Raw Milk

Balance Between Hazards and Benefits

Luis Augusto Nero, Antonio Fernandes De Carvalho

• Publisher: Academic Press
• Year: 2018
• Print ISBN: 9780128105306
• eBook ISBN: 9780128105313
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

Raw Milk: Balance Between Hazards and Benefits provides an in-depth nutritional and safety analysis of raw milk. This high-quality reference is comprised of contributions from global researchers highly specialized in the field. The book is divided into five sections that address the characteristics of raw milk, production guidelines and concerns, the benefits and hazards of raw milk, and the current market for raw milk. Topics include production physiology and microbiology, rules and guidelines for production, the world market for raw milk and its products, and consumer acceptance. A final section identifies future trends and research needs related to raw milk.- Provides current information related to raw milk's characteristics- Presents worldwide coverage of raw milk production and government guidelines- Addresses the benefits and hazards related to raw milk consumption- Analyzes the worldwide economic impact of raw milk production and consumption

Chapter 1

Raw Milk Physiology

Christopher H. Knight, Department of Veterinary and Animal Sciences, University of Copenhagen, Copenhagen, Denmark

Abstract

The physiology underpinning the development of the mammary gland and the synthesis and secretion of milk is reviewed from the perspective of consumption of raw milk. The review considers the origins and purpose of lactation, then proceeds to address the mammary gland itself. The different physiological processes that together constitute lactation are reviewed, starting with the development of the mammary gland during juvenile life and, especially, during gestation. Lactogenesis (the initiation of secretion) follows, then the secretory process itself and storage of the product prior to removal, which includes the process of milk ejection. The review aims to identify those elements of lactation physiology that are most relevant to safe consumption of raw milk, and also to be prospective rather than retrospective wherever possible.

Keywords

Lactation physiology; raw milk safety

1.1 Introduction

Consumption of raw milk is contentious and often ill-informed. Is raw milk “better” than other milk? Is it more hazardous? Raw milk is generally taken to be milk that has not been pasteurized, but the comparison that consumers will draw is usually with a processed milk that is not only pasteurized but also standardized, homogenized, and semi-skimmed. Although raw milk is not processed in a technological sense, it is subject to biological processing. The aqueous exocrine secretion of the mammary epithelial cell (milk) is subject to enzymatic and other modification during storage in the udder, during the milking process, once it reaches the farm’s bulk tank, when mixed with milk from other farms, and during packaging and distribution to consumers. My brief in this short chapter is to describe the physiology of “raw milk” (of course, lactation physiology remains the same whatever product is being consumed) and I shall start by considering the origins and purpose of lactation, then turn to consider the mammary gland itself. The development of the mammary gland during juvenile life and, especially, during gestation is followed by lactogenesis (the initiation of secretion), secretion itself, storage of the product prior to removal, and then milk ejection. During the lactation secretory cells are subject to developmental remodeling (especially just after parturition), repeated mechanical disruption during alveolar filling and emptying, potential attack by pathogenic bacteria, and finally, at the end of the lactation, regression and involution. All of these processes have been extensively research and reviewed before, and rather than simply repeating that exercise I shall try to identify those elements of lactation physiology that are most relevant to safe consumption of raw milk and also to be prospective rather than retrospective wherever possible.

1.2 Historical Context and the Evolution of Lactation as a Multipurpose Physiological Survival Mechanism

The teaching of lactation physiology is generally rudimentary, restricted to a short text at the end of a reproductive biology chapter, but the mammary gland is used as a model system by many scientific disciplines. As a consequence, literature searches that include the term “lactation physiology” return an incredibly large amount of recent literature focused on molecular investigations of mammary biology and breast cancer, but relatively little mechanistic explanation of lactation as a biological process. Fortunately the older literature, most of which is still valid, was excellently reviewed relatively recently in a series of papers published in the Journal of Mammary Gland Biology and Neoplasia (hereafter JMGBN; Neville, 2009). A good grasp of the topic can also be gained from the suitably short but informative textbook Physiology of Lactation (Mepham, 1977a) or the more recent and comprehensive “Lactation and the Mammary Gland” (Akers, 2002). For the purposes of this chapter, the different physiological processes that together constitute lactation are shown in Fig. 1.1.

Figure 1.1 An overview of the different physiological processes that together constitute lactation.

There is debate regarding the evolutionary origins of lactation, some holding that the porous nature of synapsid’s eggs drove the need for an aqueous skin secretion (Oftedal, 2012), while others believe that the mammary gland has evolved as a specialized component of the immune system (Mclellan et al., 2008). The reason for mentioning this is to highlight the multitasking nature of lactation, for it is quite evident that milk provides a triad of nutrition, protection, and hydration. The relative importance of these functions will vary to some extent from species to species, a simple example being the transfer of passive immunity in colostrum, which is much less essential in humans than in cows because of transplacental transfer to the human fetus prepartum. This reveals another important aspect, namely, the interaction and interdependency of gestation and lactation. From an evolutionary point of view, some species have attached similar importance to each of these two reproductive phases whereas other have not; marsupials give birth to incredibly immature young and have an almost total reliance on lactation (the pouch has famously been described as “a womb with a view”; Renfree, 2006), while at the other end of the spectrum young guinea pigs are born so mature that lactation is almost an optional accessory. In most species the mammary gland develops during pregnancy in order to meet the requirements of the neonate (not the mother!) so endocrine mechanisms have evolved to ensure that the fetus(es) can influence mammary growth and hence their own destiny (Knight, 1982). The secretion of the mammary gland contains species-appropriate amounts of energy in the form of fat and carbohydrate, and once again the importance attached to each will vary (Oftedal, 1984). Breastmilk supplies the majority of energy for the slow-growing human neonate as carbohydrate (lactose), while the milk of pinnipeds (seals) is incredibly rich in fat to provide for rapid growth and thermoregulatory insulation. Cow’s milk is intermediate. Milk protein (caseins and whey proteins) also varies across species, the most notable feature being the low amounts present in breastmilk and equid milks. That is not to say that the importance of milk protein is less in these species: A major feature of proteins and especially peptides present in different milks is their regulatory role or bioactivity, which may require only trace quantities. “May” is an important word. On the one hand, minor components in any food can sometimes elicit major allergic or other reactions, but on the other hand, the digestive system is designed to digest, in this case to individual amino acids. Interest in health-promoting bioactivities in milk has been intense for several decades (see, e.g., Weaver, 1997), but suffice to say that the European Food Safety Authority has not yet licensed any milk product as having health-promoting effects. A recent review of breastmilk composition and bioactivity emphasizes the potential impact of oligosaccharides as probiotics (Andreas et al., 2015) and the potential biological role of specific individual fatty acids, in particular conjugated linoleic acid (CLA) (reviewed by Kim et al., 2016) demonstrates that milk is a diverse food with a broad spectrum of potential bioactive effects, and that is without even considering all of its minerals and trace elements. A detailed description is certainly beyond the scope of this chapter, but we shall consider how the appearance of bioactive factors might be affected by different physiological mechanisms. Another important property of milk is the way in which the nutrient is delivered, calcium being a case in point; nanoclusters of calcium phosphate are present as substructures in casein micelles at concentrations well above their solubility (Lenton et al., 2015). Occasionally a particular milk might have a specific or even unique characteristic. For instance, there is growing evidence that camel milk can deliver significant quantities of biologically available insulin (Meena et al., 2016).

Having decided to secrete milk and agreed on what to put into it, for how long should lactation continue? The consensus appears to be around 10 months in cattle and at least 6 months in women, but the physiological reality is that this is another “flexible feast” (Knight, 2001). For species that have no alternative way of feeding their young, the answer has to be “for as long as the neonate needs milk,” and where ungulates are concerned this could vary enormously as a result of food availability (seasonally breeding deer will “skip” breeding for a full year if conditions are bad, since the fawn requires to be nursed for longer; Loudon et al 1983). This introduces another concept that is important for any discussion of raw milk. The biologically intended consumer of milk is the neonate, but in this book we are mainly considering older consumers. The nutritional requirements of the human neonate are normally well met by breastmilk, although there is sometimes debate about the detail (Knight, 2010) and increasing recognition that human lactation can fail (Marasco, 2014). The optimal requirements of older consumers will be very different and dependent on many lifestyle-related factors, especially other diet and exercise. There has long been an epidemiologically based belief that consumption of milk, and especially milk fat, is detrimental to health, in particular cardiovascular health. More recent reexamination of the evidence has shown this to be untrue (Thorning et al., 2016); milk is good for you!

1.3 The Physiology of Mammary Gland Development

1.3.1 Juvenile Development

The process of mammary growth and development is known as mammogenesis and mainly occurs during gestation under the stimulatory influence of ovarian steroids and placental lactogen. Nevertheless, there are “critical windows” of mammary development occurring earlier in life (Knight and Sorensen, 2001), which are related to the complex multicellular nature of the gland. At birth and well into juvenile life, the gland consists of relatively isolated ductular epithelial tissue lying in a fat pad. The mammary epithelium will not grow outside of that fat pad, and starting in early life there is clear evidence of regulatory interaction between the mesenchymal elements of the fat pad (and the extracellular matrix that is formed from it) and the epithelial cells of the ducts (Kratochwil, 1986; Faulkin and Deome, 1960). Later, however, an excess of adipose tissue has local inhibitory effects on the growth of the duct system. This has been demonstrated in the prepubertal heifer where it is proposed to be due to disturbance of the growth hormone (GH): insulin-like growth factor axis (Sejrsen et al., 2000). Whether this growth inhibition fully explains the subsequent reduction in milk yield observed in overfed heifers is open to debate; the udder develops progressively but slowly over a long period of time and it is not always possible to discount age-related differences in animals that grow at different rates (Daniels et al., 2009). Nevertheless, there is clear evidence in obese mice of an inhibitory effect of excess adipose deposition in the mammary fat pad on both the development and subsequent function of the secretory tissue (Flint et al., 2005). Given the growing concern surrounding lactation inadequacy in obese women (reviewed by Nommsen-Rivers, 2016) this is an area deserving of further investigation.

1.3.2 Gestational Mammogenesis

In all species studied there is exponential proliferation of mamm...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Chapter 1. Raw Milk Physiology
• Chapter 2. Physicochemical Characteristics of Raw Milk
• Chapter 3. The Microbiology of Raw Milk
• Chapter 4. Regulations and Production of Raw Milk
• Chapter 5. Conventional and Emerging Clean-in-Place Methods for the Milking Systems
• Chapter 6. Alternative Processing Procedures and Technological Advantages of Raw Milk
• Chapter 7. Nutritional Aspects of Raw Milk: A Beneficial or Hazardous Food Choice
• Chapter 8. What Bacteriocinogenic Lactic Acid Bacteria Do in the Milk?
• Chapter 9. Artisanal Products Made With Raw Milk
• Chapter 10. Alternative Dairy Products Made With Raw Milk
• Chapter 11. Infectious Diseases in Dairy Cattle
• Chapter 12. Foodborne Pathogens and Zoonotic Diseases
• Chapter 13. Chemical Residues and Mycotoxins in Raw Milk
• Chapter 14. Cow’s Milk Protein Allergy and Lactose Intolerance
• Chapter 15. Consumers Acceptance of Raw Milk and its Products
• Chapter 16. Challenges for Production and Consumption of Raw Milk and Raw Milk Products
• Index