448 Chapter 17
starter, (b) removes air from the medium to
produce a more conducive medium for
microaerophilic lactic cultures to grow, (c)
effects thermal breakdown of milk con-
stituents, especially proteins, releasing pep-
tones and sulfhydryl groups which provide
nutrition and anaerobic conditions for
yogurt culture, and (d) denatures whey pro-
teins of milk, thereby enhancing the viscos-
ity, leading to a custard - like consistency in
the product.
Physical changes in the proteins as a
result of heat treatment have a profound
effect on the viscosity of yogurt. Whey
protein denaturation, of the order of 70% to
95%, enhances water absorption capacity,
thereby creating a smooth consistency, high
viscosity, and stability from whey separation
in yogurt.
Homogenization. Homogenization is
usually conducted by applying pressure in
two stages. The fi rst stage pressure, of the
order of approximately 14 MPa (2,000 psi),
reduces the average milk fat globule diameter
size from approx. 4 μ m (range 0.1 to 16 μ m)
to less than 1 μ m. The second stage uses
3.5 MPa (500 psi) and is designed to break
the clusters of fat globules apart to inhibit
creaming in milk. Homogenization aids in
texture development and alleviates the sur-
face creaming and wheying off problems.
The homogenized mix is brought to 43 ° C
(110 ° F) by pumping through an appropriate
heat exchanger. It is then collected in fermen-
tation tanks.
Fermentation. Fermentation tanks for the
production of cultured dairy products are
generally designed with a cone bottom to
facilitate draining of relatively viscous fl uids
after incubation. Using bulk starters at the
4% inoculum level, the incubation period is
2.5 to 3 hours at 43 ° C (110 ° F). During fer-
mentation, lactose content of the mix is
reduced by approximately 30%. However, a
signifi cant level of lactose (4.2%) survives in
yogurt. Normally, the fermentation period is
terminated by a temperature drop to 4 ° Cfruit preparations are now preferred by yogurt
manufacturers.
Yogurt Processing
Production of Yogurt Starters
Frozen culture concentrates are available
from commercial culture suppliers. Many
plants use frozen direct - to - vat or freeze - dried
direct - to - vat cultures for yogurt production.
However, for cost savings, large yogurt man-
ufacturers prefer to make bulk starters in
their own plant from frozen or freeze - dried
bulk cultures. The medium for bulk starter
production is antibiotic - free NFDM reconsti-
tuted in water at the 10% to 12% solids level.
Following reconstitution of nonfat dry milk
in water, the medium is heated to 90 ° C to
95 ° C (194 ° F to 203 ° F) and held for 30 to 60
minutes. The medium is then cooled to 43 ° C
(110 ° F) in the vat.
The incubation period for yogurt bulk
starter ranges from four to six hours, and
the temperature of 43 ° C (110 ° F) is main-
tained by holding hot water in the jacket of
the tank. The progress of fermentation is
monitored by titratable acidity measure-
ments at regular intervals. When the titratable
acidity is 0.85% to 0.90%, the fermentation
is terminated by turning the agitators on
and replacing warm water in the jacket with
iced water. Circulating iced water drops the
temperature of the starter to 4 ° C to 5 ° C (39 ° F
to 41 ° F).
Mix preparation. Yogurt mix is prepared
from the ingredients by standardization of
milk for fat and MSNF content. The addition
of stabilizers (gelatin, starch, pectin, agar,
alginates, gums, and carrageenans) and
sweeteners impacts physical properties.
Heat treatment. Heat treatment at 85 ° C
(185 ° F) for 30 minutes or 95 ° C to 99 ° C
(203 ° F to 210 ° F) for seven to 10 minutes is
an important step in manufacture. The heat
treatment (a) produces a relatively sterile
medium for the exclusive growth of the