Food Biochemistry and Food Processing

16 Biochemistry of Seafood Processing 365

ducing agent (e.g., sodium hydrosulfite or another
legally permitted additive) (Tomlinson 1966, Koi-
zumi and Matsura 1967, Grosjean et al. 1969).

CAROTENOIDS

The carotenoids contribute to the attractive yellow,
orange, and red color of several important fish
and shellfish products. The more expensive seafoods
such as lobster, shrimp, salmon, and red snapper
have orange-red integument and/or flesh from caro-
tenoid pigments. For example, the red color of sal-
mon is directly related to the price of the product.
Carotenoids in seafoods may be easily oxidized by a
lipogenase-like enzyme (Schwimmer 1981).

MELANINOSIS(MELANINFORMATION)

Melaninosis (melanin formation) or “blackspot” in
shrimp during postmortem storage is caused by phe-
nol oxidase and has economic implications (Simp-
son et al. 1987). Consumers consider shrimp with
blackspot to be defective. This problem can be over-
come by the use of appropriate reducing or inhibit-
ing agent(s).
A comprehensive review of the biochemistry of
color and color change in seafood was presented by
Haard (1992a). Readers should refer to this refer-
ence for detailed information on this subject.

BIOCHEMICAL INDICES

Postharvest biochemical events in fish can be classi-
fied into two phases: metabolic (enzymatic) and
microbial (Eskin et al. 1971). A discussion of micro-
bial events is not the objective of this chapter. If
interested, please consult references in this chapter
or other sources. Abundant literature is available.
Physical and instrumental methods for assessing sea-
food quality were reviewed by Sorenson (1992).
The metabolic (enzymatic) changes result from
the activity of enzymes remaining in the fish flesh
after death. Metabolites from these enzymatic
changes can be used as indices of freshness and can
be monitored by biochemical or chemical methods.
The major metabolites coming from the actions of
inherent enzymes in the fish themselves are lactic
acid, nucleotide catabolites, collagen and myofibril-
lar protein degradation products, dimethylamine for-
mation, free fatty acid accumulation, and tyrosine.
Methodology for the analysis of these metabolites

can be found in government publications such as

Official Methods of Analysis of the American As-

sociation of Official Chemists in the United States.

LACTICACIDFORMATION WITHLOWERING

OF PH

It is well known that, in animals, lactic acid accumu-

lates during postmortem changes because of gly-

colytic conversion of storage glycogen in the muscle

after cessation of respiration, and finfish, crusta-

ceans, and shellfish are no exception. The metabolic

pathway of glycogen degradation in animals was

presented in Chapter 1, Food Biochemistry—An

Introduction. The accumulation of lactic acid can

cause a drop in pH. Both lactic acid and pH can be

measured without difficulty using modern instru-

mentation (Jacober and Rand 1982).

NUCLEOTIDECATABOLISM

Postmortem dephosphorylation of nucleotides by

autolysis in fish has been studied for many years

as an index of quality. Nucleotide degradation com-

mences with death and proceeds at a temperature-

dependent rate (Spinelli et al. 1964, Eskin et al.

1971). Adenosine nucleotides are rapidly deaminat-

ed to inosine monophosphate (IMP) and further de-

graded from inosine to hypoxanthine during storage

(Jones et al. 1964). There is no hypoxanthine in

freshly caught fish and marine invertebrates. Ac-

cumulation of the metabolite hypoxanthine has at-

tracted considerable attention for many years as an

index of fish freshness. Hypoxanthine content can

be determined by applying the colorimetric xanthine

oxidase (EC 1.2.3.2) test. The rate of postmortem

degradation of adenosine nucleotides in marine fish

and invertebrates differs with the species and muscle

type. For this reason, hypoxanthine analysis is of

limited use for the evaluation of quality in certain

species. It is most useful for the analysis of pelagics,

redfish, salmon, and squid but is of little value for

the estimation of lean fish quality (Gill 1992).

Because hypoxanthine is a metabolite fairly close

to the end of the nucleotide degradation, Saito et al.

(1959) proposed the use of K-value, defined as the

ratio of inosine (HxR) plus hypoxanthine (Hx) to the

total amount of adenosine triphosphate (ATP) and

related compounds (ADP, AMP, IMP, HxR, and Hx)

in a fish muscle extract. Many researchers have de-

monstrated that K-value is related to fish freshness.