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牛乳的尿素氮含量與乳熱安定性之相關
李素珍 張菊犁 吳明哲
2002/12/05 提供
行政院農業委員會畜產試驗所新竹分所利用乳成分與體細胞數合併測定儀Milk
Scan 4000及Fossomatic 5000（FOSS公司，丹麥製）檢測乳脂肪、蛋白質、乳糖、總固形物、尿素氮、檸檬酸含量與體細胞數等。利用Corning
酸鹼度及酸度（Corning pH and ion meter, 英國製）依常法測pH及滴定酸度（中國國家標準，1982）。取定量乳樣封口，37℃預熱後，140℃油浴1分鐘，記錄產生顆粒或凝固之時間（Davies
and White, 1966）。相同乳樣以毛細管電泳法分離乳蛋白質（依Cattaneo
et al., 1996法稍修正之）。等電點4.6使酪蛋白沉澱，測酪蛋白及乳清蛋白質量。測定乳中總鈣量（IDF，1992）。添加凝乳?測定游離鈣量，測定乳中總磷量（IDF，1990）。
乳之pH在6.6至6.8之間，牛乳中尿素氮含量介於5.3∼40.1
mg/dL之間時，不影響乳熱安定性。尿素氮量與αs1-酪蛋白量呈顯著負相關（P＜0.05）。近年許多研究牛乳之蛋白質與尿素氮含量，來評估日糧提供乳牛之粗蛋白質及熱能是否恰當（Jonker
et al., 1998；Godden et al., 2001 b）或評估乳牛之繁殖性能，許多報告認為乳尿素氮含量19
mg/dL為影響母牛繁殖效率的關鍵量，超過者其受孕率低（Quaife,
1994；Butler et al., 1996）。牛乳中尿素氮量會受日糧（Baker et
al., 1995；Jonker et al., 1998；Godden et al., 2001 a；Godden et al., 2001 b）、季節及泌乳期（Carlsson
et al., 1995）之影響，而乳中尿素氮量與乳牛利用日糧中粗蛋白質及熱能之效率有關（Jonker
et al., 1998；Godden et al., 2001 b），且與乳牛繁殖性能有關（Witter
et al., 1999；Melendez et al., 2000），為近年國外研究乳牛營養重視項目之一。Muir
and Sweetsur（1976）認為40 %
牛乳之熱安定性的變異與乳中尿素氮量有關，添加尿素於牛乳中，使乳尿素氮量介於19∼45
mg/dL之間，其熱安定性比未添加尿素者佳，乳中尿素氮含量愈高者熱安定性愈佳。而Banks
et al.（1984）添加0∼10 mg/dL
尿素於乳中，發現含較低尿素氮之牛乳熱凝固時間較短。常乳之熱安定性，除尿素氮外，與其他乳成分均無顯著相關（Holt
et al., 1978）。
參考文獻
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