Introduction
Principles of Marinating
Marinated Herring
Marinated Shellfish
Marinated Mussels
Pasteurized Mussels
Sterilized Mussels
Spiced Herring
This note outlines the principles of marinating, that is preserving in a solution of acid and salt, and describes the manufacture of marinated herring and marinated shellfish. In addition, a method used in northern Europe for preserving fatty fish in salt, sugar and spices is also briefly described.
Marinades are semipreserves; acid, usually acetic acid, and salt are added to the fish to retard the action of bacteria and enzymes, resulting in a product with a characteristic flavour and an extended but limited shelf life. Marinating also makes the flesh of the fish firmer in texture; the more salt that is added, the firmer the flesh becomes. The aim is to make a product that has a pleasant taste without being too tough, and one that is safe to eat after a reasonable shelf life. The amounts of acid and salt required can be reduced when the product is kept chilled until eaten.
The measure of acidity or alkalinity of a product is known as the pH, where a pH of 7 is neutral and a pH of 1 is very acid; at a pH of 4·5 or below, that is moderately acid, all food poisoning bacteria, and most spoilage bacteria, are prevented from growing, and marinated products at a pH of 4·5 will keep for several months at a temperature of 4°C.
Some bacteria and enzymes will remain active in marinades throughout storage, even in the presence of acid and salt, and eventually the flesh will break down completely. This residual action is desirable in some semi-preserves, for example those in which salt alone, or a mixture of salt and sugar, is used to preserve the fish; the products of bacterial and enzymic action produce the typical flavour of the product.
Marinated herring are either boned herring or herring fillets preserved in a solution containing acetic acid and salt, sometimes flavoured with sugar and spices. Marinated herring have long been popular in northern Europe, and increasing quantities are now being marketed in Britain and North America.
The raw material
Good marinades can be made only from good quality raw material. Herring containing 5-15 per cent fat are suitable for marinating; they should be chilled immediately after capture, and should either be marinated or quick frozen for later use within 1 day of capture. The herring should be chilled either in ice in shallow boxes or in refrigerated sea water. Prolonged storage before processing, even at chill temperature, can increase the incidence of blood discoloration of the flesh.
Herring frozen within 1 day of capture and properly cold stored for up to 7 months at - 30°C can be used to make a good marinade.
Preparation
The herring are washed on arrival at the factory, typically in a drum washer and, if soft, they are soaked in a 10 per cent salt solution for 1 hour to harden them. They are then headed, gutted, and boned or filleted to suit the type of product required. The fish or portions of fish are then rinsed in a 5 per cent salt solution to remove traces of blood from the flesh.
Marinating
The boned herring or fillets are first immersed in a relatively strong solution of acetic acid and salt in tanks or barrels in a cool room for up to 3 weeks. This first soaking, known in the trade as the finishing bath process, is vitally important in making a good quality product; the composition of the solution and the ratio of fish to liquid have to be right. The amount of acetic acid used determines the degree of preservation, and sufficient salt has to be used to keep the fish flesh firm. When the proportion of fish to liquid is increased, the amount of acid absorbed by the fish is reduced; since a high ratio of fish to liquid is desirable to improve output, the concentration of acid in the solution has to be increased accordingly.
The ratio of fish to liquid in traditional open wooden vats or barrels usually ranges from 1:1 to 1·5:1. Any further increase in the proportion of fish is impracticable because the strength of the solution required raises its specific gravity above that of the fish, with the result that the fish float and protrude from the surface of the liquid. In closed vats or barrels the ratio of fish to liquid can be increased to 2·3:1.
At the end of the time in the finishing bath, the concentration of acetic acid in the fish and in the liquid should be at least 2·5 per cent; to achieve this in open vats containing a ratio of 1·5:1 of fish to liquid, the initial composition of the solution should be 4 per cent acetic acid and 10 per cent salt. For closed vats containing a ratio of 2·3:1 offish to liquid, the initial solution should contain 7 per cent acetic acid and 14 per cent salt.
The herring are put into the vats a few at a time, and the liquid is also added in small amounts to prevent rapid dilution. When an open vat is full, the contents are stirred to ensure complete mixing and to prevent fish sticking together; a full closed vat is rotated. The fish absorb acetic acid and salt from the liquid until equilibrium is reached; this takes about 1 week in a warm room, 3 weeks in a cool one, but the fish can be left stored in the finishing baths for up to 6 months at 3°C if required. The vats should be inspected regularly, and any loss of liquid made up by adding more of the right strength to ensure the fish are completely covered at all times. Herring can be removed from a vat in batches for packing throughout the 6 months storage period, provided some of the liquid is also removed with each batch.
Packing
After marinating, the flesh of the herring is white, opaque and firm, but not tough. The fish are inspected, and any with brown or red discolorations are discarded. They are then trimmed if required, packed in glass jars and covered with a pickle or sauce; spices and vegetables are also added in some speciality products. There is a wide range of end products that can be made; typical examples are Bismarck herrings and rollmops.
The ratio of fish to covering liquid should be between 1:1 and 2:1, and the liquid should contain 1-2 per cent acetic acid and 2-4 per cent salt. The acid taste of a marinade can be reduced by substituting citric or tartaric acid for some or all of the acetic acid, but care must be taken to ensure that the pH of the product does not exceed 4·5. Since not all acids are equally strong, expert advice should be taken where necessary when substituting for acetic acid. Vinegars produced by fermentation are also used instead of acetic acid to give a milder product; the acid content of vinegars can vary considerably, and again care should be taken to ensure that the vinegar when diluted is the equivalent of a 1-2 per cent acetic acid solution.
A typical pack for marinated herring that has wide appeal in Britain is as follows. Fillets of herring from the finishing bath are packed into wide-mouthed screwtop glass jars each containing the following spices: a bay leaf, a whole clove, a red pepper, a white pepper and two coriander seeds. The top of the jar is filled with slices of pickled onion, the contents of the jar are covered with a solution of 1 per cent acetic acid and 2 per cent salt until the jar is full and the lid is screwed on. This pack has a shelf life of about 1 month at 3°C.
The marinating process for shellfish is different from that for herring in that the meats are heat processed at some stage. Three types of product are made; a marinade of the cooked meats and a pasteurized product are both semipreserves, while the third is a heat sterilized product. The three species of shellfish generally used for marinades in Britain are cockles, mussels and whelks; since the processes are virtually the same for each species, only the treatments for mussels are described in detail.
Preparation
Live mussels in shell are washed, packed in wire mesh baskets and immersed in boiling water for 4-6 minutes, depending on size, in order to destroy the natural enzymes that would otherwise cause the meats to break up during marinating. Boiling time should be reckoned from the moment the water returns to the boil after the mussels have been immersed.
The meats are removed from the shells, the beards are removed, and the meats soaked in water or a weak brine containing 2-3 per cent salt for 2-3 hours in order to get rid of any remaining sand or grit.
Packing
The cleaned cooked meats are drained and packed into containers, typically glass jars, and covered with a vinegar containing 4-6 per cent acetic acid, so that the final acid content of the pack is between 1·8 and 2·2 per cent; the pH of the pack should not be higher than 4·2. When using 4 per cent acetic acid, or vinegar containing 4 per cent acetic acid, the ratio of mussel meats to liquid should not exceed 1·5:1 in order to ensure the required final acidity.
The vinegar for covering the mussels is often spiced before being added; a typical method of preparation is as follows: to 5 litres of vinegar containing 4 per cent acetic acid are added
|
bay leaves |
12 g |
|
white pepper |
6 g |
|
mustard seed |
6 g |
|
cloves |
3 g |
|
fennel |
3 g |
|
paprika |
1 g |
Spoilage
Three forms of spoilage are commonly encountered in packs of improperly prepared marinated mussels
1. Bacterial spoilage affecting the mussel meats and the clarity of the covering liquid.The first two forms of spoilage are probably interrelated, and can be minimized by carefully controlling the acid content and the packing process; the acidity of the pack should be checked in an adequate number of samples as a matter of routine, and jars should be inspected to ensure that the meats are held below the surface of the liquid. The third form is usually due to inadequate cooking of the mussels in shell.
2. Mould growth on the surface of the meats.
3. Softening and breakdown of the meats due to enzymic action.
Shelf life
Since there is no heat processing of the packed jars, this type of pack has a limited shelf life, usually 1-2 months at room temperature; the pack is normally date stamped.
Preparation
Live mussels are boiled or steamed for long enough to cause the shells to open. The meats are extracted and the beards removed. The meats are thoroughly washed and then soaked for 24 hours in a solution of 4 per cent
acetic acid or in vinegar containing 4 per cent acetic acid; the ratio of meats to acid should be 1:1. Meats soaked in acid require less subsequent heating to destroy spoilage bacteria than untreated meats; the texture and taste of the soaked meats are thus less affected by the heating process.
Packing
The meats are drained, packed into jars, and covered with vinegar containing 3 per cent acetic acid. The jars are closed and transferred to the pasteurizing tanks.
Pasteurizing
The temperature in the centre of the jars has to be raised to 70°C, and then held there for 20 minutes. This is best done by standing the jars in a water bath in which the temperature of the water and the product are initially about the same; the water is then heated gradually so that the temperature difference between the product and the water remains small until the centre of the pack reaches 70°C. It is then fairly easy to maintain the water bath at 70°C during the holding period of 20 minutes. The jars are removed from the bath and allowed to cool in air.
The finished pack must be sufficiently acid to prevent the growth of food poisoning organisms and to prevent spoilage; the acid content of the pack should be not less than 1·8 per cent, with a pH not higher than 4·2. Routine checks should be made during production to ensure the acidity is correct.
Shelf life
Pasteurized mussels will keep in good condition for at least 6 months. Jars in store for long periods should be kept in the dark, because direct light will discolour the meats.
Preparation
The mussels should be cooled and the meats extracted and cleaned as described for marinated mussels. The meats are then soaked in a weak brine containing 3 per cent salt for up to 3 hours. They are drained and then immersed in a vinegar and salt solution, containing 2 per cent acetic acid and 3 per cent salt, for 3 days. The solution may acquire a bluish tinge during this time, but this does not affect the flavour of the product.
Packing
The meats are packed in glass jars and covered with a spiced vinegar that has been mixed with an equal quantity of water and heated to 70°C; the spiced vinegar should be prepared in the manner described for marinated mussels. The jars are then sealed.
Sterilizing
The closed jars are heat processed in water or steam under pressure in an autoclave or retort; a 125 g jar requires 25-30 minutes at 105°C or 60 minutes at 100°C. Heating and cooling of the jars should be done carefully to avoid breakages.
Shelf life
The shelf life of sterilized mussels is comparable with canned foods, but the jars should not be exposed to strong light during storage in order to avoid discoloration of the contents.
A number of speciality semipreserves akin to marinades are made in northern Europe using salt alone or salt and sugar as the main preservatives; these products are at present not made or marketed in any quantity in Britain. There are three main groups of products: Scandinavian anchovies, which are made from sprats or herring, not anchovies, preserved in spiced brine; a similar product from Germany called Anchosen is preserved in sugar and salt: spiced herring, also called tidbits or gaffel-bidder, made from herring preserved in sugar and salt: fermented herring preserved in salt.
One method of manufacture of spiced herring is given here as a guide to the processing involved.
Large fat herring are nobbed and roused in a mixture of dry salt, sugar and spices, often on board the fishing vessel, and then packed in 100-kg barrels. The barrel contains about 80 kg herring, 10 kg salt, 8 kg sugar and 1·5 kg spices. Typical composition of 100 kg mixed spices is as follows
|
black pepper, powdered |
34 kg |
|
herbs, mixed |
18 |
|
cloves, powdered |
15 |
|
nutmeg powder |
7 |
|
mustard powder |
6 |
|
hops, powdered |
6 |
|
ginger, powdered |
6 |
|
cinnamon powder |
5 |
|
sandalwood, powdered |
3 |
During the maturing process, enzymes from the fish and from bacteria break down the protein in the fish flesh until it is soft and pleasant to eat. Flavorous substances are also formed during this time. When the herring are judged to be ready, normally after about 6 months at 3°C, they are taken out, filleted, skinned and packed into jars or cans. The fillets are sometimes cut into small pieces for packing. A typical recipe for the covering liquid is as follows
|
acetic acid (1 %) |
0·5 litre |
|
red wine |
0·5 |
|
water |
0·06 |
|
ginger |
100 g |
|
cinnamon |
70 |
|
white pepper |
20 |
|
mace |
20 |
