Proteases are fun! A protease is an enzyme that breaks down protein by destroying the linkages between amino acids. Proteases destroy those linkages, which are called peptide bonds, via hydrolysis, in which the oxygen of one of our cells' oh-so-abundant water molecules stuffs its way into a chemical bond, booting off one of the components (in not-quite-accurate layman's terms, that is, but it'll do). One of the more interesting things about many proteases is the catalytic triad, or a trio of amino acids that work together to catalyze, or speed up, this hydrolysis. Their chemical and biological mechanisms are pretty nifty (particularly the classic serine-histidine-aspartate triad), but what is even cooler is that species all over the genetic map possess proteases with catalytic triads in a beautiful example of convergent evolution. What this means is that the catalytic triad arose independently about two dozen times because of the sheer chemical elegance of those arrangement(s). Even better, some species of protease-packed plants are edible.
Kiwi, papaya, and pineapple are some of the most notable protease-containing foods; nota bene that none of them can be set in gelatin unless canned or otherwise preserved first, since the peptide bonds that give gelatin its structure will rapidly be destroyed. Industrially, the papaya protease--conveniently named papain--is used as a meat tenderizer. Ficin, a protease derived from the fig tree itself rather than the fruit, can also do this.
"Yes, Hannah," you're saying. "I know all about papain, and pineapple's ability to dissolve gelatin has been the bane of housewives since the 1950s." Fine. But did you know that everyone's favorite rhizome also contains a protease?
Ginger protease, or zingibain, can actually be used to make a milk pudding (or cheese, but that's another story). Here's how it works: normally, milk proteins called caseins aggregate into balls called micelles. Micelles of any sort have hydrophilic, or water-friendly, components on the outside, and hydrophobic components pointing inward. In this case, kappa-casein (heretofore written as K-casein) is on the outside of the micelles. Zingipain cleaves K-casein (specifically, at a proline residue), leaving a hydrophobic component called para-K-casein behind... and all of a sudden, those beautiful micelles stick together, and the milk sets.
Unfortunately, zingibain is a fairly tim'rous beastie. If heated above 150 Farenheit, it denatures--or loses its structure--almost completely, per this paper. It also degrades fairly quickly; the half-life is only 20 minutes at about 85 degrees Farenheit, but according to this paper, the addition of simple ascorbic acid--also known as vitamin C--dramatically increases that half-life. The science of that phenomenon is adequately explained in both the citations above.
That first paper also mentions that zingibain has its peak activity in a very narrow temperature window of 60 to 65 Celsius... and yet I saw so many recipes for this that called for milk to be "nearly simmering" or something like that. Sheesh. Break out the candy thermometer!
Ginger milk pudding
325 mL milk
40 g sugar
37 g ginger juice
In a small saucepan, mix the milk and sugar. Heat gently; you want to add the ginger juice at a target temperature of 145F. Meanwhile, grate the ginger (I used my delightful microplane) and squeeze out the juice using your hands, cheesecloth, or a strainer. When the milk and sugar are at the appropriate temperature, pour the ginger juice in over the surface of the milk and do not stir. Allow to set for about seven minutes, and enjoy the sweet taste of science.