Water, fat, protein, sugar (lactose), minerals, vitamins, and enzymes make up milk, which is a complex biological fluid with seven primary components. It’s a clear, white fluid with fat in the form of an emulsion, protein and some mineral particles in colloidal suspension, and lactose in real solution with minerals and soluble proteins. Milk’s opacity is caused by the presence of suspended fat, protein, and mineral particles. Depending on the fat’s carotene level, the colour ranges from white to yellow. Milk has a nice, slightly sweet flavour and smell. Calcium, phosphates, and riboflavin are all abundant in this food.
Physical Properties of Milk
Colour and optical properties
Due to light scattering by fat globules and casein micelles, milk appears turbid and opaque. The way light is scattered by molecules has an impact on optical characteristics. When the wavelength of light matches the magnitude of the particle, light scattering occurs. As a result, smaller particles scatter shorter wavelength light and vice versa. Because casein micelles scatter shorter wavelengths of visible light (blue) more than longer wavelengths (red), skim milk appears somewhat blue. The creamy colour of cow milk is due to beta-carotene, a carotenoid precursor of vitamin A. The presence of riboflavin in whey gives it a greenish tint. Milk’s refractive index is an optical property that ranges from 1.3440 to 1.3485 at 20 degrees Celsius. The contribution of the various constituents is additive, and the relationship between milk solids content and refractive index is linear.
Flavors of milk
Milk’s inherent sweetness is due to the interaction of its constituents. Off-flavors develop quickly in milk due to a variety of circumstances. The feed that animals eat may have certain unfavourable flavours. Fruity, barny, malty, or acid flavours result from bacterial development in milk. Unnatural flavours can also be caused by enzyme activities, with lipase action being a prime example. Milk may have a cardboard flavour due to oxidative processes. Cooked flavours may result from the processing of milk.
Specific gravity and density
Water is lighter than milk. Cow milk has a specific gravity of 1.018 to 1.036, while buffalo milk has a specific gravity of 1.018 to 1.038. Though specific gravity changes with temperature (lower at higher temperatures and higher at lower temperatures), the rate of change is not constant.
At 20°C, the density of milk fluctuates between 1.027 and 1.033 kg/cm3. The density of milk is used to compute other physical parameters such as dynamic viscosity and to estimate the solids content. It is affected by the temperature at the time of measurement, the sample’s temperature history, the sample’s composition (especially fat content), and the presence of air.
The amount and state of dispersion of the solid ingredients, mostly casein and fat, determine the viscosity of milk. Whole milk has a viscosity of roughly 2.0 cP at 25°C. Temperatures below 65°C increase viscosity due to denaturation of whey proteins, whereas temperatures above 65°C increase viscosity due to increased voluminosity of casein micelles. Casein micelle voluminosity increases or decreases in response to changes in milk pH. Agitation has a variable effect on viscosity. At times, agitation promotes partial coalescence of fat globules, raising viscosity, whereas at other times, agitation disperses fat globules that have undergone cold agglutination, lowering viscosity.
Milk’s surface activity is influenced by proteins, fat, phospholipids, and fresh fatty acids. The surface tension of milk is increased by homogenization and heat sterilisation. At 20°C, milk has a surface tension of 50 dyne/cm.
Freezing and boiling points of milk
Cow and buffalo milk have freezing values of -0.512 to -0.572oC and -0.521 to -0.575oC, respectively. The freezing point of milk is mostly used to estimate the amount of water supplied. Milk has a boiling point of 100.17oC.
Acidity and pH
Freshly drawn milk has a pH of 6.5 to 6.7 and 0.14 to 0.18 percent titratable acid, which is computed as lactic acid. Freshly drawn milk has no established acidity, and the slightly lower than neutral pH is due to the presence of carbon dioxide, citrate, casein, and other substances.
Heat stability of milk
The amount of time required to produce coagulation at a particular temperature, or the temperature required to induce coagulation at a given period, is referred to as heat stability. The milk system’s stability at the high processing temperatures to which milk is subjected for the creation of particular products is critical. Milk’s thermal stability is governed by its caseins and salt balance. Heat stability is greatly influenced by the addition of citrates, phosphates, and calcium.
Composition of Milk
Individuality of the animal, breed variation, seasonal changes, weather, age and health of the animal, managerial practises such as nature and quality of feed, stage of lactation, the quarter of the udder from which milk is drawn, and different fractions of milking are all factors that influence the chemical composition of milk.
Toned milk :Toned milk (also known as single toned milk) is milk that has been made by combining whole milk with water and skim milk powder. In order to make it, whole buffalo milk is blended with reconstituted spray dried skim milk. It should have a fat content of 3% and an SNF content of 8.5 percent.
Double toned : milk Similar to Toned milk, except that, according to PFA standards (1976), Double Toned milk in India must contain a minimum of 1.5 percent fat and 9.0 percent Solids – Not-Fat.
Standardized milk: This is milk that has had its fat and/or solids not-fat content modified to a predetermined level. Standardization can be accomplished by partially skimming the fat from the milk using a cream separator, or by admixing fresh or reconstituted skim milk in the appropriate amounts.
Vitaminized/Irradiated: milk Vitaminized milk is milk that has been supplemented with one or more vitamins. Irradiated milk is milk that has had its vitamin D concentration raised by being exposed to Ultraviolet radiation. (Mineralized milk is milk that has had minerals added to it.)
Humanized Milk: Humanized milk is entire cow or buffalo milk that has had its chemical composition altered to mimic human milk.