| Walnut is cultivated widely in China,and its planting land and output rank the first in the world.Walnut dreg is the by-product after pressing walnut oil,and it has high protein content and is regarded as a high-quality plant protein resource.At present,walnut dreg is usually used as animal feed or is directly discarded as a result of polluting environment seriously.However,few studies are reported about the exploitation of walnut dreg.In this study,cold-pressing walnut dreg was fermented by mixed strains,and then fermentation and drying characteristics were determinated.The results were as follows:(1)The optimum parameters of fermentation by the mixed strains of Bacillus subtils natto and Torlaspora delbrueckii were as follows:inoculation amount of 3%,proportion of inoculums of 1:1,fermentation temperature of 31?,?and fermentation?time?of?36?h.In this fermentation condition,the NK activity and L~*value were 1801.45?U/g and 57.34,respectively.During the fermentation process,L~*value,b~*value and polyphenols content were decreased.In addition,soluble protein content and cell counts were increased at the beginning of fermentation,then remained stable,while a~*value,pH,nattokinase activity,volatile base nitrogen and polypeptide content were increased.After fermentation for 60 h,L~*value,a~*value,b~*value,polyphenols content,soluble protein content,polypeptide content,volatile base nitrogen content,nattokinase activity,pH,Bacillus subtils natto cell counts and Torlaspora delbrueckii cell counts eventually maintained at 41.92,6.98,16.65,10.52?mg/g,1.50?mg/g,2.49%,66.19?mg/100g,2034.85?U/g,6.54,4.4×10~7?cfu/g,5.8×10~9?cfu/g,respectively.(2)The optimum parameters of fermentation by the mixed strains of Bacillus subtils natto and Lactobacillus plantarum were as follows:inoculation amount of 9%,proportion of inoculums of 1:1.5,fermentation temperature of 42?,and fermentation time of 36?h.In this fermentation condition,the NK activity and L~*value were 1871.83?U/g and 39.56,respectively.During the fermentation process,L~*value,b~*value and polyphenols content were decreased,while pH was decreased at the beginning of fermentation and then increased over time.In addition,soluble protein content and cell counts were increased at the beginning of fermentation and then remained stable with time.However,a~*value,nattokinase activity,volatile base nitrogen and polypeptide content were increased.After fermentation for 60 h,L~*value,a~*value,b~*value,polyphenols content,soluble protein content,polypeptide content,volatile base nitrogen content,nattokinase activity,pH,Bacillus subtils natto cell counts and Lactobacillus plantarum cell counts eventually maintained at 38.59,7.04,15.61,10.34?mg/g,1.64?mg/g,4.03%,61.70?mg/100g,1880.35?U/g,5.77,3.3×10~8?cfu/g,6.7×10~9?cfu/g,respectively.(3)The free amino acids compositions of cold-pressing walnut dreg during fermentation by mixed strains were investigated.The total free amino acids content of walnut dreg was12.56 mg/g.Four essential amino acids were detected and the total essential amino acids content was 0.15?mg/g.The total free amino acids content was up to 24.17?mg/g after cooking,mainly including Ala(62.31%)and Gly(33.60%).Owing to Ala and Gly were utilized by the microbial metabolism,the total free amino acids content was decreased to 3.12?mg/g after fermentation for 60?h by mixed strains of Bacillus subtils natto and Lactobacillus plantarum.However,the kinds of free amino acids were increased to all of 18 while the content of essential amino acids was 63.14%.Compared with the walnut dreg,the contents of bitter amino acids,flavor amino acids and tasteless amino acids were increased while the content of sweet amino acids was decreased after fermentation.In addition,volatile components of walnut dreg,cooking treatment and fermention for 60?h were investigated.Total of 74 kinds of volatile components were detected during the whole walnut dreg solid-state fermentation process.The main volatile components in walnut dreg were olefins(78.44%),Aromatic hydrocarbons(6.39%),esters(4.15%)and Aldehydes(1.01%).The main volatile components in walnut dreg after cooking were olefins(68.57%),esters(8.47%),Aldehydes(3.73%)and alcohols(1.49%).The main volatile components after fermentation for 60?h were olefins(75.36%),esters(5.79%),alcohols(1.66%)and heterocyclic compounds(0.81%).Compared with those in walnut dreg,the contents of esters,alcohols and acids were increased after fermentation.(4)The drying characteristics of the nattokinase fermentation products were investigated.The quality of nattokinase products fermented by the mixed strains of Bacillus subtils natto and Torlaspora delbrueckii(Bacillus subtils natto and Lactobacillus plantarum)dried by freeze drying was the best,followed by vacuum drying.Compared with the control,the nattokinase activity of fermention products was decreased by 56%-65%by freeze drying for24 h and vacuum drying at 40? for 60 h. |
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