牛乳中之乳脂肪球係由乳脂肪球膜（milk fat globule membrane; MFGM）包覆其中心之三酸甘油脂（triglyceride）所構成，而此生物膜中蛋白質約佔總組成份之70%，其中某些蛋白質對乳脂肪球之分泌與穩定極為重要。然而，牛乳加工過程中之均質處理會明顯改變乳脂肪球膜之構成，而且酪蛋白與乳清蛋白亦會成為新的乳脂肪球膜外層之主要成分，並可能在人類胃腸消化過程中導致脂肪酶（lipase）之分解作用不完全。本研究之目的在於探討生乳、HTST鮮乳以及均質HTST鮮乳進行體外模擬消化試驗中，於胃蛋白酶（pepsin）及胰蛋白酶（trypsin）存在與否之條件下檢測胰腺酶（pancreatic lipase）對乳脂肪之水解情形。由胰腺酶配合胰蛋白酶之體外模擬消化試驗結果顯示，不論胰蛋白酶存在與否，均質HTST鮮乳所釋出之游離脂肪酸濃度均高於生乳及HTST鮮乳，但經模擬胃蛋白酶消化之結果則無顯著差異。經胃蛋白酶消化後之生乳、HTST鮮乳以及均質HTST鮮乳於解脂作用前期之乳脂水解率均會提高，再添加胰蛋白酶則無法更進一步提高乳脂消化前期之水解率。於含有胰蛋白酶之胰腺酶消化試驗中，HTST之鮮乳和均質HTST鮮乳於解脂作用前期之乳脂水解率均高於無蛋白酶處理之組別。綜上所述，胃、腸之蛋白酶於體外乳脂肪消化過程中確實扮演一輔助角色，可提高牛乳脂肪於消化初期之水解率。
Milk fat globules are composed of a triglyceride core and a natural biological membrane, milk fat globule membrane (MFGM). Proteins account for 70% of MFGM. Certain of those proteins are important for the secretion and structural stability of milk fat globule. However, composition of MFGM is restructured after homogenization, and the newly formed membrane mainly consists of caseins and whey proteins. The aim of this study was to examine the lipolysis of milk fat in raw milk, HTST milk, and homogenized HTST milk by pancreatic lipase during in vitro digestion, presence of pepsin and trypsin or not. The results of in vitrolipase-trypsin digestion indicated that whether presence trypsin or not, homogenized HTST milk showed higher level of free fatty acids release than raw milk and HTST milk. Releasable free fatty acids were not significantly different among raw milk, HTST milk, and homogenized HTST milk on pepsin presence digestion. During in vitro digestion, the hydrolysis rate of milk fat in raw milk, HTST milk, and homogenized HTST milk increased on early stage of lipolysis after pepsin digestion. After pepsin digestion, however, trypsin treatment could not further increase milk fat hydrolysis rate on in vitro digestion early stage of lipolysis. Hydrolysis rate of homogenized HTST milk and HTST milk fat on early stage of trypsin presence in vitro lipase digestion were higher than trypsin absence in vitro lipase digestion. In conclusion, gastrointestinal proteases play a coordinative role to increase fresh milk fat hydrolysis on early stage of in vitro digestion.
