Dahi and yoghurt

complete description about dahi (curd) and yoghurt
and microorganism realated to it

1. DAHI N YOGHURT

2. DAHI Definition : according to ISI (1980) now designated as BIS, dahi is a product obtained by lactic acid fermentation of cow or buffalo milk or mixed milk through the action of single or mixed strains of lactic acid bacteria or by lactic acid fermentation accompanied by alcoholic fermentation by yeast As per PFA rules (1965), dahi or curd is a product obtained from pasteurized or boiled milk by souring natural or otherwise, by a harmless lactic acid or other bacterial culture. Dahi may contain additional cane sugar and it should have the same percent of fat.  

3. This is an equivalent of yoghurt made from boiled milk after inoculation with mixed starter known as Jaman ( khatta ) which consists of the left-over dahi from the previous lot. However, it differs from yoghurt in having less acidity. The different starters used in the manufacture of dahi constitutes a complex and heterogeneous flora such as L. lactis subsp. lactis, L. lactis subsp. cremoris, S. thermophilus, L. delbrueckii subsp. bulgaricus, L. plantarum and lactose fermenting yeasts. A good quality dahi is of firm and uniform consistency with a sweet aroma and clean acid taste. The surface is smooth and glossy and a cut surface is trim and free from cracks and air bubbles

4. Dahi

5. History: Dahi has been an extremely popular fermented food in the Indian subcontinent which includes India, Pakistan, Bangladesh, Nepal, Sri Lanka etc. This product figures prominently in the ancient Hindu scriptures. The medicinal value of the product has been well documented. Dahi may be consumed directly either sweetened or salted and spiced. It is also consumed with other foods such as rice and chapatti (wheat loaf). Dahi has assumed a special place in the Indian population, who prefer to take dahi once or twice a day along-with morning or evening meals.   Composition : The composition of dahi depends upon the type of milk used and the manufacturing conditions. Following is average composition of dahi made from buffalo milk.

6. Composition of Dahi Water (%) : 85 – 88 Fat (%) : 5 - 8 Protein (%) : 3.2 - 3.4 Lactose (%) : 4.6 - 5.2 Ash (%) : 0.7 - 0.75 Lactic acid (%) : 0.5 - 1.0 Ca (%) : 0.12 - 0.14 P (%) : 0.09 - 0.11 (d) Microflora of dahi : The starters of khatta ( jaman ) used for the preparation of dahi is a mixture of lactococci and lactobacilli. The organisms commonly found in the inoculums are L. lactis subsp. cremoris , L. lactis subsp. lactis , S. thermophilus , L. acidophilus , L. delbreuckii subsp. bulgaricus and L. helveticus.

7. In order to produce a good quality product with a firm and uniform texture, a glossy surface and sweet aroma, the starter inoculum should be free from any contaminating flora. The recommended level of inoculum is 2.0-2.5% with 1-2% acidity to obtain good quality dahi. Different workers have recommended different combinations of lactic starters for the preparation of dahi, e.g. the National Culture Collection Centre at Karnal recommends the use of LF-40 (a mixed culture) as dahi culture for the preparation of dahi of superior quality . (e) Special attributes of dahi : In Indian system of medicine (Ayurveda), the use of dahi has been strongly advocated for curing ailments like dyspepsia, dysentery and other gastrointestinal disorders. The product is also believed to improve appetite, vitality and increases digestibility.

8. Some of the beneficial effects of dahi are attributed to the antibacterial components formed during the fermentation and the low pH that prevents the growth of putrefactive and other undesirable organisms including potential pathogens. (f) Nutritional value of dahi : As a result of metabolic activity of dahi starters, quantitative nutritive changes occur in milk during dahi-making process. There are appreciable changes in mineral and vitamin contents of dahi depending on the type of organism used for fermentation. A mixed culture of L. delbrueckii subsp. bulgaricus and Leuco . cremoris decreased thiamine, riboflavin and nicotinic acid in milk during dahi fermentation. However, single culture of L. lactis subsp. lactis subsp. cremoris raised the thiamine concentration from 2 to 20% over that of milk. Similarly, riboflavin content of dahi made from whole buffalo milk is almost fourfold that of skim milk dahi.

9. Classification of Dahi: Dahi may be classified into two types: For churning into Desi butter (makkhan); For direct consumption For direct consumption, dahi may be classified into: 1. Whole milk Dahi 2. Skim milk dahi 1. Sweet dahi 2. Sour dahi 3. Sweetened dahi

10. (g) Method of preparation : The different steps involved in the preparation of dahi at household level have been given in the flow diagram below:

11. SRIKHAND Definition : Srikhand is sweetened-dewatered dahi. This product is extremely popular in Western and some parts of Southern India. It has a distinctive rich flavour and fairly long shelf-life. This product too had its origin in ancient India just about the time when dahi was introduced. (b) Composition: Although, there are slight variations in the method of preparation of srikhand, the approximate average composition of this product is given below. Moisture (%) : 34.48 — 35.66 Fat (%) : 1.93 — 5.6 Protein (%) : 5.33 — 6.13 Reducing sugar (%) : 1.56 — 2.18 Sucrose (%) : 55.55 — 58.67

12. Preparation : During the preparation of srikhand, dahi is suspended in a muslin cloth until all the free water has drained off. The semisolid mass (called chakka ) is then whisked with sugar through a fine cloth, colored and scented with saffron or rose water and flavored with cardamom, if desired. In order to further extend the shelf-life of srikhand, a preparation known as 'Srikhand wadi' , which is essentially a desiccated srikhand, is made. The dewatered dahi is mixed with an equal amount of sugar by weight and dried in an open pan at low heat. When the mass begins to harden, it is tested for stickiness. The non-sticky product is flavored and colored. Powdered sugar is then further added as desired. The product is mixed, rolled, cut into shapes and packed like biscuits.   Microflora of Srikhand : The Microflora of srikhand is more or less like that of dahi. It includes dahi starter and the contaminants. Yeasts may be the most predominant contaminants and the low quality sugar may be the possible source of these organisms.

13. YOGHURT Definition: Yoghurt is defined as a coagulated milk product obtained by lactic acid fermentation of milk with or without additions brought about by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus . The starter organisms in the final product must be viable and abundant. The classical fermentation process initiated by these organisms impart healthy and nutritious properties to the product. The fermentation brought about by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus results in the solid, custard-like texture and an elevated level of developed acidity. Yoghurt as the final product should contain significant numbers of both rods and cocci originally used as the starters. The ability of the cultures to remain viable in the end product is ascribed to their symbiotic growth, metabolic and cellular by-products and a lack of heat treatment to the product.

14. The FAO/WHO standards No. 11(a) and 11(b) define yoghurt and flavored yoghurt respectively as the coagulated milk product obtained by lactic acid fermentation through the action of L. delbrueckii subsp. bulgaricus and S. thermophilus from milk. (b) Types Of Yoghurt : Following different types of yoghurt preparations are manufactured all-over the world: 1. Type based on chemical composition According to FAO/WHO (1973), yoghurt may be designated on the basis of fat content as full (above 3.0%), medium (3.02- 0.5%), or low (0.5% or below) fat yoghurts. 2. Types based on method of production Yoghurt can also be classified as (i) set yoghurt, and (ii) fluid yoghurt, based on method of manufacture and physical structure of the coagulum.

15. Set Yoghurt : It is the product where incubation/fermentation of the milk takes place in the retail container and hence the characteristic coagulum is a continuous semi-solid mass. Stirred yoghurt : It is the type of yoghurt produced after the fermentation has been carried out in bulk and then the coagulum is broken prior to cooling and packaging by stirring. Fluid yoghurt : It can be considered as stirred yoghurt with low viscosity, e.g. 11% total solids or less.

16. 3. Types based on flavour On the basis of flavour, three different types of yoghurt are in vogue. Natural or plain yoghurt which is the traditional type with sharp acidic taste. Fruit yoghurt that can be made by the addition of fruits and sweetening agents to the natural yoghurt. Flavored yoghurt is another type in which fruit ingredient is replaced with synthetic flavoring and coloring compounds.

17. Types based on post-incubation processing Various types of modified yoghurts can now be prepared based on post-incubation processes as detailed below: Pasteurized Yoghurt : This type of yoghurt can be processed by conventional methods but after fermentation, yoghurt is heat treated to extend its shelf-life. Frozen Yoghurt : This again is prepared by conventional manner but is subsequently deep-frozen to at least 20°C. It requires higher level of sugar and stabilizers for maintaining the consistency of the coagulum during freezing. Dietetic Yoghurt : These are low-caloric, low lactose or vitamin/protein fortified yoghurts. Concentrated Yoghurt : It has total solids of around 24%. Dried Yoghurt : It has total solids between 90-94%.

18. Microflora of yoghurt : There are controversial reports concerning the original Microflora of yoghurt. The presence of various physiological groups was reported in earlier investigations but these reports also pointed out that S. thermophilus and L. delbrueckii subsp. bulgaricus were the most predominating and played a vital role in the production of yoghurt. The microflora of yoghurt can be divided into three groups: (i) Essential microflora : It consists of S. thermophilus and L. delbrueckii subsp. bulgaricus. (ii) Non-essential microflora : It is represented by homo-fermentative lactic acid strains other than in group (i) and by hetero-fermentative lactic acid bacteria. Some of them may be used beneficially for supplementing the original flora, e.g. L. acidophilus, Bifidobacterium bifidum , Propionibacterium shermanii and L. lactis subsp. diacetylactis .

19. Contaminants : Yeasts ( Candida mycoderma , C. tropicalis , C. Krusei ), molds, coliforms and other undesirable micro-organisms may be present as contaminants. Associative action of starter cultures : Finished yoghurt is the end product of symbiotic interaction of S. thermophilus and L. delbrueckii subsp. bulgaricus growing at temperatures, in the range of 40-45 o C. A proportion of 1:1 of the rods and cocci forms is considered to be the optimum for flavour and texture production. However 1:5, 1:10 or 2.1:1.2 were also found to be favorable. Associative growth of rod- coccus mixed culture,

20. also results in greater acid production than single culture growth. It has been established that numerous amino acids liberated from casein by protease produced by L. delbrueckii subsp. bulgaricus stimulate the growth of S. thermophilus . Similarly, it has been demonstrated that acid production by L. delbrueckii subsp. bulgaricus is enhanced by formate and carbon dioxide produced by S. thermophilus thus, a two-stage growth pattern by yoghurt bacteria during acidification of milk has been proposed .

21. S. thermophilus grows faster during the early part of the incubation due to the stimulatory effect of the amino acids liberated (particularly glutamic acid and proline ) by L. delbrueckii subsp. bulgaricus , thereby removing oxygen and producing acid, CO 2 and formate in milk. After the growth of S. thermophilus is slowed by increasing concentrations of lactic acid, the more acid tolerant L. delbrueckii subsp. bulgaricus increases in numbers due to the stimulatory effect of the compounds generated by lactococci .

22. Therapeutic value of yoghurt : The therapeutic value of yoghurt was recognized by Metchnikoff as early as in 1900's when he claimed that yoghurt bacteria inhibited the growth of anaerobic spore formers in the large intestine thus, preventing putrefactive process from taking place in the alimentary tract and thereby prolonging life. By virtue of being a fermented product resulting from acidification of milk by S. thermophilus and L. delbrueckii subsp. bulgaricus , yoghurt has now been shown to have an antagonistic effect against a number of pathogenic and spoilage organism both in vivo and in vitro.

23. Defects of Yoghurt : The following types of defects are more common to yoghurt. Flavor defects : the most common flavor defect in yoghurt is the absence of typical yoghurt flavor and aroma, mainly because of inadequate acid formation. In spite of the desired balance of cocci and rods in the mother culture, optimum flavour development occurs only after the acidity reaches about 0.85%. However, beyond 0.95% acidity, a product is obtained which is too sour. Aroma compounds are formed over a considerably wider range of acidity. The absence of typical yoghurt flavour and aroma can also result from the use of strains of L. delbrueckii subsp. bulgaricus which produce little flavour and aroma substances.

24. Unclean and bitter flavour: These defects in yoghurt result from using poor quality milk or contaminated starter cultures. A few strains of L. delbrueckii subsp. bulgaricus can cause a bitter taste. Slow acid production : Slow acid production by yoghurt cultures may be due to bacteriophage attack on S. thermophilus . Although phage resistant cultures can be obtained, the body of the yoghurt made with these cultures is not as firm as desired.

25. Weak curd formation : Weak curd formation is a serious problem with yoghurt made from milk of normal solids content. A weak curd is most likely to result if the solids content of the milk is low or if a sufficient amount of the milk is from cows which are early in the lactation cycle. Addition of small amount of rennet may also increase firmness. Whey separation : This defect is caused by incorrect salt balance in the milk and can be controlled by reducing the heat treatment or by adding small amounts of calcium chloride in the milk.

26. Method of manufacture : The common flow diagram for manufacture of different types of yoghurt is given below: