Food Biochemistry and Food Processing (2 edition)

BLBS102-c19 BLBS102-Simpson March 21, 2012 13:33 Trim: 276mm X 219mm Printer Name: Yet to Come

19 Biochemistry of Seafood Processing 355

agent (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).

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

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 pH

It 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 in

Chapter 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 Catabolism

Postmortem 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 Proteins

During 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 Degradation

In the tuna canning industry, it was long recognized that the

appearance of honeycombing in precooked tuna was an index of