(Article researched and published by Ice Cool Services Ltd).
Ice is the oldest refrigerating medium known yet the most advanced supermarkets in the world still use it and despite rapid advances in the science of mechanical refrigeration they’re using more than ever. Why?
It is because ice remains the ‘chiller of choice’ for foods that require both a low temperature and a high moisture level for optimal preservation. Those food products that benefit the most from ice storage are the same ones that are increasingly in demand as ingredients of a healthier diet, leafy green vegetables, broccoli, asparagus and fresh fish.
Chilling alone won’t preserve them unless the chilled product is enveloped in moisture.
Ice is everything when it comes to fish. If you sit fish in cold air alone you desiccate it. The dry air attracts the moisture from the fish – known as ‘drip-loss’
The same with fresh fruits cut an apple and it shrivels. An hour later the cut is brown because of oxidation and loss of moisture. If you set the same apple on a mound of ice for an hour it will stay white. The apple retains its moisture because the air around it is saturated with moisture from the melting ice and can’t absorb any more from the apple.
What is being demonstrated is a combination of physical phenomena that occurs whenever ice is placed in an environment warmer than the ice itself.
The first phenomenon, explained in Newton’s Second Law of Thermodynamics, is that heat always migrates from areas of greater concentration to areas of lesser concentration. When a warm object is placed next to a colder one the warm object gives up its heat to the colder one until both stabilise at the same temperature. When the colder object is ice two additional phenomena come into play melting and evaporation. The ice changes from a solid to a liquid state in a process that consumes enormous amounts of energy that has to be drawn from the adjacent warmer substances. If the ice is surrounded by air a second transformation occurs as the newly formed melt water turns into a gas called water vapour.
Ice that is about to melt has a temperature of 0ºc and water that has just melted has a temperature of 0ºc. In order to change a kilogram of ice at 0ºc to a kilogram of water at 0ºc you have to ‘borrow’ 93 watt of energy from the adjacent produce. This is called the “Latent Heat of Fusion”.
The 93 watt of heat energy drawn from the produce doesn’t make the ice warmer as would happen if the chilling agent were a cold brick. Instead the heat energy is used to melt the ice into water which carries the excess heat away as it drains. As long as ice remains adjacent to the produce the produce will remain at 0ºc.
As quickly as water forms on the surface of the ice it evaporates into the surrounding air until the air can hold no more. If fresh fish or a delicate frond of spinach is resting on melting ice moisture from the product will not be attracted into the ambient air because moisture generated by the ice has already saturated the evaporative capacity of the air.
Putting ice under a product will keep the product wet reduce moisture loss and drying out. An additional benefit for fresh fish is that the melt water continually washes surface bacteria from the fish preventing odour.
ICE FOR FRESH FISH DISPLAY – WHY?
• Maintains constant low temperature 0ºc.
• Provides high moisture level preventing drying out of the product.
• Melting ice continually ‘washes’ fish removing surface bacteria and preventing odour.
• Ice as a display medium is conveniently renewed and disposed of on a daily basis (no environmental issues).
• Customers traditionally expect fresh fish to be displayed on ice. Ice reinforces the concept of “fresh”.
• Compressed flake ice, free flowing and crystalline - as specifically developed by Ice Cool Services Ltd for the UK Supermarkets - enhances a fish display whilst providing all the benefits above.