Processing is essential for producing foods which are microbiologically safe, have high nutritional quality and reduced levels of potentially toxic compounds. In many food items, such as baked or roasted products, thermal treatment is indispensable for determining the specific nutritional and sensory properties, in particular texture, flavour and colour. Recent data have indicated that thermal treatment may induce the formation of health-promoting components, such as antioxidants and antimicrobial agents, which have not been studied in detail so far. On the other hand, processing may also lead to the formation of heat-induced contaminants, such as mutagenic heterocyclic amines and acrylamide, particularly in fried potatoes.

The various reactions occurring in heated foods which lead to the formation of new compounds involve lipids, carbohydrates and proteins and/or free amino acids. The possible reaction pathways are interrelated and strongly dependent on the composition of the raw food matrices, on the treatment conditions as well as on the surface area exposed of food. Interestingly, the final consequence of thermal treatment is, in many cases, browning of the food but the chemical nature of many of the compounds responsible for colour formation is still not known.

The mechanisms of colour formation During thermal treatment are known to be based on the the reaction between reducing carbohydrates and free amino groups. This non-enzymatic browning reaction is called the Maillard Reaction (MR) which is boosted by low water activity, high temperature and prolonged heating time. Initial phases lead to the formation of the early MR Products (MRPs), most of which are coloured, possess low molecular masses and are responsible for the aroma profile of the foods. In later stages, the final MRPs are formed, which are mainly melanoidins, heterogeneous, high molecular mass materials which deeply influence the colour of the food products.

As the Maillard reaction is all-pervasive in food, MRPs are widely consumed dietary components. From the food perspective, MRPs contribute to the stability (increased shelf-life) of foods through their antioxidant properties and they play an important role in the acceptability of foods through their colour and textural properties. In addition, dietary MRPs were demonstrated to affect human health, i.e. through their antioxidant or possess antimutagenic activity. They also may play a role in the binding of nutritionally important metal ions or potentially harmful dietary components, e.g., heterocyclic amines.

In the human body, the final products are known as Advanced Glycation Endproducts (AGEs) and these materials were first noted as yellow-brown fluorescent pigments in the lens of the human eye. Probably, the most relevant pathological consequence of the MR in human body is due to the amino-carbonyl mediated cross-linking of proteins. Recently, a new class of dideoxyosones which are important crosslink precursors have been identified. Formation of such compounds proceeds both in foodstuffs and in vivo. The role of the dideoxyosones and their follow-up products as potential 'glycotoxins' or perhaps 'beneficial compounds' in foods as well as the fate of these structures in the course of the digestion and resorption process is still unclear. It should be underlined that in human plasma only the intestinal absorbed dietary AGEs can be found. Although they are only a small part of all the glycated products present in food their intake seems to play an important adverse role in the progression of atherosclerosis, diabetes, ageing and chronic renal failure. The mechanisms of these effects are largely unknown and one of the final objectives of this Action is to elucidate the effects of diet on protein modification in vivo and the implications for health/disease and ageing.