Classify industrial casein derived from skim milk. Describe the method of manufacture of acid casein.

Classification of Industrial Casein Derived from Skim Milk

Industrial casein, extracted from skim milk, is primarily classified based on the method used for its coagulation. The two main types are acid casein and rennet casein, with acid casein further sub-divided based on the specific acid used.

• Acid Casein: This type is produced by reducing the pH of skim milk to its isoelectric point (approximately pH 4.6), where casein proteins lose their charge and precipitate. Acid casein is typically insoluble in water but soluble in dilute alkalis. It is further categorized into:
  • Lactic Acid Casein: Produced by biological acidification, where starter cultures of lactic acid bacteria ferment lactose in skim milk to produce lactic acid, which lowers the pH and causes casein to coagulate.
  • Mineral Acid Casein: Obtained by the direct addition of mineral acids, such as hydrochloric acid (HCl) or sulfuric acid (H2SO4), to skim milk to achieve the required pH for precipitation.
• Rennet Casein: This casein is produced by enzymatic coagulation using rennet (chymosin) or other milk-clotting enzymes. These enzymes specifically cleave kappa-casein, destabilizing the casein micelles and causing them to coagulate. Rennet casein has different functional properties compared to acid casein, particularly in its calcium content and ability to form plastics.

Method of Manufacture of Acid Casein

The manufacturing process of acid casein from skim milk involves several critical steps designed to isolate and purify the casein protein. The goal is to achieve precipitation at the casein's isoelectric point (pH 4.6), where it is least soluble. The process can use either mineral acids or biological fermentation. Here, we describe the mineral acid method as a common industrial practice.

1. Raw Material Preparation

The starting material is fresh, pasteurized skim milk. Skim milk is preferred because the removal of milk fat (down to 0.1% fat content) prevents rancidity and ensures higher purity and better storage stability of the final casein product. Pasteurization (typically 72°C for 15-20 seconds) is essential to eliminate undesirable microorganisms while minimizing denaturation of milk proteins.

2. Preheating

The pasteurized skim milk is preheated to an optimal temperature, usually between 40-45°C. This temperature range facilitates efficient coagulation and curd formation during the acidification step, leading to a better quality curd with desirable characteristics.

3. Acidification and Precipitation

This is the most crucial step. A food-grade mineral acid (e.g., dilute hydrochloric acid or sulfuric acid) is slowly added to the preheated skim milk under continuous agitation. The acid lowers the pH of the milk gradually. As the pH approaches 4.6 (the isoelectric point of casein), the casein micelles lose their negative charge and aggregate, forming solid curds. The careful control of pH and temperature is vital to ensure uniform and efficient precipitation.

• Isoelectric Point: At pH 4.6, casein proteins have a net zero charge, leading to maximum instability and precipitation.

4. Separation of Curd and Whey

Once the casein has fully coagulated, the mixture consists of solid casein curds and liquid whey (containing lactose, whey proteins, and minerals). The curd is separated from the whey using various methods, such as decantation, screens, or centrifugal separators. This step aims to maximize casein recovery while minimizing moisture content in the curd.

5. Washing

The separated casein curd undergoes multiple washing cycles with cold, potable water. This washing is critical to remove residual acid, lactose, whey proteins, and soluble minerals that would otherwise affect the flavor, color, and storage stability of the final product. Counter-current washing systems are often employed to conserve water and maximize washing efficiency. The washing temperature is typically maintained between 35-60°C.

6. Dewatering and Pressing

After washing, the casein curd still contains a high percentage of moisture (around 85%). Mechanical pressing, using hydraulic presses or screw presses, is employed to reduce the moisture content significantly, usually to about 50-60%. This dewatering step is vital to reduce the energy requirements for subsequent drying and to prepare the curd for milling.

7. Grinding/Milling

The dewatered casein curd is then mechanically ground or milled into smaller, more uniform particles. This increases the surface area, facilitating efficient drying and producing a consistent product texture.

8. Drying

The milled casein particles are dried to a final moisture content of 8-12%. Common industrial drying methods include fluid bed dryers or band dryers. Controlled drying temperatures (e.g., 50-55°C in the first stage and around 65°C in the second stage) are maintained to prevent protein denaturation and ensure product quality. Rapid and uniform drying is essential for preventing microbial growth and maintaining the functional properties of casein.

9. Final Processing and Packaging

The dried casein is then further milled and sieved to achieve the desired particle size (e.g., 30, 60, or 90 mesh) as per market requirements. Finally, it is packed in moisture-proof bags, typically in quantities up to 50 kg, to ensure its quality and extend its shelf life.

The production of acid casein is a precise process, yielding a versatile protein ingredient used extensively in both food and non-food industries due to its excellent emulsifying, binding, and film-forming properties.