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

6.  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.

7.  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

9. 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.

10.  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.

11.  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.

12.  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.

13.  Classification of Dahi:
 Dahi may be classified into two types:
1. For churning into Desi butter (makkhan);
2. 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

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

16.  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

17.  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.

20.  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.

21.  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.

22.  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.

23.  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.

24.  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%.

25.  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.

26.  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 40-45oC. 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,

27.  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.

28.  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, CO2 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.

29.  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.

31.  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.

32.  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.

33.  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.