Dahi, lassi and milk.

Milk: what the pack tells you, and what it does not

There are two different products sold as milk and they behave nothing alike. Chilled pasteurised milk has been heated just enough to knock back spoilage organisms, so it lives in a chiller and has a short life. UHT milk in a carton has been heated much harder and sealed sterile, which is why it sits on a shelf for months — but the moment you open it, it is ordinary milk again and belongs in the fridge on the same short clock as any other.

Kachi doodh from a dodhi is a third case, and the household habit of boiling it before use exists for good reason. Boiling shifts the flavour slightly — it is why chai made from boiled milk tastes rounder — and it also thickens the milk a little as water evaporates. Shop-bought pasteurised milk does not need boiling, though plenty of families boil it anyway out of habit and for the taste in chai.

Two things to check at the shelf: that the pack is genuinely cold to the hand rather than merely in a cold cabinet, and that the carton is square. A carton that has gone slightly barrel-shaped is under pressure from inside, and pressure inside a sealed pack of milk is never good news.

Dahi: why it weeps, and when it has actually gone

A pool of thin liquid on top of a tub of dahi is whey, and it is completely normal — it separates out as the set relaxes and as the tub gets jostled. Pour it off if you want a thicker dahi for raita, or stir it back in for lassi. What is not normal is a sharply sour or yeasty smell, a fizzing when you stir, a pink or grey patch, or fur of any colour. Those go in the bin, tub and all.

Full-fat dahi behaves better in cooking than a low-fat one, which splits more readily when it meets heat. To put dahi into a salan without it curdling, whisk it smooth first, take the pan off the heat, stir a spoon of the hot masala into the dahi rather than the other way round, then add it back and keep the flame low. A spoonful of besan whisked into the dahi also helps hold it together.

Strained yogurt, sold as Greek-style, is dahi with much of the whey drained out. It is thicker and less watery, which makes it useful for a raita that has to sit out on the table, and for marinating gosht where you do not want extra liquid loosening the masala.

Setting dahi and making lassi at home

Home-set dahi needs a spoon of live dahi as a starter, milk warmed to about blood heat, and somewhere warm to sit undisturbed. In a Lahore summer that is six hours on the counter under a cloth; in January it may want ten and a wrapped towel. The moment it sets, move it to the fridge — every extra hour warm makes it sourer, and dahi that has over-set past sour will not come back.

Lassi is dahi loosened with chilled water and churned until it froths. Roughly two parts dahi to one of water for a thick sweet lassi, more water for a drinking namkeen one. Sweet lassi wants sugar dissolved before the ice goes in; namkeen lassi wants salt, roasted zeera crushed fine, and a pinch of kala namak. Fresh lassi separates within a few hours, so make what you will drink, keep it covered and cold, and stir it again before pouring.

How long each keeps once it is opened

The dates printed on dairy assume an unopened pack in a properly cold fridge. Once you open something, the clock resets to a shorter one, and one habit changes it more than any other: always use a clean, dry spoon. A spoon that has been in something else, or is still wet from the sink, introduces exactly what dahi is best at growing.

• Chilled milk, opened: roughly two to three days, kept on a middle shelf and not in the door.
• UHT milk, opened: the same short life as chilled milk, whatever the date on the carton says.
• Dahi: about a week from opening, with a clean dry spoon every time and the lid back on.
• Fresh lassi: a same-day drink. It separates, and stirring only partly brings it back.
• Paneer: two or three days, submerged in water that you change daily.
• Hard cheese such as cheddar: weeks, wrapped in paper inside a loose bag so it can breathe without drying.

Where dairy goes in the fridge

The moulded bottle rack in the fridge door is the worst place in the fridge for milk, because the door warms every time it opens and swings back and forth all day. Put milk and dahi on a middle shelf towards the back, keep lids on properly, and keep dahi away from anything strongly aromatic — dairy fat picks up smells with startling efficiency, and a tub of dahi stored next to cut onion will taste of it by morning.