Agriculture Reference
In-Depth Information
adsorbed on a resin bed, other materials fl ushed away and the glycosides eluted
with an alcohol. After concentration, the glycosides are spray-dried to yield a
crude extract. The composition of this varies, but might well comprise 50%
stevioside, 25% rebaudioside A and about 10% other glycosides, with the balance
being uncharacterised plant components.
The most-highly prized glycoside is rebaudioside A. This can be purifi ed from
crude extract by dissolving the latter in aqueous alcohol from which, under
appropriate conditions, rebaudioside A crystallises selectively at high purity (97%+
rebaudioside A with <3% other glycosides and no signifi cant other materials).
Physico-chemical properties
Pure steviol glycosides are white powders, but crude stevia extracts may be cream
or brownish.
Stevioside is sparingly soluble in room temperature water (0.13%).
Rebaudioside A is normally purifi ed by crystallisation from alcoholic solution and
the resultant solvated solid can exhibit very high temporary solubilities (>20%) at
room temperature, but the system is metastable and the glycoside forms a hydrate
that can crystallise at any point. The true equilibrium solubility at room temperature
is 0.8%, although even the hydrated form of rebaudioside A becomes abruptly
very soluble (30-50%) - and stable - in water above about 60°C.
The glycosides are stable when stored dry, though care may be needed as
rebaudioside A is markedly hygroscopic. As regards stability in solution, most
attention has been paid to stevioside and rebaudioside A. The latter is most stable
between pH values 4-8, with decreasing stability in more acid systems and
markedly lower stability at pH 2 - although this is rather an academic point as no
foodstuffs are this acidic.
Stability in solution is strongly temperature dependent and, as Fig. 3.4
demonstrates, ranges over fi ve orders of magnitude between refrigeration and
boiling temperatures. The decomposition of rebaudioside A is fi rst-order and Fig.
3.4 shows the calculated time to 25% loss under different conditions of pH and
temperature. 25% is an arbitrary limit, chosen because, in the case of aspartame,
this degree of loss is used in the industry to indicate end-of-sweetness shelf life.
However, it should be noted that the breakdown products of rebaudioside A are
themselves steviol glycosides and are thus HPS in their own right. This means that
the loss of sweetness is much less than the analytical loss of rebaudioside A would
suggest. To put the stability of rebaudioside A in the context of other sweeteners, it
is substantially more stable than aspartame, neotame or advantame in virtually all
products and processes, and is similar to the stability of sucralose in many.
Older suggestions that steviol glycosides are not photo-stable (Chang and
Cook 1983) have recently been disproved (Clos
et al.
2008).
Metabolism
The steviol glycosides have no energy value. They all break down under the
infl uence of colonic bacterial glucosidases to release steviol. Humans absorb
steviol via the hepatic portal system, transform it to steviol glucuronide in the
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