Abstract:
This phenomenon that water in the foods transforms into ice crystals during frozen storage, might have influence on the gel properties of soybean isolate protein (SPI) and sensory quality of frozen foods. In addition, heat effect generated during sterilization, which is the important procedure to process SPI, would change gel properties of SPI. Therefore, the effects of different heat treatments on the gel properties of SPI during frozen storage were carried out using Texture Analyzer and SDS-PAGE to provide a theoretical basis for endowing SPI-containing foods with better quality during frozen storage. In this study, three types of SPI, which were prepared by three different heat treatments, namely low temperature with long time (65 ℃, 30 min), high temperature with short time (90 ℃, 10 min) and ultra-high temperature sterilization (135 ℃, 15 s), hereafter were referred to as 65SPI, 90SPI and USPI. The changes in gel properties of three types of SPIs were investigated during frozen storage at -18 ℃. The SPI without any heat treatment was regarded as the control. The results showed that the gels of 65SPI and 90SPI had higher hardness with lower elasticity and water holding capacity (WHC), compared to the control SPI gel, while for USPI gel, the hardness, elasticity and WHC were relatively low. On the other hand, before freezing, 65SPI and 90SPI had more proteins incorporated into the gel network than the control SPI, whereas less protein of USPI participated in the formation of gel network. SDS-PAGE results further revealed that almost all β subunits and basic polypeptides (B) of 65SPI and 90SPI were incorporated into the gel network before freezing, while for USPI and SPI, only part of β and B subunits took part in the formation of gel network. However, no marked changes were detected in the values of chemical interactions of gels formed from three types of SPI before freezing. After freezing, the hardness of gels was increased continuously with the increase of frozen storage time, while the elasticity and WHC had apparent trends to decline. Furthermore, SPI and USPI had more soluble proteins involved into the formation of gel network during frozen storage, as compared to that before freezing, , but only slight increase of soluble proteins was detected in 65SPI and 90SPI gels. The β and B subunits, which were obviously observed on the SDS-PAGE profiles of soluble proteins of SPI and USPI gels before freezing, could not be observed any more during frozen storage, however, no changes were observed from SDS-PAGE results of 65SPI and 90SPI gels whether it was before or after freezing. It could also be indicated from the analysis results of chemical interactions that, there were great changes in SPI and USPI gels after freezing, while for 65SPI and 90SPI gels, no marked changes were detected. Moreover, no matter before or after freezing, acid polypeptides (A) was the only subunit to participate the gel formation mainly through hydrogen bonds, while α′, α, β, B subunits were incorporated mainly by hydrophobic interactions. During the frozen stage, the formation of ice crystals in gels leads to the enrichment of protein and the formation of coarse gel structure through protein-protein interactions, which significantly decreases the water holding capacity of gels and enhances gel hardness. In general, heat treatment exerted no effect on improving the gel properties of SPI during frozen storage.