Stability of low density lipoprotein particles affect the formation of off-flavor in thermal egg yolk.

This study aimed to investigate the role of stability of low density lipoprotein (LDL) spherical particles as well as oxidation of lipids and proteins on formation of off-odor in heated egg yolk (EY). Off-odor attributes and volatile components of EY with different thermal temperature (25 °C-90 °C) were investigated by sensory evaluation, gas chromatography-mass spectrometry (GC-MS) and GC-ion mobility spectrometry (IMS). The off-flavor and volatile compounds increased significantly when EY were heated at 60 °C-65 °C. Destruction of LDL particle structure was presented by confocal laser scanning microscopy with thermal treatment at 60 °C. A positive correlation (r = 0.899, p < 0.05) was shown between oil exudation and off-odor of heated EY. Moreover, oxidation of lipids and proteins, protein aggregation, and changes of protein secondary structure (increased ß-sheets and decreased α-helices) in LDL particles were all found when heating temperature rose to 65 °C. It was speculated that unstability of LDL particles resulted in the leakage of lipids; then oxidative products of lipids took part in the formation of off-odor volatiles in heated EY.

Succinylation Improves the Thermal Stability of Egg White Proteins.

Succinylation can improve the thermal stability of various proteins. In this study, succinylated egg white protein (SEWP) samples with different succinylation degrees were prepared by adding various succinic anhydride additives to egg white protein (EWP). The thermal stability of SEWP and the conformational structure under various succinylation degrees were investigated. With the increase in succinylation degree, the turbidity of heated SEWP solution (90 °C for 30 min) markedly declined. The heated SEWP solution with high succinylation degree (37.63%, 66.57%, and 72.37%) was transparent. Moreover, the result of differential scanning calorimetry confirmed that the thermal stability of succinylated EWP increased. The results of intrinsic fluorescence spectra and Fourier-transform infrared spectroscopy illustrate that succinylation changed the conformational structure of EWP. Succinylation increased the electrostatic repulsion and decreased the surface hydrophobicity, and it changed the aggregation morphology of EWP. Cross-linked spherical aggregates of low succinylation degree transformed to thready aggregates of a high succinylation degree. Thus, succinylation improved the thermal stability of EWP.

Effect of Enzyme Modified Soymilk on Rennet Induced Gelation of Skim Milk.

In this study, soymilk was hydrolyzed to different degrees with flavourzyme, and then soymilk and enzyme modified soymilk at various levels were added to skim milk respectively, to generate a mixed gel using rennet. Rheological properties, scanning electron microscopy imaging, and physical and chemical indexes were examined to reveal the effect of enzyme modified soymilk on rennet induced gelation of skim milk. Results showed that soymilk inhibited the aggregation of skim milk, led to a decrease in storage modulus (G'), significantly increased moisture content and curd yield, and the resulting network was coarse. Enzyme modified soymilk with a molecular weight below 20 kDa led to a more uniform curd distribution, which counteracted the reduction of G' and allowed for the formation of a stronger gel. Both the moisture content and the curd yield increased with the addition of soymilk and enzyme modified soymilk, and overall the effect of adding a high degree of hydrolysis of enzyme modified soymilk was superior. Compared to untreated soymilk, the addition of a certain amount of enzyme modified soymilk resulted in a new protein structure, which would improve the texture of blend cheese.

Effect of Carrageenan on Physicochemical and Functional Properties of Low-Fat Colby Cheese.

The effect of carrageenan (κ-carrageenan, ι-carrageenan, and λ-carrageenan) on the physiochemical and functional properties of low-fat Colby cheese during ripening was investigated. Protein, fat, and moisture contents; the soluble fractions of the total nitrogen at pH 4.6; protein and fat recovery; and the actual yield and dry matter yield (DM yield) were monitored. Hardness, springiness, and the storage modulus were also evaluated to assess the functional properties of the cheese. Moreover, the behavior of water in the samples was investigated to ascertain the underlying mechanisms. The results indicated that 0.15 g/kg κ-carrageenan had no significant effect on the actual yield and DM yield, and physiochemical and functional properties of low-fat Colby cheese. The protein content increased in the low-fat cheese and low-fat cheese containing κ-carrageenan, and the moisture in the nonfat substance (MNFS) decreased in both samples, which contributed to the harder texture. The addition of 0.3 g/kg ι-carrageenan and 0.3 g/kg λ-carrageenan improved the textural and rheological properties of low-fat cheese by 2 ways: one is increasing the content of bound and expressible moisture due to their high water absorption capacity and the other is interfering with casein crosslinking, thereby further increasing MNFS and the actual yield.

Microbial floral dynamics of Chinese traditional soybean paste (doujiang) and commercial soybean paste.

Traditional soybean paste from Shandong Liangshan and Tianyuan Jiangyuan commercial soybean paste were chosen for analysis and comparison of their bacterial and fungal dynamics using denaturing gel gradient electrophoresis and 16S rRNA gene clone libraries. The bacterial diversity results showed that more than 20 types of bacteria were present in traditional Shandong soybean paste during its fermentation process, whereas only six types of bacteria were present in the commercial soybean paste. The predominant bacteria in the Shandong soybean paste were most closely related to Leuconostoc spp., an uncultured bacterium, Lactococcus lactis, Bacillus licheniformis, Bacillus spp., and Citrobacter freundii. The predominant bacteria in the Tianyuan Jiangyuan soybean paste were most closely related to an uncultured bacterium, Bacillus licheniformis, and an uncultured Leuconostoc spp. The fungal diversity results showed that 10 types of fungi were present in the Shandong soybean paste during the fermentation process, with the predominant fungi being most closely related to Geotrichum spp., an uncultured fungal clone, Aspergillus oryzae, and yeast species. The predominant fungus in the commercial soybean paste was Aspergillus oryzae.