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Step 1: extraction of free sugars.

In the determination of available carbohydrates by direct analysis of all individual carbohydrates, the first step is extraction of free sugars.

A variety of extraction techniques is used. The purpose of extraction is two-fold: to extract the sugars (mono- and disaccharides) and oligosaccharides completely and to minimize extraction of interfering substances. Free sugars are generally soluble in hot aqueous alcoholic solvents. Proteins and polysaccharides (e.g. starch) are insoluble in these solvents when the percentage of alcohol reaches 70-80%. Alcohols used are mostly methanol and ethanol, but also isopropanol is sometimes used.

Besides sugars, the alcoholic extract may contain some lipids, pigments, free amino and organic acids present in the sample. These substances need to be removed because they can interfere with the subsequent measurements of sugars (see next page). For example pigments and other colouring matters can interfere with colorimetric procedures, proteins can interfere with reducing reactions etc. In addition, in most sugar methods high concentrations of alcohol will interfere. Therefore, evaporation of alcohol may be necessary. Sometimes also a degassing procedure can be necessary, for example in case of carbonated soft drinks.