Microbiology of Fermented Food Production ◾ 175
manipulation, strains that grow rapidly, produce desirable characteristics, and that are resistant to
some phages have been developed. Current information on genome sequences of Lactococcus and
Leuconostoc strains and their phages will help develop better strains in the future.
Microbial Problems
Because of too much acetaldehyde production (especially if biovar diacetylactis is used), green
(yogurt flavor) may develop. A slimy texture implies contamination with bacteria that produce
slime (Alcaligenes faecalis) or that starter cultures (some Lac. lactic strains) are slime formers (exopolysaccharides). A yeasty flavor implies contamination with lactose-fermentating yeasts, and a
cheesy flavor alludes to contamination with proteolytic psychrotrophs (during storage). Proteolysis
by proteases of contaminants in starters can also cause development of a bitter flavor, especially
during storage (also see Chapter 20).
Microbiology of Yogurt Fermentation
Characteristics
Plain yogurt has a semisolid mass resulting from the coagulation of milk (skim, low, or full fat)
by starter-culture bacteria. It has a sharp acidic taste with a flavor similar to walnuts and a smooth
mouthfeel. The flavor is a result of the combined effects of acetaldehyde, lactate, diacetyl, and
acetate, but 90% of the flavor is a result of the acetaldehyde.
Many types of yogurt are available in the market, for example, plain yogurt, fruit yogurt, flavored and colored yogurt, blended yogurt, sweetened yogurt, heated yogurt, frozen yogurt, dried
yogurt, low-lactose yogurt, and carbonated yogurt. 3,5
Processing
Yogurt is generally fermented in batches, but a continuous method has also been developed. The
batch process for a low-fat (2%) plain yogurt is as follows:
1. Homogenized milk (12% TS) + stabilizer (1%). The stabilizer is added to give desired gel
structure.
2. Heated to 185°F (85°C) for 30 min, and cooled to 110°F (43.3°C). Heating helps destroy
vegetative microbes and slightly destabilize casein for good gel formation.
3. Starter added, incubated at 110°F (43.3°C) to pH 4.8 for ca. 6 h, acidity ca. 0.9%. Starter
used as either direct vat set (frozen) or bulk culture (2%–3%).
4. Quickly cooled to 85°F (29.4°C) in ca. 30 min to slow down further starter growth and acid
production, especially by Lactobacillus species, agitated, and pumped to filler machine.
5. Packaged in containers and cooled by forced air to 40°F (4.4°C). Final cooling by forced air
results in a rapid drop in temperature to stop the growth of starters.
6. Held for 24 h; pH drops to 4.3.
Starters (Controlled Fermentation)
Frozen concentrates or direct vat set starters can be used. Normally, Lab. delbrueckii ssp. bulgaricus
and Str. thermophilus are used. Some processors also combine these two with other species, such
manipulation, strains that grow rapidly, produce desirable characteristics, and that are resistant to
some phages have been developed. Current information on genome sequences of Lactococcus and
Leuconostoc strains and their phages will help develop better strains in the future.
Microbial Problems
Because of too much acetaldehyde production (especially if biovar diacetylactis is used), green
(yogurt flavor) may develop. A slimy texture implies contamination with bacteria that produce
slime (Alcaligenes faecalis) or that starter cultures (some Lac. lactic strains) are slime formers (exopolysaccharides). A yeasty flavor implies contamination with lactose-fermentating yeasts, and a
cheesy flavor alludes to contamination with proteolytic psychrotrophs (during storage). Proteolysis
by proteases of contaminants in starters can also cause development of a bitter flavor, especially
during storage (also see Chapter 20).
Microbiology of Yogurt Fermentation
Characteristics
Plain yogurt has a semisolid mass resulting from the coagulation of milk (skim, low, or full fat)
by starter-culture bacteria. It has a sharp acidic taste with a flavor similar to walnuts and a smooth
mouthfeel. The flavor is a result of the combined effects of acetaldehyde, lactate, diacetyl, and
acetate, but 90% of the flavor is a result of the acetaldehyde.
Many types of yogurt are available in the market, for example, plain yogurt, fruit yogurt, flavored and colored yogurt, blended yogurt, sweetened yogurt, heated yogurt, frozen yogurt, dried
yogurt, low-lactose yogurt, and carbonated yogurt. 3,5
Processing
Yogurt is generally fermented in batches, but a continuous method has also been developed. The
batch process for a low-fat (2%) plain yogurt is as follows:
1. Homogenized milk (12% TS) + stabilizer (1%). The stabilizer is added to give desired gel
structure.
2. Heated to 185°F (85°C) for 30 min, and cooled to 110°F (43.3°C). Heating helps destroy
vegetative microbes and slightly destabilize casein for good gel formation.
3. Starter added, incubated at 110°F (43.3°C) to pH 4.8 for ca. 6 h, acidity ca. 0.9%. Starter
used as either direct vat set (frozen) or bulk culture (2%–3%).
4. Quickly cooled to 85°F (29.4°C) in ca. 30 min to slow down further starter growth and acid
production, especially by Lactobacillus species, agitated, and pumped to filler machine.
5. Packaged in containers and cooled by forced air to 40°F (4.4°C). Final cooling by forced air
results in a rapid drop in temperature to stop the growth of starters.
6. Held for 24 h; pH drops to 4.3.
Starters (Controlled Fermentation)
Frozen concentrates or direct vat set starters can be used. Normally, Lab. delbrueckii ssp. bulgaricus
and Str. thermophilus are used. Some processors also combine these two with other species, such