176 ◾ Fundamental Food Microbiology
as Lab. acidophilus and Bifidobacterium spp., Lab. rhamnosus, or Lab. casei. However, in general,
they do not compete well in growth with the two yogurt starters. Therefore, they are added in
high numbers after fermentation and before packaging. They may not survive well when present
in yogurt with the regular yogurt starter cultures.
For a good product, the two starter species should be added at a Streptococcus:Lactobacillus
cell ratio of 1:1; in the final product, the ratio should not exceed 3:2. However, Lactobacillus
cells are more susceptible to freezing and freeze-drying. In a frozen concentrate starter for use
as DVS, the survivors may not be present in a desired ratio unless they are properly preserved
(see Chapter 14).
Growth
For balanced growth of the two species, the fermentation is conducted at approximately 110°F
(43.3°C). At this temperature, both acid and flavor compounds are produced at the desired level.
If the temperature is raised above 110°F, the Lactobacillus sp. predominates, causing more acid and
less flavor production; at temperatures below 110°F, growth of Streptococcus sp. is favored, forming
a product containing less acid and more flavor.
The two species show symbiotic growth while growing together in milk. Initially, Streptococcus
sp. grows rapidly in the presence of dissolved oxygen and produces formic acid and CO 2 . The
anaerobic condition, formic acid, and CO 2 stimulate growth of Lactobacillus sp., which has good
exoproteinase and peptidase systems and produces peptides and amino acids from milk proteins
(outside the cells) in the milk. Some of the amino acids, such as glycine, valine, histidine, leucine, and methionine, are necessary for good growth of the Streptococcus sp., which lacks proteinase enzymes. Streptococcus sp. gets these from the milk and grows rapidly until the pH drops to
approximately 5.5, at which time the growth of Streptococcus sp. slows down. However, growth
of Lactobacillus sp. continues fairly rapidly until the temperature is reduced to 85°F (29.4°C), following a drop in pH to 4.8. At 85°F, both grow slowly, but Streptococcus sp. has the edge. At 40°F
(4.4°C) and a pH of approximately 4.3, both species stop growing.
The two species also have a synergistic effect on growth rate, rate of acid production, and
amounts of acetaldehyde formation when growing together as compared with when growing
individually. The species growing separately in milk produce approximately 8–10 ppm acetaldehyde; when grown together, acetaldehyde production increases to a desirable level of 25 ppm
or higher.
Biochemistry
Lactose Metabolism
Both species have a constitutive β-galactosidase system, and lactose (transported by permease
systems) is hydrolyzed to glucose and galactose (Figure 15.2). Both species are homofermentative
and produce lactate from glucose by the EMP pathway. Lab. delbrueckii ssp. bulgaricus strains have
enzymes for the Leloir pathway to metabolize galactose, but while actively metabolizing glucose
they do not utilize galactose well. Most Str. thermophilus strains do not have the enzymes of the
Leloir pathway (or have a very weak system) and thus do not metabolize galactose. As a result,
galactose is excreted outside, causing its accumulation in yogurt.
as Lab. acidophilus and Bifidobacterium spp., Lab. rhamnosus, or Lab. casei. However, in general,
they do not compete well in growth with the two yogurt starters. Therefore, they are added in
high numbers after fermentation and before packaging. They may not survive well when present
in yogurt with the regular yogurt starter cultures.
For a good product, the two starter species should be added at a Streptococcus:Lactobacillus
cell ratio of 1:1; in the final product, the ratio should not exceed 3:2. However, Lactobacillus
cells are more susceptible to freezing and freeze-drying. In a frozen concentrate starter for use
as DVS, the survivors may not be present in a desired ratio unless they are properly preserved
(see Chapter 14).
Growth
For balanced growth of the two species, the fermentation is conducted at approximately 110°F
(43.3°C). At this temperature, both acid and flavor compounds are produced at the desired level.
If the temperature is raised above 110°F, the Lactobacillus sp. predominates, causing more acid and
less flavor production; at temperatures below 110°F, growth of Streptococcus sp. is favored, forming
a product containing less acid and more flavor.
The two species show symbiotic growth while growing together in milk. Initially, Streptococcus
sp. grows rapidly in the presence of dissolved oxygen and produces formic acid and CO 2 . The
anaerobic condition, formic acid, and CO 2 stimulate growth of Lactobacillus sp., which has good
exoproteinase and peptidase systems and produces peptides and amino acids from milk proteins
(outside the cells) in the milk. Some of the amino acids, such as glycine, valine, histidine, leucine, and methionine, are necessary for good growth of the Streptococcus sp., which lacks proteinase enzymes. Streptococcus sp. gets these from the milk and grows rapidly until the pH drops to
approximately 5.5, at which time the growth of Streptococcus sp. slows down. However, growth
of Lactobacillus sp. continues fairly rapidly until the temperature is reduced to 85°F (29.4°C), following a drop in pH to 4.8. At 85°F, both grow slowly, but Streptococcus sp. has the edge. At 40°F
(4.4°C) and a pH of approximately 4.3, both species stop growing.
The two species also have a synergistic effect on growth rate, rate of acid production, and
amounts of acetaldehyde formation when growing together as compared with when growing
individually. The species growing separately in milk produce approximately 8–10 ppm acetaldehyde; when grown together, acetaldehyde production increases to a desirable level of 25 ppm
or higher.
Biochemistry
Lactose Metabolism
Both species have a constitutive β-galactosidase system, and lactose (transported by permease
systems) is hydrolyzed to glucose and galactose (Figure 15.2). Both species are homofermentative
and produce lactate from glucose by the EMP pathway. Lab. delbrueckii ssp. bulgaricus strains have
enzymes for the Leloir pathway to metabolize galactose, but while actively metabolizing glucose
they do not utilize galactose well. Most Str. thermophilus strains do not have the enzymes of the
Leloir pathway (or have a very weak system) and thus do not metabolize galactose. As a result,
galactose is excreted outside, causing its accumulation in yogurt.