340 Chapter 13
fermentation is terminated when pH reaches
4.4 to 4.5. Fruits can be incorporated into the
bottom of cups (set - style yogurt) and the
inoculated yogurt mix added to the top fol-
lowed by fermentation, or after fermentation
(stirred - style yogurt) by mixing the fruit
preparation with plain yogurt (Nauth 2006 ).Physicochemical Changes During
Yogurt Manufacture
During fermentation, certain physicochemi-
cal changes take place to the milk constitu-
ents that alter the properties of the yogurt mix
and give the characteristics of the fi nal
product. These changes include acidity, vis-
cosity, and, to a lesser extent, fl avor. The
increase in acidity is mainly due to the pro-
duction of lactic acid and, to a lesser extent,
acetic acid from lactose hydrolysis through
the homofermentative pathway (Cogan 1995 ,
Van de Water 2003 ). Increasing viscosity
results from the destabilization of the casein
complex, when the casein micelles fl occulate
at or near their isoelectric point and the
colloidal calcium phosphate partially solu-
bilizes due to increased acidity (Nauth
2006 ). The result of this destabilization is
an irreversible gel (Tamime and Robinson
2007 ) that is responsible for the texture of
yogurt.
Flavor development also takes place
during yogurt manufacture. The fl avor com-
pounds in yogurt consist mainly of acetalde-
hyde and diacetyl. Acetaldehyde is produced
from the amino acid threonine in milk as well
as threonine produced by the lactobacilli
during the proteolysis of milk proteins.
Diacetyl is produced during yogurt fermenta-
tion by Str. thermophilus and, to a lesser
extent, Lb. delbrueckii ssp. Bulgaricus, prob-
ably in a manner similar to the mechanism of
diacetyl production in soft cheeses such as
quark. Because yogurt starter culture does
not metabolize citric acid, the precursor of
diacetyl, i.e., pyruvic acid, is formed during
sugar fermentation (Walstra et al. 2006 ).minimize air incorporation to reduce possible
foaming, and enable easy cleaning and sani-
tation of the unit (Tamime and Robinson
2007 ).
Milk evaporation is normally carried out
under vacuum conditions and at a high tem-
perature to avoid damage to the milk consti-
uents while removing the desired amount of
water. Membrane concentration of milk,
either whole or skim, is an alternative method.
This is acchieved by ultrafi ltration or reverse
osmosis. Milk constituents that pass through
the membrane are called the permeate, and
the constituents that do not pass through the
membrane are called retentate (Tamime and
Robinson 2007 ).
The yogurt mix is heat treated to destroy
and/or eliminate pathogens and other vegeta-
tive cells, produce factors stimulatory or
inhibitory to the yogurt starter culture, and
change the physicochemical properties of
milk constituents that are relevant to yogurt
making (Chandan and O ’ Rell 2006b ). Severe
and extensive heat treatment of milk can
cleave the calcium phosphate complexes
with casein micelle, resulting in destabiliza-
tion, aggregation, and precipitation (Chandan
2006 ). This heat treatment and the resulting
interactions improve the water binding
capacity of the protein system (Nauth 2006 ).
Inoculation and fermentation of the yogurt
mix is the fundamental step that gives yogurt
its fi nal characteristics. After heat treatment,
the yogurt mix is cooled to between 40 ° C and
45 ° C (104 ° F and 113 ° F), the optimum tem-
perature for mixed starter culture (Lucey
2004 ). The starter culture can vary from 0.5%
to 6%, depending of type of yogurt and fer-
mentation system. Incubation or fermenta-
tion of yogurt mix can be carried out for as
little as 2 hours when inoculated with 5%
starter and at 43 ° C to 45 ° C (109 ° F to 113 ° F)
to more than 6 hours if inoculated with 0.5%
to 1.5% starter (Nauth 2006 ). The fermenta-
tion time can be longer, 16 to 18 hours, if the
mix is incubated at 30 ° C (86 ° F) with 3%
inoculum (Tamime and Robinson 2007 ). The