Freezing Technology

5.1.1.3 Discolouration

In freshly caught fish the blood has a brick red color of oxy hemoglobin. This color gradually changes to the red brown or brown color of met hemoglobin. Myoglobin suffers the same kind of changes. Frozen tuna and sword fish may exhibit green and brown discolouration on looking. Uncooked fish meat contains three derivatives of myoglobin. The pigment responsible for the pink colour in normal cooked meat of tuna is hemochrome, derived from the reaction of myoglobin with non-heme constituents. Greening is due to pigments resulting from the oxidation of hemochrome that occurs when the meat is unduly exposed to oxidative condition during and after cooking. Greening of frozen sword fish may be related to take H₂S produced by putrefactive bacteria. Proper evisceration and removal of blood immediately after the catch reduce the risk of discolouration. The undesirable discolouration in yellow fin tuna meat can be averted, if the fish is frozen at full rigor, stored at a temperature of -23⁰C to -27⁰C, and defrosted by still air at 10⁰C.

a. Carotenoid pigments:

Some fishes like salmon, and some crustaceans, contain carotenoid pigments in the flesh. The red color astaxanthin tends to fade away during frozen storage. In crustacean, like lobster, pigment is limited to the surface of the meat, the changes during frozen storage lead to yellow discolorations.

b. Fish oils:

The oils & fats naturally occurring in fish vary with species from colourless, as in herring through yellow to red, as in salmon. Fat herring, with a layer of oil just under the skin, may develop a yellow color during poor storage condition & which can be seen through the skin. Such fish is said to have “rusted”.

c. Black spots:

Shrimp, lobster etc, develop so called black spot when stored fresh. Bruises and rough handling increase the occurrence of this discolouration, which is caused by an enzyme, tyrosinase acting on polyphenolic compounds, present in free amino acids like tryptophan,tyrosine and form black 'melanin' pigments.

Flavor changes:

Systematic studies of flavor changes in frozen fish are lacking. Freshly processed fish, frozen and cold stored, is generally considered first to lost it’s characteristic fresh fish flavour, becoming bland & insipid; there after it develops flavors variably described as “cold stored” “Salt fishy’ etc.

Break down of connective tissue:

During some occasion, fish fillets are produced in which the muscle flakes will separate when the fish is filleted. This defect, often called “gaping” appears to result from a breakdown in the connective tissue and associated components which hold the muscle cells together. Although the main myofibrillar proteins, actin and myosin are believed to remain largely intact, many of the cytoskeletal protein like connectin, nebulin and desmin are believed to be degraded. The problem is, by and large, connected with freezing at sea and fillet production from frozen whole fish.