Natural low-calorie sweeteners - Natural Food Additives, Ingredients and Flavourings - page 60

Physico-chemical properties
Thaumatin is a pale-brown hygroscopic powder with a characteristic odour. The
solubility of thaumatin is high - more than 20% can be dissolved in room
temperature water. It is also readily soluble (10% or more) in most polar solvents
such as ethanol, isopropanol, glycerol and propylene glycol, provided the water
content of the solvent is 20% or more. In aqueous solution the protein has an
absorption maximum at 277 nm and this has been used as the basis of a
spectrophotometric assay (Coiffard et al. 1997). Owing to a high proportion of
basic amino acids, the isoelectric point of thaumatin is high at pH 11.
The tertiary structure is stabilised by the presence of eight disulphide bridges
(de Vos et al. 1985; Ogata et al. 1992). These lock the molecule in shape and
confer resistance to denaturation whether by heat or pH extremes. The maintenance
of tertiary structure is critical to thaumatin's technical function, as cleavage of just
one disulfi de bridge results in a loss of sweetness (Iyengar et al. 1979).
The heat stability of thaumatin depends on the matrix. The freeze-dried solid is
stable while, in solution, the sweetness of thaumatin I is retained after being
boiled for one hour at a pH value below 5.5. At these acid pH values, the sweetener
is claimed to be stable to heat processes including pasteurisation, canning, baking
and UHT (Gibbs et al. 1996; Lord 2007). In purifi ed form, however, the protein
aggregates and loses its sweetening power at a temperature of 70°C at pH 7.0
(Kaneko and Kitabatake 1999).
Detailed study of reaction kinetics in aqueous solution has shown the
breakdown to be fi rst order (Coiffard et al. 1997). The maximum stability is
apparently around pH 2, but this is more acid than any foodstuff and the authors
did not test the more relevant value of pH 3. In this same study the stability of
thaumatin was adequate but not impressive. Time for 10% loss at 20°C was
43 days at pH2, 19.7 days at pH 4 and only 9.3 days at pH 6. These fi gures imply
a shelf life at pH 4 of about 60 days and at pH6 of less than 40 days, before 25%
of sweetness is lost (a degree of loss often used as an indicator of the limit of
sweetness shelf life).

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Metabolism
Thaumatin is digested as any other dietary protein and has an energy content
of 4 kcal/g.

Sensory properties
Authorities differ on the potency of thaumatin. Lord (2007) quotes 3000 at about
7.5% SE, while others say 1600 at 7% SE (van der Wel and Loeve 1972). The
potency is much higher at lower concentrations, for example it is claimed to be
about 6000 at 1% SE (Pearce and Roth 2002).
The maximum sweetness attainable with thaumatin in water is about 10% SE
(DuBois et al. 1991). However, even 7% SE actually represents an unrealistically
high level of sweetness to aim for with this molecule, because of both the dynamics
of its perception and the presence of side tastes. Thus thaumatin's sweetness is
signifi cantly delayed in onset compared to sucrose, and has a pronounced linger