26.5 Cheese, yogurt and aging

The kinds of cheese and yogurt, and how to buy and use them, are covered with dairy. see chapter 14: Kinds of cheese

How acid and rennet set milk protein is covered with the other proteins. see chapter 7: Milk and soy

Where they came from

There is a story that desert nomads first made cheese by carrying milk in bags made from goat stomachs, where the rennet inside curdled it. Plausible as it sounds, the story is a myth. A similar story is told of yogurt: herders kept milk in bags made of animal gut, enzymes from the gut soured and thickened it, and the soured milk kept longer.

Cheese making goes back to Mesopotamia, around 8,000 years ago. By about 2000 B.C.E. most milk was drunk fermented, as something like a thin yogurt. Land that suited goats and sheep did not suit cows, so regions grew their own styles. In medieval Europe, religious houses made cheese both to eat in place of meat and to sell.

What is in milk

Milk is mostly water, with four main components:

• the sugar lactose
• fat
• the protein casein
• whey proteins

Casein floats in milk as tiny clusters, each made of hundreds of casein molecules held together with calcium. These clusters are called micelles.

Making cheese means drying milk down. Casein, fat and minerals end up 6 to 12 times as concentrated as in the milk, depending on the cheese. Most cheeses go through four steps:

• Making acid.
• Setting the curd.
• Removing water.
• Salting.

Three ways to set a curd

The curd forms when the casein clusters are made to clump together. There are three ways to do it.

• Rennet. Rennet holds an enzyme, chymosin, that snips one of the casein proteins, kappa-casein, so that part of it drifts away from the cluster. Without it, the clusters stop pushing each other away and stick together, with calcium acting as a bridge between them. A gel forms, the curd firms, and liquid seeps out. Cheddar and gouda depend on this.
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• Acid. Acid also makes the clusters clump. Lactic acid bacteria can supply that acid naturally. Cottage cheese, quark and cream cheese are made this way. An acid curd is more fragile than a rennet curd, because it loses its calcium.
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• Heat and acid. Heat changes the shape of the whey proteins so they join the casein, and then acid brings both out of the liquid together. Up to 90% of the protein is recovered. Paneer, ricotta and queso blanco are made this way.
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Whey proteins do not clump with rennet or acid alone, so they mostly drain away with the liquid.

In the kitchen, ricotta, mascarpone and crème fraîche (a thick soured cream) are set with acid at high heat and need no bacterial culture. They are known as "quick" or "kitchen cheeses".

Rennet is found in the stomachs of ruminant animals. It is sold as liquid, powder or tablets, and there are plant, animal and synthetic kinds. Liquid rennet from calves must be kept refrigerated; farmer's cheese and cheddar can both be set with it.

Raw milk or pasteurized

Raw milk is milk straight from the animal that has not been pasteurized. In the United States, some states allow raw milk products and some do not, so check the rules where you work. Raw milk is much richer in flavor and creaminess. Use it at once, ideally still warm; U.S.D.A. guidelines require raw milk to be processed within three days of milking.

Milk, and anything containing it, is a potentially hazardous food. The general rule for raw foods:

• Many carry pathogens.
• You cannot tell by look, taste or smell.
• Use processed or pasteurized versions wherever possible.
• Never serve such foods to high-risk customers.

A small cheese kitchen

Enough equipment to make at least six cheeses costs around five hundred dollars to start. Much of it is already in a working kitchen.

• A double boiler, or a saucepan set in a stockpot, and an accurate thermometer.
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• A curd knife; a flat cake-icing spatula does the job.
• Cheesecloth for straining, and plastic draining mats; sushi mats work.
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• Stainless steel bowls and ladles, and plastic tubs with lids for ripening cheese in the cold room.
• Cultures and molds to suit the recipe, such as a mesophilic culture, Penicillium candidum and Roqueforti.
• Calcium chloride, which makes the curd set more firmly in milk that has lost some strength to pasteurization.
• Rennet, and kosher salt or cheese salt with no anti-caking agent and no iodine.
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• Cheese molds, baskets, and presses or weights to drain off more whey and give the cheese its shape.
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From milk to curd

• Add the culture. Sprinkle the bacterial culture over the milk at the temperature the recipe gives, let it soak in for a few minutes, then stir gently with up-and-down strokes. Adding the culture is called inoculation.
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• Let it ripen. Leave the milk to rest while the bacteria turn lactose into acid.
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• Add rennet, then leave the milk undisturbed while the casein binds together. That binding is called flocculation.
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• Test for a clean break. Push a finger straight down into the set milk and lift it out sideways. When the curd splits cleanly over both sides of your finger, it is ready to cut.
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• Cut the curd, first with vertical strokes through the whole batch, then horizontal ones, to the size the recipe gives. Let the curds rest five minutes. Cutting speeds up the souring.

Curd size sets how moist the cheese will be: bigger pieces hold more whey. Some curds set by lactic acid alone, such as camembert's, are too delicate to cut, so they are ladled straight into baskets to drain.

After resting, the curds usually sink and a yellowish whey rises. Rennet curds are often cooked in their whey until they mat into a cake at the bottom of the pan, as for cheddar; other cheeses simply sit in the whey at a set temperature. Depending on the style, the curd may then be milled (broken or cut into pieces), salted, and put into hoops or pressed to squeeze out more whey. The cheeses are then turned on a schedule, and some are salted again whole.

Some cheeses, such as Colby and Gouda, have their curds washed, which leaves them moister and less acidic. Salt goes in different ways: Gouda is soaked in brine, while feta is salted on the surface.

Ripening: where flavor comes from

Flavor sources in ripening cheese.

The breakdown of fat, called lipolysis, is a key step. It splits fats into smaller acids and related compounds that are strong in both smell and flavor.

In Swiss cheese, bacteria called Propionibacterium ferment sugar into propionic acid, which gives the cheese its typical aroma and flavor. These bacteria are facultative: they breathe oxygen when they have it and ferment when they do not.

Lactic acid protects a cheese as well as flavoring it. By lowering the pH it keeps out unwanted bacteria and molds, while other products of the bacterium Lactococcus lactis add the subtle aromas that set one cheese apart from another.

Camembert can be aged in a covered plastic "cheese cave" at 60ºF, which lets its white mold grow. Some cheeses are coated in wax for storage and aging.

Yogurt

In the United States, the FDA defines yogurt as milk fermented by two bacteria, Lactobacillus bulgaricus and Streptococcus thermophilus. Both make lactic acid from the lactose in milk.

Yogurt's tartness is usually put down to lactic acid, but several other compounds add to the tang.

• Acetaldehyde. Yogurt bacteria make most of it from threonine, an amino acid, which they split into acetaldehyde and glycine. Goat's milk has more glycine than cow's milk, and goat's milk yogurt holds much less acetaldehyde.
• Diacetyl, made by both yogurt bacteria, gives a distinct buttery taste, here and in other fermented milks.
• Acetoin, a close relative of diacetyl, adds a mild, creamy note.

Living yogurt cultures release lactase in the gut, which goes on breaking down the lactose in the yogurt; this makes yogurt edible for people who cannot digest lactose. To make it keep longer, makers often heat yogurt after fermenting it, which kills those live cultures.

The two yogurt bacteria are killed by stomach acid and digestive enzymes. So makers often add other strains, called probiotics — for example Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium — for which there is evidence of reaching the gut intact. Alongside them they may add prebiotics, such as inulin, which help those bacteria grow in the gut.

Kefir

Kefir is a fizzy fermented milk drink. It is started with kefir grains: a living mix of lactic acid bacteria and yeasts held together in a mass of protein, fat and complex carbohydrate.

The bacteria act first, turning lactose into lactic acid; the pH drops and the milk is preserved. Yeasts and acetic acid bacteria join in, and propionic acid bacteria break part of that lactic acid down further, to propionic acid, just as in Swiss cheese. In a second fermentation, yeasts that can eat lactose turn it into ethanol and carbon dioxide, which gives kefir its fizz. The alcohol stays low, usually 0.2–0.3%. Its flavor, like yogurt's, owes much to diacetyl, acetoin and acetaldehyde.

Storing them

Dairy products are stored at 2–4°C (36–39°F), covered, in their own part of the fridge, and rotated as new stock arrives. Mold grows on cheese kept warm, dark and a little damp, and dairy that is spoiling turns darker and smells sour.