Injury Mechanisms And Conditions Optimization Of Lactobacillus Bulgaricus SP1.1in Low Temperture Drying Processes

Lactic acid bacteria (LAB) starter cultures is an indispensable part in dairy industry.LAB starter cultures have been largely processed by vacuum freeze drying. Currently,however, vacuum freeze drying has problems including low efficiency, substantialenergy consumption, and high costs. So spray drying has attracted attention as analternative way. Because of the direct contact with the hot air, the survival rates of LABwere too low. Reducing the drying temperature was proved as an effective method toincrease the survival rates. The suitable drying temperature and effective protectantswere obtained by simulating a low temperature drying process. Studies on the injurymechanisms of LAB during this process has been finished at the same time. Thisresearch could be used in any efforts to realize the LAB starter culture dried by spraydrying.In this study, Lactobacillus bulgaricus sp1.1was dried. Critical water content ofsamples was10%by measuring sp1.1drying curve under37℃,42℃and47℃. Thelowest water content of sp1.1were6.80±0.07%、6.12±0.13%and5.15±0.14%after24hdrying under37℃,42℃and47℃. According to the sp1.1survival rates andcharacteristic of producing acidity, the best drying temperature was37℃. Under37℃drying temperature, lowest water content of sample was6.8%, survival rate of sp1.1was(1.05±0.10)×10-3%, and pH of12%(w/w) skim milk (sp1.1inoculation quantity was1%(v/v)) decreased to4.5after24h fermentation.Under37℃drying temperature, changes of sp1.1(water content is88%,20%,10%,6.8%) extracellular/intracellular enzymes (lactate dehydrogenase, β-galactosidase)activity, membrane permeability/fluidity and cell shapes were measured. Injurymechanisms of sp1.1were get during this process. Before drying to10%(critical watercontent), death of sp1.1was caused by the increasing permeability and decreasingliquidity of cell membrane. Changes of membrane caused lactatedehydrogenase/β-galactosidase enzymes activity decreased inner and increased outer.When bound water was lost, cell membrane began to shrink, cell surface roughnessincreased, cell appeared collapse, even fracture. Due to the cell shrink and membraneviscosity increase, leak of intracellular substances (including intracellular enzyme)became harder.13selected substances were monosaccharides (glucose, galactose and fructose),disaccharides(maltose, lactose, sucrose and trehalose) and mixtures (maltodextrin,acacia gum, peptone, gelatin, beef powder and yeast extract). The results showed the multi-component mixture increased the lowest water content after drying, while thesugars had little effects. Compared with control group, trehalose could reduce the watercontent after drying. Beef powder, yeast extract powder, lactose and trehalose couldincrease the survival rates of sp1.1after drying. Yeast extract, peptone, lactose andfructose could enhance the ability of sp1.1acid production. Among those substances,lactose and trehalose were the best two protective agents. Adding lactose and trehaloseas a protective agent, survival rates of sp1.1were respectively (2.86±0.12)×10-2%and (1.63±0.11)×10-2%, with a water content7.18±1.99%and5.34±0.43%, after37℃drying.