Effects Of Water Activity On Carbon Source Utilization Capacity Of Specific Spoilage Organisms Isolated From Lightly Salted Large Yellow Croaker

Large yellow croaker（Larimichthys crocea）belongs to the family Sciaenidae and the genus Pseudosciaena,which ranks the first in mariculture in China.At present,its processed products are rich in forms,and the lightly salted large yellow croaker is popular among consumers for its good flavor and convenient edible characteristics.Hafnia alvei（H.alvei）and Proteus vulgaris（P.vulgaris）are specific spoilage organisms（SSOs）of lightly salted large yellow croaker during low temperature storage,which can utilize and decompose carbohydrate,protein,fat and other energy substances,produce small molecule substances and bad smell through metabolic pathway,and shorten the shelf life and circulation radius of products.Under the effect of temperature,p H,water activity and antimicrobial agents,microorganisms will continuously consume energy to maintain internal balance,which can lead to cell death when beyond its tolerance range.Therefore,it is of great significance to explore the energy utilization and growth kinetics of SSOs from lightly salted large yellow croaker under different stress conditions from the perspective of energy metabolism,which is of great significance for optimizing product preservation and quality control technology.In order to explore the effect of water activity on the carbon metabolism of SSOs in lightly salted large yellow croaker,we studied Hafnia alvei and Proteus vulgaris,and Biolog GENⅢmicroplate was used to obtain the optical density（OD）of the two strains under the condition of water activity（Aw）at 0.92,0.93,0.94,0.95,0.96,0.97,0.98,respectively,at 5℃and 25℃,the growth curve of strain was fitted with the modified Gompertz model to obtain kinetic parameters,and the metabolic rate was measured by using average well color development（AWCD）as indicator to describe the dynamic properties of carbon utilization.The main results are as follows.1.The effect of water activity on the carbon source utilization ability of Hafnia alvei and Proteus vulgaris（5℃）At 5℃,H.alvei and P.vulgaris mainly used carbon sources at Aw of 0.98 and0.97,and sugars,carboxylic acids and amino acids were all used better,and there was no significant difference in the types of sugars and carboxylic acids.when Aw was 0.98,the overall carbon source utilization ability and activity of the two strains were stronger than that at Aw（0.97）,and when Aw was 0.97,H.alvei mainly used monosaccharides and partial disaccharides,P.Vulgaris mainly uses some disaccharides and polysaccharides.When Aw was 0.98,the mean value ofμ_max of the utilization of sugars,carboxylic acids,amino acids,amines/amides,fatty acids/esters and others by H.alvei and P.vulgaris was significantly higher than that when Aw was 0.97,which were0.00709,0.00874,0.00744,0.0105,0.00629 h^-1and 0.0100,0.0133,0.00911,0.0160,0.00686 h^-1 respectively.When Aw was 0.98,α-D-glucose and L-rhamnose,D-glucuronic acid and D-galacturonic acid,L-histidine and D-serine,N-acetyl neuraminic acid and N-acetyl-D-glucosamine,D-glucose-6-PO₄ were the most easily utilized sugars,carboxylic acids,amino acids,amines/amides and fatty acids/esters,and others of H.alvei and P.vulgaris,respectively,and the class and quantity of carbon utilization are different from that when Aw was 0.97.2.The effect of water activity on the carbon source utilization ability of Hafnia alvei（25℃）At 25℃,H.alvei could utilize carbohydrate,amino acid and carboxylic acid with different Aw（0.92-0.98）,and the utilization rate of carboxylic acid（37.3%）and amino acid（32.6%）were higher.When Aw was 0.98,the maximum specific growth rate(μ_max)was the largest,the lag period（λ）was the smallest,the overall carbon source utilization ability and activity were the strongest,followed by 0.96,and the overall carbon source metabolism ability of the remaining group was 0.95,0.92,0.93,0.94,0.97,from strong to weak.Sugars（L-rhamnose,L-fucose）when Aw was 0.98,carboxylic acids（D-glucuronic acid,D-galacturonic acid,acetic acid）when Aw was0.98 and 0.97;amino acids（D-serine,L-arginine）when Aw was 0.98 and 0.96,amines/amides and fatty acids/esters（N-acetyl-D-glucosamine,N-acetyl-D-galactosamine,N-acetyl neuraminic acid）when Aw was 0.98 and 0.97;the others（D-glucose-6-PO₄,inosine）were highly utilized when Aw was 0.98.With the increase of water activity,carbon source metabolism was stronger,and the trend of increase and decrease was shown in the two parameters,respectively.Water activity had a great influence on the utilization of different types of carbon sources of Hafnia alvei,and had a significant correlation with the changes ofμ_maxandλof various carbon sources.3.Effect of water activity on carbon source utilization of Proteus vulgaris at 25℃At 25℃,the carbon source utilization ability and activity of Proteus vulgaris were the strongest when Aw was 0.98,followed by Aw was 0.96,and the carbon source metabolism ability of the rest groups were 0.95,0.94,0.93,0.97,0.92,from strong to weak.When Aw was 0.97 and 0.98,theμ_max of all kinds of carbon sources was higher,λwas smaller,while that of carbohydrates,carboxylic acids,amino acids and others was higher.The utilization of carbohydrates and amino acids gradually decreased with the decrease of Aw.The utilization of carboxylic acids was basically the same at Aw（0.92-0.96）,and that was the lowest at Aw（0.94）;the utilization of amines/amides and fatty acids/esters was the highest at Aw（0.98）.Among carbohydrates,gentiobiose,D-trehalose,D-maltose and D-cellobiose were better used at Aw of 0.98 and 0.97,respectively;among carboxylic acids,D-glucuronic acid,D-galacturonic acid,saccharic acid and citric acid were better used at Aw of 0.98 and 0.97,respectively;among amino acids,L-serine,D-serine,D-aspartic acid and L-aspartic acid were better used at Aw（0.98,0.97）,respectively.Among amines/amides and fatty acids/esters,N-acetylneuraminic acid,N-acetyl-D-galactosamine,methyl pyruvate and N-acetyl-D-glucosamine were better used at Aw of 0.98;among other types,glycerol,D-glucose-6-phosphate and inosine,inositol and glycerol were better used at Aw of 0.98 and Aw of 0.97.