Module 3. Microorganisms associated with milk

Lesson 11

MICROORGANISMS ASSOCIATED WITH RAW MILK AND THEIR SIGNIFICANCE – I

11.1 Introduction

Milk, like many other foods and environment around us, contains bacteria that can be classified into three categories:

· Beneficial

· Spoilage

· Harmful or pathogenic

Beneficial bacteria help in making food products, for example, cheese, yogurt, and buttermilk.

Spoilage causing bacteria leads to bad smell/ taste and poor look of the product. Most bacteria in fresh milk from healthy animals are usually harmless. However, rapid changes in the health of a milk animal or of the handler, or contaminants from polluted water, dirt, manure, air, cuts and wounds can make raw milk potentially dangerous, if these factors introduce harmful bacteria to milk, as it provides an excellent medium for the growth of bacteria. Milk is sterile, when it is secreted in udder but gets contaminated before it leaves udder. However, contaminating bacteria are usually harmless except in case of mastitis, but can lead to different spoilage problems. Microorganisms causing spoilage may be categorized as psychrotrophs, mesophiles or thermophiles. These microbes are of great significance in milk as:

· Information on microbial counts of milk can be a diagnostic criteria for the judgement of its overall quality

· If multiply, bacteria can cause spoilage of milk much faster

· Milk is highly susceptible to contamination with pathogens and precautions must be taken to minimize it and to destroy pathogens, if these gain entry

· Certain types of microbes produce chemical changes that are desirable in the production of different dairy products, for example cheese, yogurt etc.

11.1.1 Harmful or pathogenic bacteria

The consumption of raw milk was prevalent in the society even prior to the industrial revolution and discovery of pasteurization. Before that nobody knew about the infectious microbes and their association with water, milk and milk based products and the various diseases they cause. Initially, no testing procedure was available to determine the quality of raw milk and also to discriminate between healthy and infectious milk. With the advent of modern scientific techniques, now it is possible that the milk is free from harmful microbes but many problems may still persist like eradication of their toxic metabolites. Significant work has been done to combat all these issues like enforcement of sanitation and testing procedures at farm level and implantation of various practices at commercial level. However, pasteurization continues to be an effective method in control of these unseen problems. These days with innovative techniques one can identify microorganisms at genomic level to establish preventive measures effectively.

11.2 Microflora of Raw Milk

Milk provides a wealth of nutritional benefits, but raw milk can harbour dangerous microorganisms that can pose serious health risks to the consumers.

Milk from cows, sheep, or goats that has not been pasteurized to kill harmful bacteria is raw milk. This raw, unpasteurized milk can carry dangerous bacteria such as Salmonella, E. coli, and Listeria that are responsible for causing numerous food-borne illnesses.

Milk has been considered as the most significant part of our food, as it contains all essential nutrients, suitable pH, and water activity for the growth of microbes. Hence, there is a high risk for these products to get contaminated with spoilage causing microbes. Hence, it is worth to know more about its chemistry, spoilage and different preservation methods, so that microbes responsible for negative effects could be identified. Milk is a good source for the growth of microorganisms therefore, it is essential to understand more about the types of microbes associated with milk, their control measures and beneficial uses.

Milk when secreted into the udder of an animal is considered to be as sterile; and it gets contaminated later from the microflora residing in the udder tissues and most of the times the contaminants are non-infectious except in case of mastitis. Milk becomes more contaminated, when it comes in contact with an external environment during milking, transportation and storage. Other factors such as utensils, machines, handlers, water, air and soil, also contributes greatly to the micro flora of milk. This way bacteria, yeast and moulds get entry into milk constituting the normal microflora. The number of contaminants added from various sources depends on the level of precaution taken to avoid contamination.

The presence of these non-pathogenic microbes in milk is not very serious, but if these multiply quickly, these may cause souring of milk, putrefaction and development of different taints, undesirable flavours and colours of milk and milk products (Table 11.1). Thereby, it is essential to develop the suitable preventative measures to check spoilage of such highly nutritious food commodities that are perishable in nature.

Table 11.1 Bacterial types commonly associated with milk

Name of micro-organism	Activity associated
Pseudomonas	Spoilage
Brucella	Pathogenic
Enterobacteriaceae	Pathogenic and spoilage both
Staphylococci
Staphylococcus aureus	Pathogenic
Streptococcus
S. agalactiae	Pathogenic
S. thermophilus; L. lactis ssp. lactis	Acid fermentation
L. lactis ssp. lactis biovar diacetylactis	Flavour production
L. lactis ssp. cremoris; Leuconostoc lactis	Acid production
Bacillus cereus	Spoilage
Lactobacillus
L. lactis; L. bulgaricus; L. acidophilus	Acid production
Propionibacterium	Acid production
Mycobacterium tuberculosis	Pathogenic

 11.3 Microbial Contaminants from the Surroundings

1. Animal and its udder: The animal and its teats contribute greatly to the microflora of milk that includes those that are resided on skin and also that are present in the niche of animal, where it is reared and/ or milked.
2. Utensils and sanitary practices: The hygienic conditions inside milking system influence total microbial load of raw milk. Milk residues left behind on equipments may support the growth of different microbes that on subsequent milking may lead to spoilage milk. These may account for 100,000 to a billion microbes per millilitre in the cracks, scratches of utensils etc.

The microbial load of raw milk finally will be concluded on the basis of different contaminants in the surroundings. Milk microflora contains a mixed population of lactic acid bacteria, mesophiles and thermophiles, thermoduric, psychotropic and pathogenic microbes. The milk microflora will be influenced by a number of environmental parameters that affects the growth microbes.

11.4 Psychrotrophs

Psychrotrophs are the microorganisms that have the ability to grow near freezing temperature in the range of 0 to 7°C. The typical representatives of this class are both Gram negative and positive microorganisms.

Gram negative includes: Pseudomonas, Achromobacter, Aeromonas, Serraita, Alcaligenes, Chromobacterium and Flavobacterium spp.

Gram positive includes: Bacillus, Clostridium, Corynebacterium, Streptococcus, Lactobacillus and Microcobacterium spp.

The quality of raw milk has been considerably maintained by refrigeration on farms, and in processing plants. However, the current practices in India for the collection and storage of the raw milk favor the growth of psychrotrophs that are, able to grow below 7°C, regardless of their optimal growth temperature. Besides, their rapid growth ability in refrigerated milk, psychrotrophs produces heat stable extracellular proteases, lipases, phospholipases etc. Psychrotrophs are those bacteria that can grow at 7°C, although their optimal growth temperature is higher. During cold storage after milk collection these dominate the micro-flora, and their extracellular enzymes, mainly proteases and lipases, contribute to the spoilage of dairy products. The extracellular enzymes can resist pasteurization (72°C for 15 s) and even ultra-high temperature (138°C for 2 s) processing. The lipases, by hydrolyzing triglycerides, cause flavour defects associated with fat breakdown in cream, butter and cheese. Proteases are associated with bitterness in milk, gelation of sterilized milk, and reduced yields of soft cheese. Most proteases can degrade caseins and are highly heat stable. Overall, psychrotrophs play a greater role in spoilage of refrigerated milk. The numbers of psychrotrophs that develop after milk collection depend on the storage temperature and time. Under sanitary conditions, <10% of the total microflora are psychrotrophs in contrast to >75% under unsanitary conditions. Pseudomonas is the most prevalent psychrotroph in raw milk.

11.4.1  Significance of psychrotrophs in milk

Psychrotrophs are not only important from the spoilage point of view but also emerged as psychrotrophic pathogens like Listeria monocytogenes, Yersinia enterocolitica etc. that made the situation more severe. Over the last few decades, listeriosis has become a major food-borne disease, as L. monocytogenes can cause meningitis, septicemia, and abortion. Nearly, 20 to 30 % of people, who get listeriosis die from it and there is a longtime between food consumed and symptoms developed. Therefore, the control of psychrotrophs is important to stop spoilage, as well as health hazards to the consumers.

The cold storage of milk favours the growth of psychrotrophs. Following pasteurization or other heat treatment of raw milk to destroy or remove the microbes, spore germination or recontamination can still cause quality deterioration; furthermore, heat-resistant extracellular proteinases and lipases produced by psychrotrophs before processing are a major spoilage factors during storage of milk. The detection and control of psychrotrophs and their heat resistant enzymes are major concerns of quality control in dairy industry.

Thermo-resistant psychrotrophs: These are the psychrotrophs that survive pasteurization. For example - Arthrobacter, Microbacterium, Streptococcus, Corynebacterium, Clostridium; are spore formers and anaerobic. Among these thermo-resistant psychrotrophs; Bacillus spp. (sporeformer) dominates and secretes extracellular heat resistant proteinases, lipases and phospholipases (lecithinases). Control of psychrotrophs and their enzymes in milk can be achieved by:

· Following hygienic practices in different aspects of milk handling

· Minimizing storage time of milk

· Following effective HACCP system

· Strict maintenance of refrigeration at 4°C

· Use of non thermal techniques like use of high pressure for preservation

· Combination of heat treatment with aseptic filling or packaging

· Use of natural preservation by bacteriocins

· Treatment of milk with carbon dioxide or nitrogen

· Thermization of milk before chilling and storage

The suggested standards for psychrotrophs in raw milk are:

· Less than 10⁴ cfu per ml - Satisfactory

· Above 10⁴ cfu per ml – Unsatisfactory