Cheese Fermentation

Cheese Fermentation

Cheese is a milk product obtained by coagulation of milk protein. Cheese can be defined as the consolidated curd of milk solids in which milk fat is entrapped by coagulated casein.  Typically, the yield of cheese from milk is 10%. 

Essentially there are Nine key steps in cheese-making:

1.      Pretreatment of raw milk

2.      Addition of lactic acid starter bacteria and coagulant

3.      Coagulation followed by cutting the curd

4.      Cooking to temperatures from 32°C to 54°C, which, together with acid production, assist expulsion of whey from the curd

5.      Separation of the curds and whey

6.      Molding and pressing the curd at low (soft cheese) or relatively high (hard cheese) pressure

7.      Salting or brining

8.      Ripening at temperatures of 6°C to 24°C to allow the characteristic flavor and texture to develop

9.      Cheese packing

These key steps are described below

Pretreatment of raw milk

Milk is the basic ingredient and the quality of cheese depends on milk.  So the biological, physical and chemical properties of milk have to be controlled.  Also the milk for cheese production must be free from antibiotics and sanitizing agents that might interfere with fermentation.

The milk may contain some suspended materials like leucocytes or particulate matter.  Techniques like homogenization, clarification, filtration, centrifugation and vacuum treatment are done for the clarification for the removal of suspended particles in milk, removal of leucocyte and particulate matter.

Milk of good quality, having low bacterial count is used for cheese making. Milk with high microbial count may result in development of undesirable flavors in cheese.  Milk also contains enzymes such as amylase, protease, lipase, peroxidase, catalase, etc.   The microflora and enzymatic activity in milk can be controlled by pasteurization where milk is treated at 72 ° C for 15 seconds or 63 ° C for 30 minutes. After heat treatment, the milk is cooled down to the fermentation temperature which is generally 29–31^o C when the starter organisms are added.

Fresh milk with low calcium content may need addition of calcium chloride which is done after pasteurization.

Addition of lactic acid starter bacteria and coagulant

During this step, the milk proteins are coagulated to form a solid curd. In milk protein 82 % is casein and 18 % is whey protein.  The curd formed during the coagulation of milk contains fat globulins, water soluble material and water.

Coagulation of milk can be done by using Lactic starter culture and rennet enzyme.  The starter culture may be either mesophilic starters such as strains of Lactococcus lactis and its subspecies or thermophilic starters such as Lactobacillus helveticus, Lb. casei, Lb. lactis, Lb. delbrueckii subsp. bulgaricus and Strep. thermophiles are used in the production of cheeses like Emmental and Parmesan where a higher incubation temperature is employed. 

The role of starter organisms is both crucial and complex. Their central function is the fermentation of the milk sugar lactose to lactic acid to decrease the pH.  The acidic pH increases the shelf-life and safety of the cheese and gives a sharp, fresh flavour. The acid also aids in moisture expulsion and curd shrinkage which help to achieve the final cheese texture.

During the growth of starter culture in milk, the milk is acidified by fermentation of lactose to lactic acid. The bacterial culture is mixed properly with milk till acidity reaches to 0.17 to 0.2 % in about 1 hour. 

Now the enzyme rennet is added in milk.  The time of renneting and the amount added differ with cheese type. Rennet is a preparation from the fourth stomach or abomasom of suckling calves, lambs or goats.   Rennet contain the proteolytic enzyme rennin or chymosin which cleaves k-casein. k casein is important in maintaining the colloidal stability of milk proteins, so the casein becomes unstable and aggregates in the presence of calcium ions to form a gel. As the gel forms it entraps fats and ultimately forms white creamy lumps, known as curd. Microbial rennets are available, produced from fungi such as Mucor miehei, Mucor pusillus and Endothia parasitica.  The lactic acid and rennet cause the milk to curdle, which separates the curds (made of milk solids, fats and proteins) and whey (mostly water).

Coagulation followed by cutting the curd

After 30–45 min, coagulation of the milk is complete and the process of whey expulsion can be started.  Curd is a coagulated solid part formed after coagulation and whey is the water and water soluble part. Whey contains few proteins, lactalbumin, globulin, and yellow-green riboflavin (vitamin B2).  The curds will be cut into approximately 1 cm cubes for whey expulsion. 

Cooking to temperatures from 32°C to 54°C

Whey expulsion is further assisted by scalding when the curd is heated to 32°C to 54°C, the curd will shrink and become firm.  The starter organisms continue to produce acid which aids curd shrinkage.

Separation of the curds and whey

Now curd can be readily separated from the liquid whey by holding the mixture in cheese cloth. Molding and pressing the curd

Manual or mechanical cheddaring is done where curd blocks are piled up to compress and fuse the curds, expelling more whey.  This will be done at low pressure for soft cheese or relatively high pressure for hard cheese.  At the end of cheddaring, the curd has a characteristic fibrous appearance.

Salting or brining

The blocks of curd are then milled into small chips and 1.5 - 2% w/w salt is added and other ingredients may be added, such as colouring agents, herbs, or microbial inoculum for ripening.   Salt is important for the taste and also aids in moisture and acidity control. It also he1p to limit the growth of undesirable proteolytic bacteria. The salted curd is again made into blocks which are then pressed to expel trapped air and whey.

Ripening of Cheese

Finally, the cheese is ripened or matured at 10^oC to allow flavor development for up to months or year.  Ripening involves the modification of proteins and fats by microbial and milk proteases and lipases that remain in the young cheese.  Transformation of elastic and chalky curd with an acidic flavour to ductile full flavored cheese is achieved through proper cheese ripening. The acidity of the curd is increased up to 4.7.   During cheese ripening process various changes in physical, chemical and microbial profile takes place. There will be breakdown of proteins, carbohydrates, lipids, fats and sugar resulting in contribution of flavour and texture to the cheese. The development of flavour and texture of cheese also depends upon pH, salts, temperature, humidity and composition of cheese.

Proteases and peptidases from the starter culture and rennet release free amino acids such as glutamic acid and leucine and peptides which contribute to the cheese flavour.

In some cases, a high proportion of hydrophobic amino acid peptides will result in a bitter taste of the cheese.

Some countries allow acceleration of flavour development through the addition of commercial enzyme preparations.

Lysozyme may be added in the manufacture of hard-cooked Emmental, Gouda and Gruyere-type cheeses to prevent growth of the spoilage organism, Clostridium tyrobutyricum

Propionibacterium freundenreichii ssp. shermanii is used for the production of Swiss type cheeses, e.g. Emmental and Gruyere. These bacteria modify the flavour and generate gas bubbles that result in holes or eyes within the cheese.

Blue-veined cheeses such as Danish Blue, Gorgonzola, Roquefort and Stilton use Penicillium roqueforti. Young cheeses are usually punctured with stainless steel rods to promote fungal growth by increasing oxygen levels within the cheese and then stored under controlled humidity at around 9°C. Cheeses are ripened for up to a year during which they develop the characteristic blue veins, and the aroma and flavour develop due to fungal metabolites.

Surface-ripened soft cheeses such as Camembert and Brie use Penicillium camemberti, which originates naturally from the environment, or the surface of the cheese may be sprayed with a spore inoculum. The mold grows on the surface of the cheese for 1–6 months to produce the characteristic white crust or rind and the hydrolytic enzymes secreted into the cheese modify the flavour and texture.

Propionibacterium spp. Yeasts, micrococci, and Brevibacterium linens impart the characteristic flavor of Limburger cheese.

Cheese packing

Cheese is normally packed with a protective coating such as vegetable oil or special plastic films. The cheese is cut into rectangular form and coated with a plastic film, packed and sold in the market.

The shelf life of cheese varies with the type but is much superior to that of milk. This is due to the reduced pH (around 5.0), the low water activity due to whey removal and the dissolution of salt in the remaining moisture. Under these conditions yeasts and molds are the main organisms of concern which are effectively controlled by traditional procedures to exclude air such as waxing or by modern practices such as vacuum packing.

Problems encountered in Cheese manufacture

Bacteriophage infections of starter cultures is a major problem in cheese fermentations. When this occurs, acidification slows or even stops causing financial losses to the producer as well as an increased risk that pathogens might grow.  An important source of phage in cheese making is the starter culture themselves which carry lysogenic phages within them that may be induced into a virulent state. The use of phage inhibitory media, defined strains, the rotation of strains and phage-insensitive bacterial strains are used to tackle the situation. However, phage infections continue to cause problems within the industry.

Types of cheese

There are various types and flavors of cheese and about a thousand types of cheese have been described depending on the properties and treatment of the milk, the method of production, conditions such as temperature, and the properties of the coagulum, and the local preferences. Classification of cheeses is difficult due to this diversity. The most successful approach of classification of cheese is based on moisture content, milk type and the role of microorganisms in cheese ripening.  They are mainly soft cheese, Semi soft cheese, hard cheese and very hard cheese as given in the table.

Moisture Content	Type	Examples
50–80%	Soft cheeses – unripened	Cottage, Cream, Mozzarella
	Soft cheeses – ripened	Camembert, Brie, Caciotta,
	Soft cheeses – cooked, salt-cured or pickled	Feta, Domiati
39–50%	Semi soft cheeses - ripened principally by internal mould growth	Roquefort, Stilton, Danish Blue
	Semi soft cheeses - ripened by bacteria and surface micro-organisms	Limburger, Trappist
	Semi soft cheeses - ripened primarily by bacteria	Brick, Gouda, Edam
<39%	Hard cheeses - without eyes, ripened by bacteria	Cheddar, Caciocavallo
	Hard cheeses - with eyes, ripened by bacteria	Emmental, Gruyere
<34%	Very hard cheeses	Parmesan, Romano, Grana

 

References

• Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers
• Dairy Microbiology – Handbook, Richard K. Robinson, Third Edition, Wiley Interscience Publications
• Industrial Microbiology: An Introduction, M J. Waites, N L. Morgan, J S. Rockey, G Higton
• Food Microbiology, Third Edition, Martin R. Adams and Maurice O. Moss University of Surrey, Surrey, Guildford, UK.