366 Part III: Muscle Foods
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 chro-
matography (HPLC) offers accurate and reliable re-
sults but is not suitable for routine analysis. Since
adenosine nucleotides rapidly break down and disap-
pear within 24 hours postmortem, the K-value can be
simplified as (HxRHx)/(IMPHxRHx).
Karube et al. (1984) called it the K 1 value and report-
ed 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 OFMYOFIBRILLARPROTEINS
During chilled storage of fish and marine inver-
tebrates, it is common to notice textural changes
before bacterial spoilage. It is believed that these
textural changes are caused by the autolytic degra-
dation of myofibrillar proteins. This was demon-
strated 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 pic-
ture of protein degradation.
COLLAGENDEGRADATION
In the tuna canning industry, it was long recognized
that the appearance of honeycombing in precooked
tuna was an index of deterioration or mishandling.
It was demonstrated that the appearance of honey-
combing was related to collagen degradation (Frank
et al. 1984). Mackerel was also shown to develop
honeycombing when poorly handled (Pan et al.
1986). Freshwater prawn developed mushiness rap-
idly due to poor handling after catch. It was also
demonstrated that this mushiness problem was re-
lated to collagen degradation (Nip et al. 1985). It
should be noted that all these problems are observ-
able only after the fish or prawn have been cooked
and are only detected visibly. It is believed that these
problems developed before bacterial spoilage of the
fish. Analysis of collagen degradation is also te-
dious, involving hydrolysis of the extracted collagen
and analysis of hydroxyproline.
DIMETHYLAMINEFORMATION
Trimethylamine oxide (TMAO) is commonly found
in large quantities in marine species of fish, espe-
cially the elasmobranch (Jiang and Lee 2004) and
gadoid (Bonnell 1994) species. After death, TMAO is
readily degraded to dimethylamine (DMA) through
a series of reactions during iced and frozen storage.
DMA is typically observed in frozen gadoid species
such as cod, hake, haddock, whiting, red hake, and
polluck (Castell et al. 1973). TMAO degradation
with DMA formation was enhanced by the presence
of an endogenous enzyme (TMAOase) in the fish
tissues, as observed in cod muscle by Amano and
Yamada (1965). It should be noted that TMAO
degradation can also be bacterial. Readers should
consult the reviews by Regenstein et al. (1982),
Hebard et al. (1982), Hultin (1992b), or other litera-
ture elsewhere.FREEFATTYACIDACCUMULATIONPostmortem lipid degradation in seafood, especially
fatty fish, proceeds mainly due to enzymatic hydrol-
ysis, with the accumulation of free fatty acids.
About 20% of lipids are hydrolyzed during the shelf
life of iced fish. The amount of free fatty acids is
more or less doubled during that period, mostly
from phospholipids, followed by triglycerides, cho-
lesterol esters, and wax esters (Sikorski et al. 1990b,
Haard 1990).TYROSINEACCUMULATIONThe accumulation of lactic acid accompanied with a
drop in pH causes the liberation and activation of
inherent acid cell proteases, cathepsins (Eskin et al.
1971). Tyrosine has been reported to accumulate in
stored fish due to autolysis. Its use as an index of
freshness has been proposed by Shenouda et al.
(1979) because of its simplicity in analysis. However,
it was shown that the pattern of tyrosine accumulation
was similar to that of the total volatile base (TVB), an
indicator of bacterial spoilage. Therefore, its use as a
biochemical index (before bacterial spoilage) is not
sufficiently sensitive and specific enough to assess
total fish quality (Simpson and Haard 1984).BIOCHEMICAL AND
PHYSICOCHEMICAL CHANGES
IN SEAFOOD DURING FREEZING
AND FROZEN STORAGEFreezing is widely used to preserve and maintain the
quality of food products for an extended period of