In “My Grandmother’s Secret Is Bacteria”, Ammi’s spoonful of yesterday’s curd turns warm milk into new curd overnight, a cold windowsill stops it dead, and milk that’s too hot kills it. On the phone from a mehendi, she called the bacteria “they”, as if they were lodgers. She was more right than she knew. “They” are not one kind of bacterium but a household, and the way they live together is one of the neatest partnerships in the kitchen.
In most curds there are at least two main types. One, Streptococcus thermophilus, is made of tiny spheres that grow in chains like strings of beads. The other is a Lactobacillus, rod-shaped and longer. Commercial yoghurt is usually made with exactly these two. Home-made dahi is often a richer mix, with other relatives of theirs joining in, but the core idea is the same.
Both live by fermenting lactose, the sugar in milk. Lactose is two smaller sugars joined together, glucose and galactose, and the first thing the bacteria do is split it apart with an enzyme called lactase. That is the same enzyme most babies make to digest milk and many adults stop making, which is why some people feel unwell after a glass of milk. Then the bacteria break the glucose down, step by step, collecting a little energy along the way, and release lactic acid as the end product. They can’t use oxygen to burn the sugar completely, so this is their whole livelihood: a little energy from each sugar, and a lot of acid as waste.
Here’s where the partnership comes in. When the spoonful goes into warm milk, it’s the round Streptococcus that gets going first. It grows fast, uses up the little oxygen in the milk, and releases small amounts of substances, including formic acid and carbon dioxide, that the Lactobacillus happens to need to grow well. The Lactobacillus, in turn, is better at chopping up milk’s proteins into small pieces, which the Streptococcus can’t easily do for itself and needs as food. Each feeds the other. Together they acidify milk much faster than either can alone. Microbiologists have studied this pairing for decades as a textbook example of two species cooperating.
They also take turns. As the acid builds and the milk’s pH falls, the Streptococcus slows down; it doesn’t cope well with very sour conditions. The Lactobacillus is tougher and carries on, pushing the acidity further. That’s why curd left too long in a warm kitchen gets sharper and sourer: the Lactobacillus has taken over the late shift. Somewhere along the way, around a pH of 4.6, the milk’s casein proteins lose the charge that kept them apart and set into a gel, exactly as the story described.
The acid does one more important job. Most of the bacteria that make people ill grow poorly in sour conditions, so once the curd bacteria have made the milk acidic, they have effectively locked the door behind them. The milk has been claimed. This is why curd keeps longer than milk, and why fermenting has been a way of preserving food for thousands of years.
Which brings us back to the windowsill and the too-hot milk. Each of these bacteria has a comfortable range. S. thermophilus, as its name suggests, likes it warm, and both work best somewhere between about thirty-five and forty-five degrees. On a December windowsill, their enzymes slowed almost to nothing, so nothing much happened. In milk close to boiling, the heat unravelled the proteins they’re built from, and they died. The narrator’s eventual trick, letting the milk cool until a finger could stay in it for a count of ten, landed it right in their comfortable range.
And Ammi’s katori? A home starter kept going night after night for years is a living culture, and like any population it can shift. The balance between species changes a little with the seasons, the milk and the warmth of the kitchen, so every family’s curd is slightly different from every other’s. In a sense, the spoonful she saves each night is an heirloom: a small, invisible community of lodgers, passed down from batch to batch, that has been working the night shift in her kitchen for longer than the narrator has been alive.