Occurrence of furfuraldehydes during the processing of Quercus suber L. cork. Simultaneous determination of furfural, 5-hydroxymethylfurfural and 5-methylfurfural and their relation with cork polysaccharides

A spectrophotometric method was used for simultaneous determination of furfural (F), hydroxymethylfurfural (HMF) and methylfurfural (MF) based on the calculation of the derivative of the spectrum of absorption of the products of their reaction with 2-thiobarbituric acid. This methodology was applied to cork samples obtained from different industrial processing steps for stopper production: cork before autoclaving (BA), cork submitted to one autoclaving procedure (1A), cork 1A washed (1AW) and cork 1A washed and dried (1AWD). The content of furfuraldehydes in cork samples depends on the effect of heat treatments during processing. The formation of F in the autoclaving procedure might be due to the degradation of unbound or cell wall loosely bound pentoses, such as xylose and arabinose. The lower relative amount of fucose and rhamnose is in accordance with the low values determined for MF, and the higher amount of HMF in relation to F showed that the cellulose should also be attacked. In order to minimize the amount of furfuraldehydes in cork stoppers formed by heat treatments, washing and soft drying is necessary. The application of this simple and inexpensive methodology to the cork collected at different manufacturing steps allows controlling the possible thermal abuse of the cork.