How is Cheese Made?
Introduction
Cheese-making basically consists of the removal of moisture from a rennet coagulum.
The three main factors affecting the properties of the final cheese are (i) the level of fat and protein present in the milk, (ii) the scalding (“cooking”) of the curd and (iii) the rate and extent of acid production in the cheese. The information given here is for the production of British type cheeses, e.g. cheddar types.
Basic Stages
The basic processes involved are :-
- Heat treatment of milk (Pasteurisation)
- Inoculation of milk with lactic acid producing bacteria.
- The setting of the milk (forming a “coagulum”) using a rennet solution
- Cutting the cheese curd to form curds and whey
- The separation of the curds (cheese solids) and the whey (liquid).
The same basic processes are used in the manufacture of all hard cheese varieties. It is the variations within each process that determines the type of cheese produced.
The Ingredients Added to Milk to make Cheese
Starter Cultures
Cheese is really a form of fermented milk, and acid production is carried out by starter cultures. Lactic acid bacteria can be divided into two main groups - the lactobacilli and streptococci.
Colour
Only two types of colouring are allowed in cheese : annatto (E160b and carotene (E160a)
Coagulants/Rennet
By adding rennet the structure of the milk protein (casein) is changed radically to form a solid curd.
Salt
Sodium chloride, the common salt used at home for cooking and seasoning food.
The Stages of Cheese making
Milk for Cheese makingThe milk used must be of excellent quality both compositionally and microbiologically and must also be free from any taints or antibiotics.
Milk Storage
Once collected from the farm the raw milk is stored in large silos at the dairy.
Pasteurisation.
Prior to use in the dairy, the milk is pasteurised to destroy pathogenic and other micro-organisms which may cause defects in the cheese. It also gives a more uniform milk. Pasteurisation involves heating the raw milk to 72’C for a minimum of 15 seconds
Vat Fill
After pasteurisation, the milk is cooled to typically 32’C and pumped into the vat.
The cheese vat is where the rennet and starter are added to the milk to initiate coagulation of the curds. The vat is also where the curds are cut and the whey is drained off.
Addition of starter
Starter cultures (lactic acid bacteria) are added to the milk in the vat. Typically they are added at the start of vat fill and once in the milk, they start to grow and generate lactic acid. The time that the starter culture is allowed to grow in the milk is called the “ripening period”.
Renneting
Once the required ripening time has been reached, the rennet is added to the milk in the vat. This coagulates the milk and produces a “gel”. Rennet is an enzyme extract which causes the protein in the milk to bind together giving a semi-solid coagulum. The milk protein (casein) forms a “matrix” in which the whey and the milk fat are enclosed.
Setting
The period of time that the rennet is allowed to coagulate the milk is called the set. The longer the set-time, the firmer the “set”. Typically for a cheddar this would be 45 minutes.
Cutting the curd
Once the correct degree of firmness (determined by the cheese maker) has been achieved, the coagulum is cut into small particles using blades. It is this cutting action which starts to separate the curds and whey. Cutting provides a large surface area from which whey can escape.
Scald
After a set period of stirring, heat is applied to the vat by steam injection into the jacket of the cheese vat. The purpose of this heating stage is to cause the curd particles to contract and allow further expulsion of whey and more importantly, heat is used to regulate the growth of the lactic acid bacteria and control acid development in the curd.
Stirring
Once the curd has been scalded to the required temperature the curds and whey are stirred continuously until the pitch. The pitch is when the curds and whey are flushed onto the cheese table and the process of separating the whey from the cheese curds begins.
Cheese makers often refer to the “rennet to pitch time” and it is an important stage in the cheese making process.
Draining the Whey
Whey is expelled from the curd grains during the final stirring period, primarily due to continuous development of lactic acid, but also by the mechanical effect of stirring. The duration of final stirring depends upon the cheese variety and is dictated by the desired moisture content. As soon as the required acidity of whey and firmness of the curd have been attained, the residual whey is drained off.
Cheddaring
After separation, the curd settles into a solid mass on the table. For a cheddar type cheese the curd is allowed to “knit” together. This process is known as “cheddaring”. The objectives of this stage are 3 fold:- (i) to allow acidity development to continue, (ii) to permit drainage of the whey from the curd and (iii) to develop the required consistency and texture in the curd.
Acidity Development in the cheese
During the “cheddaring” stage, the acidity development is monitored regularly by titration or pH checks.
Texture Development
For cheddar the curd is often cut into large strips or blocks which are turned and then piled on top of each other to assist with the Cheddaring process. For crumbly regional cheese, the smaller pieces are regularly stirred to try and prevent the curd knitting together. Cheddar curd should develop a cooked “chicken-breast” texture.
Milling the Curd
Once the correct acidity has been achieved, the curd is milled. The purpose is to reduce the curd into smaller pieces to allow correct distribution of the salt throughout the cheese prior to pressing.
Salting
Salt is added to cheese in one of 3 ways:- (i) by addition of salt to the curd , (ii) by placing block cheese into a brine solution,(iii) by applying dry salt to the block cheese surface.
For most British cheese varieties, method (i) is used, whereas most continental cheeses use brining.
Mould Filling
The salted, milled curd is filled into cheese moulds which are then lidded, pre-pressed and then put into the press overnight. Pressing helps to expel further whey from the curd,
shapes the cheese, forms the correct texture and degree of firmness and allows the curd pieces to “knit” together to form a block of cheese.
Once the cheese has been pressed correctly the block is knocked-out, bagged, vacuum-sealed, labelled and palletised.
Most hard cheese varieties are matured for a certain period of time. It can vary from a few week for a Cheshire cheese to several years for a vintage cheddar. Generally the cheese is stored at temperatures of between 8 - 10’C and the higher the storage temperature, the faster the cheese matures.