lactic acid, rennet, casein
For most cheeses produced worldwide, cow's milk is used, however milk of other animals, especially goat and sheep is also widely used. The quality of milk used in (semi-) industrial cheese making is rigidly controlled in Europe. The majority of cheeses are made from heat-treated or pasteurised milk (either whole, low-fat or non-fat). If non-pasteurised milk is used, the cheese must be ripened for at least 60 days at a temperature of not less than 4°C to ensure safety against pathogenic organisms. Pasteurisation requirements for milk used to make specific cheese varieties are regulated differently in each country.
Cheese making involves a number of main stages that are common to most types of cheese.
The cheese milk is pre-treated, possibly preripened after addition of a bacteria culture appropriate to the type of cheese, and mixed with rennet.
The enzyme activity of the rennet causes the milk to coagulate to a solid gel known as coagulum. This is cut with special cutting tools into small cubes of the desired size – in the first place to facilitate expulsion of whey. During the rest of the curd making process the bacteria grow and form lactic acid, and the curd grains are subjected to mechanical treatment with stirring tools, while at the same time the curd is heated according to a preset programme.
The combined effect of these three actions – growth of bacteria, mechanical treatment and heat treatment – results in syneresis, i.e. separation of whey from the curd grains. The finished curd is placed in cheese moulds of metal, wood or plastic, which determine the shape of the finished cheese.
The cheese is pressed, either by its own weight or more commonly by applying pressure to the moulds. Treatment during
curd making and pressing determines the characteristics of the cheese. The actual flavour of the cheese is determined during the ripening of the cheese.
Different steps in cheese making are discussed below.
Pasteurisation
Before the actual cheese making begins, the milk usually undergoes pre-treatment designed to create optimum conditions for production.
Milk intended for types of cheese which require more than one month for ripening need not necessarily be pasteurised, but usually is.
Milk intended for unripened cheese (fresh cheese) must be pasteurised. This implies that cheese milk for types needing a ripening period of at least one month need not be pasteurised in most countries.
Milk intended for original Emmenthal, Parmesan and Grana, some extra hard types of cheese, must not be heated to more than 40°C, to avoid affecting flavour, aroma and whey expulsion. Milk intended for these types of cheese normally comes from selected dairy farms with frequent veterinary inspection of the herds.
Although cheese made from unpasteurised milk is considered to have a better flavour and aroma, most producers (except makers of the extra hard types) pasteurise the milk because its quality is seldom so dependable that they are willing to take the risk of not pasteurising it.
Pasteurisation must be sufficient to kill bacteria capable of affecting the quality of the cheese, e.g. coliforms, which can cause early “blowing” and a disagreeable taste. Regular pasteurisation at 72 – 73°C for 15 – 20 seconds is most commonly applied.
However, spore-forming microorganisms in the spore state survive pasteurisation and can cause serious problems during the ripening process. One example is Clostridium tyrobutyricum, which forms butyric acid and large volumes of hydrogen gas by fermenting lactic acid. This gas destroys the texture of the cheese completely (‘blowing'), not to mention the fact that butyric acid is unsavoury.
More intense heat treatment would reduce that particular risk, but would also seriously impair the general cheese making properties of the milk. Other means of reducing thermotolerant bacteria are therefore used.
Traditionally, certain chemicals have been added to cheese milk prior to production to prevent “blowing” and development of the unpleasant flavour caused by heat-resistant spore-forming bacteria (principally Clostridium tyrobutyricum). The most commonly used chemical is sodium nitrate (NaNO3), but at production of Emmenthal cheese, hydrogen peroxide (H2O2) is also used. However, as the use of chemicals has been widely criticised, mechanical means of reducing the number of unwanted microorganisms have been adopted, particularly in countries where the use of chemical inhibitors is banned.
Starter cultures
The starter culture is a very important factor in cheese making; it performs several duties.
Two principal types of culture are used in cheese making:
– mesophilic cultures with a temperature optimum between 20 and 40°C
and
– thermophilic cultures which develop at up to 45°C.
The most frequently used cultures are mixed strain cultures, in which two or more strains of both mesophilic and thermophilic bacteria exist in symbiosis, i.e. to their mutual benefit. These cultures not only produce lactic acid but also aroma components and CO2. Carbon dioxide is essential to creating the cavities in round-eyed and granular types of cheese. Examples are Gouda, Manchego and Tilsiter from mesophilic cultures and Emmenthal and Gruyère from thermophilic cultures.
Single-strain cultures are mainly used where the object is to develop acid and contribute to protein degradation, e.g. in Cheddar and related types of cheese.
Three characteristics of starter cultures are of primary importance in cheese making, viz.
– ability to produce lactic acid
– ability to break down the protein and, when applicable,
– ability to produce carbon dioxide (CO2).
The main task of the culture is to develop acid in the curd.
When milk coagulates, bacteria cells are concentrated in the coagulum and thus in the cheese.
Development of acid lowers the pH, which is important in assisting syneresis (contraction of the coagulum accompanied by elimination of whey).
Furthermore, salts of calcium and phosphorus are released, which influence the consistency of the cheese and help to increase the firmness of the curd.
Another important function performed by the acid-producing bacteria is to suppress surviving bacteria from pasteurisation or recontamination bacteria which need lactose or cannot tolerate lactic acid.
Production of lactic acid stops when all the lactose in the cheese (except in soft cheeses) has been fermented. Lactic acid fermentation is normally a relatively fast process. In some types of cheese, such as Cheddar, it must be completed before the cheese is pressed, and in other types within a week.
If the starter also contains CO2-forming bacteria, acidification of the curd is accompanied by production of carbon dioxide through the action of citric acid fermenting bacteria. Mixed strain cultures with the ability to develop CO2 are essential for production of cheese with a texture with round holes/eyes or irregularly shaped eyes. The evolved gas is initially dissolved in the moisture phase of the cheese; when the solution becomes saturated, the gas is released and creates the eyes.
The ripening process in hard and certain semi-hard cheeses is a combined proteolytic effect where the original enzymes of the milk and those of the bacteria in the culture, together with rennet enzyme, cause decomposition of the protein.
Other additions before making the curd
Calcium chloride (CaCl2)
If the milk is of poor quality for cheese making, the coagulum will be soft. This results in heavy losses of fines (casein) and fat as well as poor syneresis during cheese making.
5 – 20 grams of calcium chloride per 100 kg of milk is normally enough to achieve a constant coagulation time and result in sufficient firmness of the coagulum. Excessive addition of calcium chloride may make the coagulum so hard that it is difficult to cut.
For production of low-fat cheese, and if legally permitted, disodium phosphate (Na2PO4), usually 10 – 20 g/kg, can sometimes be added to the milk before the calcium chloride is added. This increases the elasticity of the coagulum due to formation of colloidal calcium phosphate (Ca3(PO4)2), which will have almost the same effect as the milk fat globules entrapped in the curd.
Carbon dioxide (CO2)
Addition of CO2is one method of improving the quality of cheese milk. Carbon dioxide occurs naturally in milk, but most of it is lost in the course of processing. Adding carbon dioxide by artificial means lowers the pH of the milk: the original pH is normally reduced by 0.1 to 0.3 units. This will then result in shorter coagulation time. The effect can be utilised to obtain the same coagulation time with a smaller amount of rennet.
Saltpetre (NaNO3 or KNO3)
Fermentation problems may be experienced if the cheese milk contains butyric-acid bacteria ( Clostridia) and/or coliform bacteria.
Saltpetre (sodium or potassium nitrate) can be used to counteract these bacteria, but the dosage must be accurately determined with reference to the composition of the milk, the process for the type of cheese, etc., as too much saltpetre will also inhibit growth of the starter. Overdosage of saltpetre may affect the ripening of the cheese or even stop the ripening process.
Saltpetre in high doses may discolour the cheese, causing reddish streaks and an impure taste. The maximum permitted dosage is about 30 grams of saltpetre per 100 kg of milk.
In the past decade usage of saltpetre has been questioned from a medical point of view, and in some countries it is also forbidden.
Colouring agents
The colour of cheese is to a great extent determined by the colour of the milk fat, and undergoes seasonal variations. Colours such as carotene and orleana, a natural anatto dye, are used to correct these seasonal variations in countries where colouring is permitted.
Green chlorophyll (contrast dye) is also used, for example for blueveined cheese, to obtain a “pale” colour as a contrast to the blue mould.
Different steps in cheese making are discussed below.
Pasteurisation
Before the actual cheese making begins, the milk usually undergoes pre-treatment designed to create optimum conditions for production.
Milk intended for types of cheese which require more than one month for ripening need not necessarily be pasteurised, but usually is.
Milk intended for unripened cheese (fresh cheese) must be pasteurised. This implies that cheese milk for types needing a ripening period of at least one month need not be pasteurised in most countries.
Milk intended for original Emmenthal, Parmesan and Grana, some extra hard types of cheese, must not be heated to more than 40°C, to avoid affecting flavour, aroma and whey expulsion. Milk intended for these types of cheese normally comes from selected dairy farms with frequent veterinary inspection of the herds.
Although cheese made from unpasteurised milk is considered to have a better flavour and aroma, most producers (except makers of the extra hard types) pasteurise the milk because its quality is seldom so dependable that they are willing to take the risk of not pasteurising it.
Pasteurisation must be sufficient to kill bacteria capable of affecting the quality of the cheese, e.g. coliforms, which can cause early “blowing” and a disagreeable taste. Regular pasteurisation at 72 – 73°C for 15 – 20 seconds is most commonly applied.
However, spore-forming microorganisms in the spore state survive pasteurisation and can cause serious problems during the ripening process. One example is Clostridium tyrobutyricum, which forms butyric acid and large volumes of hydrogen gas by fermenting lactic acid. This gas destroys the texture of the cheese completely (‘blowing'), not to mention the fact that butyric acid is unsavoury.
More intense heat treatment would reduce that particular risk, but would also seriously impair the general cheese making properties of the milk. Other means of reducing thermotolerant bacteria are therefore used.
Traditionally, certain chemicals have been added to cheese milk prior to production to prevent “blowing” and development of the unpleasant flavour caused by heat-resistant spore-forming bacteria (principally Clostridium tyrobutyricum). The most commonly used chemical is sodium nitrate (NaNO3), but at production of Emmenthal cheese, hydrogen peroxide (H2O2) is also used. However, as the use of chemicals has been widely criticised, mechanical means of reducing the number of unwanted microorganisms have been adopted, particularly in countries where the use of chemical inhibitors is banned.
Starter cultures
The starter culture is a very important factor in cheese making; it performs several duties.
Two principal types of culture are used in cheese making:
– mesophilic cultures with a temperature optimum between 20 and 40°C
and
– thermophilic cultures which develop at up to 45°C.
The most frequently used cultures are mixed strain cultures, in which two or more strains of both mesophilic and thermophilic bacteria exist in symbiosis, i.e. to their mutual benefit. These cultures not only produce lactic acid but also aroma components and CO2. Carbon dioxide is essential to creating the cavities in round-eyed and granular types of cheese. Examples are Gouda, Manchego and Tilsiter from mesophilic cultures and Emmenthal and Gruyère from thermophilic cultures.
Single-strain cultures are mainly used where the object is to develop acid and contribute to protein degradation, e.g. in Cheddar and related types of cheese.
Three characteristics of starter cultures are of primary importance in cheese making, viz.
– ability to produce lactic acid
– ability to break down the protein and, when applicable,
– ability to produce carbon dioxide (CO2).
The main task of the culture is to develop acid in the curd.
When milk coagulates, bacteria cells are concentrated in the coagulum and thus in the cheese.
Development of acid lowers the pH, which is important in assisting syneresis (contraction of the coagulum accompanied by elimination of whey).
Furthermore, salts of calcium and phosphorus are released, which influence the consistency of the cheese and help to increase the firmness of the curd.
Another important function performed by the acid-producing bacteria is to suppress surviving bacteria from pasteurisation or recontamination bacteria which need lactose or cannot tolerate lactic acid.
Production of lactic acid stops when all the lactose in the cheese (except in soft cheeses) has been fermented. Lactic acid fermentation is normally a relatively fast process. In some types of cheese, such as Cheddar, it must be completed before the cheese is pressed, and in other types within a week.
If the starter also contains CO2-forming bacteria, acidification of the curd is accompanied by production of carbon dioxide through the action of citric acid fermenting bacteria. Mixed strain cultures with the ability to develop CO2 are essential for production of cheese with a texture with round holes/eyes or irregularly shaped eyes. The evolved gas is initially dissolved in the moisture phase of the cheese; when the solution becomes saturated, the gas is released and creates the eyes.
The ripening process in hard and certain semi-hard cheeses is a combined proteolytic effect where the original enzymes of the milk and those of the bacteria in the culture, together with rennet enzyme, cause decomposition of the protein.
Other additions before making the curd
Calcium chloride (CaCl2)
If the milk is of poor quality for cheese making, the coagulum will be soft. This results in heavy losses of fines (casein) and fat as well as poor syneresis during cheese making.
5 – 20 grams of calcium chloride per 100 kg of milk is normally enough to achieve a constant coagulation time and result in sufficient firmness of the coagulum. Excessive addition of calcium chloride may make the coagulum so hard that it is difficult to cut.
For production of low-fat cheese, and if legally permitted, disodium phosphate (Na2PO4), usually 10 – 20 g/kg, can sometimes be added to the milk before the calcium chloride is added. This increases the elasticity of the coagulum due to formation of colloidal calcium phosphate (Ca3(PO4)2), which will have almost the same effect as the milk fat globules entrapped in the curd.
Carbon dioxide (CO2)
Addition of CO2is one method of improving the quality of cheese milk. Carbon dioxide occurs naturally in milk, but most of it is lost in the course of processing. Adding carbon dioxide by artificial means lowers the pH of the milk: the original pH is normally reduced by 0.1 to 0.3 units. This will then result in shorter coagulation time. The effect can be utilised to obtain the same coagulation time with a smaller amount of rennet.
Saltpetre (NaNO3 or KNO3)
Fermentation problems may be experienced if the cheese milk contains butyric-acid bacteria ( Clostridia) and/or coliform bacteria.
Saltpetre (sodium or potassium nitrate) can be used to counteract these bacteria, but the dosage must be accurately determined with reference to the composition of the milk, the process for the type of cheese, etc., as too much saltpetre will also inhibit growth of the starter. Overdosage of saltpetre may affect the ripening of the cheese or even stop the ripening process.
Saltpetre in high doses may discolour the cheese, causing reddish streaks and an impure taste. The maximum permitted dosage is about 30 grams of saltpetre per 100 kg of milk.
In the past decade usage of saltpetre has been questioned from a medical point of view, and in some countries it is also forbidden.
Colouring agents
The colour of cheese is to a great extent determined by the colour of the milk fat, and undergoes seasonal variations. Colours such as carotene and orleana, a natural anatto dye, are used to correct these seasonal variations in countries where colouring is permitted.
Green chlorophyll (contrast dye) is also used, for example for blueveined cheese, to obtain a “pale” colour as a contrast to the blue mould.
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