Qualitative tests for carbohydrates
There are several rapid tests available to characterise whether there is sugar or
a carbohydrate in a given sample. These assays are relied on chemical
reactions of carbohydrate’s functional groups: hydroxyl and carbonyl group.
For hydroxyl group, the concentrated acid (i.e., sulfuric acid or hydrochloric
acid) is used in the assays to produce furfural and its derivatives. Such
products can then react with phenol compounds (e.g., napthol, resorcinol and
orcinol) and yield a distinct colourful product (see Figure 2.1). In addition, the
free carbon atom (position 1) of the carbonyl group (as an aldose or ketose)
has the reducing property and is able to reduce Cu2+, Ag+, and Fe(CN)6
sugars with such a property are called ―reducing sugars‖.