What are polyphosphates?
How do they help in fish processing?
What can polyphosphates not do?
Which polyphosphates are used?
How are they applied?
Are polyphosphates injurious to health?
Are there patent restrictions?
This note describes what polyphosphates are, what use they are in fish processing, how to apply them and what regulations govern their use. The questions of patent restrictions and possible health hazards are also briefly discussed.
Polyphosphates are legally permitted additives that are widely used to aid processing or to improve eating quality of many foods, particularly meat and fish products. Phosphates are also used in making baking powder and cola drinks, and great quantities are used in fertilizers and detergents. Phosphates are present normally in all living things and are an essential component of our diet. A phosphate is a salt of phosphoric acid; when a number of simple phosphate units are linked to form a more complex structure, this is known as a polyphosphate. The phosphates used in foods may be simple phosphates, pyrophosphates containing two phosphate units, tripolyphosphates containing three units, or polyphosphates containing more than three phosphate units.
Many benefits have been claimed to result from the use of polyphosphates. This section describes those effects for which good evidence exists. The following section tells what polyphosphates will not do.
The main value of polyphosphates lies in improving the retention of water by the protein in fish. The manner in which they do this is not clearly understood, but their effect is mainly on the surface of the fish. Other substances such as common salt give similar results but with undesirable flavour effects.
Polyphosphate treatment of fish before freezing often reduces the amount of thaw drip, that is the liquid released when frozen fish is thawed. Good quality fish, properly frozen and cold stored, normally develops little thaw drip; therefore application of polyphosphate to such material is generally only of slight value. Poor quality fish when frozen and thawed may drip much more, and treatment will reduce the loss to some extent, but this is not sufficient reason for using polyphosphates; poor quality fish should not be frozen, since the product will be poor irrespective of treatment. Good quality fish that is frozen and then stored at too high a temperature may have a high thaw drip loss, and again polyphosphate treatment before freezing can reduce the loss, but this does nothing to prevent the corresponding deterioration in flavour and texture; there is no substitute for good cold storage. Many retail products, especially breaded ones, are cooked from the frozen state without prior thawing; thaw drip from such products is generally of no great significance and the value of phosphating is doubtful.
Chilled fillets lose water slowly during processing and distribution. The loss is usually small, but it may be increased by high temperature, excessive pressure on the product, and by delay in distribution. Polyphosphate treatment of the fillets will reduce drip loss, but will have no appreciable action on the effects of spoilage on odour and flavour; treatment does not reduce the need for rapid handling or the need for keeping the fish chilled. There are two instances where polyphosphate treatment of chilled fillets or portions may have a specific advantage. First, chilled fillets of good quality from thawed sea frozen fish often lose a high amount of drip during distribution; polyphosphating after filleting can substantially reduce this loss, and can also dramatically improve the appearance of the fillets by changing the dull, often brownish, surface that is typical of sea frozen material to a glossy bluish white. The dull surface of poor quality fillets from stale fish can also be given a gloss by treatment with polyphosphate, but it must be emphasized that the improvement is only in appearance; the poor quality remains unchanged. Secondly, polyphosphate treatment can improve the appearance of prepacked chilled fillets by preventing the accumulation of unsightly drip within the pack. The treatment of prepacked fillets is described in Advisory Notes 51 and 52.
Many retail products such as fish fingers and portions are cut from frozen blocks of bonefree flesh known as laminated blocks. The preparation of these blocks from boneless, skinless fillets is simplified by polyphosphate treatment. The polyphosphate swells some surface protein to give a lubricating layer which enables the fillets to be packed easily into moulds, without leaving voids which could affect the shape and weight of products cut from the blocks. However, polyphosphates are not an essential ingredient of laminated blocks; quantities of blocks have been manufactured without addition of polyphosphates.
Blocks are also made from minced fish flesh, or a proportion of mince may be incorporated in fillet blocks. Polyphosphate mixed with the mince acts as a binding agent by swelling protein to give the product an improved texture. A similar binding effect occurs when the treatment is applied to fillets that are particularly liable to break up on thawing, for example hake fillets.
The use of polyphosphates in laminated blocks cannot be justified on the grounds that drip is reduced; not only is thaw drip not a problem with products cut from laminated blocks, but the action of cutting a block greatly reduced the antidrip effect of the polyphosphate on the surface of the fish.
The first and universal effect of all polyphosphate treatment is to increase the weight of the fish by retaining water. The weight gain is of no technological benefit, and represents to the producer a gain in weight of product sold. This weight gain, which amounts to selling added water, clearly must not be the aim of polyphosphate treatment; polyphosphates should be added to fish only for technically justifiable purposes.
Polyphosphate treatment makes fillets slippery and difficult to handle; although slipperiness is an advantage when making laminated blocks, it may be a nuisance under some circumstances.
Polyphosphates cannot significantly improve the eating quality of fish, although claims in this respect are often made. Excessive treatment of small products such as shellfish or thin fillets can even result in undesirable flavour changes and sloppy texture.
Polyphosphates have no preservative action on chilled fish, nor do they give any protection against cold store deterioration.
Most processors now use proprietary mixtures containing appropriate polyphosphates. These mixtures may contain pyrophosphate, tripolyphosphate or compounds with more than three phosphate units; these last have the advantage of being highly soluble in water, so that the supplier can offer concentrated solutions that are readily diluted to the required strength by the processor. Mixtures based on tripolyphosphate often contain other phosphates to make the solution less alkaline and so less likely to cause skin irritation.
Proprietary solutions should be prepared according to the makers instructions. If sodium tripolyphosphate is used as the active ingredient, a satisfactory mixture can be made by dissolving 5 kg of food grade sodium tripolyphosphate and 5 kg of glassy sodium phosphate in 90 litres of water. The solution should be chilled to avoid warming the fish and to increase the life of the solution.
The simplest way of applying polyphosphate is to immerse a batch of fillets or pieces of fish contained in an open mesh basket, agitating them gently so that all surfaces come into contact with the solution. Alternatively the fish can be conveyed mechanically through a long shallow tank of solution; suitable machines are available that can be incorporated in a continuous processing line. The solution should be renewed daily, or more often if badly discoloured or contaminated. A third possibility is to tumble the fillets gently in a slowly rotating drum containing a small amount of solution, say 5 per cent of the weight of the fish; most of the liquid is taken up by the fish in a few minutes. Minces can be treated by adding either solution or dry powder at some stage in the mincing or mixing procedure.
Polyphosphate treatment should be just enough to produce the desired technological effect and no more. The solution prepared as described above contains 5 per cent of the active ingredient, sodium tripolyphosphate; weaker solutions have less ability to reduce drip although they have some value, for example in improving the appearance of fillets of sea frozen fish or in helping to make good fillet blocks. Immersion time in a batch or continuous process should be no longer than necessary; usually a minute or so is enough. Longer immersion time leads to unjustified increase in weight and the risk of flavour or texture deterioration. Since polyphosphate acts on the surface of fish, fillets should not be cut or trimmed after treatment.
Most polyphosphates added to food are broken down to single phosphate units in the stomach when the food is eaten; indeed, many are converted to single units in the food before it is eaten, for example in chill storage or during cooking. Thus it is clear that most phosphates added to food are nutritionally equivalent to the phosphates naturally present in food, and are likely to present little hazard to health. Nevertheless it is possible for essential food components to be harmful if taken in excess, and international medical authorities have recommended that the daily consumption of all phosphates should not exceed a certain level. It is believed that the amounts presently being eaten by the typical consumer are well below this limit. Some countries do have limits on the amount that can be added and exporters must bear this in mind. The subject is under regular review both in the UK and in the EEC and it is possible that restrictions will be imposed in the future.
The use of a wide range of phosphates to control thaw drip in
fish, excluding shellfish, is governed in the UK by a patent granted to one firm
of phosphate manufacturers in 1961; this patent will lapse in 1977.