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Butterfat is the natural fat that already lives in milk. When cream is churned for a long time, that fat clumps together and becomes butter — so butterfat is the part of milk that can turn into butter. You meet the word on food labels and in farming or nutrition talk: milk with more butterfat tastes richer; skimmed milk has had much of it taken out. It is slightly technical kitchen-science English, not what you say at breakfast (people just say the milk is rich or creamy). Milk fat means the same thing. You do not add butterfat to milk; it is already there.
乳脂（牛奶裡本來就有的脂肪，不是後來加進去的奶油）
原始詞典資料1 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
butterfatnoun/ˈbʌtəfæt/
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The fatty components of milk and other dairy products.
- The first change which milk undergoes in the stomach is coagulation. The casein and butterfat separates from the serum, which is soon absorbed unchanged into the blood, according to the laws of diffusion. [Quoting Wagner's Manual of Physiology.]
- By submitting pure solid butter-fat to powerful pressure in a linen cloth at the temperature of 60°, a slightly yellow transparent oil will flow out, and a solid white fat will remain behind; the first is called oleine or butter-oil, and the second margarine; and of these two substances the pure fat of butter almost entirely consists, although, as may be supposed, their relative proportions are liable to great variations.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。