Food Biochemistry and Food Processing

1 Food Biochemistry—An Introduction 15

lipids in these seeds are of less importance than the
activity of
-amylase.

BIOGENERATION OFFRESH-FISHODOR

The main enzymes involved in biogeneration of the
aroma in fresh fish have been reported as the 12- and
15-lipoxygenases (Table 1.8) and hydroperoxide
lyase. The 12-lipoxygenase acts on specific polyun-
saturated fatty acids and produces n-9-hydroperox-
ides. Hydrolysis of the 9-hydroperoxide of eicos-
apentenoic acid by specific hydroperoxide lyases
leads to the formation of mainly (Z,Z)-3-6-nonadi-
enal, which can undergo spontaneous or enzyme-
catalyzed isomerization to (E,Z)-2,6-nonadienal.
These aldehydes may undergo reduction to their
corresponding alcohols. This conversion is a signifi-
cant step in the general decline of the aroma intensi-
ty due to the fact that alcohols have somewhat higher
odor detection thresholds than the aldehydes (John-
son and Linsay 1986, German et al. 1992).

BIOCHEMICALLY INDUCED
FOOD FLAVORS

Many fruits and vegetables produce flavors that are
significant in their acceptance and handling. There
are a few well-known examples (Table 1.11). Garlic
is well known for its pungent odor due to the enzy-
matic breakdown of its alliin to the thiosulfonate
allicin, with the characteristic garlic odor. Straw-
berries have a very typical pleasant odor when they
ripen. Biochemical production of the key compound

responsible for strawberry flavor [2,5-dimethyl-4-

hydroxy-2H-furan-3-one (DMHF)] is now known. It

is the result of hydrolysis of terminal nonreducing

-D-glucose residues from DMHF-glucoside with

release of -D-glucose and DMHF. Lemon and

orange seeds contain limonin, a bitter substance that

can be hydrolyzed to limonate, which creates a less

bitter taste sensation. Many cruciferous vegetables

such as cabbage and broccoli have a sulfurous odor

due to the production of a thiol after enzymatic

hydrolysis of its glucoside. These are just some

examples of biochemically induced fruit and veg-

etable flavors. Brewed tea darkens after it is exposed

to air due to enzymatic oxidation. Flavors from

cheese fermentation and fresh-fish odor have al-

ready been described earlier. Formation of fishy

odor will be described later (see below). Readers

interested in this subject should consult Wong

(1989) for earlier findings of chemical reactions.

Huang’s review on biosynthesis of natural aroma

compounds derived from amino acids, carbohy-

drates, and lipids should also be consulted (2005).

BIOCHEMICAL DEGRADATION

AND BIOSYNTHESIS OF PLANT

PIGMENTS

DEGRADATION OFCHLOROPHYL INFRUIT

MATURATION

Green fruits are rich in chlorophylls that are gradu-

ally degraded during ripening. Table 1.12 shows

Table 1.11.Selected Enzyme-Induced Flavor Reactions

Enzyme Reaction
Alliin lyase (EC 4.4.1.4), (garlic, onion) An S-alkyl-L-cysteine S-oxide →an alkyl
sufenate
2-aminoacrylate
-glucosidase (EC 3.2.1.21) (strawberry) Hydrolysis of terminal nonreducing -D-glucose
residues with release of -D-glucose
[2,5-Dimethyl-4-hydroxy-2H-furan-3-one
(DMHF)-glucoside →DMHF]
Catechol oxidase (EC 1.10.3.1), (tea) 2 Catechol
O 2 →2 1,2-benzoquinone
2 H 2 O
Limonin-D-ring-lactonase (EC 3.1.1.36) (lemon Limonoate-D-ring-lactone
H 2 O →limonate
and orange seeds)
Thioglucosidase (EC 3.2.1.147) (cruciferous A thioglucoside
H 2 O →A thiol
a sugar
vegetables)
Sources:Wong 1989, Eskin 1990, Chin and Lindsay 1994, Orruno et al. 2001, IUBMB-NC website (www.iubmb.org).