7 Carbohydrates 235
O OCH 2 OHOHOHOHO OHCH 2 OHOHOH
MaltoseOCH 2 OHOHOHOHOKojibioseOOHCH 2 OHOHOHFig. 7.5Regioisomers: structures of maltose and kojibiose
to low-molecular weight compounds such as furanic compounds (furfural, HMF),
cyclotene, etc., and also to coloured products. These reactions are accelerated by
temperature giving rise to caramelization. In winery, caramelization can be found
in some dessert wines (Kroh 1994) and in barrels where wood is toasted.- Reactions with bases:in basic medium, reducing sugars undergo isomerization
reactions through enediol intermediates (Lobry De Bruyn-Van Ekenstein reac-
tion) so glucose is partially converted to fructose and mannose; a number of
by-products is also produced.- Maillard reaction:carbonyl compounds, mainly reducing sugars, can react with
free amino groups (from peptides, proteins or amino acids) giving rise to Maillard
reaction. This reaction takes place in a high number of foods and can produce
not only the formation of desired colours and flavours but also some potentially
toxic compounds. After alcoholic and malolactic fermentations in wine, dicar-
bonyl compounds are obtained, which are susceptible of participating in Maillard
reaction (Pripis-Nicolau et al. 2000). The formation of some flavour components
in different wine model systems (low pH, aqueous medium, and low tempera-
tures) such as glucose with alanine, arginine and proline (Kroh 1994) or carbonyl
(acetoin and acetol) and dicarbonyl (glyoxal, methylglyoxal, diacetyl and pentan-
2,3-dione) compounds with different amino acids (Pripis-Nicolau et al. 2000; de
Revel et al. 2004; Marchand et al. 2002) has been studied. Moreover, the influ-
ence of this reaction in the aroma of sweet fortified wines (Cutzach et al. 1999)
and the production of Amadori compounds in Japanese white wines have been
described (Hashiba 1978). Several by-products of Maillard reaction are identi-
cal or similar to some produced in degradation reactions catalysed by acids or
bases.- Oxidation: free carbonyl groups are able to reduce alkaline solutions of metal
(mainly copper) salts to the free metal or to the oxide, to give aldonic acids.
Thus glucose gives rise to gluconic acid (Fig. 7.6). When the hydroxyl group