Food Chemistry

984 22 Spices, Salt and Vinegar

provide or enhance the sour, acidic taste of food
(cf. 8.12.5).

22.3.1 Production

Vinegar is produced microbiologically from
ethanol or by dilution of acetic acid.

CH 3 CH 2 OH+O 2

−→ CH 3 COOH+H 2 O+494 kJ (22.11)

22.3.1.1 Microbiological Production

Acetobacterspecies are cultivated in aqueous
ethanol solution or, to a lesser extent, in wine,
fermented apple juice, malt mash or fermented
whey. Ethanol, as shown in Fig. 22.2, is dehy-
drogenated stepwise to acetic acid; the resulting
reduced form of the cosubstrate methoxatin
(PQQH 2 ) is oxidized via the respiratory chain.
Part of the energy formed by oxidation is
released as heat which has to be removed by
cooling during the processing of vinegar. If
there is an insufficient supply of oxygen, the
microorganisms disproportionate a proportion
of the acetaldehyde, the intermediate compound
(cf. Fig. 22.2) in this aerobic reaction pathway:

2CH 3 CHO−→ CH 3 COOH+CH 3 CH 2 OH
(22.12)

Fig. 22.2.Oxidation of ethanol to acetic acid byAceto-
bacterspecies (according toRehm, 1980)

Fermentation of ethanol is conducted as a top fer-

mentation and increasingly as a submerged oxi-

dative process. In top fermentation the bacteria

are cultivated on spongy, porous laminated car-

riers (usually beechwood shavings) with the alco-

holic solution trickling down over carrier surfaces

while a plentiful supply of air is provided from

below. The fermentation is stopped at a 0.3% by

volume residual ethanol level to avoid overoxi-

dation, i. e., oxidation of acetic acid to CO 2 and

water.

22.3.1.2 Chemical Synthesis

Acetic acid is usually synthesized by catalytic

oxidation of acetaldehyde:

CH 3 CHO+^12 O 2

cat.

−−→CH 3 COOH (22.13)

Acetaldehyde is obtained by the catalytic hydra-

tion of acetylene or by the catalytic dehydrogena-

tion of ethanol. Formic acid and formaldehyde

are by-products of acetic acid synthesis. They are

removed by distillation. Chemically pure acetic

acid is diluted with water to 60–80% by vol-

ume to obtain the vinegar essence. The essence

is a strongly corrosive liquid and is sold with spe-

cial precautions. It is diluted further with water

for production of food grade vinegar.

22.3.2 Composition

There are 5–15.5 g acetic acid in 100 g of vine-

gar. The blending (or adulteration) of fermented

vinegar with synthetic acid can be detected

by mass spectrometric determination of the

(^13) C/ (^12) C-isotope ratio (cf. 18.4.3); fermented
vinegar has 5‰ more^13 C isotope than acetic
acid synthesized petrochemically. In addition
fermented vinegar can be distinguished from
synthetic vinegar by analyzing the accompanying
compounds. With this method fermented vinegars
of different origin can also be distinguished from
each other; e. g. spirit vinegar (fermented from
aqueous ethanol) from wine, apple, malt and/or
whey vinegar. The fermented vinegars contain
metabolic by-products of Acetobacter strains,
such as amino acids, 2,3-butylene glycol and