| The relationship between lipase Ⅷ from pseudomonas species in the raw milk and fat separation of UHT milk during storage, and the measures to lower the activity of the enzyme and fat separation were studied. By analyzing the change of lipase Ⅷ activity from psedomonas species in the raw milk under cold storage condition, it was found that the lipase Ⅷ activity increased as the numbers of psychrotrophic bacteria increase, the correlation coefficient was 0.7469. The result also showed that there was remaining lipase activity in the milk after the treatment by UHT. By adding sterile lipase Ⅷ into the UHT treated milk and following the fat separation extent of the milk during storage, it was found that fat separation extent was closely related to the content of lipase Ⅷ added. The higher of lipase content, the severe the fat separation extent of milk (R~2=0.9583). The feature of lipase Ⅷ was also studied. It was found that pH had big influence on lipase activity. When pH was 6, the lipase activity was significantly lower than when its pH was 10 (P < 0.01). To lower the lipase Ⅷ activity, both low temperature treatment at 50℃,55℃,60℃ for 20 minutes and high temperature treatment 100℃,110℃,120℃,130 ℃ and 140 ℃ for 10 minutes were effective (P < 0.01), and addition of Mg2+ or Fe2+ could also reduce the activity of lipase significantly(P < 0.01). Based on the above results, orthogonal array method was used for experiment design to optimize the low ～ heat treatment conditions for inactivating the lipase Ⅷ added to UHT milk and for inhibition of fat separation of UHT milk. The result showed that theoptimium condition to reduce the fat separation of UHT milk was low temperature heat treatement at 55℃for 15 minutes, and the optimum emulsifier to delay fat separation of UHT milk was a complex emulsifier (mixture of mono～glycerol fatty acid esters and sucrose fatty acid esters) with the HLB at 12 or 14 at the dosage of 0.1%.
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