Cheesemaking is not only a chemical process, but it also requires desired microbial growth and enzymatic conversion of food components, which qualifies it as a fermented food.¹  Enzymes and lactic acid bacteria (LAB) are responsible for a wide range of reactions and metabolic pathways that result in textures, flavours, and aromas that many consumers enjoy, while providing proteins, lipids, carbohydrates, vitamins, and minerals that contribute to a healthy diet (see Chapter 11). Nonstarter lactic acid bacteria (NSLAB) also find their way into cheese and contribute to the final product (see Chapter 3).

Cheese starts with milk of course, though consumers sometimes opt for non-dairy “cheeses” (see Chapter 12). Most cheeses and other dairy products worldwide come from bovine milk, but milks of goats, sheep, buffalo, mare, and yak are important for human diet and subsistence in many parts of the globe (see Chapter 11). Although more people worldwide drink or consume goat milk and its products than those of any other species,^2,3  cows produce the most milk in developed and western countries, and a few species such as goats, sheep, and buffalo supply lesser amounts of milk. In developing countries, minor dairy species such as yak, mare, camel, reindeer, llama, donkey, and moose are nutritionally and economically important.⁴  The milk and milk products of non-bovine dairy species such as goats serve three types of markets in many parts of the world: (i) home consumption, (ii) specialty gourmet interests, and (iii) medical needs.³  The compositional differences between the milks of these species lead to contrasts in the resulting cheeses.

The procedures for making cheese differ in details from one variety to the next (see Chapter 4), but most follow a general outline.⁵  The process starts by adding a starter culture (a specified mixture of LAB strains) to milk, causing lactic acid to form and the pH to drop (see Chapter 2). The cheesemaker then adds rennet (an enzyme preparation that breaks the κ-casein molecule), and the milk coagulates into a solid curd with liquid whey. The action of the bacteria and enzymes subsequently increases when the milk is heated, and continues throughout the manufacturing process and during aging (see Chapter 5). Much of cheesemaking involves removal of whey from the curd by draining and pressing. The storage conditions, especially temperature, humidity, and the type of wrapping or packaging (if any), will affect some characteristics of the product. Some operations employ accelerated ripening to cut down on refrigeration and inventory expenses (see Chapter 6).

The smallest artisanal cheesemakers use the same basic procedures as the largest commercial producers, with the difference in scale resulting in changes in the final product (see Chapter 10). Most of the varieties of cheese in the world are artisanal in nature and unique, and cheese lovers seek them out.

Scientists are still elucidating some of the chemical and microbial processes that go into cheesemaking, which are quite complicated. For instance, we do not know the exact origins of some compounds responsible for flavour and aroma. However, we do know a great deal about the conversion of liquid milk to solid cheese, and this book will describe the chemistry behind it.

Microflora, the activity of milk enzymes, and relatively small variations in cheese composition may influence cheese quality but become dominant only under conditions where the principal determinants (moisture, salt, and pH) are within appropriate limits.^6,7  Considering the continued and growing presence of dairy foods in consumers’ diets, it is important for food manufacturers to create high quality products in order to maintain a competitive advantage. Cheese quality is determined by its major attributes, such as appearance, microbiological safety, flavour, texture, functionality, and nutritive value, as described below.^7,8 

Appearance is especially important from the consumers’ perspective since the appearance of cheese is usually the only attribute for assessing cheese quality when they make a purchase. Appearance attributes include colour, the presence or absence of mould as appropriate for the variety, and the presence or absence of eyes or other openings. The consumer knows from experience if the appearance of cheese is defective or not, and if the product does not look right, its flavour, texture, and functionality will be suspect. As a result, retailers strive to make the appearance of cheeses acceptable to consumers.⁶  Even the package greatly affects consumers’ attitudes toward the cheese inside.⁹ 

Aged cheese is a very safe dairy food, whereby the consumer can have confidence on the safety of the product. Most frequently, US and Canadian cheese-related outbreaks of foodborne illness are due to post-pasteurization contamination or use of raw milk, but these cases are quite infrequent.¹⁰  Since consumers cannot assess the microbiological quality of cheeses on their own, they must rely on the manufacturer, retailer, or public health inspectors to ensure the microbial safety. In addition, the consumers are heavily dependent on the nutrition label of the package of the product when they make purchasing decisions.

The flavour of milk, cheese, and other dairy products is mainly due to chemical reactions, including Maillard reactions and proteolysis, and lipolysis of cheese nutrients.^11,12  Cheese flavour is a vital part of quality control and overall consumer appeal.^13,14  A deeper understanding of flavour and its causes in cheese often comprises identifying specific flavour compounds and isolating the specific reaction or metabolic pathways that produce them.¹⁵  Maturation directly affects the flavours of cheeses and involves the interplay of several factors, such as proteolytic and lipolytic enzymes, storage temperature and period, salting, pH of the curd, and humidity.^11,16,17  The specific catabolic compounds generated during ripening processes will influence the characteristic flavours of cheeses (see Chapter 7). The products of these reactions result in a vast array of flavour compounds.^18,19  These reactions take place throughout the cheesemaking process and are concentrated in the ripening phase.^11,20  Most of these reactions occur due to both endogenous (naturally present in the milk) and exogenous (added to the milk) enzymes.²¹  Proteases and lipases mediate many of the most important flavour-generating reactions in goat and cow milk cheeses by lipolysis and proteolysis.^11,17  Numerous biochemical and physical changes can occur in cheeses during manufacture, distribution, and storage processes due to ripening and degradation of nutrients in the products.¹¹  Catabolism of carbohydrates, proteolysis, and lipolysis are the primary processes in cheese ripening, with a variety of chemical, physical, microbiological, textural, and rheological changes taking place, usually under controlled environmental conditions.^11,16,17,22 

Lactose is the main carbohydrate in cheese curds. Fresh cheese curd contains approximately 1% lactose, which the starter LAB converts to lactic acid (mainly the l-isomer), usually within 24 h.⁶  The l-lactic acid is racemized to dl-lactic acid by NSLAB or catabolised to CO₂ and H₂O in mould-ripened and smear-ripened cheese.⁶  Propionibacterium freudenreichii subsp. shermanii breaks down lactic acid into propionic and acetic acids, CO₂, and H₂O in Swiss-type cheese, where the CO₂ causes the formation of characteristic eyes in such cheese. The pH of most cheeses increases during aging due to the catabolism of lactic acid and, in the case of smear- or mould-ripened varieties, to the production of NH₃ at the surface, which diffuses into the cheese. The pH of Camembert cheese, for instance, increases from around 4.6 to 7.5.⁶  The pH of Cheddar cheese does not normally increase, probably because the catabolism of residual lactose (1%) in the curd reduces the pH to a value below the buffering maximum of cheese (pH 5.2). Washing the curd or reducing the lactose content of milk for Cheddar cheese avoids the pH increase post-manufacture. Cheddar has a mild flavour, while high-lactose Cheddar has a harsh, strong flavour.²³ 

Aging of cheese unavoidably results in lipolysis. However, little lipolysis occurs in most cheese varieties since LAB or NSLAB are weakly lipolytic, as is indigenous milk lipase if raw milk is used.⁶  Extensive lipolysis occurs in some hard Italian-type cheeses, e.g., Provolone and Pecorino varieties, for which added pre-gastric esterase is responsible, and in Blue cheese because of the action of Penicillium roqueforti.⁶  Fatty acids break down into lactones or esters that give characteristic flavours to varieties such as Provolone and Pecorino. P. roqueforti converts FA to methyl ketones in blue-veined cheese, and these account for the characteristic peppery taste of these varieties. Some methyl ketones break down into secondary alcohols, causing off-flavours.⁶ 

Proteolysis is probably the most important biochemical event that gives a major impact to flavour and texture of most cheese varieties, especially cheeses that undergo ripening by internal bacteria.^11,16  Chymosin in the rennet initially hydrolyses the casein, and plasmin, an endogenous enzyme resistant to pasteurization, attacks β-casein. High-cook varieties almost completely inactivate chymosin, and in this case plasmin is the principal agent of primary proteolysis.⁶  Chymosin and plasmin produce peptides that reduce to smaller peptides and amino acids by proteinases and peptidases of starter LAB and NSLAB. Small peptides contribute positively to the background brothy flavour of cheese, while some peptides are bitter. Furthermore, in proteolysis cheeses, many amino acids give characteristic flavour and serve as substrates for many catabolic reactions, such as decarboxylation, deamination, and transamination catalysed by the enzymes of LAB, NSLAB, and secondary culture.⁶ 

Flavour is an important characteristic of cheese, but its physical behaviour is also key. People do not want to consume food that does not feel right in their mouth. The relative amounts of protein, fat, and water are the main factors in the cheese structure. The chemical processes taking place in cheese also affect its structure, including the size of fat globules, their arrangement in the casein matrix, the presence of calcium lactate crystals, and more (see Chapter 8). The physical attributes of cheese come under the categories of texture, rheology, and functionality (see Chapter 9).

Texture is a sensory property of food, derived from its structure and manifested by the five senses.²³  It is particularly important to cheese lovers, who derive much of their enjoyment from the way the cheese feels as they eat it (see Chapter 10). Sensory panels provide information on texture (and flavour), while instrumental tests such as texture profile analysis (TPA) offer non-biased results. Scientists constantly strive to match the results of human sensory testing with instrumental results.^24–26  Terms found in both sensory testing and TPA include hardness, fracturability, adhesiveness, springiness, and cohesiveness.

Scientists define rheology as the study of the flow and deformation of matter.²⁷  Cheese is an interesting material to test due to its unique structure, and it is not surprising that one of the earliest studies concerning the rheology of food (published in 1937) dealt with cheese.²⁸  The field has expanded greatly since these tests of compression under constant load. Other tests include creep (sudden imposition of stress), stress relaxation (removal of force during a creep experiment), steady shear (internal portions sliding past each other), and small amplitude oscillatory shear (stress and strain varying harmonically with time).²⁹  The experiments provide information on mechanical properties related to the strength of the protein bonds.

The functionality of cheese includes sliceability, adherence (stickiness), meltability, and stretchability. Sliceability and adherence are of concern mainly to processors who have to cut or shred cheese blocks, while meltability and stretchability are mainly related to the use of cheese as an ingredient.⁸  For instance, the primary quality attributes of mozzarella include its meltability and stretchability in foods such as pizza.³⁰  Other parameters such as wheying off (whey escaping from the cheese after packaging) and oiling off (the oil appearing on a layer of heated cheese) may be important in some varieties and applications.

The remaining chapters of this book will more fully describe the chemistry that goes into the preparation and consumption of cheese, beginning with the starter culture and coagulants needed to convert milk into curds and whey (Chapter 2). NSLAB play a part in cheesemaking, which is the subject of Chapter 3. Chapter 4 covers the manufacture of cheese in detail and Chapter 5 describes the ripening that occurs during storage. The cheesemaker may accelerate ripening in some cases, and Chapter 6 deals with that aspect. As mentioned above, cheese does not have to come from cow milk, and Chapter 11 shows comparisons of cheeses from cow, goat, and sheep milk. Chapter 7 provides a review of the generation of flavour compounds, Chapter 8 covers the cheese microstructure, and Chapter 9 provides an overview of the rheology and texture of cheese. Artisanal cheesemakers are responsible for many varieties, as shown in Chapter 10. Chapter 11 includes the production, consumption, and nutrition of cheese, and Chapter 12 reviews the expanding market of non-dairy cheese.