Insights on Curd and its Production Process

 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. 

Dahi is produced from heat treated milk after inoculation with certain species of lactic acid bacteria added to milk 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. Judicious combination of acid producing and flavour producing microorganisms in the starter helps in the production of Dahi with a firm body and good flavour.

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.

The PFA Act defines dahi or curd as a semi-solid product, obtained from pasteurized or boiled milk by souring (natural or otherwise), using a harmless lactic acid or other bacterial cultures. Dahi may contain additional cane sugar. It should have the same minimum percentage of fat and solids-not-fat (SNF) as the milk from which it is prepared. Where dahi or curd, other than skimmed milk dahi, is sold or offered for sale without any indication of the class of milk, the standards prescribed for dahi prepared from buffalo milk shall apply.

The Bureau of Indian Standards (BIS) specifications for fermented milk products are based on the type of culture used in their preparation. Mild dahi is made from mesophillic lactococci. Leuconostocs may be adjunct organisms for added buttery odour and flavour. Sour dahi contains additional cultures belonging to the thermophillic group, which are generally employed in the manufacture of yoghurt. These thermophillic organisms grow rapidly at 37-45oC, producing dahi in less than 4 hours.

Like dahi, yoghurt is a semi-solid fermented product made from a standardized milk mix by the activity of a symbiotic blend of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus cultures. The body and texture of yoghurt depends largely on the composition of milk employed in its manufacture. Although milk of various mammals can be used for making cultured dairy products, their industrialized production is mainly based on milk of cows and buffaloes. 

Classification of Curd (Dahi) 

Broadly speaking, Curd  may be classi­fied into two types:

I. For churning into desi (or indigenous) butter (makkhan);

II. For direct consumption.

Curd  for direct consumption may be further classified into:

(a) Whole milk Curd ;

(b) Skim milk Curd ;

(c) Sweet (or mildly sour) Curd ;

(d) Sour Curd ;

(e) Sweetened Curd .

Composition of Curd

Components	Whole milk Dahi	Skim milk Dahi
	%	%
Water	85-88	90-91
Fat	5 - 8	0.05 - 0.1
protein	3.2-3.4	3.3-3.5
Lactose	4.6-5.2	4.7-5.3
Lactic acid	0.5-1.1	0.5-1.1
Ash	0.7-0.75	0.7-0.75

                     Table. Chemical composition of Curd

Curd made from buffalo milk produces a thick bodied product because of its high SNF content. It is recommended to make Curd / yoghurt from a mix containing 11-13 percent SNF. The increased protein content in the mix results in a custard like thick consistency following the required fermentation. Higher milk solids also keep the product from wheying off. Curd prepared from whole milk contains about fat 5- 8, protein 3.2 – 3.4 lactose 4.6 – 5.2, Ash 0.70 – 0.72, and titratable acidity 0.60 –0.80 percent.

Curd Processing Steps

1. Reception of milk: Fresh, good quality milk is received and analyzed for SNF and fat%.
2. Pre-heating: Done at 30-40°C
3. Standardization: 5% to 3.0% fat and 10% solids not fat.
4. Preheating (Optional): It is an optional step to heat milk up to 60°C
5. 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.
6. Pasteurization: Milk is heated to 85-90°C for 15-30 minutes and temperature bought down to 3-4°C.
7. Pre-heating: Pasteurized milk is preheated to 40-45°C and transferred to inoculation tank.
8. 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)
9. Packaging: The inoculated milk is then packaged in separate cups with lids. Curd is generally packaged in polyethylene, polypropylene, polystyrene packaging material and plastic cups
10. 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.
11. 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
12. Storage: It is stored in the cold store below 6°.

Method of Manufacture

There are two ways of in which curd can be produced and they are as follows:

i) Traditional method: In this method curd is prepared at small scale, either in the consumer’s household or in the confectionary (Halwais) shop. In the household, the milk is boiled, cooled to room temperature, inoculated with 0.5 to 1.0 percent starter (previous day’s curd or butter milk) and then incubated undisturbed for setting for about overnight. In cold weather, the curd setting vessel is usually wrapped up with woolen cloth to maintain appropriate temperature. In the confectionary shops, the method employed for preparation of curd is more or less same except that the milk is concentrated in a open pan before inoculation and usually curd is set in a earthenware.

ii) Standardized method: Process on the basis of scientific lines has been developed for curd making in the organized sector. Fresh, sweet, good quality milk is received, pre-heated and subjected to filtration and clarification. The milk is standardized to 2.5 to 3.0 percent fat and 10 percent solids not fat, pre-heated to 60oC and homogenized single-stage at a pressure of 176-kg/sq cm. The milk is heated to 85– 90oC for 15-30 minutes, cooled to 22-25oC and inoculated with 1-2 percent of specific curd starter culture. It is then filled in suitable packaging containers of the appropriate size and incubated at 22-25oC for 16-18 hours. After proper setting of the curd, the acidity of curd reaches 0.6 to 0.7 percent and a firm curd is formed. The curd is cooled by circulating chilled water or air around the containers and then transferred to cold room maintained at about 4-5oC.

Fig. Flowchart of standardized method of curd production

Curd  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.

Yoghurt vs Curd Difference

The main difference between Yogurt and Curd is that the fermentation of milk in Yogurt is caused by using a bacteria known as “Yogurt Cultures” that is consisting of lactobacillus bulgaricus, streptococcus thermophiles, whereas the edible acidic substances that can curdle the milk for making Curd are lemon juice or vinegar.