Answer to Question #165554 in Cell Biology for Emmanuella

Question

 you could describe the metabolic process of the bacteria that is used during cheese making, indicating the type of respiration that is happening during the process and the chemical reactions necessary for the formation of lactic acid.

Answer 1

In the past, lactic acid was produced by naturally occurring bacteria. Nowadays they are inoculated artificially, by specially selected bacteria termed starters. Indeed lactic acid formation is necessary in all kinds of cheese. For cheese prepared at temperatures less than 40°C strains of Lactococcus lactis are used. For those prepared at higher temperatures the more thermophilic Streptococcus thermophilus, Lactobacillus bulgaricus, and Lact. helveticus are used.

The lactic acid fermentation is anaerobic process.

The production of lactic acid from lactose and water may be summarized as: C₁₂H₂₂O₁₁ (lactose) + H₂O (water) → 4 CH₃CHOHCOOH (lactic acid)

Lactic acid has the following effects:

(i) It causes the coagulation of casein at pH 4.6, the isoelectric point of that protein, which is used in the manufacture of some cheeses, e.g. cottage cheese.

(ii) It provides a favorably low pH for the action of rennin the enzyme which forms the curd from casein in other types of cheeses.

(iii) The low pH eliminates proteolytic and other undesirable bacteria.

(iv) It causes the curd to shrink and thus promotes the drainage of whey.

(v) Metabolic products from the lactic acid bacteria such as ketones, esters and aldehydes contribute to the flavor of the cheese.