How Cheese is Made
Cheese is one of the many products we owe to bacteria: they are responsible for the formation and texture of cheese, for giving cheeses their distinctive tastes and often even for making holes in them! Here’s how cheese is made in 4 easy steps.
Step 1: Splitting Milk Into Curds & Whey
The first step in making cheese is separating milk into two parts - curds (the milk solids) and whey (the liquid left behind). There are three main ways of doing this: using a starter culture of bacteria, adding rennet to the milk, or using acid.1,2,3
Option 1: Using a starter culture
A starter culture consists of lactic acid-producing bacteria - exactly which species of bacteria used in the starter culture depends on the type of cheese you want to make. All starter cultures convert the milk sugars (lactose) into lactic acid. As the milk gets more acidic, its proteins (casein) begin to stick together, forming ‘curd’ - hence, this process is called ‘curdling’ milk.
Option 2: Adding Rennet
Rennet is a mix of digestive enzymes that cut the proteins in the milk into ‘pieces’, which causes them to stick together - curdling the milk. Originally, rennet came from calf stomachs. While many cheesemakers still use animal-based rennet, vegetable and microbial-based forms of rennet are now also used.
Option 3: Using Acid
Not all cheeses are made with bacteria or rennet. Some cheeses are made by adding acid at high temperatures to cause milk to curdle. For example, the Indian paneer is prepared with lemon juice, while Mexican queso fresco with vinegar.
By-products of cheese production: The whey leftover from cheesemaking contains, among other things, milk sugar (lactose), milk proteins, minerals and fat. Sometimes for this leftover whey is discarded, but it can also be processed into whey cheeses or even whey protein supplements.4
Step 2: Draining The Curd
The curd is collected together and pressed firmly. The remaining whey then flows out and the curds tangle better together. The cheese gets its round shape here. Sometimes additional ingredients, such as green herbs, nettle, fenugreek, mustard seeds, peppercorns, nuts or algae, are also added to change the colour, texture or flavour of the cheese.
The cheesemaker only performs this step for hard cheeses. Soft cheeses are not pressed and therefore contain more moisture (whey). Traditional cheese makers fill the mould with curd with a little whey left in it and let the whey drain slowly. In cheese factories, the brew is usually sieved to remove the whey faster.
Step 3: Brining and Coating Cheese
Then the cheese wheel is brined, either in a salt bath or by sprinkling salt water over it. This gives the cheese a salty taste and improves the shape and texture, as well as giving the cheese a longer shelf life.
Hard cheese is then coated with a protective layer which serves as packaging. It is traditionally made from paraffin linseed oil, though today it is often a plastic layer that often contains an anti-fungal antibiotic - usually natamycin (E235). This agent is not allowed with organic cheeses.8
Soft cheese only has a natural rind. Specific moulds added deliberately by the cheesemaker grow on it, such as Geotrichum and Penicillium in traditional Camembert, and produce a protective natural rind.6
Step 4: Ripening Cheese
The ripening period during cheese making can vary even for one type of cheese. Soft cheeses usually only ripen for a few weeks or even days, mozzarella takes about 2 weeks, traditional camembert ripens between 4 to 5 weeks, while factory camembert is sometimes ready after just 7 days.
The duration of the ripening determines, among other things, the taste of the cheese. The longer the cheese ripens, the more lactose is converted to lactic acid and the spicier the taste will be. The ripening time also determines the texture of the cheese, although this evolution differs per cheese type. While the blue cheese gorgonzola is creamier after 2 months of ripening, and hard and spicy after 3 months, camembert gets smoother as the ripening time increases.6
There is also such a thing as a "fast-ripening starter", a bacterial culture that has a different composition than the classic starter culture used to curdle milk in Step 1. This makes the ripening process a lot faster and the cheese tastes "old" after only 8 months, even if truly old cheese usually takes longer than 10 months to mature. Though, cheese connoisseurs are usually not a fan of the specific taste of quickly ripened cheese.2
How cheese gets its holes
Known in the cheesemaking business as ‘eyes’, the holes in some cheeses are the work of bacteria too. In Swiss cheeses such as Emmentaler, gas-forming bacteria (Propionibacterium) convert the lactic acid produced by the bacteria in the starter culture into carbon dioxide (CO2), producing gas bubbles that give rise to Emmental’s trademark ‘swiss cheese’ structure.
Gouda gets its eyes from another bacteria, from the genera Leuconostoc or Lactococcus, which instead convert citric acid into CO₂. This citric acid is naturally present in small amounts in milk. In addition to CO₂, the bacteria also produce other substances during this process that contribute to the taste, like diacetyl (which gives Gouda its buttery flavour) or propionic acid and acetic acid (which make swiss cheese taste sweet and nutty).7
Have you ever tried making cheese yourself? Let us know in the comments below!
The author originally wrote this text for the Belgian outlet Eos Tracé. Read the piece in Dutch here.