|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
An interest in fish silage is related to the desire to make maximum use of waste fish and fish offal in situations where the quantity involved, or the transport costs, prohibit conversion into fish meal. In small-scale fisheries in the tropics, this situation is common. Daily and/or seasonal gluts of fish occur and, because of transport difficulties and inadequate processing facilities, these surplus fish are often underused. The quantities involved do not permit profitable fishmeal manufacture since even the most modest fishmeal plant requires regular supplies of several tonnes of raw material per day for viable operation. Ironically, countries in this situation are often importing substantial quantities of fish meal to support their expanding animal production industries.
In countries where investment capital is available and fish waste is concentrated in one area, the obvious solution is reduction to fish meal. Where this is not possible, the fish could be utilised by the cattle, pig and poultry industries in the form of silage. The technology of fish silage production is simple; essential equipment is cheap; and the scale of production may be varied at will. These are distinct advantages in developing countries.
Silage production relies on the fact that at acidic pH, the microbial flora of fish is eliminated or greatly reduced and the enzyme systems in the fish which break down fish protein are able to function more efficiently. Fish silage methods can be divided into two major groups:
1. those employing acids, either mineral and/or organic, to lower the pH and to produce the conditions necessary for silage production, and
2. those employing a process of fermentation with the generation of organic acids to conserve the product.
The acid ensilage of fish offal was developed originally from a method invented by A. 1. Vertanen in the 1920s. Sulphuric and hydrochloric acids were used to acidify fish waste and the product was neutralised with chalk. Methods using organic acids, where the pH can be higher and neutralisation is unnecessary, have also been investigated. In the preparation of acid silage, the choice of preservative is between a mineral acid, mineral acid mixtures, organic acids such as formic or propionic, or mixtures of inorganic and organic acids. The choice will depend upon the cost and availability of the acids and the conditions under which the product is prepared. Formic acid is usually more expensive than the common mineral acids but it produces silages which are not very acidic and which do not require neutralisation before use. Care must be exercised with silages made from mineral acids and with all acids in the concentrated form. Equipment, tanks and machinery used for the production or storage of silage must be acid-resistant. Formic acid is not only acid in nature but it also has bacteriocidal properties which means that the quantities required are less than if mineral acids alone are used. It has already been said that the preparation of fish silage is a fairly simple process. An outline of the steps involved in preparation is as follows:
1. The raw material should be as fresh as possible (this may include whole fish, filleting waste, offal or other suitable protein material).
2. The fish are comminuted by mincing, cutting or chopping (this operation may be manual or mechanical).
3. For manual preparation 10 - 15 kg quantities of minced fish are placed in a suitable container (this must be acid-resistant).
4. The minced fish is acidified with mineral acid or with formic acid to the required pH. The mix is constantly stirred until the desired pH is reached.
Note: The optimum amounts of mineral acid to lower the pH and formic acid must be determined by experiment if they are used in combination. Generally, addition of sufficient mineral acid to reach pH 3 plus 0.5 per cent formic acid has been found to be acceptable in many situations. If formic acid is to be used alone, a concentration of 3 per cent formic acid by volume to weight of fish seems to be acceptable.
5. The container is left, preferably covered, for the fish mix to liquefy. This can take 3 or 4 days but the rate depends on the species of fish and the degree of comminution as well as the temperature at which the mixture is kept. It should be stirred daily.
Experience in the UK has shown that the successful production of fish silage, irrespective of scale, requires certain conditions:
1. The material should be reduced in size preferably to pieces no larger than 3 - 4 mm in diameter.
2. Acid must be thoroughly dispersed throughout the minced fish to avoid pockets of untreated material where bacterial spoilage can continue.
3. Periodic agitation is necessary to bring about rapid liquefaction.
4. Temperatures of at least 20°C are desirable since, below this, liquefaction takes place rather slowly. The enzymes responsible for liquefaction can be inactivated at higher temperatures but samples heated to 40°C have been found to liquefy rapidly.
Equipment used for silage production can vary considerably and, on a small scale, it might be sufficient to pulp the raw material, add the acid manually, mix in a suitable container, and store in a warm place. For larger-scale production, however, a mincer capable of reducing material to the required size is necessary, together with suitable heavy-duty mixing equipment, to ensure that a uniform mixture of fish and acid is made. For safety, a pump or measuring device for handling the acid is advisable. Suitable tanks are required for initial liquefaction of the fish, together with other tanks for bulk storage of the finished product and formic acid.
After liquefaction, oil removal may be necessary where fish with a high fat content are used. To do this, it is necessary to raise the temperature of the silage to 65 - 70°C when coarse suspended solids can be removed by decantation; this is followed immediately by centrifugation to remove the oil. In many situations, this process would be too costly and it may be possible to skim a certain amount of the oil from the top of the silage but experience has shown that oil is generally emulsified and that the formation of a distinct easily separated fraction does not occur.
The liquid nature of fish silage has always presented difficulties in the transportation and distribution of the product. Where production is for a local pig farm, then this may not matter but, where the silage is to be moved long distances or be fed to livestock which require a dry food e.g. poultry, there may be problems. Recent work at TPI has concentrated on the production of a dry product. The liquid silage is mixed with a powdered or granular carbohydrate source (cassava, wheat meal, rice, bran etc) which absorbs some of the moisture and makes it possible to sun dry the resultant paste. The end product is a dry powder or granular material which contains not only nitrogenous protein constituents but also an energy source from the carbohydrate added. This dry material can be sacked or bagged and is much easier to handle than the straight liquid fish silage.
Fermented fish silages rely on the biological production of lactic acid by bacteria to lower the pH. In general, lactic acid bacteria such as Lactobacillus plantarum ferment sugars to organic acids (primarily lactic), thus lowering the pH of the mixture.
Fish contain only small quantities of fermentable carbohydrates and it is usually necessary to add suitable carbohydrates for the bacteria to convert to acid. Addition of mixtures of malt and cereal meal, molasses and cereal meal, malt and tapioca meal, and molasses and tapioca meal have all proved successful.
The fermentation process for conversion of carbohydrate to lactic acid is anaerobic and can be divided into three stages:
1. The starch of the carbohydrate source is hydrolysed to maltose by alpha and beta amylase.
2. Maltose is broken down to glucose by maltase.
3. Glucose is converted to lactic acid by bacteria. Small amounts of other substances such as acetic acid and alcohol are also formed.
Lactic acid bacteria can be divided into two types: (a) homofermentative, which convert one molecule of glucose to two of lactic acid, and (b) heterofermentative, which convert one molecule of glucose to one molecule of lactic acid plus ethyl alcohol and water. It is, therefore, better to use a homofermentative bacterium if possible. Since fish do not contain many lactic acid bacteria themselves, it is essential to add a starter culture, usually of lactobacilli, for successful fermentation. In addition, it is also necessary to add a source of amylase since the first step in the fermentation relies on the hydrolysis of carbohydrate. In most processes, the amylase is provided by the addition of malt to the mixture since malt is a rich source of amylases.
In essence, the production of fermented silage requires that the fish be comminuted in the same way as for acid silage. A carbohydrate is then mixed with the fish and a starter culture of a suitable bacterium added.
The fermentation should be carried out in full airtight containers so that conditions are anaerobic and successful fermentation is indicated by a rapid drop in pH, as the lactic acid is formed, and the production of gas. The anaerobic conditions may encourage the growth of Clostridium spp. which could be of public health significance; however, if the conditions are allowed to become aerobic, yeasts capable of growth at low pH may develop resulting in the loss of protein.
There are other methods used for production of fermented fish products which use other micro-organisms and/or salt. These are covered in the chapter on fermented fish products.