Understanding Milk: Composition, Properties, and Quality

Milk, a staple beverage in many diets, appears white or yellowish due to the interaction of light with its components. The color of milk results from the scattering and absorption of light by milk fat globules and protein micelles. This is why skim milk, which has minimal fat, still retains its white hue. The yellowish tint in milk can be attributed to carotene, absorbed from the diet during grazing, present in the fat phase, and riboflavin in the aqueous phase.

Milk's flavor profile is mildly sweet with a faint odor and taste. Milk fat is present as droplets or globules, surrounded by a membrane and emulsified in milk serum, also known as whey. These fat globules tend to separate and float to the top as cream after prolonged storage or centrifugation. However, homogenization breaks these fat globules into much smaller sizes, preventing cream separation even after long periods of standing.

The proteins in milk, known as micelles, are dispersed within the milk serum. These micelles are primarily calcium salts of casein molecules. Additionally, milk contains lipoprotein particles, or milk microsomes, which include remnants of cell membranes and microvilli, as well as somatic cells, mainly leucocytes. Various proteins, carbohydrates, minerals, and other ingredients are dissolved in the milk serum.

The specific density of milk, which ranges from 1.029 to 1.039 at 15°C, varies with its composition. Higher fat content decreases the density, while higher amounts of protein, milk sugar, and salts increase it. Skim milk, having most of the fat removed, possesses a higher specific density compared to whole milk.

One notable property of milk is its freezing point, which ranges from -0.53 to -0.55°C. This relatively constant value is useful for detecting adulteration, such as the addition of water to milk.

In summary, milk's unique characteristics, from its color and taste to its density and freezing point, result from its complex composition of fats, proteins, carbohydrates, and minerals. Understanding these properties not only highlights the nutritional importance of milk but also ensures its quality and authenticity in the food industry.
Understanding Milk: Composition, Properties, and Quality

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Common Microorganisms Found in Milk and Dairy Products

Pathogenic microorganisms have the ability to spread to humans through the consumption of milk and various dairy items. A range of illnesses, including tuberculosis, brucellosis, diphtheria, scarlet fever, Q-fever, and gastroenteritis, can be contracted via these milk-related products.

The deterioration of dairy goods presents a significant global economic challenge. The volume of microorganisms present and the frequency of bacterial pathogens within food items function as indicators of food quality. Many of the risks linked to microorganisms in dairy products, like butter, cheese, and yogurt, stem from raw milk.

Domesticated animals serve as a primary reservoir of pathogens that can be transmitted to milk. Staphylococcus aureus, Salmonella spp., Listeria monocytogenes, Escherichia coli O157:H7, and Campylobacter are the most frequently encountered potential pathogens linked to milk and dairy products in industrialized nations.

Raw milk provides an environment conducive to the growth of bacteria that can be managed or eradicated through the pasteurization procedure.

Undesirable alterations in butter can arise due to the proliferation of Flavobacterium spp., Pseudomonas putrefaciens, and Shewanella putrefaciens. Pseudomonas fragi and occasionally Pseudomonas fluorescens growth can result in fruity odors, rancidity, and proteolytic activity.

In the manufacturing of hard cheeses, specific bacteria, such as L. monocytogenes, S. aureus, and Salmonella, can remain viable without subsequent proliferation.

The potential danger posed by pathogenic bacteria has been significantly mitigated, and the occurrence of outbreaks associated with milk and dairy products has consistently decreased. This is predominantly attributable to contemporary milk production practices that underscore sanitary measures, enhanced udder health, herd evaluations, appropriate cooling, meticulous handling and storage of raw milk, and the near-universal adoption of pasteurization.
Common Microorganisms Found in Milk and Dairy Products

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HTST pasteurization system

Milk and dairy products are an important source of nutrients. They are considered as the main food products in the human diet.

Milk should be free from toxins and pathogens and also it must retain all nutrients and vitamins with extended period of storage life. To achieve the above said criteria, milk is heated to a particular temperature for a brief time then cooling it down once more quickly. Pasteurization is the process of heating the product to a predetermined temperature and holding it until all or nearly all objectionable microorganisms, which may be present, are killed.

HTST (High temperature short time) pasteurization is an effective method of making milk safe for consumption, without unduly changing either its sensory characteristics or its nutritional value. As a continuous process, it makes use of energy regeneration and it is capable of both scales of operation and energy efficiencies that cannot be matched by alternative processes.

This method involves heating milk rapidly to 72◦C, keeping it for a few seconds (usually 15 s), and cooling it down immediately. The time and temperature conditions depend on several factors, such as size, shape, and type of food. The HTST method results in a higher retention of quality characteristics.

For HTST pasteurization, the following controls are very important:
• Flow rate
• Temperature
• Pressure
HTST pasteurization system

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Evaporated Milk

According to Codex Standard, evaporated milks are milk products which can be obtained by the partial removal of water from milk by heat, or by any other process which leads to a product of the same composition and characteristics.

Evaporated milk is the name for sterilized unsweetened condensed milk and it is fresh cow's milk from which a considerable portion of the water has been removed. Evaporated milk has a creamy consistence and it differs from ordinary milk in which containing slightly more than twice the number of solids.

Nicolas Appert was the first person to preserve milk in concentrated form in the early nineteenth century. First, he concentrated the milk by boiling it in a water bath over a fire, then poured it into glass bottles after cooling. Final process was sterilized the product by heating the bottles for 2 h in a boiling water bath.

The evaporation process involves evaporation, concentration, homogenization, and sterilization of whole milk. The process also concentrates the nutrients and the energy content of the evaporated milk.

The nutrient composition of evaporated low-fat milk is approximately 7.5–9.0% fat and 18–22% non-fat milk solids. During sterilization, the whole milk is exposed to an ultra-high temperature (UHT) process and then packaged aseptically.
Evaporated milk

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Reconstituted milk

“Reconstituted milk or milk product” means a product resulting from the addition of potable water to the dried or concentrated form of milk or milk products in the amount necessary to re-establish the appropriate water-to-solids ratio to achieve similar end product characteristics and appropriate milk product composition as per the standards for that product.

The milk is prepared from butterfat, dried skim milk and water by means of emulsifying apparatus. Reconstituted milks become necessary when there are seasonal shortages of fresh milk or when the local demand depends entirely on imported milk powder.

Reconstituted milk products are used as concentrated sources of milk components for standardization of milk supplies to improve the consistency and efficiency of manufacturing operations.
Reconstituted milk

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Fortification of dairy products

Milk in its natural form is almost unique as a balanced source of man's dietary need. The various steps in processing and storage have a measurable impact on some specific nutrients.

Fortification is defined as the practice of deliberately increasing the content of an essential micronutrients such as essential vitamins and minerals to food. The fortification of food products has been practiced for more than 80 years.

The micronutrients which are commonly used in the fortification of the food products such as milk and its products are:
• Vitamins and co-vitamins
• Essential minerals
• Essential fatty acids
• Essential amino acids
• Phytonutrients
• Enzymes

Vitamins are compounds which play a role as cofactors in the body. Fermented milk products such as yogurt can be thought as vitamin sources. Vitamin concentrates with vitamins A and D are used for fortification of fluid milk.

Vitamin fortification has a long history in fluid milk in the United States to reduce rickets in children, and the FDA mandated in the 1990s that fortified fluid milks must be within 100% to 150% of label claims to address documented variability in vitamin amounts.

Iron-fortified yogurt has a relatively high iron bioavailability. Iron fortification of milk and dairy product is considered as a potential approach to prevent the iron deficiency disorder , since dairy foods are an important part of the daily diet in most parts of the world.

Iron fortification of milk or dairy products induces several bio physicochemical modifications with important consequences. Iron fortification of food is regarded as the most cost-effective method for reducing the prevalence of nutritional iron deficiency. Iron deficiency induces anemia, alters mental development, decreases immunity, impairs cognitive scores in children and leads to poor pregnancy outcome.
Fortification of dairy products

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What is skimmed milk?

Milk is highly nutritious food that provides nearly all the substances essential for good health in people of all ages. About 4% of whole milk is fat. The fat that separates from milk is cream. Milk which has had the fat or cream removed is called skimmed milk.

Skimmed milk, which may contain 0.5% fat or less, has been a popular beverage for many years. Skimmed milk is as nutritious as whole milk, but contains much less fat and cholesterol than whole milk.

The fat-soluble vitamins – A, D, E and K - are removed with the fat when milk is skimmed. Consequently, they are present in only trace amounts in skimmed milk and in reduced amounts in semi-skimmed milk. Usually this milk is fortified with vitamins A and D.
What is skimmed milk?

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Milk in United States

Milk is one of the few foodstuffs consumed without further processing, and it is generally regarded as the most perfect food.

Milk is the whole, fresh, clean, normal lacteal secretion of the mammary glands for the feeding of the young of mammals. In the United States, the primary milk in commerce is cow’s milk.

Dairy products account for around 27% of the value of production and 4% of agricultural export earnings. Technological change, economies of scale and increased productivity have led to a large concentration of production: 5% of all dairy farms (those with more than 500 cows) supply 60% of all milk produced in the United States..

The responsibility for insuring the quality of this food product rests primarily with the individual states rather than the federal government.

The following definitions from its Grade A Pasteurized Milk Ordinance (1965) has been adopted by a majority of the states:

Milk is hereby defined to be lacteal secretion, practically from colostrum, obtained by the complete milking of one or more healthy cow, which contains not less than 8.25 milk solids non-fat and not less than 3.25% milk fat.

Dairy processors are increasingly using milk as a raw material from which individual component can be extracted. Processors are refining raw milk and isolating its individual components in a manner analogous to the refinement of crude petroleum into its various individual component products.
Milk in United States  

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Spray drying of milk

Dry milk was referred to as milk powder until the mid 1960s, when the designation was changed by the American Dry Milk Institute to dry milk in the United States.

Drying be done by spraying atomized droplets of milk into chamber through which heated air is circulated (spray drying).

Milk to be spray dried should receive at least a minimum temperature pasteurization before evaporation and further processing, though higher temperature heat treatments are commonly used in order to influence product characteristics.

In the more modern and more widely used process, concentrated milk is sprayed through an atomizer into the top of a large chamber with air at 120 – 205 °C blown in from the base. The particles dry rapidly and are collected at the bottom of the chamber.

Dried milk (usually the spray dried type that contains about 5% of moisture) may be re-humidified to slightly higher moisture content after drying.

This treatment agglomerates the fine milk particles  to form clumps of milk powder, which results in a powder that dissolves or disperses in water much more rapidly that the finely powdered dried milk. It is therefore, considered to be an instantly double product.

Original spray dryer designs were for single stage drying in which all of the moisture is removed in the main drying chamber. Such designs are now obsolescent and have been replaced by two and three stage dryers in which the drying process is completed in fluidized bed dryers.
Spray drying of milk

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Milk and dairy foods for good health

Milk is the product obtained by simple or multiple milking of mammals that are kept for the purpose of milk production.

While milk products are made entirely from milk. In certain cases, additives can be used during the manufacture. Milk and many of the products made from it are staples in the diet of people throughout the world.

Although fluid whole cow; milk is a liquid food (87% water), it contains an average of 13% total solids and 9% solids-non-fat, an amount comparable to the solids content of many other foods.

More than 100 different components have been identified in cow’s milk. Important nutritional contributions of milk and milk products are calcium, protein, potassium, vitamin A, vitamin B12, riboflavin, niacin and phosphorus.  Milk can also make a contribution to dietary vitamin D intake, especially when fortified.

An ever increasing variety of milk and other dairy products is available to meet the taste, nutrition, health and convenience demands of consumers. Milk products include cream, butter cheese, casein, yoghurt, clabber, gelato, ice cream and others.
Dairy products of varied fat content, of low or reduced lactose content, fortified with nutrients such as vitamin A, D and calcium, and processed to improve keeping quality are available.

The fat content in different milk products varies greatly, raging from a few percent in yoghurt to 80% in butter. It is suggested to eat moderate amounts of these foods and choose lower fat alternatives.

Dairy fats contain a range of lipids that may have health-promoting properties including omega-3 LS-PUFA, gangliosides, sphingolipids, etc.
Milk and dairy foods for good health

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Chemical properties of milk

The processability and functionality of milk and milk products are determined by the properties and concentrations of its principal constituents: protein, lipids, lactose and salts.

Milk fat occurs in the form of droplets or globules, surrounded by a membrane and emulsified in milk serum (also called whey). Milk fat is composed of approximately 400 fatty acids.

The fat globules (called cream) separate after prolonged storage or after centrifugation. The fat globules float on the skim milk.

Homogenization of milk so finely divides and emulsifies the fat globules that cream separation does not occur even after prolonged standing. The key attributes of milk flavor are influenced by its emulsion of fat globules in a colloidal protein solution, accompanied by a slight sweet and salty taste from lactose and minerals, primarily calcium.

The majority of milk proteins are synthesized from amino acids. Proteins of various sizes are dispersed in milk serum. They are called micelles and consist mostly of calcium slats of casein molecules.

Furthermore, milk contains lipoprotein particles, also called milk microsomes, which consist of the residues of cell membranes, microvilli, etc., as well as somatic cells, which are mainly leucocytes.

Various proteins, carbohydrate, minerals and other ingredients are solubilized in milk serum.

Milk contains many acidic and basic groups that result in buffering action over a wide pH range. The principal buffer components in milk are soluble phosphate, colloidal calcium phosphate, citrate, bicarbonate and casein.
Chemical properties of milk

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Metabolic food disorder

Several types of food intolerances occur with the two most important types being metabolic food disorders and idiosyncratic reactions.

Metabolic food disorder is an adverse reaction to a food or food component that occurs through some effect of the substance on the metabolism (the processes by which a substance is handled in the body) of the individual.

The individual with metabolic food disorder display increased sensitivity to certain chemicals in foods because they lack an enzyme necessary to metabolize that particular chemical or because they have a genetic abnormality that makes them especially susceptible to the toxic effects of a particular foodborne chemical. 

Lactose intolerance is the best example of the first type of metabolic food disorder, Lactose is a disaccharide found in milk.

Lactose-intolerant individuals have low level of the enzyme, beta-galactosidase, in their small intestine. It is the result of inability to digest lactose.

Since the lactose cannot be digested in the small intestine, it is fermented in the large intestine resulting in abdominal cramps, flatulence and diarrhea.
Metabolic food disorder

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Fat in milk

The fat content and its type are of considerable commercial and nutritional importance to the dairy industry. 

Biologically, due to the high percentage of carbon in fats, they are ‘stored’ nutrients with the highest energy or calorific value of all food constituents.

Milk fat occurs in the form of droplets or globules, surrounded by a membrane and emulsified in milk serum (also called whey).

The fat globule membrane acts as the emulsifying agent for the fat suspended in milk. The membrane protects the fat from enzymes and prevents the globules coalescing into butter grains.

The fat globules (called cream) separate after prolonged storage or after centrifugation. The fat globules float on the skim milk.

Homogenization of milk so finely divides and emulsifies the fat globules that cream separation does not occur even after prolonged standing.

About 98% of milk fat is a mixture of triacyl glycerides. The partial glycerides (diglycerides and monoglycerides) and free fatty acids are probably partly left over from the biosynthesis process.
Fat in milk

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Physical and Properties of Milk

Physical and Properties of Milk
Milk is a white or yellow, opaque liquid. The color is influenced by scattering and absorption of light by milk fat globules and protein micelles.

Therefore, skim milk also retains its white color.

A yellowish, i.e., yellow-green, color is derived from carotene (ingested primarily during pasture grazing) present in the fat phase and from riboflavin present in the aqueous phase.

Milk tastes mildly sweet, while its odor and flavor are normally quite faint.

Milk fat occurs in the form of droplets or globules, surrounded by a membrane and emulsified in milk serum (also called whey).

The fat globules (called cream) separate after prolonged storage or after centrifugation. The fat globules float on the skim milk.

Homogenization of milk so finely divides and emulsifies the fat globules that cream separation does not occur even after prolonged standing.

Proteins of various sizes are dispersed in milk serum. They are called micelles and consist mostly of calcium salts of casein molecules.

Furthermore, milk contains lipoprotein particles, also called milk microsomes, which consist of the residues of cell membranes, microvilli, etc., as well as somatic cells, which are mainly leucocytes.

Various proteins, carbohydrate, minerals and other ingredients are solubilized in milk serum.

The specific density of milk decreases with increasing fat content, and increases with increasing amounts of protein, milk sugar and salts.

The specific density of cow’s milk ranges from 1.029 to 1.039 (15 degree C). Defatted (skim) milk has a higher specific density than whole milk.

The freezing point of milk is -0.53 to -0.55 degree C. This rather constant value is a suitable test for detection of watering of milk.
Physical and Properties of Milk

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