Natural low-calorie sweeteners - Natural Food Additives, Ingredients and Flavourings - page 59

that individuals limit their intake to 100 mg/day (SCF 2003b). The Japanese
government recommends no more than 200 mg/day. In the Netherlands, where
liquorice confectionery is enormously popular, the Dutch Nutrition Information
Bureau advises against a daily glycyrrhizin consumption in excess of 200 mg,
assumed to correspond to 150 g of liquorice confectionery (Fenwick et al. 1990).

3.3.4 Thaumatin

Structure, source
Thaumatin is a mixture of closely-related proteins extracted from the fruit of
Thaumatococcus daniellii Benth (katemfe fruit). Five isoforms of the protein
have been found in the fruit, namely thaumatins I, II, III, a and b, all of which are
sweet (Kinghorn et al. 2010). Thaumatin I and II are the main forms. Each consists
of a single polypeptide chain of 207 amino acids and they differ only in four
amino acids. The principal protein is thaumatin 1, while thaumatin 2 comprises no
more than 45% of the mix (Pearce and Roth 2002). They have molecular masses
of 22 209 and 22 293 respectively. This size is similar to that of other small
proteins such as casein, trypsin and papain.
T. daniellii grows wild as a shrub on the rainforest fl oor in western Africa. The
plant occurs widely throughout Nigeria, Ghana and Cameroon, and on the Ivory
Coast, and has been introduced to the rainforests of northern Australia. It attains
3-4 m in height with large papery leaves up to 46 cm long. Its spectacularly red,
trigonal fruit is about 4 cm in diameter and contains up to three large, shiny, black
seeds each having a white or light yellow aril (an outgrowth of the point of seed
attachment) at its apex, surrounded by a transparent jelly. The aril is the source of
thaumatin, while the jelly can be a problem during extraction of the sweetener
because it swells during the process.
Traditionally, fruit is harvested from the wild. Arils are excised from the
fruit by hand in the country of origin and then frozen for despatch to the UK
for extraction. There they are extracted with water or a dilute, near-neutral buffer
(van der Wel and Loeve 1972). The extract is concentrated by reverse osmosis or
ion exchange chromatography, then freeze-dried to produce thaumatin of high
purity.
Other processes have been suggested, including the precipitation of the
aluminium adduct of thaumatin from solution (Higginbotham 1977) and the
preliminary freeze-drying of the fruit. The latter process makes the arils brittle and
facilitates their mechanical separation from the rest of the fruit by shaking and
screening. Separated arils are milled or crushed to powder and extracted with
dilute acid (pH 2.7-3.0) (Daniels and Higginbotham 1980). After extraction, the
aqueous solution obtained is ultra-fi ltered and freeze dried to yield thaumatin.
Transgenic barley has been shown to be a viable source of thaumatin with
yields in excess of 2 g thaumatin per kg kernel material (Stahl et al. 2009). Despite
the attractions of being able to produce virtually unlimited amounts of thaumatin
in a temperate climate, the process has yet to be commercialised.

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