Bitter Apricot Kernel Protein Gels Induced By Transglutaminase And Its Controlled Release Properties

As a by-product of the apricot kernel oil processing industry,bitter apricot(Armeniaca sibirica(L.)Lam.)kernel meal contains approximately 50%protein,which can be utilized for protein preparation,especially as a source of plant proteins.In recent years,most researches have focused on the extraction and solubility,emulsifying properties of bitter apricot kernel protein,however,there is a little information on the gel properties of bitter apricot kernel protein and its formation mechanism.Therefore,bitter apricot kernel meals were used as raw materials to extract protein in this study,and bitter apricot kernel protein gels were formed induced by transglutaminase(MTGase).Based on this,the effect of MTGase concentration on physicochemical and structural properties were investigated,and the gel formation mechanism was discussed.Moreover,the release of riboflavin from MTGase induced bitter apricot kernel protein gels and its encapsulation efficiency were investigated under gastric and intestinal conditions.The obtained research contents and results were as follows:(1)The effects of MTGase concentration,pH,cross-linking temperature and treatment time on the gel strength,cohesiveness and springiness of bitter apricot kernel protein were investigated on the basis of single factor.Then,Box-Behnken response surface model was employed to optimize the MTGase crosslinking parameters with gel strength as an indicator.The optimum cross-linking conditions were as follows:MTGase concentration 17 U/g,cross-linking temperature 43?,pH 7.2,cross-linking time 2.5 h.Under these optimized condition,the gel strength of bitter apricot kernel protein was arrived at(135 ± 7.14)g.(2)The effect of MTGase concentration on physicochemical properties were investigated.SDS-PAGE analysis showed that the subunits of bitter apricot kernel proteins were crosslinked to form higher molecular polymers(>180 kDa),while 20,38,39,40 kDa fractions were more sensitive to MTGase.SEM displayed a denser gel network as MTGase crosslinked,with smaller and more compaction pores.With the increasing MTGase concentration,the gel solubility decreased to 36.84%(pH=7),while water holding capacity kept at 4.9 g/g and oil holding capacity increased to 3.66 g/g at 16 U/g.Both emulsifying activity and emulsion stability showed an increase at the first,then decreased.In vitro digestibility suggested that the digestibility of gel was decreased from 82.5%to 40%as MTGase crosslinked,which was related to the steric hindrance caused by protein aggregation and crosslinking.(3)The effect of MTGase concentration on structural properties were investigated.The crosslinking degree showed that the formation of Glu-Lys isopeptide bonds were formed by MTGase crosslinking,resulting in a decrease in free amino group contents and an increase in the crosslinking degree of protein.FTIR analysis found that bitter apricot kernel protein gel was dominated by ?-sheets(52.79%),and the a-helices were declined accompanied by increasing ?-sheets and ?-turns under MTGase treatment.With the addition of MTGase,the SHF contents were decreased from 10.29 ?mol/g to 1.97 ?mol/g(p<0.05),followed by increasing S-S and SHT contents.MTGase significantly increased H0 index from 0 to 12 U/g(p<0.05),then dropped gradually.The hydrophobic interactions might be the most important non-covalent bonds force in the formation of gel network,while the contribution of ionic bands and hydrogen bonds were less.(4)The controlled release properties of MTGase induced bitter apricot kernel protein gels on riboflavin were investigated.The results suggested that MTGase induced gels had a higher capacity to load and encapsulate riboflavin,reaching a maximum encapsulation efficiency on 97.32%at 8 U/g.The addition of MTGase could reduce the erosion and swelling degree of the gel matrix in SGF,while increase the swelling degree in SIF.Released properties showed that MTGase induced gels decreased the release amounts of riboflavin,and the release characteristic in SIF was higher than SGF.Gastrointestinal digestion indicated that the MTGase induced gels could resist the pepsin digestion and gastric fluid erosion,prevent the early release of sensitive compounds in the stomach,while making them available in the small intestines.