Fish preservation

Fish preservationis the method of increasing theshelf lifeoffishand otherfish productsby applying the principles of differentbranches of sciencein order to keep the fish, after it has landed, in a condition wholesome and fit for human consumption.Ancient methods of preserving fish includeddrying,salting,picklingandsmoking. All of these techniques are still used today but the more modern techniques offreezingandcanninghave taken on a large importance.
Fish curing includes and of curingfishbydrying,salting,smoking, andpickling, or bycombinationsof these processes have been employed since ancient times. On sailing vessels fish were usually salted down immediately to prevent spoilage; the swifter boats of today commonly bring in unsalted fish. Modernfreezingandcanningmethods have largely supplanted older methods ofpreservation. Fish to be cured are usually first cleaned, scaled, and eviscerated. Fish are salted by packing them between layers ofsaltor byimmersioninbrine. The fish most extensively salted arecod,herring,mackerel, andhaddock. Smoking preserves fish by drying, bydepositionof creosote ingredients, and, when the fish are near the source of heat, by heat penetration. Herring and haddock (finnan haddie) are commonlysmoked.Kippersare split herring, andbloatersare whole herring, salted and smoked.Sardines,pilchards, andanchoviesare small fish of the herring family, often salted and smoked and then preserved inoil. Fish are dried under controlled conditions oftemperature,humidity, andair velocity. Since the dried product is relatively unappetizing and rehydrating slow, other preservation methods are common.
In the past, fishing vessels were restricted in range by the simple consideration that the catch must be returned to port before it spoils and becomes worthless. The development ofrefrigerationand freezing technologies transformed thecommercial fishingindustry: fishing vessels could be larger, spending more time away from port and therefore accessing fish stocks at a much greater distance. Refrigeration and freezing also allow the catch to be distributed to markets further inland, reaching customers who previously would have had access only to dried or salted sea fish.
Canning, developed during the 19th century, has also had a significant impact on fishing by allowing seasonal catches of fish that are possibly far from large centres of population to be exploited. For example:canned sardines.
Preservationtechniques are needed to prevent fishspoilageand lengthenshelf life. They are designed to inhibit the activity of spoilagebacteriaand themetabolicchanges that result in the loss of fish quality.Spoilage bacteriaare the specific bacteria that produce the unpleasant odours and flavours associated with spoiled fish. Fish normally host many bacteria that are not spoilage bacteria, and most of the bacteria present on spoiled fish played no role in the spoilage.To flourish, bacteria need the right temperature, sufficient water and oxygen, and surroundings that are not too acidic. Preservation techniques work by interrupting one or more of these needs. Preservation techniques can be classified as follows.
If the temperature is decreased, the metabolic activity in the fish frommicrobialorautolyticprocesses can be reduced or stopped. This is achieved byrefrigerationwhere the temperature is dropped to about 0 °C, orfreezingwhere the temperature is dropped below -18 °C. On fishing vessels, the fish are refrigerated mechanically by circulating cold air or by packing the fish in boxes with ice.Forage fish, which are often caught in large numbers, are usually chilled with refrigerated or chilled seawater. Once chilled or frozen, the fish need further cooling to maintain the low temperature. There are key issues with fish cold store design and management, such as how large and energy efficient they are, and the way they are insulated andpalletized.
An effective method of preserving the freshness of fish is to chill with ice by distributing ice uniformly around the fish. It is a safe cooling method that keeps the fish moist and in an easily stored form suitable for transport. It has become widely used since the development of mechanicalrefrigeration, which makes ice easy and cheap to produce. Ice is produced in various shapes; crushed ice and ice flakes, plates, tubes and blocks are commonly used to cool fish.Particularly effective isslurry ice, made from microcrystals of ice formed and suspended within a solution of water and afreezing point depressant, such as common salt.
A more recent development ispumpable ice technology. Pumpable ice flows like water, and because it ishomogeneous, it cools fish faster than freshwater solid ice methods and eliminates freeze burns. It complies withHACCPandISOfood safety and public health standards, and uses less energy than conventional freshwater solid ice technologies.
Thewater activity, aw, in a fish is defined as the ratio of thewater vapourpressure in the flesh of the fish to the vapour pressure of pure water at the same temperature and pressure. It ranges between 0 and 1, and is a parameter that measures how available the water is in the flesh of the fish. Available water is necessary for the microbial and enzymatic reactions involved in spoilage. There are a number of techniques that have been or are used to tie up the available water or remove it by reducing the aw. Traditionally, techniques such asdrying,saltingandsmokinghave been used, and have been used for thousands of years. These techniques can be very simple, for example, by using solar drying. In more recent times,freeze-drying, water-bindinghumectants, and fully automated equipment with temperature and humidity control have been added. Often a combination of these techniques is used.
Heat or ionizing irradiation can be used to kill thebacteriathat cause decomposition. Heat is applied by cooking, blanching or microwave heating in a manner that pasteurizes or sterilizes fish products. Cooking or pasteurizing does not completely inactivate microorganisms and may need to be followed with refrigeration to preserve fish products and increase their shelf life. Sterilised products are stable at ambient temperatures up to 40 °C, but to ensure they remain sterilized they need packaging inmetal cansorretortable pouchesbefore the heat treatment.
Microbial growth and proliferation can be inhibited by a technique calledbiopreservation.Biopreservation is achieved by addingantimicrobialsor by increasing theacidityof the fish muscle. Most bacteria stop multiplying when thepHis less than 4.5. Acidity is increased byfermentation,marinationor by directly adding acids (acetic, citric, lactic) to fish products.Lactic acid bacteriaproduce the antimicrobialnisinwhich further enhances preservation. Other preservatives includenitrites,sulphites,sorbates,benzoatesandessential oils.
Spoilage bacteria andlipid oxidationusually need oxygen, soreducing the oxygenaround fish can increase shelf life. This is done bycontrollingormodifying the atmospherearound the fish, or byvacuum packaging. Controlled or modified atmospheres have specific combinations of oxygen, carbon dioxide and nitrogen, and the method is often combined with refrigeration for more effective fish preservation.
Two or more of these techniques are often combined. This can improve preservation and reduce unwanted side effects such as thedenaturationof nutrients by severe heat treatments. Common combinations are salting/drying, salting/marinating, salting/smoking, drying/smoking, pasteurization/refrigeration and controlled atmosphere/refrigeration.Other process combinations are currently being developed along themultiple hurdle theory.
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