1. Fermented Dairy Products
Fermented dairy products can be divided into two groups;
1. Fermented milk products
2. Cheeses
⮚In fermented milk products, all milk constituents are
retained in the final products.
⮚In cheeses, a large portion of milk constituents is
removed in whey to obtain the final product.
2. 1. YOGHURT
TYPES OF YOGHURT
I. Set yoghurt
II. Stirred yoghurt
III. Drinking yoghurt
IV. Flavored yoghurt
V. Concentrated yoghurt
VI. Frozen yoghurt
Plain yogurt has a semisolid mass due to coagulation of
milk (skim, low, or full fat) by starter culture bacteria.
3. • Yoghurt is produced by the controlled fermentation of
milk by two species of bacteria Lactobacillus bulgaricus
and Lactococcus thermophilus (streptococcus
thermophilus)
• The lactose in milk is fermented to lactic acid and it
causes the characteristic curd to form
• Streptococcus thermophilus bring the pH of the milk down
to 5.5
• Lactobacillus bulgaricus convert lactose to lactic acid
• Proteolytic enzymes produced by L . bulgaricus break
down milk protein into peptides
• These peptides stimulate the growth of L . Thermophilus
which in turn produces formic acid and carbon dioxide.
These are the growth stimulants for the L . Bulgaricus
4. PRODUCTION OF YOGHURT
• Raw milk is taken which have low count of organisms and substances
which suppress the development of starter microbes e.g., Streptococcus
thermophilus
• Standardization of fats: Adjustment of the fat content to meet legal
requirements. This involves the removal of fat content of milk , mixing
full cream milk with skim milk or by using standardizing centrifuge
• Homogenization: Done by first pre-treating it to 60 to 65 ˚C and passing
it
through an homogenizer to stabilize the product and provide smooth
texture
• Pasteurization: To reduce spoilage microorganisms the homogenized
milk is heated at 74-78 ˚C for 16-19 sec.
• Heat treatment: this involves heating the milk at 90 to 95 ˚C for
5minutes to improve the texture of yoghurt
• Cooling and inoculation: after heating the substrate is cooled to
incubation temperature 42 ˚C to 45 ˚C and add starter culture followed
by incubation for 5 h at 47 ˚C.
• Packaged in containers, cooled by forced air to 4.4 ˚C to stop the growth
6. Biochemistry
Lactose Metabolism
▪ Both species have a constitutive β- galactosidase
system and lactose is hydrolyzed to glucose and
galactose.
▪ Both species are homofermentative and produce
lactate from glucose by glycolysis.
▪ Both species do not metabolize galactose, as a result,
galactose is excreted outside causing its accumulation
in yogurt.
7. Flavor Production
▪ The major flavor in yogurt is acetaldehyde with some
diacetyl and acetate.
▪ Acetaldehyde is produced from glucose via pyruvate by
Streptococcus spp. And from threonine by Lactobacillus
spp.
Formate Production
▪ Formate (necessary for Lactobacillus growth) is produced
by Streptococcus thermophilus from pyruvate by the
action of formate lyase.
Pyruvate-------------- Formate + Acetate
8. Slime Formation (Glycan)
▪ β-galactosidase in some strains of Streptococcus
thermophilus polymerizes glucose to produce
oligosaccharides and glycan, which may give a viscous
texture to yogurt.
▪ Milk proteins in the presence of proteinases and
Lactobacillus yields peptides some of which cause
bitter flavor.
▪ Peptides by the action of peptidases produces amino
acids necessary for Streptococcus, also threonine can
be used to produce flavor by Lactobacillus.
9. Microbial Problems
• In plain yogurt, flavor problems can be
associated with the concentration of
acetaldehyde. A low concentration gives a
chalky and sour flavor. Too much acetaldehyde
gives a green flavor.
• Too much diacetyl gives a buttery aroma.
• Too much acid production during storage are
associated with bitter flavor.
10. •Production of exopolysaccharides by the
starter can give a viscous and ropy texture.
•Growth of yeast during storage can also
produce fruity flavor especially in yogurt
containing fruits and nuts.
•During long storage, molds can grow on
the surface.
11. 2. Cultured buttermilk
It is a fermented milk product
made by using mesophilic
starters.
Cultured buttermilk is the
fluid remaining after ripened
or sour cream is churned into
butter using mesophilic
starters.
12. • Different products are produced by using different stains
of lactic acid bacteria as starter cultures and different
fractions of whole milk as the starting substrate
• Sour cream uses streptococcus cremoris or S. lactis for
producing lactic acid and leuconostoc cremoris for
characteristic flavor
• Cream is starting substrate , butter is normally made by
churning cream that has been soured by lactic acid
bacteria
• Streptococcus cremoris or S. lactis is used to produce
lactic acid rapidly and leuconostoc citrovorum produce
necessary flavors
13. PRODUCTION OF CULTURED BUTTERMILK
• Fresh or skim milk is taken and standardized
• Milk is homogenized at 200 kg/cm2 at 15 ˚C then heat
treated at 90 ˚C for 13 mins to get satisfactory end product
• After that, it is cooled to 23 ˚C and inoculated with a mixed
culture having Lactococcus lactis and Leuconostoc
citrovorum as primary producers of aroma and flavor
• The inoculation may range from 1 to 2%
• When acidity reaches 0.9% the product is gently mixed ,
cooled bottled and stored at 5 ˚C
• The fermentation time may be around 16 to 20 hrs.
• The final product is a viscous drink with pleasing aroma
and flavor
