My Grandmother's Secret Is Bacteria

Every night of my life, my grandmother has saved one spoonful of curd. It goes into a small steel katori with a plate on top, and the katori goes into the corner of the kitchen nearest the stove. In the morning it goes into the warm milk, and by evening there is a new bowl of curd, thick enough to stand a spoon in. I had seen this so many times that I’d stopped seeing it at all.

Then, last December, Ammi went to my aunt’s wedding in Lucknow for a week, and left the curd to me.

I did everything she did, more or less. I boiled the milk. I let it cool, though not as much as she does, because I was in a hurry. I put in the spoonful. And then, because the kitchen corner was crowded, I put the bowl on the windowsill, which in December, at night, was the coldest place in the flat. In the morning I had a bowl of slightly sour, completely liquid milk. The next night I tried again with the milk still very hot, to make up for it, and got the same thing.

I phoned her. She was at a mehendi and could not hear me well, but she understood the problem immediately. “Too cold, then too hot,” she said. “They don’t like either.” Then the line went and I was left wondering who “they” were.

They, it turns out, are bacteria. The spoonful of old curd is full of them, mostly kinds called Lactobacillus and Streptococcus thermophilus, alive and waiting. Put them in warm milk and they start eating its sugar, lactose. They don’t burn it completely, the way we burn food with oxygen; they ferment it, taking a little energy from each sugar molecule and throwing out what’s left as lactic acid. One night’s worth of that waste product is what makes curd sour.

The acid is also what makes it solid. Milk’s main proteins, the caseins, float about in tiny clusters, each one coated with a fuzz of negative electric charge. Like charges repel, so in fresh milk the clusters keep politely apart, and the milk stays liquid. Acid is a supply of hydrogen ions, which are positive, and as the bacteria pour it out they cancel the clusters’ charge bit by bit. Fresh milk has a pH of about 6.6. Somewhere around 4.6, the caseins have lost almost all of their charge, nothing holds them apart any more, and they link up into a soft, wobbly net that traps the liquid inside it. That net is curd.

But bacteria are fussy. They work best somewhere between about 35 and 45 degrees, roughly the warmth of the corner by the stove, where they can divide every hour or so and the crowd keeps doubling. On my windowsill they had slowed almost to a stop, and in milk that was still nearly boiling, the heat had wrecked their proteins and killed them. Even Ammi’s boiling of the milk first, which I had copied without thinking, turned out to matter: it clears out other microbes that would compete with hers, and it loosens the milk’s other proteins so they join the casein net and make the curd thicker. There was nothing wrong with my milk, or my spoonful. I had just given a crowd of living things a terrible place to work.

On the third night I let the milk cool until I could hold my finger in it and count to ten, stirred in the spoonful, wrapped the bowl in a towel and put it back in the corner by the stove. In the morning it had set. It was slightly runnier than Ammi’s, and I ate the entire bowl standing at the counter anyway.

When she came home she checked the corner first, before she’d even taken her shoes off. The katori was there, plate on top, with one spoonful inside. “Good,” she said. And that, from her, is a medal.