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  Ripening or souring the milk
All of the cheese produced on the farm was made from unpasteurised milk. This meant that the first step happened all by itself. Bacteria naturally present in the milk would start souring it, converting lactose (milk sugar) into lactic acid. In cheese made from pasteurised milk these acidifying bacteria must be added.

Although pasteurised milk will acidify if left in the fridge too long, the bacteria that grow are unknown bacteria, possibly dangerous, certainly smelly. The types of bacteria added to acidify milk in cheesemaking are carefully controlled.

Curdling and coagulation
curdling the milk using enzymes found in rennet. Curdling occurs when certain milk proteins clump together forming a web or matrix that traps water and fat.

The use of rennet may have been humankinds first venture into biotechnology. Rennet was probably discovered when curd was observed in the stomachs of young animals during butchery, or possibly when the stomachs of certain animals were used as handy bags for carrying milk around.

Cheesemaking was either carried out in a calfs stomach or with a piece of stomach in a vessel. In time cheesemakers learned to extract the enzymes using brine and now we can produce vegetarian rennet in a laboratory.

The active enzyme in rennet is special because it only acts on one type of milk protein: casein proteins. These occur in milk as clumps known as micelles, held together by a calcium glue. The micelles have negative charges over their surface which makes them repel each other and so they stay separated in the milk. To form curds, either rennet or an acid is used to overcome these negative charges and create a network of casein proteins.

If an acid is used (for example vinegar, lemon juice or bacterially produced lactic acid) the micelles are broken up, the negative charges removed, and some of the calcium glue is lost into the whey. This allows the casein proteins to join together but in a weak network, forming brittle curds that are deficient in calcium.

Acid-curdled cheeses, such as cottage cheese, are usually eaten fresh, with the whey drained off and some salt added. They tend to have little flavour because many flavour-producing enzymes do not work well in these acidic conditions.

By contrast, rennet removes the negative charge on the micelles but does not break them up, allowing them to join together in a much stronger network and form more elastic curds. Cheese made from these curds can be matured for long periods of time, developing complex flavours.

On the farm we saw little of this fascinating science as, after adding the rennet, we went off to drink a morning coffee and eat hunks of fresh bread with honey for breakfast, leaving the rennet to do its work.

Cutting and cooking
Breaking up the curd helped remove the whey, giving a firmer cheese: the more was removed, the firmer the cheese. To make the farms special Tomme cheese the curds were gently cooked in their whey, giving a slight rubbery texture. The very rubbery texture of cheeses like Gruyere comes from more intensive cooking.

The small gas stove was lit under the giant cauldron and the curds and whey were warmed. They were stirred by hand and as the mixture heated I could feel the curds become rubbery between my fingers as more whey was driven from the network of protein molecules.

Salt can be added at this point to provide not only taste but also to inhibit the growth of spoilage microbes and draw out yet more water. Along with salt, mould spores can be added in various ways depending on the type of cheese to be produced.

Moulding and pressing the curds
This process of moulding the cheese is critical: the shape of the mould, the application of pressure and the proportion of whey removed all have major effects on the final cheese, especially on its texture.

Maturing or ripening
This is the final and slowest transformation. Here we can see how milk and cheese live on a knife edge between life and decay. If the milk is not pasteurised then there is life and growth at every stage, from animal to milk to cheese. But in the end it is a form of decay that gives cheese its flavours and textures. As cheeses age and decay they develop flavour and character, mould and even personalities.

However, given too much time a cheese can become harsh and coarse.

The lives of soft cheeses like Brie and Camembert are meteoric: their prime comes and goes within weeks. The white surface mould that characterises them produces an enzyme that penetrates the cheese, breaking down the casein protein network and transforming the chalky curd into the runny, smelly substance that we enjoy. Left too long, however, and the protein structure collapses into an unpalatable, stinky mess.

For harder cheeses the ageing process is prolonged for a few months to a year, or even, for really dry cheeses like Parmesan, several years. With more time, the decay of the casein proteins allows more flavour-producing chemicals to form.

First, the proteins break into medium-sized pieces called peptides and then into smaller amino acids. These can in turn be broken down into various, highly flavoured molecules called amines. At each stage more complex flavours are produced. For example amines can have smells ranging from spoiling meat to sulphur-like smells, or even ammonia. Though these scarcely sound appetising, bare hints of them build the complexity and richness of cheeses flavours and smells.

Alongside the breaking down of proteins, fats can also be degraded, particularly by blue-cheese moulds such as P. roqueforti. These fats become fatty acids, which in turn become smaller molecules with many of the characteristic smells and flavours of blue cheeses. Some fatty acids have a peppery effect on the tongue and an intense sheep or goat aroma.

The more diverse the cast of ripening enzymes, the more complex the resulting collection of protein and fat fragments and the richer the flavour. A flavour that is still of the land but has developed a life of its own.

Back on the mountainside in the French Pyrenees we would remove the cheese from the mould and transfer it to a cave kept at the correct humidity and temperature by the mountain air and a giant waterfall near its mouth. The cave smelled damp but clean. There was an earthy smell but also a background aroma of cheese that grew the longer you stood there, into an overwhelming, mouth-watering, nose-filling sensation. The walls, the shelves, the cheeses, everything was alive with enzymes, bacteria and moulds all working together to produce the incredibly complex flavours in cheese.

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