Food Biochemistry and Food Processing (2 edition)

BLBS102-c23 BLBS102-Simpson March 21, 2012 13:44 Trim: 276mm X 219mm Printer Name: Yet to Come

23 Dairy Products 429

Table 23.1. Characteristics of Lactic Acid Bacteria Used in Fermented Dairy Products

Temperature for Growth (◦C)

Morphology

Lactose

Fermentation Minimum Optimal Maximum

Lactic Acid

Production^1

Primary

Metabolic

Products

Lactococcus lactisssp.

Lc. lactisssp.lactis Cocci Homofermentative 8–10 28–32 40 ∼0.9 Lactate

Lc. lactisssp.

cremoris

Cocci Homofermentative 8–10 22 37–39 ∼0.9 Lactate

Leuconostoc

Ln. mesenteroides

ssp.cremoris

Cocci Heterofermentative 4–10 20–25 ∼ 37 Trace Lactate, diacetyl,

carbon dioxide

Ln. lactis Cocci Heterofermentative 4–10 20–25 ∼ 37 ∼0.8 Lactate, diacetyl,

carbon dioxide

Streptococcus

S. thermophilus Cocci Homofermentative 20 40 50 ∼0.9 Lactate,

acetaldehyde

Lactobacillus

Lb. delbrueckiissp.

bulgaricus

Rods Homofermentative 22 40–45 52 ∼2.5 Lactate,

acetaldehyde

Lb. helveticus Rods Homofermentative 20–22 42 54 ∼2.5 Lactate

Lb. delbrueckiissp.

lactis

Rods Homofermentative 18 40 50 ∼1.2 Lactate

Lb. acidophilus Rods Homofermentative 20–22 37 45—48 ∼1.0

Sources:Walstra et al. 1999, Stanley 1998.

(^1) Percentage in milk in 24 h at optimum temperature.
the formation of lactic acid from the monosaccharides include
the hydrolysis of the hexose diphosphates to glyceraldehyde-3-
phosphate by aldolases, the formation of pyruvate from phospho-
enolpyruvate by pyruvate kinase, and the reduction of pyruvate to
lactate by lactate dehydrogenase. Heterofermentative lactic acid
bacteria metabolize lactose to produce carbon dioxide, acetic
acid, and ethanol, in addition to lactic acid according to the
reaction
Lactose+2H 3 PO 4 +2ADP→2Lactic Acid+2Ethanol
+2CO 2 +2ATP+H 2 O
The heterofermentative lactic acid bacteria lack al-
dolases, therefore, the sugars are metabolized through
6-phosphogluconate pathway rather than the glycolytic path-
way. In this pathway, glucose-6-phosphate is oxidized to
6-phosphogluconate, which is decarboxylated to a pentose-
5-phosphate and carbon dioxide by phosphoketolase. The
pentose-5-phosphate is converted to glyceraldehyde-3-
phosphate, which enters the glycolytic pathway to form lactic
acid and acetyl-phosphate, which is metabolized to acetalde-
hyde. The acetaldehyde is reduced to ethanol by alcohol
dehydrogenase. However, several bacteria, includingLacto-
coccus lactis ssp. lactisbiovar.diacetylactis, Streptococcus
thermophilus, andLactobacillus delbrueckiissp. bulgaricus
lack the dehydrogenase and accumulate acetaldehyde.
The production of acid by the lactic acid bacteria has a sig-
nificant impact on the safety and quality of the cultured dairy
products. The reduction in pH increases the shelf life and safety
of the cultured dairy products through the inhibition of spoilage
and pathogenic microorganisms. Acid production by these lactic
acid bacteria is critical for the precipitation of the casein pro-
teins in the formation of several cultured dairy products, such as
yogurt, sour cream, and unripened cheeses. These bacteria may
also contribute to the degradation of proteins and lipids through
proteolytic and lipolytic reactions to further develop the unique
texture and flavor characteristics of the cultured dairy products.
These reactions are especially important in ripened cheeses.
The selection of the appropriate type and level of starter cul-
tures is imperative for the development of the appropriate flavor
characteristics.
Metabolism of citrate byLactococcus, Leuconostoc,and
other lactic acid bacteria results in the formation of diacetyl