Milk and dairy products have been an integral part of human diet throughout the world since ancient times. Bovine milk constitutes 90% of the world’s total milk production, but milk from buffaloes, sheep and goats is also consumed. Milk is a rich source of nutrients and provides a highly favourable growth environment for many microorganisms. Milk is sterile when secreted, but gets contaminated by bacteria while still in the udder. Inside the udder, bacteria are few in numbers and mostly harmless, but further infection occurs during handling and storage. Microorganisms in milk come from three main sources – the udder, the teats and their surroundings, and the milking process and equipment (Adams & Moss 2008). This paper examines the factors which facilitate the rapid spoilage of milk, lists the microorganisms involved and the types and mechanism of spoilage caused by them.
Factors Facilitating the Growth of Bacteria in Milk
All microorganisms have different water requirements, but most of them need a minimum water content of 18 to 20% to proliferate. High water content of milk (up to 87%) facilitates the growth of bacteria. Foods with an acidic pH (5 or below) can be attacked by mould but are resistant to bacterial growth. As milk has an approximately neutral pH of 6.6 to 6.8, it is a good medium for growth of bacteria. Milk spoils easily if not refrigerated as ample oxygen and hot, humid conditions are ideal for the growth of microbes. The physical structure of milk allows it to be easily invaded by microbes (as it has no external barrier). Chemical composition of food also determines the rate and type of spoilage possible. The presence of certain vitamins can encourage the growth of microbes while the absence of some vitamins can provide resistance to microbial growth (Pommerville 2010). Raw milk possesses certain anti-microbial factors to resist bacterial growth, but their concentration is very low in cow’s milk. Therefore, they are not very effective against microbial attack (Adams & Moss 2008).
Chemical Composition of Milk
Milk is an aqueous solution containing proteins, fats and carbohydrates with many vitamins and minerals. Water is the major constituent of milk (about 87 %). The rest of it is composed of fat, protein and lactose along with small quantities of minerals and vitamins (calcium and vitamins A, D and B12). Carbohydrates constitute about 5 % of milk. The major one is a disaccharide of glucose and galactose, called lactose or milk sugar. It is found exclusively in the milk of mammals. Proteins in milk are of two types – casein α and β (about 2.5 %), and whey proteins (0.6 %). Caseins are highly susceptible to proteolysis. Whey proteins include serum albumin, immunoglobulins, α-lactalbumin etc, and are less susceptible to proteolysis. Milk also contains some non-protein nitrogens like urea, peptides and amino acids (Montville & Matthews 2008). Fats make up about 4 % of milk. Milk fat is also known as butterfat, and 99 % of it is in the form of triglycerides. The rest is composed of diglycerides and phospholipids. The amount of fat can vary with the diet of the animal (Ranken, Baker & Kill 1997).
Some microbial inhibitors are also found in raw milk; these include lactoferrin, the lactoperoxidase system, lysozyme, folate and vitamin B12 binding system. The antimicrobial activity of lactoferrin is lessened by the presence of citrate in cow’s milk, as citrate competes with lactoferrin in binding the iron, which is vital to its action as an antimicrobial agent. The lactoperoxidase system is the most effective microbial inhibitor present in milk. It inhibits lactic acid bacteria, coliforms and various pathogens by catalyzing the oxidation of thiocyanate and reduction of hydrogen peroxide, thus facilitating the formation of hypothiocyanite (Montville & Matthews 2008).
Types of Bacteria Found in Milk
The bacteria normally found in milk can be either gram positive or gram negative. Gram positive bacteria in milk include species of Lactobacillus, Corynebacterium, Mycobacterium, Propionibacterium and Streptococcus. Gram negative bacteria include Staphylococcus, Pseudomonas, Salmonella, Escherichia, Enterobacter, Proteus, Aerococcus, Micrococcus and Shigella. The microorganisms found in raw milk come from three sources: the udder interior, the teat and its surroundings and the process of milking and handling. Bacterial count can be especially high in the presence of mastitis, an inflammation of mammary glands (Adams & Moss 2008).
What is Spoilage?
Spoilage refers to destruction of good qualities. When food is spoiled, there happens deterioration in texture, flavour, colour or odour, and it becomes unappetizing or unsuitable for consumption. Microbiological spoilage of milk occurs due to the action of microbes and production of certain chemicals as by-products of microbial metabolism such as gas, pigments, foul odours and flavours. It involves breakdown of proteins, carbohydrates and fats by microorganisms. Microorganisms grow at an exponential rate and, because of this, microbial spoilage is sudden in onset. The figure below depicts microbial growth with time (Adams & Moss 2008, p.120).
Spoilage of milk occurs due to growth of bacteria
The main bacteria responsible for spoilage are as follows:
- Lactic acid bacteria. These bacteria act on lactose and ferment it to produce lactic acid and acetic acid, which gives a sour taste to milk. These include Streptococci and Lactobacilli. A sour flavour can be detected when the acidity reaches 0.2-0.3%. A large quantity of acid can change the structure of protein and make it solidify into a type of curd. This acidic curd is known as sour curd. This is a common type of spoilage that occurs in the kitchen refrigerator or in the supermarket. The bacteria causing it are the ones which survived the pasteurization process (Pommerville 2010).
- Coliforms. Coliforms in milk are mainly represented by Escherichia and Enterobacter species. The actions of these bacteria result in a bitter taste and foul odour of milk. These are facultative anaerobes which act on lactose to produce lactic acid and gas (carbon dioxide and hydrogen). The acid causes the protein to curdle, and the gas causes the curds to be forced apart, so that sometimes they violently explode out of the container. This is called stormy fermentation. Coliforms are usually killed in the process of pasteurization. Therefore, their presence in pasteurized milk indicates insufficient pasteurization or contamination after pasteurization (Pommerville 2010).
- Enzyme producing bacteria. These are spore producing bacteria of the genus Bacillus. Their spores survive pasteurization and can germinate later. The extracellular enzymes of these bacteria hydrolyze protein and fat, producing proteases and lecithinases which cause sweet curdling and bitty cream defect in pasteurized milk (Ranken, Baker & Kill 1997). Sweet curdling is coagulation without any noticeable acid production or foul odour. Eventually, the enzymatic degradation of casein produces a bitter flavour and the growth of bacterial colonies may be seen as ‘buttons’ at the bottom of the carton. Bitty cream defect is the appearance of small proteinaceous fat particles seen floating on the surface of hot drinks and adhering to the surface of glasses (Montville & Matthews 2008).
- Psychotropic bacteria. Another group of enzyme producing bacteria is the psychotropic bacteria, most of which are gram-negative, but some are gram-positive Bacilli. The process of pasteurization destroys these bacteria but the enzymes produced by them are not deactivated completely. The residual enzymes act on proteins and fats in milk producing bitter, fruity and rancid flavours. When the population of psychotropic bacteria reaches 106 to 107 CFU/ml, there is sufficient concentration of enzyme to produce spoilage (Montville & Matthews 2008).
- Gram negative rods: such as Klebsiella, Alcaligenes and Enterobacter, are capsule producing organisms that multiply even at low temperatures and cause milk ropiness (Pommerville 2010).
- Grey rot of milk is caused by Clostridium species. The release of H2 S from cysteine produces the characteristic rotten-egg smell of milk. H2S reacts with minerals to produce a grey or black sulphide compound.
Other types of milk spoilage occur by deposition of blue or green pigment of Pseudomonas species, or the red pigment of Serratia marcescens (Pommerville 2010).
After collection, raw milk is quickly cooled and refrigerated until further treatment. It may then be subjected to a number of treatments to reduce the amount of microorganisms. The treatments popularly used are pasteurization, HTST (high temperature, short time) heating and the UHT or Ultra high temperature heating (Jay, Loessner & Golden 2005).
Milk is a good source of nutrients for both humans and microbes and is easily spoiled. Therefore, in order to extend the shelf life of milk, it is important to maintain good sanitation, clean equipment at the milking farms, good hygiene of cows, storage of milk at sufficiently low temperature after collection, pasteurization, timely marketing and refrigeration. The process of pasteurization helps to eliminate or markedly reduce the number of many spoilage microorganisms, but new techniques are needed to address the problem of post-processing contamination. We need better and more economical techniques to detect microorganisms that survive the processing treatment. Emphasis should be on developing such techniques that do not significantly alter the chemical composition and physical properties of milk like colour and taste. New techniques should combine convenience of use, assurance of high quality and good nutrition along with microbiological safety.