in Chapter 9. They differ from the other acids and esters described here in
that they are usually present in amounts sufficient to exert an effect on
flavour and on product pH, thus potentiating their own action by
increasing the proportion of undissociated acid present.
Benzoic acidoccurs naturally in cherry bark, cranberries, greengage
plums, tea and anise but is prepared synthetically for food use. Its
antimicrobial activity is principally in the undissociated form and since it
is a relatively strong acid (pKa4.19) it is effective only in acid foods. As a
consequence, its practical use is to inhibit the growth of spoilage yeasts
and moulds. Activity against bacteria has been reported but they show
greater variability in their sensitivity.
Inhibition by benzoic acid appears multifactorial. The ability of the
undissociated molecule to interfere with membrane energetics and func-
tion appears to be of prime importance since growth inhibition has been
shown to parallel closely the inhibition of amino acid uptake in whole
cells and membrane vesicles. Some inhibition may also result from
benzoic acid once it is inside the cell as a number of key enzyme activities
have also been shown to be adversely affected.
Parabens (para-hydroxybenzoic acid esters) differ from the other
organic acids described here in the respect that they are phenols rather
Figure 4.11 Structures of organic acid food preservatives
Chapter 4 101