Correct defrosting means higher yield and improved product quality

The defrosting of frozen raw materials is an everyday routine in fish processing companies. Although it is a highly sensitive process which has an influence on raw material yield and product quality it is rarely paid the attention it deserves. In other words, there are some reserves here, particularly since the defrosting process often proves to be a “technological bottleneck” within operational production procedures.

A recently conducted survey of 155 Norwegian seafood companies showed that three quarters of the companies questioned had integrated defrosting into their production processes. 93% of the companies performed this production step without any specific controls, however, i.e. without the usual detailed operational instructions. This would seem to imply that there is some catching up to be done here – and not only in Norway. Because seafood products are frozen there is the logical problem of defrosting them later on. From a physical point of view, defrosting is the reverse process of freezing, with the heat flow going in the opposite direction: whilst during freezing heat is taken out of the product it has to be put back into it during defrosting. Defrosting is thus a process that requires energy, and the necessary energy comes from the product’s environment: once there is more energy or heat surrounding the product than in the product itself the water contained within the product begins to melt and the product thaws.

Despite several parallels it would be wrong, however, to see freezing and defrosting as quasi identical processes, be it in different directions. Defrosting is in many respects considerably more complicated than freezing, particularly with regard to the ability to influence and control the process when dealing with such sensitive foods as fish and seafood. A high temperature difference between the product and the cooling medium during the freezing process does not constitute a risk and can even be of advantage (during cryogenic freezing, for example, liquid nitrogen at a temperature of -196°C or carbonic acid at a temperature of -79°C is sprayed onto the product) but excessive temperature differences should be avoided during the defrosting process. Admittedly, a high temperature difference would certainly speed up the thawing process but it could also damage the product. The degree of heat that flows into a product during defrosting should be just sufficient for the ice crystals to melt. At the same time, the process should be as fast as possible because otherwise enzymatic decomposition processes in the tissue are accelerated and the development of micro-organisms encouraged.

Defrosting is in many respects considerably more complicated than freezing, particularly with regard to the ability to influence and control the process when dealing with such sensitive foods as fish and seafood.

Defrosting time hard to predict

Defrosting thus takes considerably more time than freezing, for one thing because the temperature difference between the frozen fish and its environment has to be kept as low as possible for the above reasons; and for another because the defrosting speed gets slower and slower over the course of the process. This is because heat conduction is lower in thawed fish than in frozen fish and so the more fish is thawed, the longer the defrosting process takes. The frozen fish thaws from the outside inwards. During the course of the thawing process a temperature difference is created in the fish’s body. Whilst it is already thawed and soft on the outside it can still be frozen and hard on the inside. In order to completely defrost the fish, heat has to flow through the already thawed body layers from the outside into the inside which is still frozen. The thicker the layer of thawed fish is the more time it takes for the ice in the product core to melt. The temperature differences within the product can amount to 15, 20 or more degrees depending on how far the thawing process has progressed and this makes exact control of the overall process relatively difficult. As a rule, however, the defrosting process is considered to be complete when the temperature of the product core is about minus 1°C, at which point it can be assumed that all the ice crystals in the product interior have changed back to water.

It is difficult to predict just how long this will take. Defrosting speed depends on several factors, such as the chosen defrosting method, the type and thickness of the frozen product, and the conditions of heat transfer. A simple rule of thumb states that seafood products should be defrosted as fast as possible and as slowly as necessary. In spite of the difficulties mentioned above it is important to recognize the optimal thawing point because both products that have been defrosted for too long and products that have not been sufficiently defrosted and are still frozen in their core can lead to economic losses. For example, excessive defrosting can make the flesh soft and limp. The product’s typical colour, aroma and flavour properties are lost. And the activity of some enzymatic and chemical spoilage processes increases. On the other hand, fish that is not completely thawed is much more difficult to process and fillet. Filleting yield decreases because the frozen core of the product makes it more difficult to guide the cut along the backbone.


Defrosting can influence the germ count in a product

The biggest economic losses come from the defrosting process itself, however, due to drip loss and the development of harmful micro-organisms which can reduce the potential shelf-life of the product.

It may at first seem paradox that large numbers of micro-organisms can develop during defrosting, bearing in mind that probably more germs are killed during the defrosting process than during freezing and storage. The reason for this lies in the immense stress to which the bacteria are subjected during the defrosting phase. Before the ice crystals turn back into water they “expand” slightly, i.e. their diameter increases slightly. This increase in volume constitutes a considerable mechanical strain on the bacteria. Added to this is the osmotic stress that occurs when the previously frozen tissue fluid melts, thereby “diluting” the ambient medium around the bacteria. Although a very large number of germs are killed or at least inactivated during this process a considerable number of them naturally still survives. And this is where the risk lies because in the course of the thawing process the outer layers of the product quickly reach temperatures at which these germs can flourish again. In general, the higher the defrosting temperature the higher the risk of microbial spoilage. But there are also psychrophilic bacteria (cold-loving bacteria) which reach their optimum at low temperature ranges. For these reasons it is important to control temperature constantly during thawing and to keep the process as short as possible. A long time before the germ count reaches a level that presents a real health risk seafood can already become inedible and thus no longer sellable due to bacterial contamination.

The other big complex of conceivable changes to which a product is subject during defrosting can in the broadest sense best be termed “moisture changes”. These, too, can lead to economic losses. They do not only come about during thawing, however, but strictly speaking already during freezing and frozen storage of the product. Overall they can alter the product’s physical, chemical and biochemical properties. Nearly all of these changes are related to the water that seafood products contain in their tissue. This water can be redistributed and re-absorbed or completely evaporated by sublimation (sublimation: direct conversion of a substance from a solid to a gaseous state without going through a liquid state). In this way, even frozen products can lose surface moisture and dry out locally. Freezer burn which is caused by incorrect storage is nothing other than local water loss in a product.


Fast shock freezing reduces drip loss

Whilst defects such as freezer burn are already recognisable in the product in its frozen state other damages do not become visible until the product is thawed. Mass loss – also called thawing loss – usually occurs during defrosting. This results on the one hand from the melting of the glaze and the rinsing water with which the product was cleaned prior to freezing. This mass loss does not have a negative effect on net weight because this water share was added to the product during the course of freezing. However, the tissue’s ability to bind water can be reduced through freezing and frozen storage, for example if the product was not frozen quickly enough or if it was stored under strongly fluctuating temperatures so that sharp ice crystals have formed and penetrated the cell walls and membranes. In this case, cell and tissue fluid escapes during thawing. This is what is called drip loss. It can account for 3 to 5% of net fish mass and adds up to huge economic losses when one considers that when defrosting one tonne of fish about 30 to 50 kg of the original fish weight are lost.

But there are other reasons for economic losses, too: drip loss also reduces product quality. The milky liquid that drips off the product contains numerous soluble nutrients, especially proteins that are thus lost and hence reduce the product’s nutritional value. And that is one of the reasons why great effort has gone into reducing drip loss during thawing. Paradoxically, drip loss is particularly high when the product is thawed according to recommendations, i.e. relatively quickly, in order to protect the tissue. Although fast thawing helps maintain the typical properties of the product better the tissue has less time to reabsorb the drip loss. Reabsorption of cell and tissue fluid is a slow process that takes several hours, particularly in the case of fish, whose muscle structure has a relatively low absorption capacity for liquids anyway. Slowing down the defrosting times does not offer an alternative, however, because as described above this could lead to damages to the product elsewhere. In practice, it is impossible to get around having to make a compromise here. This makes it all the more important to do as much as possible in advance to prevent drip loss later on. Fast shock freezing and constant control of temperatures during frozen storage and transport are the best way to keep drip loss to a minimum.


Thaw rigor can damage products

The seafood industry endeavours to process and freeze the catch as fast as possible to enable optimum maintenance of freshness. Some companies have developed their technology and logistics to such an advanced stage that the fishes and ready fillets are already frozen prior to the start of rigor mortis. This pre-rigor freezing has a particular effect called “thaw rigor” in which rigor mortis occurs during or directly after thawing and sometimes damages the muscle tissue. Whilst this is rarely the case in whole fishes it can frequently lead to defects in fillets. Pre-rigor frozen fillets that are thawed at relatively high temperatures often display a particularly intense rigor. As soon as the water has melted in their interior the muscles contract strongly and the fillet bends and squeezes out a larger quantity of drip. This effect can be seen particularly when pre-rigor fillets are not first thawed but prepared, i.e. heated in their frozen state. In extreme cases the fillet can then become hard and tough. Gaping often occurs, too: the connective tissue between the individual muscle areas tears so that the fillets gape at these points. Gaping also leads to larger quantities of drip loss.

The easiest way to avoid thaw rigor is to leave the product for a sufficiently long time in frozen storage. If the product is stored for at least eight weeks at a temperature of minus 28°C or colder, rigor mortis will occur already in the frozen state so that it is already over when the product thaws. If a pre-rigor fillet is stored for less than eight weeks it should be thawed slowly at room temperature. Slow defrosting allows rigor mortis to take place gently in semi-frozen state without the described muscle contractions.


Increase attention to thawing

In everyday practice defrosting of frozen raw materials is rarely a process to which much attention is paid, however. As a rule it is only an auxiliary process through which the raw material must pass prior to further processing. Often defrosting is even seen to be a bottleneck which can be an obstacle during production if the process is not exactly fitted into the timing of the technological processes within a company. It is not only for economic reasons that it is worthwhile to pay more attention to thawing: correct defrosting can also have a positive influence on product quality. Whilst smaller raw material quantities can mostly be thawed “by hand” according to set routines (instructions should define times, temperatures, maximum defrosting quantities and throughput of defrosting medium) for larger quantities it is wise to consider investment in a defrosting unit. Such automatic plants defrost frozen products according to set programmes and mostly they pay off quickly due to consistent and often better product quality.

Part 2 of our series will look at common techniques of industrial thawing.