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- Water content and its role: Milk averages 87 percent water by weight, serving as the continuous phase that dissolves lactose, minerals, and water-soluble vitamins. Water activity influences microbial growth and must be controlled during concentration and drying.
- Fat globules and lipid profile: Milk fat exists as emulsified globules ranging from 0.1 to 20 micrometers in diameter, surrounded by a natural membrane comprised of phospholipids and proteins. The lipid fraction contains approximately 65 percent saturated, 30 percent monounsaturated, and 5 percent polyunsaturated fatty acids.
- Protein fractions and casein micelles: Two major protein groups define milk functionality: caseins (about 80 percent of total protein) form colloidal micelles stabilized by calcium phosphate, while whey proteins (beta-lactoglobulin, alpha-lactalbumin) remain soluble. Caseins precipitate at pH 4.6, the basis for cheese and yogurt manufacture.
- Lactose as the primary carbohydrate: This disaccharide (glucose plus galactose) accounts for about 4.8 percent of bovine milk and provides substrate for lactic acid bacteria in fermented products. Lactose intolerance in consumers affects product formulation, driving demand for lactose-free alternatives.
- Mineral and vitamin distribution: Calcium, phosphorus, magnesium, potassium, and sodium are present in ionic and colloidal forms. Milk supplies B vitamins and fat-soluble vitamins A, D, E, and K. Seasonal and feeding variations affect these micronutrient levels.
- Enzymes naturally present: Indigenous enzymes such as lipase, protease, and alkaline phosphatase serve as indicators of thermal treatment adequacy. Phosphatase testing validates proper pasteurization because the enzyme is inactivated at 63 degrees C for 30 minutes.
- Seasonal and breed variations: Holstein cows produce higher volumes with lower solids content (about 3.5 percent fat, 3.2 percent protein), while Jersey and Guernsey breeds yield richer milk with 5 percent fat or more. Summer grazing alters fatty acid profiles compared to winter feeding.
- Impact of processing on composition: Standardization adjusts fat-to-protein ratios for specific products. Evaporation and drying remove water to create concentrated and powdered forms, while membrane filtration can enrich protein or reduce lactose.
- Microbiology of raw milk: Raw milk microbiota typically includes lactic acid bacteria, psychrotrophic Pseudomonas, and coliforms from environmental sources. Psychrotrophs proliferate during refrigerated storage, producing heat-stable lipases and proteases that survive pasteurization and cause spoilage. Rigorous sanitation and rapid cooling to 4°C immediately after milking are essential to inhibit bacterial growth and preserve shelf life.
- Physical properties influencing processing: Key physical properties include a pH of approximately 6.6-6.7, density of 1.030-1.035 g/mL, and a freezing point near; 0.522°C, which is legally monitored to detect water adulteration. Heat capacity and thermal conductivity values are essential for designing pasteurization and sterilization equipment.
- Flavor, color, and appearance: Milk's white appearance arises from the scattering of visible light by colloidal casein micelles and fat globules. Carotenoids from feed impart a creamy yellow hue, more pronounced in Jersey milk. The characteristic flavor combines sweetness from lactose, saltiness from minerals, and a clean mouthfeel from the emulsified fat system.
Equipment Specification
Equipment: Infrared Milk Analyzer (Mid-Infrared Spectroscopy)
- Measurement Principle
- Absorption at specific wavelengths correlates with fat, protein, lactose, and total solids
- Accuracy
- +/- 0.06 percent for fat, +/- 0.05 percent for protein
- Standard
- ISO 9622 / IDF 141 for raw milk analysis
Quick Revision
- Milk is 87 percent water with fat globules emulsified in a protein-stabilized system.
- Casein micelles and whey proteins determine functional properties for processing.
- Lactose provides substrate for fermentation but causes intolerance in some populations.
- Enzymes like alkaline phosphatase verify pasteurization effectiveness.
- Breed and season influence fat and protein levels, requiring standardization.