H H pH C C C Hho

Figure 1. Structure of calcium propionate.

How then, does calcium propionate exerts its anti-fungal activity? Recall that weak organic acids can exist in either the acid (undissociated) or salt (dissociated) form (Figure 2, upper panel). When the pH is equal to the pKa (the dissociation constant) of the acid, the concentration of undissociated and dissociated forms (at equilibrium) will be the same. When the pH is above the pKa, the equilibrium shifts to the salt or dissociated form. Conversely, when the pH is below the pKa, the acid or undissociated form predominates. It is this uncharged, undissoci-ated, lipophilic form of the acid that can diffuse across the cytoplasmic membrane. Of course, once the acid reaches the cytoplasmic environment, where the pH is usually near neutral, it dissociates, in accord with its pKa, forming the anion and releasing a proton. Not only does the organism have to spend valuable energy (in the form of ATP) to expel these protons, but their accumulation in the cytoplasm eventually will result in a decrease in the intracellular pH to a point that is inhibitory to the cell.

276 Microbiology and Technology of Fermented Foods Box 8-3. How Propionates Preserve Bread (Continued)

Figure 2. Mode of action of propionate. The cell membrane is permeable only to the acid form (top panel). The ratio of propionate (A~) to propionic acid (HA) at a typical bread pH of 5.2 can be calculated from the Hender-son-Hasselbalch equation (middle panel). If 0.2% propionate is added to bread, the concentrations (%) of propionate and propionic acid at other pH values can be similarly determined (lower panel).

Figure 2. Mode of action of propionate. The cell membrane is permeable only to the acid form (top panel). The ratio of propionate (A~) to propionic acid (HA) at a typical bread pH of 5.2 can be calculated from the Hender-son-Hasselbalch equation (middle panel). If 0.2% propionate is added to bread, the concentrations (%) of propionate and propionic acid at other pH values can be similarly determined (lower panel).

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