole egg pH adjustment on functional and gel characteristicsa Quality
7.18 (Cont)
pH 6.5
Functional FC FS BV Gel textural FM NF SF Gel color L a b
40.77 4.68ab 89.05 2.55a 197.25 3.42bc 4.60 0.37cd 0.98 0.05c 0.40 0.08cd 64.6 0.89c )6.03 0.14b 14.10 0.44c
7.0
37.14 5.82bcd 36.35 0.57cd 86.57 2.63b 90.08 0.71a 194.00 7.71bc 191.87 6.56c 3.27 0.15e 0.98 0.04c 0.27 0.02d
4.35 0.40d 0.99 0.06c 0.34 0.03d
72.80 0.38a )4.52 0.13a 22.87 0.11a
72.86 0.51b )6.24 0.16c 18.72 0.19c
7.5
8.0
8.5
34.42 1.38d 90.81 0.46a 204.00 6.78b
43.84 1.37a 88.32 0.51ab 215.25 1.92a
37.52 1.67bcd 42.44 2.6ab 89.78 0.58a 86.65 1.49ab 216.75 9.63a 217.75 2.49a
4.85 0.23c 1.00 0.04c 0.48 0.04c
5.69 0.90b 1.09 0.05b 0.69 0.14b
64.51 0.87c )6.13 0.09bc 14.03 0.13c
62.43 1.01d )6.17 0.12bc 11.82 0.14d
9.0
5.92 0.36b 1.10 0.06b 0.77 0.11b
6.32 0.26a 1.19 0.02a 1.10 0.07a
57.18 0.78e )6.47 0.14d 9.29 0.10e
51.27 0.32f )7.07 0.07e 6.56 0.06f
a Mean S.D., mean value form four replications. af: means in the same row not followed by the same letter are signi®cantly dierent (P < 0.05). Cont: control, FC: foaming capacity (%), FS: foam stability (%), BV: bake volume (ml), FM: ®rmness (KN), NF: strain at failure, SF: stress at failure (KN/cm2 ), L: Hunter L, a: Hunter a, b: Hunter b.
coagulates upon heating to maintain a stable foam structure. From the egg protein components, the globulins facilitate foam formation, ovomucin and the ovomucin-lysozyme complex attributes foam stability. The possible eects of pH on these components might be responsible for the FC and FS alterations. In the preparation of angel food cake, when globulins and ovomucins were removed from the egg white, the cake volume decreased. Many factors have been reported to aect the foam stability, Forsythe (1957) indicated that small amounts of fat in the ¯our decreased the FS of eggs and the yolk contains 32±36% lipids on a solids basis (Marion, Woodroof & Cook, 1965; Chung & Stadelman, 1965). 3.2. Textural properties of liquid whole egg gel In general, the FM, NF, and SF of LWE gel increased as the pH increased (Table 1). Denser structures were observed as pH increased and more porous structures were observed at lower pH values. The results suggested that textural properties of egg protein gel in foods were strongly dependent upon the pH. Beveridge et al. (1980) reported that the higher ®rmness of heat induced albumen coagulum was observed at higher pH values between pH 7.0 and 9.0 with shearing force using a multiple blade shear compression cell. They also reported that ®rmness increased with increasing pH due to sulfhydryl-disul®de exchange, which accelerated at pH 9.0; however, formation of puy coagulum was observed at a pH value below 7.0 due to evolution of gas and expansion of gel. Hermansson (1982) reported that a ®ne and uniform gel matrix is formed with high gel strength, with small
and water-binding pores at high pH. She also indicated that the net charge of proteins increases as pH increases, causing the proteins to aggregate with low gel strength, thus, forming a coarse network with large pores and minimal water-binding when heated at their isoelectric point. Baldwin, Matter, Upchurch and Breidenstein (1967) and Beveridge et al. (1980) also reported that acidi®ed albumen gels were puy and foamy. 3.3. Color attributes of liquid whole egg gel Hunter L and b values of LWE gel decreased (P < 0:05) and Hunter )a values (intensity of greenness) increased (P < 0:05) with increased pH (Table 1); which means that the LWE gels become darker, more green and less yellow in color with increased pH. A greenishblack discoloration of hard cooked egg yolk surface is the major color problem due to formation of FeS (Baker, Dar¯er & Lifshiz, 1967). They suggested that the enzyme cystine desulfhyrase might be responsible for the occurrence of H2 S in egg white, then iron in the egg yolk reacts with H2 S to form FeS. Formation of FeS is sometimes not desirable in food products. As alkalinity increases, the egg yolk favors FeS formation. 3.4. Correlations between quality parameters of liquid whole egg with various pH The correlations between pH and FC or FS of LWE were low. However, there was a positive and highly signi®cant correlation (P < 0:01) between BV and pH (Table 2). The correlation coecient between BV and pH was 0.798. The result indicates that BV of LWE increased as pH increased. The FM, NF and SF of LWE
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Y.I. Chang, T.C. Chen / Journal of Food Engineering 45 (2000) 237±241
Table 2 Correlations among functional and gel parameters of liquid whole egg as aected by pH adjustment QPa
pH
FC
FS
FM
NF
SF
BV
L
a
b
pH FC FS FM NF SF BV L a b
1
0.195 1
)0.037 )0.873 1
0.707 0.366 )0.110 1
0.803 0.169 )0.016 0.739 1
0.893 0.248 )0.077 0.845 0.927 1
0.798 0.205 )0.058 0.654 0.777 0.804 1
0.873 )0.352 0.204 )0.708 )0.726 )0.880 )0.756 1
0.789 )0.121 0.099 )0.553 0.734 )0.815 0.507 0.714 1
0.861 0.365 0.223 )0.688 )0.728 )0.874 )0.776 0.988 0.708 1
a
QP (quality parameters): FC: foaming capacity (%), FS: foam stability (%), BV: bake volume (ml), FM: ®rmness (KN), NF: strain at failure, SF: stress at failure (KN/cm2 ), L: Hunter L, a: Hunter a, b: Hunter b. N 28. * Correlations are signi®cant at P < 0:05. ** Correlations are highly signi®cant at P < 0:01.
gel were positive and highly correlated (P < 0:01) with pH changes. The correlation coecient between pH and FM, NF, SF, of LWE gels were 0.707, 0.803 and 0.893, respectively. This indicates that FM, NF, and SF of LWE gels increased as pH increased. Hunter L, a and b values of LWE gels were highly correlated (P < 0:01) with pH changes. The correlation coecient between pH an L, a and b values of LWE gels were 0.873, 0.789, and 0.861, respectively. This indicates that the color of LWE gels became more dark, more greenish and more yellowish as pH increased. The FC and FS were not correlated (P > 0:05) with other properties such as BV, FM, FS, NF, SF, L, a and b values for LWE. However, there was a negative and highly signi®cant correlation (P < 0:01) between FC and FS of LWE. Therefore, FM, SF and Hunter b value of LWE gels were indicators to determine the changes in texture and color with pH changes.
4. Summary The FC and FS of LWE altered slightly as pH changed and the trend was nonlinear. The BV of LWE increased from pH 7.0 to 8.0. The FM, NF, and SF increased as the adjusted pH increased. A more dense gel structure was observed as the pH increased and more porous gel structure was exhibited as the pH decreased. In addition, as pH increased, the gel became darker and more greenish in color. Hunter b values for LWE gels could be used as quality indicators to evaluate the changes of texture and color with pH changes. Eggs provide many desirable attribute as a food ingredient because they are considered polyfunctional, or usually perform in more than one capacity. Results demonstrated that the functional and gel characteristics of liquid whole egg can be altered by pH alteration.
References Arnt®eld, S. D., & Bernatsky, A. (1993). Characteristics of heatinduced networks for mixtures of ovalbumin and lysozyme. Journal of Agricultural and Food Chemistry, 41, 2291±2295. Baker, R. C., Dar¯er, J., & Lifshiz, A. (1967). Factors aecting the discoloration of hard-cooked egg yolks. Poultry Science, 46, 664±672. Baldwin, R. E., Matter, J. C., Upchurch, R. E., & Breidenstein, D. M. (1967). Eects of microwaves on egg whites. I. Characteristics of coagulation. Journal of Food Science, 32, 305±309. Beveridge, T., Arnt®eld, S., Ko, S., & Chung, J. K. L. (1980). Firmness of heat induced albumen coagulum. Poultry Science, 59, 1229± 1236. Bornstein, A., & Bartov, I. (1966). Studies on egg yolk pigmentation. I. A comparison between visual scoring of yolk color and colorimetric assay of yolk carotenoids. Poultry Science, 45, 287±296. Chung, R. A., & Stadelman, W. J. (1965). A study of variations in the structure of the henÕs egg. British Poultry Science, 6, 277±282. Forsythe, R. H. (1957). Some factors aecting the use of eggs in the food industry. Cereal Science Today, 2, 211±216. Germ, A. C. (1994). Improvement of solubility and foaming properties of egg yolk protein and its separation from yolk lipid by liquid± liquid extraction. In J. S. Sim, & S. Nakai, Egg uses and processing technologies: new development (pp. 329±339). Wattingford, Oxon, UK: Cab internal. Hermansson, A. M. (1982). Gel characteristics-structure as related to texture and water binding of blood plasma gels. Journal of Food Science, 47, 1965±1972. Hickson, D. W., Alford, E. S., Gadner, F. A., Diehl, K., Sanders, J. O., & Dill, C. W. (1982). Changes in heat-induced rheological properties during cold storage of egg albumen. Journal of Food Science, 47, 1908±1911. Hsieh, Y. L., & Resenstein, J. M. (1989). Texture changes on hating spray-dried egg white. Journal of Food Science, 54, 1206±1208. Hsieh, Y.L., & Resenstein, J. M. (1993). Failure deformation and stress relaxation of heated egg white gels. Journal of Food Science, 58, 113±115, 123. Janssen, H. J. L. (1971). Methods of determination of the quality of chicken egg white. Voedingsmiddelen Technologie, 2 (36), 11±13. Kucukoner, E. (1995). Eect of various commercial fat replacers on the physico-chemical properties and rheology of low fat cheddar cheese. Ph.D. Dissertion, Mississippi State University, Mississippi State, MS.
Y.I. Chang, T.C. Chen / Journal of Food Engineering 45 (2000) 237±241 Marion, J. E., Woodroof, J. G., & Cook, R. E. (1965). Some physical and chemical properties of eggs from hens of ®ve dierent stocks. Poultry Science, 44, 522±534. Norris, M. E., & Cotterill, O. J. (1971). Sponge cake preparation. In Recommended methods for the analysis of eggs and poultry meat (p. 68). Lincoln, NE: University of Nebraska College of Agriculture and Home Economics. SAS (1995). SAS users' guide: statistics, 1995 edition. Cary, NC: SAS Institute.
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Silversides, F. G., Twizeyimana, F., & Villeneuve, P. (1993). Research note: a study relating to the validity of the Haugh unit correction for egg weight in fresh eggs. Poultry Science, 72, 760±764. Su, H. P., & Lin, C. W. (1993). A new process for preparing transparent alkalised duck egg and its quality. Journal of Science and Food Agriculture, 61, 117±120.
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Functional and gel characteristics of liquid whole egg as aected by pH alteration Y.I. Chang, T.C. Chen * Poultry Science Department, Mississippi State University, Mississippi State, MS 39762, USA Received 28 October 1999; accepted 20 March 2000
Abstract Studies were conducted to evaluate the eect of pH alteration of liquid whole egg (LWE) from newly laid shell eggs on their functional and gel characteristics. The pH of LWE was altered either with 1 N NaOH or 1 N HCl to 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0 and tested for their foaming capacities (FCs), foam stabilities (FSs), and baked volumes (BVs). Gels were also prepared and tested for ®rmness (FM), strain at failure (NF), and stress at failure (SF), as well as Hunter color values. The FC and FS of LWE were altered by pH alterations. As the pH increased, the FM, NF and SF of gels increased and the structure became more dense. Hunter L (lightness) and b (yellowness) values of the gel decreased and Hunter )a (greenness) values increased as the pH of LWE increased. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Liquid whole egg; Functional; Gel; pH; Color
1. Introduction Eggs provide many desirable attributes as a food ingredient because they are considered polyfunctional. Three-dimensional egg protein gels are stabilized by intermolecular linkages such as disul®de cross-link, hydrogen bonds, and hydrophobic interaction with heating for a desired texture in many food systems. Egg proteins are responsible for many functional properties such as foaming, whipping, and viscosity building (Hsieh & Resenstein, 1989). Eggs are used in food related capacities for coagulation, foaming, emulsi®cation, color and ¯avor. Hickson et al. (1982) reported that the pH of egg albumen was the main factor in governing the rheological properties of gels formed during the heat treatment of albumen at 80°C. Textural properties of protein gel were greatly aected by pH. Hermansson (1982) reported that a ®ne, uniform gel matrix was formed at high pH with a high gel strength. Beveridge, Arnt®eld, Ko and Chung (1980) reported that ®rmness of albumen gel increased with increasing pH due to sulfhydryl-disul®de exchange, which was accelerated up to pH 9.0. Su and Lin (1993) reported that the transparency of duck egg gel could be increased through appropriate alkal*
Corresponding author. Tel.: +1-601-325-3382. E-mail address: [email protected] (T.C. Chen).
ization from pH 8.0±8.5 (fresh egg) to 11.5±12.8. Arnt®eld and Bernatsky (1993) reported that gel network formation of the ovalbumen±lysozyme mixture was not aected at pH 5.5. However, the mixture gel network increased at pH 7.0 and 8.5. Texture is one of the major quality attributes of foods. Compression tests are most commonly used to determine the functional properties of gelling protein. Color is the ®rst visual impression of the acceptability of food products. Color from egg yolk aects the acceptability of many food products, especially baked products (Bornstein & Bartov, 1966). The pH of the shell egg and egg products is an important factor in quality retention, processing, and performance. The pH is one of the most useful indicators for shell egg during storage due to the pH increase of albumen (Silversides, Twizeyimana & Villeneuve, 1993). The objective of this study was to investigate the eect of adjusted pH with naturally existing ranges on the functional properties and gel characteristics of liquid whole egg.
2. Materials and methods 2.1. Liquid whole egg Liquid whole egg (LWE) was prepared from newly laid shell eggs. The pooled LWE was homogenized and
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 6 6 - 2
238
Y.I. Chang, T.C. Chen / Journal of Food Engineering 45 (2000) 237±241
adjusted to pH 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0 with 1 N HCl or 1 N NaOH. A Sargent±Welch pH meter (Model LS) was used to measure pH. To minimize the possible dilution eect, the acid or alkali added to the LWE was less than 2% (V/V). 2.2. Gel preparation Gel samples of LWE at various pH levels were prepared by pouring the liquid phase of the protein into cellulose casings (Nojax Casing, size 22-diameter 20 0:5 mm), sealing and cooking in boiling water for 30 min. The prepared gels were cooled to room temperature (24 1°C) and cut into 2 cm length for the gel quality measurement. 2.3. Measurement The foaming capacity (FC) and foam stability (FS) of the LWE, baked volume (BV) of sponge cake, ®rmness (FM), stress at failure (SF), strain at failure (NF) and Hunter L, a, and b values of the prepared gels were measured. 2.4. Foaming capacity and foam stability Twenty milliliters of LWE at 24 1°C were placed in 100 ml stoppered graduated cylinders and shaken twice for 30 s at a rate of two strokes per second. The volume of foam and liquid phase were measured 30 s after the second shaking. Drainage was measured after standing for 60 min. The following formulae were used to determine FC and FS (Janssen, 1971; Germ, 1994). FC % FV=ILV 100; FS % ILV ÿ DV=ILV 100; Drainage ml LVM ÿ LVS; where FV is the volume of foam (ml), ILV the volume of initial liquid phase (ml), DV the volume of drainage (ml), LVM the volume of liquid phase after 60 min (ml), and LVS is the volume of liquid phase after 30 s (ml). 2.5. Baked volume of sponge cake Sponge cakes were prepared (Norris & Cotterill, 1971). Sixty four grams of LWE were mixed with 44.7 g of sugar, 25.7 g of cake ¯our and 50 g of the butter was poured into rectangular loaf pans and baked in a preheated oven at 177°C for 21 min. After cooling, the cake volume was measured by using a glass bead solid (3 mm, Fisher Scienti®c, Pittsburgh, PA) displacement method.
2.6. Firmness, stress and strain of gel The prepared gel samples were placed on the ¯at hold plate of an Instron Universal Testing Machine Model 1130 with compression accessories and compression was carried out at room temperature (24 1°C). Gel samples were cut straight with a uniform geometry (2 cm height 2 cm diameter). Gel properties were measured using the Instron set at the following conditions: (1) calibration, 10 kg load cell, (2) cross head speed, 5 cm/min, (3) chart speed, 10 cm/min, (4) range, 0±10, and (5) deformation, 70%. Firmness (KN) was expressed as the compressive force at 70% deformation. The stress at failure (SF, KN/cm2 ), and the strain at failure (NF) of gels were calculated by using the following equation (Hsieh & Resenstein, 1993; Kucukoner, 1995). SF fF t1 ÿ dh=hg=pr2 ; where SF is the stress at failure, F t the compressive force at failure point (kN), dh the deformation height of sample, h the initial sample height, dh/h the strain at failure, and r is the radius of initial cross-section. 2.7. Color The Hunter color values of the prepared gel samples were measured using a Hunter color and color dierence meter (Model D-25: Hunter Associate Laboratory) with reference plate L 92:4;
a ÿ0:7;
b ÿ0:9:
2.8. Data analysis Analysis of the data was performed using the general linear model (GLM) procedure of SAS software (SAS, 1995). Means, where signi®cant dierences occurred (P < 0:05), were separated using the least square dierence (LSD) test. Data were then treated using the PROC COR procedure of SAS software (SAS, 1995) to determine the correlation coecient between all parameters and adjusted pH. 3. Results and discussion 3.1. Functional properties of liquid whole egg The FC and FS of LWE altered slightly as pH changed and the trend was nonlinear. The BV of LWE increased (P < 0:05) from pH 7.0 to 8.0; however, there were no BV changes between pH 6.5 and 7.0, and among pH 8.0, 8.5, and 9.0, respectively (Table 1). Eggs are good foaming agents because they develop a large foam volume which is relatively stable for cooking, and
Y.I. Chang, T.C. Chen / Journal of Food Engineering 45 (2000) 237±241
239
Table 1 Eects of liquid whole egg pH adjustment on functional and gel characteristicsa Quality
7.18 (Cont)
pH 6.5
Functional FC FS BV Gel textural FM NF SF Gel color L a b
40.77 4.68ab 89.05 2.55a 197.25 3.42bc 4.60 0.37cd 0.98 0.05c 0.40 0.08cd 64.6 0.89c )6.03 0.14b 14.10 0.44c
7.0
37.14 5.82bcd 36.35 0.57cd 86.57 2.63b 90.08 0.71a 194.00 7.71bc 191.87 6.56c 3.27 0.15e 0.98 0.04c 0.27 0.02d
4.35 0.40d 0.99 0.06c 0.34 0.03d
72.80 0.38a )4.52 0.13a 22.87 0.11a
72.86 0.51b )6.24 0.16c 18.72 0.19c
7.5
8.0
8.5
34.42 1.38d 90.81 0.46a 204.00 6.78b
43.84 1.37a 88.32 0.51ab 215.25 1.92a
37.52 1.67bcd 42.44 2.6ab 89.78 0.58a 86.65 1.49ab 216.75 9.63a 217.75 2.49a
4.85 0.23c 1.00 0.04c 0.48 0.04c
5.69 0.90b 1.09 0.05b 0.69 0.14b
64.51 0.87c )6.13 0.09bc 14.03 0.13c
62.43 1.01d )6.17 0.12bc 11.82 0.14d
9.0
5.92 0.36b 1.10 0.06b 0.77 0.11b
6.32 0.26a 1.19 0.02a 1.10 0.07a
57.18 0.78e )6.47 0.14d 9.29 0.10e
51.27 0.32f )7.07 0.07e 6.56 0.06f
a Mean S.D., mean value form four replications. af: means in the same row not followed by the same letter are signi®cantly dierent (P < 0.05). Cont: control, FC: foaming capacity (%), FS: foam stability (%), BV: bake volume (ml), FM: ®rmness (KN), NF: strain at failure, SF: stress at failure (KN/cm2 ), L: Hunter L, a: Hunter a, b: Hunter b.
coagulates upon heating to maintain a stable foam structure. From the egg protein components, the globulins facilitate foam formation, ovomucin and the ovomucin-lysozyme complex attributes foam stability. The possible eects of pH on these components might be responsible for the FC and FS alterations. In the preparation of angel food cake, when globulins and ovomucins were removed from the egg white, the cake volume decreased. Many factors have been reported to aect the foam stability, Forsythe (1957) indicated that small amounts of fat in the ¯our decreased the FS of eggs and the yolk contains 32±36% lipids on a solids basis (Marion, Woodroof & Cook, 1965; Chung & Stadelman, 1965). 3.2. Textural properties of liquid whole egg gel In general, the FM, NF, and SF of LWE gel increased as the pH increased (Table 1). Denser structures were observed as pH increased and more porous structures were observed at lower pH values. The results suggested that textural properties of egg protein gel in foods were strongly dependent upon the pH. Beveridge et al. (1980) reported that the higher ®rmness of heat induced albumen coagulum was observed at higher pH values between pH 7.0 and 9.0 with shearing force using a multiple blade shear compression cell. They also reported that ®rmness increased with increasing pH due to sulfhydryl-disul®de exchange, which accelerated at pH 9.0; however, formation of puy coagulum was observed at a pH value below 7.0 due to evolution of gas and expansion of gel. Hermansson (1982) reported that a ®ne and uniform gel matrix is formed with high gel strength, with small
and water-binding pores at high pH. She also indicated that the net charge of proteins increases as pH increases, causing the proteins to aggregate with low gel strength, thus, forming a coarse network with large pores and minimal water-binding when heated at their isoelectric point. Baldwin, Matter, Upchurch and Breidenstein (1967) and Beveridge et al. (1980) also reported that acidi®ed albumen gels were puy and foamy. 3.3. Color attributes of liquid whole egg gel Hunter L and b values of LWE gel decreased (P < 0:05) and Hunter )a values (intensity of greenness) increased (P < 0:05) with increased pH (Table 1); which means that the LWE gels become darker, more green and less yellow in color with increased pH. A greenishblack discoloration of hard cooked egg yolk surface is the major color problem due to formation of FeS (Baker, Dar¯er & Lifshiz, 1967). They suggested that the enzyme cystine desulfhyrase might be responsible for the occurrence of H2 S in egg white, then iron in the egg yolk reacts with H2 S to form FeS. Formation of FeS is sometimes not desirable in food products. As alkalinity increases, the egg yolk favors FeS formation. 3.4. Correlations between quality parameters of liquid whole egg with various pH The correlations between pH and FC or FS of LWE were low. However, there was a positive and highly signi®cant correlation (P < 0:01) between BV and pH (Table 2). The correlation coecient between BV and pH was 0.798. The result indicates that BV of LWE increased as pH increased. The FM, NF and SF of LWE
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Y.I. Chang, T.C. Chen / Journal of Food Engineering 45 (2000) 237±241
Table 2 Correlations among functional and gel parameters of liquid whole egg as aected by pH adjustment QPa
pH
FC
FS
FM
NF
SF
BV
L
a
b
pH FC FS FM NF SF BV L a b
1
0.195 1
)0.037 )0.873 1
0.707 0.366 )0.110 1
0.803 0.169 )0.016 0.739 1
0.893 0.248 )0.077 0.845 0.927 1
0.798 0.205 )0.058 0.654 0.777 0.804 1
0.873 )0.352 0.204 )0.708 )0.726 )0.880 )0.756 1
0.789 )0.121 0.099 )0.553 0.734 )0.815 0.507 0.714 1
0.861 0.365 0.223 )0.688 )0.728 )0.874 )0.776 0.988 0.708 1
a
QP (quality parameters): FC: foaming capacity (%), FS: foam stability (%), BV: bake volume (ml), FM: ®rmness (KN), NF: strain at failure, SF: stress at failure (KN/cm2 ), L: Hunter L, a: Hunter a, b: Hunter b. N 28. * Correlations are signi®cant at P < 0:05. ** Correlations are highly signi®cant at P < 0:01.
gel were positive and highly correlated (P < 0:01) with pH changes. The correlation coecient between pH and FM, NF, SF, of LWE gels were 0.707, 0.803 and 0.893, respectively. This indicates that FM, NF, and SF of LWE gels increased as pH increased. Hunter L, a and b values of LWE gels were highly correlated (P < 0:01) with pH changes. The correlation coecient between pH an L, a and b values of LWE gels were 0.873, 0.789, and 0.861, respectively. This indicates that the color of LWE gels became more dark, more greenish and more yellowish as pH increased. The FC and FS were not correlated (P > 0:05) with other properties such as BV, FM, FS, NF, SF, L, a and b values for LWE. However, there was a negative and highly signi®cant correlation (P < 0:01) between FC and FS of LWE. Therefore, FM, SF and Hunter b value of LWE gels were indicators to determine the changes in texture and color with pH changes.
4. Summary The FC and FS of LWE altered slightly as pH changed and the trend was nonlinear. The BV of LWE increased from pH 7.0 to 8.0. The FM, NF, and SF increased as the adjusted pH increased. A more dense gel structure was observed as the pH increased and more porous gel structure was exhibited as the pH decreased. In addition, as pH increased, the gel became darker and more greenish in color. Hunter b values for LWE gels could be used as quality indicators to evaluate the changes of texture and color with pH changes. Eggs provide many desirable attribute as a food ingredient because they are considered polyfunctional, or usually perform in more than one capacity. Results demonstrated that the functional and gel characteristics of liquid whole egg can be altered by pH alteration.
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