Research On Aroma-producing Properties Of Yeast And Its Effect On Flavor Quality Of Fermented Meat Products

Naturally fermented meat products,represented by dry-cured ham,are very popular around the world for its high nutrition and unique flavor.However,due to the long production cycle,poor quality stability and safety controversy of natural fermentation process,the standardization degree of dry-cured ham industry still needs to be improved.Yeast is a large group in the microbial community of dry cured ham,which is closely related to the formation of flavor quality and sensory characteristics of dry cured ham.Therefore,this study evaluated the aroma production properties of yeast isolated from traditional dry-cured ham at different processing stages to select yeast starters that could contribute to the standardized production of dry-cured ham.The objective was to provide theoretical guidance and strain resources for improving the quality of dry-cured ham and promoting the standardized production level of dry-cured ham.The main research results are as follows:（1）Changes of yeast population structure during dry-cured ham processing.High-throughput sequencing results showed that Penicillium and Debaryomyces were the most abundant dominant fungi in dry-cured ham.The results of ITS and 26S rDNA sequencing results showed that Debaryomyces hansenii,Candida zeylanoides,Yarrowia bubula and Yarrowia alimentaria were the four yeast species isolated from dry cured ham.Among them,D.hansenii is the most abundant yeast in dry cured ham,which is isolated in all stages of dry cured ham processing,and mainly dominates in the middle and late processing.（2）Evaluation of fermentation characteristics of yeast isolated from dry-cured ham.Based on preliminary screening,21 yeast strains with good lipase and protease activities,10%NaCl tolerance,150 mg/L NaNO₂tolerance and low temperature tolerance were obtained to evaluate their aroma-producing ability.Yeast strains were grown in a meat model medium which conditions was similar to fermented sausage and volatile compounds were identified.The result showed that inoculated D.hansenii and C.zeylanoides significantly promoted the production of volatile compounds including alcohols,aldehydes,ester and acids.Among them,D.hansenii S25 isolated from the maturing phase of dry-cured ham showed the highest production of alcohol and aldehyde compounds,and D.hansenii S20 showed the highest number of key volatiles with OAV>1.According to the PLS analysis,D.hansenii S25 and D.hansenii S20 were closely correlated with8 important volatile compounds（VIP>1）.The aroma-producing capacity of 11 strains of D.hansenii was evaluated in a meat model medium containing dry cured ham components.The results showed that the production of aldehydes from fat oxidation in inoculated groups was significantly inhibited,except for D.hansenii S2.According to the principal component analysis（PCA）,D.hansenii S26 and S20 had the strongest relationship with volatile compounds produced by microbial metabolism.Therefore,the strains of D.hansenii S20,S25 and S26 isolated from the mature phase of dry-cured ham have potential as starter for meat products.（3）The effects of autochthonous strains D.hansenii S20 and S26 on the physicochemical indexes and volatile components of fermented pork pieces were studied.The results show that:among the microbial indexes,the number of yeasts in the experimental group inoculated with starter was significantly higher than control during the whole process.In addition,the yeast inoculation inhibited the growth of Enterobacteriaceae in the experimental group,and significantly affected the number of lactobacillus and staphylococcus.Among the physical and chemical indexes,yeast promoted p H value,redness value（a*）and hardness value of fermented pork pieces,and thus affected the color and texture.But the effect on water activity（Aw）was not significant（p>0.05）at the end of fermentation.Among the protein degradation indexes,the non-protein nitrogen content of the experimental group inoculated with yeast was significantly higher（p<0.05）than that of the control group before,during and after processing.The results of free amino acid analysis showed that the total content of free amino acids and several kinds of flavor amino acids in the experimental group inoculated with yeast were significantly higher（p<0.05）than control.At the same time,SDS-PAGE results showed that the protein degradation level of the fermented meat pieces in experimental groups were higher than control group.Among the analysis of flavor quality,the electronic tongue response values of umami,saltiness,richness,aftertaste-A and Aftertaste-B in D.hansenii S26 were significantly higher（p<0.05）than control group C.However,there was no significant difference in the response values of saltiness,aftertaste-B and richness between the D.hansenii S20 and the control.According to SPME-GC-MS analysis,49,59 and 52 volatile compounds were detected in control,D.hansenii S20 and D.hansenii S26 group,respectively.Comparing with the control,yeast starter promoted the production of aldehydes,esters,alcohols,ketones and acids in fermented pork pieces.The PCA result of volatile compounds in fermented pork pieces at the end of processing showed that there were significant differences in volatile compounds between the control and the two experimental groups as well as the two experimental groups inoculated with D.hansenii.