Ginger Milk Curd: How the Milk Coagulates (no magic)

A while back, I discovered that the process of making Ginger Milk Curd depended on enzymes, found in ginger, that break down proteins (proteases). Since then, I've always wondered how an enzyme that breaks down proteins would cause the proteins to solidify. I did a little research and some of inferring. So below are the cliff notes.



I have to note that I didn't find an exact mechanism for ginger, but I did find one for rennet (a set of enzymes produced in the mammalian stomach and often used in industry to produce cheese). After reading some articles and wikipedia, I think the enzymes in ginger and the enzymes in rennet likely work in a similar, but obviously not identical fashion.

So why does milk solidify in the presence of certain proteases?

In cow's milk, the main type of protein found are caseins. Three of these caseins are unstable in water based solutions such as milk. So how does milk remain so uniform and liquid? (At least at the macroscopic level, below is milk magnified 450x. Picture is from The Wonderful Microworld)



If the caseins are so unstable, we would expect milk to be a solid. The insoluble parts would clump together. It turns out that milk remains a homogeneous solution (to the naked eye) because of the fourth type of casein, K-Casein. K-Casein stabilizes the other three and prevents them from aggregating. Precisely how this work is debated. The basic idea consists of a ball of casein surrounded by a protective coating of K-Casein. Since K-Casein is in the way, the other caseins can't lump together. Effectively, K-Casein acts as a stabilizing barrier. There are many proposed models of the specifics, but this post is likely boring as it is, so I won't get into them.

Now, what would happen if we got rid of the K-Casein? The other caseins wouldn't be stabilized and would fall out of solution and aggregate with one another. This is exactly what happens when proteases are added to milk. They destroy K-Casein by breaking it into little pieces. The net result is the formation of a solid... or in our case, a tasty gingery treat! Such proteases (besides the one in rennet) are found in ginger, papaya, figs, pineapple, and kiwi fruits. Papaya Milk Curd, anyone?

Fun fact: Extracts from papaya, figs, and pineapple have been traditionally used to treat intestinal worms. The proteases discussed earlier have been shown to break down the skin of some of these worms. (Please don't try to self-treat worms with pineapple juice.)

For the scientifically inclined:

Casein Functionality from Ohio State University (Food Science 822)

Milk Proteins from Ohio State University (Food Science 822)

Sandra S, Alexander M & Dalgleish DG (2007). The rennet coagulation mechanism of skim milk as observed by transmission diffusing wave spectroscopy. Journal of Colloid and Interface Science 308(2), 364-373.

Stepek G, Behnke JM, Buttle DJ & Ducel IR (2004). Natural plant cysteine proteinases as anthelmintics? Trends in Parasitology 20(7), 322-327.

Su HP, Huang MJ & Wang HT (2009). Characterization of ginger proteases and their potential as a rennin replacement. Journal of the Science of Food and Agriculture 89(7), 1178-1185.