Fresh or frozen? The science of freezing food

So just why do we freeze food? Good question. At this point, we could have tried to bluff an answer, but on reflection decided it was better to ask the white coated boffins at the British Nutrition Foundation to talk us through the science.

They were very keen to help - we suspect they don't get out much - so the first thing we had to do was listen to an overview of the different methods of food preservation.

It seems food preservation prevents or delays spoilage and stops the growth of harmful organisms which would make the food unsafe, by changing the environment that the food is in. This can include traditional methods such as pickling and sun-drying, and more modern approaches such pasteurization and irradiation.

But, somewhat pointedly bringing them back to the subject at hand, we learnt that freezing is based on two principles:

• Very low temperatures inhibit growth of micro-organisms and stop enzymic or chemical activity
• The formation of ice crystals draws away available water from food, therefore preventing the growth of micro-organisms.

Domestically - and to be honest in most pubs - food is frozen by placing it in a freezer and allowing heat transfer to occur by conduction, that is, the heat is removed from the food. This process can take several hours, depending on the nature of the food being frozen.

Other forms of freezing have been developed by the food industry to speed up freezing time. This reduces the risk of micro-organism growth. Examples include:

• Blast freezing: This involves batches of food being subjected to a constant, steady stream of cold air, at -40C or lower, in a tunnel or cabinet. Many bigger pub kitchens have blast chillers
• Scraped heat exchange: Products such as ice cream are frozen using this method to reduce the formation of large ice crystals. The product is churned against a cold surface at high speed
• Cryogenic freezing: Liquid nitrogen or carbon dioxide is sprayed directly onto small food items such as soft fruits or prawns. Owing to the liquids' extremely low temperatures, -196ÞC and -78ÞC respectively, freezing is almost instant.

The nutritionists then tried to move the conversation on to other forms of preservation, such as modified atmosphere packaging. But when they whipped out some vacuum packed nuts to demonstrate, we decided it was time to move on. Thanks to the British Nutrition Foundation, www.nutrition.org.uk