Skim milk is valued for its high-quality casein protein which may be precipitated from the skim milk to make acid, rennet and co-precipitate casein. Casein is the principal milk protein which constitutes 80% of the total milk protein. Broadly, there are two types available in the market viz. edible and industrial casein. The above two types can be acid or rennet based on precipitation done. Industrial casein is used to make plastics (buckets, costume jewellery, buttons, etc), adhesive, paper coating, films, medicines, bristles, paint, etc. Edible casein is a long-established dairy by-product. It is having extensive use in the food industry. This article aims to sensitise readers with manufacturing steps of edible casein.
High-quality casein can be produced only from good quality skim milk. Skim milk should not have more than 0.5% fat (as per FSSAI). The developed acidity should not be greater than 0.15% lactic acid. Excessive heating of skim milk before precipitation causes assorted interactions among skim milk constituents(lactose, casein and whey proteins). This also gives casein brownish colour. Therefore, adequate preheating must be ensured. To reduce microbial load without pasteurisation, microfiltration can be done. Handling of raw material should be carefully controlled to produce good quality casein.
Acidification (in case of Acid Casein)
Casein exists in milk as ‘calcium caseinate-calcium phosphate’ complex. When acid is added to milk, this complex breakdown. Calcium is removed from casein micelles and calcium phosphate is liberated insoluble form. At pH 5.3, precipitation of casein occurs. However, maximum precipitation occurs at pH 4.6 which is also an isoelectric point of casein. Acidification of milk can be performed by any of the following methods:-
- Inoculation of milk with lactic acid-producing bacteria like Lactococcus lactis ssp. lactis or Lactococcus lactis ssp. cremoris. The incubation time of the starter is 16-18 hours.
- Direct acidification method involves the addition of dilute hydrochloric acid, phosphoric acid, sulphuric acid, or lactic acid or another organic acid.
- Indirect acidification method includes various patented technologies like usage of ‘ion exchange resins in the acid form’, ‘addition of acidified whey’, electrodialysis, etc. However, these technologies are not economically feasible therefore not used extensively in the food industry.
For lactic acid casein, pasteurised skim milk is cooled to 22-26°C further at pH 4.6 with the production of lactic acid (produced by precipitation occurs. In the case of mineral acids, it should be noted that greater the extent of dilution of acid, better will be the quality of casein.
Rennet addition (in case of rennet casein)
The rennet casein is prepared by precipitating skim milk by action of the enzyme ‘chymosin’ or a suitable rennet substitute. Since rennet casein is precipitated at neutral pH therefore it has a higher mineral content than acid casein (especially Calcium). Pasteurized skim milk is mixed with rennet or rennet substitute at 30°C (or lower). After 20-30 minutes gel is formed which is cut and then coagulum is stirred while being heated to about 50-60° C. The high temperature used here deactivates the enzyme. Cooking time is about 30 minutes.
Temperature of precipitation
Temperature affects the curd characteristics. At a temperature of less than 35°C, the curd formed has a very soft and fine texture, further, it takes considerable time to settle down and there are losses at the time of washing. At a temperature of more than 35°C and less than 38°C, along with gentle stirring the curd formed has a coarse texture.
At 43°C, curd has a texture similar to chewing gum (lumpy and coarse).
Best way to get curd with good characteristics is to keep temperature less than 38°C or keep pH around 4.1.
Draining whey and washing of curd
After precipitation whey should be separated. Longer the curd is in contact with whey, more difficult is to wash acid and remove salts.
Washing should be done just after the removal of whey from the curd. Water used to wash curd should be equal to the quantity of whey drained. Three separate times washing to be performed with a duration of 15-20 minutes each. The pH of the wash water should be around 4.6 for the first two washings. This will prevent the formation of a gelatinous layer over the curd particles in highly acidic water and softening as well as redispersion of the curd in alkaline water. Third wash should be done with neutral water. The temperature of the very first washing should be equal to precipitation temperature. This will give good curd shrinkage thus enhancing sensory characteristics of the final product. Wash water in case rennet casein is neutral.
After washing, the curd is dewatered (water is removed mechanically). This is done to remove excess water from the curd before drying. It is highly recommended to remove water as much as possible since drying is a relatively expensive process. In an aim to produce a minimum water content and maximum friability of the curd, the temperature of the last wash should be carefully controlled. As the excessive temperature of last washing releases more water during dewatering and make the finished product of plastic consistency. In traditional methods, the pressing of curd should not be for less than 12-15 hours with 3-4 kg/sq cm pressure.
Milling and drying
Pressed curd should be milled quickly to avoid mould contamination. The pressed curd should be milled into uniform size particles to ensure a smooth surface drying. Ideally, 57°C can be kept for air entering the tunnel dryer. Once the drying process has been started it should not be interrupted. The process of drying continues until the desired moisture level is achieved.
Tempering, grinding, sieving and packaging
Tempering is the operation which involves holding of casein for a certain period (24 hours). It is done to achieve efficient cooling as well as hardening of the casein this ensures even distribution of moisture throughout the batch. Tempering is accompanied by agitation.
Cooled and tempered casein is then grounded. Cooling is essential to prevent ‘burn on’ rollers. The objective of grinding and sieving is to produce uniform sized particles. Commonly used mesh sizes for casein are 30-40 mesh casein, 60 casein and 90 mesh casein. After being properly blended, it is packed into multi-paper bags equipped with plastic liners and stored.
Inferior quality of skim milk used and faulty manufacturing processes can lead to defects. Overheating and the presence of excess fat in casein during drying causes browning in the finished product. The excessive-high temperature used during cooking curd and overheating of casein during drying can reduce the solubility of the final product.
The intensive investigations in manufacturing technologies over the decades has led to the introduction of improved manufacturing technology of edible casein. There exist huge opportunities for this byproduct due to its extensive use in the ice cream industry, instant breakfast, protein hydrolysates, imitation milk, whipping powder, coffee whiteners, etc and availability of the good amount of skim milk (as a byproduct) in the dairy industry.
- Outlines of Dairy Technology by Sukumar De
- Dairy Technology Volume 2 by Shivashraya Singh
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