酸奶中的神秘液体

When you open a box of yogurt， youll often see liquid floating on the top. Some people mix it back into the yogurt， and others drain it off. What is it， and where does it come from？

To solve the mystery of the liquid in yogurt， lets take a few steps back to the milk that yogurt is made of. Milk has two different types of proteins. Most of the proteins are casein proteins， and the rest are whey proteins.

If you look at a glass of milk， you cant tell the casein from the whey. But that changes when special bacteria are added to milk to make yogurt. Yogurt-producing bacteria ferment the sugars in milk and produce an acid. As a result， the mixture is more acidic than before， and this higher acidity solidifies the casein proteins. The whey proteins， on the other hand， dont solidify， and separate out from the casein as liquid.

Yogurt you buy in the store has a stabilizer， usually fruit pectin， to keep the solid casein and liquid whey mixed together. But some of the whey still separates out from the casein， and thats what you see when you open the yogurt container.

Whey contains not only proteins but also B vitamins， including riboflavin， niacin， and thiamin， which cant be found in the solid portion of the yogurt. Because of the nutritional value of whey， your best bet is to mix it back into the rest of the yogurt， then gobble it down！

打开一盒酸奶，你常常会发现有一层液体浮在表面。有的人把它拌入酸奶中，有的人则将这层液体倒掉。这层液体究竟是什么东西，它又是从哪来的呢？

为了揭开酸奶中这层液体的神秘面纱，让我们往回追溯，去看看制作酸奶的原料牛奶吧！牛奶有两种不同类型的蛋白质。大部分蛋白质是酪蛋白，其余是乳清蛋白。

单单看一杯牛奶，我们是无法从中分辨出酪蛋白和乳清蛋白的。但是往牛奶中加入特殊的细菌来制作酸奶时，情况就不一样了。制作酸奶用的细菌会发酵牛奶中的糖并制成酸。因此，混合物比之前更酸，更高的酸度使得酪蛋白凝固。而另一方面，乳清蛋白则不会凝固，它会从酪蛋白中以液体的形式分离出来。

你在超市买到的酸奶中含有一种稳定剂，通常是果胶，它的作用是让固体的酪蛋白和液体的乳清混合在一起。但是部分乳清仍然从酪蛋白中分离出来，那就是你在打开酸奶包装时所看到的液体。

乳清不仅含有蛋白质还有维生素B，维生素B包括核黄素，烟酸和硫胺素，这些物质反倒无法在酸奶的固体部分中获得。因为乳清的营养价值，你最佳的选择是把它和酸奶搅拌在一起，然后一饮而尽！