Lactic Acid Fermentation Quiz: Dairy Science and Cell Metabolism

Lactic acid bacteria primarily ferment lactose, the disaccharide sugar naturally found in milk. The bacteria first break lactose into glucose and galactose using the enzyme beta-galactosidase, then ferment these monosaccharides through glycolysis to produce lactic acid as the main end product.

Lactic acid fermentation is an anaerobic process, meaning it occurs in the absence of oxygen. Lactic acid bacteria use glycolysis to break down sugars and regenerate NAD+ through lactic acid production rather than using an electron transport chain that would require oxygen.

Homolactic fermentation, carried out by organisms such as Lactobacillus bulgaricus, produces lactic acid as the sole end product from glucose. This is in contrast to heterolactic fermentation, which produces lactic acid alongside ethanol and carbon dioxide. Homolactic fermentation is most common in yogurt and cheese production.

Yogurt, cheese, and sour cream are all produced through lactic acid fermentation. In each case, lactic acid bacteria convert lactose into lactic acid, which lowers pH, causes casein proteins to coagulate, and creates characteristic flavors and textures. Sweet cream butter is made by churning cream without fermentation.

As lactic acid bacteria produce lactic acid, the pH of milk drops. When pH approaches the isoelectric point of casein, which is approximately 4.6, the negatively charged casein micelles lose their repulsion, causing them to aggregate and form the semisolid curd structure characteristic of products like yogurt and cheese.

Commercial yogurt production relies on a symbiotic relationship between Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These two species stimulate each other's growth in a mutualistic interaction, producing lactic acid along with compounds such as acetaldehyde that give yogurt its characteristic flavor and aroma.

Lactic acid bacteria use glycolysis to break down glucose into pyruvate, yielding ATP and NADH. Since there is no electron transport chain under anaerobic conditions, pyruvate is reduced to lactic acid using NADH, regenerating NAD+ so that glycolysis can continue to produce energy for the bacteria.

Beta-galactosidase, commonly called lactase, hydrolyzes lactose into its component monosaccharides, glucose and galactose. These simpler sugars are then metabolized through glycolysis. The activity of this enzyme is essential because lactic acid bacteria cannot directly ferment the intact disaccharide lactose.

Lactic acid fermentation preserves dairy products by lowering pH, which inhibits spoilage organisms and pathogens. The organic acids and aromatic compounds produced, including lactic acid and acetaldehyde, develop characteristic flavors. Protein content is not removed; rather, proteins are modified through coagulation.

Heterolactic fermentation differs fundamentally from homolactic fermentation in its end products. While homolactic fermentation produces only lactic acid, heterolactic fermentation produces lactic acid, ethanol, and carbon dioxide. This difference arises because heterolactic bacteria use the phosphoketolase pathway rather than standard glycolysis.

During cheese making, once lactic acid bacteria cause milk to curdle, the resulting mass is cut to release whey, which is the liquid portion containing water-soluble proteins, lactose, and minerals. The whey is then drained away from the solid curd, which is further processed into cheese. Whey itself is often processed into whey protein products.

The acidic environment created by lactic acid accumulation inhibits many spoilage organisms and pathogens that cannot tolerate low pH conditions. This natural preservation mechanism is why fermented dairy products such as yogurt and cheese have significantly longer shelf lives than fresh milk under similar storage conditions.

During lactic acid fermentation, pH drops as lactic acid accumulates, casein proteins coagulate when the isoelectric point is reached, and the product develops a characteristic sour or acidic taste from the organic acids produced. Lactose concentration decreases as it is consumed by bacteria as a carbon source.

Mesophilic lactic acid bacteria have an optimal growth temperature range of approximately 20 to 30 degrees Celsius. These bacteria are used in producing cheeses such as cheddar and gouda. In contrast, thermophilic cultures used in yogurt and Swiss cheese production grow optimally at higher temperatures between 40 and 45 degrees Celsius.

Acetaldehyde is the main volatile compound responsible for the distinctive fresh, tangy aroma of yogurt. It is produced during fermentation primarily by Lactobacillus bulgaricus. Diacetyl contributes to the buttery flavor of some fermented dairy products like sour cream and cultured butter, but acetaldehyde defines yogurt aroma.