杏吧原创

FOOD used to be for savouring. But today it seems food is something to be saved from 鈥� at least in Britain. First people were laid low by Salmonella in eggs, then Listeria in cheese, and within the past few months 20 people have died after eating beef contaminated with Escherichia coli.

And these scary stories are just the tip of an iceberg. According to the government鈥檚 Communicable Diseases Surveillance Centre in North London, reports of food poisoning in England and Wales have increased fourfold in the past 15 years to around 80 000 cases a year.

In the face of this bacterial onslaught, the spotlight has picked out farmers and abattoir owners, with politicians pressing them to clean up their act. Behind the scenes, however, scientists have been doing their bit to find new ways to kill contaminants in food before it reaches the shops. They have developed a weapon that needs neither chemicals nor heat yet knocks out bacteria to leave food with the colour, taste and nutrients that nature intended.

The weapon is pressure 鈥� and no small increase at that. Researchers are subjecting food to as much as 9000 times atmospheric pressure. By putting the squeeze on from all sides, most foods simply compress slightly with only a few turning into pur茅ed mush. Right now the technique is being tried on everything from meat and dairy products to fruit and vegetables in the hope that this kind of preservation will finally help to stem the tide of food poisoning.

But interest in high pressure is not some knee-jerk reaction to the recent upsurge in food poisoning cases. The initial driving force came from another direction altogether. Advocates of the technique say the real benefit for choosing high pressure over other preservation techniques, such as heat treatment, is that it leaves nutrient, flavour and colour molecules relatively unscathed.

Take canned peas, as an example. They are treated at about 120 掳C for up to an hour, leaving them pale shadows of their former selves and making it necessary to use artificial additives to restore their colour. Other canned vegetables and fruits suffer similar fates and taste quite different from fresh produce. High pressure could also help to improve fresh fruits and vegetables, which are cleaned up at present by washing them in a chlorine solution.

Although high-pressure processing is only now gaining in popularity, its roots go back about a hundred years to pioneer Bert Hite of West Virginia University and his attempts to squeeze to death the microorganisms in milk. Despite some successes, however, interest in the technique curdled after the First World War. 鈥淚t鈥檚 the lost frontier of food processing,鈥� says Donald Johnston of Queen鈥檚 University, Belfast.

Johnston and other food scientists around the world are now trying to whip up enthusiasm in the food industry for the rediscovered technology. And they have had some success. In Japan, for example, the delights of pressure-treated, fresh-tasting fruit jams and yoghurts are already becoming popular delicacies. Kunio Kimura, research director at Meidi-Ya Food Factory near Osaka, says that apart from safety, the main attractions of pressure treatment for Japanese consumers are 鈥渇reshness, colour, and less nutrients destroyed during processing鈥�. Sales of Meidi-Ya鈥檚 High Pressure 鈥淪鈥� foods continue to increase despite prices that are roughly double those of conventional products.

Another successful pressure-treated dish is the New Year delicacy yomogimochi, a steamed rice paste mixed with a wild herb, which is normally prepared at home from fresh ingredients for children. In the past, commercial yomogimochi has been heat treated, which destroys the flavour, says Rikimaru Hayashi of Kyoto University. 鈥淗igh-pressure yomogimochi tastes the same as our mothers鈥� food,鈥� he adds. 鈥淗igh pressure is used to kill bacteria but keep the flavour.鈥� And it鈥檚 not just Hayashi who prefers the high-pressure version: the Japanese bought 100 million yen鈥檚 worth (拢490 000) last year.

Elsewhere in the world, scientists working on high-pressure processing have less on the table to show for their work. They reckon that European and American manufacturers are more reluctant than their Japanese counterparts to gamble on the high-cost pressure vessels needed for mass production.

In Europe, people don鈥檛 buy foods because they have been produced in a high-tech way. But this is not true in Japan, says Richard Earnshaw of the Campden & Chorleywood Food Research Association, near London. 鈥淚n Japan, technology can sell food and drink,鈥� he says. Earnshaw is one of a group of European researchers who gathered at a London conference on pressure treatment organised by Britain鈥檚 Ministry of Agriculture, Fisheries and Food a few weeks ago in London.

So how does high pressure inactivate microorganisms in our food? It鈥檚 a key question that is preoccupying researchers round Europe. One programme funded by the European Commission, for example, is trying to establish the best conditions for killing the main food-poisoning bacteria such as Listeriaand E. coli. 鈥淲e need to be sure we understand what controls the inactivation and be absolutely sure that the bugs cannot repair themselves and come back to life when we least expect it,鈥� says Earnshaw.

Killing conditions

So far the signs are that it will be relatively easy to kill vegetative microorganisms such as E. coli, which reproduce by dividing. But spore-forming bacteria, such as Clostridium botulinum 鈥� which causes botulism 鈥� are proving more resistant to high pressures.

The idea that a chemical system responds to minimise the effects of a change in conditions is well known in science 鈥� it鈥檚 called Le Chatelier鈥檚 principle. So, for example, if the pressure goes up, chemical reactions take place so that the system takes up less volume. The difference between this response and what happens when the same system is heat-treated comes down to which type of chemical bonds are broken.

Heat breaks covalent bonds, which hold atoms together as molecules (see Diagram). This is why heat alters the flavour of foods. Flavour molecules such as esters are held together by covalent bonds and break down into different chemicals 鈥� so the taste changes.

By contrast, high pressure affects only a few covalent bonds. For the most part it disrupts ionic bonds, which exist between electrically charged chemical groups. As the pressure rises, water molecules flow between the positively and negatively charged groups and bind to them 鈥� because this reduces the volume of the system. The influx of water breaks the bonds in two.

Ionic bonds keep the chain-like structures of protein molecules in their biologically active form. Once these are broken, the proteins unravel and become useless. Whether it is this mechanism that finishes off bacterial cells is still a matter of debate, but there is a majority view. 鈥淔or vegetative microbes, pressure damages the cell membrane,鈥� says Daniel Farkas of Oregon State University. 鈥淲hen the cell cannot repair the damage and the cell contents are lost, reproduction ceases.鈥� So is it a case of cells imploding or spilling their guts? Nothing quite so dramatic, says Earnshaw. 鈥淢icrobial inactivation . . . is not through gross cell disruption,鈥� he says. 鈥淢icrobes are [probably] inactivated by subtle changes in their membrane structure, which disturbs their energy-generating processes.鈥�

Brian Brooker of the Institute of Food Research in Reading, Berkshire, takes the story a step further. 鈥淢embranes become leaky after pressure treatment and inactivation of membrane ATPase has been demonstrated in some species,鈥� he says. The ATPase enzyme pumps protons out of the cells, and if it stops working the bacteria accumulate protons and the internal pH falls, causing death by acid.

But this is still not the whole story, says Brooker. High pressure disrupts other cell structures too, including nucleic acids and ribosomes 鈥� the organs that manufacture proteins. 鈥淭he protein synthesising apparatus seems particularly sensitive to pressure,鈥� Brooker says. 鈥淚n E. coli, for example, protein synthesis is totally inhibited at about 70 megapascals.鈥�

Whatever the process, killing vegetative cells looks straightforward. By contrast, destroying spore-producing bacteria is more complicated. While a single blast of high pressure can kill active cells, it is not enough to kill the dormant spores. So researchers have turned to a variation on their theme 鈥� repeated squeezing. They have found that the first few pressurisations cause the spores to germinate and lose their resistance. 鈥淪pores become sensitive to pressure or heat,鈥� says Brooker. According to Johnston, 鈥渁 multiple pulse treatment, cycling pressure between ambient and 600 megapascals鈥� has proved successful.

Decontaminating food is more about forcing bacterial numbers to below harmful levels than about killing them off altogether. This is because whichever method you use, a few hardy individuals from a colony always survive, a phenomenon known as tailing.

The right mix

But just as a combination of drugs can kill stubborn infections, so mixing high pressure with heat processing, chilling or some other decontamination technique could reduce numbers of bacteria in food close to zero. 鈥淐ombination processes could be effective,鈥� says Dietrich Knorr of the Berlin University of Technology.

But hang on a minute. While we are destroying microorganisms with high pressure, what happens to the cells in the food itself? Surely these will be innocent casualties? Well, apparently not. Because microorganisms are inactivated partly by disrupting their vital functions, cells that are not alive don鈥檛 suffer. And membrane disruption and unravelled proteins don鈥檛 seem to have much effect on nutrients or the important qualities of taste and colour. 鈥淐hanges might also occur in the food cells, but in terms of overall structure and chemistry, the foods are not significantly altered,鈥� says Earnshaw. There are some messy 鈥� and potentially nasty 鈥� exceptions though.

When squeezed from all sides, food cells do compress because of the natural compressibility of water. 鈥淲ater is compressed by about 15 per cent at 600 megapascals,鈥� says Knorr. But if any air is present, things start to go to pulp. 鈥淰acuoles and intercellular air are very compressible,鈥� says Earnshaw, who puts fruit and vegetables under pressure. While most of his subjects stay solid, a few 鈥� such as cucumbers 鈥� end up as mush.

At the London meeting, Earnshaw laid out a variety of pressure-treated foods that had suffered somewhat for the experience. Grapes had hardened, cabbage softened and the mushrooms and potatoes had oxidised and gone brown. Rapid browning takes place because the enzyme responsible, polyphenol oxidase, speeds up its work rate at high pressure. The way to prevent browning is to get rid of any oxygen, says Earnshaw. This could be done by, say, packing the food in a vacuum or in pure nitrogen. Earnshaw is confident that methods will be found to overcome all the unwelcome side effects of high pressure.

So we can expect delicate fruit and vegetables, such as pears and mushrooms, to survive pressurisation once researchers get the conditions right. And getting those conditions right is not just of cosmetic interest. Some fruit stones contain toxins, including cyanide. It goes without saying that pressure treatment must be designed so that it does not free these toxins or concentrate them. 鈥淚 don鈥檛 believe that there will be any problems,鈥� says Earnshaw.

For the most part, the health effects of high-pressure treatment are likely to be positive. 鈥淰itamins such as vitamin C are partially destroyed by heat processing,鈥� says Earnshaw. 鈥淧ressure processing gives good retention of vitamin C.鈥� In Japan, you can buy high-pressure mandarin and grapefruit juices, and there are plants for pressure processing orange juice in both Spain and Texas. Fruit juices are one of the few types of food tested so far that undergo a change in taste when exposed to high pressure. Fortunately for the producers, many consumers think the taste is better 鈥� grapefruit juice, apparently, is less bitter.

According to Johnston, the benefits of high pressure will not be confined just to fruit and veg. Dairy and meat products could also become more pleasing to the palette. Ultra high-temperature treatment kills a far higher proportion of the bacteria in milk than the more gentle pasteurisation, so it stays safer for longer. UHT milk, on the other hand, tastes little like the natural product. Johnston thinks that pressure processing could give us a long-lived alternative to UHT milk that tastes more like the real thing 鈥� probably by putting the squeeze on milk that has already been pasteurised.

Pinta pressure

And what about replacing pasteurisation itself with pressure treatment? Johnston is less optimistic: 鈥淧ressurised pintas on the doorstep are probably a long way off.鈥� It will take time, he says, to prove pressure鈥檚 ability to control microorganisms on its own, although it is effective against the Listeria found in raw milk. Devotees of skimmed milk will also take a long time to convince because pressure-induced changes in protein chemistry turn the liquid a yellow-green colour.

Pressurised milk could turn out to be of more help to cheesemakers. The chemical and structural alterations to proteins that take place in milk during pressure treatment mean techniques and probably textures will change. High pressure also accelerates the actions of ripening enzymes, such as rennet, so cheese could arrive on your table earlier than in the past. At present, cheeses are artificially aged by adding these enzymes. Applying pressure could simply speed up this ageing process further, or reduce the need for the extra enzymes.

An alternative high-pressure dessert might be a nice firm yoghurt. 鈥淧re-treating milk before processing enhances gel strength and viscosity,鈥� according to Johnston. When pressure breaks the ionic bonds of the milk proteins, it increases their ability to bind water molecules. Practically speaking, this means that yoghurt will not separate into creamy and watery components after just a few days in the fridge.

Meanwhile, meat-eaters will be pleased to know they haven鈥檛 been forgotten. Pressure could be used to control bacteria in fresh meat and to improve the texture of restructured meat, says Johnston. Research by Hayashi and others has shown that squeezing beef, chicken and pork can kills contaminants such as E. coli. But it may still be some years before we can tuck into pressure-treated steaks and burgers because the commercial equipment has still to be developed.

According to several studies in Japan and the US, high-pressure breaks the interconnections between the muscle proteins actin and myosin in meat, making it more tender. It also increases water uptake, helping to make the meat juicier. And on a purely aesthetic note, pressure-treated meat retains its pinkish colour longer in the butcher鈥檚 shop.

Proponents of high-pressure technology clearly believe it could improve the menu from starter to mints. But high pressure isn鈥檛 going to revolutionise the food industry overnight. Heat processing of food is an established technology, which in most cases is effective at cutting down bacterial numbers to harmless levels. More importantly, it is cheap. High-pressure processing, although cheaper in terms of the energy it uses, needs expensive pressure vessels.

Researchers believe that high-cost foods such as avocado and p芒t茅 will be pressurised first, before being taken up across the board. In Japan, people initially saw pressure-treated jams and yoghurts as novelties. Today, they are hooked on the idea, and the technique is spreading as it becomes less of a commercial risk.

The attitude is different in the US and Europe, says Farkas. 鈥淧eople see pressure preservation as a `silver bullet鈥� to be used where other preservation methods cannot deliver the [standards] needed for a valuable food product,鈥� he says. But once high-pressure technology takes off, other areas will benefit. 鈥淚nitially the benefits will probably be for fruit and vegetable juices and fruit pur茅es,鈥� says Brooker. 鈥淏ut the next biggest market will be for the processing of milk prior to the manufacture of yoghurt and cheese.鈥�

As well as treating the foods we all eat today, new products and processes could be just as important for high pressure to pay its way. Knorr is working on rapid freezing of foods to prevent structural damage. Normal freezing is slow and creates large ice crystals which damage cell structures. Knorr wants to chill produce to about -20 掳C under pressure to prevent freezing. When he then removes the pressure, the water freezes rapidly into tiny ice crystals which are less damaging to the food cells.

Farkas is pursuing a different goal, working with the US Department of Defense to develop pressure-treated field rations. The bottom line is that the troops may be able to look forward to more exotic menus than in the past: spaghetti with meat sauce, Spanish rice, yoghurt with peaches, fruit cocktail and lemon pudding. All came through microbiological testing with flying colours, but they are now waiting for their flavours to be retested after three months in storage.

If army rations aren鈥檛 quite to your taste, here鈥檚 a future menu from the Under Pressure Restaurant to whet your appetite 鈥� all guaranteed free from bacterial contamination. For starters, fully flavoured organic soup, pressurised p芒t茅, or squeezed fruit juices; for the main course, tenderised juicy steak served with a selection of soft, raw vegetables and for dessert, creamy yoghurt or fresh fruit ice cream, followed by rapidly matured cheese. And if you wanted to be ironic, you could wash it all down with a glass of squash.