Milk microbiology is a vast ocean and is too comprehensive to be explained in one or two paragraphs. I want to point out that there is a 1000-page book on lactic acid bacteria. However, I will explain briefly so that you can have an idea.
Microorganisms are the only reason we cannot consume raw milk. Likewise, we have to keep milk cold because of microorganisms. Raw milk is a favorable environment for the development of microorganisms in terms of nutrients and environmental properties such as pH, water activity and oxygen availability.
Theoretically, milk from a healthy animal is considered sterile; it does not contain any microorganisms. However, this is not possible in practice. Even if the animal is healthy, while the milk is still in the udder, microorganisms enter through the nipple and contaminate the milk.
In this way, there are approximately 300-4000 microorganisms in one milliliter of milk at the very first moment. However, if there is an infection in the animal’s udder, many microorganisms can pass into the milk from the infection.
After milking, the milk is flooded with microorganisms from its environment. Every environment that milk comes into contact with or is in, such as milking equipment, barn environment, milking workers, air, storage and processing equipment, causes microorganisms to contaminate milk.
The downside is that milk is also a very nutritious medium for microorganisms, and if the milk is at the optimum temperature where microorganisms can grow, the microorganisms multiply very quickly. Their number doubles every 20 minutes on average, so milk becomes unusable after about 5-6 hours.
From this point of view, it is understood how significant the number of microorganisms contaminating milk and the storage conditions of milk are. The milk produced by a clean milking system and a disinfectant-conscious producer contains an average of 1,000-10,000/mL microorganisms, which is normal. If the environment and milking system are not clean and the producer is not conscious, this value can go up to 1,000,000 /mL depending on the degree of contamination.
Controlling the temperature during the storage phase of the milk is as important as the number of microorganisms transmitted at the beginning. There is a considerable difference between keeping milk at 4oC and leaving it at 25oC. While the number of microorganisms increases slightly in milk stored at 4oC for one day, the number of microorganisms increases approximately 1,200 times in milk kept for one day at 25oC.
This means that the cleaning and disinfection at the beginning becomes meaningless if the milk is not stored at the appropriate temperature. Therefore, milk must be stored at the proper temperature, even if the cleaning conditions are followed in milking and storage.
On the other hand, raw milk cannot be stored for a long time, even at 4oC. Because at this temperature, psychrotrophic bacteria (bacteria that can grow in cold) can develop, albeit relatively slowly, and render the milk unusable. Raw milk can be stored at 4oC for a maximum of 2 days. Therefore, raw milk should be processed as soon as possible.
After this introduction, discussing the types of microorganisms found in milk is necessary. Due to the diversity of the farm environment in which the milk is milked, the animals, workers, air and other habitats in which it is milked, almost most of the known microorganism species can be transmitted into the milk. However, some species dominate the milk environment after a certain period.
There are microorganisms in milk that can spoil milk or harm human health, as well as microorganisms that are beneficial to human health, such as probiotics.
Apart from bacteria, yeast, mold, virus and rickettsia from other microorganism classes can also be found in raw milk. As with bacteria, the transmission routes are animals, milking workers, barn environment and processing equipment.
Bacteria
Bacteria found in milk can be grouped as follows;
1. Lactic acid bacteria; It is the most important group of bacteria found in milk. From this group, Lactobacillus, Lactococcus, Streptococcus, Enterococcus, Leuconostoc and Pediococcus genara are found extensively in milk. They break down lactose and release lactic acid. The lactic acid formed also increases the acidity of the milk and when the lactic acid accumulates in sufficient amounts, it causes the casein to coagulate.
This situation is undesirable when processing milk into drinking milk. However, in yogurt, this is the opposite. In yogurt production, lactic acid bacteria are added as a starter culture and the acidity increases to coagulate the casein. (For more detailed information on starter cultures, see Starter Cultures; History, Definition and Classification)
The most common members of lactic acid bacteria in milk are Lactococcus lactis ssp. lactis, Lactococcus lactis ssp. cremoris, Lactobacillus bulgaricus, Streptococcus thermophilus, Lactobacillus fermentum, Lactobacillus paracasei, Lactobacillus plantarum, Leuconostoc mesenteroides, Enterococcus faecalis, Enterococcus faecium and Enterococcus durans. (For more detailed information on lactic acid bacteria, see Lactic Acid Bacteria (LAB); Definition, Classification and Characteristics)
2. Proteolytic bacteria; In fact, this group is quite large. Moreover, while one strain of a specie shows proteolytic activity, another strain of the same specie may not show the same activity.
I studied the proteolytic activity in my doctoral thesis. I have seen that some Enterococcus faecalis strains have proteolytic activity while some Enterococcus faecalis strains have no proteolytic activity.
Generally, many bacterial species with proteolytic activity belonging to Pseudomonas, Bacillus, Proteus, Clostridium and Enterococcus genera are found in raw milk. However, in addition to the Enterococcus genus, some other strains of lactic acid bacteria may also have proteolytic activity. Bacteria such as Pseudomonas fluorescens, Bacillus cereus and Clostridium sporogenes are the most common bacteria with proteolytic activity in milk.
As a result of the proteolytic activity exhibited by these bacteria, peptides with bitter and undesirable tastes and aromas are formed from the proteins that are broken down. On the contrary, proteolytic activity and bitter peptides formed due to this activity are desirable in some cheese types.
3. Lipolytic bacteria; There is an approach in food microbiology: “A bacterium with proteolytic activity is likely also have lipolytic activity.” Although this approach is valid mainly, in addition to the strains mentioned above with proteolytic activity, some bacteria belonging to Alcaligenes, Serratia, Staphylococcus and Micrococcus genera and some strains of lactic acid bacteria, which are a lot in the milk, may also exhibit lipolytic activity.
It is helpful to mention the following point here; The lipase enzyme naturally found in milk is primarily responsible for the rancid (bitter) taste due to lipolytic activity. The effect of the lipolytic activity of microorganisms is lower.
4. Propionic acid bacteria; Bacteria of the genus Propionibacterium form this group. The main species found in milk are Propionibacterium freudenreichii, Propionibacterium theonii and Propionibacterium jensenii.
5. Thermophilic bacteria; The optimum temperature for developing bacterial species in this group is 45oC, and they can grow at around 55oC. Many species in this group belong to the genus Corynebacterium, Micrococcus, Streptococcus, Bacillus and Enterococcus.
Some of the thermophilic bacteria survive pasteurization, so that they can be found in pasteurized milk. Streptococcus thermophilus and Corynebacterium lactium are thermophilic bacteria may found in pasteurized milk.
6. Thermoduric bacteria; Bacteria that can survive at pasteurization temperature fall into this group. Therefore, such bacteria are also found in pasteurized milk. Most species belonging to the Clostridium, Bacillus, Micrococcus, Streptococcus, Lactobacillus and Microbacterium genera are thermoduric.
7. Psychotrophic bacteria; Bacteria included in this group can grow at 7oC and below. Moreover, they exhibit lipolytic and proteolytic activity at these temperatures, causing the formation of undesirable taste and odor. Therefore, they can cause problems in the cold storage of raw milk and butter.
Some species of Pseudomonas, Achromobacter, Flavobacterium, Alcaligenes, Micrococcus, Lactobacillus, Enterobacter, Serratia and Arthrobacter are psychrotrophic. The most well-known and frequently encountered examples are Pseudomonas fluorescens and Pseudomonas fragi.
8. Color-forming bacteria; Colonies formed by some bacteria have different colors. Although these bacterial colonies are may found in raw milk, they are more prominent in dairy products such as cheese and yogurt (unless effective pasteurization or contamination is present).
To put it briefly,
• Pseudomonas syncyanea causes gray-blue color,
• Species belonging to Pseudomonas synxantha and Flavobacterium genus cause yellow color,
• Serratia marcescens, Brevibacterium erythrogenes and Micrococcus roseus cause red color,
• Pseudomonas putrefaciens and Pseudomonas fluorescens cause brown color formation in milk.
By the way, molds and bacteria should not be confused with each other. Mold colonies can also be of different colors. To distinguish between mold and bacteria colonies, it should be remembered that “mold colonies have feather-like micelles, while bacterial colonies do not have feather-like micelles.”
9. Gas-forming bacteria; Bacteria from this species produce carbon dioxide (CO2) and hydrogen (H2) gas due to their metabolic activities. Most species belonging to the genera Leuconostoc, Escherichia, Enterobacter, Proteus, Bacillus and Clostridium can form gas.
The gases these bacteria create can cause swelling, especially in cheese packaging. With effective pasteurization and compliance with hygiene rules, the problem can be eliminated.
In this context, it is worth remembering that any dairy product with swollen packaging should never be purchased or consumed.
Gas-forming microorganisms mainly cause the bubbles seen in the cheese mass. These bubbles’ shape, size and density tell a lot to the experts on the job.
10. Coliform group bacteria; This group of bacteria includes species found naturally in raw milk but undesirable in dairy products. If a dairy product does not contain coliform bacteria, it is accepted that the product was produced under clean conditions and an effective pasteurization process was applied. In this respect, the presence of coliform group bacteria is an important quality indicator for dairy products.
In other words, if coliform bacteria are found in a dairy product, the product was not produced in clean conditions, was exposed to severe contamination after production, or was not effectively pasteurized.
This group’s most known and familiar members are Escherichia coli, Enterobacter aerogenes, Klebsiella pneumonia and Citrobacter freundi. Reproduction of coliform bacteria causes gas, lousy odor and unpleasant taste in milk.
11. Probiotic bacteria; Probiotic bacteria are very beneficial to human health, and this group can be called the “jewels of the microorganism world.” They are naturally rare, like jewelry. However, since milk cannot be consumed and raw and it should be pasteurized, boiled or UHT heat-treated, they are unlikely to be found in drinking milk.
However, probiotic bacteria can be found with high probability in village-type yogurt, cheese or butter. In fact, probiotic bacteria sold commercially today are mostly isolated from dairy products produced in undisturbed and natural areas.
It is impossible to say “this species is definitely probiotic” for a bacterial species. Bacteria should be evaluated based on strain and all probiotic properties should be examined individually.
However, strains with probiotic properties mainly belong to the genera Lactobacillus, Bifidobacterium, Streptococcus, Enterococcus, Leuconostoc, Pediococcus, Bacillus, Bacteriodes and Propionibacterium.
Lactobacillus fermentum, Lactobacillus plantarum, Lactobacillus jonhsonii, Lactobacillus rhamnosus, Lactobacillus helveticus, Lactobacillus brevis, Bifidobacterium bifidum, Bifidobacterium breve, Bifidobacterium infantis and Bifidobacterium longum are the most known probiotic species.
However, as I mentioned above, for example, a strain of Lactobacillus rhamnosus does not necessarily mean that it is probiotic; the probiotic characteristics of that strain should be examined.
12. Pathogenic bacteria; Pathogenic bacteria are a group of bacteria that harm human health and cause diseases. It is a very comprehensive subject and I want to mention only genera and species since it is beyond the purpose of explaining this subject.
Salmonella enteritidis, Salmonella newport, Salmonella typhimurium, Salmonella typhi, Salmonella paratyphi, Brucella suis, Brucella abortus, Brucella melitensis, Shigella dysenteriae, Vibrio cholera, Escherichia coli, Campylobacter jejuni, Enterococcus faecalis, Streptococcus viridians, Streptococcus pyogenes, Streptococcus agalactia, Yersinia enterocolitica, Listeria monocytogenes, Corynebacterium diphtheria, Mycobacterium tuberculosis, Clostridium perfingens, Clostridium botulinum, Bacillus cereus and Staphylococcus aureus are the main pathogenic bacteria that can be found in raw milk.
It is aimed to destroy all of these pathogens in milk by pasteurization, UHT, or boiling as we do at home.
In general, these microorganisms must reach a certain number to cause human disease. For example, let’s assume that there is no effective pasteurization of raw milk or that the milk is contaminated after pasteurization;
In this case, Staphylococcus aureus must reach the level of 106-107 units/mL, Shigella dysenteriae must reach the level of 101-102 units/mL, Salmonella typhi must reach the level of 105 units/mL, Salmonella paratyphi must reach the level of 103-104 units/mL and Escherichia coli must reach the level of 108 units/mL to may cause disease. (106; 1 Million. The expression 106 -107 units/mL means that there are between 1 million and 10 million bacteria in 1 milliliter of milk)
Molds
Molds are another class of microorganisms found in raw milk. Transmission routes are the same as for other types of microorganisms. In addition, the presence of mold spores suspended in the air causes mold to be transmitted from the air very quickly.
Mold species belonging to Penicillium, Oospora, Aspergillus, Geotrichium, Mucor, Rhizopus and Cladosporium can be found widely in raw milk. The applied heat treatment destroys them. However, some molds produce toxins that are not affected by heat treatment. In other words, even if the mold cell is destroyed by heat treatment, the toxin it produces is still effective.
Some types of mold are essential for producing “moldy cheese,” which is give their name. The main species used as moldy cheese starter culture are Penicillium roqueforti, Penicillium camemberti, Penicillium caseicolum and Oospora lactis. Another essential feature of molds is that some species can produce antibiotics. In this respect, they are very beneficial for health.
However, as mentioned earlier, it cannot be said, for example, that “a strain of Penicillium roqueforti necessarily produces antibiotics.” On the contrary, it may also be producing toxins. Therefore, strains belonging to the species should be examined individually and their characteristics revealed.
Yeasts
Yeasts are a class of microorganisms that are inevitably found in raw milk and, with one or two exceptions, mostly spoil the milk and render it unusable. Exceptions are the types used as starter cultures for kumiss, kefir and some cheese varieties such as Camembert and Brie.
Generally, yeasts of the genus Torulopsis, Kluyveromyces and Saccharomyces are commonly found in raw milk. Yeasts are destroyed when pasteurization, UHT or boiling process is applied to milk.
As it is known, many yeast species can synthesize B-group vitamins (except vitamin B12). Therefore, dairy products in which yeast is used as a starter culture become more affluent regarding B group vitamins.
On the other hand, some species can ferment ethyl alcohol, so alcohol accumulation occurs in the product over time. This situation is found in kefir and kumiss.
Viruses
In terms of viruses, there is no virus-related health problem when an adequate heat treatment (pasteurization, UHT or home boiling) is applied. The most critical viruses found in milk are Aphta epizootica, Hepatitis A (infectious jaundice) and bacteriophages.
Bacteriophages do not have any adverse effects on human health. Still, as their name suggests, they destroy bacteria, including bacteria used in yogurt, buttermilk and cheese as starter cultures and this situation poses a severe problem for the dairy industry.
A 30-second heat treatment at 70-75oC can destroy bacteriophages. However, a business with a bacteriophage problem must undergo severe disinfection. Starter culture rotation is the most common procedure.
Ricketts
Coxiella burnetti from the Rickettsia class is critical regarding dairy technology. Because, the most heat-resistant pathogenic microorganism found in milk is Coxiella burnetti. Therefore, all heat treatments are applied based on this microorganism.
Because it is the most heat-resistant microorganism, if this microorganism is destroyed, all other pathogens will be destroyed as well. Thus, the risk of milk-borne diseases is eliminated. (Except for anthrax, the cells of Bacillus anthracis are destroyed at pasteurization temperature, but it is necessary to boil milk for about 15 minutes to destroy their spores. Anthrax is a severe disease and the main source of transmission to humans is the consumption of sick animal meat.)
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Raw milk, Pasteurized milk or UHT milk?
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