Milk microbiology is a vast field, far too broad to cover in a paragraph or two. There is even a 1000-page book devoted to lactic acid bacteria alone. Here, I will give a brief overview to introduce the subject.
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.
The number of microorganisms initially contaminating milk and its storage conditions are therefore important. Milk obtained using a clean milking system by a producer who understands cleaning and disinfection contains an average of 1,000–10,000 microorganisms/mL, which is normal. If the surroundings and milking equipment are not clean and the producer lacks this awareness, the count can reach 1,000,000 microorganisms/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 4°C and leaving it at 25°C. While the number of microorganisms increases slightly in milk stored at 4°C for one day, the number of microorganisms increases approximately 1,200 times in milk kept for one day at 25°C.
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 4°C. 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 4°C for a maximum of 2 days. Therefore, raw milk should be processed as soon as possible.
The farm environment, the animal being milked, workers, air and other surroundings can introduce a very wide range of microorganisms into milk. After some time, however, certain species become dominant.
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.
Besides bacteria, raw milk can contain other microorganisms, including yeasts, molds, viruses and rickettsiae. As with bacteria, contamination can originate from animals, milking workers, the barn environment and processing equipment.
Bacteria
Bacteria found in milk can be grouped as follows;
1. Lactic acid bacteria; These form the most important bacterial group in milk. Bacteria belonging to the genera Lactobacillus, Lactococcus, Streptococcus, Enterococcus, Leuconostoc and Pediococcus are abundant in milk. They break down lactose to produce lactic acid. As lactic acid accumulates, the milk’s acidity increases, eventually causing 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; This is a broad group. One strain of a species may show proteolytic activity while another strain of the same species may not.
I examined proteolytic activity in my doctoral thesis. Some Enterococcus faecalis strains showed this activity, whereas others did not.
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; A common observation in food microbiology is that “a bacterium with proteolytic activity is also likely to have lipolytic activity.” Although this often holds true, the ability varies between strains. In addition to the genera mentioned above, some bacteria belonging to Alcaligenes, Serratia, Staphylococcus and Micrococcus, as well as some lactic acid bacterial strains abundant in milk, can show 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 45°C, and they can grow at around 55°C. Many species in this group belong to the genus Corynebacterium, Micrococcus, Streptococcus, Bacillus and Enterococcus.
Some thermophilic bacteria survive pasteurization and can therefore occur in pasteurized milk. Examples include Streptococcus thermophilus and Corynebacterium lactium.
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. Psychrotrophic bacteria; Bacteria included in this group can grow at 7°C 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; Some bacteria form colonies of different colors. Although these bacteria may be present in raw milk, their colored colonies are more noticeable in dairy products such as cheese and yogurt when pasteurization has been inadequate or contamination has occurred. Briefly:
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.
Molds and bacteria should not be confused. Mold colonies can also have different colors. To distinguish them, remember that mold colonies have a fuzzy, thread-like mycelium, while bacterial colonies lack this appearance.
9. Gas-forming bacteria; Bacteria in this group can produce carbon dioxide (CO2) and hydrogen (H2) through their metabolic activities. Many species in 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 pneumoniae and Citrobacter freundii. Reproduction of coliform bacteria causes gas, unpleasant odors 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 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, Bacteroides and Propionibacterium.
Lactobacillus fermentum, Lactobacillus plantarum, Lactobacillus johnsonii, 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 cholerae, Escherichia coli, Campylobacter jejuni, Enterococcus faecalis, Streptococcus viridians, Streptococcus pyogenes, Streptococcus agalactiae, Yersinia enterocolitica, Listeria monocytogenes, Corynebacterium diphtheriae, Mycobacterium tuberculosis, Clostridium perfringens, 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 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 molds are essential for producing mold-ripened cheeses. Important species include Penicillium roqueforti, Penicillium camemberti, Penicillium caseicolum and Oospora lactis. Another important characteristic of some molds is their ability to produce antibiotics, which makes them valuable in medicine.
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 genera 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 richer in B vitamins.
In Turkish, the word “maya” can be confusing: it denotes yeasts as a class of microorganisms, but it is also used for materials that initiate fermentation or coagulation. Yogurt starter consists of bacteria, while cheese rennet is an enzyme preparation. Kefir starter contains microorganisms from the yeast group as well. These different meanings should not be confused.
Some yeast species also carry out alcoholic fermentation, causing ethanol to accumulate in the product over time. This occurs 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, ayran (a yogurt drink) and cheese as starter cultures and this situation poses a severe problem for the dairy industry.
A 30-second heat treatment at 70-75°C can destroy bacteriophages. However, a business with a bacteriophage problem must undergo severe disinfection. Starter culture rotation is the most common procedure.
Rickettsiae
Coxiella burnetii from the Rickettsia class is critical regarding dairy technology. Because, the most heat-resistant pathogenic microorganism found in milk is Coxiella burnetii. 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. (Milk and other products from animals suspected of having anthrax must not be consumed or treated at home in an attempt to make them safe.)
References
Akgül, H. İ. (2020). Tereyağlarından ekzopolisakkarit (EPS) üreten laktik asit bakterilerinin izolasyonu ve tereyağı üretiminde starter kültür olarak kullanılabilirliğinin araştırılması. Atatürk Üniversitesi, Fen Bilimleri Enstitüsü.
Üçüncü, M. (2024). Süt ve Mamülleri Teknolojisi (8. baskı). Sidas Medya.
Metin, M. (2014). Süt Teknolojisi: Sütün Bileşimi ve İşlenmesi (13. baskı). Ege Üniversitesi Basımevi.
Erkmen, O. (Ed.). (2021). Gıda Mikrobiyolojisi (6. baskı). Efil Yayınevi.
Ray, B., & Bhunia, A. (2021). Temel Gıda Mikrobiyolojisi (5. baskıdan çeviri; D. Heperkan, Çev. Ed.). Nobel Akademik Yayıncılık.
U.S. Food and Drug Administration. (n.d.). Raw milk misconceptions and the danger of raw milk consumption. FDA.
World Health Organization, Regional Office for Europe. (2016). Anthrax: Questions and answers.
Halil İbrahim Akgül
Halil Ibrahim Akgul, Ph.D., is an Assistant Professor of Food Engineering at Bayburt University, where he works in the Division of Food Sciences within the Faculty of Engineering. He received his B.Sc. in Food Engineering from Akdeniz University in 2010, followed by an M.Sc. and a Ph.D. in Food Engineering from Atatürk University in 2015 and 2020, respectively. His formally recognised academic fields are Food Sciences and Engineering, Food Technologies, Dairy and Dairy Products Technology, and Food Microbiology.
Dr. Akgul has more than fifteen years of professional, research, and academic experience in food science. Between 2011 and 2013, he worked as the food engineer responsible for operations at the Pilot Dairy and Bread Factory of Atatürk University’s Directorate of Agricultural Enterprises. This industrial experience, combined with his subsequent academic research, gave him extensive practical and scientific knowledge of milk processing, dairy production, food quality, hygiene, and product development.
His research is primarily focused on milk and dairy products, dairy microbiology, starter cultures, lactic acid bacteria, exopolysaccharides, butter technology, and the physicochemical, microbiological, sensory, and nutritional properties of foods. His studies have examined the quality and fatty-acid composition of traditional butter, aroma formation in butter produced with different starter cultures, raw-milk quality, the characterisation and antioxidant activity of bacterial exopolysaccharides, traditional dairy products such as çökelek and karın kaymağı, colostrum powder, and the use of fruit-derived ingredients in ice cream production.
His academic record includes peer-reviewed research articles published in national and international journals, including Mljekarstvo, Acta Scientiarum Technology, the Journal of Food Safety and Food Quality, the International Journal of Food Engineering, the Atatürk University Journal of Agricultural Faculty, and ATA Food Journal. Four of the research articles currently listed in his university profile were published in SCI-Expanded-indexed journals. He has also contributed to twelve national and international conference papers and presentations covering dairy products, food microbiology, butter quality, fatty-acid composition, starter cultures, exopolysaccharide-producing bacteria, colostrum, traditional foods, and functional ice-cream formulations.
Dr. Akgul has participated as a researcher or adviser in eleven completed scientific and applied research projects. These projects have addressed the modernisation of pilot dairy-processing facilities; the production and quality characteristics of colostrum powder; the physicochemical and microbiological quality of butter and raw milk; the identification of exopolysaccharide-producing lactic acid bacteria; the development and production optimisation of starter cultures for mould-ripened cheese; the standardisation of traditional dairy products; and the use of sloe berry and persimmon fibre in ice-cream production.
His teaching experience spans undergraduate, associate-degree, and graduate education. The courses he has taught include Food Chemistry, Analytical Chemistry, General Chemistry, Organic Chemistry, Fermented Dairy Products Technology, Butter Production and Processing Technology, Healthy Living and Nutrition, Organic Agriculture and Food, Popular Science, Digital Literacy, and Information Technologies. Through these courses, he combines fundamental food chemistry and engineering principles with dairy technology, nutrition, scientific literacy, and the practical communication of scientific knowledge.
He is the founder and Editor-in-Chief of Gıda ve Sağlık, a multilingual digital publication devoted to food, nutrition, health, food chemistry, food microbiology, and culinary science. Through this platform, he draws upon his academic background, teaching experience, research activity, and practical knowledge of food production to present scientifically grounded information in a detailed and accessible form for an international readership.