Food Biochemistry and Food Processing (2 edition)

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1 Introduction to Food Biochemistry 19

Table 1.16.Selected Enzyme-induced Flavour Reactions

Enzyme Reaction

Alliin lyase (EC 4.4.1.4; garlic, onion) AnS-alkyl-l-cysteineS-oxide→An alkyl sufenate+
2-Aminoacrylate
β-Glucosidase (EC 3.2.1.21; strawberry) Hydrolysis of terminal non-reducingβ-d-glucose residues with
release ofβ-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+2H 2 O
Limonin-d-ring-lactonase (EC 3.1.1.36; lemon and
orange seeds)

Limonoate-d-ring-lactone+H 2 O→Limonate

Thioglucosidase (EC 3.2.1.147; cruciferous vegetables) A thioglucoside+H 2 O→A thiol+Asugar

Source: Wong 1989, Eskin 1990, Chin and Lindsay 1994, Orruno et al. 2001, IUBMB-NC website (www.iubmb.org).

after it is exposed to air due to enzymatic oxidation (discussed
earlier). Flavours from cheese fermentation and fresh-fish odour
have already been described earlier, and formation of fishy odour
will be described later.
Colour is an intrinsic property of foods, and therefore, a
change in colour is often caused by a change in quality. Vi-
sion is the most important sensory perception in selecting food
and appreciating its quality (Diehl 2008). Chlorophylls are the
most abundant natural pigments and are responsible for the
green colour of plants (Marquez Ursula and Sinnecker 2008) and
are the biomolecules responsible for capturing light energy in
its transformation into chemical energy during photosynthesis.
Light is able to be absorbed very efficiently due to the presence
of many conjugated double bonds within the large, multi-ring
chlorophyll structure. Disappearance of chlorophyll during fruit
ripening and leaf senescence indicates slowing of photosynthe-
sis. The loss of green colour is due to a loss of chlorophyll
structure via two main stages: First, various reactions produce
greenish chlorophyll derivatives and second, oxidative reactions
result in opening of ring structures, thereby causing colourless
products (Diehl 2008).
Carotenoids are the most widely distributed group of pig-
ments, and although they are not produced by the human body,

they are essential to human health. Vitamin A/β-carotene are
carotenoids critical to a healthy diet. Additionally, carotenoids
may help reduce the risk of cancer and heart disease (Bertram
1999), are important natural colourants in foods and are used
as sources of red, yellow and orange food colouring (Otles and
Cagindi 2008). Carotenoids are fat-soluble pigments that pro-
vide the colour for many common fruits such as yellow peaches,
papayas and mangoes. During post-harvest maturation, these
fruits show intense yellow to yellowish orange colours due to
synthesis of carotenoids from its precursor isopentyl diphos-
phate, which is derived from (R)-mevalonate. Isopentyl diphos-
phate is a key building block for carotenoids (Croteau et al.
2000), and Table 1.17 lists the sequence of reactions in the
formation of (R)-mevalonate from acetyl-CoA and from (R)-
mevalonate to isopentyl diphosphate.
Flavonoids are not only a group of compounds responsible for
various red, blue or violet colours of fruits and vegetables, they
are also related to the group of bioactive, anti-plant pathogen
compounds called stilbenes. Stilbenes have a common precur-
sor oftrans-cinnamatebranching out into two routes, one that
leads to flavonoids and the other leading to stilbenes (Table
1.18). Table 1.18 gives the series of reactions in the biosyn-
thesis of naringenin chalcone, the building block for flavonoid

Table 1.17.Mevalonate and Isopentyl Diphosphate Biosyntheses

Enzyme Reaction

Acetyl-CoAC-acetyltransferase (EC 2.3.1.9) 2 Acetyl-CoA→Acetoacetyl-Co-A+CoA
Hydroxymethylglutaryl-CoA-synthase (EC 2.3.3.10) Acetoacetyl-CoA+Acetyl-CoA+H 2 O→
(S)-3-Hydroxy-3-methylglutaryl CoA+CoA
Hydroxymethylglutaryl-CoA reductase (EC 1.1.1.34) (S)-3-Hydroxy-3-methylglutaryl-CoA+2 NADPH 2 →
(R)-Mevalonate+CoA+2 NADP
Mevaldate reductase (EC 1.1.1.32) (R)-Mevalonate+NAD→Mevaldate+NADH 2
Mevalonate kinase (EC 2.7.1.36) (R)-Mevalonate+ATP→(R)-5-Phosphomevalonate+ADP
Phosphomevalonate kinase (EC 2.7.4.2) (R)-5-Phosphomevalonate+ATP→
(R)-5-Diphosphomevalonate+ADP
Diphosphomevalonate decarboxylase (EC 4.1.1.33) (R)-5-Diphosphomevalonate+ATP→Isopentyl
diphosphate+ADP+Pi+CO2

Source: Croteau et al. 2000, IUBMB-NC Enzyme website (www.iubmb.org).
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