T. Uniacke-Lowe

Objective assessment of cheddar cheese quality

Chemical and physical analyses of cheese are required to objectively assess cheese ripening. Statistical Multivariate Analysis of HPLC and free amino acid data for each of 60 Cheddar cheeses, varying in age and quality, were used to objectively classify the cheeses according to maturity, flavour quality (defective or not) and texture. Additional information was obtained from compositional analysis and gel electrophoresis. The total concentration of free amio acids was more effective than HPLC analysis for discriminating between mild, mature and extra-mature Cheddar cheeses whereas HPLC discriminated more effectively between defective and non-defective.

Physicochemical and acid gelation properties of commercial UHT-treated plant-based milk substitutes and lactose free bovine milk

Physicochemical and acid gelation properties of UHT-treated commercial soy, oat, quinoa, rice and lactose-free bovine milks were studied. The separation profiles were determined using a LUMiSizer dispersion analyser. Soy, rice and quinoa milks formed both cream and sediment layers, while oat milk sedimented but did not cream. Bovine milk was very stable to separation while all plant milks separated at varying rates; rice and oat milks being the most unstable products. Particle sizes in plant-based milk substitutes, expressed as volume mean diameters (d4.3), ranged from 0.55μm (soy) to 2.08μm (quinoa) while the average size in bovine milk was 0.52μm. Particles of plant-based milk substitutes were significantly more polydisperse compared to those of bovine milk. Upon acidification with glucono-δ-lactone (GDL), bovine, soy and quinoa milks formed structured gels with maximum storage moduli of 262, 187 and 105Pa, respectively while oat and rice milks did not gel. In addition to soy products currently on the market, quinoa may have potential in dairy-type food applications.

Salts of Milk

The salts of milk are mainly the phosphates, citrates, chlorides, sulphates, carbonates and bicarbonates of sodium, potassium, calcium and magnesium. Approximately 20 other elements are found in milk in trace quantities, including copper, iron, lead, boron, manganese, zinc, iodine, etc. Strictly speaking, the proteins of milk should be included as part of the salt system since these carry positively and negatively charged groups and can form salts with counter-ions; however, they are not normally treated as such. There is no lactate in freshly drawn milk but may be present in stored milk and in milk products. Many of the inorganic elements are of importance in nutrition, in the preparation, processing and storage of milk products due to their marked influence on the conformation and stability of milk proteins, especially caseins, in the activity of some indigenous enzymes and to a lesser extent in the stability of lipids.

Enzymology of Milk and Milk Products

Like all other foods of plant or animal origin, milk contains several indigenous enzymes. The principal constituents of milk (lactose, lipids and proteins) can be modified by exogenous enzymes, added to induce specific changes; being a liquid, milk is more amenable to enzyme action than solid foods Exogenous enzymes may also be used to analyse for certain constituents in milk. In addition, milk and most dairy products contain viable microorganisms which secrete extracellular enzymes or release intracellular enzymes after the cells have lysed. Some of these microbial enzymes may cause undesirable changes, e.g., hydrolytic rancidity in milk and dairy products, bitterness and/or age gelation of UHT milk, bittiness in cream, malty flavours or bitterness in fluid milk, or they may cause desirable flavours, e.g., in ripened cheese.

Chemistry and Biochemistry of Cheese

Cheese is a very varied group of dairy products, produced worldwide; cheesemaking originated in the Middle East during the Agricultural Revolution, about 8,000 years ago. Cheese production and consumption, which vary widely between countries and regions is increasing in traditional producing countries and is spreading to new areas. Milk for cheese is coagulated by acidification, or more commonly, by enzymatic action, and the resulting gel is processed to encourage moisture loss. Curds are salted and ripered βr up to two years during which time extensive biochemical reactions occur leading to the development of flavour and texture.

Proteomic Comparison of Equine and Bovine Milks on Renneting

Rennet-induced coagulation of bovine milk is a complex mechanism in which chymosin specifically hydrolyzes κ-casein, the protein responsible for the stability of the casein micelle. In equine milk, this mechanism is still unclear, and the protein targets of chymosin are unknown. To reveal the proteins involved, the rennetability of equine milk by calf chymosin was examined using gel-free and gel-based proteomic analysis and compared to bovine milk. RP-HPLC analysis of bovine and equine milks showed the release of several peptides following chymosin incubation. The hydrolyses of equine and bovine casein by chymosin were different, and the major peptides produced from equine milk were identified by mass spectrometry as fragments of β-casein. Using two-dimensional electrophoresis, equine β-casein was confirmed as the main target of calf chymosin over 24 h at 30 °C and pH 6.5. The gel-based analysis of equine milk discriminated between the different individual proteins and provided information on the range of isoforms of each protein as a result of post-translational modifications, as well as positively identified for the first time several isoforms of κ-casein. In comparison to bovine milk, κ-casein isoforms in equine milk were not involved in chymosin-induced coagulation. The intensity of equine β-casein spots decreased following chymosin addition, but at a slower rate than bovine κ-casein.

Production and Utilization of Milk

Milk is a fluid secreted by the female of all mammalian species, of which there are more than 4,000, for the primary function of meeting the complete nutritional requirements of the neonate of the species. In addition, milk serves several physiological functions for the neonate. Most of the non-nutritional functions of milk are served by proteins and peptides which include immunoglobulins, enzymes and enzyme inhibitors, binding or carrier proteins, growth factors and antibacterial agents. Because the nutritional and physiological requirements of each species are more or less unique, the composition of milk shows very marked inter-species differences. Of the more than 4,000 species of mammal, the milks of only ~180 have been analysed and of these, the data for only about 50 species are considered to be reliable (sufficient number of samples, representative sampling, adequate coverage of the lactation period). Not surprisingly, the milk of the principal dairying species, i.e., cow, goat, sheep and buffalo, and the human are among those that are well characterized. The gross composition of milks from selected species are summarized in Table 1.1; very extensive data on the composition of bovine and human milk are contained in Jensen (1995).

Physical Properties of Milk

Milk is a dilute emulsion consisting of an oil/fat dispersed phase and an aqueous colloidal continuous phase. The physical properties of milk are similar to those of water but are modified by the presence of various solutes (proteins, lactose and salts) in the continuous phase and by the degree of dispersion of the emulsified and colloidal components. The principal physical properties of milk include its density, redox properties, colligative properties, surface activity buffering capacity, rheological behaviour, conductivity, thermal properties and color.

Vitamins in Milk and Dairy Products

Vitamins are organic chemicals required by the body in trace amounts but which cannot be synthesized by the body. The vitamins required for growth and maintenance of health differ between species; compounds regarded as vitamins for one species may be synthesized at adequate rates by other species. For example, only primates and guinea pigs require ascorbic acid (vitamin C; see Sect. 6.4) from their diet; other species possess the enzyme gluconolactone oxidase which is necessary for the synthesis of vitamin C. The chemical structures of the vitamins have no relationship with each other. Vitamins may be classified based on their solubility in water. Water-soluble vitamins are the B group [thiamine, riboflavin, niacin, biotin, pantothenate, folate, pyridoxine (and related substances, vitamin B6)] and cobalamin (and its derivatives, vitamin B12) and ascorbic acid (vitamin C) while the fat-soluble vitamins are retinol (vitamin A), calciferols (vitamin D), tocopherols (and related compounds, vitamin E) and phylloquinone (and related compounds, vitamin K). The water-soluble vitamins and vitamin K function as co-enzymes while vitamin A is important in the vision process, vitamin D functions like a hormone and vitamin E is primarily an antioxidant.

Water in Milk and Dairy Products

The water content of dairy products ranges from ~2.5 to ~94 % (w/w) (Table 7.1) and is the principal component, by weight, in most dairy products, including milk, cream, ice cream, yogurt and most cheeses. The moisture content of foods (or more correctly their water activity, see Sect. 7.3), together with temperature and pH, are of great importance to food technology. As described in Sect. 7.8, water plays an extremely important role even in relatively low-moisture products such as butter (~16 % moisture) or dehydrated milk powders (~2.5 to 4 % moisture). Water, the most important diluent in foodstuffs, has an important influence on the physical, chemical and microbiological changes which occur in dairy products, and is an important plasticizer of non-fat milk solids.