Effects of ph and neutral salts on the formation and quality of thermal aggregates of ovalbumin. A study on thermal aggregation and denaturation

Abstract

The effect of NaCl and CaC12 on the aggregation and denaturation temperature of ovalbumin was examined in the pH interval 3-11. It was found that ovalbumin had maximal thermal stability between pH values of 6-10. NaCl did not affect the denaturation temperature, while a small decrease was observed with CaClz even at the low concentration range investigated. Both salts extended the pH interval of thermal aggregation, but at the alkaline side of the isoelectric point CaClz suppressed the aggregation temperature more than did NaCI. The aggregation properties of S-ovalbumin were also investigated and found to be similar to those of ovalbumin, although aggregation occurred at a higher temperature level. The quality of the aggregates was described by their dry matter content. Aggregates appeared as transparent gels, opaque gels, gel-like precipitates and precipitates. The addition of NaCl gave predominantly gel aggregates at the alkaline side of the isoelectric point, while precipitates dominated when CaC12 was added. Gelling is an intermediate phenomenon which occurs between the two extremes of precipitating and nonaggregating conditions. Special conditions, in terms of electrostatic repulsion between the protein molecules, are required for gelling, which could be achieved by manipulating with pH, type of salt, salt concentration or added detergent. The difference between aggregation and denaturation temperature correlated with the dry matter content of the aggregates formed. When aggregation temperature was well below denaturation temperature, the resulting aggregates had a high dry matter content (precipitates). Gelling, on the other hand, was observed at an aggregation temperature equal to or above the denaturation temperature.

Thermally Induced Gelation of Native and Modified Egg White‐Rheological Changes During Processing; Final Strengths and Microstructures

“…A high net charge repulsion between proteins, limited intermolecular interactions and a low dry matter content have been suggested as conditions required for the formation of translucent gels (Hegg et al, 1979;Hegg, 1982;Egelandsdal, 1980). The present results, however, indicate that molecular interactions were present in the translucent gels producing areas of dense aggregates.…”

“…2) may have been related to further changes in ovalbumin. DSC data using heating rates 20 times those applied in this study yielded denaturation temperatures for ovalbumin ranging from 79 -84'C (Hegg et al, 1979;Donovan et al, 1975;Karmas and DiMarco, 1970) which suggest that completion of the denaturation process and aggregation of ovalbumin could be responsible for the rapid increase in rigidity above 8O'C. In the context of this discussion it must be mentioned that heating rate, in general, has an effect on the observed response in thermal studies of protein systems.…”

Preparation of Heat‐induced Transparent Gels from Egg White by the Control of pH and Ionic Strength of the Medium

“…Ovalbumin, the main constituent of egg white, gives a transparent solution, transparent gel, turbid gel or turbid suspension, depending on the pH and ionic strength of the heating medium (Hegg et al, 1979;Egelandsdal, 1980;Hatta et al, 1986). A similar effect has been observed with other proteins (Clark and Lee-Tuffnell, 1986).…”

Preparation of Transparent Egg White Gel with Salt by Two‐step Heating Method

“…The conditions used in this study (80°C pH 2.5 and low ionic strength) were adequate to denature the protein (Hegg et al, 1979) but were selected to minimize the aggregation of the denatured proteins. Therefore, a transparent solution was obtained on the first heating when the original solution was slightly diluted; probably, each protein species in the egg white forms a soluble aggregate on the first heating, as does ovalbumin (Kitabatake et al, 1987).…”