15 Technology of Cheese Manufacture –I: Milk quality, Ingredients andClassification of cheese

Definition of cheese

As per Food Safety and Standards Authority of India (FSSAI), cheese means the ripened or unripened soft or semihard, hard and extra hard product, which may be coated with food grade waxes or polyfilm, and in which the whey protein / casein ratio does not exceed that of milk. Cheese is obtained by coagulating wholly or partly milk and/ or products obtained from milk through the action of nonanimal rennet or other suitable coagulating agents and by partially draining the whey resulting from such coagulation and/ or processing techniques involving coagulation of milk and/ or products obtained from milk which give a final product with similar physical, chemical and organoleptic characteristics. The product may contain starter cultures of harmless lactic acid and / or flavour producing bacteria and cultures of other harmless microorganisms, safe and suitable enzymes and sodium chloride. It may be in the form of blocks, slices, cut, shredded or grated cheese.

The cheese may contain maximum of 3000 ppm sorbic acid or its Na, K or Ca salts calculated as sorbic acid, and or 12.5 ppm of nisin, either singly or in combination. Natamycin may be used for surface treatment only.

(i)   Ripened cheese is cheese which is not ready for consumption shortly after manufacture but which must be held for some time at such temperature and other conditions as will result in necessary biochemical and physical changes characterizing the cheese in question.

(ii)    Mould Ripened cheese is a ripened cheese in which the ripening has been accomplished primarily by the development of characteristic mould growth through the interior and/ or on the surface of the cheese.

(iii)    Unripened cheese including fresh cheese is cheese which is ready for consumption shortly after manufacture.

The specifications for cheese as per FSSAI are collated in Table 1.

There are hundreds of varieties of natural cheese now being produced worldwide. However, the vast majority of cheese consumed in India remains limited to only a few varieties such as Cheddar as processed cheese or processed cheese spread, Swiss, Gouda and Mozzarella.

Principle of Cheese manufacture

Cheese  is   a   concentrated  protein   gel,   which   occludes   fat  and   moisture.   Its  manufacture essentially involves gelation of cheese milk, dehydration of the gel to form a curd and treatment of the curd (e.g. dry stirring, cheddaring, texturisation, salting, molding, pressing). The molded curd may be consumed fresh (shortly after manufacture, for example within 1 week) or matured for periods of ~2 weeks to 2 years to form a ripened cheese.

Just after manufacture, rennet-curd cheeses are usually matured or ripened by holding under specific conditions of temperature and humidity for periods which range from ~ 2 to 4 weeks for soft cheeses (for Camembert-type cheeses) to ~ 2 years for some hard cheeses (for Parmesan- style cheeses). During this period, physico-chemical changes take place which transform the ‘rubbery/chewy’  textured  fresh  cheese  curd  to  the  finished  cheese  with  the  desired  quality characteristics, e.g. a soft, smooth, short and adhesive texture with a mushroom-like flavour and creamy mouth-feel for Camembert, or a long, elastic sliceable texture and mild, sweet flavour for Leerdammer cheese.

Ingredients used in cheese making

Milk and milk solids

The main ingredient used in cheese making is milk. Cheese is made using cow, buffalo, goat, sheep or a blend of these milks. However, in India cheese can only be made from cow milk, buffalo milk or a combination of the two milks as per FSSAI.

Milk derived ingredients like white butter, skim milk powder, whey powder, butter milk powder, chakka (partly whey drained fermented curd) are permitted for use in preparation of ‘Processed cheese spread’.

Starter cultures

Cultures for cheese making are called lactic acid bacteria (LAB) because their primary source of energy is lactose in milk and their primary metabolic product is lactic acid. Starter cultures are used early in the cheese making process to assist coagulation by lowering the pH prior to rennet addition. The metabolism of the starter cultures contribute desirable flavor compounds, and help prevent the growth of spoilage organisms. Starters play a role in the acidification of cheese milk to the desired pH during manufacture. In addition, starter bacteria play an important role in the maturation and flavour development of cheese. Current starter technologies include genetically modified starters, adjunct starters and fast-acid starters, which are available commercially as liquid, frozen or dried. Frozen cultures are available in concentrated or unconcentrated forms and inoculated directly into milk, so called ‘direct vat cultures’. Dried starters may be spray-dried or preferably freeze-dried.

Typical   starter   bacteria   include    Lactococcus     lactis subsp. lactis or cremoris, Streptococcus salivarius subsp. thermophilus, Lactobacillus delbruckii subsp. bulgaricus, Lactobacillus helveticus and so on.

Adjunct cultures are used to provide or enhance the characteristic flavor and texture of cheese. Common adjunct cultures added during manufacture include Lactobacillus casei and Lactobacillus plantarum for flavor in Cheddar cheese, or the use of Propionibacterium freudenreichii for eye formation in Swiss. Adjunct cultures can also be used as a smear for smearing the outside of the formed cheese, such as the use of Brevibacterium linens of Gruyere, Brick and Limburger cheeses.

Yeasts and molds are used in some cheeses to provide the characteristic color and flavor. Torula yeast is used in the smear for the ripening of Brick and Limburger cheese. Examples of molds include Penicillium camemberti in Camembert and Brie, and Penicillium roqueforti in Blue cheeses.

Milk coagulant

The type of coagulant used depends on the type of cheese desired. For acid cheeses, an acid source such as acetic acid (the acid in vinegar) or glucono-delta-lactone is used; others include lactic, citric, malic and phosphoric acids. For rennet cheeses, calf rennet or, more commonly, rennet produced through microbial bioprocessing (rennet substitutes) is used.

The two basic methods for clotting milk for cheese manufacture are by rennet or acid, leading to the respective terms, rennet- or acid-coagulated cheeses. In general, acid-coagulated cheeses are soft, whereas rennet-set cheeses are firm.

Chymosin is the principal milk-clotting enzyme used in cheese making. There are several coagulants of animal and microbial origin. These include calf chymosin, bovine and porcine pepsins, proteases from Mucor meihei, Mucor pusillus, or Cryophonectria parasitica and fermentation derived chymosins. These enzymes, often called rennets, hydrolyze the micelle-stabilizing κ-casein at or near the Phe(105)–Met(106) bond. The rennet altered milk coagulates in the presence of Ca2+ at ~30°C. Rennet-hydrolyzed κ-casein gives a hydrophobic para-κ-casein (f1–105) that remains with the curd, and a hydrophilic glycol-macropeptide (f106–169) that is lost in the whey. The activity of chymosin and other milk clotting enzymes is pH dependent; activity increasing as the pH is lowered. It is reported that < 10% of the enzyme used to clot milk remains active in cheese and plays a role in the initial stages of ripening. The residual activities of the microbial enzymes M. meihei and M. pusillus are independent of the milk pH at setting.

Flavourings

Flavorings may be added depending on the cheese type. Some common ingredients include herbs, spices, hot and sweet peppers, horseradish and port wine.

Miscellaneous food additives

Calcium chloride is sometimes added to the cheese milk (@ 0.01-0.02% by weight) to improve the coagulation properties of the milk.

Salt (NaCl) is always used as a seasoning, and helps in prolonging the shelf life of cheese too. Brine salted cheese are kept in brine.

Annatto or β-carotene colourant may be used, as required, to produce cheese having uniform yellow colour day to day, to cope up with the variation in the colour of cheese based on the feed of the animal or the effect of season.

Transglutaminase enzyme has been advantageously used to improve the yield of specific cheese varieties.

Classification of cheese

I. Classification based on the source of milk

Some cheeses are categorized by the source of the milk used to produce them or by the added fat content of the milk from which they are produced. Most of the world’s commercially available cheese is made from cow’s milk, however some produce cheese from goat and sheep milk. Well-known examples include Roquefort (produced in France) and Pecorino Romano (produced in Italy) from ewe’s milk. One farm in Sweden produces cheese from moose’s milk. Sometimes cheeses marketed under the same name are made from milk of different animal i.e. Feta style cheeses are made from sheep’s milk in Greece and from cow’s milk elsewhere. Double cream cheeses are soft cheeses of cow’s milk enriched with cream so that their fat on dry matter content (FDM) is 60–75% or, in the case of triple creams, at least 75% FDM.

II. Classification of cheese based on method of coagulation

The cheese varieties may be classified into few families depending on the way in which milk is coagulated. Majority of cheeses are coagulated by rennet, where the milk is converted to a gel by the action of an enzyme – Rennet. Rennet coagulated cheeses include the majority of major international varieties (e.g. Cheddar, Gouda, Mozzarella, Swiss, Blue, Camembert). The milk may be coagulated by direct acidification producing another family of cheeses. Acid-coagulated varieties, including Cottage cheese and Quark, differ from yoghurt by the fact that some moisture is removed in the form of whey, whereas yoghurt has nearly the same moisture as milk.

Acid curd cheeses

In the manufacture of acid-curd cheeses, the milk gel is cut or broken, and whey is removed by various means viz., centrifugation, ultrafiltration (UF) and/or straining the broken gel in muslin cloth bags. In some varieties (e.g. Cream cheese), whey separation is enhanced by heating the broken gel to temperatures of ~80◦C prior to centrifugation or to ~50◦C prior to UF or straining. In the manufacture of Quark cheese, the temperature of the concentrated curd (~18% TS) is cooled rapidly to < 8◦C to limit hydrophobic interactions between the proteins to minimize defects such as sandy, chalky, or grainy mouth-feel, and/or wheying-off. Manufacture of Cream cheese involves high heat treatment of the curd (~80◦C), addition of NaCl (~0.5g/100g) and hydrocolloids (blend of xanthan and guar gum, ~0.3g/100g), mixing, homogenization and cooling.

III.    Classification based on Firmness (moisture content)

All cheeses, whether rennet or acid set, can be classified as soft, semi-soft (semi-hard), hard, or very hard, depending on their moisture content. Categorizing cheeses by moisture content or firmness is a common practice. The lines between ‘Soft’, ‘Semi-soft’, ‘Semi-hard’, and ‘Hard’ are arbitrary. The factor that controls cheese hardness is moisture content, which depends on the pressure with which it is packed into moulds, and also based on the aging time.

Semi-soft cheese: Semi-soft cheeses and the sub-group, Monastery cheeses have high moisture content and tend to be mild-tasting. Some well-known such cheese varieties include Havarti, Munster and Port Salut.

Medium-hard cheese: Cheeses that range in texture from semi-soft to firm include Swiss-style cheeses such as Emmental and Gruyere. The same bacteria that give such cheeses their eyes also contribute to their aromatic and sharp flavours. Other semi-soft to firm cheeses includes Gouda, Edam, Jarlsberg and Cantal. Cheeses of this type are ideal for melting and are often served on toast for quick snacks or simple meals.

Semi-hard or hard cheese: Harder cheeses have lower moisture content than softer cheeses. They are generally packed into moulds under greater pressure and aged for a longer time than the soft cheeses. Cheeses that are classified as semi-hard to hard include Cheddar, originating in the village of Cheddar in England. Cheddar is one of a family of semi-hard or hard cheeses (including Cheshire and Gloucester), whose curd is cut, gently heated, piled, and stirred before being pressed into forms. Colby and Monterey Jack are similar but milder cheeses; their curd is rinsed before it is pressed, washing away some acidity and minerals. A similar curd-washing takes place when making the Dutch cheeses Edam and Gouda.

Natural cheese is most often classified according to moisture content. Very high moisture cheeses, such as cream and cottage, are not aged and, thus, are often called ‘Fresh cheeses’. Hard cheeses also referred to as ‘grating cheese’ such as Parmesan and Pecorino Romano are firmly packed into large forms and aged for months or years.

The classification of cheese based on moisture content and the cheese categorized based on its texture is depicted in Tables 2 and 3 respectively.

Much cheese is consumed in the form of processed cheese, e.g. on cheeseburgers. Processed cheese is produced by shredding and melting young, natural cheese together with emulsifying salts into a smooth molten mass which is then cooled and molded into the shape desired.

V.   Classification based on non-dairy ingredients used

Cheese analogues: Another commercial product is analogue (or imitation cheese). Cheese analogues are cheese-like products formulated from ingredients, usually a form of casein powder known as rennet casein and a source of fat (either butterfat or usually vegetable oil). Cheese analogues are used mainly as ingredients in consumer-ready meals.

VI. Classification based on brining/pickling

Brine/Pickled cheeses: Brined or pickled cheese is matured in a solution of brine in an airtight or semi-permeable container. This process gives the cheese good stability, inhibiting bacterial growth, even in hot countries. Brined cheeses may be soft or hard, varying in moisture content and in colour and flavour, according to the type of milk used. All such brined cheeses are rindless, and generally taste clean, salty and acidic when fresh, developing some piquancy when aged, and most will be white. Varieties of brined cheese include Feta, Halloumi, Serene and Telemea, a variant of Brinza. Brined cheese is the main type of cheese produced and eaten in the Middle East and Mediterranean areas.

VII. Classification based on mold used for ripening

Mold cheeses: There are three main categories of cheese in which the presence of mold is an important feature. These are soft-ripened cheeses, washed rind cheeses and blue cheeses.

Soft-ripened: Soft-ripened cheeses begin firm and rather chalky in texture, but are aged from the exterior inwards by exposing them to mold. The mold may be a velvety bloom of Penicillium candida or P. camemberti that forms a flexible white crust and contributes to the smooth, runny, or gooey textures and more intense flavour of such aged cheeses. Brie and Camembert are made by allowing white mold to grow on the outside of a soft cheese for a few days or weeks. Goat’s milk cheeses are often treated in a similar manner, sometimes with white molds (Chèvre-Boîte) and sometimes with blue.

Washed-rind: Washed-rind cheeses are soft in character and ripen inwards like those with white molds. Washed-rind cheeses are periodically cured in a solution of salt water brine and/or mold-bearing agents that may include beer, wine, brandy or spices, making their surfaces amenable to bacteria Brevibacterium linens (the reddish-orange “smear bacteria”) that impart pungent odour and distinctive flavour, and produce a firm, flavourful rind around the cheese. Washed-rind cheeses can be soft (Limburger), semi-hard, or hard (Appenzeller).

Smear-ripened: Some washed-rind cheeses are also smear-ripened with solutions of bacteria or fungi, most commonly Brevibacterium linens, Debaryomyces hansenii and/or Geotrichum candidum which usually gives them a stronger flavour as the cheese matures. Many, but not all, of these cheeses have a distinctive pinkish or orange colouring of the exterior. Unlike other washed-rind cheeses, the washing is done to ensure uniform growth of desired bacteria or fungi and to prevent the growth of undesired molds. Notable examples of smear-ripened cheeses include Munster and Port Salut.

Blue: Blue cheese is created by inoculating a cheese with Penicillium roqueforti or Penicillium glaucum. This is done while the cheese is still in the form of loosely pressed curds, and may be further enhanced by piercing a ripening block of cheese with skewers. The mold grows within the cheese as it ages. These cheeses have distinct blue veins, which gives them their name and, often, assertive flavours. The moulds range from pale green to dark blue, and may be accompanied by white and crusty brown molds. The cheese texture can be soft or firm. Some of the renowned cheeses of this type include Roquefort, Gorgonzola and Stilton.

Type of milk used for cheese making

Majority of the cheese is made from cow’s milk. However, buffalo milk is also used for cheese making in few countries including Italy, India, Pakistan, etc. Cow milk is more suited for preparation of Cheddar cheese compared to buffalo milk.

Sheep’s milk is more suited than cows’ milk for the production of piquant-flavoured cheeses, such as Pecorino Romano owing to the higher concentration of short-chain fatty acids (C4:0, C6:0, C8:0, C10:0 and C12:0) in its milk fat, which contributes to such flavour profile. The low carotenoid content of sheep’s and goat’s milk relative to cows’ milk is also more suited to the manufacture of white-coloured cheese varieties, such as Manchego and Roquefort cheeses. However, cows’ milk can vary dramatically in carotenoid content from ~4 to 13 mg/g fat, depending on breed, feed type and stage of lactation. In contrast, sheep’s milk is unlikely to be suitable for the manufacture of Cheddar cheese, in which the rich straw-yellow colour, relatively low level of lipolysis and non-rancid flavour are key quality criteria.

Owing to its low ratio of as1- to as2-casein, goat milk gels much more slowly than cow milk on rennet addition and forms markedly weaker gels and curds, and is consequently much more suited for the manufacture of soft cheese, but less suited for manufacture of hard cheeses, such as Emmental, Gouda, Mozzarella and Cheddar. Apart from the altered proportions of individual caseins, other factors such as the (generally) lower contents of calcium and total casein may also contribute to the relatively poor rennet coagulation characteristics of goat milk.

Composition of milk affecting the cheese quality and yield

The gross composition of cheese milk, especially the concentrations of casein and fat, has a major influence on several aspects of cheese manufacture, including rennet coagulability, gel strength, curd syneresis, cheese composition, yield and quality.

Casein: Casein, which is typically present at 2.5% in cows’ milk, is the main structural protein of both rennet- and acid-induced milk gels.

Whey protein: Whey protein in cows’ milk is typically ~ 0.6–0.7%. On heat-induced denaturation, the whey proteins can interact via thiol–disulphide bonds with other whey proteins and with κ-casein. High heat treatment of milk (≥ 82◦C for 26 s) is generally undesirable for rennet-curd cheeses as denatured protein at levels of ≥ 25% of total impedes the ability of the milk to gel on rennet addition, causes marked deterioration in melt properties of the cheese.

Milk fat: The principal fatty acids in milk fat on a total weight basis are palmitic, oleic and myristic. While the shorter chain fatty acids (C4:0 to C12:0) are present in lower quantities on a weight basis, they are primarily responsible for the piquant flavour of hard Italian cheeses, such as Parmesan and Romano, or the sharp goaty/sheep-like flavours of soft goat milk cheeses. These fatty acids are hydrolyzed from the milk fat triacylglycerols by lipase enzymes, formed during maturation.

Minerals: The insoluble (colloidal salts associated with the casein) and soluble (serum) salts exist in equilibrium. While the soluble citrate and phosphate competes with the casein for calcium ions, the polyvalent casein is the main player controlling the equilibrium concentrations of salts.

The calcium content of milk changes with stage of lactation and season. Variation in calcium content of milk is expected to affect the level of intact casein, citrate, pH, casein micelle size, ionic strength that interactively affect rennet gelation of milk and cheese making efficiency yield.

Apart from variations in the levels of gross constituents (protein, fat), seasonal variation can also occur in the ‘quality’ of the protein in terms of its ability to form a gel with satisfactory curd firming and syneretic (wheying off) properties and to produce cheese curd of satisfactory moisture content. Late-lactation milk generally gives poor rennet coagulability (low curd firmness), impaired curd syneresis, high moisture Cheddar cheese and lower recovery of milk fat to cheese. Low lactose levels in milk generally coincide with high SCC and levels of plasmin activity, and may be indicative of udder infection.

Factors affecting the quality of milk for cheese manufacture

The quality of milk for cheese manufacture is affected by five key parameters, namely composition, microbiology, somatic cell count (SCC), enzymatic activity and levels of residues/contaminants.

Milk enzymes: Milk enzymes (trypsin like proteinase, plasmin proteinase, etc.) originate from a number of sources, viz., milk itself, bacterial contamination (proteinase and lipase from psychrotrophic bacteria) and somatic cells present in milk. Proteinase and lipase enzymes can have significant effects on cheese making properties, yield and quality.

Microbial quality and contamination of milk: Contamination outside the udder can originate from two main sources, namely the environment of the cow and milk contact surfaces. Milking heavily soiled cows could result in bulk milk counts exceeding 1 ×104 cfu/ ml; contamination of milk by unclean teats can contaminate the milk with heat-resistant bacterial spores, which can be problematic for the cheese industry. Clostridium species (Clostridium tyrobutyricum – sporeformer) can cause problems with late gas-blowing development in some types of cheese; main source of clostridia in milk is feeding of poor-quality silage. Refrigerated storage of milk is conducive to the growth of psychrotrophic bacteria. These bacteria typically come from the cows’ environment such as dirt and manure. Heat treatment of milk (thermization, pasteurisation) at the dairy may destroy the psychrotrophic bacteria, but not necessarily the products of their metabolism (free fatty acids – FFA) or their enzymes that can adversely affect rennet coagulation properties of the milk, cheese yield and quality. Psychrotrophic bacteria produce extracellular enzymes capable of hydrolyzing proteins and fats of milk. Thus, they can increase the likelihood of off-flavours and odours and cause changes in body, texture and colour.

Somatic cell count of milk: Factors that contribute to increases in somatic cell count (SCC) of bulk milk include subclinical mastitis, advance in stage of lactation, lactation number, stress and poor nutrition. Somatic cells are present at low levels (< 100 × 103 cells per ml) in normal milk from healthy animals during mid lactation. Increasing SCC in the range 1 × 105 to 6 × 105 cells per ml of milk resulted in an increase in rennet coagulation time and reduction in curd-firming rate and cheese curd firmness; adversely affecting the cheese yield.

Genetic variants of milk: Compared to the AA variants, the BB genotypes of both κ-casein and β-lg are generally associated with a higher concentration of casein and superior rennet coagulation properties, as reflected by higher curd firming rates and gel firmness after a given renneting time. The BB variants of κ-casein and β-lg have been associated with superior cheese making properties, as reflected by the higher recovery of fat, a lower level of curd fines, and higher cheese yields for a range of varieties, including Cheddar, Edam, Gouda, Mozzarella and Camembert.

Chemical residues in milk: The presence of chemical residues and contaminants in milk is of public health concern and a cause of economic loss to the cheese industry. A range of veterinary drugs including antibiotics are commonly administered to animals to combat diseases, the most prevalent being mastitis. Milk indicated as positive for antibiotic residues on receipt at dairy companies is discarded. Antimicrobial residues (i.e. antibiotics) can lead to partial or complete inhibition of acid production by starter cultures, inadequate ripening and ageing of cheese and cause defects of flavour and texture of cheese products. Other sources of contaminants to milk include cleaning and disinfecting agents (trichloromethane, iodine) and compounded animal feeds (mycotoxins).

Pre-treatment of milk for cheese making

Cheese making is favoured if milk is subjected to few pre-treatments viz., bactofugation, microfiltration, standardization for casein/fat ratio, homogenization, pasteurization, thermization and so on.

Bactofugation of milk: The introduction of the bactofuge has helped to control the quality of milk in regions of the world where the bacteriological quality of the milk is poor. The bactofuge is a high-speed centrifuge specially designed for removing bacteria and bacterial spores from milk at high temperatures; decreasing the bacterial numbers by > 95.0%. About 6.0% of milk solids are present in the sludge, which is UHT treated at 130–140°C/few seconds and added back to the cheese milk, to avoid losses.

Standardization of cheese milk: Milk is standardized by adjusting the casein to fat (C/F) ratio (usually 0.7 for Cheddar and Gouda-type cheeses) in milk to control the percent fat on dry matter (FDM) (prescribed in legal standards) to produce consistent cheese. Methods to increase the casein content of cheese milk include addition of skim-milk powder, condensed skim, milk protein concentrate or caseinates or the removal of fat as cream.

Pasteurization of milk: Cheese milk must be adequately pasteurized (72°C/15 s). In few countries, the use of raw milk for cheese making is still prevalent. The pasteurization temperature adopted should be at minimal temperature-time combination as dictated by law, since higher temperatures has an adverse influence on rennetability and thus cheese texture.

Thermization of milk: Extended cold storage of milk may lead to development of high psychrotrophic population that produces proteolytic and lipolytic enzymes resistant to pasteurization. These enzymes can reduce cheese yield and may adversely affect cheese quality.

Thermization of milk at sub pasteurization temperature (i.e. 57-68oC for 10-15 s) on reception at cheese factory reduces the number of psychrotrophs in milk and hence may prove advantageous with regard to cheese yield.

Homogenization of cheese milk: Usually cheese milk is not subjected to homogenization treatment. However, it may be beneficial in few cheese varieties like Blue, Feta, Akawi, etc. and especially in increasing the moisture retention in case of low-fat cheeses. Usually low fat cream (15-20% fat) is subjected to homogenization, referred to as ‘partial homogenization’. Recombined milk cheese is invariably made employing homogenization. Usually lower homogenization pressures (25-50 kg/cm2) are adopted to avoid deleterious effect on the body and texture of cheese. Homogenization leads to whiter cheese and improves the flavor too. Use of homogenized milk helps in increasing Mozzarella cheese yield.

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