Milk and dairy products form a significant part of the human diet. They are rich sources of nutrients such as proteins, fats, vitamins and minerals; ironically, it is because of this that these products are susceptible to rapid microbial growth.
In some instances, this microbial growth may be beneficial, while in others it is undesirable. Dairy products are vulnerable to spoilage or contamination with pathogens or microbial toxins; therefore, the microbiology of these products is of key interest to those in the dairy industry.
Although milk produced from the mammary glands of healthy animals is initially sterile, microorganisms are able to enter the udder through the teat duct opening.
Gram-positive cocci, streptococci, staphylococci and micrococci; lactic acid bacteria (LAB), Pseudomonas spp. and yeast are most frequently found in milk drawn aseptically from the udder; corynebacterial are also common.
Main type of Milk organism in Milk
Microorganisms present in milk can be classified into two main groups: pathogenic and spoilage organisms, although some may play a dual role (e.g. Bacillus cereus).
Pathogenic organisms are those capable of inducing food poisoning, thus posing a threat to public health. By virtue of their enzymes (e.g. protease, peptidase, lipase, esterase, oxidase, polymerase, Beta-galactosidase), spoilage organisms are capable of hydrolysing milk components. Such reactions can lead to spoilage of milk, manifested as off-flavours and odours, and changes in texture and appearance.
Intrinsic Growth factors for microorganism in milk
a)Nutrient Requirements
While the nutrient requirements are quite organism-specific, the microbes important in food require
- Water
- Energy Source
- Carbon/ Nitrogen Source
- Vitamins
- Minerals
Milk and dairy products are generally very much rich in nutrients which provide an ideal growth media for microorganisms.
b) Moisture Content
All microbes need water but the amount necessary for growth varies between species. The amount of food that is available in food is expressed in term of water activity (aw) where the aw of pure water is one. Each microorganism has a minimum, optimum and maximum value of aw or growth and survival. Generally, bacteria dominate with high aw food; yeast & mould have low aw. The water activity of liquid milk is approx. 0.98aw.
C) pH
Most microbes have approx. neutral pH .Yeast can grow at an acidic pH compared to bacteria .Moulds can grow at a wide range but prefer an acidic condition. Milk has a pH of 6.6 which is optimal for the growth of many microbes.
d)Available Oxygen
Microbes can be classified on the basis of oxygen requirements-
- Obligate Aerobes- Oxygen Required
- Facultative-Grow in the presence or absence of oxygen
- Aero Tolerant- Oxygen not required for growth but not harmful
Obligate Anaerobes-Grow only in the complete absence of oxygen. If present can be lethal.
e)Biological Structure
Physical barriers such as skin, rinds, feathers etc. provide protection to plants and animals against microbial invasion however milk has no such barrier thus becomes more susceptible to the microbes.
f)Antimicrobiological Constituents
Lactoperoxidase, Lactoferrin, Lysozymes, Xanthin’s are some non-immunological proteins which are present in milk and inhibit the growth of microorganisms.
Extrinsic Growth factors for microorganism in milk
a) Temperature
As a group, microorganisms are capable of growth over an extremely wide temperature range. According to temperature, these microbes are classified as
- Psychotropic
- Mesophylls
- Thermophylls.
b) Relative Humidity
Relative humidity plays an important part in the growth of microbes. If the percentage humidity increases in the atmosphere than the microbial growth increases because microbes need a humid substrate for growth. All enzymes need water for their working.
Microflora of raw milk
The types of organisms present in raw milk are influenced by temperatures and time of storage as well as methods of handling during and after milking.
Spoilage organisms
The growth of psychotropic bacteria is of major concern when raw milk is kept at low temperature, which is the normal practice in the modern dairy industry .
During the growth of these bacteria, heat-stable enzymes such as proteases and lipases are formed and consequently cause protein and lipid breakdown and related defects.
Therefore, the metabolic processes of psychrotrophs are considered more important than the total numbers of organisms.
Gram Negative Bacteria
1.Pseudomonas spp.
- Species of the Pseudomonas genus are the most important because of their ability to produce heat-stable enzymes (particularly proteases and lipases) during growth under refrigerated storage .
- They are motile, gram-negative rods, with the ability to grow at temperatures just above freezing, despite their optimum growth temperature being between 25 and 30◦C .
- This genus is represented by species with short generation times (growth rate) at 0–7◦C , their generation times can be even shorter in the presence of air .
- The generation times of the most rapidly growing psychotropic Pseudomonas spp. isolated from raw milk are 8–12 h at 3◦C and 5.5–10.5 h at 3–5◦C .
- These growth rates are sufficient to cause spoilage within 5 days at these temperatures if the milk initially contains only 1 colony count unit (cfu) mL−1 .
2.Enterobacteriaceae
- Enterobacteriaceae account for 5–33% of psychotropic microflora present in raw milk.
- These organisms are small, motile, gram-negative rods.
- Their optimum growth temperature (>30◦C) tends to be higher than that of pseudomonas, but they adapt well to growth at refrigeration temperature .
- Coliforms belonging to this group are able to ferment lactose with the production of acid and gas at 32◦C within 48 h.
- Members of the genera Enterobacter and Klebsiella are most often associated with coliform spoilage.
Other psychotropic bacteria
- Other types of psychrotrophs commonly found in raw milk include Flavobacterium, Achromobacter, Aeromonas, Alcaligenes and Chromobacterium.
- They, like pseudomonads, are gram-negative rods capable of growing at low temperature.
- Acinetobacter and Psychrobacter are of limited spoilage potential because their growth at low temperatures is slower than that of other psychrotrophs and so they are often overgrown by Pseudomonas spp.
Gram-positive bacteria -Spore forming bacteria
- Spore-forming bacteria in raw milk are predominantly Bacillus spp. Bacillus contamination levels, although variable, are found up to 105 cfu mL−1.
- The optimum growth temperature for most Bacillus spp. is 20–40◦C , although, for some, such as Bacillus stearothermophilus (now Geobacillus stearothermophilus), it is higher.
- Therefore, the generation and lag times of psychotropic Bacillus spp. at 2–7◦C are considerably longer than those of Pseudomonas spp, although psychotropic spore-formers may become the dominating microflora in spoiled milk at 10◦C .
- Of the Bacillus spp. found in milk, B. licheniformis, B. cereus, B.subtilis and B. megaterium are most commonly isolated.
- These gram-positive motile, spore-forming, rod-shaped organisms have also been implicated as the cause of a variety of proteolytic defects
- B. cereus is a common contaminant of raw milks , being present in over 80% of raw milk samples . There is a distinct seasonality in the occurrence of the organism in raw milk supplies with the highest levels being in late summer and early fall , indicating that contamination occurs at the farm.
- B. cereus usually gives rise to milk spoilage defects such as bitty cream, sweet curdling and various off-flavours. Populations in raw milk vary seasonally.
- In temperate climates, clostridia are at higher levels in raw milk collected in the winter than that collected in the summer, because in winter, in many countries, cows are housed and lie on spore-contaminated bedding materials and are more likely to consume spore-laden silage.
Lactic acid bacteria
Spoilage of raw milk resulting from growth of acid-producing fermentative lactic acid bacteria occurs when storage temperatures are sufficiently high for these microorganisms to outgrow psychotropic bacteria or when product composition is inhibitory to gram-negative aerobic organisms .
This particularly occurs in countries where milk is still stored on-farm and transported in unrefrigerated containers. Spoilage can occur, especially inmhot weather, before milk is delivered to the factory.
Species of Streptococcus, Enterococcus, Lactobacillus, Leuconostoc, Lactococcus and Pediococcus are involved.
Lactococcus lactis subsp. lactis is the main species responsible for spoilage of raw milk at 10–37◦C, being able to produce acid (about 0.25%, mostly lactic acid but also small amounts of acetic and propionic acids) to cause milk to sour .
Some Enterococcus isolates can grow at 7◦C and have detectable proteolytic activity. These microorganisms constitute only a minor population of the microflora in raw milk, but their numbers may be proportionally higher in pasteurised milk because of their resistance to pasteurisation.
Pathogenic organisms of Milk
Numerous milkborne pathogens have been isolated from raw milk. The prevalence of these varies considerably, depending on geographical area, season, farm size, number of animals on farm, hygiene and farm management practices ).
Although the growth of these pathogens in milk is known to be inhibited by cooling and competing for non-pathogenic microorganisms, outbreaks of illness caused by Campylobacter jejuni, Shiga toxin-producing Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, Salmonella spp. and Yersinia enterocolitica have been reported following consumption of raw milk .
Pathogenic organisms in raw milk are of two types: those that are involved in bovine mastitis and those that externally contaminate milk.
The bacteria causing mastitis, which vary geographically and with different animal husbandry practices, include S. aureus, Streptococcus agalactiae, Str. dysgalactiae, Str. uberis, Listeria spp. and E. coli .
The staphylococci are of concern to the dairy industry and public health as some of them, particularly S. aureus, produce heat-stable enterotoxins that can cause food poisoning. Str. agalactiae causes bacteraemia and meningitis, which are potentially fatal to infected infants (.Salmonellae and thermoduric Campylobacter strains are the most common pathogenic bacteria from sources external to the udder.
Salmonellae, particularly, continue to be a major concern for the dairy industry because they have been implicated in numerous outbreaks of illness. The emergence of multiple antibiotic-resistant strains of Salmonella ser. Typhimurium DT104 should be of greater concern to public health authorities since this organism is reportedly resistant to antibiotics commonly used in medical and veterinary practice.