Food Biochemistry and Food Processing (2 edition)

BLBS102-c01 BLBS102-Simpson March 21, 2012 11:8 Trim: 276mm X 219mm Printer Name: Yet to Come

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).