COOKING up a storm

The Food Refrigeration and Process Engineering Research Centre (FRPERC) is set within the tranquil, if rather unlikely, surroundings of Bristol University's School of Veterinary Science. Originally the government's meat research laboratory, the centre now concentrates on research of interest to the food industry, and derives some 40 per cent of its funding from industrial consultancy.

As the centre's name implies, its work is divided between refrigeration and chilling, and more process-based work such as the decontamination of meat and vegetables. Industrial work tends to be of the troubleshooting variety, explains research associate Judith Evans - companies tend to come to the centre with a problem at their plant, such as an item not freezing evenly, and the centre then works out a solution. The research tends to be more long-term, such as the design and construction of novel refrigeration systems.

Machines that go ping

Among the more recent arrivals to the food processing industry is the use of microwave ovens. Industry and public alike jumped at their potential to speed up the cooking of ready meals and other foodstuffs. However, explains food engineer Mark Swain, the advantages are matched by risks.

Because microwaves heat and cook food by exciting its molecular structure, generating heat inside the food itself rather than applying heat to its outside surface, it doesn't cook in the same way as it would in a conventional oven. This raises the possibility that it might not be cooked properly (leading to food poisoning scares in the mid-'80s); or that it might cook unevenly and scorch in some places while remaining raw in others.

The microwave facility at FRPERC operates in two areas - domestic microwaves and industrial-scale ovens. `One of the most recent projects was funded by the European Union's measurement and testing programme,' explains Swain; `the EU wanted us to develop a standard test to characterise the temperature distribution of food cooked in microwave ovens.'

To this end, the centre has developed two new technologies. The first of these is a series of `standard' artificial foods, made from powdered hydrophilic polymers mixed to a gel with water and salt, and which mimic the properties of a typical cook-chill food (`It's probably closest to lasagne, except it's blue,' says Swain), rice, or a mixture of both. These are studded with temperature probes and `cooked' in commercially-available microwaves to check their uniformity of cooking and repeatability of cooking results.

The other technology is a `standard' microwave oven. This £35000 lump of steel and aluminium occupies roughly the same space as two large chest-freezers stacked on top of each other. The bulk is stuffed with features to make the oven's performance as reproducible as possible, says Swain; for example, both the cavity walls and microwave generator are water-cooled. This is used to give feedback on how food cooks. `It actually has rather bad distribution, and it scorches food on the sides,' Swain admits, `but it always scorches the same amount, and in the same position.'

The centre's industrial work includes a project with Bristol University's microwave communications department, which is attempting to model how microwave energy is distributed in cavities and how it is dissipated once it enters food. This is proving extremely complex, says Swain; the amount of data needed to simulate food effectively is very large, and processing it is a Herculean task.

Other projects include work on pasteurising food that is sealed inside cartons - `the tricky bit is not blowing the lid off,' says Swain; and the use of beam focusing - using a single mode of microwaves, which is very fast, but requires absolute precision in locating the food inside the beam.

Refrigeration system that blows hot and cold

Refrigeration is taken for granted today, but it's a complex process, and probably the most widespread direct use of the laws of thermodynamics.

The cooling effect comes from the energy difference between a high-pressure liquid expanding into a low-pressure gas. The sticking-point, especially today, is the choice of working fluid. For many years, the refrigerants of choice have been the stable, non-toxic and relatively cheap CFCs and HCFCs - but not for much longer; their potential to destroy the ozone layer has rendered them unusable. Other candidates have also appeared - hydrocarbons such as butane - but although these are easy to compress and cheap, they pose a risk of explosion should they escape from the refrigeration circuit.

But the laws of thermodynamics are universal - any gas will do. So why not use one that's completely harmless, plentiful, and free? Why not use the air? That's the problem that Steve Russell has been addressing.

Air cycle refrigeration isn't a new technology, but it's only relatively recently that it has been efficient enough to be a viable option. Originally, the compression - up to about 30 bar - was achieved with rather slow reciprocating pumps and expanders. But the FRPERC team has replaced this equipment with the far more efficient rotary compressors, originally developed by the aerospace industry - in fact, Russell's rig uses an old aerospace compressor. `It's a complete transfer of technology,' comments Russell; up to 80 per cent efficiency can now be achieved.

Despite these advances, air cycle is still far less efficient than vapour compression. This is mainly because the refrigerant remains a gas throughout the cycle, so the latent heat used to evaporate a liquid is not available. However, it still offers significant advantages. Within a single-stage system it can achieve temperatures far lower than conventional chilling systems - `we can go down to -100 degrees C,' says Russell; `with conventional single-stage vapour compression, the best you can get is -40 degrees C.' This would be a big advantage for the food industry - low temperatures mean fast freezing times, and that means better quality frozen food and reduced weight loss; with vapour compression chilling, the weight lost from freezing meat is worth 20-50 times as much as the energy consumed to freeze it.

Steam-cleaning is the key

Salmonella, listeria, E coli... the list of so-called `killer bugs' seems to be lengthening by the day. And for food producers, the pressure to do something about food poisoning bacteria, from both government and the public, is growing. Keen both to do something and to be seen to be doing something, producers of meat, fresh vegetables and semi-cooked products are approaching FRPERC to help them develop methods of decontaminating their produce.

The research, under Graham Purnell, focuses on several different methods for killing bacteria on both meat and vegetables. The `benchmark' method, against which the others are assessed, is immersion in hot water; others involve the use of infra-red and ultraviolet radiation, hot air and, particularly, steam.

`We have a variety of projects using steam,' comments Purnell. `We use it at atmospheric pressure, below atmospheric and under pressure, and in combination with organic acids and with vacuum cooling.' Steam is useful because it condenses as it hits a cold food surface; this can transfer a large amount of heat into the surface, which can reduce the bacteria count on the surface of the food by 3-4 log cycles.

Combining the effects of steam with a spray of organic acid, such as acetic or lactic acid, seems to reduce the count by a further 2 log cycles. However, Judith Evans points out, this also introduces another problem. Under European Union rules, the acid treatment renders the food `processed' and it can no longer be described in shops as `fresh'. Consumers show a definite preference for `fresh' over `processed' food - which might make supermarkets reluctant to take food decontaminated in this way.