Natural low-calorie sweeteners - Natural Food Additives, Ingredients and Flavourings - page 63

be that the S,S form is not sweet and the recorded taste is the result of a trace
presence of R,R .
However, Sclerochiton ilicifolius is scarce and the root bark contains only
minute amounts of monatin (0.007%) that can be isolated (Van Wyk and Ackerman
1992; Vleggaar et al. 1992). Consequently, various processes have been described
as alternatives to extraction from roots for the possible bulk production of monatin.
Some of these use traditional organic chemical reactions (Van Wyk and Ackerman
1990, 1992; Holzapfel and Olivier 1993; Holzapfel et al. 1994; Abushanab and
Arumugam 1999) or a chemo-biocatalytic route of synthesis (Rousseau et al.
2011), while others rely on either in vivo synthesis using genetically modifi ed
organisms (Anderson et al. 2007) or in vitro enzymatic transformations (Abraham
et al. 2009). Enzymatic methods should allow the product to be described as
natural, at least in the US. The latter processes also have the potential advantage
of yielding pure single isomers.

Physico-chemical properties
The free acid form of monatin is likely to have low solubility in water, but simple
metal salts such as sodium or potassium monatin are readily soluble. The majority
of information regarding the physico-chemical properties of monatin comes from
patent applications. There are few peer-reviewed data published. Monatin is more
stable than aspartame (Abraham et al. 2009), but does have some stability issues
in solution. For example, the amino acid can form a lactone (reversible) or a
lactam (irreversible) (Roy 2010). The former is still potently sweet, but not the
latter (Ikeda et al. 2007). Lactamisation is promoted by heat and acid conditions.
Also of concern is the oxidation of monatin. Accelerated by light and the
presence of oxygen, oxidation not only gives rise to loss of monatin, but to the
generation of 3-methyl indole (skatole) and, consequently, a musty taste at levels
above 0.5 mg/l (Evans and Goulson 2010). Yellow discolouration of light-
coloured drinks and sediment formation can also occur (Roy 2010).
As well as procedures to reduce dissolved oxygen, enzyme-modifi ed
isoquercitrin (EMIQ) and myricitrin are among key antioxidants suggested for
use with monatin to reduce oxidation (Evans and Goulson 2010). However,
ascorbic acid - often used in antioxidant systems - is reported to promote the
development of unwanted pink hues in clear beverage systems (Roy 2010).
Synthetic antioxidants, although well-known and effective, are obviously less
appealing for use with a natural sweetener.
In addition to the above, monatin is a free amino acid and might, therefore, be
expected to take part in Maillard browning reactions with reducing sugars. The
extent to which this is of practical signifi cance is not apparent from the literature.

￿ ￿ ￿ ￿ ￿

Metabolism
It is anticipated that safety studies will show monatin is metabolised as any other
free amino acid. This would mean that its calorie content would be very low to
zero, it should have no effect on blood glucose, no insulin demand, and should not
be fermented by oral bacteria and, hence, should be non-cariogenic.