STARTER CULTURES AND FERMENTED MILK PRODUCTS

Lesson 22. DESIRABLE PROPERTIES OF DAHI AND FACTORS INFLUENCING IT , COMPOSITION OF DAHI

Module 13. Dahi

Lesson 22

DESIRABLE PROPERTIES OF DAHI AND FACTORS INFLUENCING IT , COMPOSITION OF DAHI

22.1 Composition of Dahi

The composition of dahi depends upon the type of milk and manufacturing conditions.The average composition of dahi is depicted in Table 2.1.

Table 22.1 Composition of dahi

22.2 Desirable Properties of Dahi and Factors Influencing It

A good quality dahi made from whole milk has a creamy layer on the top, the rest being made up of a homogenous body of curd and the surface being smooth and glossy, while the cut surface should be firm and free from cracks of gas bubbles and it should have a pleasant acid taste with sweetish aroma.

22.2.1 Quality of milk as a raw material for dahi making

The type and quality of milk used for dahi making has great influence on the quality of dahi as it affects the starter activity as well as sensory characteristics of the dahi. The milk should be fresh, clean and free from developed acidity and any off flavours. It should be free from antibiotic residues and inhibitory substances which can adversaly affect the starter activity. The chemical composition, status of fat,protein, minerals, enzymes and vitamins, etc. of dahi can vary with the type of milk used in its manufacture. Milks of different species of mammals have been used for the production of dahi depending on the availability. Treatments like filtration,clarification, bactofugation, etc. can be applied to make the milk clean and bacteriologically better. Milk with abnormal composition like colostrum, late lactation milk and mastitic milk should not be used as it adversely affects the growth of starter culture which subsequently results into poor quality product.

22.2.2 Additives

Except sugar, no other additives are legally permitted in dahi. However, various additives such as starch,alginate, gelatin, CaCl₂ and MgCl₂ are frequently used to control the texture and also to reduce whey separation in dahi.

22.2.3 Homogenization

Homogenization of milk is anessential step in the commercial level preparation of dahi. This step increases the viscosity and coagulum stability. It also improves the mouth feel of the product, and thus increases the organoleptic quality of dahi. If, not done hygienically this step can serve as a source of contamination in dahi.

22.2.4 Heat treatments given to milk

Generally milk is heated to higher temperatures than the normal pasteurization. This higher temperature is necessary to produce :

• relatively sterile and conducive environment for the growth of starter culture
• to denature and coagulate whey proteins to enhance the viscosity and texture of the final product.
• The heat treatment also affects the activity of the culture by causing stimulation and inhibition of lactic cultures during fermentation of milk depending on changes in O-R potential.
• denaturation of whey proteins and destruction of inhibitors

22.2.5 Starter cultures

Starter cultures used for dahi making is of utmost importance owing to the inevitable role (Table 2.2) it plays during the entire manufacturing process. The type and characteristics of starter organisms used in the production of dahi and their associative growth during the fermentation process plays an important role in determining the overall quality of the final product. Some important starter culture criteria, which can decide the dahi quality include activity and purity of the culture, composition of starter microflora, type of fermentation, growth temperature, etc.

Table 22.2 Role of starter culture in dahi making

The ability of the starter culture to perform its function during the manufacture of dahi depends on its purity and activity. Almost any standardized test of metabolic activity is suitable for testing the activity, but the rate of acid production is easy to measure and is considered the most suitable test. Depending on the requirement the starters are tested for flavour production also. Starter cultures should be periodically examined for contamination by extraneous organisms. The presence of contaminants is highly objectionable as they not only reduce starter activity but also bring about undesirable changes in the dahi. Microscopic examination of stained smears of cultures may give preliminary idea about the contaminating organisms but conclusive detection of certain groups of organisms should be done by cultural methods. Now a days commercial dahi makers use DVS starters for direct vat inoculation. These starters avoid the need for propagation of the starter cultures.

22.2.6 Inoculation and incubation

The amount of inoculum to be used for the production of cultured milk products should be optimum. The use of minimum amount of culture causes greater fluctuations in acid production and high amount will result in defect in the texture and aroma of the final product. An optimum amount of inoculum will result in normal course of lactic acid fermentation. The amount of inoculum can vary depending on the method of production. A short set production method requires more amount of inoculum where as a long set requires less amount of inoculum. Different research workers have tried rate of inoculum varying from 1 to 10% for making dahi. The incubation temperature and period depends on the optimum growth of starter culture ,manufacturing schedule (short set or long set),etc. Mesophilic starters are generally incubated between 20°C to 30°C where as for thermophiles the temperature range is 37°C to 45°C.The inoculated milk is either retail packed or set in bulk. The packaging before or after setting depends upon whether the product is stirred type or set type.

22.2.7 Cooling and packaging of dahi

The product is immediately cooled after setting to less than 5°C to arrest excessive acid development and ‘whey-off’. Cooling should give minimum mechanical damage to the curd. In India, earthern pots or kulhars are used for setting as well as selling large quantities of dahi. Dahi packaged in earthern pots has a firm body and these packages are eco friendly and easily degradable after disposal,but the limitations are, heavy, breakable and can cause excessive shrinkage of the product during storage due to moisture seepage through pores. Polystyrene cups are the most convenient packaging material for set dahi in case of commercialization. The other commonly used plastics for packaging of dahi are polypropylene, polyvinylchloride and polyvinyledene chloride. The polystyrene containers are clean in appearance, give a good shining look, light in weight and unbreakable, but causes wheying off in dahi during storage and are nonbiodegradable.