Natural low-calorie sweeteners - Natural Food Additives, Ingredients and Flavourings - page 48

Fig. 3.3 Structure of steviol glycosides. Steviol nucleus shown (steviol, R 1 = R 2 = H) with

the various substituent groups listed for each sweetener.

than 12 hours' daylight encourages fl owering, which also depletes the leaves of
sweet glycosides. Commercial growth is also confi ned to areas of low labour cost
because the plant does not germinate reliably from seed. Consequently, 98% of
the world crop is grown from cuttings. These are taken, established and
transplanted to crop fi elds by hand because the plant is also intolerant of soil
compaction that occurs when mechanised methods are used.
Stevia typically takes about four months to grow from cutting to harvestable
crop. Two crops per year may be taken where it is grown as an annual, but most
annual Asian crops are harvested but once, typically at about 5% fl owering. Where
conditions permit, S. rebaudiana is treated as a perennial with three 'harvests' per
year. The fi rst of these is mainly to encourage future leaf growth and is not a
serious source of leaves. The second cut is normally the most productive, with a
third, smaller harvest taken later.
The output of this effort is dried leaves, often produced by small-scale farmers
who grow stevia as a short-term cash crop. It is uneconomical to transport leaves
far, so most are sold to local wholesalers who may also run an extraction facility.
Primary extraction involves steeping dry leaves in water, a process suffi cient to
dissolve the steviol glycosides. Excess plant material is fi ltered off and fl occulants
such as calcium salts are added to precipitate soluble proteins. The clarifi ed liquor
is sometimes subjected to resin chromatography where the steviol glycosides are

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