By Franz Scheurer
Yesterday I attended a most interesting lecture by Ken Harada at the Sydney Fish Market entitled Freezing Training. It dispelled a great many myths and misconceptions and taught me just how ill informed even a dedicated foodie can be.
Ken started off by showing us a graph of enzymic and bacterial spoilage of seafood according to the temperature. As you would expect, at +18șC damage occurs quickly and below -14șC it happens much slower. However, between -2șC and -5șC seafood spoils quicker than if it is kept between +4șC and -1.8șC.
The reason for this is that seawater has a salinity of 3.5%, which means it doesnt start to freeze until -2șC. Cells contain water and there is also so-called free water between the cells. Between -5ș and -10șC only 70% of the water in the seafood freezes. As the free water freezes, it compacts and isolates the 3.5% of salt. This creates areas of high salt that require much lower temperatures to freeze. Amino acids are stored in cells as a spiral, which holds the water, once denatured the spiral breaks and water escapes. At -5șC water in the cells starts to escape and become free water, which in turn leads to the cells collapse. This is why, when you squeeze a fillet of fresh fish no water comes out, but if you do this to a thawed piece of fish (that has deteriorated during freezing) water will be extracted.
Most domestic chest freezers are struggling to reach -10șC and most freezer compartments in domestic fridges are more likely to run at -5șC.
The speed of freezing is another important factor in preserving the quality of the produce. The object is to pass from 0șC to -10șC as quickly as possible. Crystals stop forming at -10șC, but may still change at lower temperatures (as will be explained below). An ice slurry with 3.5% salinity helps to bring the temperature of the produce down to 0șC before the freezing process. Freezing creates crystals, which increase in size as the temperature decreases, until -10șC. The longer it takes to freeze the produce completely, the greater the number of crystals created. The higher the temperature frozen product is stored at, the greater the number, and size, of crystals created. Therefore, more (and larger) crystals will be created by storing at -5șC than at -10șC. Items that are snap-frozen using liquid nitrogen will show no change in cell structure (Ken Harada claims that an experiment he conducted with snap freezing goldfish and replacing them in their standard habitat a while later proved this, with the fish happily swimming as if nothing happened). By the time 8 hours have passed to freeze seafood to -15șC by conventional (non-snap freezing) technology, the proteins have already been denatured and oxidation of the fatty acids has occurred. The faster the danger zone (0°C to -5°C) can be passed, the better the resulting product.
Temperature fluctuations are another problem. Every time the temperature rises, the smallest crystals will start to melt. They then attach themselves to the larger, yet unmelted crystals, once the temperature is lowered again. These newly enlarged crystals break through cell walls and, once proteins have been denatured in this way, the texture of the fish will never be the same. Oxidation follows closely, leading to a distinct paint smell, a result of rancid oils.
Fatty acids are stored as either saturated or unsaturated fats. Saturated means that each Carbon atom is connected to four non-Carbon atoms, typically three hydrogen and one glyceride. These are very robust molecules. Unsaturated means that Carbon atoms form a chain where one arm is always connected to another Carbon atom. These unsaturated elements are long, fragile chains ending in glycerides, which contain the Omega elements that are so important to our health. They will break easily under the strain of increased size, resulting in oxidation of the fats, and in rancid fatty acids and a tainted taste.
Although seafood stored at -5șC can be kept for 3 to 4 months without poisoning the consumer, a one month period is far more realistic if you value the texture and taste of your seafood. At -18șC the maximum safe time is increased to 1 year. At -30șC bacterial spoilage stops totally, although enzymic spoilage will continue, albeit at a very slow rate. Only at cryogenic temperatures (-70șC) is the damage totally controlled. A product can be kept with minimum quality changes at -60șC for 4 years and at -30șC for 12 months.
Note that green prawns and other shellfish are often treated by metabisulphite (the law allows 30 parts per million). Many people who experience allergic reactions to shellfish are more likely to be allergic to the metabisulphite than the shellfish. This can be overcome by soaking the shellfish in a 3.5% salt solution for about one hour. As the contamination is only surface based, it will leach out into the saline solution. Your taste buds will notice the difference too.
What does all this mean in real terms?
Chilling fish by resting it on ice in the fridge is still the best method of maintaining the original quality, taste and texture. Freezing fish is the next best thing, but make sure it is done quickly, stored at a suitable temperature, and packed with the least amount of air in the package to insure no dehydration occurs. (i.e. freezer burn)
In real terms: unless you own a professional freezer, buy your seafood fresh and consume it quickly, enjoying the full gamut of flavours and textures that make this such a rewarding food.
For more information call the Sydney Fish Market Fish Line on 02 9004 1122