Module 1. Hygienic milk

Lesson 1

INTRODUCTION AND SIGNIFICANCE OF DAIRY MICROBIOLOGY

1.1 Introduction

As per the definition, a branch of biology that deals with the study of microorganisms and their different activity is termed as microbiology.  Since ages, these microorganisms are playing a potential role in human welfare both as useful and harmful biological agents. This leads to an extensive study of these micro-organisms with the aim of understanding their growth and nutritional requirements for their production or destruction both.

The different branches of microbiology are:

·         Agricultural and soil microbiology

·         Aquatic and marine microbiology

·         Dairy and food microbiology

·         Environmental microbiology

·         Industrial microbiology

·         Medical and pharmaceutical microbiology

·         Rumen microbiology

·         Space microbiology

1.2 Definitions

Milk and milk products occupy a more significant role in the human food profiles. The study of microorganisms that are associated with milk and milk products in all aspects is defined as “Dairy Microbiology”.

1.2.1 Milk

Milk is described as a whole, fresh, clean, lacteal secretion obtained from the complete milking of healthy milch animal (excluding this milk is obtained within 15 days before or five days after calving to render the milk practically colostrum-free) containing the minimum prescribed levels of fat and solids non-fat.

1.2.2 Milk hygiene

Milk hygiene is concerned with the production of clean, wholesome milk that is free from bacteria or other disease causing micro-organisms and maintenance of this condition from farm to the consumers.

In general milk is considered as the most nutritious and complete food for neonates and adult human beings both. An approximate composition of major constituents in milk of different animal origins is given in Table 1.1.  

Table 1.1 Percent composition of milk of different animal origins

Source (Animal)

Percent composition

Total Protein

Casein

Whey protein

Fat

Carbohydrate

Buffalo

4.0

3.5

0.5

7.5

4.8

Cow

3.5

2.8

0.7

3.7

4.8

Goat

3.6

2.7

0.9

4.1

4.7

Sheep

5.8

4.9

0.9

7.9

4.5

Dairy industry provides an excellent example where bacteria, yeasts, moulds and viruses are very im­portant in determining the quality of final product. The control and destruction of undesirable microorganisms, as well as their intentional introduction and utilization are problems that need special attention. The nutritional qualities of milk make it a desirable food for humans and other young animals. However, these nutritional values also permit growth of many microorganisms, some of which cause undesir­able changes in milk and its products. Milk’s sanitary qualities are influenced by many factors in the course of production, processing, and delivery to the consumers.

An example of dairy industry which is dependent to a larger extent on the desirable enzymatic changes caused by microorganisms is cheese making. The flavours and the texture of cheese are largely ascribed to the conversion of milk constituents by various species. Few bacterial and mould species are added intentionally to cheese during manufacturing and much of the microbial activity in that takes place during ripen­ing are due to microbial species that enter the milk by chance at different stages. The taste and aroma are competitive assets of butter. Starter cultures, that are mixed bacterial cultures selected for acid- and flavor-producing ability, are important in the manufacture of flavored butter. The conditions in which optimum flavor develops and the relationships with microbes are well known, and certainly the changes brought about by the starters are desirable.

Yogurt, sour cream, and buttermilk are some of the examples of fermented milks made by different intervention of microorganisms to milk, cream, and skim milk, respectively. Each of those products has a characteristic microflora that is partly responsible, for its characteristic flavor and texture. It is also possible to ferment milks having desired properties (i.e. flavor and acid-producing) by using starter cultures.

Microbial conversion of certain constituents of milk into economically valuable products like vitamins, solvents, and food adjuncts can be done commercially these days. On the other hand, milk constituents that have no economic significance, or that are usually wasted, must be converted to stably oxidized and non-obnoxious substances prior to their dis­charge into the environment. Here also microbial activity is responsible for the desired changes in the organic constituents of dairy wastes dung of sewage treatment.

Microbes are undesirable in milk or its products, if these deteriorate flavor or texture, and produce diseases. It is important that dairy microbiologists understand the facts that, influence the deterioration of milk and the methods through which this type of deterioration can be prevented. Souring of milk or cream is undesirable under majority of situations, but souring (i.e. production of lactic acid) is essen­tial also for the manufacture of cheese or cultured milk products. Discolorations, slimi­ness, ropiness, putrefaction, rancidity, gassiness, and many other defects are caused by different microbes that grow in dairy products. Milk occasionally has been the car­rier of microbes that can cause disease. However, these agents of disease can be effectively controlled. As the new processes and products are devel­oped by the dairy scientists, these must be checked to ensure that these are safe for consumers.

Many ordinances and other regulations under which milk is produced and handled specify quantitative, qualitative, and microbial standards. These standards have shown that under desirable conditions, the numbers of microbes gaining entrance to the product will not ex­ceed certain levels and, also, that proper handling of these products will not permit growth beyond certain levels. To a certain degree, the numbers of microbes permitted on these standards usually are far much below the numbers necessary to cause spoilage. Practically this results into a gradual up-gradation of microbial standards for different dairy products. Because of this type of control, the packaged milk produced by dairy industries is of high microbiological quality and hence, safe to the consumers.

1.3 Significance of Microbes in Milk

·         Information on microbial load  can be used to assess its sanitary quality and the conditions of production

·         Bacteria, if  permitted to multiply, cause spoilage of milk

·         Milk is potentially susceptible to contamination with pathogens, however precautions must be taken to destroy  them

·         Certain microbes produce chemical changes that are desirable in the production of cheese, yogurt and fermented milk products.

There is a need for knowledge of the microbiology of different dairy products as shown by the three distinct considerations placed below:

1.3.1 Spreading of diseases via milk

Different studies have proved that diseases in both man and animals are sometimes caused by the pathogens spread by milk. Although, the presence of such microorganisms is not specific only to foods from dairy but from a variety of other sources also. However, it is especially serious with dairy products because these are mostly consumed without being heated and thus without destruc­tion of harmful micro-organisms. Accordingly, the sources of pathogens in dairy products, condi­tions under which they grow, and methods of prevention or destroying them is essential. Such a knowledge is important to the manufacturer of dairy products, be­cause of the responsibility of producing only safe foods, and it is of extreme importance for the supervisor of dairy plants, as the inspection is the final protection efforts to the consumer. Although, the manufacturer and supervisor should ensure consumer's safety, but it is also desirable that the consumers are well aware to decide whether a particular dairy product is satisfactory or not? Similarly, the farmer using milk or milk products for animals feeding should know the probability of introducing diseases into his herds through such products.

The hygienic milk production practices, proper handling and storage of milk and pasteurization have decreased the threat of milk-borne diseases, such as tuberculosis, brucellosis, and typhoid fever. However, there have been a number of milk-borne illnesses resulting from the ingestion of raw milk, or dairy products made with milk that was not properly pasteurized or was poorly handled leading to post-processing contamination. The following bacterial pathogens are of major concern to in raw milk and other dairy products:

·         Bacillus cereus

·         Campylobacter jejuni

·         Escherichia coli

·         Listeria monocytogenes

·         Salmonella spp.

·         Yersinia enterocolitica

It should be noted that moulds, mainly the species of Aspergillus, Fusarium, and Penicillium can also grow in milk and dairy products. If conditions permit, these moulds may produce mycotoxins that can be a health hazard to the consumers.

1.3.2 Spoilage of milk

Micro-organisms, as a result of their growth or biochemical activities, cause undesirable changes in milk and, are responsible for spoilage. The producer of milk should be aware of the sources of micro-organisms causing rapid changes, conditions favoring their growth, and methods of preventing their activity. The manufacturer of milk products must contend with problems similar to those of  pro­ducer of milk and additional ones also, as butter, cheeses, etc. are frequently  stored for longer periods, during which may further decrease quality. The problem of spoilage is especially important with the cheeses they require ripening, since conditions must be favorable for growth of certain desirable microorganisms and may also allow the growth development of undesirable ones.

The initial microbial quality of raw milk is quite crucial for the production of good quality dairy foods. Spoilage is a term used to describe the deterioration of a food’s texture, color, odor or flavor to the point, where it becomes unsuitable for human consumption. Microbial spoilage of food often involves the degradation of protein, carbohydrates, and fats by the microorganisms or their enzymes.  In milk, the microorganisms that are mainly involved in spoilage are psychrotrophs. Most psychrotrophs are destroyed by pasteurization, however, some like Pseudomonas fluorescens; Pseudomonas fragi can produce proteolytic and lipolytic extracellular enzymes that are heat stable and capable of causing spoilage. Some species and strains of Bacillus, Clostridium, Corynebacterium, Arthrobacter, Lactobacillus, Microbacterium, Micrococcus, and Streptococcus can survive pasteurization and grow at refrigeration temperatures that can cause spoilage problems in milk and milk products.

1.3.3 Developing desirable microorganisms in some milk products

The pleasing flavor of dairy products as butter, cheeses, and fermented milks, and the desired texture of many dairy products are largely due to the development of certain microorganisms during manufacture and ripening. An understanding of the action of these organisms, conditions favoring their growth, etc., is important and necessary if products having a uniformly high quality are to be pro­duced and marketed. The manufacturing methods used for certain dairy products were developed empirically even before the role of micro­organisms was understood, but various valuable improvements have resulted from knowledge of the action of the important micro-or­ganisms.