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Mozzarella (Fresh Stretched-Curd Cheese)

Name: Mozzarella (Fresh Stretched-Curd Cheese)
Prep time: 30 min
Cook time: 40 min
Total time: 1 h 10 min
Yield: 4 servings
Cuisine: Italian
Ingredients: 4 L whole milk; 6 g citric acid; 2 ml liquid rennet; 2 L water; 40 g salt; 2 tbsp extra virgin olive oil; 0.5 tsp coarse salt; 6 leaves basil

Italy's fresh cheese, set with rennet and then stretched in hot water. Acid-set curd loses the calcium from its casein and turns grainy, while rennet cuts one spot on the casein surface and leaves the calcium inside. That is why this curd can later be worked in hot water until it flows and stretches.

Ingredients

4servings

The curd (makes about 400g)
whole milk · pasteurised; UHT will not set4 L
citric acid · dissolved in 250ml cold water6 g
liquid rennet · diluted in 50ml cold water; animal or microbial2 ml
The stretch
water · at 80°C2 L
salt · for the stretching water40 g
To serve (optional)
extra virgin olive oil2 tbsp
coarse salt0.5 tsp
basil6 leaves

Steps

Cool the milk to 12°C and pour in the dissolved citric acid, stirring for 30 seconds. The acid is not the setting agent here — it takes the milk from about pH 6.5 to about 5.9, which is where the curd rennet later makes will be able to stretch. If the milk flecks or curdles at this stage the acid went in too fast.
⏲ 30 sec
Over a low heat bring it slowly to 32°C, stirring occasionally so the base does not catch. 32°C is where rennet works best; go past it and the curd tightens.
⏲ 7 min
Off the heat, pour in the diluted rennet and stir up and down for 20 seconds only. Then stop touching it. Rennet works by waiting for casein it has already cut to knit together, and stirring while it knits tears the network — douhua is left alone after the pour for the same reason.
⏲ 20 sec
Cover and leave 5–10 minutes. It sets as one mass. Slide a finger in at an angle and lift: when it splits cleanly and clear whey runs into the split, it is ready (the clean break). If it still smears like porridge, give it another 5 minutes.
⏲ 10 min
Cut a 2cm lattice right down to the base with a long knife. Leave 5 minutes, then stir very gently while bringing it to 41°C. Whey runs out of the cut faces and the pieces shrink and firm.
⏲ 10 min
Drain the pieces in a colander. Do not throw the whey away — heated to 80°C it can be used instead of the stretching water, which loses less salt and flavour out of the cheese. Slice the curd 1cm thick into a bowl.
⏲ 5 min
This is where the dish is. Take a small piece, hold it in 80°C water for 20 seconds and pull it between both hands. If it draws out past 20cm like taffy, it is ready. If it snaps, the water is not hot enough to soften the curd: bring it to 85–90°C and try again. With citric acid and no culture the acid dose sets the pH, so waiting will not lower it; if it still will not stretch, use 1–2g more citric acid next time.
⏲ 20 sec
Pour the salted 80°C water over the curd and work it with two wooden spoons, folding and pulling. Stop the moment the pieces have fused into one glossy mass — thirty seconds to a minute. Keep going and fat and water are driven out and it turns rubbery.
⏲ 1 min
Hold the mass in one hand and push a ball up between thumb and forefinger, then twist it off with the other hand (mozzare, to cut off — the name comes from here). Drop it straight into cold brine to fix the shape.
⏲ 5 min
It is at its best the same day, at room temperature. With coarse salt, olive oil and basil it is simply caprese. Keep any leftovers in the brine in the fridge and use within two days.

Variations

Burrata: a mozzarella skin tied around a filling of torn mozzarella and cream. The cheesemaking ends where this recipe ends; the rest of it is stuffing a bag.
Smoked (affumicata): 30 minutes in cold smoke after the brine, colouring the skin only.
For pizza: a wet ball weeps in the oven and soaks the base. For margherita pizza, slice it and drain it in a sieve for 30 minutes first.

More detail and the reasons why (2)

Mozzarella di bufala campana (DOP): the original, made from water buffalo milk, which carries far more fat and protein than cow's milk and gives a whiter, tangier, wetter cheese. The cow's-milk version has a different name in Italy — fior di latte — and that is what this recipe makes.
Letting it go further: in the traditional method, with a starter culture, leave the curd at room temperature for another 4–6 hours without stretching and the pH keeps falling. Past about 5.2 too much calcium has gone and it will not stretch at all, only crumble. With citric acid alone, as here, the acid dose sets the pH and waiting does not lower it.

Tips

The fridge is this cheese's enemy. Cold, the fat firms and the flavour shuts down. Thirty minutes at room temperature before serving (serving temperature).

More detail and the reasons why (6)

There are three ways to set milk, and which one you pick decides what the cheese can do afterwards. Rasgulla uses acid: it neutralises the charge holding the casein micelles apart so they clump, and in doing so the calcium that braced the inside of each micelle leaves with the whey. What is left is grainy and will never flow again. Douhua uses a calcium salt, bridging between soy proteins to stand a whole pot up as one gel. Rennet is the third, and its chymosin cuts one specific spot on kappa-casein, the hairy layer on the micelle surface that keeps them apart. The hairs go, the micelles knit — and the calcium stays inside. That is the only reason a stretchable curd exists at all.
This cheese sits between two states (compare saganaki). An acid-set cheese has almost no calcium bridging left to loosen, so heat never makes it flow. A freshly renneted curd has the opposite problem: too much calcium, so it is rubbery and will not stretch either. Stretching only works in the narrow band between them, and the pull test in step 7 is how you find out whether you are in it.
So pH 5.2 is this dish's doneness. Not a temperature and not a time. Too little acid lost and it snaps like rubber; too much and it crumbles like chalk. A pH meter tells you exactly; a torn-off test piece tells you well enough.
Do not over-stretch it. Stretching aligns the casein into fibres, and going on past that point squeezes the fat and water trapped between those fibres straight out. Glossy and in one piece is the end of the step.
UHT milk cannot be used. At 135°C the whey proteins denature and coat the casein surface, so rennet cannot reach the spot it needs to cut. It failing to set is not a mistake, it is that milk.
Keep the whey. Heated to 80°C it makes a better stretching liquid than water, because less comes out of the cheese. Boil the rest again with acid and you have ricotta — the name means "re-cooked".