Thermal processing and allergenicity of foods

Many thousands of years ago, when humans decided to stop eating foods exclusively in their raw state and started cooking them, they certainly did not intend to reduce their allergenicity. Heat treatments were, and still are, used for increasing food qualities in terms of safety and storage ability by inactivation of microbes and toxins, in nutritional value through improvement of digestibility and in pleasure through improvements of appearance, texture, flavour, and taste. In the past few years, consumers and the food industry have been facing the challenge of a rapid and substantial increase in food allergies characterized by an increase in the prevalence and also in the severity of the clinical manifestations. Additionally, there has been an increase in the number of foods incriminated in allergic reactions. Food allergy has thus become a major public health concern with important socioeconomic consequences. It has been observed that food processing, and particularly thermal processing, could also reduce the allergenicity of some foods as an indirect effect and it has even been proposed that this might serve for a better management of the allergenic risk of foods. The rationale is apparently quite simple. Heat treatments alter the structure of proteins, i.e. the allergenic constituents of a food and, as a consequence, alter their allergenic potential and therefore the allergenicity of the whole food. Although no clear and general relationship between the structure of a protein and its allergenicity has been established, this view assumes, albeit not explicitly, that allergenicity is an intrinsic property of a protein because of or strictly related to particular structural features. This view, however, does not take into account the qualitative and quantitative variability of the allergen repertoire of a whole food, the multiplicity of epitopes on a given allergen or the genetic/geographic variability of the immune response in atopic human beings. Significant alterations in protein structure do occur during heat treatments, the nature and extent of such changes being dependent on the temperature and duration of the thermal processing as well as on the intrinsic characteristics of the protein and the physico-chemical conditions of its environment (e.g. pH). Typically loss of tertiary structure is followed by (reversible) unfolding, loss of secondary structure (55–70 C), cleavage of disulphide bonds (70–80 C), formation of new intra-/intermolecular interactions, rearrangements of disulphide bonds (80–90 C) and the formation of aggregates (90– 100 C) (1). These modifications reflect a progressive passage to a disorganized structure with denaturation of the proteins that adopt an unfolded, random-coil conformation. The denatured molecules associate to form aggregates and then gels resulting in a modification of the surface properties and an increase in size. Besides those physical transformations, chemical modifications of the protein may also occur at high temperatures (100–125 C and higher). These may involve formation of covalent bonds between the lysine residues of a protein and other constituents of the food matrix leading to various adducts. Advanced glycation endproducts, carboxymethyl lysine, malondialdehyde and 4-hydroxynonenal formed through protein–sugar interactions (i.e. by the Maillard reaction) and the crosslinking of oxidized lipid products with proteins are commonly found and may contribute to the formation of new immunologically reactive structures. It appears that there are no general rules regarding the consequences of thermal treatment on allergenicity. Some allergens or, more properly, some allergenic foods, are described as heat stable (e.g. milk, egg, fish, peanuts, and products thereof), while others are considered partially stable (e.g. soya bean, cereals, celery, tree nuts, and their products) or labile (fruits of the Rosaceae family and J.-M. Wal Laboratoire d'Immuno-Allergie Alimentaire INRA SPI-CEA de Saclay F-91191 Gif sur Yvette France