Z

cell wall membrane

Figure 2-18. Proteolytic system in lactococci. Milk casein is hydrolyzed by a cell envelope-associated proteinase (PrtP) to form oligopeptides. These oligopeptides are then transported across the membrane by the oligopeptide transport system (Opp). The intracellular oligopeptides are then hydrolyzed by cytoplasmic peptidases (e.g., PepA, PepC, PepN, and PepX) to form amino acids. Extracellular di- and tripeptides and free amino acids in milk are transported by di- and tripeptide transporters (DtpT, DtpP) and amino acid (AA) transporters. The intracellular di- and tripeptides are then hydrolyzed to amino acids. Adapted from Hutkins, 2001.

Other Metabolic Systems

Although sugar metabolism, and the lactic and ethanolic fermentations in particular, form the basis for the manufacture of most fermented foods and beverages, other metabolic activities are also important.As noted above, protein metabolism serves the needs of the cells and provides flavor attributes in cheese. In other dairy products, such as buttermilk and sour cream, lactic acid bacteria perform fermentations that yield diacetyl, a compound that imparts the characteristic flavor of these products. In Swiss-type cheeses, the propionic acid fermentation results in formation of several metabolic end-products that likewise give these cheese their unique features. Finally, while fungi do not ferment substrates in the classical sense (see above), they nonetheless convert proteins and fats into an array of products with strong aromatic and flavor properties.

The citrate fermentation

Several species of lactic acid bacteria can ferment citrate and produce the flavor compound diacetyl. Initially, citrate is transported by a pH-dependent, PMF-mediated citrate permease (CitP).The intracellular citrate is split by citrate lyase to form acetate and oxaloacetate (Figure 2-19).The acetate is released directly into the medium, whereas the oxaloacetate is decar-boxylated to form pyruvate and CO2.The pyru-vate concentration, however, may increase beyond the reducing capacity of lactate dehy-drogenase, causing a glycolytic bottleneck due a shortage of NADH. Therefore, the excess pyru-vate is removed by a decarboxylation reaction catalyzed by thiamine pyrophosphate (TPP)-de-pendent pyruvate decarboxylase. The product, acetaldehyde-TPP, then condenses with another molecule of pyruvate, via a-acetolactate synthase, forming a-acetolactate.The latter is unsta ble in the presence of oxygen and is non-enzymatically decarboxylated to form diacetyl.

Because the citrate-to-diacetyl pathway does not generate ATP, it would appear to hold no metabolic advantage for the organism. In fact, it costs the cell energy (supplied by the PMF) to transport citrate. Despite these observations, it is now clear that the cell does derive energy during the citrate fermentation (Box 24). First, during the intracellular oxaloacetate decarboxylation reaction, a cytoplasmic proton is consumed, causing the cytoplasmic pH and the ApH component of the PMF to increase. Second, when both citrate and a fermentable sugar are present, the efflux of lactate can drive uptake of citrate.Moreover,since lactate is monovalent and citrate (at physiological pH) is divalent, the citrate permease acts as an electrogenic precursor-product exchanger (i.e., making the inside more negative and the outside more positive), resulting in a net increase in the A^ or electrical component of the PMF. Collectively, therefore, citrate fermentation results in an increase in the metabolic energy available to the cell.

The propionic acid fermentation

The propionic acid fermentation is performed by P. freudenreichii subsp. shermanii and related species. In fermented foods, the relevance of this pathway is limited to the manufacture of citrate acetate 4^|citrate lyase oxaloacetate

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