Research On The Directed Vat Set (DVS) Of Xiaoshan Pickle Radish

Directed Vat Set(DVS)is a series of highly concentrated freeze-dried starter with high content of live bacteria and stable quality,which can realize the standardization and large-scale production of fermentation food.Xiaoshan pickle radish is one of the famous Chinese traditional fermented vegetables and a typical semi-dried femented vegetable.In this study,the lactic acid bacteria were isolated and identified from Xiaoshan pickle radish and the DVS was developed.The lactic acid bacterial flora during processing of Xiaoshan pickle radish were investigated.The samples were pickled with the product from three different markets by spontaneous fermentation.There were no significant difference between the number of bacteria and pH value in samples from different product sites(p<0.05).A total of 387 gram-positive and catalase-negative isolates were obtained.All isolates were identified as Lactobacillus sakei and Leuconostoc lactis by physiological tests and 16S rDNA gene sequencing.Leuc.lactis was the dominated species in the initial stages of fermentation,but in late stages the number of L.sakei remarkly increased.The results showed that the two strains had tolerance to bile salt and simulated gastrointestinal fluid,scavenging DPPH free radicals and hydroxyl free radicals,and were insensitive to H2O2.All of them were negative for amino acid decarboxylase,which could degrade nitrite and inhibit the growth of Escherichia coli and Staphylococcus aureus.There was no antagonism between them.The culture medium of Lactobacillus sakei and Leuconostoc lactis were optimized by response surface methodology.The result showed that L.sakei could growth well in the medium of MRS elementary culture medium with an addition of 6.69%maltose as carbon source,1.5%yeast extract as nitrogen source,0.04%cysteine as proliferation factor,0.2%CH3COONa as buffer salt.The suitable initial pH should be 6.0 and culture temperature was 37?.The optimal ratios of enrichment medium for Leuc.lactis was MRS elementary culture medium with 3.42%glucose as carbon source,5%extractum carnis as nitrogen source,10%carrot juice as proliferation factor,0.5%KH2PO4 as buffer salt.The suitable initial pH should be 5.24 and culture temperature was 37?.After the proliferation culture,the number of L.sakei and Leuc.lactis in the fermentation broth was 1.22×1011cfu/mL and 8.76×1010cfu/mL,2.57 times and 3.02 times higher than those before the culture.It was feasible for the established model due to the consistent results between the prediction and experiments.Both membrane ultrafilitration and centrifugation were effective separation and concentration method and the method in this study was centrifugation.The optimum centrifugal conditions for Lactobacillus sakei was 4000r/min for 15min and the centrifugal yield was 84.30%.The optimum centrifugal conditions for Leuconostoc lactis was 5000r/min for 15min and the centrifugal yield was 91.80%.The optimization formulation of cryoprotectants for Lactobacillus sakei and Leuconostoc lactis were optimized with the way of response surface methodology.The research concluded the optimum of cryoprotectants as follows:the skim milk 19.24g/100mL,Vc 1.86 g/100mL,manganese sulfate 0.39g/100mL for L.sakei and the skim milk 23,79g/100mL,sodium glutamate 4.37g/100mL,manganese sulfate 0.08g/100mL for Leuc.lactis.The colony number of L.sakei and Leuc.lactis freeze-dried powder at 4? and-18?were significantly higher than that at room temperature(p<0.05).In the storage period of twelve months LDH activity and P-galactosidase activity of L.sakei and Leuc.lactis maintained in a stable condition.The optimum ratio of L.sakei and Leuc.lactis was 1:1,and the inoculation volume was 0.15%."Yidaozhong" radishes were inoculated with four fermentation methods,such as natural fermentation,DVS starter,commercial starter and standard strains.The results showed that the composition and quantity of volatile flavor substances and free amino acids in radishes of DVS starter was the most close to the pickled radishes of natural fermentation,showing good sensory quality and be equal to radishes of the natural fermentation.The pH value of the radishes of DVS starter was 3.9,and the content of nitrite was significantly lower than that of the other three fermentation methods(p<0.05).The content of Vc was significantly higher than that of other three fermentation methods(p<0.05).During the six-month storage process,the expansion bag rate was 45%in pickled radishes of natural fermentation and only 10%in pickled radishes of DVS starter.During storage the hardness and brittleness of the radishes of DVS starter was significantly higher than that of the natural fermented radishes(p<0.05).The nitrite decreased of both Xiaoshan radishes,and the number of Escherichia coli was lower than 30 MNP/100g.