What is fish silage?
How is fish silage made?
What acids can be used?
The composition of fish silage
How long does fish silage keep?
How is fish silage used?
The pros and cons of making silage
Where might silage be the answer?
This note explains what fish silage is, and describes briefly its manufacture, storage and use.
The note deals mainly with silage made from white fish offal, but the use of fatty fish as raw material is also mentioned. The idea of fish silage is not new; the first work was done in Sweden in the 1930s, and Denmark started commercial production about 10 years later. Although fish silage industries exist now in Denmark and in Poland, there is little or no production elsewhere.
The advice given here is based largely on UK pilot scale trials involving the manufacture of about 10 tonnes of silage.
Fish silage as described here is defined as a liquid product made from whole fish or parts of fish that are liquefied by the action of enzymes in the fish in the presence of an added acid. The enzymes break down fish proteins into smaller soluble units, and the acid helps to speed up their activity while preventing bacterial spoilage.
Silage made from white fish offal does not contain much oil, but when it is made from fatty fish like herring it may be necessary to remove the oil at some stage.
There are other methods of making liquid fish protein, for example by adding enzymes or bacteria, but these are not described here.
The raw material is first minced; suitably small particles can be obtained by using a hammer mill grinder fitted with a screen containing 10 mm diameter holes. Immediately after mincing, 3·5 per cent by weight of 85 per cent formic acid is added, that is 35 kg or about 30 litres of acid to one tonne of fish. It is important to mix thoroughly so that all the fish comes into contact with acid, because pockets of untreated material will putrefy. The acidity of the mixture must be pH 4 or lower to prevent bacterial action. After the initial mixing, the silage process starts naturally, but occasional stirring helps to ensure uniformity.
The production tank can be of any size or shape provided it is acid resistant; some steel containers used for making or carrying the silage may need a polyethylene liner to prevent corrosion. Concrete tanks treated with bitumen are suitable for holding large quantities. The size and number of tanks depend on the amount and type of raw material available.
The rate of liquefaction depends on the type of raw material, its freshness, and the temperature of the process. Most species can be used, but sharks and rays are rather difficult to liquefy, and should be mixed in with other species. Fatty fish liquefy more quickly than white fish offal, and fresh fish liquefy much more quickly than stale fish. It should be possible in most installations to mince and add the acid immediately the raw material is received, thus avoiding slow liquefaction of stale fish. The warmer the mixture, the faster the process; silage made from fresh white fish offal takes about two days to liquefy at 20°C, but takes 5-10 days at 10°C, and much longer at lower temperatures. Thus in winter it would be necessary to heat the mixture initially, or to keep it in a warm area until liquid.
Minced untreated fish must be kept covered to keep out flies; once the acid has been added, flies are not attracted to the mixture.
Once the silage is prepared it can be handled like any other liquid, and transported in bulk or in containers. It can also be blended with cereals to make a semidry feed. Silage made from white fish offal should be stirred as it is removed from the production tank to obtain a uniform batch, since a bone-rich layer tends to settle at the bottom of the tank after a time. Silage made from fatty fish is more homogeneous and there is little separation even after prolonged storage, but the oil in it deteriorates very rapidly; if the oil has to be removed and used for other purposes, it can be separated by heating and centrifuging.
Fish silage can be concentrated to reduce its bulk, but more experimental work needs to be done to assess the commercial advantage of such a process.
A simple system for the manufacture of fish silage.
Several acids can be used, either alone or in combination. Hydrochloric or sulphuric acid can be used; they are reasonably cheap, but a lower pH is required with these mineral acids than with some organic ones, and this means greater corrosion problems, and the silage has to be neutralized before use. Formic acid, an organic acid, is a good choice because preservation is achieved at a slightly higher pH, it has some bacteriostatic action, and the silage need not be neutralized before adding it to the feed, but it is more expensive than mineral acids.
Acids must be handled with care, and formic acid is no exception; operators should always wear rubber gloves and goggles. The acid storage tank, made of resistant material, should be inaccessible to unauthorized people.
The composition of fish silage is very similar to that of the material from which it is made. A typical analysis of white fish offal is 80 per cent water, 15 per cent protein, 4·5 per cent ash and 0·5 per cent fat, and the composition of silage from offal is virtually the same. Whole fatty fish like sprats and sand eels have a higher protein and fat content, and correspondingly lower water and ash content.
Samples from a batch of silage for analysis should be taken only after thorough mixing to ensure that they are representative. Acidity should be measured when making large batches; with formic acid the pH should be 3·6-4; if it is above 4 more acid should be added; if it is below 3·8 less acid could probably have been used, with a saving in cost. The exact amount of acid has to be found by experience, but the proportion given earlier is a good guide.
Fish silage of the correct acidity keeps at room temperature for at least two years without putrefaction. The protein becomes more soluble, and the amount of free fatty acid increases in any fish oil present during storage, but these changes are unlikely to be significant nutritionally. Fish silage in any event would probably not be stored commercially for more than about 6 months. Silage becomes smoother in consistency during storage, and develops a pleasant malty odour.
Fish silage is used in the same way as fish meal in animal feed. Fish meal contains about 65 per cent protein whereas fish silage contains about 15 per cent, so that about four times as much silage is required for the same protein intake. The most suitable outlet for silage appears to be in pig farming, since it can be used in liquid feeding systems. Silage can be used alone, or with fish meal; feeding trials show that pigs grow as fast on silage as on meal, and the quality and flavour of the meat is good. Fish silage is used in the Danish pig industry, and most nutritional work has been done there. Other animals have been fed on silage with good results; cow's milk and butter are without taint, and egg production from hens is high.
The main advantage of the fish silage process is that, in areas where there is no fish meal factory, fish offal and waste fish can be utilized instead of being thrown away.
The advantages and disadvantages of making fish silage instead of fish meal can be summarized as follows.
Capital cost of meal plant is fairly high; the cost of silage equipment is fairly low.
Processing of meal requires engineers and technical staff; silage can be made by unskilled workers.
Smell is a problem when making meal, unless specially equipped plant is used; there is no smell when making silage.
Transport of meal is cheap, because the stable concentrated powder is low in bulk; silage is more expensive to carry because the liquid, which contains all the water that was in the fish, is four or five times as bulky as meal.
Marketing of fish meal is long established and the product is well known; silage is little known in the UK and if anything more than local production and use is envisaged, some marketing effort would be required. Silage manufacture might sometimes serve as a preliminary step towards making fish meal by proving the existence of a sufficient supply of raw material before making a large investment.
The silage process is most likely to be successful in areas where fish offal or waste fish is regularly available, but the cost of sending it to the nearest meal plant is prohibitive, and where there are farms, particularly pig farms, close by.
There is no limit to the size of a silage plant; a batch can
be made in one oil drum or in a tank holding several tonnes. Each situation has
to be considered on its own merits, but the larger the potential production and
the greater the distance the silage has to be carried the more likely it is that
meal manufacture is more appropriate.