BLBS102-c19 BLBS102-Simpson March 21, 2012 13:33 Trim: 276mm X 219mm Printer Name: Yet to Come
19 Biochemistry of Seafood Processing 355agent (e.g., TMAO). This greening problem can be effectively
prevented by the use of a reducing agent (e.g., sodium hydro-
sulfite or another legally permitted additive) (Tomlinson 1966,
Koizumi and Matsura 1967, Grosjean et al. 1969).CarotenoidsThe 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
carotenoid pigments. For example, the red color of salmon
is directly related to the price of the product. Carotenoids in
seafoods may be easily oxidized by a lipogenase-like enzyme
(Schwimmer 1981).Melanosis (Melanin Formation)Melanosis (melanin formation) or “blackspot” in shrimp dur-
ing postmortem storage is caused by phenol oxidase and has
economic implications (Simpson et al. 1987). Consumers con-
sider shrimp with blackspot to be defective. This problem can
be overcome by the use of appropriate reducing or inhibiting
agent(s).
A comprehensive review of the biochemistry of color and
color change in seafood was presented by Haard (1992a). Read-
ers should refer to this reference for detailed information on this
subject.BIOCHEMICAL INDICES
Postharvest biochemical events in fish can be classified into
two phases: metabolic (enzymatic) and microbial (Eskin et al.
1971). A discussion of microbial events is not the objective of
this chapter. If interested, please consult references in this chap-
ter or other sources. Abundant literature is available. Physical
and instrumental methods for assessing seafood 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 myofibrillar protein degradation products, DMA
formation, FFA accumulation, and tyrosine. Methodology for
the analysis of these metabolites can be found in government
publications such as Official Methods of Analysis of the Amer-
ican Association of Official Chemists in the United States.Lactic Acid Formation with Lowering of pHIt is well known that, in animals, lactic acid accumulates during
postmortem changes because of glycolytic conversion of storage
glycogen in the muscle after cessation of respiration, and fin-
fish, crustaceans, and shellfish are no exception. The metabolic
pathway of glycogen degradation in animals was presented inChapter 1 of this book. The accumulation of lactic acid can cause
a drop in pH. Both lactic acid and pH can be measured with-
out difficulty using modern instrumentation (Jacober and Rand
1982).Nucleotide CatabolismPostmortem dephosphorylation of nucleotides by autolysis in
fish has been studied for many years as an index of quality.
Nucleotide degradation commences with death and proceeds
at a temperature-dependent rate (Spinelli et al. 1964, Eskin
et al. 1971). Adenosine nucleotides are rapidly deaminated to
IMP and further degraded from inosine to hypoxanthine dur-
ing storage (Jones et al. 1964). There is no hypoxanthine in
freshly caught fish and marine invertebrates. Accumulation of
the metabolite hypoxanthine has attracted considerable attention
for many years as an index of fish freshness. Hypoxanthine con-
tent 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 hypox-
anthine (Hx) to the total amount of ATP and related compounds
(ADP, AMP, IMP, HxR, and Hx) in a fish muscle extract. Many
researchers have demonstrated that K-value is related to fish
freshness. However, its limitation is that it is difficult to analyze
all these nucleotides and their metabolites by simple procedures.
The use of high performance liquid chromatography offers ac-
curate and reliable results but is not suitable for routine analysis.
Since adenosine nucleotides rapidly break down and disappear
within 24 hours postmortem, the K-value can be simplified as
(HxR+Hx)/(IMP+HxR+Hx). Karube et al. (1984) called it
the K 1 -value and reported that this value is strongly correlated
to the K-value proposed by Saito et al. (1959). Like the K-value,
the K 1 -value is species dependent (Huynh et al. 1990).Degradation of Myofibrillar ProteinsDuring chilled storage of fish and marine invertebrates, it is
common to notice textural changes before bacterial spoilage. It
is believed that these textural changes are caused by the autolytic
degradation of myofibrillar proteins. This was demonstrated in
ice-stored freshwater prawn (Kye et al. 1988) and in other finfish
(Shewfelt 1980). Analysis of myofibrillar proteins is tedious and
time-consuming using electrophoresis, but it will give a definite
picture of protein degradation.Collagen DegradationIn the tuna canning industry, it was long recognized that the
appearance of honeycombing in precooked tuna was an index of