Casein

Casein is the primary component of milk proteins. It constitutes a major portion of the total protein content in milk, particularly from ruminant animals. Approximately 76–86% of milk protein consists of casein. This property makes it a fundamental protein source in the dairy industry, especially in cheese production. In milk, casein exists as large colloidal structures called micelles, along with calcium and phosphate ions. These micelles determine the biochemical and functional properties of casein. While casein accounts for about 80% of milk proteins, the remainder is composed of whey proteins.

Casein exhibits amphoteric properties due to the presence of both acidic and basic groups in its chemical structure. When the pH of milk decreases, casein precipitates and forms curds. This characteristic renders casein indispensable in cheese production. During cheese manufacturing, casein separates from whey proteins and forms the curd, which constitutes the primary protein structure of cheese.

Chemical Structure of Casein (Ersöz, 2024)

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Structural Properties and Casein Micelles

In milk, casein exists as a colloidal dispersion known as casein micelles. The diameter of these micelles typically ranges from 20 to 600 nanometers. The structure of micelles determines the functional properties and digestibility of milk proteins. Approximately 93% of casein micelles consist of protein, while the remaining 7% comprises inorganic substances including calcium, phosphate, magnesium, sodium, and citrate. Calcium and phosphate are present in the highest concentrations and form complexes with casein as colloidal calcium phosphate. These complexes are referred to as calcium caseinate-phosphate.

Casein micelles are composed of smaller submicelles, usually 10–15 nanometers in diameter, linked together by calcium bridges. The core of the micelle structure is formed primarily by αS1-casein and β-casein, while κ-casein predominates on the surface. This structural arrangement influences casein’s solubility, coagulation properties, and digestion behavior.

Casein Micelles and Submicelle Groups (Demirel, 2019) 

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Casein Fractions

Casein is divided into four main fractions with distinct characteristics: αS1-casein (αS1-casein), αS2-casein (αS2-casein), β-casein (β-casein), and κ-casein (κ-casein). These fractions differ in quantity and biological properties.

αS1-Casein

It is the most abundant among milk proteins, accounting for approximately 40% of total casein. αS1-casein consists of 199 amino acids and has a hydrophobic structure rich in proline. It is encoded by the CSN1S1 gene and has nine variants designated A through I. The B and C variants are the most common. Genetic variations influence milk yield, protein content, and cheese-making properties.

αS2-Casein

It constitutes approximately 14% of total casein and contains 207 amino acids. Variants A, B, C, and D are classified based on the number of bound phosphate groups. These variants are breed-specific. For example, variant A is common in European cattle, variant B in Taurus and Indicus cattle, variant C in yak cattle, and variant D is more frequent in certain French dairy breeds.

β-Casein

It is the second most abundant casein fraction after αS1-casein and plays a crucial role in milk coagulation. β-casein accounts for approximately 20% of total casein. Composed of 209 amino acids, β-casein has 13 variants: A1, A2, A3, A4, B, C, D, E, F, H1, H2, I, and G. The A1 and A2 variants are the most prevalent. A1 β-casein produces a biologically active peptide called beta-casomorphin-7 (BCM-7) during digestion. It has been suggested that BCM-7 may affect the nervous, immune, and endocrine systems and could be associated with certain chronic diseases. A2 β-casein produces BCM-7 at very low levels. Human milk, goat milk, and sheep milk generally contain proteins similar to A2 β-casein.

κ-Casein

It constitutes approximately 13% of total casein and plays a critical role in micelle stability. As a glycoprotein, κ-casein is insensitive to calcium and resides on the micelle surface, maintaining the colloidal stability of milk. κ-Casein is one of the key components in milk coagulation. It consists of approximately 169 amino acids and has variants A through J. Variants A and B are the most common. Genetic variations influence milk yield, protein content, cheese-making time, and milk composition.

Biological Functions of Casein

Casein is a slowly digested protein. Its gradual breakdown in the digestive system provides a sustained release of amino acids. This property helps prevent muscle protein breakdown over extended periods, exerting a muscle-protective effect. Peptides formed during casein digestion possess antioxidant, antihypertensive, and immunomodulatory properties. Additionally, casein plays a vital role in the transport of calcium and phosphorus, enhancing their bioavailability.

Casein has the capacity to bind soluble salts of heavy metals. Complexes formed with mercury, lead, copper, zinc, and aluminum facilitate the elimination of these toxins from the body. Therefore, casein consumption has been associated with protective effects against metal poisoning.

Casein and Health

The health significance of casein proteins, particularly regarding β-casein variants, remains a subject of debate. It has been suggested that dairy products containing A1 β-casein may be associated with certain chronic diseases, including type 1 diabetes, cardiovascular diseases, neurological disorders, and Sudden Infant Death Syndrome (SIDS). However, further scientific research is needed to confirm these associations.

A2 β-casein significantly reduces these potential risks. Consequently, some dairy producers prefer to produce milk containing A2 β-casein. The presence of A2 β-casein-like proteins in human milk and other animal milks suggests potential advantages of these milks in infant nutrition.

Dietary sources of casein play a significant role in meeting protein requirements. Casein intake is particularly recommended for athletes, elderly individuals, and those in prolonged fasting states due to its muscle-protective effects. However, excessive consumption of casein-containing foods may carry certain risks. High protein intake can impose additional strain on the kidneys and liver, lead to unwanted mineral losses, and negatively affect skeletal health. Therefore, casein-derived protein intake should be integrated into a balanced diet.

Casein-containing foods should also be consumed with caution in certain health conditions. In individuals with lactose intolerance, casein-containing dairy products may cause digestive issues. Additionally, literature reports suggest possible associations between β-casein variants, particularly A1, and certain chronic diseases. Therefore, the source and quantity of casein intake should be carefully evaluated.

Casein and Nutrition

Casein proteins serve essential functions in cheese production, including curd formation, texture development, and flavor influence. Due to these properties, casein is regarded as a fundamental protein in the dairy industry, both technologically and nutritionally. Casein is considered a high-quality protein source and plays a significant role in nutrition due to its slow digestion. In the stomach environment, casein gradually coagulates, enabling controlled release of amino acids into the bloodstream. This feature is advantageous for individuals with prolonged protein needs. Key nutritional attributes of casein include:

• Sustained amino acid release: Casein’s ability to form curds in the stomach ensures a slow, prolonged release of amino acids into the bloodstream, which is particularly important for preventing muscle protein breakdown.
• Muscle protein preservation: Its slow digestion supports long-term muscle protein maintenance, making it valuable in sports nutrition.
• Biologically active peptides: During digestion, casein generates peptides with antioxidant, antihypertensive, and immunomodulatory properties.
• Mineral transport: Casein plays a critical role in transporting calcium and phosphorus, enhancing their bioavailability.
• Balanced amino acid profile: Casein is particularly rich in glutamine, an amino acid essential for muscle repair and immune function.

Due to its slow digestion, casein is especially beneficial when consumed before sleep, as it helps suppress muscle protein breakdown over extended periods and supports muscle development. However, its relatively low branched-chain amino acid (BCAA) content and lactose content may pose disadvantages in certain contexts. In individuals with lactose intolerance, casein-containing products can cause digestive discomfort.

Casein-Containing Foods

Casein is found primarily in milk and dairy products. The amount and quality of casein vary depending on the milk source, processing method, and animal species.

• Cow’s milk: One of the richest sources of casein. In cow’s milk, casein proteins constitute approximately 78–80% of total milk proteins. When milk pH decreases, casein precipitates to form curds.
• Cheese: During cheese production, milk casein coagulates to form curds, which constitute the protein structure of cheese. The casein content in cheese varies according to the type of milk and production method. Cheeses are among the most concentrated sources of casein.
• Yogurt and kefir: These fermented milk products contain casein. During fermentation, casein is partially broken down into peptides and amino acids, enhancing digestibility.
• Milk powder and protein powders: In industrial processing, casein is concentrated in milk powder and casein protein powders. These products are widely used in sports nutrition and dietary supplements.
• Other dairy products: Casein is also present in butter, cream, and milk-based desserts, though concentrations vary depending on processing.

Some plant-based products and alternative protein sources do not contain casein; therefore, casein intake is derived exclusively from milk and dairy products. In individuals with lactose intolerance, consumption of casein-containing dairy products may lead to digestive issues.

Milk as a Food Source Rich in Casein (Anadolu Agency)

Casein Genetics and Industrial Importance

Casein proteins vary according to genetic variants, which directly influence milk yield, protein content, cheese-making time, and milk composition. For example, the κ-casein BB genotype increases milk yield and shortens cheese-making time. Variants of the αS1-casein gene affect milk protein concentration and yield.

Industrially, casein is used not only in the production of cheese, yogurt, ice cream, and other dairy products but also as a food additive, nutritional supplement, and technical material. Due to its film-forming properties, casein is also employed as a packaging material and adhesive.