CHEESE TECHNOLOGY

Module 3. Milk quality in relation to cheesemaking

Lesson 6
SELECTION AND COMPOSITION OF MILK

6.1 Introduction

Cheese making commences with the selection of milk. The quality of raw milk used for cheese making has an important bearing on the quality of cheese resulting from it. Milk quality influences starter growth, rennet coagulation, manufacturing methods, development of taints and other defects in body and texture of cheese and other changes taking place during ripening of cheese. Milk quality is assessed in terms of sensory, microbiological and chemical quality attributes. As soon as milk is received at the reception dock of the cheese factory, it is evaluated for its odor and appearance. It must not possess any objectionable odor and must be free from extraneous matter. Once the milk has been accepted for further processing, it is assessed for the following group of factors.

6.2 Quantitative Factors

6.2.1 Composition of milk

The composition of milk affects mainly the yield and texture of cheese. The composition of milk affects coagulum properties thereby influencing yield and texture. The milk constituents of prime importance in cheese making are milk proteins (particularly casein), milk fat and mineral salts (particularly calcium). Influence of each of these factors on cheesemaking is discussed below:

6.2.1.1 Milk proteins

Casein, the major milk protein, exists largely in micellar form in milk. It constitutes about 70-80% of the total milk protein. The casein micelles are composed of α-casein, β-casein, κ-casein and some minor components. In addition, genetic variants of these components are also found. The composition and concentration of such a mixture is not constant and subjected to variations due to influence of species, breed, animal health, stage of lactation, climate, time of year and environmental factors. These alterations in the concentration of total casein, relative proportion of its constituents and genetic variants, size of the casein micelle and mineral make up of the casein micelles affect cheesemaking properties of milk such as clotting time, curd strength, syneresis, cheese yield, proteolysis of cheese and composition of the cheese.

During cheesemaking, serum proteins and lactose are lost in whey. So, milk high in casein is desirable. Milk high in whey protein content may delay clotting time as β-lactoglobulin undergoes aggregation when subjected to heat and may react with κ-casein. Apart from longer clotting time, the interaction between β-lactoglobulin and the casein tends to cause softer curds which lose moisture more slowly. Cheese milk containing B variant of β-and κ-casein give shorter clotting time, higher curd strength as compared to the milk devoid of this variant. Size of the casein micelle also affects clotting time. Smaller micelles having more κ-casein give shorter clotting time than the larger ones.

6.2.1.2 Milk fat

The amount, composition and nature of milk fat are affected by the same factors as discussed in the case of milk proteins. Alterations in milk fat composition and the amount of milk fat have direct influence on cheesemaking characteristics and quality of cheese like rennet coagulation time (RCT), coagulum strength, syneresis, yield, flavor and texture of cheese and cheese composition.

Increase in the fat content of milk gives reduced syneresis and thus longer times are required to achieve desired moisture content in cheese. Too low fat in milk is also not desirable as it poses problem of moisture retention and results in hard and dry body cheese. Texture of cheese is also affected by the fat content. Low fat milk gives cheese with leathery texture and lacks mellow, velvetiness while high fat milk produces too soft, buttery and greasy texture. Fat is also important for flavor development of cheese. During ripening, lipolysis of fat results in formation of fatty acids, which impart to cheese, its peculiar flavor.

The physical and chemical nature of milk fat also plays a significant role in cheese making. Fat is incorporated in the casein matrix. This incorporation of fat is dependent on size of fat globule and its composition. Smaller sized globules are easy to incorporate in curd than larger ones. Milk fat with higher melting point is better incorporated in the curd.
The milk used for cheese making must be of uniform quality in terms of its fat and casein content. So, it is always desirable to pool all the milk supply prior to standardization to a definite casein/fat ratio. In case of Cheddar cheese making, milk is standardized to a casein/fat ratio between 0.67 and 0.72.

6.2.1.3 Milk salts

The milk salts are normally classed as ash. Ash contains a large proportion of the metallic components, potassium, sodium, calcium, zinc, chromium, and nickel as well as the non-metallic elements such as sulphur, chlorine, phosphorous, iodine, etc. The salts in milk which are of primary importance to the cheesemaking process are calcium and magnesium salts of phosphoric and citric acid. Calcium makes casein complex with phosphates. Calcium content of milk greatly influences rennet coagulation time, strength of the clot and body and texture of cheese. The quantity of calcium affects the size of the casein aggregates. More calcium content in milk leads to increased micelle size of the casein. Variation in concentration of calcium as well as magnesium, phosphates, citrates and sodium has a direct influence on RCT of milk. High soluble phosphates, citrates and sodium and low soluble calcium and magnesium and also low proportion of casein bound calcium have been found to give slow coagulation of milk by rennet.

6.2.2 Qualitative factors

Chemical qualities of milk which affect the cheese quality fall into three main groups:

A. Quality that inhibits starter growth

B. Quality that affects coagulation

C. Quality that produces taints, gas-holes, etc., in cheese.

Starter is said to be 90% of importance of cheesemaking. This statement emphasizes the importance of starter organisms in cheesemaking. Therefore, the milk used for cheese must be free from inhibitory substances of physiological origin, preservatives, and antibiotics. Presence of even 0.01 IU/ml antibiotics in milk can affect the starter growth.

In cheesemaking one of the most important factors is the time taken for coagulation of milk with rennet. Mixing of mastitis milk, colostrum or late lactation milk adversely affects the coagulation. Therefore, milk must be free from these contaminants.

Mastitis milk is high in serum albumins and often high in sodium and chloride content as well. It is low in fat, casein, lactose, phosphate, potassium and calcium content than the normal milk. Since it is low in casein it produces low yield and moist cheese. It has high concentration of immunoglobulins, several enzymes (e.g. proteinase, protease-peptone and catalase) and leucocyte count. Content of short chain fatty acids are more. Lipolysis in milk increases with initial count, but later decreases with high cell count. Mastitis milk has a changed casein composition like less β-casein, more γ and κ-casein, presence of para-κ- casein in comparison to good quality milk.

Late lactation milk resembles sub-clinical mastitis milk biochemically. It is slightly alkaline, high in albumin and chloride and low in casein, lactose and calcium and its effect on coagulation of milk with rennet is similar to that of mastitis milk.

Colostrum is grossly different from the normal milk. It is very rich in proteins, vitamin A, and sodium chloride, but contains lower amounts of carbohydrates, lipids and potassium than normal milk. It contains many antimicrobial substances like lactoferrin and lactoperoxidase (LP), which inhibit starter cultures used in cheesemaking. Therefore, colostrum must not be mixed with the milk at least for three days but preferably up to 15 days to avoid coagulation delays.

6.3 Natural Inhibitory Substances and Antibiotic Residues in Milk

Raw milk contains a number of natural inhibitory systems such as immunoglobulins, lactoferrin, lysozyme and LP system. Presence of such inhibitory systems in milk can influence cheesemaking properties, particularly starter growth and acid development. Other than these natural inhibitors, some antibiotics may also gain entry to milk as a result of the medical treatment of animals for various diseases. The presence of antibiotics again affects acid development and may also cause starter failure leading to various defects like high moisture in cheese, early and late blowing, weak and pasty body, cracks, open texture and sponginess.