Study On The Changes Of Texture And Flavor Of Oil Furu And Their Formation Pathways During Fermentation

Oil furu is a special fermented soybean curds with fine texture and flavor,which is found and popular in southwest China such as Yunnan,Sichuan and Guizhou Province.There are two distinct differences from other furu:1)lactic acid bacteria?LAB?replaces Ca²⁺and Mg²⁺as coagulants during the preparation of soybean curd for oil furu;2)rapeseed oil is used to provide a special flavor instead of water in ageing solution during post-fermentation/ripening.However,traditional natural fermentation is still adopted during the production of oil furu,and it leads to unstable quality and flavor and unguaranteed microbial safety of oil furu.High saltiness greatly limits their consumption,and excessive sodium intake increases the risk of developing hypertension,stroke,stomach cancer and renal diseases.In this paper,the fermented strains from the traditional natural fermented oil furu were isolated and identified,the changes of the quality,texture and flavor of traditional fermented oil furu and oil furu fermented with pure strains?1 isolated strain and 3 common strains for furu production?were compared,the advantages of each fermented strain were clarified,the correlations among protein and fat catabolism,texture and flavor of oil furu were analyzed,and the formation pathways of texture and flavor of oil furu were clarified.Meanwhile,the low-salt oil furu fermented with the isolated strain was made,and the effect of salt content and fermentation time on their quality,microbial flora,texture and flavor were studied to provide theoretical basis for the development and production of low-salt oil furu.The main research results of this paper were as follows:1.Strains from traditional natural fermented oil furu were selected and identified using morphological and molecular biological methods,and the quality and microbial flora of traditional fermented oil furu and oil furu fermented with pure strains were studied.The results showed that 67 strains were isolated from natural fermented oil furu,and they were identified as Mucor racemosus.The contents of water-soluble protein,amino nitrogen,total acid,salt and moisture of oil furu fermented with isolated strain M2 were similar to those of the traditional natural fermented oil furu;the counts of molds,lactic acid bacteria and total microbial population in oil furu fermented with pure strains were lower than that of the traditional natural fermented oil furu;no pathogenic bacteria were detected in the former while escherichia coli?10 MPN?was found in the traditional natural fermented oil furu.These indicated that the isolated strains could be used for the production of oil furu instead of traditional natural fermentation to meet the quality and microbial safety requirements.2.The textural,microstructural and rheological properties of oil furu fermented with different strains during fermentation were studied using texture analyzer,confocal laser scanning microscopy,scanning electron microscope and rheometer.The results showed that the hardness of oil furu decreased first,then increased,and then decreased with fermentation time,and the orders of hardness of oil furu fermented for 180 days were traditional fermented oil furu>isolated strain>Mucor racemosus>Mucor wutungkiao>Actinomucor.Protein aggregates and pores in oil furu gradually decreased during fermentation,and the protein phase turned into continuous and uniform.The effects of pure strains on the microstructure of oil furu were similar,and all of them were superior to traditional fermented oil furu.The elastic and viscous modulus of oil furu continuously decreased with post-fermentation time,yield stress decreased gradually,and spreadability gradually improved.The orders of yield stress of oil furu were traditional fermented oil furu>isolated strain>Mucor racemosus>Mucor wutungkiao>Actinomucor.The results showed that oil furu fermented with Actinomucor showed the best spreadability.3.The flavor compounds of oil furu during fermentation were analyzed using SPME-GC-MS,HPLC and LC-MS/MS.The results showed that 73 volatile flavor compounds were detected,including 2 phenols,6 aldehydes,5 ketones,10 acids,26 alcohols,21 esters and 3alkanes;esters and alcohols were the main volatile flavor compounds in oil furu and post-fermentation was the main stage for ester formation.The relative odor activity values?ROAV?of ethyl caprylate,2-dodecenol,4-methylvaleric acid,?E?-2-octenal,2-pentylfuran,isopulegol and linalool were highest,and they made the most contribution to the overall aroma profile of oil fufu.The total contents of volatile flavor compounds in furu were 11.65?traditional fermented oil furu?,32.34?isolated strain?,31.13?Mucor racemosus?,25.11?Actinomucor?and22.25 mg/kg?Mucor wutungkiao?,indicating that pure fermentation had a positive effect on aroma profile of oil furu instead of traditional natural fermentation.The highest contents of umami amino acids were found in oil furu fermented with Actinomucor and Mucor wutungkiao;the highest contents of sweet amino acids were found in oil furu fermented with isolated strain and Mucor racemosus;the highest contents of bitter amino acids were found in traditional fermented oil furu and oil furu fermented with Actinomucor,while the lowest was in Mucor wutungkiao.After fermentation,the most kinds of tasty peptides were found in oil furu fermented with isolated strains,followed by Mucor racemosus,Actinomucor,Mucor wutungkiao and traditional fermented oil furu,and 51 peptides were the same among them;70peptides of oil furu fermented with isolated strains and Mucor racemosus showed the highest similarity.The results showed that isolated strains and Mucor racemosus could substitute traditional natural fermentation to produce oil furu with better aroma and taste.4.To analysis the effect of protein catabolism on texture and flavor during the fermentation of oil furu,the changes of protein hydrolysis in the fermentation process of oil furu were analyzed using SDS-PAGE and degree of protein hydrolysis,and the correlations among protein hydrolysis,texture and flavor of oil furu were analyzed.The results showed that protease activity of oil furu increased and then decreased during fermentation,and oil furu fermented with Mucor wutungkiao showed highest protease activity.The degree of protein hydrolysis of oil furu increased with fermentation time,and it was more than 80%after fermentation for 180days.SDS-PAGE showed that the protein bands above 15kD basically disappeared,while the bands below 15kD significantly became darker.According to the Pearson correlation between the texture and protein hydrolysis of oil furu,the degree of protein hydrolysis and protease activity showed a significantly negative correlation?P<0.01,r=-0.735?,hardness was negatively correlated with the degree of protein hydrolysis?P<0.01,r=-0.88?,yield stress was strongly and negatively correlated with the degree of protein hydrolysis?P<0.01,r=-0.676?,elasticity and viscosity modulus were positively correlated with the degree of protein hydrolysis?P<0.01,r<-0.6?,free amino acids were strongly and positively correlated with degree of protein hydrolysis?P<0.01,r>0.6?,and benzaldehyde,phenyl ethanol and ethyl phenylacetate were positively correlated with phenylalanine?P<0.01,r>0.4?.Based on protein catabolism,the formation pathways of texture and flavor of oil furu were proposed:1)Soybean protein molecules were induced to form the stable gel network structure by the acids produced by metabolism of lactic acid bacteria;2)during the fermentation of oil furu,the protease secreted by mucor acted on the structure of the protein gel network,constantly destroyed its complete structure and led to the changes of textural,microstructural and rheological properties of oil furu;3)meanwhile,tasty peptides and free amino acids were produced by protein catabolism,and amino acids were further metabolized to form volatile flavor compounds,which gave oil furu specific aromas.5.To clarify the effect of fat catabolism on the flavor of oil furu during fermentation,the hydrolysis and metabolites of fat acids during fermentation were analyzed,and the correlations among hydrolysis of fat acids and flavor of oil furu were revealed.The lipase activity of oil furu increased and then decreased during fermentation,and oil furu fermented with isolated strain showed highest lipase activity.Fifteen fatty acids and 13 free fatty acids were detected in oil furu,and their contents slowly increased with fermentation time;the main fatty acids were linoleic acid,oleic acid,palmitic acid,linolenic acid and stearic acid,and the main free fatty acids in oil furu were linoleic acid,oleic acid,palmitic acid,linolenic acid and stearic acid.There were no changes in the contents of 16 fatty acids in oil ageing solutions during fermentation?P>0.05?,while the contents of free fatty acids increased with time.The results of GC-MS showed that free fatty acids produced by the hydrolysis of fat directly provided the specific aroma for oil furu;free fatty acids reacted with ethanol to form ethyl ester flavor substances;in addition,free fatty acids were oxidized to produce?-ketoic acid,decarboxylated to form methyl ketones,and then reduced to form secondary alcohols.Pearson correlation results showed that there was a strong correlation between free fatty acid and volatile acids and esters compounds?P<0.01,r>0.6?,and a significantly positive correlation between free fatty acid and 2-methyl alcohols and ketones?P<0.01,r>0.6?.Based on the change of flavor substances,protein and fat catabolism in oil furu,the formation path ways of flavor compounds in the oil furu were illustrated:1)catabolism of protein and fatty acid in the oil furu;2)migration of compounds in red pepper,zanthoxylum and ageing solution.6.Low-salt oil furu was prepared with isolated strains,and the effects of different salt contents on the quality,texture,flavor and sensory properties of oil furu were analyzed.The results showed that the quality,protease and lipase activity,the degree of protein hydrolysis and the release of free fatty acid were improved by lowering salt content.The hardness of low-salt oil furu decreased,the microstructure became more delicate and smoother,and the spreadability were improved with fermentation time and lowering salt content.In addition,the flavor of low-salt oil furu was analyzed during ripening,and the results showed that the aroma and taste compounds were similar,while their contents increased with lowering salt content.Considering the change of the indexes during the fermentation of low-salt oil furu,7%and 9%salt could be recommended for the actual production of low-salt oil furu.