Dairy Science & Technology · Dairy Microbiology · also Dairy Microbiology II, Microbiology of Milk Products
The microflora, spoilage and safety of each dairy product family, from cream to ice cream to traditional products.
In the exams: ICAR AICE subject 36 Units 3 and 5; ICAR AIEEA-PG group 16 (microbiological quality of cream, butter, ice cream, concentrated and dried milks, infant foods and indigenous products)
Each dairy product is a different environment for microorganisms. Cream is rich and near neutral; butter confines bacteria to tiny water droplets; sweetened condensed milk and milk powder have too little available water for most organisms; ice cream is frozen; fresh cheeses are moist and only mildly acid; ripened cheeses change over months as salt, acid and ripening flora develop; fermented milks are acid and full of live bacteria; traditional heat-desiccated products are concentrated but often handled by hand. Microbiology of Dairy Products examines, product by product, which organisms survive processing or enter afterwards, which of them grow, how they spoil the product or make it unsafe, and how the microbiological standards that govern each product are set and applied.
The subject builds directly on Microbiology of Raw and Market Milk, which covers sources of contamination, temperature groups, psychrotroph enzymes, heat treatment and post-pasteurisation contamination, and Starter Cultures and Lactic Acid Bacteria, which covers the organisms used to make fermented products and cheeses. Pathogens and toxins are treated in depth in Dairy Pathogens, Toxins and Microbial Risk, and general food microbiology, indicator organisms, methods and sampling plans in Food Technology's Food Microbiology. Product technology is in the product subjects (fat-rich products, concentrated and dried milks, frozen desserts, cheese, fermented milks, traditional products). This manual refers to those subjects and concentrates on the microbial ecology and quality of each product.
The subject is examined in Units 3 and 5 of ICAR AICE-JRF/SRF subject 36 (Dairy Microbiology) and in group 16 of ICAR AIEEA-PG, which names the microbiological quality of cream, butter, ice cream, concentrated and dried milks, infant foods and indigenous products and the microbiology of cheese and fermented milks. Food analyst and food safety officer papers test microbiological standards. Expect questions linking each defect to its organism and cause, on control measures, on standards and sampling plans, and simple calculations of counts and acceptance probabilities.
The microflora of raw cream reflects that of the milk from which it was separated, concentrated somewhat because bacteria tend to rise with the fat globules. Cream is a rich medium, close to neutral pH, and supports rapid growth if it is not cooled. Because fat protects microorganisms against heat, cream is pasteurised at higher temperatures or for longer than milk; regulations such as the United States Pasteurized Milk Ordinance set higher minimum conditions for products with more fat or added sweeteners, and in practice cream is often heated well above the minimum, which also inactivates lipases and helps keeping quality. Pasteurised cream spoils in the same ways as pasteurised milk, mainly through post-pasteurisation contamination with Gram-negative psychrotrophs, which cause bitterness, rancidity and off-flavours, and through spore-formers that survive heating. Bacillus cereus is particularly important in cream: its lecithinase damages fat globule membranes and causes bitty cream, and its proteases cause sweet curdling, as described in Microbiology of Raw and Market Milk. Cultured (sour) cream is fermented with mesophilic lactic cultures and is protected by its acidity, but it is susceptible to yeasts and moulds at the surface. UHT cream is commercially sterile but can show age thickening and rancidity from heat-stable bacterial enzymes carried over from raw milk.
Butter is a water-in-oil system in which the aqueous phase, about 16 per cent of the product, is divided into tiny droplets, as described in Chemistry of Dairy Products. Microorganisms are confined to these droplets, and most droplets contain no bacteria at all; those that do are limited by the small volume of nutrients, by salt (in salted butter, salt in moisture of about 8 to 10 per cent or more), by low pH in lactic butter, and by low temperature in storage. Butter made from pasteurised cream under good hygiene therefore has a long shelf life when refrigerated or frozen. Poor working, which leaves large or connected water droplets, allows growth. The typical spoilage organisms are psychrotrophic and lipolytic. Surface taint, a putrid, cheesy defect on the surface of unsalted butter, has been attributed to Shewanella putrefaciens (formerly Pseudomonas putrefaciens). Rancidity from microbial lipases is caused by Pseudomonas fragi and other psychrotrophs and by lipolytic yeasts such as Yarrowia lipolytica. Moulds, including Penicillium, Cladosporium, Alternaria and Geotrichum species, grow on surfaces and in wrapping, producing coloured spots, especially under humid storage. Coliform counts and yeast and mould counts are used as hygiene indicators for butter. Although butter is a low-risk product, outbreaks of listeriosis have been traced to butter contaminated after pasteurisation, showing that the processing environment must be controlled even for products that do not support rapid growth.
Evaporated milk is sterilised in the container or by UHT and aseptic packing, so it should be commercially sterile; spoilage indicates under-processing or leakage of containers after processing. Under-processed evaporated milk can be spoiled by thermophilic and mesophilic spore-formers: Geobacillus stearothermophilus and Bacillus coagulans cause coagulation with acid but no gas (flat-sour spoilage), especially if cans are stored warm, and Clostridium species cause gas production and swelling of cans. Leaking containers admit a mixed flora of non-spore-formers. Sweetened condensed milk is not sterilised; it is preserved by its high sugar content, with a sucrose ratio of about 62.5 to 64.5 per cent and a water activity of roughly 0.83 to 0.86, as described in Lactose, Milk Salts and Vitamins. Most bacteria cannot grow at this water activity, but osmophilic (osmotolerant) yeasts can, producing gas that blows the cans; moulds such as Aspergillus and Penicillium species grow on the surface if oxygen is present in the headspace, forming the brownish button defect; and micrococci and other organisms have been associated with thickening. Control depends on hygienic manufacture after the heat treatment, the correct sucrose ratio, sound containers, minimal headspace oxygen and storage at moderate temperatures. Microbiological specifications for these products commonly set limits for total counts, coliforms, and yeasts and moulds.
Milk powders have water activities of about 0.2 to 0.3, far too low for any microbial growth, so their microbiology is a matter of survival and contamination rather than growth. The flora of a powder consists of organisms that survived the heat treatment of the milk and the drying, mainly spores of Bacillus, Geobacillus and Anoxybacillus species and thermoduric non-spore-formers, and organisms that contaminated the product after drying. Thermophilic spore-formers are a particular concern: they grow in biofilms in the warm sections of evaporators and preheaters during long production runs, as described in Microbiology of Raw and Market Milk, and their spores pass into the powder; many buyers set limits for thermophilic and highly heat-resistant spores. Post-drying contamination from the dryer, fluid bed, cyclones, powder conveying systems and the factory environment is the main route for Salmonella and Cronobacter, both of which survive for long periods in dry powders. A large outbreak of staphylococcal food poisoning in Japan in 2000 was traced to skim milk powder made from concentrate held at warm temperature during a power failure, allowing Staphylococcus aureus to grow and form enterotoxin, which survived later heating; the toxin, not the bacteria, caused illness. Control of powder plants therefore emphasises temperature control of concentrates and balance tanks, short run times, separation of wet and dry areas, controlled dry cleaning of dry zones, environmental monitoring for Salmonella and Cronobacter, and testing of finished product.
Infant formula is the sole source of nutrition for vulnerable consumers, so its microbiological safety is held to the highest standard. Liquid ready-to-feed formulas are commercially sterile, made by UHT treatment and aseptic packing or retort sterilisation. Powdered infant formula, however, cannot be made sterile: the dry-blending of heat-sensitive ingredients and the drying process leave the possibility of low-level contamination. The two pathogens of greatest concern are Cronobacter species, especially Cronobacter sakazakii (formerly Enterobacter sakazakii), and Salmonella. Cronobacter is a Gram-negative member of the Enterobacteriaceae that is unusually resistant to drying; it causes rare but severe infections in neonates, especially premature and low-birth-weight infants, including meningitis, septicaemia and necrotising enterocolitis, with high case fatality and serious long-term effects in survivors. Salmonella in powdered formula has caused outbreaks in several countries, including a large recall of products from a French factory in 2017. Other hazards include Bacillus cereus, whose spores survive processing and can grow in reconstituted formula held warm, and, rarely, Clostridium botulinum. Enterobacteriaceae are monitored as indicators of process hygiene, because their presence suggests conditions under which Cronobacter and Salmonella could enter.
International control of powdered infant formula is set out in the Codex Code of Hygienic Practice for Powdered Formulae for Infants and Young Children and in national rules such as the European Union microbiological criteria. These require absence of Cronobacter in 10-gram samples and absence of Salmonella in 25-gram samples, tested in a large number of samples from each lot, with Enterobacteriaceae used as a process hygiene criterion. Manufacturers apply a zoning approach that separates high-hygiene dry processing areas from wet areas, control water in dry zones, which would allow Cronobacter and Salmonella to multiply in residues, use dry cleaning where possible, control incoming dry-blended ingredients, and run intensive environmental monitoring programmes. Because testing cannot prove absence at very low contamination levels, these process controls are the main protection. The World Health Organization and the Food and Agriculture Organization also advise caregivers on safe preparation: powdered formula should be reconstituted with water that has been boiled and cooled to no less than 70 degrees Celsius, which kills Cronobacter and Salmonella that may be present, fed promptly, and any feed not consumed within about two hours discarded. Sterile liquid formula is recommended for high-risk infants in hospitals where possible. Milk-based foods for older infants and young children, such as follow-on formulas and growing-up milks, are subject to similar but less stringent controls in many jurisdictions. Human milk banks pasteurise donated milk, usually by the Holder method at 62.5 degrees Celsius for 30 minutes, to inactivate pathogens while retaining much of its biological activity.
Ice cream mix is a rich medium of milk solids, sugar, fat, stabilisers and sometimes egg, and it is pasteurised at higher temperatures than milk because sugar and solids protect bacteria; the US Pasteurized Milk Ordinance, for example, sets minimums of 69 degrees Celsius for 30 minutes or 80 degrees Celsius for 25 seconds for frozen dessert mixes. After pasteurisation and homogenisation, the mix is aged cold and frozen, and freezing and frozen storage stop microbial growth, though they kill few organisms; bacteria present in the frozen product survive for long periods. The microbiological quality of ice cream therefore depends on the quality of the pasteurised mix and on contamination after pasteurisation. Important sources are ingredients added after pasteurisation, such as fruits, nuts, chocolate pieces, biscuit, colours and flavours, which must themselves be of good microbiological quality or treated; the ageing tanks, freezers, filling machines and pipework, which are difficult to clean and can harbour biofilms; and the processing environment, especially for Listeria. Soft-serve ice cream is a particular hygiene problem because the mix is held in machines at chill temperatures for long periods, and machines that are not cleaned properly allow bacterial growth. Coliforms and total counts are used as hygiene indicators, and many countries set microbiological standards for ice cream, commonly including limits for total viable count and coliforms and absence of Salmonella and Listeria in specified samples.
Ice cream has caused some of the largest dairy outbreaks. In 1994, an outbreak of Salmonella Enteritidis infection in the United States, estimated to have affected about 224,000 people, was traced to ice cream premix transported in tanker trucks that had previously carried raw liquid egg and were not adequately cleaned; the premix was not re-pasteurised after transport. In 2015, contamination of ice cream products with Listeria monocytogenes at a large US manufacturer was linked to illnesses and deaths over several years, leading to a recall of all its products and showing how Listeria can persist in a processing plant. Other outbreaks have involved Staphylococcus aureus in products handled after heating, and Salmonella and hepatitis A from contaminated ingredients or handlers. Because freezing does not kill pathogens, and because Listeria can grow slowly in mixes and in machines held at chill temperatures, prevention depends on proper pasteurisation of the mix, re-pasteurisation of any mix that leaves the closed system, control of ingredients added after pasteurisation, hygienic design and cleaning of freezers and filling equipment, environmental monitoring for Listeria, and good hygiene in retail soft-serve operations. Melted and refrozen ice cream poses a quality problem rather than a safety problem unless it was contaminated, but temperature abuse in distribution can allow growth in partly thawed products. Frozen desserts made with non-dairy fats or plant-based ingredients are subject to the same principles.
Fresh cheeses are unripened cheeses eaten soon after manufacture: cottage cheese, quarg, cream cheese, ricotta, fresh mozzarella, Latin American queso fresco and queso blanco, and South Asian paneer and chhana. They are high in moisture, usually 50 to 80 per cent, low in salt and either mildly acid (cottage cheese and quarg, pH about 4.5 to 5) or close to neutral (paneer, queso fresco, ricotta). With few hurdles to microbial growth, they have short shelf lives, often one to a few weeks under refrigeration, and their safety depends on pasteurisation of the milk and prevention of contamination afterwards. Spoilage is most often caused by psychrotrophic Gram-negative bacteria such as Pseudomonas, which make cottage cheese curd slimy and give bitter, fruity or putrid flavours, by coliforms, which produce gas and unclean flavours, and by yeasts and moulds, which grow at the low pH of acid-set cheeses and on surfaces. Cottage cheese is especially vulnerable because the cooked, washed curd is mixed with a cream dressing after the main heat step; the dressing, washing water and equipment are frequent sources of psychrotrophs. Practices that extend shelf life include chlorinating or acidifying wash water, high-quality pasteurised dressings, carbon dioxide in the package headspace, protective cultures that inhibit spoilage organisms, and strict cold chains.
Fresh cheeses made from raw milk, or contaminated after pasteurisation, have caused serious outbreaks of listeriosis. In 1985, Mexican-style soft cheese produced in California and contaminated with Listeria monocytogenes caused about 140 cases with many deaths, mostly among pregnant women and newborn infants; investigations implicated raw milk entering the pasteurised product. Since then, queso fresco and similar cheeses made with unpasteurised or recontaminated milk have repeatedly been linked to listeriosis, brucellosis and other infections, particularly in communities where these cheeses are traditional. Because fresh cheeses near neutral pH support the growth of Listeria during refrigerated storage, they are classed as ready-to-eat foods able to support its growth and are subject to strict criteria. Staphylococcus aureus can grow and form enterotoxin in paneer and other fresh cheeses handled by hand and kept warm, and Escherichia coli and other coliforms are common indicators of poor hygiene in paneer and chhana made in small units. Indian standards set microbiological limits for paneer, including coliforms, S. aureus and yeasts and moulds, and many processors vacuum-pack paneer and keep it chilled to extend its shelf life. Good practice for all fresh cheeses combines pasteurised milk, verified pasteurisation, hygienic handling of curd after heating, clean packaging, chilled storage and environmental monitoring for Listeria in the plant.
In ripened cheeses, the microflora changes over the ripening period. Starter bacteria, which reach about a billion per gram in the young curd, stop growing as lactose is used up and salt is added, and their numbers then decline as cells die and lyse, releasing the intracellular enzymes that contribute to flavour, as described in Starter Cultures and Lactic Acid Bacteria. Meanwhile non-starter lactic acid bacteria (NSLAB), which enter at low numbers from the milk, which survive pasteurisation in small numbers, or which come from the plant environment, grow slowly from perhaps a hundred per gram to ten million or more per gram within a few months and dominate the flora of mature hard and semi-hard cheeses. NSLAB are mainly facultatively heterofermentative mesophilic lactobacilli, such as Lacticaseibacillus paracasei, Lacticaseibacillus casei and Lactiplantibacillus plantarum, with pediococci and sometimes obligately heterofermentative species. They use energy sources other than lactose, such as citrate, amino acids, nucleotides from lysed starter cells and glycerol from fat, and they contribute to flavour development but can also cause defects, such as calcium lactate crystals from racemisation of lactate, open texture and slits from gas production by heterofermentative strains, and biogenic amines. Adjunct cultures are added partly to steer the NSLAB population towards desirable strains.
Many varieties also depend on secondary flora deliberately added or encouraged, whose roles are described in Starter Cultures and Lactic Acid Bacteria: propionibacteria in Swiss-type cheeses; Penicillium camemberti and Geotrichum candidum on surface-mould cheeses; Penicillium roqueforti in blue cheeses; and yeasts and coryneform bacteria in smear-ripened cheeses. Surface-ripened soft cheeses are microbiologically distinctive because the surface flora raises the pH from acid to near neutral during ripening, creating conditions in which Listeria monocytogenes can grow if present; several listeriosis outbreaks have been associated with soft surface-ripened cheeses, including a large outbreak in Switzerland in the 1980s linked to a smear-ripened soft cheese. Hard and semi-hard cheeses, in contrast, combine several hurdles: low pH, salt in moisture of commonly 4 to 6 per cent, low water activity, low redox potential, competition from a large population of lactic bacteria, and long ripening, during which many pathogens die. For example, a cheese with 1.8 per cent salt and 38 per cent moisture has a salt in moisture of about 4.5 per cent. This is the basis of rules, such as the United States requirement that cheese made from unpasteurised milk be held for at least 60 days at no less than about 1.7 degrees Celsius before sale, though research has shown that some pathogens can survive this period in some cheeses. Raw milk cheese safety is discussed further in Dairy Pathogens, Toxins and Microbial Risk.
Gas defects are among the most costly microbial faults of cheese. Early blowing appears within the first days of manufacture or brining. It is caused mainly by coliforms, which ferment lactose to acid, carbon dioxide and hydrogen, and sometimes by yeasts; it produces many small holes, a spongy texture, swollen cheeses and unclean, faecal or yeasty flavours. It reflects poor milk hygiene, inadequate pasteurisation or post-pasteurisation contamination combined with slow starter activity, which leaves lactose and a high pH available for the contaminants. Late blowing appears after weeks of ripening in hard and semi-hard cheeses such as Gouda, Emmental and Grana types. It is caused by Clostridium tyrobutyricum, an anaerobic spore-former whose spores survive pasteurisation and germinate in the cheese, where the organism ferments lactate to butyrate, carbon dioxide and hydrogen. The gas produces irregular cavities, cracks and splits, sometimes bursting the cheese, and butyric acid gives a rancid, cheesy off-flavour. Very low numbers of spores, of the order of a few hundred per litre of milk, can be enough to cause late blowing in susceptible cheeses. The spores come mainly from silage fed to cows, especially poorly fermented silage, through the cows' faeces to the teats and into the milk, as described in Microbiology of Raw and Market Milk.
Late blowing is controlled at several points. On the farm, avoiding silage feeding, or ensuring that silage is well made and teats are cleaned thoroughly, reduces spore numbers; several protected-designation cheeses, such as Emmental and Parmigiano Reggiano, forbid silage feeding. In the plant, bactofugation or microfiltration of cheese milk removes most spores, as described in Microbiology of Raw and Market Milk. Inhibitors are used where permitted: sodium or potassium nitrate added to cheese milk inhibits clostridial germination and growth, though its use is restricted in many countries because of concern about nitrate and nitrite residues; hen egg white lysozyme, which lyses vegetative clostridia, is used in some cheeses such as Grana Padano. Nisin-producing or other protective cultures and rapid lactate consumption by the flora are also used, and brining and ripening temperatures are managed to slow clostridial growth. Other microbial defects of cheese include slits and openness caused by heterofermentative NSLAB that produce carbon dioxide, especially in Cheddar ripened at elevated temperatures; unwanted propionic acid fermentation in non-Swiss cheeses; bitterness from excessive proteolysis by starters or coagulant; biogenic amines such as histamine and tyramine produced by decarboxylating lactobacilli and enterococci, which can cause illness in sensitive people; pink and other discolourations from pigmented bacteria; and surface defects from moulds and yeasts, including rind rot. Each defect is controlled by identifying the organism, its source and the conditions that favour it.
Fermented milks are made by fermenting milk with specific lactic acid bacteria, sometimes with yeasts, and their microbiology starts with the culture, as described in Starter Cultures and Lactic Acid Bacteria and Fermented Milks Technology. Yoghurt is made with Streptococcus thermophilus and Lactobacillus delbrueckii subspecies bulgaricus; the Codex standard for fermented milks requires the specific starter organisms to be viable, active and abundant, with a total of at least ten million colony-forming units per gram at the end of shelf life, and at least a million per gram of any additional labelled organism. Dahi, the traditional fermented milk of South Asia, is made by back-slopping or with defined cultures; its flora typically includes mesophilic lactococci, sometimes with leuconostocs, lactobacilli and S. thermophilus, and household dahi varies widely with the inoculum and temperature. Misti dahi is sweetened and caramelised before fermentation. Kefir is fermented with kefir grains or with grain-derived starters containing lactic acid bacteria, acetic acid bacteria and yeasts, which produce lactic acid, carbon dioxide and small amounts of ethanol. Koumiss is traditionally made from mare's milk with lactobacilli and lactose-fermenting yeasts and contains more ethanol and carbon dioxide than kefir. Counting the starter organisms in yoghurt uses selective media, such as M17 agar for S. thermophilus and acidified MRS agar for L. bulgaricus, with incubation conditions that favour each.
The low pH of fermented milks, usually about 4.0 to 4.6, together with lactic acid, the live culture and cold storage, prevents the growth of most spoilage bacteria and pathogens, so their spoilage is dominated by yeasts and moulds and by over-acidification. Yeasts tolerant of acid and cold, such as Kluyveromyces marxianus, Debaryomyces hansenii, Torulaspora and Saccharomyces species, cause gas, bulging of containers, yeasty and fruity flavours; they often enter with fruit preparations, sugar or the factory air, or grow in poorly cleaned filling equipment. Moulds such as Penicillium, Mucor, Rhizopus and Geotrichum grow on the surface, especially of set products and containers with headspace. Post-acidification, the continued production of acid by Lactobacillus bulgaricus during cold storage, makes yoghurt increasingly sour and is reduced by choosing mild cultures and keeping the product cold. Coliforms in fermented milks indicate contamination after fermentation, since they die gradually in acid conditions. Although fermented milks have a good safety record, they are not a substitute for pasteurisation: acid-tolerant pathogens such as Escherichia coli O157 and Listeria monocytogenes can survive for days in yoghurt, and outbreaks have occurred, including botulism from an under-processed hazelnut conserve added to yoghurt in the United Kingdom in 1989. Milk for fermented products should therefore be heat-treated, and ingredients added after fermentation must be safe.
Yeasts and moulds are the main spoilage organisms of dairy products in which bacteria are restricted by low pH, salt, low water activity or competition: fermented milks, cheeses, butter, sweetened condensed milk and some desserts. Many dairy spoilage yeasts tolerate acid, salt and cold and use lactose, lactate, citrate, protein or fat; important genera include Kluyveromyces, Debaryomyces, Candida, Yarrowia, Torulaspora, Rhodotorula and Saccharomyces. They cause gas, bloating of packs, yeasty, fruity and alcoholic flavours, slime and discolouration. Moulds need oxygen and so grow mainly on surfaces, but some tolerate very low oxygen and high carbon dioxide; Penicillium species (such as P. commune and P. roqueforti, which grows in vacuum-packed cheese), Mucor, Cladosporium, Aspergillus and Geotrichum are common. Moulds produce visible colonies, off-flavours, softening and, in some cases, mycotoxins. Mould growth on cheese can produce toxins such as cyclopiazonic acid and sterigmatocystin, though their practical significance in cheese is debated; the main mycotoxin concern in dairy, aflatoxin M1, comes from the animal's feed, as described in Dairy Pathogens, Toxins and Microbial Risk. Yeast and mould contamination comes mostly from the air of the plant, especially near cooling, cutting and packing operations, from ingredients such as fruit, nuts and sugar, from brine in cheese plants and from poorly cleaned equipment.
Control of yeast and mould spoilage combines hygiene and product protection. Air in filling and packing areas is filtered and kept at positive pressure, and condensation, which drips spores onto products, is prevented. Brine for cheese is filtered, pasteurised or treated with ultraviolet light, and its salt concentration and pH are controlled. Packaging excludes oxygen through vacuum packing or modified atmosphere packaging with carbon dioxide, which inhibits many moulds, although some moulds and yeasts tolerate it. Preservatives are permitted in some products in some countries: natamycin (pimaricin), a polyene antifungal produced by Streptomyces natalensis, is applied to the surfaces of hard and semi-hard cheeses and sausages to prevent mould growth without affecting the internal bacterial flora, its use being limited to surface treatment with maximum residue levels; sorbic acid and sorbates are permitted in some cheeses and other products. Protective cultures of lactic acid bacteria or propionibacteria that inhibit yeasts and moulds are increasingly used as alternatives. Low and stable storage temperatures slow growth. Yeasts and moulds are counted on selective media that suppress bacteria, such as dichloran rose bengal chloramphenicol agar, oxytetracycline glucose yeast extract agar or acidified potato dextrose agar, usually incubated at about 25 degrees Celsius for 5 days. Air sampling with settle plates or air samplers, and environmental swabbing, locate sources of contamination. Microbiological specifications for many products include limits for yeasts and moulds.
Traditional dairy products of South Asia are made in large volumes, often by small units and street vendors, and their microbiological quality varies widely. Khoa, made by heating milk in open pans until most of the water evaporates, is effectively sterilised of vegetative bacteria by the prolonged boiling, but it is then cooled, scraped, kneaded and stored by hand, often at ambient temperature, so it is recontaminated. With a moisture of about 25 to 35 per cent and a water activity high enough for many organisms, khoa supports growth of Staphylococcus aureus, coliforms, micrococci, yeasts and moulds, and its shelf life at room temperature is only a few days. Khoa-based sweets such as burfi and peda, which contain added sugar, have lower water activity and keep longer, but mould growth on their surface is common. Paneer and chhana, made by acid-heat coagulation, are moist and near neutral in pH and have the hazards described in chapter 5. Rasogolla, chhana balls cooked in sugar syrup, keeps reasonably well in syrup and is canned for longer life. Studies of market samples have often found high counts, coliforms and S. aureus in these products, reflecting handling and storage rather than the processes themselves. Improvements include mechanised manufacture, hygienic cooling, vacuum or modified atmosphere packaging, refrigeration, and the use of permitted preservatives where allowed.
Ghee contains less than about 0.5 per cent moisture and is microbiologically stable; its deterioration is chemical, as described in Milk Lipids, although ghee with higher moisture or residue can support mould growth. Shrikhand, made from strained dahi (chakka) with sugar, has a low water activity and acid pH, so it keeps reasonably well under refrigeration but is spoiled by yeasts and moulds. Lassi and buttermilk are perishable acid drinks. Kulfi, a traditional frozen dessert made from concentrated milk, is often made and sold in small units without effective pasteurisation after mixing, and has the risks described for ice cream. Outside South Asia, dried fermented milk products such as kashk and qurut, heat-concentrated products such as dulce de leche, and smoked or dried cheeses such as chhurpi rely on low water activity, acidity, salt or smoking for preservation, and their safety depends on the raw material and hygiene. Across all traditional products, the common hazards are recontamination after heating, especially by hand, storage at ambient temperature, the use of raw milk in some products, and contaminated packaging. The microbiological standards set for many of these products by national regulators, such as India's Food Safety and Standards Authority, include limits for total counts, coliforms, S. aureus, yeasts and moulds, and absence of Salmonella and Listeria in specified samples. Training of small producers in hygiene and the provision of refrigeration are often more effective than testing in improving their safety.
Microbiological criteria define the acceptability of a product, a batch or a process on the basis of the presence, absence or number of microorganisms or toxins per unit of food. Codex principles for microbiological criteria distinguish the microorganism or toxin, the analytical method, the sampling plan, the limits and the action to be taken. Two types of criteria are widely used: food safety criteria, which apply to products placed on the market and concern pathogens such as Salmonella and Listeria, and process hygiene criteria, which indicate whether the production process is working properly and concern indicator organisms such as Enterobacteriaceae, Escherichia coli, coagulase-positive staphylococci and total counts. Sampling plans, developed by the International Commission on Microbiological Specifications for Foods (ICMSF), specify the number of sample units (n) to test and how many may give unacceptable results. In a two-class plan, each unit is classed as acceptable or not against a limit m, and the lot is accepted if no more than c units exceed m; for pathogens, c is usually 0. In a three-class plan, results below m are acceptable, results between m and M are marginal, and results above M are unacceptable; the lot is accepted if no unit exceeds M and no more than c units are marginal. For example, with n = 5, c = 2, m = 10 and M = 100 per gram, results of 5, 20, 50, 80 and 9 give three marginal units and lead to rejection. Sampling plans and their statistics are covered more fully in Food Technology's Food Microbiology.
National and regional standards differ, so they must be read from current regulations; the following examples show their structure. Under the European Union regulation on microbiological criteria, Listeria monocytogenes in ready-to-eat foods must not exceed 100 colony-forming units per gram during shelf life, and must be absent in 25 grams of product before it leaves the producer's control if the food can support growth and the producer cannot show that the limit will be met; foods with pH of 4.4 or less, water activity of 0.92 or less, or pH of 5.0 or less together with water activity of 0.94 or less are regarded as unable to support growth. Salmonella must be absent in 25 grams in cheeses, butter and cream made from raw or low-heat-treated milk, in milk powder, whey powder and ice cream, typically with five samples. Process hygiene criteria cover Enterobacteriaceae in pasteurised milk and dried products, E. coli in raw-milk cheeses and butter, and coagulase-positive staphylococci in cheeses and powders, with testing for staphylococcal enterotoxins when counts exceed 100,000 per gram. Infant formula criteria are described in chapter 3. Under the United States Pasteurized Milk Ordinance, Grade A pasteurised milk must have a standard plate count of no more than 20,000 per millilitre and coliforms of no more than 10 per millilitre. India's Food Safety and Standards Authority sets microbiological standards for milk and milk products, including process hygiene and food safety criteria for each product category. The probability that a sampling plan detects a contaminated lot depends on the number of samples: with 2 per cent of units contaminated and c = 0, five samples accept the lot about 90 per cent of the time, sixty samples about 30 per cent.
A student uses this subject in dairy microbiology practicals and product courses. You will count total viable organisms, coliforms, yeasts and moulds, spores and starter organisms in cream, butter, ice cream, cheese, yoghurt, powders and indigenous products; examine defective products and relate defects to organisms; and compare results with standards. Learn, for each product, the main spoilage organisms and hazards, how they enter and why they grow, and the standards that apply, because examiners ask these links and plant work depends on them.
Five years into work as a microbiologist or quality manager in a dairy plant, you set and verify product specifications, investigate gas, mould or off-flavour complaints, trace late blowing to silage or failed bactofugation, find the source of yeasts in yoghurt in a fruit preparation or a filler, run environmental monitoring for Listeria and Salmonella in cheese and powder plants, decide whether a batch meets criteria under a sampling plan, and advise small producers of paneer and khoa on hygiene and packaging.
A senior quality director with twenty-five years of experience designs the microbiological control system for a range of products and plants: hygienic zoning in powder and infant formula factories, shelf-life targets and challenge testing for fresh cheeses and fermented milks, raw-milk and spore specifications for cheese and powder, sampling plans and positive-release policies, and the response to contamination incidents and recalls. Such a person deals with regulators and major customers and decides when the evidence justifies changing a process or supplier.
A researcher with long experience studies the microbial ecology of dairy products with culture and sequencing methods, the sources and persistence of spoilage organisms and pathogens in plants, the role of non-starter and secondary flora in cheese quality and safety, protective cultures and natural antimicrobials against yeasts, moulds and Listeria, and predictive models of shelf life. Such a person advises industry and standard-setting bodies on criteria and hygiene. The test of such work is whether it reduces spoilage, waste and illness in real factories and markets, so it is usually done with industrial partners and validated in commercial conditions.
This manual covers the microbial ecology and quality of each product family; pathogens and toxins are treated in Dairy Pathogens, Toxins and Microbial Risk, raw milk and processing microbiology in Microbiology of Raw and Market Milk, starters in Starter Cultures and Lactic Acid Bacteria, and methods and sampling statistics in Food Technology's Food Microbiology. National standards are given only as examples (EU, US, Codex) because they change and differ; India's FSSAI product-by-product microbiological limits are not reproduced. Outbreak figures are those widely cited in reviews and may differ between sources. Microbiology of non-dairy fermented and plant-based alternatives, probiotic survival and detailed predictive models are not covered.
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.
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