Food Biochemistry and Food Processing (2 edition)

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1 Introduction to Food Biochemistry 13

to PPO for subsequent action on the phenolic ring of Tyr. Phe-

nolics are hydroxylated, thus producing diphenols that are then

subsequently oxidised to quinones:

2 Tyrosine O 2 +BH 2 →2 Dihydroxylphenylamine+O 2

→2 o-Benzoquinone+2H 2 O

The action of PPO can be desirable in various food products,

such as raisins, prunes, dates, cider and tea; however, the extent

of browning needs to be controlled. The use of reducing com-

pounds is the most effective control method for PPO browning.

The most widespread anti-browning treatment used by the food

industry was the addition of sulfiting agents; however, due to

safety concerns (e.g. allergenic-type reactions), other methods

have been developed, including the use of other reducing agents

(ascorbic acid and analogues, Cys, glutathione), chelating agents

(phosphates, EDTA), acidulants (citric acid, phosphoric acid),

enzyme inhibitors, enzyme treatment and complexing agents

(e.g. copolymerisedβ-cyclodextrin or polyvinylpolypyrroli-

done; Sapers et al. 2002). Application of these PPO activity

inhibitors is strictly regulated in different countries (Eskin 1990,

Gopakumar 2000, Kim et al. 2000). Oxidative browning is one

of three types of browning reactions important in food colour, the

other two being non-oxidative/Maillard browning and carameli-

sation (covered above and extensively in food chemistry texts;

see Damodaran et al. 2008).

Enzymatic Texture Modifications

Transglutaminase (TGase, EC 2.3.2.13, protein-glutamine-y-

glutamyltransferase) catalyses acyl transfer between R group

carboxyamides of glutamine residues in proteins, peptides and

various primary amines; theε-amino group of Lys acts as acyl ac-

ceptor, resulting in polymerisation and inter- or intra-molecular

cross-linking of proteins via formation ofε-(-y-glutamyl) Lys

linkages via exchange of the Lysε-amino group for ammonia

at the carboxyamide group of a glutamine residue. Formation

of covalent cross-links between proteins is the basis for TGase-

based modification of food protein physical properties. The pri-

mary applications of TGase in seafood processing have been

for cold restructuring, cold gelation of pastes and gel-strength

enhancement through myosin cross-linking.

Quality Index

Trimethylamine and itsN-oxide have long been used as indices

for freshness in fishery products. Degradation of trimethylamine

and itsN-oxide leads to the formation of ammonia and formalde-

hyde with undesirable odours. The pathway on the production

of formaldehyde and ammonia from trimethylamine and itsN-

oxide is shown in Figure 1.3.

Most live pelagic and scombroid fish (e.g. tunas, sardines and

mackerel) contain an appreciable amount of His in the free state.

In post-mortem scombroid fish, the free His is converted by the

bacterial enzyme His decarboxylase into free histamine. His-

tamine is produced in fish caught 40–50 hours after death when

Trimethylamine

Trimethylamine N-oxide reductase

Trimethylamine dehydrogenase +H

+H^2 O, NADH

2 O, flavoprotein - NAD+

Trimethylamine

N-oxide

Trimethylamineoxidealdolase

Formaldehyde

Dimethylamine

Dimethylamine + H 2 O, FAD

dehydrogenase

• FADH

Methylamine Formaldehyde

Amine

dehydrogenase + H 2 O

• Formaldehyde

Ammonia

Figure 1.3.Degradation of trimethylamine and itsN-oxide.

TrimethylamineN-oxide reductase (EC 1.6.6.9), trimethylamine

dehydrogenase (EC 1.5.8.2), dimethylamine dehydrogenase (EC

1.5.8.1), amine dehydrogenase (EC 1.4.99.3). (From Haard et al.

1982, Gopakumar 2000, Stoleo and Rehbein 2000, IUBMB-NC

website (www.iubmb.org).)

fish are not properly chilled. Improper handling of tuna and

mackerel after harvest can produce enough histamine to cause

food poisoning (called scombroid or histamine poisoning), re-

sulting in facial flushing, rashes, headache and gastrointestinal

disorder. These disorders seem to be strongly influenced by

other related biogenic amines, such as putrescine and cadaver-

ine, produced by similar enzymatic decarboxylation (Table 1.9).

The presence of putrescine and cadaverine is more significant in

shellfish, such as shrimp. The detection and quantification of his-

tamine is fairly simple and inexpensive; however, the detection

and quantification of putrescine and cadaverine are more com-

plicated and expensive. Despite the possibility that histamine

may not be the main cause of poisoning (histamine is not stable

under strong acidic conditions such as the stomach), it is used

as an index of freshness of raw materials due to the simplicity

of histamine analysis (Gopakumar 2000).

Urea is hydrolysed by the enzyme urease (EC 3.5.1.5), pro-

ducing ammonia, which is one of the components measured by

total volatile base (TVB). TVB nitrogen has been used as a qual-

ity index of seafood acceptability by various agencies (Johnson

and Linsay 1986, Cadwallader 2000, Gopakumar 2000). Live

shark contains relatively high amounts of urea, thus under im-

proper handling urea is converted to ammonia, giving shark meat

an ammonia odour, which is a quality defect.