Most common as well as traditional dairy based product, Curd, locally known as Dahi, is consumed widely all over the world. Curd falls under the category of fermented dairy product, produced from heat treated milk after inoculation with certain Lactic Acid Bacteria (LAB) in the form of starter culture. Lactic acid bacteria multiply, grow and produce lactic acid, acetic acid, and carbon dioxide by utilizing available lactose in milk. Some bacteria use citric acid of milk to produce certain volatile organic compounds mainly diacetyl, which is mainly responsible for flavor of Dahi.
Due to its rich nutritional profile and high consumer demand, it is commercially produced to meet the market demands. A new revolution in the industrial manufacturing of Dahi is using Dahi as functional food.
Principle behind Curd Production
Curd has live cultures and the lactose in the milk is converted to lactic acid by the action of starter cultures and the lactic acid act as preservative for the milk and the low pH (4.5- 5.0) also inhibit the growth of harmful micro-organism, thus increasing the shelf life of the product. Dahi is manufactured using single or mixed cultures, Lactococcus cremoris, Lactococcus diacetylactis, along with Leuconostoc species, Lactococcus lactis, a combination of acid and flavor producing bacteria. These bacteria are responsible for imparting firm body, sweetness, and a mild acidic flavor to the Dahi increasing its acceptability to the consumer.
Process Flow chart of Curd:
Curd Processing Steps
- Reception of milk: Fresh, good quality milk is received and analyzed for SNF and fat%.
- Pre-heating: Done at 30-40°C
- Standardization: 5% to 3.0% fat and 10% solids not fat.
- Preheating (Optional): It is an optional step to heat milk up to 60°C
- Homogenization: Milk is subjected to high pressure pump forcing milk through extremely small orifice for even distribution of fat globules. Homogenization reduces the cream layer formation during incubation and single-stage Homogenizer, or double stage homogenizer can be used as per process requirement. After homogenization all the fat globules of the milk has an average size below 1 micron.
- Pasteurization: Milk is heated to 85-90°C for 15-30 minutes and temperature bought down to 3-4°C.
- Pre-heating: Pasteurized milk is preheated to 40-45°C and transferred to inoculation tank.
- Inoculation: Milk is inoculated with 1-2% of specific curd starter culture at 37°C. The incubation tanks are insulated, to ensure that the temperature remains constant during the incubation period. The tanks can be fitted with pH meters to check the development of acidity (4.2 – 4.5)
- Packaging: The inoculated milk is then packaged in separate cups with lids. Dahi is generally packaged in polyethylene, polypropylene, polystyrene packaging material and plastic cups
- Incubation: Cups are arranged in crates which are then transferred to hot room (37˚C) while the fermentation process proceeds. In case of set curd, incubation is done when the product is in its final retail container at 30-42°C for around 4-5 hours, while for stirred types of products it can be done within the inoculum/incubation tank and then packed in pouches.
- Cooling: The pH of the milk in the cups should be regularly checked and when it reaches 4.4-4.5, these crates are transferred to room with temperature 3-4oC for proper setting
- Storage: It is stored in the cold store below 6°C
Difference between Industrial curd and Homemade curd
Dahi produced on domestic levels has undefined cultures, thus, difference in texture and taste is often observed. While in commercial manufacturing, process set points are defined and followed to get consistent product.
Difference between Yoghurt and Curd
Quite common confusion is that yogurt and curd are same. But that is not the case.