Ever since the discovery in 2002 that the potential carcinogen acrylamide could be produced in certain heat-processed foods, attention has been focused on chemical contaminants that can be generated during processing. Since then, as well as learning a great deal about how acrylamide is produced in foods and how it can be controlled, researchers have been finding that other potentially harmful compounds can be detected in a surprisingly wide range of processed foods. The public health significance of this is still not clear, but the European Commission has recently embarked on a three-year survey of processing contaminants in retail foods in an attempt to learn more about their occurrence. It seems certain that processing contaminants will remain an emerging food safety issue for some time yet.

Process contaminants have existed in foods since the first discovery that cooking food over a fire improves its taste and palatability. Applying high temperatures to food generates a host of new chemical compounds, some of which may have a beneficial effect, while others may be toxic. This has been known for many years, and some process contaminants have been widely studied. For example, polycyclic aromatic hydrocarbons (PAHs), such as benzo[a]pyrene, are a large group of potentially carcinogenic chemicals that can be produced whenever organic material is partially burnt. PAHs are a well known hazard in grilled and barbecued foods, especially when overcooking and carbonisation occurs, and this was first reported more than 40 years ago.

The mechanisms of PAH generation are still not fully understood, but incomplete combustion of melted fat dripping onto the heat source is thought to be one way that PAHs can be produced. Vertical charcoal grills, where the fat does not drip onto the burning charcoal, have been shown to dramatically reduce PAH levels in grilled foods. Cooking at lower temperatures for longer times, choosing leaner meats, and avoiding overcooking also help to control PAH levels. PAHs are also found in smoked foods, especially those produced using traditional smokehouses, and more surprisingly, in vegetable oils. Vegetable oils can become contaminated via the seed drying process when combustion gases in the dryer come into contact with the seeds. A range of practical methods to minimise PAH generation in processed foods has been available for some time, but the discovery of acrylamide in foods has sparked renewed interest in some other well known process contaminants and has also led to the discovery of some others that were previously unknown.

Chloropropanols

Chloropropanols are another group of potentially carcinogenic chemicals that may be present in a range of commercially and domestically produced foods. The most commonly found chloropropanol in food is 3-monochloropropane-1,2-diol (3-MCPD), first detected in acid-hydrolysed vegetable protein (HVP) in the late1970's and then later in foods containing acid HVP, such as soy sauce. Further investigation showed that the contaminant was being produced during processing by a reaction between a source of chlorine and lipids, especially at high temperatures. Once this had been established, it was possible to virtually eliminate the problem by changing the manufacturing process for HVP, either by switching from acid to enzyme hydrolysis, or by using an over-neutralisation technique following acid hydrolysis. Subsequent surveys showed greatly reduced 3-MCPD levels in HVP and soy sauces.

Despite this, 3-MCPD is now causing renewed concern following the discovery that it can also be present at low levels in a number of other heat-processed, or fermented foods and ingredients that do not contain any HVP. These include bread and other bakery products, coffee, some cooked and cured meat and fish products, certain cheeses and roasted barley malt. Several mechanisms for the production of 3-MCPD in these foods have been proposed -- in bread it is thought to be a reaction during baking between chlorine in added salt and glycerol from yeast or flour -- but in some other foods the mechanism in unclear. While strategies for reducing 3-MCPD levels in bread revolve around cutting the amount of salt added, the lack of any firm knowledge about the mechanisms of production in other foods means that effective reduction techniques are yet to be developed. The European Commission Scientific Committee on Food has recommended a Tolerable Daily Intake (TDI) that will not cause any measurable damage to health for 3-MCPD of 2µg/kg bodyweight per day over a lifetime. It is likely that any future legislative limits for 3-MCPD in foods other than HVP and soy sauce will be set with this TDI in mind.

Acrylamide

Acrylamide is the processing contaminant that sparked all the recent interest in this branch of food safety. First reported to be produced in certain starchy foods during high temperature heat processing in 2002 by researchers at the Swedish National Food Administration, acrylamide is yet another potential carcinogen produced during food processing. Presumably present in susceptible foods for hundreds, if not thousands of years, its presence was only discovered by an investigation into unexplained exposure to acrylamide in people being used as control subjects in medical studies.

Since that discovery, acrylamide has been found at varying levels in a wide range of cooked foods, especially potato chips and French fries, bread and baked cereal products and roasted and ground coffee. A very recent Swiss report also detected appreciable levels of acrylamide in dried fruits. So much information has been collected now that a database of acrylamide survey data has been set up on a web site on behalf of the WHO and FAO.

The mechanism of acrylamide production during processing has also been widely studied, and the principal route is now considered to be a reaction between the amino acid asparagine and reducing sugars, such as glucose and fructose, during the Maillard browning that occurs during cooking at high temperature. Once this was understood it opened up possibilities for reducing acrylamide production by various means, notably by cooking at lower temperatures and limiting browning during cooking. Reducing the levels of asparagine in raw materials like potatoes is also proving to be a promising area for investigation. In fact, so much progress has been made in understanding acrylamide production in foods that the Confederation of the Food and Drink Industries of the EU (CIAA) has produced an 'Acrylamide Toolbox' of practical techniques for limiting acrylamide generation during processing of susceptible foods, designed specifically for processors. This can be downloaded from the CIAA web site. No legislative limits have yet been set for acrylamide levels in specific foods, but food safety authorities in Europe and elsewhere generally recommend that industry should continue to make efforts to reduce levels as far as is practical.

The rate of progress in dealing with the problem of acrylamide in foods has been remarkable, and much has been achieved since 2002. Furthermore, this is largely as a result of close collaboration between industry, food safety authorities and the academic community and serves as an example of what can be achieved by this approach. Equally encouraging are the results of some recent epidemiological studies on acrylamide in the diet. For example, the preliminary results of a very large American study announced in August this year indicate that foods containing acrylamide are unlikely to be involved in breast cancer in women. However, the news is not exclusively good. A recently reported survey of food manufacturers in the UK suggested that the bigger operators have implemented changes to processes, product formulations and ingredients to achieve reductions in acrylamide levels. But smaller manufacturers are much less likely to have made changes, despite being aware of the problem. It seems that many small businesses are reluctant to make potentially problematic processing changes until required to by customers or enforcement authorities, perhaps fearing a negative impact on sales.

Ethyl carbamate

Ethyl carbamate is yet another potential carcinogen, but differs somewhat from the other processing contaminants mentioned so far in that its occurrence appears to be restricted to fermented food and drinks, particularly alcoholic beverages. Its presence was first identified in 1985 when elevated levels were discovered during a Canadian survey of alcoholic drinks. This finding prompted the Canadian authorities to introduce a guideline limit of 150 µg/litre for distilled drinks. Ethyl carbamate is thought to be formed naturally during fermentation processes.

After the initial Canadian finding in 1985, further surveys of fermented foods, including bread and yoghurt as well as alcoholic beverages, were conducted elsewhere. The data from these surveys suggests that by far the greatest exposure to ethyl carbamate in the diet comes from alcoholic drinks, especially whisky. Control measures have since been introduced by the drinks industry to reduce ethyl carbamate concentrations as much as possible. These have included changes to the whisky distilling process and using barley varieties that have lower concentrations of the precursor compounds for ethyl carbamate, such as cyanates. Excluding light from distilled spirits with coloured glass bottles also helps to reduce the production of ethyl carbamate.

Furan

Furan is a very volatile organic compound present naturally in tobacco smoke and also used as an industrial solvent -- it is a very different compound from the large group of chemicals known collectively as 'furans', which includes various antimicrobials (nitrofurans) and dioxin-like toxins. Concern over furan in foods dates back only to 2004 when an FDA survey of heat-processed foods in the USA revealed that low levels of furan could be found in a number of products processed in closed containers, such as cans and jars. Little is known about how furan is formed during processing, although one suggestion is that it is derived from oxidation of polyunsaturated fatty acids at high temperatures. There is also a dearth of information on the likely exposure of consumers to the contaminant, but the fact that it can occur in infant foods processed in jars is a cause of concern.

Furan is currently classified as a possible carcinogen, and its presence in products intended for babies is clearly a worry, although it is so volatile that levels fall quickly when containers are opened. Current research is focusing on collecting data on occurrence and exposure to furan in the diet and on establishing its significance to human health. EFSA has recently requested data to set up a 'furan database' for use in future risk assessments.

Advanced Glycation End Products

Like acrylamide, advanced glycation end products (AGEs) are produced during the later stages of the Maillard browning reaction in foods cooked at high temperatures, but unlike acrylamide they occur mainly in cooked meat and dairy products. AGEs are a group of compounds formed from glycated proteins -- examples of AGEs include carboxymethyl lysine (CML), methylglyoxal (MG) and pentosidine. Their presence in grilled, fried, or broiled meats and cheeses has been known for some time and they can also be produced in pasteurised and retorted food products. AGEs are also known to be toxins and have been linked to various conditions, including inflammation, insulin resistance, diabetes, kidney disease and Alzheimer's disease, but until quite recently, the significance of AGEs in the diet was almost entirely unknown. Then in April 2007, researchers from the Mount Sinai School of Medicine in the USA published the results of a study into AGE levels in the blood of 172 healthy people of varying ages in the Journal of Gerontology: Medical Sciences. They expected to find higher levels in older people, and this turned out to be the case, but they also found that some healthy younger people had AGE levels in their blood that would normally be associated with those found in diabetics. Furthermore, they were able to establish a direct link between increased dietary intake of AGEs and higher levels of the toxins in the blood.

As study director Dr Helen Vlassara commented, "Excessive intake of fried, broiled and grilled foods can overload the body's natural capacity to remove AGEs, so they accumulate in our tissues, and take over the body's own built in defences, pushing them towards a state of inflammation. Over time, this can precipitate disease or early aging." Dr Vlassara suggests that there is a link between the wide availability and consumption of foods containing AGEs and the current epidemics of diseases such as diabetes. She goes so far as to suggest that information on AGE levels in foods should be shown on nutrition labels so that consumers can limit their own intake, and also advocates steaming and boiling of meats to limit AGE formation.

The knowledge gap

Some food safety experts consider that not nearly enough is known about processing contaminants in general and that there is a pressing need for more research into how they are produced, their real significance to public health and how their production during food processing can be limited and controlled. The European Commission has responded by launching a major three-year survey of processing contaminants in retail foods. This will concentrate on collecting data about 3-MCPD, acrylamide, ethyl carbamate and furan. The purpose of this exercise is not only to estimate consumers' exposure to these compounds, but also to monitor how effectively the food industry is reducing their levels in its products.

The food industry is acknowledged to have reacted constructively to the potential health threat of acrylamide in foods, but the recent discovery of furan in canned foods and the possible role of dietary AGEs in a range of diseases show that there is no room for complacency. It will probably never be possible to remove all processing contaminants from foods unless consumers are prepared to abandon cooked products altogether. But the more information industry has about these compounds, the more it will be able to identify the real hazards and develop practical controls.

Useful web links

The CIAA Acrylamide Toolbox http://www.ciaa.be/pages_en/documents/acrylamide.asp

Acrylamide Infonet http://www.acrylamide-food.org/index.htm