| Soy protein products usually undergo serious decline in their quality during storage, which challenges the commercial producers and users. For our country, the exports of soy protein products need to experience long process of sea transportation. This process inevitably causes the losses in protein quality, thereby results low price and small profits. Therefore, it is very vital significance for us to study the effect factors, mechanism, and the control pathways of the decline of quality in soy proteins during storage. However, so far, the knowledge on these issues is very limited. The objective of this dissertation study is to figure out the main effect factors of the storage stability of soy proteins, to reveal the mechanism of the quality losses and to search the efficient approach for controlling.To determine effect of storage conditions on protein storage stability, SPI was stored under different temperatures(4 and 37 oC) and lighting conditions, with various moisture contents(3.7% and 7.0%, w/w) and different oxygen levels. Low temperature storage induced insignificant changes of SPI except slightly decrease of free amino acid(P < 0.05); while high temperature storage caused almost a half decline in both protein solubility and free amino acid after 8 weeks, as well as significant increase of soluble aggregates. Although they slenderly promoted protein oxidation(protein carbonyls), high moisture content, light condition, or high oxygen level did not reduced the protein solubility or increase the soluble aggregates. Base on these results, the SPIs was all spray dried and performed the storage at 37 oC.Raw soy powder, defatted(n-hexane : ethanol 9:1) soy powder and enzyme-inactivated(105 oC, 30 min) powder were stored for 25 weeks. The proteins in these powders occurred limited increase of carbonyls and decrease of surface sulfhydryl groups(SH)(P < 0.05), but kept unchangeable in their protein solubility and aggregation. SPIs were extracted from soybeans(control) and three batches of commercial low-defatted soy flakes which obtained in different seasons and then stored. The control SPI showed high storage stability. While SPIs extracted from different batches of the commercial soy flakes exhibited quite different initial qualities and storage stability. The higher protein denaturation degree was induced in initial soy flakes, the lower protein solubility was obtained before or after storage. Comparing one of the commercial material-extracted SPI with the control sample, which shared similar initial protein solubility and enthalpy(ΔH) but different aggregation degree, the decrease of protein solubility in higher aggregated former sample was almost twice of that in the control after 10-week storage.To investigate the influence of different processing-caused damages on SPI storage stability, SPI samples were subjected to various p H values(p H 5–8) before spray-drying, heated before and after drying, or dried with lyophilization or spray-drying and then stored. Higher p H value induced lesser SH and higher protein aggregation in their initial samples, yet the p H value(at the range of p H 6–8) showed limited effect on changes of protein carbonyl and protein solubility during storage. SPI experienced heat treatments(100 oC, 15–45 min) in the solid state caused obvious protein denaturation, yet just led to slightly protein oxidation and aggregation, the protein solubility changed insignificantly during 8-week storage. However, SPI heated before spray-drying(8%, w/v; 80/90 oC, 30 min) largely enhanced the protein denaturation, oxidation, and aggregation, corresponding to accelerated protein insolubility during storage. The protein solubility almost dropped in half after a 8-week storage. Two drying methods showed no significant different in protein initial qualities and their changes during storage.Base on the above studies, in which it was found that SPI with higher initial protein aggregation always resulted in lower storage stability, the mechanism of soy protein storage instability was investigated through analysis the aggregation behaviors and interaction forces of a serious SPIs, whch shared similar initial solubilities but different degrees of aggregation, prepared by heating before spray drying. After 12-week storage, the solubility of mildly aggregated SPI(80 oC holding up to the target termerature) was almost three times of that in deeply aggregated sample(90 oC); the rates of solubility loss were positively correlated with the amounts of soluble aggregates contained in the initial samples(r2 = 0.861; P < 0.05), suggesting their nucleation and activation effects., suggesting their nucleation and activation effects. Non-covalent interactions were the main forces involved in insoluble fractions, while the disulfide bonds were abundant in soluble aggregates and their content increased markedly during storage, this effect suggested that the covalent linkages acted as blockers for hydrophobic aggregation. In addition, the protein solubility was negatively correlated with the contents of carbonyls(r2 = 0.878, P < 0.05).According to the mechanism analysis, the control strategies were then explored by using different additions which can decrease the non-covalent or covalent interactions. 15%(w/w) or more trehalose significantly reduced the initial soluble aggregation and largely decrease the decline rate of protein solubility during storage, after 12-week storage, the solubility in 30% trehalose-contained SPI was twice higher of that in the 15% trehalose-contained sample. The applying of disulfide-reducer markedly improved the initial solubility of SPI, and 2.5 m M or 6.5 m M showed a great controlling effect on storage insolubility. β-carotene, tocopherol and EDTA displayed relatively low but significant effect(P< 0.05) on the insolubility.In summary, the main effect factors of storage conditions and processing damages on storage stability of soy proteins were confirmed, the mechanism of protein quality decline during storage was clarified, and the efficient controlling strategies were provided, which provide a theoretical basis to understanding the problem occurring during storage/transportation, and to suggest the references for developing control strategies to over the problem. |
PDF Full Text Request