Study On The Fermenting Characteristics Of Lactobacillus In Processing Yogurt

Yogurt fermented by lactobacillus has rich nutrition and health-care functions.L.paracasei,generally used for producing cheese,can tolerate the body's defense mechanism and can abundantly survive in the intestine.With purpose of expanding the application area of L.paracasei,this paper has studied the fermentation performance,flavoring substances and postacidification in yogurt fermented by single L.paracasei,L.bulgaricus,S.thermophilus and compound of L.paracasei,L.bulgaricus and S.thermophilus.The research contents are as follows:The first part,we used compound of L.paracasei,L.bulgaricus,S.thermophilus and single lactobacillus to ferment milk for preparation yogurt,researched the changing trends of acidity values with time,the relationship between viable count of lactic acid bacteria and acidity values,the relationship between viscosity values and acidity values in the process of fermentation.The results showed:L.paracasei and S.thermophilus had mutually catalytic role in the fermentation process.If there was L.bulgaricus,the viscosity value was higher and more stable.When the inoculated ratio of L.paracasei,S.thermophilus,L.bulgaricus were2:1:1,we fermented yogurt 5h then the acidity can reach 71.4°T,viable count can reach1.32×10~8cfu/g,viscosity values can reach 11250mPa.s.It confirmed that it was feasible to ferment yoghurt by the combination of L.paracasei,S.thermophilus and L.bulgaricus when the amount of inoculation was 2×10~6cfu/g,sucrose was 7%and fermentation temperature was40?.The second part we used SPME-GC-MS to analysis the flavoring substances in yogurt which were fermented by L.paracasei,L.bulgaricus and S.thermophilus.Used the compound of L.paracasei,L.bulgaricus,S.thermophilus and single lactobacillus to ferment yogurt,researched the relationship between diacetyl content and acidity values,the relationship between acetaldehyde content and acidity values in the process of fermentation.The results showed:L.paracasei yogurt had 23 kinds of flavoring substances which include 5 kinds of esters,2 kinds of ketones,2 kinds of acids,3 kinds of aldehydes,5kinds of alcohols,and 6 kinds of other chemical compounds;S.thermophilus yogurt had 19kinds of flavoring substances which include 8 kinds of esters,5 kinds of ketones,1 kind acid,1 kind aldehyde,2 kinds of alcohols,and 1 kind other chemical compound;L.bulgaricus yogurt had 20 kinds of flavoring substances which include 7 kinds of esters,5 kinds of ketones,5 kinds of acids,1 kind aldehyde,1 kind alcohol,and 1 kind other chemical compounds.In three kinds of lactobacillus yogurt,diacetyl were all detected out;in L.bulgaricus yogurt,acetaldehyde was not detected out,but it was detected out in the other of two lactobacillus yogurt.When the acidity was greater than 40°T,diacetyl and acetaldehyde synthesized in large quantities which was mid-fermentation.The combination of three kinds of fermentation lactobacillus was more conducive to the synthesis of diacetyl and acetaldehyde.When the inoculated ratio of L.paracasei,S.thermophilus,L.bulgaricus was2:1:1,the content of diacetyl can be up to 22.05mg/L at the end of fermentation.When the inoculated ratio of L.paracasei,S.thermophilus,L.bulgaricus was 1:1:1,the content of acetaldehyde can be up to 18.82mg/L at the end of fermentation.The third part we evaluated the quality by sensory after storage yogurts 12h at 4?,determined regularly the acidity values,viscosity values,viable count,diacetyl content and acetaldehyde content.The result showed:when three lactobacillus mixed fermented yoghurts after 12h refrigerated sensory score were all over 95 points,when inoculated ratio of L.paracasei,S.thermophilus,L.bulgaricus was 1:1:1,sensory score was the highest 97 points.In period of storage,L.paracasei was the main acid producing strain.Each group had a large change in viscosity values,L.bulgaricus played a stabilizing role in its organizational structure during fermentation.After 14d,the using of L.paracasei and L.bulgaricus can make viable count higher than the national standards.But high level of viable count only can maintain a short time,about 4d.after 4d,the viable count reduced sharply.In storage period,the content of diacetyl and acetaldehyde presented a tend of increasing followed by decreasing.After 14d,diacetyl and acetaldehyde content was still in a relatively high level.The combination of three lactobacillus fermented yoghurt had good fermentation performance,yogurt sensory score was higher,but the postacidification is serious.Under the conditions of4?,it would be best to eat yogurt in 4d,in which the quality of yogurt is best.