The Water Activity Lowering Properties of Selected Humectants in Eggs 1 ' 2

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1 The Activity Lowering Properties of Selected Humectants in Eggs 1 ' 2 YUN-CHAN LO, G. W. FRONING, and R. G. ARNOLD 3 Department of Animal Science, University of Nebraska Lincoln, Nebraska (Received for publication September 13, 1982) ABSTRACT The high and intermediate moisture sorption isotherms of whole egg, egg white, and egg yolk at 25 and with and without the addition of selected food humectants (glucose, sucrose, glycerol, propylene glycol, sodium chloride) were determined. In the pure egg systems, dehydrated egg white exhibited the highest water activity (a w ) lowering ability, dehydrated whole egg was intermediate, and dehydrated egg yolk was the lowest when used in liquid egg systems. The pattern was reversed when the humectants were employed as the a w controlling agents in the systems. On an equivalent weight basis, the a w depressing property of sodium chloride was the most effective, followed by that of glycerol, which was comparable to the moisture sorption characteristics of propylene glycol. Glucose was not as effective as glycerol or propylene glycol but was superior to sucrose as an a w lowering solute. The a w of the egg systems incorporated with egg, glucose, sucrose, or sodium chloride was lower at 45 than at. However, glycerol produced higher a w at the higher temperature when its level in the egg systems exceeded %. (Key words: water activity, humectants, eggs) INTRODUCTION The commercial application of water activity (a w ) lowering techniques to the production of shelf stable, intermediate moisture pet foods owes much of its success to the use of humectants. These hygroscopic compounds confer storage stability to the nonrefrigerated food products by virtue of their water binding abilities, which reduce or eliminate the amount of unbound water available for microbial proliferation and chemical and biological reactions. In addition, due to their plasticizing properties, incorporation of these humectants gives the resulting intermediate moisture food products highly desirable textural characteristics not present in dehydrated products. Numerous articles have reported the a w lowering effects of various food humectants in model systems (Loncin, 1975; Sloan and Labuza, 1975; Chen and Karmas, 1980; Chirife and Ferro Fontan, 1980a,b,c). Most of these systems are of binary nature, consisting of a specific solute in aqueous solution. The knowledge of the a w depressing behavior of these 1 Published as paper number 6983, Journal Series, Nebraska Agricultural Experiment Station, Lincoln, NE Project supported by the American Egg Board. 3 Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE Poultry Science 62: compounds in simple model systems has not been successfully extrapolated to real food systems for several reasons. A solute will react differently in a food system made up of multicomponents of varying hydrophilic and hydrophobic properties than it would in a simple aqueous solution. Solubility of the solute may be significantly affected in the presence of the heterogenous constituents because the level of water having true solvating properties may be altered. Distribution of the humectant in the food system may not be as uniform as in aqueous solutions due to localization, physical interaction, and chemical bonding of the solute to the food components. The presence of capillaries in some food constituents may also affect the a w depressing capability of the solute differently (Karel, 1975a). Even the mixing of two humectants in an aqueous solution may deviate from the a w lowering properties observed in these solutes when they are added independently (Bone, 1973; Karel, 1975b; Loncin, 1975). For similar reasons, the numerous a w predicting equations that have been developed empirically or theoretically from model systems have limited applicability for the estimation of a w in food systems. The current limitation of knowledge about the a w depressing behavior of solutes in various food products necessitates the actual experimental determinations of these a w in the Downloaded from at Penn State University (Paterno Lib) on April 8,

2 972 LO ET AL. specific products to predict their storage stabilities. In this study, the moisture sorption isotherms of whole egg, egg white, and egg yolk in the high and intermediate moisture ranges were examined. The a w lowering capabilities of five food humectants glucose, sucrose, glycerol, propylene glycol, and sodium chloride in these egg systems were also investigated. MATERIALS AND METHODS Preparation of Standard Moisture Sorption Isotherms. Standard sorption isotherms at two different temperatures, 25 and, were prepared by the isopiestic equilibration method of Vos and Labuza (1974), which employs microcrystalline cellulose (Avicel) as the absorbing material. Saturated solutions of salts listed in Table 1 were obtained by dissolving each salt in distilled water in a beaker until its solubility limit was exceeded. The slurried solutions were kept in an equilibration chamber at 25 or for at least 24 hr with occasional stirring to ensure true equilibrium. From the beaker 1 g of the slurry was transferred into the bottom of a Bel-Art plastic desiccator, taking extreme caution that the undissolved crystals remained submerged in the solution. One gram of Avicel was weighed (to.001 g) into a preweighed weighing dish and placed on the plate of the desiccator, which was subsequently evacuated to 26 in Hg. The saturated solution and Avicel were equilibrated at 25 or for 96 hr. Vacuum was then slowly released. Moisture content (dry weight basis) of the Avicel was determined by weighing the dish, drying at 105 C overnight, and reweighing. Standard sorption isotherm at 25 or was obtained by plotting the moisture contents of the Avicel and the a w of the saturated salt solutions shown in Table 1. Egg Sample Preparation. Fresh eggs were procured from the University's poultry farm. Salt 1 Dried eggs (whole egg standard, type W-l; egg white spray dried, type P-110; and egg yolk standard, type Y-l) were supplied by Henningsen Foods, Omaha, NE. Because the inherent glucose in the dried egg white had been removed during the dehydration process, 3.2% glucose was added to the dried egg white to restore the glucose to its original level. All the humectants used were either USP or reagent grade. The levels of dried whole egg, dried egg white, and dried egg yolk added to their respective liquid counterparts were,, 70, 80, and 90% (wet weight basis). The levels of glucose, sucrose, glycerol, and propylene glycol added to the liquid whole egg, egg white, or egg yolk were,,,, and %, whereas those of sodium chloride were 5, 10, 15,, and 25%. Two hundred grams of liquid and dried eggs (or liquid egg and humectant) were thoroughly mixed in a Hobart blender (Model N-) at a speed setting of 1. The mixture was packed into a retortable pouch (25 X 25 cm), sealed, and steamed at 100 C for 5 min. After cooling to room temperature, 1 g of the egg sample was transferred toalx 15 mm petri dish to be used for a w determination. The remaining sample was resealed and used for nonenzymatic browning measurement, the results of which will be reported in a subsequent article. Activity Determination of Egg Systems. The procedure for determination of a w in the egg systems was similar to that used for standard moisture sorption isotherm preparations except that 1 g of the egg mixture was substituted for the saturated salt solution in the bottom of the desiccator. A similar gravimetric technique was used to measure the moisture content of the egg system, employing about 5 g of sample. When glycerol or propylene glycol was the added humectant, the moisture content of pure egg was first determined. The moisture level of the egg-humectant mixture was estimated by multiplying the moisture content of the egg by the fraction of the egg used in the preparation. This method TABLE 1. activities of saturated salt solutions at 25 and Potassium sulfate Potassium nitrate Sodium benzoate Potassium chloride Potassium bromide Sodium chloride Sodium nitrate Potassium iodide Sodium bromide Magnesium nitrate activities at 'Data from Greenspan (1977), except for sodium benzoate, which is from Rockland and Nishi (1980). Downloaded from at Penn State University (Paterno Lib) on April 8, 16

3 HUMECTANTS IN EGGS 973 TABLE 2. sorption properties of whole egg, egg white, and egg yolk at 25 and Level of dried egg <%) «,a.98*> a.86 x.72 a.88 a.77 x.68 x 1.00*> a. 99 «,a.91 a.75 a 97»,b.95 b.91 b.85y.65 b.92 b.87 b.84 b.75y.65y 99*,b 97*,b.93 b.85 b.73 D a b x '^Values in the same row with different superscripts was followed to avoid possible erroneous results attributable to vaporization of the polyols from the samples dried at high temperatures (Sloan and Labuza, 1975; Favetto et al, 1979). The a w of the sample at 25 or was read from the corresponding standard moisture sorption isotherm. RESULTS AND DISCUSSION The moisture-a w relationship of the pure egg systems is shown in Table 2. At the same level of dried egg, the a w depressing ability was the greatest for egg white, intermediate for whole egg, and the poorest for egg yolk, even though egg white contained much more moisture than whole egg or egg yolk. The discrepancy in behavior could be explained by their compositional differences. The content of egg white is mostly proteins, which have a strong tendency to interact with water. However, egg yolk possesses a high level of fat, which contributes negligibly to the a w depression because of its hydrophobic properties. Van den Berg and Bruin (1981) stated that the solubility of water in fat only increased from.2% to about 1% when the room temperature was raised to 100 C. Contrary to the observation in many foods, the a w of whole egg, egg white, and egg yolk were significantly lower at 45 than at in any of the moisture levels examined. This could be attributed to higher solute solubility or swelling of the protein matrices at the higher temperature so that more water was entrapped, preventing it from evaporating into the surrounding atmosphere. Table 3 illustrates the a w depressing property of glucose in the liquid egg systems. At all glucose levels, egg white maintained the highest a w values, followed by whole egg, whereas the a w was lowest for egg yolk. This observation implies that glucose plays a more prominent role in depressing the partial vapor pressure of the egg systems than do the protein fractions. because glucose reversed the a w lowering pattern noted in the pure egg system. With the lowest moisture content in egg yolk and the highest in egg white, addition of an equivalent amount of glucose to each reduced the amount TABLE 3. sorption properties of glucose in egg systems at 25 and Level of glucose a.77 a.98*. a.96 a.92 x.87 x.81 a.94 a.89 x.84 a.79 a.76 a b.80 b.96 b.93 b.9iy.86y.80 b.92 b.ssy.81 b.78 b "Values in the same row with different superscripts Downloaded from at Penn State University (Paterno Lib) on April 8, 16

4 974 LO ET AL. TABLE 4. sorption properties of sucrose in egg systems at 25 and Level of sucrose gh 2 OZ *.".84 a 99*,a.95 x.92 a.87 a.97 x.93 x.83 a.97*.y.95 b.87 b.83 b.98'. b.95 b.93y.85 b.96y.92y.88 b.83 b.79 b ab '"Values in the same row with different superscripts of free water much more substantially in the egg yolk than in the egg white. The superiority of glucose over the egg proteins in a w depressing capability could be demonstrated by the significantly higher amount of water held by the solute at all the levels. Table 3 also shows that the a w of the egg-glucose systems was significantly lower at 45 than at, indicating a reduction in the freedom of water movement due to colligative properties as a result of further dissolution of glucose and food components with increased temperature. Similar behavior had been reported for glucose and fructose by Audu et al. (1978). They observed that the sugars became more syrupy at an elevated temperature and maintained that this phenomenon would play a vital role in food dehydration. Table 4 shows that the sorption isotherms of the egg systems in the presence of sucrose followed the same trend as those with glucose added, i.e., for any given sucrose level, the a w was in the following order: egg yolk< whole egg<egg white. However, the a w lowering capability of sucrose was less than that of glucose. The reduced a w observed with an increase in temperature in the presence of sucrose was probably due to increased hydration and interaction of the humectant with water and other food components. The moisture sorption properties of glycerol in whole egg, egg white, and egg yolk are presented in Table 5. This humectant depressed the vapor pressure of the systems far more effectively than the glucose or sucrose, especially at the higher levels. Glycerol was completely miscible with fat and water, thereby bringing the molecular configuration of the egg systems toward a more ordered orientation. However, excess quantities of sugar (concentrations above their limits of solubility) in the systems had virtually no a w depressing property. In the presence of glycerol, the a w of the egg systems was lower at 45 than at when the humectant added was less than %. When a high glycerol concentration was incorporated, the reverse was observed. This behavior could be because the increase in TABLE 5. sorption properties of glycerol in egg systems at 25 and Level of glycerol iy.73y.65 b.91 a b.71 b.87 a.82 b.59 b.43*.b.88 b.82 x.75 x.67 b.92 b.89 b a.73 a.86 a.83 a.77 a.62 a.49 a 1 Values in the same row with different superscripts "Values in the same row with different superscripts Downloaded from at Penn State University (Paterno Lib) on April 8, 16

5 HUMECTANTS IN EGGS 975 TABLE 6. sorption properties of propylene glycol in egg systems at Level of propylene glycol activity at * * Value obtained from extrapolated standard temperature could not further dissolve the humectant, because it already was in total solution at the lower temperature. The effect of propylene glycol on the moisture sorption curves of the egg systems at is shown in Table 6. Vaporization of the humectant at made it impossible to accurately determine the a w at this temperature. An attempt to measure a w using an Airguide humidity indicator also ended in failure. By comparing the data collected for propylene glycol to those in Table 5, it could be seen that the a w depressing property of propylene glycol was similar to that of the glycerol. As was the case with other humectants, an equivalent amount of the humectant depressed the aw in the order of egg yolk >whole egg>egg white. Table 7 depicts the moisture sorption isotherms of the egg samples as influenced by sodium chloride. This humectant had by far the best sorption property in the high and intermediate a w range. For example, at the % sodium chloride level, the a w of whole egg, egg white, and egg yolk at were.76,.81, and.77, respectively. The corresponding values for glucose were.95,.98, and.94; for sucrose they were.98,.99, and.97; for glycerol they were.93,.93, and.87; and for propylene glycol they were.91,.93, and.88, respectively. However, the addition of sodium chloride beyond its solubility limit (a w about.75) did not further depress the a w of the systems appreciably. This observation was in good agreement with the data obtained by Gal and Bankay (1971) when working with casein; they reported that when a protein became saturated with the ion pairs of sodium chloride, any excess salt present became crystalline and lost its ability to absorb more water molecules. Due to the dissolution effect at elevated temperatures, the same trend showing a decreased a w at, when compared to, occurred in the egg systems containing sodium chloride. Of the five humectants investigated, sodium chloride exhibited the greatest a w depressing ability in all the egg samples until its solubility limit was reached. The properties of glycerol and propylene glycol made them completely miscible with water and fat, which partially accounts for their good water binding capacity, especially at the lower a w levels. Glucose was not as effective a humectant as glycerol or TABLE 7. sorption properties of sodium chloride in egg systems at 25 and Level of sodium chloride ghjo/ a.84 a.76 a.75 a.97 x.92 a.81 x a.77 x.74 x.83 b.74 b.96y.soy.75,92 b,85 b.77 b.75y.73y a b ''Values in the same row with different superscripts Downloaded from at Penn State University (Paterno Lib) on April 8, 16

6 976 LO ET AL. propylene glycol, but it possessed better hygroscopic properties than did sucrose. On an equivalent weight basis, the a w lowering capability of the five humectants in the egg systems was inversely related to their molecular weights. All of the solutes were superior to the egg proteins in reducing the partial vapor pressure of the egg systems. REFERENCES Audu, T.O.I., M. Loncin and H. Weisser, Sorption isotherms of sugars. Lebensm. Wiss. Technol. 11: Bone, D. P., activity in intermediate moisture foods. Food Technol. 27(4): Chen, A.C.C., and E. Karmas, Solute activity effect on water activity. Lebensm. Wiss. Technol. 13: Chirife, J., and C. Ferro Fontan, 1981a. A study of the water activity lowering behavior of polyethylene glycols in the intermediate moisture range. J. Food Sci. 45: Chirife, J., andc. Ferro Fontan, 1980b. activity of aqueous lactulose solutions. J. Food Sci. 45: Chirife, J., and C. Ferro Fontan, 1980c. Prediction of water activity of aqueous solutions in connection with intermediate moisture foods: Experimental investigation of the a w lowering behavior of sodium lactate and some related compounds. J. Food Sci. 45: Favetto, G., J. Chirife, and G. Bartholomai, Determination of moisture content in glycerolcontaining intermediate moisture foods. J. Food Sci. 44: Gal, S., and D. Bankay, Hydration of sodium chloride bound by casein at medium water activities. J. Food Sci. 36: Greenspan, L., Humidity fixed points of binary saturated aqueous solutions. J. Res. Natl. Bur. Stand. Sect. A. 81A:89. Karel, M., 1975a. Stability of low and intermediate moisture foods. Page 643 in Freeze Drying and Advanced Food Technology. S. A. Goldblith, L. Rey, and W. W. Rothmayr, ed. Academic Press, New York, NY. Karel, M., 1975b. Physico-chemical modification of the state of water in foods a speculative survey. Page 639 in Relations of Foods. R. B. Duckworth, ed. Academic Press, London. Loncin, M., Basic principles of moisture equilibria. Page 594 in Freeze Drying and Advanced Food Technology. S. A. Goldblith, L. Rey, and W. W. Rothmayr, ed. Academic Press, New York, NY. Rockland, L. B., and S. K. Nishi, Influence of water activity on food product quality and stability. Food Technol. 34(4):42-. Sloan, A. E., and T. P. Labuza, Humectant water sorption isotherms. Food Prod. Dev. 9(10):68. van den Berg, C, and S. Bruin, activity and its estimation in food systems: Theoretical aspects. Page 1 in Activity: Influences on Food Quality. L. B. Rockland and G. F. Stewart, ed. Academic Press, New York, NY. Vos, P. T., and T. P. Labuza, Technique for measurement of water activity in the high a w range. J. Agric. Food Chem. 22: Downloaded from at Penn State University (Paterno Lib) on April 8, 16