Spiced Fish Products and Preservation Techniques

Fish production involves curing fish with salt, followed by smoking to preserve and flavor it. This process typically includes salting, washing, drying, and smoking the fish using specific techniques to achieve the desired texture and taste. 

Production of Salted-Smoked Fish

1. Product Description: Fish salting, drying, and smoking are choice fish processing technologies particularly suited for small-scale producers. The product is usually a dry, brown, leathery fish that has a salty taste and a characteristic flavor. It has a shelf-life of several weeks or months when stored correctly. It is a valuable product with high domestic demand.

2. Raw Material: The most important factors affecting the suitability of fish for the salt-smoking process are oil content and flesh texture. Very small fish may be dried whole; larger fish must always be cut or filleted to increase the surface area available for salt penetration and/or moisture loss. However, fish for processing must be fresh and devoid of putrid odors.

3. Preliminary Processing: The preliminary process of fish usually consists of the following steps or unit operations: evisceration, beheading (optional), scaling, cutting of fins and belly flaps, slicing of whole fish into steaks, filleting, and different combinations of these. As a result of the preprocessing, fish are obtained in the following forms:

1. Whole fish
2. Gutted fish without head
3. Gutted fish without head and fins
4. Sliced whole fish after beheading and evisceration
5. Fillet without ribs, with or without the skin
6. Fillet without ribs

Any of these could form the raw material for the production of salted, smoked fish.

4. Equipment:

1. Smoking kiln
2. Table with smooth surface
3. Cutting knife
4. Plastic buckets
5. Basins

5. The Process: Saturation of raw materials with wood smoke is the main principle of the smoking process. During this process, some water is removed from the tissue, and changes in proteins occur.

The smoked fish is ready for consumption without further culinary treatment. There are two methods of fish smoking hot and cold which give very different products.

The difference lies in stability and sensory properties, which in turn depend on the degree of fish dryness and saturation with smoke components. Smoke is produced by partial burning of some type of hardwood and is a mixture of more than a hundred chemical components.

During the smoking process, sensory features such as the color of properly smoked fish depend on the quantity and composition of the smoke components absorbed; the darker the color of the fish, the more smoke components are absorbed. The presence of antioxidants in smoke renders smoked products resistant to rancidity.

The hot-smoking process includes the preliminary processing of raw material, brining, drying to a certain loss of water content, the actual smoking process, and thermal treatment at temperatures usually 70–80°C.

The cold-smoking process involves no thermal treatment, and the entire process is carried out at temperatures below 30°C.

6. Brining or Salting: During hot-smoking, brining is carried out to ensure penetration of about 3% salt into the fish tissue. It is recommended to dip the fish for about 30 minutes. This results in a salt uptake of only 2–3% and produces a good gloss on the surface.

For longer storage life, the use of more salt is required, and levels of 8–10% salt or more in the finished product are not uncommon. A fully saturated brine contains about 360g of salt to each liter of water. Full-strength or saturated brine is called a 100° brine.

A 10° brine with 9 parts of water is sometimes used to soak fish before salting. Prepare an 80% saturated brine, add spices (e.g., onion extract, garlic extract, and pepper, optional). Place fish in brine and let it stand for 30–40 minutes.

Drain and dry on perforated trays. Hot smoke in three stages: lower heat 25–55°C for 30–60 minutes, medium heat 80–110°C for 40–50 minutes. For a kiln without temperature control, smoke for at least 5 hours. Cool and package.

7. Catfish Process Notes:

i. Raw Fresh Fish: Clean and fillet. Cut the fish from tail to head through the stomach using a sharp knife. Wash with clean water.

ii. Mix Salt and Spices: Prepare an 80% saturated brine solution. Mix with spice extracts.

iii. Cure: Dry fillets in brine for 30–40 minutes.

iv. Smoke: Place the salted fish on the smoking racks in the kiln in an orderly fashion. Smoke for at least 5 hours.

v. Cool: Cool at room temperature.

vi. Pack: Package in polythene bags and seal.

8. Process Control: Adequate cleaning and preparation of the fish are necessary to prevent contamination of the final product. Correct smoking temperature and time are critical: overheating causes excessive browning, and underheating/inadequate time may result in incomplete drying and mold growth.

The main quality characteristics of the product are color, texture, and flavor. These are each determined by the type of wood used to smoke the fish and the time and temperature of smoking. Packaging is needed to prevent contamination by dust and insects. Sealed polythene bags are suitable. The product should be stored in a cool, dry place.

Fish Muscle Composition

All true fish and some crustacean and molluscan muscle is typical striated muscle, although, in each case, the cells are inserted into sheets of connective tissue, arranged in a complicated pattern which, on heating, breaks down and gives rise to the characteristic flaky appearance of coagulated blocks (myotomes) of cells.

Two main types of muscle exist red or dark and white the former being disposed laterally along the body in discrete strips or blocks between the skin and backbone. Different species of fish contain different proportions of red and white muscle, the latter generally predominating.

In crustaceans, the blocks of striated cells are somewhat similarly segmented, whilst in mollusks, the cells, which are predominantly of a specialized smooth type, run in a complex pattern for much of the length of the muscle.

The major feature of the proximate composition of fish and shellfish is the great variability in lipid content. Lean species contain typically 0.3–1.0% lipid, most of which is phospholipid and the remainder triglyceride, whereas the total amount in fatty species can be as much as 30%.

In fatty species, the amount of lipid varies seasonally within a species and may fall to as low as 1%; these changes are accounted for entirely by changes in the proportion of triglycerides. As the amount of lipid increases, the amount of water falls in almost linear proportion, while the amount of protein remains fairly constant.

In lean fish and crustaceans, the amount of water is generally slightly greater than that in meat or chicken; in fatty fish of high lipid content, it is often less. Several mollusks have a high water content.

The proportion of protein, as in meat and chicken, is in most cases in the range of 15–18%, the remaining content of nitrogenous substances (1–3%) being made up of a multiplicity of low molecular-weight compounds.

The lipids of fish are among the most unsaturated of animal-muscle lipids. Whether phospholipids or triglycerides, they contain a high proportion of polyunsaturated fatty acids, which in most fish are very susceptible to oxidation by atmospheric oxygen during handling and processing.

In some species, the presence of natural antioxidants retards this tendency. The immediate products of oxidation are hydroperoxides, which readily break down to a series of carbonyls, several of which have rancid odors and flavors. Such reactions are among the reasons why fish is so perishable.

Fish and crustaceans contain the normal types of muscle proteins. Some mollusk muscles contain a special form of myofibrillar protein known as paramyosin. The proportion of connective tissue proteins is lower than in meat, being 3–5% of the total proteins in many species and 8–10% in stockfish.

This is likely to be one of the reasons why fish is much more tender than meat. To the food scientist and technologist, the special feature of the main fish-muscle proteins, including enzymes, is their instability vis-à-vis their meat counterparts.

Thus, fish myosins, either when isolated or when in the intact tissue, denature much more rapidly than beef or chicken myosins kept under the same conditions.

Also, the thermal shrinkage or denaturation temperatures of fish connective tissue collagen are not richly insulated, and therefore the concentration of blood chromoproteins is lower, and the concentrations of myoglobin are very low and moderate, respectively.

The array of low molecular-weight compounds in fish and shellfish is, in general, typical of any muscle. Of particular note is the high concentration of trimethylamine oxide (TMAO).

This compound is of considerable significance as an odor precursor in that it is reduced by spoilage bacteria or otherwise degraded during processing to trimethylamine (TMA), dimethylamine (DMA), and monomethylamine (MMA). TMA is a prominent contributor to the odor of stale fish.

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Fish Spoilage Mechanisms

Freshness and spoilage are two qualities that must be clearly defined. A fresh product is defined as a product whose original characteristics remain unchanged. Freshness is the property of fish that has a considerable influence on its quality.

It is the most important single criterion for judging the quality of the majority of fish products. Loss of freshness followed by spoilage is a complex combination of microbiological, chemical, and physical processes.

Spoilage is the degradation of food such that the food becomes unfit for human consumption. Food can be spoiled by a number of means, including physical, chemical, enzymatic, and microbiological.

However, the most prevalent cause of food spoilage is microbial growth and residence in the food. Spoilage, therefore, is indicative of postharvest change. This change may be graded as the change from absolute freshness to limits of acceptability and unacceptability.

Fish spoilage is usually accompanied by changes in the following physical characteristics: change in color, odor, texture, color of the eyes, color of the gills, and softness of the muscle. With regard to smell or odor, spoiled fish will generally have a fishy, sour, or ammonia-like stench.

Appearance loose, spoiled fish may appear to be dry or mushy in certain areas, and the gills may have slime. Typically, spoiled fish will also have a green or yellowish discoloration, which arises not from the spoilage metabolites but rather from oxidation of the oxygen transporter in the fish myoglobin and metmyoglobin during frozen storage from prolonged or unnecessary exposure of fish to air.

Spoiled fish will also have flesh that is soft or does not spring back when pressed upon. The spoilage process of fish starts immediately after the death of the fish. The process involves three stages:

• Rigor mortis
• Autolysis
• Bacterial invasion and putrefaction

i. Spoilage Factors of Fish: Through the action of enzymes, bacteria, chemicals present in fish, and physical phenomena, the following factors contribute to spoilage of fish:

• High moisture content
• High fat content
• High protein content
• Weak muscle tissue
• Unhygienic handling

1. Enzyme Action in Fish Spoilage

The rigor mortis is a physical effect on the muscle tissue of fish caused by chemical changes following death. In live fish, the glycogen present in the muscle is converted to CO2 and H2O after the supply of O2 to the cells. After the death of fish, the blood circulation stops, and the supply of O2 ceases.

The enzymes present in the muscle convert glycogen into lactic acid. The pH of fish muscle falls. The formation of lactic acid continues until glycogen is completely depleted. The process is known as rigor mortis.

After the completion of rigor mortis, muscle stiffness gradually decreases, accompanied by an increase in pH, leading to softening of muscle. This is followed by the breakdown of proteins by enzymes.

This is called autolysis. Thus, autolysis can be described as an internal breakdown of the structure of the protein and fat due to a complex series of reactions by enzymes. Autolysis of protein starts immediately after rigor and creates favorable conditions for the growth of bacteria.

Another important action of the enzyme is that it affects the flavor of fish. The components responsible for the taste and flavor of fish are changed by enzymatic action, such as the progressive degradation of ATP to AMP and hypoxanthine.

Hypoxanthine is produced by the breakdown of ATP, which is the main component of fish muscle nucleotide. The accumulation of hypoxanthine imparts a bitter taste in the fish muscle, accompanied by loss of fresh fish flavor. Thus, the estimation of hypoxanthine indicates the degree of freshness.

Enzymatic action also causes decomposition in the fish, known as belly bursting. This is caused by the action of digestive enzymes present in the gut of the fish. The black spot formation in shrimps is also caused by the action of enzymes on the amino acid.

The black color is due to the formation of melanin (black pigment) by the action of tyrosinase on tyrosine present in the shrimps. Black spots present a poor appearance, which is not acceptable.

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2. Bacterial Action in Fish Spoilage

The freshly caught fish is almost free from bacteria, but the surface slime, gills, and intestine may contain a considerable load of bacteria. When the fish is dead, these bacteria start attacking the flesh, causing spoilage and producing undesirable compounds.

The nature and type of bacteria present in the flesh depend upon the water from where it is caught and the methods used in handling the fish after catch. Compared to other foods, fish is unique as a substrate for microbial growth.

This uniqueness stems from several important factors:

• The poikilothermic nature of fish
• A high post-mortem pH in the flesh (typically greater than 6.0)
• The presence of non-protein nitrogen (NPN) in large quantities
• The presence of trimethylamine oxide (TMAO)

The poikilothermic nature of fish selects for bacteria that can thrive in a wide range of temperatures. For example, the microflora of temperate water fish is dominated by psychrotrophic gram-negative rod-shaped bacteria, such as those found in genera Pseudomonas and Moraxella, with only varying proportions of Gram-positive organisms such as Bacillus.

High post-mortem pH of fish flesh is caused by the fact that fish flesh is low in carbohydrate (less than 0.5%) in the muscle tissue, and only a small amount of lactic acid is produced after death. This allows pH-sensitive organisms, such as Shewanella putrefaciens, in seafood but not in other meat.

The NPN fraction of the fish flesh consists of low-molecular-weight, water-soluble nitrogen compounds, particularly free amino acids and nucleotides, that are readily available bacterial growth substrates.

The spoilage of fish is influenced most by the presence of TMAO in conditions where oxygen is not present. Some anaerobic bacteria are able to utilize TMAO as the terminal electron acceptor in an anaerobic respiration process, with TMA as the primary product.

The important changes brought about by the action of bacteria in fish are as follows:

i. Reduction of TMAO to TMA: The odorless TMAO, found in small percentages in marine fish, is reduced to offensive-smelling TMA. This contributes to the characteristic ammonia-like and fishy off-flavor in spoiled fish.

ii. Breakdown of amino acids and formation of amines: This is responsible for many off-flavors and off-odors typically found in spoiled fish. Examples include the breakdown of cysteine and methionine by certain microorganisms, leading to the formation of hydrogen sulfide and methylmercaptan, respectively, from both sulfur-containing amino acids.

The formation of primary amines such as histamine from histidine, arginine from glutamic acid, etc., may cause food poisoning in extreme cases.

iii. Breakdown of urea, found in high concentrations in the flesh of some fish, to ammonia by microorganisms is accompanied by an offensive odor.

3. Chemical Action in Fish Spoilage

The chemicals present in living things are able to change due to their either splitting up or joining together. In both cases, new chemicals are formed. These changes are called chemical reactions. The most common chemical action that causes spoilage is oxidative rancidity in fatty fishes.

This is due to unsaturated fatty acids, which are reactive with oxygen and lead to autoxidation. The primary oxidation product, the peroxide, is odorless and flavorless.

The secondary oxidation products, which comprise aldehydes, ketones, short-chain fatty acids, etc., have very unpleasant odors and flavors. These chemicals in combination yield the fishy and rancid character associated with oxidized fish lipid.

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