What Happens to Milk?
Milk, as it turns out, is a colloid: just a bunch of fat and protein particles hanging out together in a liquid. It's about 85 to 90 percent water, and in their pure form, milk's proteins do not like each other. They repel each other and evenly spread out throughout the liquid.
事实证明，牛奶是一种胶体：只是一堆脂肪和蛋白质颗粒悬浮在液体中。大约 85% 到 90% 是水，而且在纯净形式下，牛奶中的蛋白质互不相容。它们相互排斥并均匀地分布在整个液体中。
When you leave milk out in temperatures over 40 degrees Fahrenheit, however, it invites the growth of bacteria. Most milk nowadays is pasteurized, but that doesn't completely eliminate bacteria, which can also re-contaminate milk once it's opened.
然而，将牛奶放在40华氏度（大约为4摄氏度左右）以上的温度时，就会滋生细菌。现在大多数牛奶都经过巴氏消毒，但这并不能完全消除细菌，一旦打开，细菌也会重新污染牛奶。
The bacteria start eating up the milk's lactic acid, thereby decreasing the milk's PH. Decreasing the PH not only makes the milk taste sour, but it forces the milk's protein particles to coagulate together into curds. Welcome to the world of spoiled milk.
细菌开始吃掉牛奶的乳酸，从而降低牛奶的PH。降低PH值不仅会使牛奶变酸，还会迫使牛奶的蛋白质颗粒凝结成凝乳。欢迎来到变质牛奶的世界。
Opened milk will spoil within five to seven days, whether you leave it out on the counter or not. Even if you leave it unopened, fresh milk will spoil between five to 10 days. Ultra-pasteurized milk lasts between 30 to 90 days unopened but still spoils eventually.
无论您是否将其放在冰柜里，已开封牛奶都会在5-7天内变质。即使未开封，鲜奶也会在5-10天内变质。超巴氏杀菌牛奶未开封可保存 30 至 90 天，但最终仍会变质。
The long and short of it is that if you leave your milk out, you're hurrying up an inevitable process. And rotten milk won't always smell off to people.
总而言之，如果你忘记喝完牛奶，就会加速一个不可避免的过程。而腐烂变质的牛奶味并不能及时让人闻到。
Any perishables left out at temperatures of more than 40 degrees Fahrenheit for over two hours. If you've left your milk sit out for more than one hour at temperatures at or over 90 degrees Fahrenheit, it's not safe to drink.
任何易腐烂的物品在华氏40度以上的温度下放置超过2个小时就会开始变质。如果牛奶在华氏90度（约为32摄氏度）或以上的温度下放置超过1小时，则饮用这些牛奶是不安全的。
2026年3月7日于东原璞阅
