The big chill

Advances in technology still dictate tastes in food. But today, the foods are more complex. Food manufacturers are developing new products at an ever-increasing rate: marinated chickens, sauces, pizza toppings, 'kievs'. These are invariably frozen, and freezing technology has to come up with ways of delivering them.

The food technology division of BOC struggles daily with these problems in laboratories in the US and Europe. The challenges it faces are how to freeze the food quickly and hygienically; and how to avoid gumming up the works of its machinery in the process.

There are two way to freeze or chill food, explains marketing manager Scott Peterson. You can use a mechanical freezer, like domestic models but larger, where a chemical refrigerant is pumped through a series of expansions and contractions which extract heat; or you can use a cryogen generally taken to mean anything below 100 C. This is BOC's speciality. Says Peterson: 'Cryogenic freezers tend to be smaller than mechanical ones, and need less capital investment.' They also act very fast exposing something to a temperature of -196 C, the boiling point of liquid nitrogen, will freeze it solid very rapidly, and using the cold vapour generated by the liquid gives a highly controllable method of chilling. The downside to these advantages is that running costs are higher, as the cryogen must be purchased regularly; the systems are 'closed' and need to have venting systems for the expanding gases; and the gases can be hazardous, because of the temperature and their lack of breathability in the case of nitrogen, so operators need specialist training.

Nevertheless, cryogenic freezers can be very versatile. One particular problem area is 'IQF' (individually quick-frozen) foods. These are basically anything that comes in a small lump, but have to be frozen separately for example, prawns, chicken nuggets, cubes of cheese, and pizza toppings. Generally, explains Richard Leeson, business support manager for new cryogenic technologies, these are frozen by dunking them into liquid nitrogen, but this is very inefficient.

To get around this problem, BOC has developed a fluidised bed freezer. This was devised by the US division for the country's large shrimp-processing industry, but it is equally capable of coping with any IQF product, claims Leeson.

The items enter the freezer on conveyor belts at the top, and are first run through shallow trays of liquid nitrogen. This forms a crust on the surface which stops the individual items sticking together, says Leeson. The items then drop onto another conveyor to form a layer up to 6in deep. This conveyor made from stainless steel chain runs over a series of fans which blow the gas generated by the liquid nitrogen in the immersion trays up through the layer of food, fluidising the items and continually turning them over. By the time they emerge from the freezer just 212 minutes after they went in the items' temperature has dropped from 7 C to -20 C. Despite the unit's small size it's an eight-foot cube it can handle 2000lb/hr of food, claims Leeson.