Hydroxymethylfuran-2-carbaldehyde

The formation of 5-hydroxymethylfuran-2-carbaldehyde from saccharose proceeds via hydrolysis to fructofuranosyl cation and glucose (Figure 4.26). Ehmination of a proton and two molecules of water from the fructofuranosyl cation yields 5-hydroxymethylfuran-2-carbaldehyde. Under dry pyrolytic conditions and at temperatures above 250 °C, 90% of 5-hydroxymethylfuran-2-carbaldehyde originates from the fructose moiety and only 10% originates from glucose. Pentoses and L-ascorbic acid dehydrate in the same way as hexoses under addic conditions yielding furan-2-carbaldehyde (via reactive 3-deoxy-L-t/jreo-pentos-2-ulose and 3,4-dideoxypentosulos-3-ene) as the main product (see Section 5.14.6.1.5). 6-Deoxyhexoses, such as L-rhamnose, yield, analogously, 5-methylfuran-2-carbaldehyde (4-195). 

Under acidic conditions, 5-hydroxymethylfuran-2-carbaldehyde partially decomposes to laevulinic (4-oxopentanoic) acid and... 

All derivatives of furan-2-carbaldehyde are reactive substances. They are easily oxidised by oxygen to the corresponding acids, which, together with the appropriate alcohols, are also formed from furan-2-carbaldehyde derivatives by Cannizzaro reaction. Derivatives of furan-2-carbaldehyde condense together or with other carbonyl compounds and enter into non-enzymatic browning reactions. Oxidation of 5-hydroxymethylfuran-2-carbaldehyde, for example, yields 5-hydroxymethylfuran-2-carboxylic acid (4-198). Condensation... 

Flqure 4.43 Condensation of 5-hydroxymethylfuran-2-carbaldehyde and 2-hydroxy-6-hydroxymethylpyran-3-one. 

Decomposition of ketosamines starts, like most other reactions of monosaccharides, by 1,2-enoKsation that yields an amino analogue of l-ene-l,2-diol, which is l-ene-l-amino-2-ol (Figure 4.86). The reaction proceeds by elimination of the C-3 hydroxyl group and hydrolysis of the bound amino compound provides 3-deoxy-o-erythro-hexos-2-ulose and, further, 3,4-dideoxy-D-g/ycero-hex-3-enos-2-ulose, 5-hydroxymethylfuran-2-carbaldehyde in addition to other products that are formed by degradation of glucose by... 

The main products of 3-deoxy-D-er7thro-2-hexos-2-ulose degradation are (Z)-3,4-dideoxy-D-g/ycero-hex-3-enos-2-ulose (2-hydroxy-6-hydroxymethylpyran-3-one), the corresponding (E)-isomer and 5-hydroxymethylfuran-2-carbaldehyde. 

Deoxyglycos-2-uloses react differently with secondary amines than with primary amines. Usually significant amounts of cyclic compounds form, but 5-hydroxymethylfuran-2-carbaldehyde does not arise at all (or only in small amounts). For example, the reaction of proHne with 3-deoxy-D-erythro-hexos-2-ulose yields the so-called maltoxazine, a flavour-active compound occurring in beer and malt (Figure 4.91). 

It is common to find laevulinic (4-oxopentanoic) acid (8-69) in foods containing carbohydrates, as it is one of the final dehydration products of hexoses via 5-hydroxymethylfuran-2-carbaldehyde in acidic media. It is present in very high concentrations (about 1-2% tv/w) in acid hydrolysates of proteins. 

A large number of furan derivatives are produced in non-enzymatic browning reactions as dehydration products of carbohydrates and related compounds, such as ascorbic acid (y-lactones can also be considered furan derivatives). Most furans are common to many thermally processed foods. Higher levels of these compounds are also present in acid protein hydrolysates. Furan-2-carbaldehyde has the characteristic pleasant woody aroma resembling nuts, which arises from pentoses and ascorbic acid, and 5-methylfuran-2-carbaldehyde, which is a product of 6-deoxyhexoses. A fatty herbal type flavour is provided by 5-hydroxymethylfuran-2-carbaldehyde arising from hexoses. 2-Acetylfuran arising from hexoses and pentoses is an important component of a number... 

Raw or gently pasteurised milk (e.g. for 10 seconds at 73 °C) has a fine characteristic odour and sweet taste. Typical components present in low concentrations are dimethylsulfide, biacetyl, 2-methylbutan-l-ol, (Z)-hept-4-enal and ( )-non-2-enal. Milk pasteurised at higher temperatures and Ultra High Temperature (UHT) milk present the so-called cooked flavour, the appearance of which is the first measurable manifestation of the chemical changes that occur in heated milk. The substances responsible for the cooked off-flavour are sulfane and other sulfur compounds. Of particular importance are dimethylsulfide, dimethyldisulfide and dimethyltrisullide that are produced from proteins contained in the membranes of fat particles and from thiamine. Also relevant are alkane-2-ones (methylketones) generated by thermal decarboxylation of P-oxocarboxylic acids (mainly hexane-2-one, heptane-2-one and nonane-2-one), y-lactones and 5-lactones produced by dehydration of y- and 5-hydroxycarboxylic acids (mainly 8-decalactone and y- and 8-dodecalactones). Important carbonyl compounds include biacetyl, hexanal, 3-methylbutanal, (Z)-hept-4-enal and ( )-non-2-enal. In the more intensive thermal treatment of milk (sterilisation), products of the Maillard reaction play a role, such as maltol and isomaltol, 5-hydroxymethylfuran-2-carbaldehyde, 4-hydroxy-2,5-dimethyl-2 f-furan-3-one (furaneol) and 2,5-dimethylpyrazine. 

Sugar concentrations higher than 20% (found in jams and similar products) have a stabilising effect on the colour of anthocyanins, mainly due to decreased water activity. Degradation of anthocyanins is accelerated in the presence of sugar degradation products, especially by furan-2-carbaldehyde and 5-hydroxymethylfuran-2-carbaldehyde that produce complex brown-coloured condensation products with anthocyanins. 

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