Chill winds over a cold climate

New moves in Europe to tighten control of hydrofluorocarbons (HFCs) could spell the beginning of the end for today's refrigerant gases. The HFC gases now used in refrigeration and freezing systems, including air-conditioning, account for 2% of the UK's greenhouse gas emissions. As a result, political and environmental pressure is mounting on the food industry to green up its refrigeration, chilling, and freezing processes.

So what can the food industry do? One answer could well be a return to a refrigeration technology developed over a 100 years ago -- the air cycle. In the air cycle, air is compressed, cooled, and then allowed to expand and do work by blasting its way through a turbine. This rapidly reduces its temperature further -- down to -140° C. The cold air can either be used in an open system to blow cold air over food, or fed through a heat exchanger in a closed system.

Air cycle refrigeration was in widespread use in the early 1900s on board ships, and by food processors and retailers to keep food stores cool. But the size and unreliability of the compressors and turbines at the time eventually killed it off. Now, however, a team of researchers from Bristol University is hoping to get government cash to revive the air cycle. It has put forward a Department for Environment, Food and Rural Affairs (DEFRA) Link project to develop the air cycle into a combined heating and cooling system for cook-chill, cook-freeze food processing. This would use the considerable amount of heat generated in compressing the air to cook the food, and then use the cold air produced to cool it. A Japanese air conditioning manufacturer has promised to donate a newly developed turbine to kick-start the project.

Another answer may come from the world of human tissue banks. Knowledge gained by small Cambridge company Asymptote in controlling the freezing of human embryos and other medical tissues is being used to improve the quality of frozen food. By carefully controlling the freezing of fresh fruit and vegetables, cell damage is minimised and the quality and taste after thawing is considerably improved. Asymptote says that in trials on fruit salad, for instance, consumers could not distinguish between fresh and frozen/thawed samples.

And because the freezing process can be adjusted to destroy parts of the cells responsible for rotting, the shelf-life of fresh tropical fruit such as mangoes, papayas, and pineapples can be extended by freezing it and thawing it. Asymptote claims it offers an alternative to genetic modification (GM).

When the world agreed to take the 'chloro' out of CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) in the early 90s, the refrigeration and freezer suppliers gradually phased out the old ozone-busting refrigeration gases -- the CFCs and the HCFCs -- and replaced them with chlorine-free HFCs and perfluorocarbons (PFCs), known as the F-gases. But although the F-gases may not burn holes in the ozone layer, there has been mounting concern that they contribute to global warming. And so the food industry is now facing moves to phase out these gases as well.

The European Parliament has before it draft regulations to cut emissions of F-gases by a quarter by 2010. This will be done by insisting on containment systems to minimise leaks, mandatory inspection of fridge and freezer installations, and the banning of some types of cooling systems. A decision is expected early this year.

So it is beginning to look increasingly likely that synthetic refrigerant gases will eventually be phased out altogether. The alternatives are the so-called 'natural' refrigerants such as ammonia, carbon dioxide, and hydrocarbons such as butane.

Air cycle potential

But according to Steve James at Bristol University, air has potentially the best credentials of all the natural refrigerants, hence the renewed interest in the air cycle.

If it leaks, no harm is done to the environment. If it gets into food, no harm is done either because it is sterile, having been heated to 200°C during compression. Indeed, says James, the air can be injected directly into food mixes to create new products. "I've seen one that is almost like an ice-cream Aero."

Also, over extended cooling periods at lower temperatures, the air cycle can be just as efficient as conventional refrigeration. And because the air cycle can reach temperatures down to -140°C, it bridges the gap between normal refrigeration, which can get down to -40°C, and cryo-freezing using liquid nitrogen which doesn't really kick in until about -100°C.

But the air cycle's biggest potential, says James, is for combining cook/chill, cook/freeze, and blanch/freeze type operations into a single integrated process. "You can get temperatures of up to 300°C on the cooking side if you really push it. And once you combine cooking with cooling in one unit, then the overall efficiency is far better than from conventional stand-alone refrigeration and cooker systems." And it would take up far less floor space, he says.

The problem is finding a supplier of air cycle compressor/turbine units. Today, the only widespread use of the air cycle is in aircraft air-conditioning where reliable, compact compressor/turbine units have been developed -- but at a cost way beyond what the food industry is prepared to pay.

In an earlier DEFRA Link project to investigate the feasibility of the air cycle for food refrigeration, Bristol University used a prototype air cycle unit that had been developed by aerospace company Normalair-Garrett (now part of Honeywell) for air conditioning on the German ICE2 high speed trains. But Normalair-Garrett pulled out of further development, leaving food refrigeration companies without a source of low-cost compressor/turbine units.

However, this time round, James has found a Japanese firm that is prepared to develop a compressor/turbine unit for the refrigeration market. "They have produced a prototype 60kW air cycle air unit and in effect said they are prepared to help people develop it for other uses. And they are willing to donate a unit to us for the Link project. James hopes to get the go-ahead for the three-year Link project by Easter.

From IVF to frozen food

Asymptote is better known for developing techniques for freezing human embryos for invitro fertilisation (IVF) treatment than it is for making fresh fruit salad taste better. But its expertise in controlling the growth and location of ice crystals during the freezing of medical tissues could be about to revolutionise the frozen food market.

What Asymptote has done is to use its medical experience to modify conventional cryogenic food freezing tunnels to reduce the damage to the cell structure of food tissues during freezing. The result is a dramatic improvement in texture and taste when the food is thawed out. The process can be applied to meat, fish, and shellfish, as well as fruit and vegetables.

"Our background is in freezing and crystallisation for medical applications," says research director Dr John Morris. "And we found we could get very good recovery of frozen foodstuffs by treating them as medical tissue." The trick has been to control ice crystal growth on a microscopic scale. By imposing different heat transfers around a piece of tissue you can modify when and where ice crystals form in the cell tissue, he says. And by devising different freezing profiles for different tissues, Asymptote has been able to minimise cell damage and get very good recovery from a range of different frozen tissues.

The technique was initially tried out at fruit company Geest in the late 1990s. By adapting a commercial cryogenic freezing tunnel, Asymptote found it could scale up the process for different types and different sizes of tropical fruit. "Pieces of fruit would be frozen, stored frozen, then assembled into fruit salads, thawed out and put onto the supermarket shelves as fresh fruit salad," says Morris. "And you couldn't tell the difference from fresh."

There were other advantages, says Morris. The shelf-life of a fresh tropical fruit was actually extended by freezing and thawing. "You knock out the mitochondria so that the cells don't respire any more and don't make the energy for the rotting process. We would cut a mango in half, for example, keep one half fresh and freeze and thaw the other half, and then put both side by side in a fridge. We found the frozen and thawed piece would remain edible two to three times longer than the fresh piece."

Unfortunately the trials at Geest came to an end when the company pulled out of fruit, says Morris. However, Asymptote is now working with a number of refrigeration companies to commercialise its technology and expects to see equipment on the market within the next year or so.

The F-Gas Regulation will be covered at the Refrigeration and Air Conditioning show RAC05 taking place at the NEC Birmingham, March 1-3. http://www.racexhibition.com​

Asymptote, tel: 01223 421161Bristol University, tel: 0117 928 9269