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Thermal processing of fishery products for extended storage
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Sunday, 16 October, 2011, 08 : 00 AM [IST]
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Dr V VENUGOPAL
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Heating is the oldest and most reliable method of preservation for food products. Thermal processing makes foods more digestible and palatable and ensures microbiological safety. Chemical changes that take place during heating include denaturation of proteins influencing texture, oxidation of lipids, and generation of compounds that are responsible for the characteristic flavour.
Depending on the degree of thermal processing, food may have some undesirable consequences, such as loss of certain vitamins, essential amino acids and unsaturated fatty acids and formation of certain harmful substances in the treated food. The undesired changes can be significantly reduced by combination of heating at lower temperatures with other conventional methods or novel technologies including packaging that can reduce undesirable changes in the product.
Judicious combinations of these processes have resulted in novel value-added products, such as retort pouch products, cook-chill and sous vide items, coated and grilled products, surimi-based restructured products, heat / pressure processed products, etc. Advantages of these techniques are saving of energy, lesser loss of nutrients, convenient handling and better consumer satisfaction.
Introduction
Processing of fishery products has undergone significant changes in the last two decades, fuelled by changing consumer lifestyles and advances in packaging, logistics and transport.
According to the Food and Agriculture Organisation of the United Nations, processed fish for human consumption increased from 46.7 million tonnes in 1998 to 58.6 million tonnes in 2008. Freezing is the main method of processing fish, which uses about 1/5th of total fish production. Heat processing is the next major technique for value addition. Heat processed fishery products may be grouped into three, depending on the intensity of thermal energy applied. Conventional processes that depend on low to medium level heat (below 100oC) are cured products such as air dried and smoked items. Most of these produces, however, have poor quality and hence limited consumer appeal. Processing at high temperatures (above 100oC) relates to canned items, retort pouch packaged products, extrusion cooked products, battered, pre-fried and grilled products.
Combination of mild heat treatment with other processes such as chilling, use of food additives, packaging, etc. can give appealing and convenient products that can have enhanced marketability. This article will briefly deal with the major classes of heat-treated fishery products, their processing and quality characteristics followed by a discussion on the merits of thermal processing.
Traditional heat-treated processes
Traditional heat-treated processes for fishery products include boiling, curing, drying, frying, baking, smoking and grilling. Curing is a combination of one or more processes such as salting, hot air or solar pickling, smoking and marinading. While elevated temperatures enhance water evaporation and reduce water activity, presence of salt (in the case of salted products), smoke components (smoked products), or pH (pickles and marinades) in the product provide additional barriers to control growth of microorganisms.
Smoking is the process of the penetration of volatile compounds from smoke generated by burning of sawdust or chips of wood such as maple, oak etc. While a single barrier may be inefficient for the purpose, a combination of barriers efficiently prevents microbial growth through ‘hurdle technology. ’
Storage stability of smoked fish is due to presence of salt and deposition of antimicrobial and antioxidant substances from smoke, which include ammonia, furans, nitrogen oxides, and sulfur compounds, among others, constituting up to 0.5 gm per 100 gm tissue. Many of these compounds also impart characteristic flavour of smoked fish. Smoke treatment may be cold (the product temperature is maintained between 30 to 40oC) or hot (50o to 90oC). Cold smoked fish, containing about 5% salt can be kept for about 2 months at refrigerated temperatures, without microbial risks. Vacuum, modified or controlled atmosphere packaging can increase the shelf life of smoked products. Many types of dried, marinated and smoked fish including salmon with traditional seasonings are highly popular in Japan and the West. Quality of smoked fish can be improved by electrostatic filtration of smoke, and using ‘liquid’ smoke.
Heat sterilised products Canning
Thermal sterilisation on a commercial scale is used in canning of fish. The unit operations include skinning, filleting, separation of fish parts after evisceration and trimming of fins, scales and other inedible parts, brining, cooking, exhausting, hot filling, and sealing. The filling medium (brine or oil) accelerates heat transfer and avoids overcooking of fish near the can walls. Sterilisation of the filled can is done at 121oC to attain commercial sterility. The recognised minimum heat process duration to inactivate spores of lethal Clostridium botulinum is 3 min at 121oC. High acid fishery products such as marinades and pickles, which contain acetic, citric or lactic acids require heat treatment at a lower temperatures (e.g. 90oC), while fish canned in tomato juice and low acid (pH 5.3) products require full sterilisation at 121oC. The safety of the product is dictated by container seal integrity, adequate thermal process and post-process hygiene. Canning has been employed for several fish and shellfish including marine pelagic fish such as anchovy, herring, mackerel, sardine, scad, sprat, pilchard, freshwater and aquacultured fish such as carp, ide, and rohu, different species of shrimp, among others. These canned products have shelf life up to 18 months when stored at room temperature. In Europe, headed and gutted herring, small sardines and sprat canned in oil or tomato sauce are popular. Fish such as sprats may be mildly smoked prior to canning. In the US, more than 250,000 tonnes of tuna are canned, the skipjack tuna being the most popular. Some of the products include tuna canned in oil, brine, or vegetable sauce, tuna salad with vegetables, etc. Figure 1 shows process flow sheet for canning of mackerel.
Retortable pouch packaging
The conventional canning is being replaced by retort pouch packaging as a better process for convenient and consumer-friendly shelf stable products. The process operations in retort pouch packaging are comparable to canning. In the process, the dressed fish in suitable medium is sterilised in retort pouch, which is a laminate of three materials, an outer layer (normally 12 µm thick) of polyester, a middle aluminum foil, and an inner layer of polypropylene. The outer layer protects the foil, provides strength and also surface for printing details of the contents. The aluminum layer functions as a moisture, odour, light and gas barrier, while, the inner layer is the heat seal and food contact material. The retortable pouches should have high thermal resistance (up to 133oC), low gas and water permeability, and inertness towards food components.
The packaging should have high heat sealability (seal strength, 2 3.5 kg/100 mm), ability to stand burst at 7.5 kg/15 mm and resist handling stress. There are three essential rules for the safety of retort pouch processed products, namely, pouch seal integrity, adequate thermal processes to eliminate the most dangerous and heat resistant microorganisms including Clostridium botulinum spores, and hygiene.
Since during the sterilisation process, the retort pouches are unable to support internal pressure developed by expansion of headspace gas
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