| Wheat gluten(WG),a by-product of wheat starch industry,mainly used in flour products and feed industry,is a relatively inexpensive and safely edible source of plant protein.The water-insoluble characteristics of WG which has a high proportion of hydrophobic amino acids,complex molecular composition and structure features,strongly limite its application in the food industry.Enzymatic hydrolysis of WG to produce value-added peptides with a wide range of functional characteristics is one of the most attractive way to convert WG into value-added products.However,traditional methods is not sufficiently mature,the content of small peptides in protein hydrolysates is low and enzyme-hydrolyzed protein has strong bitterness,which have made them difficult to incorporate into foods.WG was used as raw material in this paper.This study was focused on the application of enzymatic technology in the preparation of low-bitterness peptide powders and discussion of debittering mechanism.According to the bitter taste characteristics of WG hydrolysates,sequential hydrolysis with endo-and exo-peptidase was designed for the preparation of low-bitterness peptide powders.In addition,deamidation-induced modification was used to improve the flavor of WG hydrolysates.In order to lay the groundwork for the targeted optimization of the Proteax hydrolysis reaction system,the release characteristics of bitter peptides of WG hydrolysates were studied.Finally,step-by-step enzymatic hydrolysis of WG was proposed to prepare low-bitterness peptide powders.Sequential hydrolysis with endo-and exo-peptidase shoud be more efficient to obtain small peptides,since they have greater hydrolysis ability relative to single endopeptidase or exopeptidase.After the bitterness of WG hydrolysates using single-enzyme treatment(Neutrase,Papain,Flavourzyme,Alcalase,Trypsin,Protamex and Proteax)was assessed by taste dilution analysis(TDA),schemes of sequential hydrolysis with endo-and exopeptidases were designed.The effects of the peptide nitrogen content,amino acid composition,molecular weight distribution and amino acid sequence on the sensory characterization and sensor response of the electronic tongue in WG hydrolysates were discussed between the schemes.The results showed the differences between the contents of bitter peptides in all the schemes were relatively low.The disparity of the bitterness between all schemes was determined by the bitter taste thresholds of bitter peptides,but not the content of bitter peptides.Bitter taste thresholds were affected by the amino acid sequences and molecular weights distribution of peptides.It was found that Pro-m hydrolysates exhibited the lowest bitterness value(1.33),while they had the highest content of peptide nitrogen(61.54%)and small peptides(53.26%)ranging from 180-500 Da in all the schemes,and their contents of soluble nitrogen reached up to 82.82%.The Proteax hydrolysis reaction system was confirmed as the basic framework in the following study.Glutaminase SD-C100 S,Glutaminase or HCl was used to deamidate Pro-m hydrolysates and catalyze the conversion of glutamine into glutamic acid respectively.The generation of umami peptide and free glutamic acid could increase umami taste and suppress bitter taste,which improved the flavor of WG hydrolysates.After deamidation-induced modification with certain enzymes or acid treatment,flavor characteristics,molecular weight distribution and amino acid composition of WG hydrolysates were determined as a comparative study.The main compositions of umami substances and the corresponding effect of suppression on bitter taste sensation were discussed in WG hydrolysates.The result showed that WG hydrolysates had a higher proportion of umami substances as the level of degree of deamidation increased,which was capable of both umami-enhancing and debittering effects.Sodium glutamate(MSG)had better hydrophilic characteristics and stronger solubility of saliva than umami peptides,and could contribute to higher suppression of bitter signal transduction than umami peptides.Umami peptides had a longer duration of action with taste buds than MSG,which enhanced the duration of umami taste and showed a strong feeling of softness and coordination.In order to lay the groundwork for the targeted optimization of the Proteax hydrolysis reaction system,the release characteristics of bitter peptides of WG hydrolysates were studied.When the release characteristics of the bitter peptides obviously changed,bitter peptides were extracted from the resulting WG hydrolysates within the above-mentioned period of time,and showed strong bitter taste.The changes of the amino acid sequence,amino acid composition and molecular weight distribution of bitter peptides were investigated during the process of enzymatic hydrolysis.Partial least squares regression(PLSR)analysis was used to explore the relationship between the isobutyl alcohol extraction,sensory data,molecular weight distribution and amino acid composition.The results showed the amino acid composition and molecular weight distribution had a more significant impact on the bitterness intensity than the amino acid sequence.The peptide fractions ranging from 180-500 Da,500-1000 Da and 1000-3000 Da gradually had a higher proportion of bitter tasting amino acids within the first 120 min of the enzymatic hydrolysis reaction.This phenomenon stopped during 120–300 min of the enzymatic hydrolysis reaction.The content of long-chain bitter peptides began to gradually reduce,especially for the peptide fraction ranging from 500-1000 Da that had the strongest bitter taste.Therefore the bitterness intensity of the bitter peptides increased first and then decreased using the Proteax hydrolysis reaction system.Enzymatic hydrolysis of WG generated the soluble hydrolysates and water insoluble aggregates using single-enzyme treatment(Proteax,Alcalase,Protamex and Trypsin).Water insoluble aggregates were used as material to generate the aggregate hydrolysates by repeating the process above.The bitterness of the soluble hydrolysates and the aggregate hydrolysates was assessed.It was found that when degree of hydrolysis kept the same size using the same single-enzyme treatment,the aggregate hydrolysates had stronger bitter taste than the soluble hydrolysates.On the basis of the findings,a step-by-step enzymatic hydrolysis technology was designed to optimize the Proteax hydrolysis reaction system.The operation for actual production was as follow: When WG was hydrolysed for 35 min by Proteax,water insoluble aggregates were removed.Glutaminase SD-C100 S was added to WG hydrolysates solution to form a multi-enzymatic system,which was optimized by Design-Expert 8.0.6 software.The Design-Expert 8.0.6 software predicted that the bitterness value of peptide powder was 0.41 when the reaction condition was the hydrolysis time of 280.79 min,hydrolysis temperature of 53.09 °C and pH of 6.94.The results are verified by the adjustment of multi-enzymatic parameters(hydrolysis time of 280 min,hydrolysis temperature of 53 °C and pH of 6.9).The finished product was prepared under the conditions described above.The bitter taste value of low-bitterness peptide powders was only 0.43. |
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