Improving the Safety and Quality of Milk

Milk Production and Processing

M. Griffiths

• Editorial: Woodhead Publishing
• Año: 2010
• ISBN:
  9781845699420
• Categoría: Technology & Engineering
• Categoría: Food Science

Consumers demand quality milk with a reasonable shelf-life, a requirement that can be met more successfully by the milk industry through use of improved processes and technologies. Guaranteeing the production of safe milk also remains of paramount importance. Improving the safety and quality of milk provides a comprehensive and timely reference to best practice and research advances in these areas. Volume 1 focuses on milk production and processing. Volume 2 covers the sensory and nutritional quality of cow's milk and addresses quality improvement of a range of other milk-based products.The opening section of Volume 1: Milk production and processing introduces milk biochemistry and raw milk microbiology. Part two then reviews major milk contaminants, such as bacterial pathogens, pesticides and veterinary residues. The significance of milk production on the farm for product quality and safety is the focus of Part three. Chapters cover the effects of cows' diet and mastitis, among other topics. Part four then reviews the state-of-the-art in milk processing. Improving the quality of pasteurised milk and UHT milk and novel non-thermal processing methods are among the subjects treated.With its distinguished editor and international team of contributors, volume 1 of Improving the safety and quality of milk is an essential reference for researchers and those in industry responsible for milk safety and quality.

• Addresses consumer demand for improved processes and technologies in the production, safety and quality of milk and milk products
• Reviews the major milk contaminants including bacterial pathogens, pesticides and vetinary residues as well as the routes of contamination, analytical techniques and methods of control
• Examines the latest advances in milk processing methods to improve the quality and safety of milk such as modelling heat processing, removal of bacteria and microfiltration techniques

Part I

Key requirements for milk quality and safety

1

Milk biochemistry

A.L. Kelly, University College Cork, Ireland

L. Bach Larsen, University of Aarhus, Denmark

Abstract:

As milk is the key base raw material for all dairy products, the safety and quality of such products are heavily influenced by the characteristics of the milk. In this chapter, the key constituents of milk (fat, protein, salts, lactose and enzymes) and their properties are described, and the factors affecting the chemical composition and processing characteristics of milk, such as diet and lactation, are discussed in detail.

Key words:

milk

composition

quality

processing

1.1 Introduction

Milk is the fluid secreted by female mammals for the purpose of providing high-level nutrition to their offspring in the first days or weeks of life. Mankind has, for millennia, domesticated a small number of mammalian species, e.g., cows, goats, sheep and buffalo, for the purpose of producing their milk over an artificially lengthened season, and consuming it either directly or after conversion into a range of dairy products. Today, a very significant proportion of food consumed worldwide has its origins in mammalian milk, and a huge and diverse dairy industry is at the forefront of the global food industry in terms of scale, economic significance and technological sophistication.

It is well known that many people worldwide, e.g. in Asia, have problems tolerating milk due to lactose intolerance. It has recently been discovered that the ability of most European adults to tolerate milk is the result of a mutation in a single gene, which gave our ancestors bearing that mutation an advantage for survival, and furthermore that the tolerance for milk in the Saudi population is the result of a different mutation leading to the same adaptation to consumption of milk (Enattah et al., 2008). The mutation results in continuous production of the enzyme responsible for the cleavage of lactose, the lactase enzyme (β-galactosidase), which is produced by cells in the intestine. This mutation is thought to have originated in the Caucasus region before people migrated to Europe after the last ice age. Even though it is sometimes said by some people that ‘milk is not intended as food for adults’, the discovery of this mutation, which is thought to have its origin in a single person from whom it was spread, strongly indicated this to be very beneficial for the survival of our ancestors. During the last ice age, it gave them the possibility of exploiting the valuable nutrients in milk from domesticated animals, which was an obvious advantage for their survival at times with limited food alternatives.

The characteristics and quality of dairy products from market milk to cheese and yoghurt depend to a large extent on the primary stage of milk secretion within the mammal, and milk is a highly variable and complex raw material for processing. Hence, understanding of the mechanism of secretion of milk, the factors affecting the composition of milk, and ways in which milk composition and yield can be manipulated are of great interest to processors and farmers alike.

The objective of this chapter is to outline the major constituents of milk and their properties, to explore the manner in which they are secreted in the mammal, and to discuss factors affecting this production, and hence the quality of milk. The focus of the discussion will be on bovine milk, as the predominant milk-supplying species in most countries.

1.2 Milk composition and constituents

Milk is an enormously complex physicochemical system, with multiple constituents in different phases and states existing in a delicate balance of forces which exists on the brink of stability. It can readily be destabilised so as to collapse into separated or altered states; indeed, these phenomena had been exploited to produce dairy products long before their scientific mechanisms were understood.

In essence, milk is a solution of dilute salts, a simple sugar and vitamins, in which fat is emulsified as globules, and which contains a complex system of proteins, most of which exist in colloidal aggregates of thousands of molecules (casein micelles), an order of magnitude smaller than the fat globules. Studying milk under progressively higher microscopic magnification thus reveals a teeming multiphase system of complex biological molecules arranged in highly structured complex entities.

1.2.1 Lactose

In concentration terms, the dominant constituent of milk is generally lactose, a disaccharide consisting of one linked molecule each of glucose and galactose, which is present at 4.5–5.0% in bovine milk. The level of lactose in milk is relatively constant, and has an influential role on milk yield, as lactose is synthesised by the mammary gland, and determines how much water is drawn into the milk. The presence of lactose makes milk a highly fermentable medium, as a large number of bacterial species (collectively termed the lactic acid bacteria) can hydrolyse lactose to lactic acid, which reduces the pH of milk and, as we will see, results in coagulation if this drop is great enough (i.e., when the pH reaches 4.6, the isoelectric point of casein). While uncontrolled or unwanted fermentation clearly results in spoilage of the milk, controlling this fermentation is the basis of production of dairy products such as cheese and yoghurt. Lactose is also of interest due to its propensity, as a reducing sugar, to undergo Maillard reactions at high temperature, leading to colour changes in milk heated to very high temperatures (e.g., during sterilisation processes), and to its crystallisation behaviour, which is principally of significance in highly concentrated dairy systems, such as evaporated milk.

1.2.2 Milk fat

The next most abundant substance in milk is generally fat, although the level of fat can vary from below 3.0% to more than 5.0%, a much greater range than that of any milk constituent. The main constituent of milk fat is triglycerides (more than 95% of the milk fat), which consist of three fatty acid molecules esterified to a glycerol molecule. Milk contains several types of fatty acids, differing in the length of the chain of carbon atoms (and classified on this basis into short-, medium-and long-chain) and numbers of double bonds, i.e., whether saturated or unsaturated (Jensen, 2002; Huppertz et al., 2008). Compared to other types of food, milk fat is characterised by a great diversity of fatty acids, with chain lengths from four carbons up to more than 20 carbons, as well as branched fatty acids produced by microbes. The chemical properties of fatty acids have considerable consequences for both the nutritional quality of milk (in terms of the healthiness or otherwise of saturated fats) and its technological properties; chain length and degree of saturation both influence the melting point of the triglyceride, and hence the ultimate hardness of milk fat at, for example, refrigeration temperature. Compared to bovine milk fat, vegetable fats such as olive oil have a far higher proportion of unsaturated fatty acids, and hence provide both softness and perceived health benefits to consumers when added to products such as margarine and dairy spreads.

Milk fat also contains low levels of mono-and diglycerides, and minor constituents such as cholesterol, sphingolipids and phospholipids. Recently, attention has been drawn to some possible beneficial effects of some of the fatty acids in milk, including conjugated linoleic acid (CLA) and short-chain fatty acids (SCFAs) (Collomb et al., 2006; McIntosh et al., 2006; Bisig et al., 2007; Chilliard et al., 2007).

Milk fat is present in the milk as milk fat globules (MFG) with diameters ranging from 0.1 to more than 10 μm. The globules contain a nonpolar core of triglycerides and cholesterol esters. The milk fat in the core is protected and rendered (almost) stable in the aqueous environment of milk by the presence of a protective coating on the surface of the spherical globules, the milk fat globule membrane (MFGM), which stabilises the emulsion and protects the triglycerides from degradation (lipolysis). The structure and composition of milk fat were reviewed by Jensen (2002), and the physical stability of milk fat globules was reviewed by Huppertz and Kelly (2006). The MFGM consists of a double-layer phospholipid membrane into which different proteins are embedded, giving the MFGM specific characteristics. These proteins include some major proteins such as butyrophilin and the enzyme xanthine oxidase, and an increasing list of minor proteins are associated with the MFGM (Reinhardt and Lippolis, 2006; Fong et al., 2007). Studies on knock-out mice, in which functional genes coding for either xanthine oxidase or butyrophilin were lacking, have indicated some functions of the proteins associated with the MFGM. In both types of mice, large droplets of lipid without a proper outer membrane were secreted, and fused together into large aggregates of lipid; t...