Study On Effect Of Heat Resistant Lipase On UHT Milk Quality

As a convenient and nutritional food, UHT(ultra high temperature) milk is very popular to consumer. Its consumption increases quickly and largely in China recently. But the fat separation and fat decomposition happen often because of the residual lipase in the UHT milk during the whole storage, this result in the deterioration of its flavor and quality. So it is necessary to research on heat-resistant lipase and its effection to shelf-life of UHT milk, which provide academic foundation for control measures.This study includes four parts. Isolation, purification and identification of psychrotrophic bacteria from raw milk, the correlation between psychrotrophs counts and lipase activity, the dynamic rule of lipase producing of psychrotrophic bacteria and the construction of UHT milk shelf-life prediction model.Ten psychrotrophic bacteria were isolated from raw milk, four strains were identified by API 20NE identification system: Pseudomanas fluorescens, Pseudomonas putida, Aeromonas hydrophia/caviae and Chryseomonas luteola. The ID and T of Pseudomanas fluorescens are 99.8% and 0.94. Four strains were purified by ammonium sulphate conten and dialyse and heat-resistent experiment. Finally, we chose Pseudomanas fluorescens as target psychrotrophic bacteria to use in following experiment.By studing on the correlation of psychrotrophs counts and lipase activity, we found that the psychrotrophs counts were positively related to lipase activity, regression equation is y=0.0133x+1.069, R²=0.7127.By studing on dynamic rule of lipase-producing of Pseudomanas fluorescens through the single factor experiments and L9(3⁴) orthogonal design, the results indicated the optimum parameter for producting lipase were: pH 7.0 , temperature 30℃and time 30h, fructore 1.0%, soybean powder: NaNO3 4:1, 0.15 % CaCl2, 0.08% Tween-80.Analysis of variance of showed that the impact factors followed order: the effect of soybean powder: NaNO3 was largest, followed by fructose and surfactant ,the last was inorganic salt.Through the quadratic orthogonal rotational design, by considering SCC(X₁), lipase activity(X₂) and temperature(X₃) as independent variable, we obtained regression equation:Y=53.94-3.46X₁-6.56X₂-3.52X₃+0.89X₁²+2.67X₂²+0.19X₃²-2.75X₁X₂+1.50X₁X₃+2.0X₂X₃. The effect of three factors on shelf life was significant. Analysis of variance of showed that the impact factors followed order: X₂(p=0.0002),X₃(p=0.0160),X₁(p=0.0173). Proteolysis, the middle diameter of fat globule on the surface increased and pH value decreased during storage. Free fat acid was analyzed by GC during storage the results showed: the amount of myristic acid, palmitic acid, stearic acid, oleic acid and linoleic acid increased in different extent. The results showed that the inflence of SSC on the decomposition of UHT milk fat was significant(p<0.05), and the inflence of psychortrophic bacteria count on the decomposition of UHT milk fat was extremely significant(p<0.01) through the quadratic orthogonal rotational design.