Quality Characteristics And Microbial Diversity Of Xuanwei Ham

Xuanwei ham is a famous traditional specialty food in China,which is favored by consumers due to its bright color and strong aroma.During ham processing,under the action of endogenous enzymes and microorganisms,a series of complex biochemical reactions will occur,and then form its unique quality.It is important to study the changes of quality characteristics of Xuanwei ham processing and the influence of microorganisms on its quality for the formation of ham quality.On account of this,Xuanwei ham in different year(1 year,2 years and 3 year)was taken as that research object in this study,the differences in sensory characteristics,physical and chemical properties,free fatty acids,small molecular compounds,volatile flavor compounds and microbial diversity of ham with different processing times were analyzed by quantitative description analysis(QDA),gas chromatography-mass spectrometry(GC-MS),ultra-performance liquid chromatography-high resolution mass spectrometry(UHPLC-QE-MS),gas chromatography-ion mobility spectrometry(GC-IMS)and high-throughput sequencing techniques,respectively.The correlation between microorganism and quality characteristics was also analyzed,and the inner relationship between microorganism diversity and quality characteristics of Xuanwei hams in different years was clarified,which provided a scientific theoretical basis for the processing and quality control of Xuanwei hams.The main research results were as follows:1.Sensory evaluation of Xuanwei ham from different years was conducted by QDA.The results showed that the color,overall acceptability,texture,taste and aroma scores of three-year-old ham were the highest,and the color,aroma,texture and overall acceptability scores were significantly different from those of one-year-old ham and two-year-old ham(P<0.05).With the extension of maturation time,salt content,pH,TBA,trimethylamine nitrogen,chewiness,hardness,a~*value,shear force,non-protein nitrogen content and proteolysis index showed an increasing trend(P<0.05),while water content,water activity,L~*value,elasticity,cohesion and resilience showed a decreasing trend(P<0.05).The b~*value of two-year ham was the highest.Water content had a significant negative correlation with salt content,nitrite residue,pH,POV,L~*,TBA,trimethylamine nitrogen,hardness,chewiness,shear force and proteolysis index(P<0.05),but a significant positive correlation with water activity,resilience,cohesion and elasticity(P<0.05).2.31 kinds of free fatty acids were detected from Xuanwei ham in different processing years by GC-MS technology.Among them,there were 8 monounsaturated fatty acids,10 polyunsaturated fatty acids,and 13 saturated fatty acids.The composition of free fatty acids in Xuanwei ham in different processing years was the same,but the content was significantly different.Palmitic acid(C16:0),palmitoleic acid(C16:1),stearic acid(C18:0),oleic acid(C18:1),linoleic acid(C18:2),γ-linolenic acid(C18:3),eicosenoic acid(C20:1),decadecanoic acid(C24:0),docosahexaenoic acid(C22:6)and decacosenoic acid(C24:1)were the main free fatty acids in Xuanwei ham.Among them,the contents of palmitic acid(C16:0),palmitoleic acid(C16:1),oleic acid(C18:1),eicosaenoic acid(C20:1)and docosahexaenoic acid(C22:6)were increased with the extension of processing time,and palmitoleic acid(C16:1),oleic acid(C18:1)and eicosaenoic acid(C20:1)had significant differences in xuanwei ham from different processing years(P<0.05).3.The small molecular metabolites in different years of Xuanwei ham were analyzed by UHPLC-QE-MS technology,271 and 145 metabolites were detected in positive and negative ion modes,respectively,including fatty acids,amino acids,nucleotides,sugars,organic acids,vitamins,etc.Among them,48 different metabolites(VIP>1.5,P<0.05)were the important components that affected the flavor difference of Xuanwei ham in different years.Through pathway analysis,it was found that 21 metabolic pathways were related to ham fermentation,and 13 of them were identified as key metabolic pathways(Impact>0.01,P<0.05).23 key metabolites such as arginine,proline,glycine,serine,inositol,phosphoric acid,tyrosine,alanine,and aspartic acid were accurately matched through database mapping of metabolites such as KEGG and Pub Chem.4.50 volatile flavor compounds were identified from Xuanwei ham in different years by GC-IMS,including 8 alcohols,17 aldehydes,16 ketones,4 esters,3 acids and 2nitrogen compounds.The combination of partial least squares discriminant analysis(PLS-DA)and principal component analysis(PCA)could distinguish the volatile flavor compounds of Xuanwei ham in different years and screen out 18 characteristic flavor compounds.With the extension of ripening time,the contents of hexanal-M,octanal and2-methylbutyraldehyde in Xuanwei ham decreased gradually,while the content of butyric acid increased continuously,while the content of acetone was the highest in three different years of Xuanwei ham.5.High-throughput sequencing technology was used to analyze the microbial diversity of Xuanwei ham in different years.At the phylum level,the dominant bacteria on the surface and inside of Xuanwei ham were Proteobacteria and Firmicutes,and the dominant fungi on the surface and inside of Xuanwei ham were Ascomycota.At the genus level,the dominant bacteria on the surface and inside of Xuanwei ham were Staphylococcus,and the dominant fungi on the surface and inside of Xuanwei ham were Aspergillus.Proteobacteria accounted for the lowest relative abundance in Xuanwei ham in the first year sample,while Firmicutes accounted for the highest.The relative abundance of Proteobacteria in Xuanwei ham was the lowest in the first year sample,and the highest in the first year sample.The proportion of Firmicutes in the first year sample was the highest,and that in the third year sample was the lowest.Actinobacteria accounts for the highest proportion of the first year sample,and the lowest proportion of the second year sample.Proteobacteria accounted for the highest relative abundance of Xuanwei ham in the first year sample and Firmicutes in the second year sample.6.Spearman correlation analysis was carried out between bacteria and fungi with the highest relative abundance and physicochemical indexes,free fatty acids,differential metabolites and flavor substances.The abundance of Halomonas,Halophilic Bacillus,Dbaly Yeast,and Cobetia on the surface of Xuanwei ham was negatively correlated with TBA,POV and trimethylamine nitrogen content,and positively correlated with water content and Aw.The abundance of Staphylococcus,Aspergillus and Arthrobacter in Xuanwei ham was positively correlated with TBA and POV,and negatively correlated with water content and Aw.The results showed that the contents of 10 free fatty acids,such as C16:0 and C18:1,had a significant positive correlation with the abundance of Candida and Vibrio in Xuanwei ham(P<0.05),but a significant negative correlation with the abundance of Carnobacterium(P<0.05).The abundance of Staphylococcus and Yamadazyma on the surface of Xuanwei ham was significantly positively correlated with C16:0,C10:0,C18:0,C20:1 and C16:1contents(P<0.05),and significantly negatively correlated with C18:3,C22:2 and C20:4 contents(P<0.05).The relative abundance of Brevundimonas and Wallemia in Xuanwei ham was significantly positively correlated with the content of Cytosine,L-Aspartic acid,Acetylcysteine,D-Glucuronic acid,Pipecolic acid and Hydroquinone.The relative abundances of Serratia and Debaryomyces on the surface of ham were significantly negatively correlated with the contents of D-glucuronic acid,acetylcysteine,cytosine,glycine,hydroquinone,hydroxypyruvate,L-aspartic acid,formononetin and arachidonic acid(P<0.05),and were significantly positively correlated with the contents of mushroom penicillin,diaminopimelic acid,nitrobenzene,alginate,1-arachidonic acid,isocholic acid,α-diphosphate and antibiotic JI-20A(P<0.05).The relative abundance of Yamadazyma and Staphylococcus was positively correlated with arginine,erucic acid and Ferulate(P<0.05),and negatively correlated with L-alanine,resin phenol andα-oxophenylacetic acid(P<0.05).The contents of Nonanal,3-methylbutanal,2-methylbutanal,Benzaldehyde-M,Pyrrole,Heptanal-M,2-heptanone-M and Hexanal-M were significantly positively correlated with the relative abundance of Halomonas,Debaryomyces,Bacteroides,Lactobacillus and Penicillium in the internal samples of Xuanwei ham,which were significantly negatively correlated with the relative abundance of Ralstonia,Brevundimonas,Wallemia and Aspergillus(P<0.05).The relative abundance of Serratia and Debaryomyces on the surface of Xuanwei ham were positively correlated with the content of benzaldehyde-M,nonanal,octanal and hexanal-M(P<0.05),and negatively correlated with the relative abundance of Aspergillus(P<0.05).