Hans-Dieter-Belitz-Institute for Cereal Grain Research

Physiological Effects of Browning Products of Bread Crust^b

Michael Lindenmeier, Thomas Hofmann and Veronika Somoza

Besides the characteristic aroma and the texture of the crumb, the brown colour of the crust is one of the most important quality parameters of freshly baked bread. The daily consumption of the staple food bread (Figure 1) is 180 g per day in Germany. Although this is a high amount compared to other foods, there are only few data on the physiological effects of the brown coloured, high molecular weight roasting products of the bread crust, which have been termed melanoidins.

Melanoidins are formed during heat treatment in the course of the Maillard reaction by reaction of reducing carbohydrates and amino acids or proteins. On the basis of their molecular weight the products of this non-enzymatic browning reaction can be classified into two groups. On one side low-molecular-weight colorants with molecular masses of <1.000 are formed, on the other side the so called melanoidins are generated, which are of brown colour and have molecular masses >100.000. Despite of numerous experimental attempts to isolate melanoidins from browned food, e.g. coffee or barley malt, only very few studies have been successful in giving insight into molecular structures of high-molecular-weight melanoidins with molecular masses of >100.000. Therefore, no information is available on the physiologigal effects of these roasting products, which are part of the daily diet.

It is well known, that heating during food preparation causes losses of essential amino acids, especially lysine, due to the reaction of amino acids and reducing carbohydrates. This causes a decrease of the biological value of the food, which is, however, completely harmless as the in western culture the daily intake of protein is rather high. However, the formation of toxic roasting products is often subject of public discussion. One example is the formation of acrylamide, which has recently been in the public focus.

Furthermore, previous studies using cell systems specific for roasting products of model mixtures gave evidence for mutagenic effects. However, there are no studies which extend these results to effects in the human organism, because neither the chemical composition of rosting mixtures is known up to now, nor is any information available on the resorption and metabolisation of these products. On the other hand, studies on extracts of roasting products have indicated health promoting effects. For these complex mixtures of compounds an antioxidative effect and a contribution to an anitcarcinogenic, chemopreventive potential is discussed.

In the present study a roasting product was used to demonstrate for the first time, that low-molecular-weight structures present in a food are absorbed in the small intestine, metabolised by the organism, and then excreted by the kidneys. Furthermore, systematic studies using fractions of bread crust and food models showed the antioxidative potential of chemically defined melanoidins and gave insight into active structural elements for the first time. Investigations with breads of rye/wheat flour mixtures revealed that the ethanol extractable melanoidins had the highest antioxidative activities in vitro, whereas the corresponding fractions isolated from the bread crumb or from flour were not able to inhibit the peroxidation of essential, unsaturated fatty acids. Then, activity-based screening of model melanoidins led to the identification of a new carbohydrate mofication of protein- bound lysin side chains, the so called pronyl-lysine (Figure 2), which has a high antioxidative activity in vitro. Further studies showed for the first time that pronyl-lysine modulates the activity of the metabolic active phase I-/phase II nzymes in epithelium cells of the small intestine. This was an experimantal proof for the chemopreventive properties of this new compound.

Figure 1: Bread - a staple food	Figure 2: Chemical structure of pronyl-lysine

Reference:
Lindenmeier, M. ; Faist, V.; Hofmann, T. Structural and Functional Characterization of Pronyl-lysine, a Novel Protein Modification in Bread Crust Melanoidins Showing in Vitro Antioxidative and Phase I/II Enzyme Modulating Activity. J.Agric. Food Chem. (2002), 50, 6997-7006.

^b AiF-Project No. 12403 N

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