| The products made by lactic acid bacteria attract significant coverage from the consumers,along with the healthy effect of lactic acid bacteria was studied,which resulted in the rapid improvement of the fermented dairy product and all kinds of lactic acid bacteria products also emerge in endlessly. So developed high energy,high stability of the culture in order to adapt to the further development of dairy products in fermentation has become an inevitable. Fine strains,appropriate growth condition and the technology research of the preservation methods of starter is three pivotal technical problems.Currently deep freezing lactobacillus starter and vacuum freeze-drying lactobacillus starter are more widely application.So it has theoretic and practical significance to study the technology and mechanism of lactic acid bacteria during deep freezing and freeze-drying.This paper respectively studied the key process parameters on deep freezing lactobacillus starter and vacuum freeze-drying lactobacillus starter,and protective agent of S. thermophilus SP1.1. To ensure that S. thermophilus SP1.1 get high survival rate of cell and fermentation vitality in the production process,so this study has a great significance to manufacture products.This study through the survival rate and fermentation vitality determined the strains freezing and freeze-drying conditions,including the harvest time of cultivating S. thermophilus SP1.1,the freezing temperature ,frozen duration in freezing process and the harvest time of cultivating S. thermophilus SP1.1,the freezing temperature,frozen duration,the temperture of final freeze-drying in vacuum freeze-drying process. The final determination of freezing and freeze-drying the best time for harvest is the eighth hour,freezing temperature 196℃for 10 min,the temperture of final freeze-drying in freeze-drying process is 0℃. In order to determineβ-galactosidase and lactate dehydrogenase of lactic acid bacteria activity change after freezing and the vacuum freeze-drying process will affect the ermentation vitality,and the influence of membrane permeability after freezing and the vacuum freeze-drying process ,we detected theβ-galactosidase and lactate dehydrogenase vitality inside and outside the cell after freezing and the vacuum freeze-drying process.The data shows,theβ-galactosidase vitality outside the cell of normal bacteria is 1.687×10-3U,after freezing,theβ-galactosidase vitality outside the cell of the bacteria with skimmed milk as protective agent is 9.022×10-3U,theβ-galactosidase vitality outside the cell of the bacteria with protective agent is 6.381×10-3U;the lactate dehydrogenase vitality outside the cell of normal bacteria is 8.84×10-3U,after freezing,the lactate dehydrogenase vitality outside the cell of the bacteria with skimmed milk as protective agent is 2.251×10-2U,the lactate dehydrogenase vitality outside the cell of the bacteria with protective agent is 1.206×10-2U. This shows,freezing has very strong physical damage to membrane,make cell membrane permeability increasing,even rupturing,leading to the two enzymes leakage.After vacuum freeze-drying,theβ-galactosidase vitality outside the cell of the bacteria with skimmed milk as protective agent is 1.350×10-2U,theβ-galactosidase vitality outside the cell of the bacteria with protective agent is 1.027×10-2U; the lactate dehydrogenase vitality outside the cell of the bacteria with skimmed milk as protective agent is 9.646×10-3U,the lactate dehydrogenase vitality outside the cell of the bacteria with protective agent is 6.833×10-2U.Thus there is more serious damage to lactic acid bacteria in vacuum freeze-drying than drying. In the stage of drying,the water between the phospholipid molecules of membrane remove gradually,a Phase transition of membrane maybe occur and after water come back,there still is some space between phospholipid molecules ,the membrane permeability increasing,leading to the two enzymes leakage. The data also shows after freezing and vacuum freeze-drying,the total vitality of the two enzymes declined,so wo think that the vitality of the two enzymes decline after the freezing and vacuum freeze-drying will reduce fermentation vigor of S. thermophilus SP1.1 .Through comparison of single factor and orthogonal experimental design,determining the appropriate formula of cryoprotectants and lyophilized protective agent. the appropriate formula of cryoprotectants is sucrose instead of half trehalose in the original formula and with the addition of Mannitol.5%,sodium tripolyphosphate1%,HASS 0.01%,Histidine1.5% ,and the appropriate pH is 7.0,the Survival rate is 94.3%. The appropriate formula of lyophilized protective agent is the original formula with the addition of Histidine 1.5%,sodium tripolyphosphate 1.5% ,HASS 0.02% ,L-sodium glutamate 0.5%,and the appropriate pH is 6.0,the Survival rate is 86.6%.Through the infrared spectral analysis,presumably,in the freezing and vacuum freeze-drying process,HASS protection mechanism of cell membrane is HASS combining cytomembrane through hydrogen bond,maintaining their membrane structure and function. |
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