By Richard E. Bleil, Ph.D.
Full disclosure, I am not the best at remembering to put out the garbage. Since I live alone, I don’t generate that much garbage anyway, so it can be many weeks between times that I actually need to take it out. This can be a problem, especially if you have old milk.
The process of rotting milk actually kind of fascinates me. It begins with a lumpy solid precipitate. Eventually it separates between the solids on the bottom and a yellowish liquid on top, and finally, through the action of microbes feasting on the bad milk, gases begin to accumulate, and the plastic bottle begins to bloat.
I especially enjoy it when I have friends over who get very (VERY) disgusted with the rotten milk still in my refrigerator. I even make it a point to keep it (for MONTHS) just to mess with their minds. When they ask why I don’t throw it away, I argue that I don’t want to put up with the smell to pour it down the drain, which, honestly, is the truth, but I still keep it just for them.
A lot of people believe that milk is white because of the calcium content. This seems reasonable; calcium carbonate is a white solid (think of white chalk or calcium carbonate based antacid tablets), but the reality is that the calcium in milk is a dissolved salt. It has no more color than a weak salt solution (think of contact lens saline solution). In fact, the reason that milk is white is because of a protein called “casein”. If you were thinking “albumin”, that is the protein in egg whites; milk is casein.
Proteins do work. For example, the protein “calbindin” binds to calcium and transports it though cell membranes. To the best of my knowledge, the only job for casein (or albumin) is to provide a food source (easily digestible I’m sure) for developing chicks in eggs and calves.
Now, proteins have two states. In the “native state” (very low entropy), the configuration of the protein is highly regulated, very specific, with almost no variation. The protein is capable of doing its work only when it is in the native state. If a protein is not in its native state, it is “denatured” (the “denatured state”). When denatured, the protein is in very high entropy, and it can take on nearly any structure.
The native state is very fragile, and the protein can be denatured by many means. For example, albumin (egg white protein) denatures readily with heat. This is why egg whites go from liquid and clear prior to cooking, to a white solid during cooking. The cooked egg white is denatured albumin.
The lumps in expired milk (the solid precipitate) is actually denatured casein. This casein is denatured because the milk has gone acidic (which is why rotten milk tastes sour; acids tend to have a sour taste, like vinegar or citric acid in lemons). Another ingredient in milk is a sugar called lactose (a source of great consternation for the lactose intolerant). Lactose will oxidize, and become lactic acid. The accumulation of lactic acid in muscles is the cause of sore muscles, but also acidifies milk. As the pH of the milk drops because of acidification via lactic acid, the casein leaves its native state, and becomes a solid in its denatured form.
As an interesting side note, my friend from India tells me that many milk-based deserts in India are based on curdled milk, because often refrigeration is lacking and milk lasts longer once it is curdled. The process of curdling involves adding (often) lemon juice, wherein the citric acid drops the pH of the milk (acidifies it) causing the casein to solidify and precipitate out. The resulting mixture is then strained through cheesecloth, and becomes the basis for sweets by adding syrups and other flavorings.
Today, there are people who use recently curdled milk in cooking. The claim is that once you cook out the sour flavor (lactic acid), the curdled milk is very similar to buttermilk. I don’t know that I recommend trying this, as I do not know how long milk is still useful after it has curdled, but, at least now, if you hear of this, you’ll understand why this should be so.
And it’s all because you stuck with me!