Dairy Science & Technology · Dairy Microbiology · also Food Safety Microbiology, Milk-borne Diseases
The bacteria, toxins and residues that make dairy products unsafe, and how outbreaks happen and are prevented. The veterinary track (milk-meat-hygiene-food-safety, epidemiology-zoonoses) covers the animal side.
In the exams: ICAR AICE subject 36 Unit 3 and subject 35b Unit 2; ICAR AIEEA-PG group 16 (food poisoning, pathogens and their control); GATE XL-U Section 2
Milk and dairy products have a long history as vehicles of disease. Before pasteurisation became routine, milk spread tuberculosis, brucellosis, scarlet fever, diphtheria and typhoid; today, in countries where milk is pasteurised and plants are well controlled, dairy foods cause a small share of food-borne illness, but outbreaks still occur, often large and sometimes fatal, and raw milk and raw milk cheeses remain an important source of infection. Dairy Pathogens, Toxins and Microbial Risk describes the microorganisms and toxins that make dairy products unsafe, how illness arises from them, the zoonoses carried in milk, aflatoxin M1, the special risks of raw milk products, antimicrobial resistance in dairy-associated bacteria, and the tools used to manage risk and to investigate outbreaks.
The subject is the food-safety core of dairy microbiology. A dairy technologist must know which hazards are relevant to each product, which process steps control them, and how to show that control is working; a quality manager must be able to decide what to do when a pathogen is found; and anyone who works in the industry must understand how an outbreak is recognised and traced. The animal side of these diseases, including veterinary control of tuberculosis, brucellosis and mastitis, is covered in the veterinary track's subjects on milk and meat hygiene and on epidemiology and zoonoses.
General food-borne pathogens, detection methods, indicator organisms and sampling are covered in Food Technology's Food Microbiology; sources of contamination, heat treatment and post-pasteurisation contamination in Microbiology of Raw and Market Milk; product-specific microbiology in Microbiology of Dairy Products; and analytical methods for residues and aflatoxin M1 in Dairy Analysis and Instrumental Methods. This manual refers to those subjects. It is examined in Unit 3 of ICAR AICE-JRF/SRF subject 36 (Dairy Microbiology) and Unit 2 of subject 35b (Food Safety and Quality), in group 16 of ICAR AIEEA-PG (food poisoning, pathogens and their control), in GATE food technology, and in food analyst and food safety officer papers. Expect questions on the characteristics, sources and control of each pathogen, incubation periods and symptoms, notable dairy outbreaks, and calculations in risk assessment and outbreak investigation.
Food-borne illness caused by microorganisms arises in three ways. In a food infection, the person eats living pathogens that survive the stomach, multiply in the intestine and may invade the tissues; Salmonella, Campylobacter, Listeria monocytogenes and pathogenic Escherichia coli cause infections. Illness depends on the number of organisms swallowed, their virulence and the host's susceptibility, and the incubation period is usually a day or more, because the organisms must multiply first. In a food intoxication, the person eats a toxin already formed in the food by microbial growth before consumption, and the living organism need not be present at all; the toxins of Staphylococcus aureus, the emetic toxin of Bacillus cereus and the neurotoxins of Clostridium botulinum cause intoxications. Because the toxin acts directly, the incubation period is short, from less than an hour to a few hours for staphylococcal and emetic B. cereus toxins, and about 12 to 36 hours for botulism, whose toxin must reach the nervous system. Heating a food that already contains a heat-stable toxin, such as staphylococcal enterotoxin or cereulide, does not make it safe. In a toxi-infection, the person eats living organisms, often in large numbers, which then produce toxin in the intestine; Clostridium perfringens and the diarrhoeal type of B. cereus illness are examples, with incubation periods of about 8 to 16 hours.
The three mechanisms call for different controls. Infections are prevented by killing pathogens, for example by pasteurisation, and by preventing recontamination afterwards, because even a few cells of organisms with low infective doses, such as Shiga toxin-producing E. coli and Campylobacter, can cause illness. Intoxications are prevented by stopping the growth of toxin-producing organisms before heating or consumption, because once a heat-stable toxin has formed no later treatment removes it; hence the importance of cooling milk, concentrates and products, of rapid acid development in cheesemaking, and of limits on staphylococcal counts. Toxi-infections are prevented by controlling the growth of spore-forming organisms during holding and cooling of cooked or heat-treated foods. Typical incubation periods help investigators identify the agent in an outbreak: about 2 to 4 hours (range 30 minutes to 8 hours) points to staphylococcal intoxication or emetic B. cereus; 6 to 72 hours, commonly 12 to 36, to Salmonella; 2 to 5 days to Campylobacter; 3 to 4 days, with a range of 1 to 10, to Shiga toxin-producing E. coli; and days to several weeks for invasive listeriosis. Symptoms also differ: vomiting dominates intoxications by staphylococcal and emetic toxins, diarrhoea and fever dominate many infections, neurological signs mark botulism, and invasive listeriosis causes septicaemia, meningitis and miscarriage. Detailed descriptions of the organisms and their detection are given in Food Technology's Food Microbiology.
Listeria monocytogenes is a Gram-positive, non-spore-forming, facultatively anaerobic short rod. Several features make it a particular problem for dairy foods. It is psychrotrophic, able to grow at refrigeration temperatures, slowly even near 0 degrees Celsius; it tolerates salt up to about 10 per cent and relatively low pH; it survives well in the environment, forms biofilms and persists for years in drains, floors, wet equipment and other niches in processing plants; and it is widespread in soil, silage, water and the intestines of animals, from which it reaches raw milk. It shows characteristic tumbling motility at about 20 to 25 degrees Celsius but not at 37 degrees Celsius, and its main virulence factor, listeriolysin O, lets it escape from host cell vacuoles and spread from cell to cell. Of its serotypes, 1/2a, 1/2b and 4b cause most human disease, with 4b prominent in large outbreaks. Listeriosis affects mainly pregnant women, in whom it causes miscarriage, stillbirth and infection of the newborn; the elderly; and people with weakened immunity. Invasive listeriosis causes septicaemia and meningitis and has a high case fatality, commonly 20 to 30 per cent, which makes it one of the most serious food-borne infections despite its rarity. Healthy adults may suffer only a self-limiting febrile gastroenteritis after high doses. In ruminants, Listeria causes encephalitis (circling disease), abortion and occasionally mastitis, and is associated with poorly fermented silage.
Dairy products have been involved in many listeriosis outbreaks. Notable examples include pasteurised milk in Massachusetts in 1983, Mexican-style soft cheese in California in 1985, a smear-ripened soft cheese in Switzerland in the 1980s, butter in Finland, and ice cream in the United States in 2015, together with many outbreaks linked to soft and fresh cheeses made from raw milk or contaminated after pasteurisation. Pasteurisation kills L. monocytogenes, so outbreaks from pasteurised products arise from post-pasteurisation contamination, usually from a persistent strain in the processing environment, as described in Microbiology of Raw and Market Milk. Control therefore focuses on the environment: hygienic design, effective cleaning and sanitising, separation of raw and finished product areas, control of water and condensation, and environmental monitoring programmes that sample food-contact surfaces and the surrounding environment in zones of decreasing proximity to the product, with corrective action when Listeria is found. Product formulation also matters: under European Union criteria, ready-to-eat foods with pH of 4.4 or less, water activity of 0.92 or less, or pH of 5.0 or less with water activity of 0.94 or less are regarded as unable to support growth, while others must meet stricter conditions, as set out in Microbiology of Dairy Products. Detection uses selective enrichment, for example in Fraser broth, followed by plating on chromogenic agars, as in ISO 11290, and molecular typing, now usually whole-genome sequencing, links isolates from patients, foods and plants.
Salmonella are Gram-negative, facultatively anaerobic rods of the Enterobacteriaceae. Almost all those causing human food-borne illness belong to Salmonella enterica subspecies enterica, which contains more than 2,500 serovars, such as Typhimurium, Enteritidis and Dublin; S. Dublin is adapted to cattle, can cause invasive disease in people and is often resistant to several antibiotics. Non-typhoidal salmonellosis usually causes diarrhoea, fever and abdominal cramps after 12 to 36 hours, and can be severe in infants, the elderly and the immunocompromised. Cattle can shed Salmonella in faeces and occasionally in milk, so raw milk and products made from it are a recognised source. Salmonella is killed by pasteurisation, but it survives for long periods in dry products and has caused outbreaks from milk powder and powdered infant formula contaminated after drying, as described in Microbiology of Dairy Products. The largest dairy outbreak of salmonellosis occurred in 1985 in the United States, when pasteurised milk from a dairy plant in Illinois contaminated with Salmonella Typhimurium, apparently through a cross-connection between raw and pasteurised milk lines, caused more than 16,000 confirmed cases and an estimated total of around 170,000 or more illnesses. Ice cream premix contaminated in egg tankers caused a large outbreak in 1994. Detection follows ISO 6579, with non-selective pre-enrichment, selective enrichment and plating on selective agars, followed by confirmation and serotyping.
Most Escherichia coli are harmless gut organisms used as indicators of faecal contamination, but several pathotypes cause disease. The most important in dairy foods are Shiga toxin-producing E. coli (STEC), also called enterohaemorrhagic E. coli (EHEC) when they cause bloody diarrhoea. They produce Shiga toxins (Stx1 and Stx2) and usually attach to intestinal cells by the intimin protein. Serotype O157:H7 is the best known, but non-O157 serogroups such as O26, O103, O111 and O145 also cause illness. STEC infection causes abdominal cramps and diarrhoea, often bloody (haemorrhagic colitis), after about 3 to 4 days, and in a minority of cases, especially children, haemolytic uraemic syndrome (HUS), with kidney failure that can be fatal or leave lasting damage. The infective dose is very low, possibly fewer than 100 cells. Cattle and other ruminants are the main reservoir, usually without illness, and shed STEC in faeces, so raw milk and raw milk cheeses are recognised vehicles; outbreaks have been linked to raw milk, raw milk cheeses and, less often, to pasteurised products contaminated after processing. STEC survive well in acid foods and can persist in fermented milks and cheeses for days to weeks. Other pathotypes, such as enteropathogenic, enterotoxigenic and enteroinvasive E. coli, are less often linked to dairy products. Pasteurisation kills all E. coli, so their control depends on pasteurisation, prevention of recontamination and, for raw milk cheeses, herd hygiene, testing and the hurdles of cheesemaking.
Staphylococcus aureus is a Gram-positive coccus that grows in grape-like clusters, is catalase-positive and coagulase-positive, and is a facultative anaerobe. It tolerates high salt, up to about 10 to 15 per cent, and lower water activity than most bacteria, growing down to about 0.86 aerobically, which lets it grow in foods such as cured products, cheese brines and partly dried dairy products where competitors are inhibited. It is a common cause of bovine mastitis and lives on the skin and in the noses of a large proportion of people, so it reaches dairy products from infected udders and from handlers. Many strains produce staphylococcal enterotoxins, of which the classical types are A to E, type A being the most frequent in food poisoning; further enterotoxin-like proteins have been described. The enterotoxins are small proteins that are very resistant to heat, surviving pasteurisation and even boiling, and to digestive enzymes. They are usually formed when S. aureus grows to large numbers, commonly above about 100,000 per gram, so toxin is a sign that the organism was allowed to grow. Staphylococcal food poisoning causes sudden nausea, vomiting and abdominal cramps, sometimes with diarrhoea, about 2 to 4 hours after eating; it is usually over within a day or two and rarely fatal. Laboratory detection uses Baird-Parker agar, on which S. aureus forms black colonies with clear zones, confirmation by the coagulase test, and immunoassays for the enterotoxins in food.
Dairy-associated staphylococcal outbreaks have involved cheese, particularly raw milk cheese and cheese in which starter activity failed, milk powder, and products handled by hand after heating, such as paneer, khoa and cream-filled sweets. The largest dairy outbreak occurred in Japan in 2000, when low-fat milk products made with skim milk powder caused illness in more than 13,000 people; the powder had been made from concentrate held at warm temperatures during an electricity failure, allowing S. aureus to grow and form enterotoxin A, which survived the subsequent heat treatments even though no live staphylococci remained. In cheesemaking, S. aureus present in the milk can grow during the early hours if acid production is slow, because the warm, nutrient-rich curd favours it; vigorous starter activity, which lowers the pH quickly, is therefore an important control, and cheeses with slow acidification should be checked for staphylococci and enterotoxin. European Union criteria set process hygiene limits for coagulase-positive staphylococci in cheeses and milk powders and require testing for enterotoxins when counts exceed 100,000 per gram. Control rests on mastitis control and milk cooling, pasteurisation, rapid acid development in fermented products, hygienic handling by healthy workers with covered wounds, and keeping products that can support growth cold. Methicillin-resistant S. aureus, including livestock-associated strains, is discussed in chapter 10.
Bacillus cereus is a Gram-positive, spore-forming, facultatively anaerobic rod found widely in soil, dust and feed. Its spores reach milk from the farm environment, survive pasteurisation and germinate afterwards, and psychrotrophic strains of the B. cereus group can grow slowly in refrigerated pasteurised milk and cream, causing sweet curdling and bitty cream, as described in Microbiology of Raw and Market Milk. B. cereus causes two kinds of food poisoning. The emetic type is an intoxication caused by cereulide, a small cyclic peptide formed in the food during growth; it is resistant to heat, acid and digestive enzymes, and causes nausea and vomiting within about 30 minutes to 6 hours. It is most often associated with starchy foods such as cooked rice and pasta, but can be formed in dairy products. The diarrhoeal type is a toxi-infection caused by protein enterotoxins (haemolysin BL, non-haemolytic enterotoxin and cytotoxin K) produced in the intestine after ingestion of large numbers of cells or spores, with diarrhoea and cramps after about 8 to 16 hours. Illness is usually associated with foods containing more than about 100,000 B. cereus per gram. In dairy products, the main concerns are pasteurised milk held too warm or too long, powdered infant formula and reconstituted formula kept warm, and dairy desserts. Control relies on milk hygiene to limit spore numbers, cold storage of pasteurised products, limits on shelf life, and prompt consumption or refrigeration of reconstituted formula.
Clostridium botulinum is an anaerobic spore-former that produces botulinum neurotoxins, the most potent toxins known; ingestion of food containing preformed toxin causes botulism, a descending flaccid paralysis that can be fatal without treatment. Its spores are common in soil and sediments. Dairy products are rarely involved because most are refrigerated, acid, salted or heat-treated in ways that prevent growth, but outbreaks have occurred, including one in the United Kingdom in 1989 caused by hazelnut yoghurt made with an under-processed hazelnut conserve, and others linked to soft cheese, mascarpone and processed cheese spreads stored at ambient temperature. Processed cheese is formulated with an appropriate combination of moisture, pH, salt, emulsifying salts and sometimes nisin to prevent the outgrowth of C. botulinum spores during ambient storage; changes in formulation must be checked for safety, for example by challenge testing. Clostridium perfringens causes toxi-infection with diarrhoea and cramps after about 8 to 16 hours when large numbers grow in cooked foods cooled slowly; it is rarely associated with dairy products. Clostridium tyrobutyricum and related species cause spoilage rather than illness, notably late blowing of cheese, as described in Microbiology of Dairy Products. Spore-formers in general are controlled by preventing spore entry, by heat treatments sufficient for the product, by formulation that prevents germination and growth, and by temperature and time control during storage.
Campylobacter jejuni, and to a lesser extent Campylobacter coli, are Gram-negative, curved or spiral, microaerophilic rods that grow best at 37 to 42 degrees Celsius and do not grow below about 30 degrees Celsius. They are the most commonly reported bacterial cause of gastroenteritis in many high-income countries. The infective dose is low, of the order of a few hundred cells, and illness, with diarrhoea that is sometimes bloody, fever and abdominal pain, follows after about 2 to 5 days. A small proportion of cases is followed by Guillain-Barré syndrome, a serious paralytic disorder. Campylobacter lives in the intestines of poultry, cattle, wild birds and other animals; cattle shed it in faeces, and raw milk is one of the most frequent vehicles of Campylobacter outbreaks wherever raw milk is consumed. Outbreaks have also followed contamination of pasteurised milk in bottles whose foil caps were pecked by birds. Because it is sensitive to heat, drying and oxygen, Campylobacter is readily killed by pasteurisation and does not grow in foods. Yersinia enterocolitica is a Gram-negative rod of the Enterobacteriaceae that, unusually for an intestinal pathogen, is psychrotrophic and can grow at refrigeration temperatures. It causes gastroenteritis and a pseudo-appendicitis syndrome, especially in children, with incubation of a few days. Pigs are the main reservoir of pathogenic strains, but milk has been implicated in outbreaks, including contaminated pasteurised and chocolate milk. It is killed by pasteurisation; its ability to grow in the cold makes post-pasteurisation contamination significant.
Cronobacter species, especially Cronobacter sakazakii, formerly called Enterobacter sakazakii, are Gram-negative rods of the Enterobacteriaceae that are unusually resistant to drying and can survive for long periods in powdered infant formula and in the dry environments of formula factories. Infections are rare but severe. In neonates, particularly premature and low-birth-weight infants and those under two months of age, Cronobacter causes meningitis, septicaemia and necrotising enterocolitis, with a high case fatality and frequent neurological damage in survivors; older infants and adults with underlying illness are occasionally affected. Powdered infant formula has been implicated in many cases, and contamination can occur during manufacture, especially from the factory environment after the heat treatment, and during preparation in homes and hospitals. Control measures, including the microbiological criteria for Cronobacter in infant formula and the WHO/FAO advice to reconstitute formula with water at no less than 70 degrees Celsius, are described in Microbiology of Dairy Products. Detection uses enrichment and chromogenic agars under ISO 22964, with molecular confirmation. The organism's ability to form capsules and biofilms, and its tolerance of dry conditions, explain its persistence in formula plants. Other Enterobacteriaceae found in the same environments are used as indicators of hygiene. Because healthy older children and adults are rarely affected, the hazard is defined by the vulnerability of the consumer as much as by the organism, which is why the most stringent criteria apply to foods for young infants.
Several zoonoses, infections transmitted from animals to people, can be carried in milk from infected animals. Mycobacterium bovis causes bovine tuberculosis and can infect people, causing zoonotic tuberculosis, which before pasteurisation was a major cause of tuberculosis of the lymph nodes, bones and abdomen in children who drank infected milk. Pasteurisation was designed in part to destroy the tubercle bacilli, which were among the most heat-resistant non-spore-forming pathogens of milk. In countries with test-and-slaughter programmes and universal pasteurisation, milk-borne tuberculosis is rare, but it remains a concern where bovine tuberculosis is common and raw milk is consumed, and zoonotic tuberculosis is a recognised public health problem in parts of Africa and Asia. Mycobacterium avium subspecies paratuberculosis, the cause of Johne's disease in cattle, is shed in milk and faeces; a possible link with Crohn's disease in people has been proposed but is not established, and the effectiveness of pasteurisation against it has been debated, though commercial pasteurisation greatly reduces it. Brucella abortus (from cattle) and Brucella melitensis (from goats and sheep, the most virulent species for people) cause brucellosis, also called undulant or Malta fever, a long-lasting febrile illness with joint pain and fatigue that can relapse. Raw milk and fresh cheeses made from raw goat and sheep milk are important sources, alongside occupational exposure of farmers and veterinarians. Brucella is killed by pasteurisation; national control relies on vaccination, testing and removal of infected animals.
Coxiella burnetii, an obligate intracellular bacterium, causes Q fever. Cattle, sheep and goats are the main reservoirs and shed the organism in birth products, faeces and milk; most human infections result from inhaling contaminated dust or aerosols, particularly around parturition, but raw milk is a possible route. Q fever ranges from a mild flu-like illness to pneumonia and hepatitis, and chronic infection can cause endocarditis. Coxiella is the most heat-resistant non-spore-forming pathogen associated with milk, and when it was recognised as a milk-borne hazard, studies confirmed that the high-temperature short-time pasteurisation standard of 72 degrees Celsius for 15 seconds inactivates it, as described in Microbiology of Raw and Market Milk. Other zoonotic agents occasionally transmitted by raw milk include tick-borne encephalitis virus, which has caused outbreaks linked to raw goat and sheep milk in parts of Europe; Streptococcus equi subspecies zooepidemicus, from raw milk and cheese; and, historically, the streptococci of scarlet fever and the agents of diphtheria and typhoid, which contaminated milk from infected people rather than animals. Highly pathogenic avian influenza A(H5N1) found in dairy cattle and their milk in the United States from 2024 raised new questions about raw milk; studies have indicated that pasteurisation inactivates the virus, and authorities advise against consuming raw milk from affected herds. These zoonoses show why veterinary disease control and pasteurisation work together: control of disease in animals reduces the hazard at its source, and pasteurisation protects consumers from what remains.
Aflatoxin M1 is the principal mycotoxin of concern in milk. It is not produced in the milk or in the product; it is a hydroxylated metabolite of aflatoxin B1, which is produced by Aspergillus flavus and Aspergillus parasiticus on feeds such as maize, groundnut cake and cottonseed cake stored under warm, humid conditions. A dairy animal that eats contaminated feed converts part of the B1 in its liver to M1 and excretes it in milk within hours; the carry-over is commonly in the range of about 1 to 3 per cent of ingested B1, higher in high-yielding cows, and M1 largely disappears from milk within a few days of removing the contaminated feed. Aflatoxin B1 is a potent human liver carcinogen, and M1 is also considered carcinogenic, though less potent than B1. Because milk is consumed in large amounts by children, regulators set low limits: the European Union limit is 0.050 micrograms per kilogram of milk, with a lower limit for infant foods, whereas Codex, the United States and India set 0.5 micrograms per kilogram. Contamination is a recurring problem in tropical and subtropical countries with warm, humid feed storage, and surveys in parts of South Asia and Africa have found many samples above the stricter limits. Analytical methods for aflatoxin M1, including immunoaffinity clean-up with HPLC and rapid immunoassays, are described in Dairy Analysis and Instrumental Methods.
Aflatoxin M1 is stable to pasteurisation, UHT treatment, drying and storage, so processing does not remove it. It is distributed between the fractions of milk in different proportions: much of it is associated with the casein, so it is concentrated in cheese relative to the milk from which the cheese was made, by a factor that varies with cheese type and moisture, while whey and butter carry less. Fermentation and ripening reduce it little, although some lactic acid bacteria bind it in experiments. Limits for cheese and other products are therefore set in some countries on a product basis or derived from the milk limit using concentration factors. Because it cannot be removed economically from milk, control depends entirely on preventing aflatoxin B1 in feed: proper drying of grain and oilseeds, storage that keeps them dry and cool, testing of feed ingredients, rejection or dilution of contaminated batches according to feed regulations, and the use of permitted mycotoxin binders, such as hydrated sodium calcium aluminosilicate and other clays, which bind aflatoxin in the gut and reduce its transfer to milk. Dairy processors protect themselves by testing incoming milk with rapid tests, especially in seasons and regions of high risk, by working with suppliers on feed quality, and by tracing contaminated milk back to farms. Other mycotoxins, such as ochratoxin A, zearalenone and those from moulds growing on cheese, are of lesser concern in milk, as noted in Microbiology of Dairy Products.
Raw milk is milk that has not been pasteurised or otherwise heat-treated. Even milk from healthy, clean, well-managed herds can contain pathogens, because animals carry Campylobacter, Shiga toxin-producing E. coli, Salmonella and Listeria in their intestines without illness and because milking can never be fully sterile. Raw milk has been the vehicle of many outbreaks of campylobacteriosis, STEC infection including haemolytic uraemic syndrome in children, salmonellosis, listeriosis, brucellosis, tuberculosis and Q fever. In the United States, where raw milk is consumed by a small minority, it has nevertheless been linked to most dairy-associated outbreaks, and one modelling study estimated that unpasteurised dairy products cause hundreds of times more illnesses per serving than pasteurised products. Claims that raw milk has nutritional or health benefits that outweigh these risks are not supported by public health authorities, which note that pasteurisation causes only minor nutritional changes. The legal status of raw milk sales varies: some countries and states prohibit retail sale for drinking, others allow sales from farms, through vending machines or under specific hygiene and testing rules, and many require warning labels. Raw milk consumption is particularly dangerous for pregnant women, young children, the elderly and people with weakened immunity. In much of South Asia and Africa, most milk is boiled at home before drinking, which provides protection similar to pasteurisation, although products made from raw milk, such as some fresh cheeses and sweets, remain risks.
Cheeses made from raw milk are a traditional and economically important part of European and other cheese cultures, and many protected-designation cheeses must be made from raw milk. Their safety depends on the hurdles of cheesemaking and ripening, which differ greatly between types. Hard, long-ripened cheeses such as Parmigiano Reggiano, Grana Padano, Comté and Gruyère combine curd cooking at high temperatures, rapid acidification, low moisture, salt, low water activity and many months of ripening, and pathogens rarely survive in them; they are regarded as low risk. Soft and fresh cheeses made from raw milk, particularly those with high moisture, near-neutral pH or surface ripening, offer few hurdles and have been linked to outbreaks of listeriosis, STEC infection, salmonellosis and brucellosis. Regulations take different approaches. The United States requires cheese made from unpasteurised milk to be aged for at least 60 days at no less than about 1.7 degrees Celsius before sale, a rule that research has shown does not guarantee safety for all types. The European Union allows raw milk cheese production under hygiene rules for raw milk, process hygiene and food safety criteria, such as those for Salmonella, E. coli and coagulase-positive staphylococci, and requires labelling of products made with raw milk. Good practice for raw milk cheesemakers includes healthy, tested herds; excellent milking hygiene and cooling; testing of milk and cheese for pathogens and indicators; vigorous starter cultures; validated ripening times; and environmental monitoring. Thermisation of milk is sometimes used as an intermediate step that reduces risk while retaining some raw milk character.
Antimicrobial resistance (AMR) is the ability of microorganisms to survive drugs that would normally kill them or stop their growth. It is a One Health problem, linking human medicine, animal production and the environment. In dairy farming, antimicrobials are used mainly to treat clinical mastitis, for dry-cow therapy at the end of lactation, and to treat other diseases of cows and calves; their use selects for resistant bacteria in animals and on farms. Feeding calves waste milk containing antibiotic residues is a particular driver of resistance in calf intestinal bacteria. Antimicrobial residues in milk are a separate issue from resistance: residues are controlled by withdrawal periods and testing, as described in Dairy Analysis and Instrumental Methods, while resistance concerns living bacteria and their genes. Dairy-associated resistant bacteria of public health concern include methicillin-resistant Staphylococcus aureus (MRSA), including livestock-associated strains such as clonal complex 398, found in dairy herds in several countries; Escherichia coli and other Enterobacteriaceae producing extended-spectrum beta-lactamases (ESBL) or AmpC enzymes, which inactivate important cephalosporins; multidrug-resistant Salmonella, such as some strains of S. Dublin and S. Typhimurium; and Campylobacter resistant to fluoroquinolones and macrolides. Resistance genes can also be carried by lactic acid bacteria, including some starter and probiotic strains, which is why the absence of transferable resistance genes is part of the safety assessment of cultures.
Resistant bacteria can pass from dairy animals to people through direct contact, through the environment, including manure and water, and through food, particularly raw milk and raw milk products; pasteurisation kills resistant bacteria as readily as susceptible ones, but post-pasteurisation contamination and raw products remain routes. Resistance genes on plasmids, transposons and other mobile elements can transfer between bacteria in the intestine, in the environment and potentially in foods, spreading resistance to organisms that were never exposed to the drug. Control requires prudent use of antimicrobials: accurate diagnosis and culture of mastitis cases, treatment guided by susceptibility testing, selective rather than blanket dry-cow therapy (treating only infected quarters or cows, often with internal teat sealants for the rest), avoiding the use of antimicrobials critically important for human medicine, such as third- and fourth-generation cephalosporins and fluoroquinolones, where alternatives exist, and not feeding waste milk to calves without treatment. Prevention of infection through vaccination, hygiene, good housing and nutrition reduces the need for antimicrobials in the first place. Many countries now monitor antimicrobial use on farms and resistance in bacteria from animals and food, for example through harmonised European Union monitoring and national action plans that follow the WHO Global Action Plan on Antimicrobial Resistance. For dairy processors, AMR is addressed through supplier requirements on antimicrobial use, residue testing, pasteurisation and the safety assessment of cultures.
Codex Alimentarius describes food safety risk analysis as three linked components: risk assessment, a scientific evaluation of the likelihood and severity of adverse health effects from a hazard in food; risk management, the weighing of policy options and the selection of control measures; and risk communication, the exchange of information among assessors, managers, industry and the public. Microbiological risk assessment has four steps. Hazard identification establishes which microorganism or toxin in which food can cause harm. Hazard characterisation describes the adverse effects and the relationship between the dose ingested and the probability of illness, the dose-response relationship; in a simple exponential model, the probability of illness equals one minus e raised to the power of minus r times the dose, where r is the probability that a single cell causes illness, so with r of 0.0001 and a dose of 1,000 cells the probability is about 0.095. Exposure assessment estimates how much of the hazard people consume, considering contamination at production, growth or death during processing, storage and preparation, serving sizes and consumption frequency. Risk characterisation combines these to estimate the risk to a population and its uncertainty. Quantitative risk assessments have been carried out for Listeria monocytogenes in ready-to-eat foods, including soft cheeses, for STEC and Campylobacter in raw milk and for Cronobacter in infant formula, and they inform the criteria and controls set by regulators.
To connect public health goals with what happens in plants, risk managers use a set of related terms. The appropriate level of protection is the level of risk a country considers acceptable. A food safety objective (FSO) is the maximum frequency or concentration of a hazard in a food at the time of consumption that provides that level of protection. Performance objectives set the corresponding maximum at earlier points in the chain, and performance criteria specify the effect a step must achieve, such as a minimum log reduction from pasteurisation. The International Commission on Microbiological Specifications for Foods expresses the logic as an inequality in log units: the initial level of the hazard, minus the sum of reductions, plus the sum of increases, must not exceed the FSO. For example, if raw material contains 2 log units of a pathogen per gram, processing achieves a 6 log reduction and recontamination and growth add 1 log, the final level is minus 3 log units per gram, which meets an FSO of minus 2 log. The ICMSF also developed the sampling plan approach described in Microbiology of Dairy Products, classifying foods into cases according to the severity of the hazard (from indicator or low-risk organisms to severe hazards such as Listeria for vulnerable groups or Cronobacter in infant formula) and whether conditions of handling would decrease, not change or increase the hazard; more stringent plans, with more samples and lower acceptance numbers, apply to more severe hazards and to foods in which the hazard can increase. Testing alone cannot assure safety, because practical sampling plans have limited power to detect low contamination, so these approaches emphasise process control backed by verification testing.
An outbreak is the occurrence of two or more cases of a similar illness linked to a common source, or more cases than expected in a place and time. Outbreaks are recognised through surveillance of laboratory-confirmed infections, through clusters of genetically related isolates detected by laboratories, and through reports from doctors, the public or food businesses. Investigation follows a standard sequence, though steps often run in parallel: verify the diagnosis and confirm that an outbreak exists; establish a case definition with criteria of person, place, time and clinical or laboratory findings; find and count cases; describe them by time, place and person, including an epidemic curve, the plot of cases by date of onset, whose shape suggests the type of outbreak (a sharp single peak indicates a point source, a prolonged or stepped curve a continuing or propagated source); generate hypotheses about the vehicle from interviews; test hypotheses with analytical studies; carry out environmental and food investigations, including inspection, sampling and traceback of suspected products through the supply chain; implement control measures, such as recalls, closure or process changes, as soon as evidence justifies them; and communicate findings and lessons. Laboratory typing of isolates from patients, foods and the environment, now usually by whole-genome sequencing, provides strong evidence linking cases to a source, and international networks such as PulseNet share typing data so that dispersed outbreaks from widely distributed products can be detected.
Analytical epidemiology tests whether illness is associated with a suspected food. In a cohort study, used when the exposed group is well defined, as at a wedding or school meal, attack rates are compared: the attack rate is the number ill divided by the number exposed. If 40 of 50 people who drank a particular milk became ill and 5 of 50 who did not, the attack rates are 80 and 10 per cent and the relative risk is 8. In a case-control study, used when cases are dispersed in the community, cases are compared with healthy controls on their past exposures, and the strength of association is given by the odds ratio: with 30 exposed and 10 unexposed cases, and 20 exposed and 40 unexposed controls, the odds ratio is (30 x 40) divided by (10 x 20), or 6. Statistical significance and confidence intervals show whether the association could be due to chance, and dose-response relationships and laboratory findings strengthen the case. Dairy outbreaks have taught recurring lessons: cross-connections between raw and pasteurised milk, as in the 1985 Illinois Salmonella outbreak; contamination after pasteurisation from persistent environmental strains, as in many Listeria outbreaks; growth of toxin-producing organisms during process failures, as in the Japanese milk powder outbreak; contaminated ingredients and transport, as in the 1994 ice cream outbreak; and the consistent role of raw milk and raw milk products. Each investigation should end with a clear account of the cause and of the changes made to prevent recurrence, shared with industry and regulators.
A student uses this subject in dairy microbiology and food safety courses. You will isolate and identify indicator organisms and selected pathogens on selective media in a suitable laboratory, run coagulase and catalase tests, review outbreak case studies, calculate attack rates, relative risks and odds ratios, and work through simple risk calculations. Learn each pathogen's key features, sources, dairy vehicles, incubation period and controls, and the major dairy outbreaks and their causes, because examiners ask them and because every food safety plan rests on them.
Five years into work as a food safety or quality manager, you write and verify the hazard analysis for each product, run environmental monitoring for Listeria and Salmonella, decide what to do when a pathogen is found on a surface or in a product, manage rapid testing for aflatoxin M1 and antibiotics at milk reception, respond to customer complaints of illness, and support regulators during investigations. You also train staff in hygiene and in the reasons behind each control, and keep the records that show auditors the system works.
A senior food safety director with twenty-five years of experience sets the food safety strategy for a dairy group: hygienic zoning and environmental programmes for high-risk plants, policies on raw milk products, supplier programmes for feed quality and antimicrobial use, crisis management and recall procedures, and relationships with regulators. Such a person leads the company's response to outbreaks and incidents and makes the decisions, sometimes under great pressure, that protect consumers and the business. Such a person also invests in people and laboratories, because food safety depends on skilled staff as much as on equipment.
A researcher with long experience studies the ecology and virulence of dairy pathogens, their persistence in plants, dose-response and exposure modelling, the transfer of antimicrobial resistance in the dairy chain, emerging hazards such as new zoonotic viruses, and genomic surveillance. Such a person builds quantitative risk assessments, advises Codex and national authorities on criteria and controls, and contributes to outbreak investigations as a scientific expert. The work is judged by whether it changes practice and reduces illness, so it is shared openly with industry, regulators and public health agencies.
This manual describes the hazards and their control; detailed methods of isolation and identification are in Food Technology's Food Microbiology and analytical methods for residues and aflatoxin M1 in Dairy Analysis and Instrumental Methods. The animal side of zoonoses and mastitis is covered in the veterinary track. Outbreak case numbers are those widely cited in reviews and may differ between sources. Incubation periods and infective doses are typical ranges and vary with strain and host. Emerging issues such as H5N1 in dairy cattle are changing quickly and should be checked against current guidance. Viral and parasitic hazards (norovirus, hepatitis A, Cryptosporidium, Toxoplasma) are mentioned only briefly. National microbiological limits other than the EU, US and Codex examples are not reproduced.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.