| Summary: | 碩士 === 國立嘉義大學 === 食品科學系碩士班 === 91 === Hen eggs were firstly separated into egg white and yolk and, then the egg yolk was homogenized and filled into inedible acetate cellulose casing. It was then soaked in 5 % of acetic acid, lactic acid and citric acid solutions, respectively for 24 hours. After acidic soaking, the weights of egg yolk in these three acid solutions increased for 22~32% with increasing soaking time during 24 hours, the lactic acid treatment having the greatest increase in weight. In addition, the pH values of egg yolk decreased from 6.0 to 3.6~2.6 with the citric acid treatment having the greatest decrease. The viscosity of egg yolk was slightly increased in the beginning, and then decreased after 2 to 24 hours’ soaking. When acid was added into egg yolk solutions directly to reach the same pH as soaking method, the result shows that as the pH get lower, the viscosity get higher. Aggregation phenomenon of proteins in egg yolk was observed due to acid osmosis effect. The turbidity increased with increasing soaking time, but the trend was level off after soaking for 2 hours. The content of soluble proteins in egg yolk decreased from 139.5 mg/ml to 117.4 mg/ml after 24 hours soaking. Initially, the surface hydrophobic intensity was slightly decreased, but it was then increased with increasing soaking time afterwards. In the case of lactic acid soaking, after 24 hours, the value of hydrophobic intensity rose to 691 So/mg protein. On the other hand, the amount of surface sulfhydryl group of fresh hen egg yolk was 5.9 mmole/ g protein, if increasing acidic soaking to 24 hours, the value was lifted to 32.5 mmole/g protein. In SDS-PAGE profile, it was not significantly different for hen egg yolk after acidic soaking. The results between acidic soaking and direct addition of acids had the same trend at the same pH value, except for viscosity. For emulsifying properties, the emulsifying activity and emulsifying stability increased with increased egg yolk concentration from 0 to 2 %, however, the trend was not so obvious after 0.5 % concentration. At initial stage, emulsifying activity and emulsifying stability was better, but it was decreased with increased soaking time at a later stage. In SDS-PAGE, to assay adsorbed and unadsorbed proteins in oil-in-water emulsions of egg yolk, it was found that at pH=4.6~5.5, egg yolk proteins adsorption was better. The results had the same trend for both acidic soaking and direct addition of acidic solutions. After acidic soaking, the hen egg yolk was heated at 100℃ for 30 minutes, the gel strength and gel rigidity before soaking was 377.2 (g*cm) and 806.2 (g/cm), after 24 hours soaking, these two values were reduced to 39 (g*cm) and 101.7 (g/cm), respectively. This indicates that the properties of gel were broken down with increased acidic soaking time, and there are no significant differences among acids. Changes of acid-soaked egg yolk on viscoelastic properties detected by dynamic rheometer showed that, if increasing soaking times, the storage modulus was decreased, and the transition temperature was decreased at the same time. The microstructure of egg yolk gel both with acidic soaking and without acidic soaking was observed by scanning electron microscopy (SEM). The results show that, at initial stage, the network structure among yolk molecules was tighter. In a later period of acidic soaking, network structure was quite hollow, and there was no noticeable difference among three acid treatments.
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