Enzyme-assisted Modification Of Soy Protein Isolates And Their In Vitro Simulated Gastric Digestion Behaviors

Due to their low level of sulfur-containing amino acids,plant proteins could be a suitable ingredient used in foods for special medical purposes(FSMP),which provide proteins and health effects that are not present in animal protein for patients.However,As the conformation and functionalities of plant proteins are quite different from those obtained from the animal source,the taste,texture,and digestibility of plant proteins do not match the requirements in FSMP,and the applications of plant proteins have limitations.Therefore,soy proteins,which are the most used plant proteins in the food industry,were chosen as the research object in this work.Phytase and glutaminase were involved in the preparation of soy protein isolates(SPI),to modify the properties of soy proteins via lowing their phytic acid content and reducing their amounts of amide groups.And a semi-dynamic system for simulating in vitro gastric digestion was used to investigate the effects of enzymatic and heat treatments in soy protein preparation on the digestibility of resulting protein products.This might provide valuable information for plant-based FSMP manufacturing that involves plant proteins that have improved digestibility.The main results are as follows.1.SPI with low phytic acid level had been prepared by involving phytase treatment during its preparation.Since the content of phytic acid was reduced from 22.83 ± 0.09 mg/g to8.72±0.07 mg/g,fewer negative charge groups were located on the surface of the protein molecules,and the isoelectric point of the soy proteins moved to a higher pH condition.As a result,the solubility of the soy protein products at pH 4.0 had been greatly improved.The in vitro simulated gastric digestion of the low phytic acid soy proteins was observed.The appearance of the digesta,subunit composition in the emptied digesta analyzed by SDS-PAGE,solid content,and the number of free amino groups released in the emptied digesta had been recorded during this process.The results indicated that the capacity of the soy proteins forming curds during acidification induced by the dropping of simulated gastric fluid had been weakened,as their phytic acid contents were remarkably reduced.The gastric digestion process of this soy protein was shortened by at least 15 min when compared to common SPI.2.Soy proteins with fewer amide groups were obtained by involving glutaminase-induced deamidation during SPI preparation.The deamidation degree of this protein product was26.79±4.21%.Their solubility of this protein when pH condition was higher than 6.0 was significantly improved,leading to a more transparent appearance of their dispersion.And their foaming capacity was also improved.When this protein product was subjected to be digested in the in vitro semi-dynamic simulated gastric digestion model,this protein formed more curds in the early stage of digestion,compared to common SPI.It spent a long time for these curds to be hydrolyzed,and this cost more pepsin activity and more simulated gastric fluid.This suggested that the enzyme-induced deamidation could not accelerate the gastric digestion process of soy proteins.3.Soy protein slurry,which had experienced phytase treatment and enzyme-induced deamidation successively,had been subjected to heat treatment conducted by incubating in boiling water and hydrothermal cooking,respectively.And two protein products SPI-EPT and SPI-EHT were obtained.Results indicated that the solubility of these two soy protein products decreased to a different degree.SPI-EPT shared a similar in vitro simulated gastric digestion process in the semi-dynamic digestion model.As SPI-EHT had poor solubility,it formed more precipitate than the other two protein products in the early stage of digestion,leading to a longer digestion process and lower digestibility.Rats were fed various soy protein products,and the number of essential amino acids in their blood that were digested from soy proteins was measured.Similar release rates of essential amino acids were recorded in the rats that had consumed SPI-EPT and common SPI.The number of essential amino acids in the serum of the rats that had consumed SPI-EHT could maintain at a higher level for a longer time.This implied that heating treatment below the boiling point of water rather than hydrothermal cooking was suitable for preparing soy proteins with better digestibility.