Fermented Milk Production

Fermented milk products are defined as products prepared from milk—whole, skimmed, partially skimmed, reconstituted, or recombined—that have undergone fermentation by the action of suitable microorganisms, resulting in a reduction of pH with or without coagulation. Fermentation extends the shelf life of highly perishable fluid milk, enhances digestibility, and improves nutritional and therapeutic value.

• Mesophilic Fermented Milks: Produced using mesophilic starter cultures that thrive best at temperatures between 20°C and 30°C. Examples include cultured buttermilk, sour cream, and traditional kefir.
• Thermophilic Fermented Milks: Produced using thermophilic cultures operating optimally at 40°C to 45°C. Yogurt, dahi, and acidophilus milk belong to this category.
• Yeast-Lactic Fermented Milks: Involve both lactic acid bacteria and lactose-fermenting or non-fermenting yeasts, yielding ethanol and carbon dioxide alongside lactic acid (e.g., kefir, kumis).

Starter cultures are preparations containing live microorganisms used intentionally to initiate and accelerate milk fermentation. They dictate the flavor, aroma, texture, and body of the final product.

• Lactococcus lactis subsp. lactis and Lactococcus lactis subsp. cremoris: Primary acid producers in mesophilic products.
• Leuconostoc mesenteroides subsp. cremoris: Key aroma and carbon dioxide producer, responsible for the characteristic diacetyl flavor in cultured butter and buttermilk.
• Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus: The symbiotic pair used in yogurt manufacturing.
• Symbiosis: Streptococcus thermophilus grows rapidly, producing formic acid and pyruvic acid which stimulate Lactobacillus delbrueckii subsp. bulgaricus; in turn, the latter releases free amino acids (valine) via proteolysis, stimulating the streptococci.

The core biochemical mechanism in fermented milk production is the conversion of lactose (milk sugar) into lactic acid by lactic acid bacteria (LAB).

• Homofermentative Lactic Acid Fermentation: Organisms such as Lactobacillus acidophilus and Streptococcus thermophilus possess aldolase enzymes and metabolize glucose exclusively into lactic acid via the Embden-Meyerhof-Parnas (EMP) pathway, yielding approximately 1.5 to 1.8 moles of lactic acid per mole of glucose consumed.
• Heterofermentative Lactic Acid Fermentation: Organisms such as Leuconostoc species lack fructose-1,6-bisphosphate aldolase and utilize the phosphoketolase pathway, producing lactic acid, ethanol (or acetic acid), and carbon dioxide.
• Proteolysis and Lipolysis: Microbial proteinases break down casein into peptides and amino acids, enhancing digestibility. Lipases release free fatty acids that contribute to complex flavor profiles.

Yogurt manufacturing is an industrial standard process requiring rigorous time-temperature combinations and hygiene control.

• Standardization: Adjusting the fat and solids-not-fat (SNF) content, typically targeting 12–14% total solids by adding skim milk powder or through vacuum evaporation.
• Homogenization: Performed at 60–65°C under 15–20 MPa pressure to prevent cream separation and enhance the viscosity and mouthfeel of the curd.
• Heat Treatment (Pasteurization/Sterilization): Heating milk to 85°C for 30 minutes or 95°C for 5 minutes. This inactivates native milk enzymes, destroys competitive microflora, and denatures whey proteins (mainly eta-lactoglobulin) to interact with \kappa-casein, improving water-holding capacity.
• Inoculation and Incubation: Cooling the milk to 42–45°C, adding 2–3% starter culture. For set yogurt, incubation occurs directly in retail containers; for stirred yogurt, incubation occurs in large vats followed by agitation, cooling, and packaging.

Quality assurance ensures consumer safety and product uniformity, while awareness of defects guides corrective troubleshooting.

• Syneresis (Wheying-Off): Separation of whey from the curd, caused by excessive acidity, insufficient SNF, rough handling of warm coagulum, or high incubation temperatures.
• Flavor Defects: Flat flavor (insufficient diacetyl), yeasty or alcoholic flavor (yeast contamination), and bitterness (excessive proteolysis by rogue psychrotrophs).
• Therapeutic Value (Probiotics): Consumption of viable probiotic strains like Lactobacillus acidophilus and Bifidobacterium bifidum improves gut microbiota, enhances immune function, lowers serum cholesterol, and aids lactose intolerance management.