Milk Fermentation Processes Across Mammalian Milks

Milk fermentation is an important traditional and industrial method for producing fermented dairy foods and beverages. It is driven mainly by microorganisms such as lactic acid bacteria (LAB), although yeasts and other microorganisms may also participate in some products.

During fermentation, microorganisms metabolize lactose and other available nutrients to produce lactic acid and, depending on the microorganisms involved, additional metabolites such as carbon dioxide, ethanol, acetate, acetaldehyde and diacetyl. The resulting changes in acidity, protein interactions, aroma and viscosity give fermented milk products their characteristic properties.

1. Basic Biochemical Mechanism of Milk Fermentation

Lactose is the principal carbohydrate in most mammalian milks. Many LAB possess enzymes that allow lactose to be hydrolyzed into glucose and galactose, which can then enter different metabolic pathways.

General lactose hydrolysis:
Lactose + H₂O → Glucose + Galactose

In homolactic fermentation, glucose is metabolized mainly to lactic acid through glycolysis:

Simplified homolactic reaction:
Glucose → 2 Lactic acid

Typical yogurt starters such as Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus produce lactic acid efficiently and work synergistically during yogurt fermentation.

In heterolactic fermentation, microorganisms can produce a mixture of lactic acid and other products, including carbon dioxide, ethanol and acetate. Some microorganisms also contribute compounds such as diacetyl that influence aroma and flavour.

Important: The exact fermentation products depend on the microorganism, substrate, temperature, oxygen availability and fermentation conditions. Therefore, fixed percentages of individual products should not be treated as universal values.

As lactic acid accumulates, milk pH decreases. When the pH approaches the isoelectric region of caseins, electrostatic repulsion between casein micelles decreases and protein aggregation occurs. This produces the characteristic gel or curd of many acid-fermented milks.

For conventional bovine-milk yogurt, the final pH is commonly around 4.0–4.6, although the exact value depends on the product and manufacturing process.

2. Major Fermented Milk Products and Their Processes

Product	Major Microorganisms	Typical Fermentation Conditions	Important Characteristics	Typical Final pH
Yogurt	Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus	Usually about 40–45 °C	Symbiotic starter growth, lactic acid production, protein gel formation and characteristic acetaldehyde aroma.	Approximately 4.0–4.6
Kefir	A mixed community of LAB, yeasts and acetic-acid bacteria associated with kefir grains.	Usually conducted at relatively mild temperatures	Acidic, mildly effervescent fermented beverage with a complex microbial community and characteristic flavour.	Usually acidic, commonly around 4–4.6
Koumiss	Mainly lactic-acid bacteria and yeasts	Traditional processes vary	Traditionally produced from mare's milk; fermentation produces lactic acid and may also produce carbon dioxide and ethanol.	Usually acidic
Viili	Lactococcus lactis and other lactic-acid bacteria; some traditional cultures also involve surface-associated microorganisms such as Geotrichum.	Relatively cool fermentation	Characteristically viscous and ropy texture with a distinctive fermented flavour.	Approximately 4–5
Dahi / Indian curd	Mixed lactic-acid bacteria from starter cultures or traditional inocula	Commonly around 30–40 °C	Acid coagulation produces a characteristic set curd. Buffalo milk often gives a particularly firm product because of its relatively high total solids.	Approximately 4.2–4.8
Cheese	Starter LAB, with additional microorganisms depending on cheese variety	Highly variable	Acidification is combined with enzymatic coagulation, curd cutting, drainage, salting and, in many varieties, ripening.	Varies widely

3. Influence of Milk Source on Fermentation

The composition of milk strongly affects fermentation because protein concentration, casein composition, casein-micelle structure, fat content, mineral balance, buffering capacity and antimicrobial components influence microbial growth and gel formation.

Buffalo milk: Its relatively high fat, protein and total-solids contents generally support the formation of a firm yogurt gel and can increase cheese yield.
Goat milk: Differences in casein composition and fat-globule characteristics can produce a softer and more delicate curd than conventional bovine milk. The exact fermentation behaviour depends on breed, composition and starter culture.
Camel milk: Camel milk can be successfully fermented, but it commonly produces a weaker and thinner acid gel than bovine milk. Differences in casein composition, casein-micelle properties, lower κ-casein content and the presence of antimicrobial proteins can contribute to its distinctive fermentation behaviour. :contentReference[oaicite:2]{index=2}
Donkey and mare milk: Their relatively low casein content and different protein composition can make the formation of a firm acid gel more difficult than in conventional bovine or buffalo milk. Nevertheless, they can be used to produce fermented beverages and traditional products.
Sheep milk: Its relatively high protein, fat and total-solids contents among many commonly used dairy milks generally favour dense fermented products and good cheese-making characteristics.
Human milk: Human milk has a distinctive carbohydrate composition that includes a large amount of lactose and numerous human milk oligosaccharides (HMOs). HMOs are important bioactive carbohydrates and can serve as substrates for particular members of the infant gut microbiota. Therefore, it is incorrect to state that human milk is not fermented because HMOs simply inhibit LAB. :contentReference[oaicite:3]{index=3}

Key concept: Milk composition does not merely determine the amount of acid produced. It also influences acidification rate, protein aggregation, gel strength, water-holding capacity, viscosity and the sensory properties of the final fermented product.

4. Modern Industrial Yogurt Process

A simplified industrial yogurt process illustrates how milk chemistry and microbial fermentation are combined to obtain the desired texture and acidity.

Milk standardization: Fat and protein levels are adjusted according to the desired product.
Homogenization: Milk is homogenized to reduce fat-globule size and improve the physical stability and texture of the product.
Heat treatment: Yogurt milk is commonly heated at a relatively high temperature to reduce microbial load and promote whey-protein denaturation. Industrial conditions vary; a commonly used treatment is around 85–95 °C for several minutes.
Cooling and inoculation: The milk is cooled to a suitable temperature and inoculated with the selected yogurt starter culture.
Incubation: The inoculated milk is held under controlled conditions until sufficient acidification and gel formation occur.
End-point control: For many set-yogurt processes, fermentation is stopped when the pH reaches approximately 4.5–4.6, although the target depends on the product.
Cooling: Rapid cooling slows microbial activity and helps stabilize the texture and flavour of the finished yogurt.

Process note: Industrial yogurt manufacture is not based on a single universal temperature or time combination. Heat treatment, starter composition, incubation temperature, fermentation endpoint and cooling conditions vary according to the type of yogurt and manufacturing system.

5. Factors Affecting Milk Fermentation

Milk composition: Protein, fat, lactose, minerals and total solids influence fermentation.
Starter culture: Different microorganisms produce different acids, aromas and textures.
Temperature: Microbial growth and metabolic activity depend strongly on temperature.
pH: Acidification controls casein aggregation and microbial succession.
Heat treatment: Heating changes whey-protein interactions and can improve yogurt gel structure.
Casein composition: The proportions and properties of individual caseins influence coagulation.
Buffering capacity: Milk components determine how rapidly the pH changes during acid production.
Antimicrobial components: Naturally occurring protective proteins can influence starter growth in some milks, particularly camel milk.

6. Conclusion

Milk fermentation is a complex interaction between microbial metabolism and milk chemistry. Although the basic principle—microbial conversion of milk carbohydrates into acids and other metabolites—is shared by many fermented milks, the final product depends strongly on the composition of the starting milk and the microorganisms used.

Buffalo, goat, sheep, camel, mare, donkey and bovine milks can all exhibit different fermentation behaviours because their proteins, casein fractions, minerals, fat globules and antimicrobial components differ. Camel milk is a particularly good example: it can be fermented successfully but generally produces a weaker and more fragile gel than bovine milk, creating distinctive technological challenges. :contentReference[oaicite:4]{index=4}

Thus, understanding milk fermentation requires the combined study of microbiology, biochemistry, protein chemistry, food processing and rheology. This scientific approach explains why the same fermentation principle can produce very different textures, flavours and nutritional characteristics across mammalian milks.

Milk Fermentation Processes Across Mammalian Milks