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

358 Part 3: Meat, Poultry and Seafoods

suggested that formaldehyde might interact with protein side

chains and form aggregates without causing cross-linking.

Lipid Oxidation and Hydrolysis

Lipids degrade by two mechanisms: hydrolysis (lipolysis) and

oxidation (Shenouda 1980, Shewfelt 1981). In frozen foods, en-

zymes 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 elevates, FFAs from the hydrolysis of lipids

start to accumulate (Dyer and Dingle 1961). FFAs have a detri-

mental effect on both the textural 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 oxidation, particularly during frozen

storage. Oxidation of PUFAs yields various oxidative products

including a mixture of aldehydes, epoxides, and ketones, which

give fish a rancid flavor (Gardner 1979). The low flavor threshold

of most aldehydes formed during lipid oxidation means they are

easily perceived by the consumer and, therefore, reduce the ac-

ceptability of the products. Rancid flavor in salmon is caused by

the formation of volatile products such as (E,Z)-2,6-nonadienal

(cucumber odor), (Z)-3-hexanal (green odor), and (Z,Z)-3,6-

nonadienal (fatty odor) (Milo and Grosch 1996). The character-

istic “seaweed” odor of fresh fish tissue results from the volatile

compounds formed during rapid degradation of site-specific hy-

droperoxides (Josephson and Lindsey 1986). The content of

lipid hydroperoxides and FFAs in salmon increases during stor-

age, and these changes are fastest when stored at− 10 ◦C (Refs-

gaard et al. 1998). Cod samples stored for 18 months at− 15 ◦C

had hepta-trans-2-enal and hepta-trans-2,cis-dienal. These com-

pounds 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 of decrease for

thiobarbituric acid reactive substances (TBARS) was abrupt be-

low glass transition temperature (Tg′), whereas the rates of de-

crease 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 hydrolysis and peroxide.

Color changes are an indicator of food quality deterioration.

Nonenzymic browning occurs as a consequence of chemical re-

actions between peroxidizing lipids in the presence of protein.

Fluorescence Schiff base adducts formed as a result of chemi-

cal reactions between tetrameric dialdehyde and amino groups

of protein (Haard 1992a, 1992b). Aubourg (1998) observed a

higher fluorescence ratio in a formaldehyde and fatty fish model

system compared with the model for formaldehyde and lean fish.

In addition to flavor and color changes, lipid oxidation products,

including FFAs and aldehydes, decrease protein solubility and

cause undesirable changes in the functional properties of pro-

teins (Sikorski et al. 1976).

Aquatic species are especially rich in long-chain PUFAs and

are more susceptible to lipid oxidation. As a result, various an-

tioxidants have been experimented with by adding antioxidants

to seafood during processing, thus minimizing or delaying the

oxidation process and prolonging shelf life of seafood. Natural

antioxidants seem to be the preferred types for recent research

studies. Rosemary extract added to sea salmon mince reduced

lipid oxidation during storage up to 6 months (Tironi et al. 2010)

and garlic and grape seed extract have also shown to have a lipid

oxidation protective effect (Yerlikaya et al. 2010). However,

grape seed extract did not score well in sensory aspects for ap-

pearance, odor, and taste. Coating of fish with chitosan film has

been demonstrated to be successful in effectively inhibiting lipid

oxidation during frozen storage. Examples are the coating of sil-

ver carp (Fan et al. 2009) and lingcod fillet (Duan et al. 2010)

with chitosan prior to frozen storage.

Lipid oxidation stability may also be achieved through feed-

ing commercially farmed fish with a diet enriched with natural

antioxidants such as tocopherol isomers (Ortiz et al. 2009).α-

Tocopheryl acetate was also used as a dietary vitamin E supple-

ment in beluga sturgeon (Huso huso) because it is fairly stable

in the feed and only becomes active as an antioxidant after the

hydrolysis of the acetate group in the fish body. It was shown

to slow down lipid oxidation of the fish muscle during frozen

storage (Hosseini et al. 2010).

Degradation of Trimethylamine Oxide

TMAO, a source of formaldehyde, is present naturally in many

marine animals as an osmoregulator and as a means of excret-

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

degraded to DMA and formaldehyde in the presence of the

endogenous enzyme (TMAOase) in the fish tissues (Bremmer

1977, Hebard et al. 1982, Nielsen and Jorgensen 2004). 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 muscle protein denaturation (Crawford et al. 1979,

Ciarlo et al. 1985, Sotelo et al. 1995). Research using both me-

chanical and sensory tests has shown that formaldehyde formed

from the degradation of TMAO increases the firmness and de-

creases juiciness of mince prepared from white muscle or fil-

lets of gadoid and nongadoid fish species (Rehbein 1988). The

presence of formaldehyde also causes a noticeable decrease in

the extractability of total proteins, particularly the myofibril-

lar 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 increases the toughness of muscle

protein and accelerates the oxidation and hydrolysis of lipids.

Summary

During freezing, the formation of ice changes the order of

water molecules in the food environment, causes dehydration

and solute concentration, and thus disturbs the conformation of

protein, leading to protein denaturation. Lipid degradation and

enzymatic degradation of TMAO during freezing affect the