This article is about the act of preserving seafood. For the American
politician, see Preserved Fish. Ancient methods of preserving fish
included drying, salting, pickling and smoking. All of these techniques are
still used today but the more modern techniques of freezing and canning have
taken on a large importance. Fish curing includes and of curing fish
by drying, salting, smoking, and pickling, or by combinations of 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. Modern freezing and canning methods have
largely supplanted older methods of preservation. Fish to be cured are
usually first cleaned, scaled, and eviscerated. Fish are salted by packing
them between layers of salt or by immersion in brine. The fish most
extensively salted are cod, herring, mackerel, and haddock. Smoking preserves
fish by drying, by deposition of creosote ingredients, and, when the fish
are near the source of heat, by heat penetration. Herring and haddock are
commonly smoked. Kippers are split herring, and bloaters are whole herring,
salted and smoked. Sardines, pilchards, and anchovies are small fish of the
herring family, often salted and smoked and then preserved in oil. Fish are
dried under controlled conditions of temperature, humidity, and air velocity.
Since the dried product is relatively unappetizing and rehydrating slow, other
preservation methods are common. History
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 of refrigeration and freezing technologies
transformed the commercial fishing industry: 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.
Preservation techniques are needed to prevent fish spoilage and lengthen shelf
life. They are designed to inhibit the activity of spoilage bacteria and the
metabolic changes that result in the loss of fish quality. Spoilage bacteria
are 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.
Control of temperature If the temperature is decreased, the
metabolic activity in the fish from microbial or autolytic processes can be
reduced or stopped. This is achieved by refrigeration where the temperature is
dropped to about 0 °C, or freezing where 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 and palletized. 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 mechanical refrigeration, 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 is slurry ice, made from micro crystals of ice formed and suspended
within a solution of water and a freezing point depressant, such as
common salt. A more recent development is pumpable
ice technology. Pumpable ice flows like water, and because it is homogeneous, it
cools fish faster than fresh water solid ice methods and eliminates freeze burns.
It complies with HACCP and ISO food safety and public health standards, and
uses less energy than conventional fresh water solid ice technologies.
Control of water activity The water activity, aw, in a fish is
defined as the ratio of the water vapour pressure 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 as drying, salting and
smoking have 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-binding humectants, and fully
automated equipment with temperature and humidity control have been added. Often
a combination of these techniques is used.
Physical control of microbial loads Heat or ionizing irradiation can be used
to kill the bacteria that 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 in metal cans or retortable pouches before the
heat treatment. Chemical control of microbial loads
Microbial growth and proliferation can be inhibited by a technique called
biopreservation. Biopreservation is achieved by adding antimicrobials or by
increasing the acidity of the fish muscle. Most bacteria stop multiplying
when the pH is less than 4.5. Acidity is increased by fermentation, marination or
by directly adding acids to fish products. Lactic acid bacteria produce
the antimicrobial nisin which further enhances preservation. Other
preservatives include nitrites, sulphites, sorbates, benzoates and
essential oils. Control of the oxygen reduction
potential Spoilage bacteria and lipid oxidation
usually need oxygen, so reducing the oxygen around fish can increase shelf
life. This is done by controlling or modifying the atmosphere around the
fish, or by vacuum 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.
Combined techniques Two or more of these techniques are
often combined. This can improve preservation and reduce unwanted side
effects such as the denaturation of 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 the multiple hurdle theory.
See: Haddock: Arbroath Smokie.
Herring: kipper, surströmming, rollmops, soused.
Salmon: smoked salmon, cured salmon, and gravlax.
Cod: stockfish, lutefisk. See also
Fish processing Dried shrimp
List of smoked foods Sharks fin
References