Food Chemistry

8.12 Antimicrobial Agents 451

Fig. 8.12.Growth inhibition ofAspergillus nigerby
benzoic acid (•), p-hydroxybenzoic acid propyl es-
ter (◦) and sorbic acid ( )

Unlike benzoic acid, the esters can be used over
a wide pH range since their activity is almost
independent of pH (cf. Fig. 8.12). As additives,
they are applied at 0.3–0.06% as aqueous alkali
solutions or as ethanol or propylene glycol so-
lutions in fillings for baked goods, fruit juices,
marmalades, syrups, preserves, olives and pick-
led sour vegetables.

8.12.3 Sorbic Acid

The antimycotic effect of straight chain car-

boxylic acids has long been known. In particular

the unsaturated acids, for example crotonic acid

and its homologues, are very active. Sorbic acid

(2-trans, 4-trans-hexadienoic acid; pK= 4 .76)

has the advantage that it is odorless and tasteless

at the levels of use (0.3% or less). The acid is

obtained by several syntheses:

• From parasorbic acid[(S)-2-hexen-5-olide);
  cf. Reaction 8.27]. The acid is present in
  berries of the mountain ash tree (Sorbus
  aucuparia).

(8.27)

• From ethanal:

(8.28)

• From crotonaldehyde obtained from ethanal
  (cf. Reaction 8.29).

(8.29)

The third synthesis is the most important.