Freezing Technology

1.5.1 Spoliage fish

1.5.1 Spoilage of fish

Spoilage of fish, post mortem, is mainly due to (1) oxidation, (2) autolysis and (3) bacteria.  The major cause of spoilage of fish is bacteria, particularly in the case of marine fishes.

The flesh and body fluids of newly caught fish are free from bacteria (except when the fish has bacterial disease). The bacteria present on skin, adhering slime gills and intestine are normally saprophytic. Once the fish is dead, these bacteria invade the fish tissue.There are three main routes for this attack.

1. From the gills into the flesh through the vasculatory (circulatory) system.
2. Through the skin by penetration.
3. Through the peritoneal lining (from the intestinal cavity).

Invation of bacteria through the first and second routes is faster. Entry through the peritonial lining can take place only after perforation of stomach and intestinal walls, which normally takes longer time.

The fish muscle contains 15 to 18% protein. Bacteria attack the protein and break it down to peptides and amino acids. Initially, bacteria live and multiply in the fish tissue, utilizing the low molecular weight compounds like carbohydrates and amino acids present in small quantities in the muscle. Due to post-mortem enzymatic break down of the macromolecules in the muscle, enough low molecular weight compounds are formed, which serve as the food of bacteria. Subsequently bacteria elaborate proteolytic enzymes, which break down proteins to peptone, polypeptides, lower peptides and finally to amino acids. Bacteria will metabolize amino acids, in different ways leading to the production of odouriferous and foul smelling compounds like ammonia, hydrogen sulphide, mercaptans, indole, amines and organic acids.

When the fish is left in ambient temperature, which is usually 28 ±4°C, tropical fishes get spoiled within 6 to 12 hrs. depending on their size. In order to prevent such spoilage, many methods are in practice. Drying, icing, freezing and use of chemicals are some of the usual methods. The basic principle involved in these methods of preservation of fish is to control the activities of the microorganisms.

 

 Typical spoilage compounds during spoilage of fresh fish stored aerobically or packed in ice or at ambient temperature

Specific spoilage organism	Typical spoilage compounds
Shewanella putrefaeciens	TMA, H2S, CH3SH, (CH3)2S, Hx
Photobacterium phosphoreum	TMA, H2S
Pseudomonas spp.	Ketones, aldehydes, esters, non-H2S Sulphides
Vibrionaceae	TMA, H2S
Anaerobic spoilers	NH3, acetic, butyric and propionic acid

 

 Substrate and off-odour / off-flavour compounds produced by bacteria during spoilage of fish

Substrate	Compounds produced by bacterial action
TMAO	TMA
Cysteine	H2S
Methionine	CH3SH, (CH3)2S
Carbohydrates and lactate	Acetate, CO2, H2O
Inosine, IMP	Hypoxanthine
Amino-acids (glycine, serine, leucine)	Esters, ketones, aldehydes
Amino-acids, urea	NH3