|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
Marinades are made by preserving fish and shellfish in a mixture of acetic acid and salt; the resulting product has an extended shelf life and characteristic flavour. Pelagic fish, with a high fat content in the flesh, such as herrings and sardines, are normally the raw material for the preparation of marinated products. Good quality marinades can only be made from high quality raw material.
The acetic acid produces the tenderness characteristic of marinades; this is largely due to the action of certain of the proteolytic enzymes which cause a partial breakdown of the proteins with the release of some free amino acids. This gives the products their characteristic taste. The fat content of the flesh is also important in giving the characteristic flavour. Some of the acetic acid combines chemically with the proteins while the remaining acid controls the pH and selectively allows the autolytic reactions to take place.
The salt (sodium chloride) causes the removal of water and coagulates the protein. It also controls the hydrolytic action and allows it to proceed within desired limits.
Marinades may be conveniently divided into three groups:
(i) Cold marinades: in which raw fish, with or without the backbone, are preserved in a mixture of acetic acid and salt. At no stage during the process are the fish heated.
(ii) Cooked marinades: the fish are placed in a hot solution (+85°C) of acetic acid and salt. At approximately 85°C, most of the bacteria are killed and the enzymes are inactivated (denatured).
(iii) Fried marinades: the fish are fried or baked before being packed in an acetic acid and salt solution. The frying kills most of the bacteria and denatures the enzymes.
Examples of each type of marinade are given below.
A pH of 4.5 is considered the optimum for most cold marinade products. At pH 4.5 and approximately 10 per cent salt, most bacterial activity is halted; some activity does occur and this contributes to the characteristic flavour of marinades. Since autolytic and bacterial activity occur, the shelf life of cold marinated products is limited, even under chill storage conditions. Eventually reactions will occur that produce off-flavours and make the product unacceptable. Shelf life at chill temperatures may be several months; at tropical ambient temperatures it may only be a few weeks.
Preparation of cold marinades - herring
The following recipe is for herring:
1. Wash the fish in a 10 per cent salt solution (brine) to harden them and remove slime.
2. Head, gut or fillet as required.
3. Wash in a 5 per cent brine to remove traces of blood from the muscle.
4. Immerse in a solution containing 7 per cent acetic acid and 14 per cent salt for up to three weeks. The strength of the solution depends on the ratio of fish to solution and the type of product required. If the fish are held at chill temperatures, the strength of the solution may be reduced. If the fish are held at high ambient temperatures, a stronger solution may be necessary to prevent spoilage. The process will proceed more rapidly at higher temperatures. If the acid and salt concentrations are too high the characteristic flavours may not be developed. The container should be full and have a tightly fitting lid; otherwise the fish may develop rancid flavours.
5. Packing: after the marinating process is completed, the flesh should be firm, white, opaque and tender. Discard any discoloured pieces. Glass jars are frequently used to pack the final product; the fish or fish pieces are packed in the jars and covered with a solution containing 1 - 2 per cent acetic acid and 2 - 4 per cent salt. The acid taste of the final product may be reduced by substituting citric or tartaric acid for some or all of the acetic acid; the pH of the final solution should not be more than 4.5. Spices, such as coriander, cloves, peppers and bay leaves, may be added to the final pack to improve the flavour.
1. Pretreatment: washing, cutting and pre-salting are similar to those processes used for cold marinades.
2. Bleaching: the fish or pieces of fish are spread on perforated trays that are immersed in a bleaching bath containing 1 - 2 per cent acetic acid at 85° C; some salt may also be added. Normally, 10 - 15 minutes immersion is adequate; a slightly longer time may be necessary for larger pieces.
3. Cooling: after bleaching, the product should be cooled with cold, clean water to remove fat and protein foam.
4. Packing: glass, porcelain or laquered cans may be used. Spices may be added to the final pack as required. With some European products, the fish are packed in a jelly. The jelly or final liquid should contain 1 - 2 per cent acetic acid and 3 per cent salt.
1. Pretreatment: cleaning, cutting and pre-salting as with cold marinades. After draining, the fish or fish pieces are breaded.
2. Frying: the breaded fish are fried for 5 - 12 minutes in fat at a temperature between 160 and 180°C. If a deep fat system is used, frying can be considered completed when the fish or fish pieces rise to the surface (the specific gravity is altered as fat is absorbed and water is lost).
3. Packing: the fish are packed in cans and covered with a brine containing 2 - 3.5 per cent acetic acid and 3 - 5 per cent salt. As with the other types of marinade, spices may be added to taste.
Marinades have a limited storage life because of the methods of preparation used. Cooked and fried marinades are not usually given sufficient heat treatment to make them sterile. Spoilage of marinated products occurs in differing ways depending upon the cause:
(i) Physical spoilage: if a pack is frozen, expansion of the contents may damage the glass jar or tin can.
(ii) Chemical spoilage: the acetic acid will attack the metal of a can if the cans are badly laquered or tinned. The action of the acid on the metal will cause the formation of hydrogen which may cause the can to swell. Metal dissolved in the acid may alter the flavour of the product.
(iii) Biological spoilage: the protein of the fish itself may be broken down to such an extent that off-flavours develop due to the action of bacteria or autolytic enzymes. If any of the spices or other additives contain sugar, bacteriological fermentation may occur.
Since marinated products are not sterile, it is essential that preparation is only carried out under hygienic conditions. All containers, working surfaces, tools and ingredients should be clean.