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！

打開一盒酸奶，你常常會發(fā)現有一層液體浮在表面。有的人把它拌入酸奶中，有的人則將這層液體倒掉。這層液體究竟是什么東西，它又是從哪來的呢？

為了揭開酸奶中這層液體的神秘面紗，讓我們往回追溯，去看看制作酸奶的原料牛奶吧！牛奶有兩種不同類型的蛋白質。大部分蛋白質是酪蛋白，其余是乳清蛋白。

單單看一杯牛奶，我們是無法從中分辨出酪蛋白和乳清蛋白的。但是往牛奶中加入特殊的細菌來制作酸奶時，情況就不一樣了。制作酸奶用的細菌會發(fā)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，乳清蛋白則不會凝固，它會從酪蛋白中以液體的形式分離出來。

你在超市買到的酸奶中含有一種穩(wěn)定劑，通常是果膠，它的作用是讓固體的酪蛋白和液體的乳清混合在一起。但是部分乳清仍然從酪蛋白中分離出來，那就是你在打開酸奶包裝時所看到的液體。

乳清不僅含有蛋白質還有維生素B，維生素B包括核黃素，煙酸和硫胺素，這些物質反倒無法在酸奶的固體部分中獲得。因為乳清的營養(yǎng)價值，你最佳的選擇是把它和酸奶攪拌在一起，然后一飲而盡！