Vitamin C astray

Submitted by Thomas Pröller, meat-n-more on 11 April 2013, Published in Spices & Ingredients

Reactivity in the Maillard degradation of vitamin C

Vitamin C is found in many foods, other it is added to extend shelf life. An air and at room temperature, it is however not stable: Partly sliced fruit is brown and the taste of food changes. In the journal Angewandte Chemie provide German scientists now a systematic study taking place during the vitamin C degradation processes.

Vitamin C, ascorbic acid, is a reducing carbohydrate and can react with amino acids, peptides and proteins. Such reactions between carbohydrates (sugars) and proteins belong to the class of so-called "Maillard reactions", named after its discoverer, Louis Camille Maillard. Maillard reactions are ubiquitous: they make as our toast crisp, provide the typical roast fragrance when searing meat and add coffee beans during roasting aroma.

However, Maillard reactions to vitamin C are less pleasant. They are involved in the browning of cut fruit and can trigger changes in the taste of food. In addition, the Maillard breakdown of vitamin C in the organism could be involved in the clouding of the lens of the eye and in the age-related loss of elasticity of the skin and tendons.

It is not really understood how vitamin C is broken down.

Marcus A. Glomb and Mareen Smuda from the University of Halle-Wittenberg have now comprehensively investigated the amine-catalyzed degradation of vitamin C in a model system. Using vitamin C molecules labeled with carbon-13 isotopes at various points, they were able to trace the resulting Maillard reaction products back to their original positions in the vitamin C structure. They also carried out experiments under an atmosphere with 18O2 oxygen isotopes and quantified all major fragmentation products. In this way they succeeded in clearing up about 75% of the Maillard-induced decomposition of vitamin C, which leads to carbonyl and dicarbonyl compounds, carboxylic acids and amide end products.

Among other things, the researchers identified N6-xylonyl-lysine, N6-lyxonyl-lysine and N6-threonyl-lysine as unique, characteristic end products of vitamin C Maillard systems. On the basis of these compounds, vitamin C-mediated Maillard reaction products can in future be distinguished from those which are derived from other reducing carbohydrates such as glucose.

The knowledge gained from the model system helps to better understand the changes that take place during the storage and processing of foods containing vitamin C - even if the reaction pathways in real systems are of course much more complex. In addition, a foundation stone was laid for understanding the negative effects of vitamin C breakdown in the organism.