16 Biochemistry of Seafood Processing 353tein from milk, eggs, and beef. Muscle from fish and
shellfish has very little connective tissue and is read-
ily hydrolyzed upon heating, resulting in a product
that is tender and easy to chew.
The forms of lipid in fish are triglycerides or tria-
cylglycerols. Triglycerides in pelagic fish contain the
long-chain polyunsaturated fatty acids 20:53 eico-
sapentoic acid (EPA) and 22:63, docosahexanoic
acid (DHA), which have many health benefits in-
cluding normal development of the brain and retina
in infants and prevention of heart disease in adults.
Pelagic fish store lipids in the head and muscle,
whereas lipids in demersal fish are stored primarily
in the liver and peritoneal lining and under the
skin, except in halibut, which has both liver and
muscle stores. The lipid content of pelagic fish varies
with the season, feeding ground, water salinity, and
spawning. Fish do not feed during spawning but
depend on lipid stores for energy. For example, the
lipid content of salmon may be 13% when the sal-
mon begins traveling upstream to spawning grounds
in the spring and only 5% at the end of the spawning
season in the fall. Herring show more seasonal vari-
ation in lipid content, with a peak level of 20% in the
spring, a drop to 10–15% during the fall spawning
season, and lows of 3–4% in the winter, because of
colder sea temperatures and a scarcity of food.
Water replaces the lipid depleted from muscle tissue
and results in fish of poor eating quality.
Livers of species of the Gadidae (cod) family con-
tain 50% lipids and are a concentrated source of
vitamins A and D and omega-3 fatty acids. Oil is
commercially extracted from the liver and purified
for medicinal use and animal feed. Livers from oth-
er species of fish (haddock, coley, ling, shark, huss,
halibut, and tuna) are also commercially processed
to extract the fish oil.
In general, processing does not change the nutri-
ent content of fish. Drying and smoking fish reduces
the water content. The process can also reduce the
lipid content of the fish if extremely high tempera-
tures are used. Drying also increases the concentra-
tion of all nutrients on a weight basis. Sodium nitrite
(usually with sodium nitrate) that is added during the
process of drying or smoking increases the content
of sodium considerably. The salmon carcass used for
smoking is lower in lipids than whole salmon.
Breading fish adds carbohydrate and yields a final
product with lower concentrations of protein, vita-
mins, and minerals than unbreaded fish.
Freezing and canning do not change the nutrient
composition of fish except for canned tuna. Most can-
ned tuna is low in lipids since the lighter colored tuna
is lower in fat and is preferred by consumers over the
deep brown-red colored tuna. Tuna is also cooked pri-
or to canning, which removes some of the lipid.
Tables 16.1 and 16.2 compare the water, protein,
and lipid content of different forms of fish.
The macronutrients in crustaceans and mollusks
differ from those in bony fish since all mollusks
contain carbohydrate, and lobster and scallops contain
small amounts of carbohydrate. Carbohydrate in shell-
fish is in the form of glycogen. The protein content is
more variable than in bony fish, and the lipid content is
comparable to that in pelagic fish. See Table 16.3.
Refer to Tables 16.4 and 16.5 for comparison of
micronutrients. Ranges are used in Tables 16.1–16.5
because data from many sources are so variable.MicronutrientsMicronutrients cover vitamins and minerals. Sea-
food is an important dietary source of vitamins A
and D and the B vitamins B 6 and B 12. Although
seafood is not high in thiamin or riboflavin, the
amounts are comparable to those found in beef. Fish
and shellfish have little or no vitamin C or folate.
Cod liver oil has been used to supplement diets
with vitamins A and D; one teaspoon (5 g) of cod
liver oil provides 12.5 μg of vitamin D and 1500 μg
of vitamin A.
Mineral contributions of fish and shellfish include
iodine and selenium; a 100 g portion contains more
than 50% of the Dietary Reference Intake (DRI) for
iodine (DRI150 μg/day) and selenium (DRI 55
μg/day) respectively. Generally, meat, eggs, and dairy
products are poor sources of iodine and selenium.
Iron and zinc are found in moderate amounts in most
seafood. Clams and oysters are concentrated sources
of zinc, with 24 and 91 mg/100 g serving, respective-
ly, compared with a DRI of 15 mg/day. Seafood, sim-
ilar to other dietary protein sources is also an excel-
lent source of potassium. Fresh seafood contains
varying amounts of sodium, from a low of 31 mg/100
g of trout to a high of 856 mg/100 g of crab. Sodium is
also added during canning and smoking fish and
other seafood. In general, seafood is a poor source of
calcium, except for whole canned salmon and sar-
dines: when their bones are included, they will pro-
vide 200–400 mg calclium/100 g portion.