Dairy Science & Technology · Dairy Processing & Product Technology · also Cultured Dairy Products, Technology of Fermented Milks
Yoghurt, dahi and the world's fermented milks: process, cultures, texture, defects and shelf life.
In the exams: ICAR AICE subjects 34 Unit 7 and 36 Unit 5; ICAR AIEEA-PG group 16 (dahi, yoghurt, kefir, koumiss, Bulgarian milk, cultured buttermilk, leben, yakult); GATE XL-U Section 2 (curd, yoghurt)
Fermented milks are the oldest processed dairy foods and today one of the largest and most innovative categories in the industry. Every culture has its own: yoghurt and its many styles, dahi and shrikhand in South Asia, kefir and kumys from the Caucasus and Central Asia, skyr and filmjölk from the Nordic countries, laban and labneh in the Middle East, amasi in southern Africa and probiotic drinks in East Asia. The science behind them is shared: lactic acid bacteria (sometimes with yeasts or moulds) ferment lactose, the pH falls towards the isoelectric point of casein, and an acid gel forms whose texture and flavour depend on the milk, the heat treatment, the culture and the handling.
This manual covers the classification of fermented milks; yoghurt in its set, stirred, drinking and strained forms; the process steps and how each shapes texture; South Asian products; yeast-lactic products; cultured buttermilk and sour cream; concentrated products; probiotic fermented milks; defects; fruit preparations; and shelf life. The microbiology of the cultures themselves (taxonomy, metabolism, phage, propagation) is treated in depth in the starter-cultures subject, and milk reception, standardisation, homogenisation and heat treatment equipment in market-milk-processing; this manual refers to them rather than repeating them.
ICAR AIEEA-PG Dairy Science names dahi, yoghurt, kefir, kumiss, Bulgarian milk, cultured buttermilk, leben and probiotic fermented milks; ICAR AICE-JRF/SRF asks for the chemistry of milk fermentation and flavour (Dairy Chemistry) and the starters and spoilage of fermented products (Dairy Microbiology); GATE XL Food Technology includes curd and yoghurt. Temperatures, times and compositions in this manual are representative textbook ranges. Standards of identity, permitted additives, minimum culture counts and labelling rules differ by country and must be checked in the current Codex standard and national regulation.
A fermented milk is milk, or a milk product, fermented by specific micro-organisms so that the pH falls and the milk coagulates, with or without other changes such as alcohol or gas production. The Codex Alimentarius Standard for Fermented Milks (CXS 243-2003) is the international reference. It requires the starter micro-organisms to be viable, active and abundant in the product to the date of minimum durability, unless the product is heat-treated after fermentation, in which case the requirement for viable organisms does not apply and the name must show the heat treatment where national law so requires. The standard defines several named products by their cultures: yoghurt (symbiotic cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus), alternate-culture yoghurt (Streptococcus thermophilus with any Lactobacillus species), acidophilus milk (Lactobacillus acidophilus), kefir (kefir grains, a defined community of lactic acid bacteria, yeasts and sometimes acetic acid bacteria) and kumys (Lactobacillus delbrueckii subsp. bulgaricus and Kluyveromyces marxianus). It also sets compositional minimums for these products, such as milk protein and titratable acidity, minimum counts of the characteristic organisms, minimum yeast counts for kefir and kumys, and a minimum ethanol content for kumys; national standards build on or differ from these values, so the current text should be consulted for exact limits.
The hundreds of traditional fermented milks can be grouped in several ways. By the micro-organisms: mesophilic lactic fermentations (cultured buttermilk, Nordic filmjölk, Indian dahi in its traditional form), thermophilic lactic fermentations (yoghurt, Bulgarian sour milk made with L. delbrueckii subsp. bulgaricus alone), therapeutic or probiotic fermentations (acidophilus milk, bifidus milks, fermented milk drinks made with Lacticaseibacillus species), yeast-lactic fermentations (kefir, kumys) and mould-lactic fermentations (Finnish viili, which carries a velvety surface layer of Geotrichum candidum and is also ropy). By texture: set, stirred, drinkable and concentrated. By composition: whole, reduced-fat and fat-free, and by concentration from ordinary to strained products such as labneh, skyr, ymer, chakka and Greek-style yoghurt. By processing after fermentation: products with live cultures, and products heat-treated after fermentation for long or ambient shelf life.
These groupings matter in practice because they determine the incubation temperature, the culture handling, the plant layout, the defects to expect and the labelling. A mesophilic aroma product made at 20–30 °C over many hours is run very differently from a thermophilic yoghurt fermented at around 40–45 °C in a few hours.
South Asia: dahi, misti dahi, shrikhand (from chakka), lassi and chhach. Middle East and Mediterranean: laban or leben, labneh, ayran (a salted yoghurt drink), and strained Greek-style yoghurts. Caucasus and Eastern Europe: kefir, prostokvasha, ryazhenka (made from baked milk). Central Asia and Mongolia: kumys from mare's milk, and shubat from camel's milk. Nordic countries: filmjölk, viili, skyr, ymer, långfil (a ropy milk). Africa: amasi, nunu, suusac from camel milk, and many household soured milks. East Asia: fermented milk drinks based on probiotic lactobacilli. The technological principles are the same; the cultures, milks, and the consumer's idea of the right flavour and texture differ.
Yoghurt is the world's most widely made fermented milk. Its two organisms grow better together than alone, a relationship called protocooperation. Streptococcus thermophilus grows first; it removes oxygen, produces formic acid and carbon dioxide, and these stimulate Lactobacillus delbrueckii subsp. bulgaricus. The lactobacillus is more proteolytic: its cell-wall proteinase releases peptides and free amino acids, including valine and histidine, that stimulate the streptococcus, which has weak proteolytic activity. Together they acidify milk faster and produce more flavour than either alone. Lactose is fermented to lactic acid; both species are homofermentative and most strains use only the glucose half of lactose, excreting galactose into the milk. The lactobacillus produces most of the acetaldehyde that gives yoghurt its characteristic 'green apple' note, and minor flavour compounds include diacetyl, acetone and acetoin. Cultures are commonly balanced so that the two organisms are present in roughly equal numbers, and the balance shifts during fermentation and storage. Culture microbiology in depth is covered in the starter-cultures subject.
Set yoghurt is inoculated, filled into retail cups and fermented in the cup in an incubation room, then cooled in a cooling tunnel or room. Its gel is undisturbed, so it is firm and sliceable, and any damage during handling shows as whey on the surface. Stirred yoghurt is fermented in a tank; when the target pH is reached, the gel is broken by stirring, cooled (often in a plate or tubular cooler designed for gentle treatment), may be mixed with fruit preparation, and filled. Its texture is a smooth, viscous fluid, and its viscosity depends strongly on how gently it is pumped, cooled and filled. Many plants cool stirred yoghurt to around 20–24 °C before filling and complete cooling in the cup, which slows fermentation but keeps viscosity high.
Drinking yoghurt is a stirred yoghurt of lower total solids that is homogenised or otherwise broken down after fermentation to a low viscosity, often with stabiliser such as pectin to prevent protein sedimentation at acid pH, and frequently fruit or sugar added. It may be sold with live culture or heat-treated for longer shelf life. Strained yoghurt has part of its whey removed after fermentation, raising the protein and total solids several-fold. The traditional method hangs yoghurt in cloth bags; industrial methods use nozzle separators, ultrafiltration of the fermented product, or ultrafiltration of the milk before fermentation followed by direct fermentation of the concentrate, which yields no acid whey. Strained yoghurt from straining produces large quantities of acid whey, which is difficult to dispose of or use because of its high acidity and mineral content; this is a significant environmental and cost issue for plants making Greek-style products.
During fermentation only part of the lactose is used: commonly a quarter to a third, because acid stops the culture long before the lactose runs out. A homofermentative culture converts one molecule of lactose (or more precisely its glucose, with the galactose excreted by many strains) to lactic acid; on a whole-lactose basis one mole of lactose can give at most four moles of lactic acid, which means 342 g of lactose gives up to 360 g of lactic acid once the water of hydrolysis is counted. Titratable acidity of yoghurt is commonly around 0.8–1.2 per cent as lactic acid at the end of shelf life, depending on the product. Proteolysis by the lactobacillus raises free amino acids and peptides; lipolysis is slight. Some B vitamins are synthesised by the culture and others are used up. Yoghurt is better tolerated than milk by many people with lactose maldigestion because the culture's beta-galactosidase, released when cells lyse in the small intestine, continues to digest lactose, and because the semi-solid product empties more slowly from the stomach. Calcium is partly solubilised as the pH falls, which does not reduce its nutritional value. These facts underlie the nutritional claims often made for fermented milks and are examined in AICE Dairy Chemistry under the nutritional and therapeutic significance of fermented products.
Texture in yoghurt is built from the milk up. Total solids are commonly raised from about 12 per cent in milk to somewhere around 14–16 per cent by adding skim milk powder, milk protein concentrate or whey protein, by evaporation, or by membrane concentration (ultrafiltration or reverse osmosis). Protein, especially casein, forms the gel network, so higher protein gives a firmer gel with less syneresis. Fat content is set by the product class; fat globules, once homogenised, become part of the gel network and add creaminess. Stabilisers such as gelatin, pectin, starch or other hydrocolloids are used in some products and some countries, but not in others where standards restrict additives; some plants rely instead on exopolysaccharide-producing cultures. The mix must be well dissolved and deaerated, because undissolved powder and air cause grainy texture and defects.
The mix is homogenised, commonly at a temperature around 60–70 °C, to prevent creaming during fermentation and to make the fat globules active fillers in the gel. The heat treatment of yoghurt milk is much stronger than pasteurisation: typical combinations are about 90–95 °C for around 5 minutes, or 85 °C for about 30 minutes in batch systems. Its purposes are to destroy pathogens and competing micro-organisms, to drive off oxygen, and most importantly to denature a large part of the whey proteins. Denatured beta-lactoglobulin associates with kappa-casein on the micelle surface, so that the micelles that aggregate as the pH falls carry whey protein with them. This increases the number of protein strands in the gel and its water-holding capacity, giving a firmer, smoother gel with much less whey separation than yoghurt made from pasteurised milk. Excessive heat or very long holding gives cooked flavour and can coarsen texture.
The milk is cooled to the incubation temperature, commonly about 40–45 °C for yoghurt, and inoculated with bulk starter (often a few per cent) or direct-vat-set frozen or freeze-dried culture. Incubation continues until the target pH is reached, commonly about 4.5–4.6, typically in some hours. As the pH approaches the isoelectric region of casein, colloidal calcium phosphate dissolves and the micelles aggregate into a three-dimensional network: an acid gel. The rate of acidification affects structure. Slower fermentation at a lower temperature generally gives a finer, more continuous network and a more viscous product, while very fast acidification at a high temperature tends to give a coarser network more prone to whey separation, although cultures and recipes differ. Vibration or movement of set yoghurt during gelation damages the network. Cooling stops fermentation: set yoghurt is cooled in the cup, stirred yoghurt is broken and cooled in the tank or in line. The point at which cooling begins controls the final acidity, because cultures keep producing acid until the product is cold.
Some strains of S. thermophilus and L. bulgaricus produce exopolysaccharides (EPS): long sugar polymers either attached to the cell as a capsule or released into the milk. Even at low concentrations they raise viscosity, improve smoothness and reduce syneresis, because they bind water and interact with the casein network; ropy strains give a stringy texture that some markets like and others reject. EPS-producing cultures allow 'clean label' products without added stabilisers. Where stabilisers are permitted, gelatin gives a firm, short texture and melts in the mouth; pectin is used particularly in drinking yoghurts, where it adsorbs onto casein particles at acid pH and keeps them from aggregating and settling; modified starches add body to stirred and low-fat products; and other hydrocolloids such as carrageenan or locust bean gum are used in some formulations. Their use is restricted by national standards for yoghurt and by religious and vegetarian requirements in the case of gelatin. Excess stabiliser gives an artificial, jelly-like or slimy texture, so the amount is kept low and matched to the product.
Dahi (curd) is the Indian subcontinent's everyday fermented milk, made in homes, by sweet-makers and by organised dairies. Household dahi is made by boiling milk, cooling it to lukewarm and inoculating it with a little of the previous day's dahi, then leaving it to set overnight; the culture is therefore an undefined mixture that varies with household and season. Traditional dahi flora includes mesophilic lactococci and leuconostocs, sometimes with thermophilic streptococci and lactobacilli. Industrial dahi uses defined cultures: mesophilic mixed cultures (Lactococcus lactis subspecies and aroma strains) give a mild, aromatic, less acid product; thermophilic yoghurt cultures give a firmer, more acid product; and many manufacturers blend the two. Milk of cow, buffalo or a blend is used; buffalo milk gives a firmer, creamier set. Dahi is sold set in cups or earthen pots, and in some regions as sour, in others as mild. Indian food standards define dahi and its fat and SNF classes by the milk it is made from; the values must be checked in the current FSSAI regulation.
Misti dahi (sweet dahi) is a traditional product of eastern India and Bangladesh. Milk is concentrated by boiling, sugar is added, and the long heating caramelises part of the sugar and develops Maillard browning, giving a light brown colour and caramel flavour. The sweetened concentrated milk is cooled, inoculated and set, traditionally in earthen pots that absorb some moisture and give a firmer surface. The product is firm, sweet and mildly acid. Industrial misti dahi uses evaporated or standardised milk with added sugar and colour development by controlled heating or caramel. Because sugar content is high, the culture must tolerate reduced water activity, and fermentation is slower than in plain dahi.
Shrikhand, from western India, is made from chakka, the strained curd obtained by hanging dahi in a muslin cloth (or by mechanical separation or ultrafiltration) to remove whey. Chakka is mixed with a large amount of sugar, together with cardamom, saffron, nuts or fruit (as in amrakhand, made with mango pulp), and then often worked through a mill to a smooth, semi-soft paste. The high sugar content lowers water activity and gives shrikhand a longer shelf life than dahi; heat treatment of the finished product (thermised shrikhand) extends it further. The main production issues are the yield and hygiene of straining, the acid whey produced, and smoothness.
Lassi is a beverage made by churning or blending dahi with water, and is either sweet (with sugar, and sometimes fruit or flavours) or salted with spices. Chhach (chaas) is the thinner, often spiced buttermilk left when butter has been churned from dahi, and is a traditional cooling drink. Packaged lassi and chhach are made from fermented milk diluted and stabilised; they may be sold live or heat-treated and must be stabilised against protein sedimentation in the same way as drinking yoghurt.
Kefir originated in the Caucasus. It is traditionally made with kefir grains: gelatinous, cauliflower-like masses in which lactic acid bacteria, yeasts and sometimes acetic acid bacteria are embedded in a matrix of protein and the polysaccharide kefiran (produced mainly by Lactobacillus kefiranofaciens). Grains are added to milk, which ferments at room temperature for about a day; the grains are then strained out, washed and reused, and they grow slowly. The resulting drink is acid, slightly effervescent from carbon dioxide produced by yeasts, contains a small amount of ethanol, and has a yeasty, refreshing flavour. The microbial community of grains is complex and varies between sources; yeasts reported include lactose-fermenting Kluyveromyces marxianus and non-lactose-fermenting species, and the bacteria include lactobacilli, lactococci and leuconostocs.
Industrial production rarely uses grains directly, because the grain flora is hard to control and the product variable. Instead, a 'mother culture' is made from grains and propagated, or defined cultures of selected bacteria and yeasts are used. Packaged kefir is a challenge because yeasts produce gas during storage, which can swell packs; manufacturers control yeast activity by culture choice, storage temperature and package design.
Kumys (koumiss) is the traditional fermented mare's milk of the Central Asian steppes and Mongolia. Mare's milk is low in fat and casein and high in lactose compared with cow's milk, so it does not form a firm curd; its whey-protein-rich composition gives a thin, finely dispersed product. Fermentation by lactobacilli (classically L. delbrueckii subsp. bulgaricus) and lactose-fermenting yeasts (Kluyveromyces marxianus) yields lactic acid, ethanol and carbon dioxide; Codex requires a minimum ethanol content for kumys, and the product is mildly alcoholic and effervescent. Traditionally it was agitated in leather bags. Because mare's milk is scarce, kumys-like products are also made from cow's milk modified to resemble mare's milk, for example by adding whey or lactose and reducing casein.
Other yeast-containing fermented milks include shubat from camel's milk and various regional sour milks in which yeasts occur naturally. In yeast-lactic products the yeasts contribute ethanol, carbon dioxide, esters and other aroma compounds, and they can raise pH slightly by consuming lactic acid. Mould-lactic products such as viili carry a surface layer of Geotrichum candidum that gives a velvety skin and a distinctive aroma, while ropy lactococci give the body its stringy texture. These products illustrate that a fermented milk is defined by its whole microbial community, not just by the lactic acid bacteria.
Traditional buttermilk is the liquid left after churning cream into butter. Cultured buttermilk, sold as a drink in North America and elsewhere, is a different product: skim or low-fat milk, heat-treated (commonly above pasteurisation to improve body), cooled to around 20–22 °C and fermented with a mesophilic aroma culture for many hours, typically overnight, until the pH falls to about 4.5–4.6. The culture contains acid-producing lactococci (Lactococcus lactis subsp. lactis and subsp. cremoris) and citrate-fermenting aroma organisms (Lactococcus lactis subsp. lactis biovar diacetylactis and Leuconostoc mesenteroides subsp. cremoris). Citrate is converted to diacetyl, which gives the buttery aroma, together with acetoin and carbon dioxide. Some producers add citrate to milk to increase diacetyl. The gel is broken by gentle stirring, sometimes salt or butter flakes are added, and the product is cooled and filled.
Diacetyl is unstable: the enzyme diacetyl reductase, present in some culture organisms, converts it to acetoin, which has little flavour, especially if the product is held warm or fermented too long. Good practice is to stop fermentation at the right pH and cool promptly. A harsh, 'green' or yoghurt-like flavour indicates excess acetaldehyde relative to diacetyl, a problem when Leuconostoc activity is too low.
Sour cream (cultured cream) is cream, commonly around 18–20 per cent fat in North America though standards differ by country, that is homogenised (often at high pressure, sometimes twice, to build viscosity), heat-treated, cooled and fermented with a mesophilic aroma culture until it thickens and reaches its target acidity. Homogenisation coats fat globules with casein, so they join the acid gel, which gives sour cream its heavy body; a small amount of rennet or stabilisers may be added where permitted. Light sour cream and dressings are made with lower fat and stabilisers. Crème fraîche, from France, is a higher-fat cultured cream with a milder acidity and buttery flavour; smetana in Eastern Europe is similar. Defects of sour cream include thin body (insufficient homogenisation or solids), whey separation, bitterness, and spoilage by yeasts and moulds. Like other cultured products it relies on low pH, cold storage and hygienic filling for shelf life.
Concentrated fermented milks have part of their whey removed, so they are thick and high in protein. Skyr, from Iceland, is made from skim milk fermented with yoghurt-type cultures (traditionally with a little rennet and back-slopped culture), then concentrated so that it is very thick and high in protein with almost no fat. Labneh, from the Middle East, is strained yoghurt, often salted, eaten as a spread, and sometimes formed into balls stored in olive oil. Greek-style yoghurt is strained yoghurt that may be full-fat or fat-free. Ymer (Denmark) is a concentrated mesophilic fermented milk. Chakka, the base of shrikhand, is strained dahi. Quark and tvorog are concentrated acid-curd products classed as fresh cheeses rather than fermented milks in some standards; the boundary is drawn differently by different countries. Codex has a separate definition for concentrated fermented milk with a minimum protein content.
Four approaches are used. Traditional straining in cloth bags is simple but slow, hard to keep hygienic and labour-intensive. Centrifugal separation of the fermented curd in a nozzle separator (the same type used for quark) is fast and continuous; the curd is then cooled and blended. Ultrafiltration of the fermented product concentrates protein while removing acid whey as permeate; the high viscosity of the concentrate limits the achievable concentration and requires suitable membranes. Ultrafiltration (or addition of milk protein) before fermentation produces a high-protein milk that is fermented directly to the final composition; this produces no acid whey, but the high buffering capacity of the concentrate slows acidification and can change flavour and texture, so cultures and process have to be adapted.
Protein fortification with milk protein concentrate or micellar casein is a simpler route used for many high-protein products. Each route gives a different texture, yield, whey stream and cost. Yields can be calculated by protein mass balance: mass of concentrate × its protein = mass of feed × its protein − mass of whey × whey protein.
Straining fermented milk produces acid whey, at a pH around 4.5, with a high lactic acid and mineral content and little protein. It cannot be processed as easily as sweet cheese whey: it is hard to dry because lactic acid and lactose make the powder sticky, and its high biochemical oxygen demand makes disposal to sewers or land restricted in many places. Uses include animal feed, biogas production, fermentation substrates, and demineralised ingredients. Choosing a concentration route that avoids or reduces acid whey is an important design decision for a plant making strained products.
Probiotics are defined by FAO and WHO, in wording later endorsed by the International Scientific Association for Probiotics and Prebiotics, as live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host. The definition applies to a strain with evidence, not to a species or to fermented milk in general. Organisms used in probiotic fermented milks include Lactobacillus acidophilus, Lacticaseibacillus casei, Lacticaseibacillus paracasei and Lacticaseibacillus rhamnosus (formerly all in the genus Lactobacillus, which was divided into 25 genera in 2020), Bifidobacterium animalis subsp. lactis and other bifidobacteria. The yoghurt organisms themselves are not usually counted as probiotics, although yoghurt cultures do improve lactose digestion in people with lactose maldigestion, a benefit recognised by regulators in some jurisdictions.
A probiotic benefit depends on enough viable cells at the time of consumption. Many guidelines and national rules refer to a minimum count per gram (often of the order of a million colony-forming units per gram or more at the end of shelf life), but the requirement and how it is expressed vary. Most probiotic organisms grow poorly in milk, because they lack the proteolytic activity of L. bulgaricus and are sensitive to acid and, for bifidobacteria, to oxygen. Strategies include: adding the probiotic at a high count alongside a faster starter; using starters without L. bulgaricus, whose post-acidification during storage kills sensitive probiotics (cultures of L. acidophilus, bifidobacteria and S. thermophilus are a common alternative); adding growth factors such as whey protein or yeast extract; using packaging with a low oxygen transmission rate; microencapsulation; and controlling storage temperature. Viability is checked through shelf life with selective media, since probiotic and starter organisms must be counted separately.
Health claims for probiotic foods are regulated differently around the world. Some regulators have approved very few claims, requiring strain-specific evidence from human studies; others permit general or structure-function claims with less evidence. Labelling may need to identify the strain, the count at the end of shelf life, and storage conditions. A manufacturer should treat the strain, the dose, the matrix and the evidence as one package and should not transfer evidence from one strain to another. In exams and in practice the important points are the definition, the strain-specificity of effects, the need for viability, and the conflict between L. bulgaricus post-acidification and probiotic survival.
Acidophilus milk, one of the first 'therapeutic' fermented milks, is milk heat-treated hard and fermented with Lactobacillus acidophilus alone at about 37 °C. The organism grows slowly in milk and the traditional product is very acid; sweet acidophilus milk avoids this by adding a high count of concentrated culture to cold milk without fermentation, so the milk tastes normal while carrying the organism. Bifidus milks use bifidobacteria, often with S. thermophilus or L. acidophilus to speed acidification. Fermented milk drinks based on Lacticaseibacillus strains are made from heat-treated skim milk fermented for a long time, then diluted with sugar syrup to a sweet, thin drink with a very high count of the organism. These products show the usual compromise in probiotic technology: the organism chosen for health effects is rarely a good acid producer, so the process must be designed around its weaknesses.
Whey separation on the surface of set yoghurt, or in the container of stirred products, is the most common texture complaint. It results when the acid casein network contracts or is damaged and expels serum. Causes include low total solids or protein; insufficient heat treatment of the milk (too little whey protein denaturation); too high an incubation temperature or too fast an acidification, which gives a coarse network; too low a final pH; mechanical damage or vibration of set yoghurt during and after gelation; and temperature fluctuations during storage. Remedies are the reverse: raise protein and solids, heat milk adequately, control fermentation rate and final pH, handle set yoghurt gently, and use exopolysaccharide-producing cultures or permitted stabilisers. Note that syneresis in fermented milks is a defect, whereas in cheese making it is a deliberate step.
Fermentation does not stop instantly when yoghurt is cooled. L. delbrueckii subsp. bulgaricus, in particular, continues to produce lactic acid slowly at refrigeration temperature, so the pH continues to fall during storage. This post-acidification makes the product progressively sourer, can cause syneresis and harms probiotic organisms. It is controlled by cooling quickly and to a low temperature, by stopping fermentation at a slightly higher pH, by using mild cultures with low post-acidification (including strains of L. bulgaricus with impaired lactose metabolism, or cultures without L. bulgaricus), and by short supply chains. Measuring pH and titratable acidity across shelf life is part of every product's validation.
Slow or failed fermentation is caused by bacteriophage attack on the culture, antibiotic residues or sanitiser residues in milk, or wrong incubation temperature. Grainy, lumpy or gritty texture comes from undissolved powder, too high an inoculum, too much whey protein added, or excessive heat; stirred yoghurt can also become lumpy if broken at too high a pH. Excessive ropiness or sliminess may come from exopolysaccharide strains in unintended amounts or from contaminants. Thin, low-viscosity stirred yoghurt results from rough pumping, too much mechanical shear or too warm a filling temperature. Flavour defects include too much acid, too little acetaldehyde (flat), too much acetaldehyde (green), bitterness from proteolysis, and yeasty or fruity flavours with blown lids from yeast contamination. Mould colonies on the surface indicate post-process contamination. Each defect points to a step in the process that can be checked.
Fruit yoghurts and other flavoured fermented milks use fruit preparations: fruit pieces or purée cooked with sugar (or other sweeteners), stabiliser (commonly pectin, sometimes starch), acid to adjust pH, colour and flavour, then heat-treated and filled hot or aseptically into sterile containers. The preparation must be microbiologically stable, because it is added to a product with no further heat treatment; yeasts and moulds from fruit preparations are a classic cause of blown lids and spoilage. It must also be compatible with the yoghurt in pH, soluble solids and viscosity, so that fruit pieces stay suspended and do not bleed colour or release water into the yoghurt, and so that the water-activity difference between fruit and yoghurt does not cause syneresis. Preparations are added commonly at around 10–20 per cent of the final product, depending on style.
Fruit-on-the-bottom (sundae-style) yoghurt has the fruit preparation dosed into the cup first and set yoghurt fermented on top. Stirred (Swiss-style) yoghurt has the fruit mixed in after fermentation, using in-line mixers or dosing pumps before the filler. Fruit-on-top and side-pot formats keep fruit separate until consumption. Flavourings, colours and sweeteners are used according to national rules; natural, nature-identical and artificial flavour categories and permitted colours differ by country. Sugar reduction is a major trend: methods include using sweeter preparations at lower dose, lactase treatment of milk (glucose and galactose are sweeter than lactose), non-nutritive sweeteners where permitted, and consumer adaptation over time.
Calculations for fruit products use simple mass balances. If a yoghurt base with no sugar is mixed with a fruit preparation, the final sugar content equals the preparation's sugar content multiplied by its proportion in the product. Similar balances give the final fat, protein and fruit content, which must match labelling and any legal minimum fruit content.
Refrigerated fermented milks with live cultures typically keep for some weeks. Their low pH (about 4.0–4.6) and refrigeration inhibit most pathogens and spoilage bacteria, so shelf life is usually limited by: yeasts and moulds, which tolerate acid and grow slowly in the cold, producing gas, off-flavours and visible colonies; post-acidification, which makes the product too sour; syneresis; loss of probiotic viability where a count is claimed; and flavour changes such as loss of acetaldehyde or development of oxidised flavours. Pathogens are not usually the limiting factor in correctly made products, but some, such as acid-adapted Escherichia coli O157:H7 or Listeria introduced after fermentation, can survive for a time, so hygiene after the heat treatment remains essential.
Key measures are: excellent hygiene of the filling area and equipment, with hygienic or ultra-clean filling and filtered air; heat-treated and aseptically filled fruit preparations; good-quality, well-sealed packaging, sometimes with a modified atmosphere or a low-oxygen barrier; rapid cooling and an unbroken cold chain; and cultures chosen for low post-acidification. Some countries permit preservatives such as sorbates in certain fermented products, while others prohibit them in yoghurt; local rules must be checked. Post-fermentation heat treatment (thermisation after fermentation) kills the culture and gives long refrigerated or ambient shelf life; these products are common in some markets, especially ambient yoghurt in China, but under Codex and many national standards they must be labelled to show they have been heat-treated after fermentation and they cannot claim live cultures.
Shelf life is set by storage trials under realistic and abusive conditions, measuring pH, titratable acidity, viscosity, syneresis, yeast and mould counts, culture or probiotic counts and sensory quality at intervals. Challenge tests may be used for new formulations. The date chosen must hold under the temperatures the product will actually meet in distribution and in the consumer's home.
Fermented milks are packed in thermoformed or preformed polystyrene or polypropylene cups, glass jars, plastic bottles for drinks, pouches and paperboard cartons, sealed with foil or plastic lids. The package must withstand the acid product, keep out light and oxygen where flavour or probiotic survival requires it, and for set yoghurt survive the incubation room and cooling tunnel without distortion. Form-fill-seal machines that make the cup from a reel of plastic and fill it in a clean enclosure are common in large plants. Lids must seal reliably, because a leaking seal admits yeasts and moulds. Kefir and other gas-producing products need packs that tolerate some pressure or vent it. Labels show the product name as defined by the standard (for example whether it is yoghurt, a heat-treated fermented milk or a dessert), fat class, ingredients including fruit and additives, live-culture statements where justified, nutrition information, storage instructions and the date mark. Claims such as 'high protein', 'no added sugar' or 'contains probiotics' must meet the relevant national rules. Recyclability of polystyrene cups and multilayer materials is an increasing concern, and many producers are moving to mono-material polypropylene or paper-based packs.
A student uses this subject to explain why yoghurt milk is heated to 90 °C rather than 72 °C, why the two yoghurt organisms are used together, and how each step changes texture; to name the organisms of kefir, kumys and buttermilk; and to do fortification, acidity and strained-yield calculations. It is a regular source of AIEEA-PG group 16 questions and of practical work in the plant semester.
A production or quality officer five years in runs yoghurt and dahi lines: sets fortification and heat treatment, chooses when to break and cool, controls viscosity through the pumps and coolers, validates shelf life, and investigates complaints of whey separation, sourness, blown lids or slow fermentation. This subject gives the cause-and-effect map behind those decisions and the mass balances for strained and fruit products.
A plant head or category director chooses between straining, separator and membrane routes for high-protein products, weighing yield, acid whey disposal and capital; decides whether to launch probiotic, ambient or reduced-sugar ranges; and answers for label claims and recalls. This subject frames those decisions in terms of product quality, regulation and cost.
A professor or research director works on culture interactions, exopolysaccharides, acid gel structure, probiotic viability and evidence, acid whey valorisation and the microbiology of traditional products. They teach and set the AICE questions on fermentation chemistry and starters, and advise regulators on definitions and claims.
This manual does not yet give: the exact Codex compositional and count limits for each fermented milk (they are described but should be read in CXS 243-2003); current FSSAI definitions for dahi, misti dahi, shrikhand and lassi; detailed culture taxonomy, phage control and propagation (in starter-cultures-lab); detailed plant equipment design; detailed health evidence for specific probiotic strains; and legal limits on stabilisers, preservatives and sweeteners, which differ by country. Times, temperatures, total solids and doses are representative ranges, not recipes.
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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