Food Biochemistry and Food Processing

16 Biochemistry of Seafood Processing 369

1981). In frozen foods, enzymes generally catalyze
lipid hydrolysis. Phospholipase A and lipases from
muscle tissues are responsible for the hydrolysis of
phospholipids and lipids in frozen fish (Shewfelt
1981, De Koning et al. 1987). As the storage time
increases and the frozen storage temperature ele-
vates, free fatty acids from the hydrolysis of lipids
start to accumulate (Dyer and Dingle 1961). Free
fatty acids have a detrimental effect on both the tex-
tural properties and flavor of fish.
Lipid oxidation is considered one of the major
factors that limit the shelf life of frozen seafood.
Fish lipids are known for their susceptibility to oxi-
dation, particularly during frozen storage. Oxidation
of polyunsaturated fatty acids yields various oxi-
dative products including a mixture of aldehydes,
epoxides, and ketones, which give fish a rancid fla-
vor (Gardner 1979). The low flavor threshold of
most aldehydes formed during lipid oxidation means
they are easily perceived by the consumer and there-
fore reduce the acceptability of the products. Rancid
flavor in salmon is caused by the formation of
volatile products such as (E,Z)-2,6-nonadienal (cu-
cumber odor), (Z)-3-hexanal (green odor), and
(Z,Z)-3,6-nonadienal (fatty odor) (Milo and Grosch
1996). The characteristic “seaweed” odor of fresh
fish tissue results from the volatile compounds
formed during rapid degradation of site-specific
hydroperoxides (Josephson and Lindsey 1986). The
content of lipid hydroperoxides and free fatty acids
in salmon increases during storage, and these
changes are fastest when stored at10°C (Refs-
gaard et al. 1998). Cod samples stored for 18 months
at15°C had hepta-trans-2-enal and hepta-trans-
2,cis-dienal. These compounds were described as
cold storage flavor (cardboard, musty) with a very
low flavor threshold (Coggins and Chamul 2004).
Brake and Fennema (1999) found that the rate
decrease for thiobarbituric acid reactive substances
(TBARS) was abrupt below glass transition temper-
ature (Tg’), whereas the rates of decrease for lipid
hydrolysis and peroxide values were moderate to
small, respectively, in frozen minced Mackerel.
They suggested that the TBARS reduction rate is
more diffusion limited than those for lipid hydroly-
sis and peroxide.
Color changes are an indicator of food quality
deterioration. Nonenzymic browning occurs as a
consequence of chemical reactions between peroxi-
dizing lipids in the presence of protein. Fluores-

cence Schiff base adducts formed as a result of

chemical reactions between tetrameric dialdehyde

and amino groups of protein (Haard 1992a,b). Au-

bourg (1998) observed a higher fluorescence ratio in

a formaldehyde and fatty fish model system com-

pared with the model for formaldehyde and lean

fish. In addition to flavor and color changes, lipid

oxidation products, including free fatty acids and

aldehydes, decrease protein solubility and cause

undesirable changes in the functional properties of

proteins (Sikorski et al. 1976), as described in the

previous section.

DEGRADATION OFTRIMETHYLAMINEOXIDE

Trimethylamine oxide (TMAO), a source of for-

maldehyde, is present naturally in many marine ani-

mals as an osmoregulator and as a means of excret-

ing nitrogen (Hebard et al. 1982). After death,

TMAO is readily degraded to dimethylamine (DMA)

and formaldehyde in the presence of the endogenous

enzyme (TMAOase) in the fish tissues (Bremmer

1977, Hebard et al. 1982). It has been postulated that

formaldehyde binds covalently to various functional

groups in proteins and hence results in a deconfor-

mation of the protein, followed by cross-linking

between the protein peptide chains via methylene

bridges (Sikorski et al. 1976). The interaction of

formaldehyde with muscle protein accelerates mus-

cle protein denaturation (Crawford et al. 1979,

Ciarlo et al. 1985, Sotelo et al. 1995). Research

using both mechanical and sensory tests has shown

that formaldehyde formed from the degradation of

TMAO increases the firmness and decreases juici-

ness of mince prepared from white muscle or fillets

of gadoid and nongadoid fish species (Rehbein

1988). The presence of formaldehyde also causes a

noticeable decrease in the extractability of total pro-

teins, particularly the myofibrillar group (Lim and

Haard 1984, Benjakul and Bauer 2000). Ishikawa et

al. (1978) suggested that depletion of TMAO accel-

erates the autoxidation reaction of lipids. Therefore,

it is clear that the degradation of TMAO both

increases the toughness of muscle protein and accel-

erates the oxidation and hydrolysis of lipids.

SUMMARY

During freezing, the formation of ice changes the

order of water molecules in the food environment,