Study On The Influence Of Ice Crystal Formation And Water Migration On The Lactobacillus Viability In Frozen Sourdough

As one of the ancient fermentation techniques,fermented products of sourdough are very popular.Compared with yeast fermentation,sourdough fermentation can endow the product with the characteristics of acidification,bacteriostasis,nutrition and rich flavor.However,due to the complex microbial strains in the system of sourdough,it is not easy to keep stable storage.At present,low-temperature freezing technology is more and more widely used in food.The combination of sourdough fermentation technology and low-temperature freezing technology greatly extends the fermentation characteristics of sourdough.However,during the freezing and freezing process of sourdough,microorganism in the system will be damaged by"mechanical damage"and"solute damage",which will reduce the activity of microorganism in the sourdough.Therefore,studying the changes of moisture and ice crystals during frozen storage of sourdough and its effects on the vitality of lactic acid bacteria can provide new ideas for maintaining fermentating capacity of traditional sourdough.Lactobacillus plantarum is one of the dominant bacterial groups in sourdough.In this paper,the metabolic characteristics of Lactobacillus plantarum M616 was studied,which lays a theoretical foundation for its application in sourdough.And studies effects of several freezing and frozen storage conditions on the viability of lactobacillus.The ice crystal formation and water migration during the freezing storage process affected the vitality of lactobacillus was investigated,at the same time.Finally,frozen sourdough was used to prepare fermented dough,and its fermentation characteristics were analyzed.The main conclusions were as follows:1.The growth curve,acid production curve,acid resistance,alkali resistance and bile salt resistance of Lactobacillus plantarum M616 were studied,and its antibacterial properties were studied.The results showed that Lactobacillus plantarum M616 were short rod-shaped.Fermented in 37^oC MRS shake flask for 30h,the p H reached 3.89;when cultured under the conditions of p H 2 and 3,the 6-hour OD₆₀₀values were 0.03 and 0.05,respectively;at the mass concentration of bile salt was 0.3 g/100 m L,the 4-hour OD₆₀₀values were 0.03;when the mass concentration of Na Cl more than 8%,the strain stopped growing.In terms of bacteriostasis,the strain had inhibitory effects on Escherichia coli,Staphylococcus aureus,and Listeria monocytogenes,and the diameter of the bacteriostatic circle reached 14 mm,15mm,and 17 mm,respectively.2.Taking Lactobacillus plantarum in frozen sourdough as the research object,the viable cell count and acid production capacity were used as the main indicators to evaluate its vitality,The effects of freezing storage time,freeze-thaw cycle,freezing rate,amount of water and antifreeze on the vitality of lactobacillus were investigated respectively,and the optimal freezing storage conditions for sourdough were optimized through response surface experiments.The results showed that he five frozen storage factors have significant effects on the viability of lactobacillus.There was a linear relationship between frozen storage time and freeze-thaw cycles and the viability of lactobacillus.The survival rate and acid-producing capacity of lactobacillus decreased remarkly with the prolongation of frozen storage time and the increase of freeze-thaw cycles.Effects of freezing rate,amounts of water,and GSH addition level on the viability of lactobacillus showed nonlinear.The optimal freezing and frozen storage conditions for maintaining the viability of lactobacillus in frozen sourdough were freezing rate 13.00 mm/h,amounts of water 45%,and GSH addition level 0.35%,which was optimized by Response Surface Method.On the selected condition,the acid-producing capacity and survival rate of lactobacillus were 1.90 and 7.8×10⁷,respectively,which indicated lactobacillus having fermentative capability.3.DSC and NMR were used to analyze the changes in the content of freezeable water and the distribution of free water and bound water in the sourdough system with different freezing time,respectively;CLSM was used to explore the changes of the ice crystal structure during the frozen storage of sourdough;and with viability and acidified capacity of lactobacillus in the frozen-stored sourdough were analyzed for correlation.The results showed that with the prolongation of frozen storage time,the number of viable lactobacillus showed a decreasing trend,the p H value of the sourdough increased and the TTA decreased,which indicated that the acidified capacity of lactobacillus was weakened;the water loss rate of the sourdough increased with the prolongation of frozen storage time,the freezable water content and the free water content was reduced.CLSM results showed that the number of small cavities in the dough gradually increases,and larger cavities were gradually formed with the prolongation of frozen storage time.Correlation analysis found that the number of lactobacillus viable was extremely negatively correlated with the total water loss rate of sourdough（p=0.000,R²=-0.996）,and the p H value and free water content were extremely significantly negatively correlated（p=0.000,R²=-0.966）.It can be seen that the total water content and free water content of frozen sourdough had a significant impact on viability and acidified capacity of lactobacillus4.Sourdough,frozen sourdough,and remove-water frozen sourdough were used to prepare fermented dough.And the characteristics of the refermented dough were analyzed.The results showed that the acidity and reducing sugar content of frozen sourdough fermented dough was greater than that of remove-water frozen sourdough fermented dough but less than sourdough fermented dough.The texture profile analysis results showed that the hardness of frozen sourdough fermented dough was less than that of remove-water frozen sourdough fermented dough,but the adhesiveness and springiness were higher than that,which showed that frozen sourdough maintained better fermenting power.