Prevention of degradation of the natural high potency sweetener (2R,4R)-monatin in mock beverage solutions

Highlights

• The natural sweetener monatin is degraded on exposure to light and metal ions.

• Decomposition involves indole ring destruction by radicals and singlet-oxygen.

• Decomposition is decreased by metal ion chelators, light filters and antioxidants.

• Monatin degradation can be limited by choice of treatments, additives and container.

Abstract

Exposure of the naturally-occurring sweetener monatin to light and metal ions results in loss of both parent monatin and total indole (monatin plus monatin lactone/lactam) in mock beverage solutions, with an accompanying decrease in sweetness. In this study potential protective strategies to prevent degradation were investigated. Metal ion chelating resin, or the chelators EDTA and desferrioxamine decreased monatin and indole loss for solutions kept either in darkness or exposed to light. Tannic acid and Chinese bayberry extract both afforded protection, but this did not arise from a light filtering effect. Plastics with defined absorbance characteristics provided protection with this being wavelength dependent; yellow transparent PET plastic was most effective. The contribution of these interventions (metal ion removal/binding; antioxidant; light absorption) was additive, with combinations providing the greatest protective effect against monatin and indole loss. These results indicate that it is possible to minimise monatin degradation by appropriate choices of treatments, additives and container.

Introduction

There is a growing demand for low calorie, high-potency sweeteners as a substitute for sucrose and high fructose corn syrups in food and beverages, and considerable effort has been invested in identifying natural products that provide sweetness and flavour without compromising taste (Bassoli et al., 2011, DuBois and Prakash, 2012, Duffy and Anderson, 1998). Monatin (Fig. 1; (1)), also known by the common name arruva, is a naturally occurring high-potency sweetener isolated from the Sclerochiton ilicifolius plant found in regions of the Transvaal, South Africa. This compound is related to the amino acid tryptophan (Trp), and has a sweet taste without contributing significant calories to formulations in which it is included (Buddoo, Rousseau, & Gordon, 2009). (2R,4R)-Monatin has been reported to be over 3000 times sweeter than sucrose at 5% sucrose equivalence making it one of the most potently sweet natural substances known (Fry, Yurttas, Biermann, Lindley, & Goulson, 2012). In addition, monatin has been described as having a remarkably clean taste (Abraham et al., 2005). Stability studies have shown that exposure of mock beverage solutions containing monatin to long-term UV/visible photolysis results in a loss of sweetness and the formation of unpleasant flavours and aromas (Storkey et al., 2014, Upreti et al., 2012). Prevention of such degradation is therefore a key consideration in the use of this compound.

Monatin loss is known to occur via multiple pathways involving both non-oxidative (cyclisation to yield the lactone (2) and/or lactam (3), Fig. 1) and oxidative pathways (Storkey et al., 2014, Upreti et al., 2012). Recent studies using UHPLC, LC–MS/MS and peroxide assays have demonstrated that degradation is accelerated by both UV/visible light, and the presence of trace transition metal ions. These studies indicated a role for singlet oxygen (¹O₂), free radicals and peroxides (both H₂O₂ and organic peroxides), and have characterised a number of hydroxylated and peroxide species formed on the indole ring (mass increases of 16 and 32 respectively), as well as multiple ring and side-chain oxidation and scission products (Storkey et al., 2014). Further evidence to support a role for ¹O₂ in the proposed photo-degradation pathways was obtained using the photosensitizer Rose Bengal as a source of ¹O₂ (Storkey et al., 2014). These results have allowed a scheme to be proposed that accounts for the majority of the known monatin degradation products (Storkey et al., 2014).

In the light of the above data, the current study aimed to investigate whether modulation of metal ion levels, addition of antioxidant compounds, or strategies that modulate light absorption (inclusion of light absorbing compounds, or use of coloured plastic containers) could minimise monatin and total indole degradation in mock beverage solutions.