26 Dairy Products 611tion. Two types of yogurt, set and stirred, are pro-
duced. Set-style yogurt is fermented following pack-
aging, with color and flavors added to the container
prior to the addition of the inoculated milk, resulting
in a gel that forms a firm, unbroken coagulum. Stir-
red yogurt is fermented in a vat prior to packaging,
with the gel structure broken following fermentation
during the addition of flavors and colors and during
the cooling and packaging stages. For both types of
yogurt, either a short incubation period at 40–45°C
for 2–3 hours or a long incubation period at 30°C
for 16–18 hours may be used to attain a pH of 4.5
or titratable acidity of 0.9% lactic acid prior to cool-
ing.
The metabolism of the yogurt cultures contributes
to the characteristic flavor of yogurt. Both bacteria
are heterofermentative and produce lactic acid from
glucose, yet are unable to metabolize galactose.
Acetaldehyde, a key flavor component of yogurt, is
produced by the degradation of threonine to acet-
aldehyde and glycine by the enzyme threonine aldo-
lase. Although S. thermophilusforms a majority of
the acetaldehyde produced, the proteolytic activity
of Lb. delbrueckiissp. bulgaricusgenerates the pre-
cursors for the formation of acetaldehyde.
Syneresis, the expelling of interstitial liquid due
to association of the protein molecules and shrink-
age of a gel network, is undesirable in yogurt. Syn-
eresis increases with an increase in incubation tem-
perature. In stirred yogurt, extensive syneresis re-
sults in a thin product. Therefore, incubation of the
yogurt at a lower temperature, such as 32°C, is rec-
ommended to maintain the desirable viscosity.
ACIDOPHILUSMILK
Acidophilus milk was developed as a fluid milk
product that provides therapeutic benefits through
the alleviation of intestinal disorders. Following
homogenization and heat treatment of the milk to
minimize contamination by other microorganisms,
the milk is cooled to 37°C and inoculated with a 1%
culture of Lactobacillus acidophilus(Fig. 26.6). The
milk is incubated at this temperature for 18–24
hours until the titratable acidity reaches 0.63–
0.72%. Lb. acidophilusis a thermophilic starter that
grows slowly in milk and has a relatively high acid
tolerance. To provide the desired therapeutic effects,
the milk should contain 5 108 cfu/mL at the time
of consumption (Tamime and Marshall 1997).
Figure 26.6.Processing scheme for acidophilus milk.