Study On Fermentation Properties Of Cold Pressed Walnut Dregs By Bacillus Subtils Natto

Walnut is cultivated widely in China and its acreage and yield ranks as first in the world.Cold-pressing walnut meal with high concent of protein is usually used as animal feed and fertilizer,while few studies are reported about the exploitation of walnut meal.In this study,cold-pressed wanlnut meal was fermented with Bacillus subtils natto,and its fermentation characteristics were studied.The results were as follows:(1)Nattokinase prepared from cold-pressing walnut dregs through solid-state fermentation by Bacillus subtils natto was investigated.Taked nttokinase activity as an indicator,the process parameters were optimized by Single-factor experiment,Plackett-Burman(PB)design and Box-Behnken(BBD)response surface methodology.The optimal parameters were that particle size of walnut meal was less than 10 mesh;amount of glucose and NaCl were 1% and 1.5%,respectively;ratio to solid-liquid was 1:1.5;soaking time was 2 h,pressure cooking time was 50 min,inoculum was 11%,fermentation time was30 h,fermentation temperature was 36oC,ripening time was 24 h,which obtained the highest nattokinase activity(1522 U/g).(2)Quality changes during fermentation of cold-pressed walnut meal by Bacillus subtils natto were investigated.Results showed that cooking treatment resulted in darker color of walnut meal,decreased hardness,increased viscosity,and lower moisture and protein content(decreased by 5-8% and 3-4%,respectively).In addition,L value,moisture content,protein content and hardness was decreased,while viscosity and polypeptide content was increased during fermentation process.Ammonia content,nattokinase activity and cell counts increased at the beginning of fermentation and then decreased over time.Enzyme and cell counts reached maximum values(1493-1726 U/g and 4.7-8.1×109 cfu/g)at 36 h after fermentation.Meanwhile ammonia content was at relative high level.After ripening,L value,hardness,moisture content,protein content,ammonia content,natto kinase activity and cell counts decreased firstly,and then showed a stable trend,eventually maintained at 5.0,16.0,60%,27%,1300 U/g,5.0 mg/g,2.0×109cfu/g,respectively.Viscosity increased over time,which was 1.4 to 2.5 times of fermentation at 60 h of post-ripening post-ripening.Polypeptide content reached maximum(6.80-7.45 mg/g)at 36 h of post-ripening.Watercontent,soaking time,inoculum and fermentation temperature during immersing had different impacts on the fermentation characteristics and the quality of the active ingredients.(3)Volatile components of walnut meal during soaking,cooking,fermentation and post-ripening were investigated.18 kinds of aldehydes,16 kinds of alcohols,11 kinds of ketones,7 kinds of acids,13 kinds of esters,15 kinds of alkanes,12 kinds of alkenes,6 kinds of furans,3 kinds of pyrazines,9 kinds of aromatic compounds and 9 kinds of miscellaneous,total of 119 kinds of volatile components divided into 11 categories were found during the whole meal walnut solid-state fermentation process.The components of main volatile in walnut meal were olefin(28.89%),alcohol(16.02%),furans(13.48%)and esters(13.04%);The components of main volatile after soaking were furans(53.91%),hydrocarbons(12.21%),pyrazine(11.34%)and ketones(8.41%);The components of main volatile after cooking were aldehydes(59.40%),furans(21.80%)and alcohol(11.89%);The components of main volatile after fermentation were aldehydes(66.82%),alcohol(11.89%)and aromatic(2.89%);The components of main volatile after post-ripening were aldehydes(67.50%),alcohol(13.31%)and alkanes(5.85%).The highest components of ingredients in serial 5stages were D-dipentene(27.51%),2-pentyl furan(42.87 %%),hexanal(46.90%),hexanal(52.20%)and hexanal(46.38%),respectively.(4)The composition of free amino acids in walnut meal,soaking,cooking,fermentation and post-ripening were investigated.The results showed that free amino acids content increased first and then decreased during the fermentation process.The content of free amino acids got maximum(33.496 mg/g)after cooking.The content of free amino acids after fermentation and post-ripening were 16.931 mg/g and 10.709 mg/g,respectively.The kinds of essential amino acids increased from 4(Met,Try,Phe and Lys)to all of 8.The content of free amino acids increased from 17.4%(material)to 64.7%(after-ripening).The contents of nonpolar amino acids(9.073 mg/g)and bitter amino acids(6.945 mg/g)were higher than other types of amino acids after the process of after-ripening.(5)Taked polypeptide content and hydrolysis degree(DH)as indicators,and the optimum process parameters were analyzed by single-factor experiment and BBD response surface methodology.Results showed that the optimum technological parameters were as following: fermentation time 84 h,feed concentration 8 g/100 mL,initial pH value 8,inoculum 11%,and fermentation temperature 33oC.The related contents of polypeptide and DH were 2.58 mg/mL and 37.5%,respectively.