STARTER CULTURES AND FERMENTED MILK PRODUCTS

Lesson 21. DEFINITION AND STANDARDS, METHODS OF PRODUCTION, TYPES OF STARTERS

Module 13. Dahi

Lesson 21

DEFINITION AND STANDARDS, METHODS OF PRODUCTION, TYPES OF STARTERS

21.1 Introduction

Since time immemorial, surplus milk has been used in India for preparing a wide variety of dairy delicacies. The first of these products developed was dahi (curds), obtained by fermenting milk. Dahi (Sanskrit:Dadhi) is considered the oldest Indian fermented milk product and is equivalent to Western Yogurt. It is believed that dahi has valuable therapeutic properties and helps curing gastrointestinal disorders. In the Indian system of medicine dahi has been recommended for curing dyspepsia, dysentery, and other intestinal disorders. Dahi is prepared and consumed in the household on a day to day basis and a huge portion of dahi production in the country is still confined to the unorganized sector. Recently several commercial dairies have come up with production of dahi on large scale and are having good market.

21.2 Definition of Dahi

PFA rules (2006), defines dahi or curd as : “It is the product obtained from pasteurized or boiled milk by souring, natural or otherwise, by aharmless lactic acid or other bacterial culture. Dahi may contain added canesugar. Dahi shall have the same minimum per cent of milk fat and milk solids-not-fat as the milk from which it is prepared. Where dahi or curd 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. Milk solids may also be used in the preparation of this product”.

21.3 Standards for Dahi

According to Bureau of Indian Standards (1978) specifications for fermented milk, dahi should have a pleasing flavour and a clean acid taste, devoid of undesirable flavour,should have firm, solid body and texture and be uniform with negligible whey separation. Other characteristics (Table-21.1) should be as follows:

Table 21.1 Specifications for dahi (Bis, 1978)

A good quality dahi made from whole milk has a cream layer on the top, the rest being made up of a homogenous body of curd and the surface being smooth and glossy, while the cutsurface should be firm and free from cracks of gas bubbles and it should have a pleasant acid taste with sweetish aroma. Composition and quality of dahi vary widely from one locality to another as it is being prepared under different domestic conditions as well as milk with variable chemical and bacteriological quality used for the preparation. However, the chemical composition of dahi has been reported as fat ranging from 5 to 8 per cent, protein 3.3 to 3.4 per cent,ash 0.75 to 0.79 per cent and lactic acid 0.5 to 1.1 per cent. Diacetyl is the major flavor compound in dahi. Table 21.2 shows the specifications recommended by FSSA, 2006.

Table 21.2 Microbiological specifications for dahi (FSSA, 2006)

21.4 Methods of Production of Dahi

The traditional method for preparation of dahi invariably involves a small scale,either in consumers’ household or in the sweet makers shop in urban areas. In the household, milk is boiled, cooled to ambient temperature and inoculated with 0.5 – 1 per cent of starter (previous day’s dahi or butter milk) and allowed to set overnight (Figure-21.1) . It is then stored under refrigeration and consumed. In cooler weather the dahi setting vessel is usually wrapped in a woolen cloth to maintain warmth. In shops, the method is more or less the same except that milk is concentrated somewhat before inoculation and the dahi is usually set in a shallow circular earthen pot, which helps in the absorption of any whey that may ooze out.

Fig. 21.1 Household method of preparation of dahi

Restaurants and sweetmeat shops make dahi by the short set method (curd within 4-6 h). They use inoculum at the rate of 2-4% followed by incubation at 42-45°C till setting of the curd. The procedure for commercial production of dahi is depicted in Figure. 21.2.

Fig. 21.2 Commercial production of dahi (short set method)

Fig. 21.3 Bowl of dahi

21.5 Starter cultures for Dahi

The inoculum used to initiate fermentation in milk is called starter. The type and characteristics of starter organisms used in the production of fermented milk is an important factor that determines the type and characteristics of the final product. Traditionally,the previous day dahi or chhash, containing an unknown mixture of lactic acid bacteria was used as starter culture. However, to manufacture dahi on large scale with predictable uniform quality, it is desirable to use known mixtures of starters. Usually the starter bacteria consist of Lactococcus lactiss ubsp. lactis, cremoris and diacetylactis, Leuconostocs,Lactobacilli sp. and Streptococcus thermophilus . Lactobacilli (Figure21.4) dominate in sour dahi due to their higher acid resistence, while streptococci dominate in sweet dahi. Surveys have also indicated that in summer dahi Lactobacilli dominate, while in winter, Streptococci dominate. Similarly in southern parts of India, where people are habituated to take sour dahi, lactobacilli and yeast dominate, while in northern parts where mildly sour dahi is preferred,Streptococci predominate.

Fig. 21.4 Starter cultures in dahi

According to BIS: 9617 (1980), the following cultures shall be used in preparing dahi.

(a) Streptococcus lactis, Streptococcus diacetylactis, Streptococcus cremoris, singly or in combination, with orwithout Leuconostoc species.

(b) Also as above, along with species of Lactobacillus such as Lactobacillus acidophilus, Lactobacillus bulgaricus and L. casei and Streptococcus thermophilus.

Starter cultures can be classified in to different groups based on certain criteria as shown in Table-21.3.The starter cultures for dahi making are available in the form of liquid,concentrated, freeze dried cultures from culture suppliers and Culture collection centres. Alternatively a Direct Vat Inoculum (DVI) concentrated culture can be used directly in fermentation vat for the preparation of dahi.

Table 21.3 Classification of starter organisms based on certain phenotype properties