Why does milk curdle when you add lemon juice to it? It also curdles when it goes sour, so I guess there’s something similar going on, but what?
First, you have to understand what milk is. Milk is mostly water, with some dissolved sugars, suspended fat (“butter’’ globules), and, most importantly for this question, various proteins.
The proteins are mostly of a kind called “caseins,’’ which have two discernable ends: One likes to be surrounded by water and hates to be surrounded by fats, and the other end is the opposite. In milk these proteins form little spheres called “micelles,’’ with the water-hating sides all facing in and the water-loving sides facing out (into the water).
There’s just one more thing to know - each protein end sticking into the water grabs electrons and winds up with a negative electric charge. All the little spheres are then negatively charged and repel each other, which stops them from clumping together. Milk stays as milk.
Now an acid is something that puts positively charged hydrogen atoms into water. If you add acid (lemon juice, vinegar, . . . whatever) to milk, the positive hydrogen atoms get attracted to the negative micelles, making them neutral. Now there’s no force to push the micelles apart, and eventually they hit each other and stick together, making a tangled mess of protein called curds. When milk goes sour, bacteria turn the main sugar in milk - lactose - into lactic acid, which does the curdling.
Milk can also be curdled by enzymes, which break the proteins up into fragments that wind up sticking together. In cheese making, the enzymes are often obtained from rennet, a substance made from mammalian stomachs, though for vegetarians there are also plant sources for suitable enzymes.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
