Allergic reactions

It's one of those things that everybody knows - food allergies are becoming more common. In fact, it's a contentious issue. There is no firm evidence that the problem is increasing, and the issue is confused by the difference between an intolerance and an allergy.

The former is any unpleasant effect caused by eating foods. The latter is a specific response to a food from the body's immune system, involving the release of immunoglobulin E antibodies that bind onto proteins present in the food.

Food allergies are believed to account for about a fifth of all food intolerances, but estimates of the prevalence of the condition vary widely. According to Fiona Angus, head of the food intolerance department at the Leatherhead Food Research Association (LFRH), around 2 per cent of adults in the UK are food-intolerant. The figure is higher for children, as some intolerance and allergies disappear in adulthood.

The list of foods that can cause adverse reactions is long, and Angus comments that you will almost certainly be able to find someone, somewhere, with an allergy to any specific food. However, the most common foods that cause allergic reactions, responsible for around 90 per cent of cases, are eggs, milk, fish, crustacea, peanuts, soya beans, wheat and tree nuts such as almonds.

The most serious form of allergic reaction is anaphylactic shock, where the immune reaction causes severe dilation of blood vessels, which impairs breathing and causes a dramatic drop in blood pressure. This fatal reaction is more usually caused by insect stings and reactions to medicines, but around a quarter of all anaphylaxis deaths are caused by foods, with peanuts and other nuts again the most common culprits.

It's only relatively recently that the food industry in the UK has realised that this is a problem. Angus traces the realisation back to one particular case in 1993, when 17-year-old Sarah Reading died of anaphylactic shock after eating a lemon meringue pie in a restaurant. It transpired that the pie contained small quantities of peanut.

Research in the following years revealed some worrying facts - even small traces of peanut products can cause anaphylaxis; the allergen is not destroyed by high temperatures; and there are no chemicals or processes that can eliminate it effectively. Food manufacturers began to consider ways that they could ensure their production was allergen-free.

Ensuring that cross-contamination does not occur is vital for any company manufacturing foods which might contain the 'top eight' allergens, and particularly peanuts. In some cases, companies have installed segregated equipment to handle recipes containing nuts, to ensure that everything made on elsewhere can be guaranteed nut-free.

The danger point is the changeover from one batch to another. Thorough cleaning of equipment, such as by clean-in-place equipment, is an obvious safeguard, but not the only one. Angus suggests that manufacture of products containing major allergens should be scheduled immediately before major clean-downs.

She also recommends using specific immunoassays to check for the presence of allergens. Allergen kits are available to check for the majority of the allergens: US-based company Neogen, for example, produces kits that check for the presence of peanut, egg, almond and cow's milk, while kits for whey, soya, clam, hazelnut, pecan, cashew, walnut and sesame will be available soon. LHRA itself offers individual assays which check for peanut, hazelnut, gluten, egg, soya and histamine. These kits can be used as routine, or to check whether existing cleaning processes are adequate, and where cross-contamination might be a problem.

Meanwhile, research is continuing on whether the allergenicity of certain foods can be reduced by processing. A pan-European project called Protall, for example, is focusing on allergenic plant proteins and their relationship to each other, and on how processing, biological activities and interactions with other food components alters these properties.

The project, which brings together some 30 scientists from 13 countries, has three main objectives - to assess the relationship between the biological activities, associated structures and allergenic potentials of known plant allergens; to investigate the effects of processing on the absorption of allergens and on their potential to trigger the immune system; and to establish criteria for the inclusion of allergen data into existing food databases.

Allergens tend to be comparatively small protein molecules, and their effects are triggered by even smaller structures called epitopes which are recognised by the immune system. There are two types of epitope - linear, whose effects are triggered by their chemical structure, and conformational, whose effects depend on their three-dimensional shape. The Protall team has found that various processing techniques can sometimes reduce the effects of these epitopes, thereby making the food less allergenic.

Safety in processing

The epitope can sometimes be removed altogether. For example, allergies to peaches are largely due to molecules found in the skin.

Therefore, ensuring that the skin is fully removed before the fruit is processed will greatly reduce allergenicity.Conformational epitopes can sometimes be rendered harmless by heating, as this causes the protein strand to unfold and destroys the structure. This process is easy for some epitopes, but others are more stubborn: lipid-transfer proteins, which occur in cereals, are very stable to heat and only break down above 90 degrees C.

Linear epitopes, meanwhile, can also sometimes be destroyed, this time using protease enzymes which cleave the protein chains, disrupting their chemical structure. However, this is not always viable: some allergens are also vital to the food's structure. Protease treatment of wheat flour, for example, makes the flour unsuitable for baking.

Enzymes can also be used to modify rather than destroy proteins, reducing their allergenicity by making it impossible for the epitopes to bind to the IgE molecules. For example, the transglutimase enzyme causes allergens in wheat to cross-link.All of these techniques can only reduce allergenicity, however. To be completely safe, the tried-and-tested methods of segregation, cleaning and monitoring are still the order of the day.