| Gel properties are important functional characteristics of soy protein isolate(SPI),which is widely used as a gelling agent to improve the texture and water holding capacity(WHC)of meat products.However,water in the gel network transforms into ice crystals during frozen storage.This phenomenon destroys the original structure,induces the formation of cellular structure,increases the hardness,and dramatically decreases the water holding capacity of gels,thereby reducing the sensory quality of frozen foods.Therefore,increasing the freezing resistance of soy proteins is crucial to widen the applications of SPI in food freezing.Therefore,in this study,TGase was applied to modify SPI and gelatin to prepare a modified SPI product.Analysis of soluble protein,total SH groups,and gel chemical interaction showed that the improvement of MSPI gel properties during frozen storage was due to the slight decrease in the amount of gel soluble proteins and total SH groups and to the chemical interaction among different gels.Overall,the modification method helped prevent the deterioration of the quality and properties of the traditional soy protein products used in frozen foods,making the method a promising industry prospect.The effects of different sterilization conditions on the physicochemical properties of Soybean Protein Isolate(SPI)during frozen storage were carried out to gain scientific knowledge and to provide a basis for supplying soy foods with better quality.SPI was prepared after sterilized at UHT(ulter high temperature(hereafter referred to as USPI).The changes in physicochemical properties of protein dispersions(2%,pH7.0)subjected to frozen storage at-18℃ were investigated.The results showed that UHT resulted in heat denaturation of proteins.Compared with the control SPI,the soluble proteins content and free SH groups of USPI had no marked changes.During frozen storage,the solubility,viscosity,free SH groups,total SH groups of protein,gel capacity and emulufing capacity trended down gradually with the increase of frozen time while particle size and exposed extent of hydrophobic domains were increased.For another,some soluble aggregations associated with disulfide bonds were observed in the SDS-PAGE electrophoresis of SPI and USPI solutions treated with frozen storage.This phenomenon might have influence on the gel properties of 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℃,30min),high temperature with short time(90℃,10min)and ultra high temperature sterilization(135℃,15s),hereafter were referred to as 65SPI,90SPI and USPI.Another six types of SPI were prepared by three different pH values(pH3,pH7 and pH9)and three different inoice strengths(I0,I0.1 and I0.2).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 and the increment of inoice strength exerted no effect on improving the gel properties of SPI during frozen storage.The functional properties(elasticity and water holding capacity)of soy protein isolate(SPI)are degraded by denaturalization during frozen storage.To solve this problem,a modified SPI was prepared by cross-linking SPI and gelatin with transglutanminase(TGase).The gel properties of the modified SPI were analyzed during frozen storage.After 20 days of frozen storage,the water holding capacity(WHC)of the SPI gels decreased from 89%to 65%,whereas that of the modified soybean protein isolate(MSPI)gels remained above 85%,which was 29.6%higher than that of the SPI gels.In addition,the hardness increment of the MSPI gels only accounted for 47.1%that of the SPI gels.The moisture distribution of the MSPI gels showed no significant change during frozen storage,and the content of unfreezing water in the MSPI gels was higher than that in the SPI gels.Confocal laser scanning microscopy revealed that the microstructure of the MSPI gels suffered minimal damage after frozen storage,whereas that of the SPI gels presented a honeycomb pattern after frozen storage.Analysis of soluble protein,total SH groups,and gel chemical interaction showed that the improvement of MSPI gel properties during frozen storage was due to the slight decrease in the amount of gel soluble proteins and total SH groups and to the chemical interaction among different gels.For the SPI modified by gelatin via cross-linking with TGase,the aggregation degree of proteins significantly decreased,and the protein-water interaction was maintained,which prevented the gel quality from deteriorating.The improvement of the antifreezing properties in the MSPI gels could be mainly attributed to the fact that the formation of ice crystals was inhibited in the water molecules associated with proteins during frozen storage,the aggregations and structural rearrangements caused by protein dehydration were prevented,and the gel structure was maintained.Overall,the modification method helped prevent the deterioration of the quality and properties of the traditional soy protein products used in frozen foods,making the method a promising industry prospect.The hydrolysis of MSPI was hydrolyzed by using papasin.The results were shown that MSPI hydrolysate has better gel freezing resistance and lower viscosity,which revealed no significant different for SPI in 4-12%protein concentration range.This revealed that antifreezing protein hydrolysate with lower viscosity can be obtained when 7S were selectively hydrolyzed by papasin. |
