Organic acids mode of action

The principal mode of action of organic acids is the release of protons from carboxylic groups, thus lowering the pH of their environment. Proton release means dissociation of the acid molecule, with the degree of dissociation depending on the pKa value. Strong acids tend to be fully dissociated and thus produce a strong drop in pH, while weak acids are only partially dissociated. This has two important consequences: (a) organic acids with two or three carboxylic groups react on changes of pH by changes in protonation of their carboxylic groups, i.e., they may 'buffer', and (b), in their undissociated state, some of these acids can pass through lipophilic cell membranes. Consequently, on a molar basis, the antibacterial efficacy of weak acids is stronger than that of strong acids. Table 5.3 reviews properties of selected organic acids. Kation effects are reported also. Protons may change the conformation of polar membrane proteins. Binding metal ions (Fe3+, Cu++, Mn++, Ca++, Mg++) by chelation is relevant only for di- and tricarboxylic acids, especially citric acid (Stratford, 1999).

Organic acids will readily lower the 'environmental' pH, and also lower the growth limits of meatborne microbes (see above), but only temporarily. Woolthuis et al. (1984), having immersed liver in 0.2M lactic acid and effecting an immediate pH drop of 0.6 units, saw the original pH restored after 5 h. Van der Marel et al. (1988) observed no pH drop of broiler carcasses treated with 2% lactic acid. Apparently, this effect is dependent on the substrate also. Hence Rohrbacher (2001) showed that after having sprayed acetic acid (0.3M) and lactic acid (0.2M), onto pork cuts (M. longissimus), these acids are nearly completely buffered within 3 h, whereas on pig skin the pH was significantly lowered (2 units) for as long as 30 h.

The effect on interior cell structures is described by the 'weak acid theory'. The principal idea is that organic acids may pass cell membranes in their undissociated form and in cytoplasm with ca. neutral pH, readily dissociate and

Table 5.3 Properties of some organic acids used as additives (after Smulders, 1987; Stratford, 1999, Saltmarsh, 2000; Naidu, 2000)

Acid

COOH*

pKa

acid.**

taste

'weak acid'***

spec. effect****

Acetic

1

4.74

1.4

++

yes

Membrane diffusion

Lactic

1

3.66

2.5

yes

Membrane diffusion

Water activity reduction

'Specific anion effect'

Malic

2

4.7/3.3

2.7

+

no

(Chelation)

Tartric

2

3.9/2.8

2.9

+

no

(Chelation)

Citric

3

5.7/4.3/2.9

2.6

++

no

Chelation

* Number of carboxylic groups; ** pH fall of a 1% bacteriological peptone solution (pH 6.2), when acids (0.5%, compared on a weight basis, Stratford, 1999) are added; this represents the strength of the acidulant effect on 'environmental pH'; *** Lipophilic character; ability to depress intracellular pH; **** Effects other than acidulant; ~/+/ ++ ... no/weak/marked generic taste.

* Number of carboxylic groups; ** pH fall of a 1% bacteriological peptone solution (pH 6.2), when acids (0.5%, compared on a weight basis, Stratford, 1999) are added; this represents the strength of the acidulant effect on 'environmental pH'; *** Lipophilic character; ability to depress intracellular pH; **** Effects other than acidulant; ~/+/ ++ ... no/weak/marked generic taste.

thus lower interior pH. This has a direct effect on proteins and nucleic acids (Smulders, 1987; Stratford, 1999), overloads membrane located 'pumps' trying to remove excess protons, thus consuming ATP and effecting energy depletion. This mechanism assumes acid molecules with lipophilic character, as is the case for acetic and to a lesser extent, lactic acid. The extracellular pH has to be near to the pKa of the acid. Values of pH < 5.5 have been found sufficient (Corry and Mead, 1996), ensuring that at least ~10% of the lactic or acetic acid molecules are undissociated, and therefore, diffusible (see Table 5.3; Stratford, 1999; Bogaert and Naidu, 2000; Marshal et al., 2000, Sharma, 2000).

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