Milk: Properties, Composition, and Controversies

Posted on Aug 16, 2024 in Biology

Physical Properties

Cow’s milk has an average density of 1.032 g/ml. It is a complex heterogeneous mixture comprising a colloidal system of three phases:

• Solution: Minerals and carbohydrates are dissolved in water.
• Suspension: Protein substances are suspended in water.
• Emulsion: Fat is present as an emulsion in water.

Milk contains a major proportion of water (about 87%). The remaining 13% constitutes the “dry matter,” which contains 35g to 45g of fat. Other main components are lactose (carbohydrate), proteins, and lipids. Milk contains different groups of nutrients, including organic components (carbohydrates, lipids, proteins, vitamins) and mineral components (Ca, Na, Mg, Cl). The organic substances are present in roughly equal amounts and constitute the main source of energy. These nutrients are divided into building blocks (proteins), and energy-yielding compounds (carbohydrates and lipids).

Chemical Properties

The pH of milk is slightly acidic (between 6.6 and 6.8). Another important chemical property is the amount of lactic acid, which is usually 0.15-0.16% of the milk.

Proximal Chemical Analysis of Milk from Various Mammals

Average composition of milk in grams per liter:

Protein substances are crucial in milk chemistry. They are classified into two groups: proteins (casein, which comprises 80% of total protein, and whey proteins, which make up the remaining 20%), and enzymes.

Enzyme activity depends on temperature and pH. Phosphatase is an indicator of proper pasteurization. Reductase, produced by microorganisms outside milk, indicates contamination. Xanthine oxidase, in combination with potassium nitrate (KNO₃), inhibits the growth of bacteria in butterfat. Lipase, inhibited by pasteurization, oxidizes fats and can give products a rancid odor. Catalase levels increase with mastitis and can be used as a microbiological indicator.

Milk Composition

Immediately after birth, female mammals produce colostrum for the first two or three days. After this period, milk synthesis continues throughout the lactation period (180 to 300 days), with an average daily production ranging from 3 to 25 liters. Milk is synthesized primarily in the mammary gland, but many constituents come from blood serum. Its complex chemical composition reflects its importance in nourishing the young. The composition of milk depends on the needs of the species during the breeding period.

Lactose

Lactose, a disaccharide, is the primary carbohydrate in milk. Small amounts of glucose, galactose, sucrose, and amino cerebrosides derived from hexosamine are also present.

Synthesized in the mammary gland by an enzyme system involving α-lactalbumin, lactose is 15% less sweet than sucrose and contributes to the overall flavor of milk. Some populations (mainly Black and Latin American mestizos) are lactose intolerant due to a lack of the enzyme lactase, which breaks down lactose into glucose and galactose.

Undigested lactose ferments in the colon, producing hydrogen, carbon dioxide, and lactic acid, leading to diarrhea, flatulence, and abdominal cramps. To address this, some manufacturers add lactase to milk products to pre-hydrolyze lactose.

Lipids or Fats

Milk fat contributes significantly to its properties. Several lipid groups are present in milk:

• Triacylglycerols
• Diacylglycerols
• Monoacylglycerols
• Phospholipids
• Free fatty acids
• Sterols and their esters
• Carbohydrates

Triacylglycerols are found as small particles called corpuscles. Milk fat contains a complex composition of nearly 400 different fatty acids. However, 96% of the total fat comprises only 14 fatty acids, with myristic acid, palmitic acid, and oleic acid being the most abundant. The high fat content in cow’s milk is attributed to cattle feed and rumen activity. In seals, the high fat content is due to their fish-based diet and adaptation to cold environments. In human milk, fat content depends on the mother’s diet during pregnancy and lactation.

Casein

Casein is the most common protein in milk, with three main types: α_s1-casein, α_s2-casein, and β-casein. β-casein is particularly important in the dairy industry for cheesemaking. It is hydrolyzed by rennet, resulting in the precipitation of para-κ-casein, which reacts with calcium to form calcium paracaseinate. Cow’s milk is rich in β-casein, while human milk contains very little.

The Micellar Phase

Caseins interact to form a colloidal dispersion of spherical particles called micelles, with diameters ranging from 60 to 450 nm (average 130 nm). The exact structure of a micelle is still debated, but a proposed model suggests that it is composed of subunits with diameters of 10-20 nm. These subunits are linked by calcium ions, with calcium phosphate binding to lysine’s NH₂ groups and calcium interacting with ionized carboxyl groups (COO^–). The submicelles are formed by interactions between α-, β-, and κ-caseins. κ-casein plays a crucial role in stabilizing the micelle and preventing calcium precipitation of other protein fractions. The stability of the micelle is attributed to hydrophobic interactions between protein molecules.

Whey

Whey is the liquid remaining after milk coagulation. It contains components not integrated into the casein curd. Two types of whey are classified by taste:

• Sweet whey: Obtained from cheeses coagulated with rennet. It is rich in lactose (4.9%), non-protein nitrogen (22%), and protein (0.8%), but low in lactic acid (0.15%). Its pH ranges from 6 to 6.2.
• Acid whey: Derived from acid-coagulated cheeses. It has a lower lactose concentration (4.3%) due to its conversion to lactic acid (0.75%) during fermentation. It has a higher proportion of non-protein nitrogen (27%) but lower protein content (0.6%) due to denaturation. Its pH is 4.6.

Whey has a lower protein content than casein but higher nutritional quality. It is commonly used in various food products after dehydration. Whey proteins are compact, globular, with molecular weights ranging from 14,000 to 1,000,000 daltons. They are soluble over a wide pH range and are sensitive to high temperatures. Some important whey proteins include:

• α-lactalbumin: An enzyme essential for lactose synthesis. It is rich in sulfur and tryptophan and denatures at 63°C.
• β-lactoglobulin: Abundant in ruminant milk but absent in human milk. It is denatured and precipitates at temperatures below 73°C and is associated with allergic reactions in infants.
• Whey acidic protein (WAP): Found in rodents and lagomorphs, it has antimicrobial properties.
• Immunoglobulins: Account for 10% of total whey proteins and provide passive immunity to newborns. They are abundant in colostrum.

Microbiological Properties

Fresh milk is an ideal substrate for various bacteria, some beneficial and others harmful. Here are some examples:

Raw milk with a bacterial count exceeding 100,000 CFU/mL is considered lower quality. Brucellosis potential is also assessed.

Nutritional Properties

Milk is a complete food due to its diverse composition of fats, proteins, and carbohydrates. Whole cow’s milk is a good source of vitamins (A, D3) and minerals (calcium, phosphorus). Vitamin D aids in calcium absorption, making milk beneficial for bone health, especially in children. Colostrum, rich in proteins and antibodies, is essential for newborn immunization but has no industrial application.

Industrial Processes

Raw milk undergoes several industrial processes to ensure safety and quality:

• Filtration: Removes impurities like blood, hair, straw, and manure.
• Homogenization: Reduces fat globule size to prevent cream separation.
• Standardization: Adjusts fat content using powdered milk or vegetable fat.
• Deodorization: Removes unwanted odors.
• Bactofugation: Eliminates bacteria through centrifugation.
• Clarification: Separates solids and sediments from milk.

Heat Treatments

Heat treatments eliminate bacteria and improve safety:

• Thermization: Reduces or inhibits enzyme activity.
• Pasteurization (SHT): Eliminates specific pathogens, such as Streptococcus thermophilus.
• Ultra-high temperature (UHT): Eliminates most bacteria, eliminating the need for refrigeration.
• Sterilization: Eliminates all microorganisms using high temperatures (140°C for 45 minutes).

Presentation of Milk on the Market

Milk is available in various forms to meet consumer preferences:

• Whole milk: 3.2% fat content.
• Skimmed milk: Less than 0.3% fat content.
• Semi-skimmed milk: 1.5-1.8% fat content.
• Flavored milk: Sweetened milk with added flavors (strawberry, chocolate, etc.).
• Galalith: Hard plastic made from casein and formaldehyde.
• Powdered milk: Dehydrated milk with 95% water removed.
• Condensed milk: Partially dehydrated milk, thicker than regular milk.
• Enriched milk: Milk fortified with vitamins, calcium, or other nutrients.

Social Controversies of Milk Consumption

Milk production and consumption have sparked controversies, particularly regarding animal welfare and potential health concerns.

Animal Welfare

Concerns about animal welfare in the dairy industry stem from practices like artificial insemination, separation of calves from mothers, and the eventual slaughter of cows when their milk production declines. The use of artificial insemination, while ensuring desirable milk characteristics, raises ethical questions about animal autonomy. The separation of calves from their mothers, often shortly after birth, is seen as a stressful and potentially traumatic experience for both. Additionally, the practice of sending dairy cows to slaughter when their milk production is no longer profitable is criticized by animal welfare advocates.

Problems Related to Milk Consumption

Some argue that cow’s milk is harmful to humans, claiming that its protein and calcium are difficult for humans to digest. Vegetarians abstain from milk due to its animal origin. However, most experts consider milk beneficial for human health. Potential problems related to milk consumption include:

• Lactose intolerance: Caused by lactase deficiency.
• Allergies: Specifically, cow’s milk protein allergy (CMPA).
• Protein intolerance: Cow’s milk protein intolerance (CMPI).

Some studies suggest a link between milk consumption and an increased risk of Parkinson’s disease.

The Current Perspective

Despite controversies, humans are the only species that consumes milk throughout life. Milk is a good source of calcium and vitamin D, essential for bone health and preventing rickets. Milk consumption is also associated with a reduced risk of arthritis, osteoporosis, and other bone-related ailments. While some studies suggest a link between high calcium intake and osteoporosis, others emphasize the importance of calcium intake for overall health. The decision to consume milk should be based on individual dietary needs, ethical considerations, and health status.