Article pp.523-526 du Vol.20 n°4-5 (2000)

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NOTE

Photodegradation kinetics under UV light of aspartame

Céline COUTEAU^{1 *}, Sanja STOJANOVIC², Françoise PEIGNE¹, Laurence J.M. COIFFARD¹

RÉSUMÉ Étude de la cinétique de dégradation après irradiation ultraviolette de l’aspartame.

L’objectif de ce travail est l’étude de la photodégradation de solutions aqueuses d’aspartame 3,4 ×10^–3M (λ_max= 257 nm, ε= 180 M^–1·cm^–1). Le dosage de l’aspartame a été effectué en chromatographie liquide haute perfor- mance. On a ainsi pu mettre en évidence que la photodégradation de cet édul- corant suivait une cinétique d’ordre un. La constante de vitesse de réaction de dégradation à pH 5 est de 3,47 ×10^–3min^–1.

Mots clés : photodégradation, aspartame, solution aqueuse.

SUMMARY

The objective of this research was to study the photodegradation of 3.4 ×10^–3M aqueous solutions of aspartame (λmax: 257 nm, ε= 180 M^–1·cm^–1). A conve- niant HPLC method was used for the quantification of aspartame in order to study the photostability. Photodegradation appeared to follow first-order kinetics. The degradation rate constant was calculated to be 3.47 ×10^–3min^–1at pH 5.

Key-words: photodegradation, aspartame, aqueous solutions.

SCIENCES DES ALIMENTS, 20(2000) 523-526

1. Laboratoire de pharmacie industrielle, UFR des Sciences pharmaceutiques, Université de Nantes, 1 rue Gaston Veil, 44035 Nantes cedex, France.

2. Department of Pharmaceutical Technology, Faculty of Pharmacy, Vojvode Stepe 450, PO Box 146, 11000 Belgrade, Yugoslavia.

* Correspondence

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1 - INTRODUCTION

Aspartame, an artificial intense sweetener is a dipeptide (L-aspartyl-L-phe- nylalanyl methyl ester) which is 200 times sweeter than sucrose (GAYTE- SORBIER, 1986).

Aspartame has been approved by the Food and Drug Administration (FDA) as a «general purpose sweetener» since 1996.

Aspartame is not metabolized to acids by bacteria of the oral cavity. It is a low caloric sweetener. Ther for, it can be used by people who must avoid sucrose because of dental caries or obesity (BOWEN, 1984; SUMITRA 1991).

Some studies show that its consumption decreases food intake (ANDERSON, 1988; ANDERSONet al., 1989).

The sweety flavour of aspartame is pure without any aftertaste. This flavour develops and fades slowly.

Previous studies established that aspartame, in aqueous solution (3.4·10^–3 M), is thermodegradable (HOMLER, 1984; COIFFARD et al., 1998). The time necessary to obtain a decrease of 10% of the initial concentration (t_90%) is about 200 days at pH 5 (COIFFARDet al., 1998). Aspartame is more thermostable in a acidic media than in basic ones. The present study was designed to further the knowledge of aspartame stability by determining the kinetics of photodegradation at the pH where aspartame is the most stable.

2 - MATERIALS AND METHODS

2.1 Materials

Aspartame (Interchemical, France) (Batch N° D - 608036), a two residue single chain peptide (L aspartic acid and phenylalanine), is a white powder without any smell. All chemicals were of analytical quality. HPLC grade acetonitrile and monosodium phosphate were obtained from Merck. Distilled water was obtained from an Autostill 4000X (Jencons) apparatus.

2.2 Experimental procedure

Solutions of aspartame at a concentration of 3.40 ×10^–3M were enclosed in spectrophotometer tubes and exposed to a UV light source in a light-stability cabinet (Original Hanau, No. 7011, Original Hanau Quarzlampen GmbH). The intensity of UV-A and UV-B was measured with an Osram apparatus (Centra- UV-Meβgerät). The intensity was maintained at 6.45 and 1.47 mW·cm^–2for UV- A and UV-B, respectively. All tubes containing aspartame solutions were covered with aluminium foil before exposure in order to eliminate the influence of heat generated by the light within the cabinet. The pH of the solution was determined with a Metrohm Herisau pH-meter, model E300B, equiped with a

524 Sci. Aliments 20(4/5), 2000 C. Couteau et al.

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Refill Ingold I 3556 (pH = 0 - 14, T = 0 - 80°C) electrode and standardized with Panreac solutions respectively at pH = 4 and pH = 10. The measurements were carried out at 20°C.

2.3 Absorbance spectrum and analyses of aspartame solutions The absorbance spectrum of aspartame was analyzed between 200 and 400 nm with a spectrophotometric method (Hitachi UV-visible double beam spectrophotometer, model U-2000). A liquid chromatographic method was used for the determination of the aspartame concentrations, initially and at various times. HPLC was carried out with a system consisting of a Waters Model 655 A-12 pump, a Waters Lambda Max model 481 LC variable-wavelength detector set at 257 nm and a Merck D-2500 model integrator controlled with a L-5000 LC controller (Hitachi). Each solution was analyzed under the following conditions: analytical column ChromCart Nucleosil 100-5 C₁₈ HD (4×250 mm, 5µm, Macherey-Nagel); volume injected 10 µL; temperature 20°C;

flow rate 1.0 mL·min^–1. The mobile phase was the following: Acetonitrile/50 mM NaH₂PO₄(pH = 3.50) (10/90 v/v). The pH of the buffer was adjusted to 3.50 with HPLC grade 85% phosphoric acid. The buffer and acetonitrile were separately filtered through a 0.45µm filter (Millipore), mixed in the desired proportions and degassed. The analyses were carried out on triplicate samples and the diffe- rence between the triplicates was below 1%.

3 - RESULTS AND DISCUSSION

The spectrum of aspartame showed a maximum at 257 nm. The optimum sensitivity of aspartame was thus obtained at this wavelength.

Each solution of aspartame (pH = 5) was injected into the HPLC column and the areas of the peaks were measured. A linear relationship was found throu- ghout the concentration interval analyzed (10 - 25 mg·L^–1; r > 0.99).

The photodegradation of aspartame was expressed as the rate of change of the area of the peak (10.5 min). We observed a gradual decrease during photo- lysis. The degradation rate constant was calculated from the slope of the line of the area of the peak (10.5 min) versus time. The percentage of substance remai- ning was calculated. The photodegradation of aspartame in aqueous solution (figure 1) follows apparent firt-order kinetics and is described by the following equation:

C/C₀= e^–kt Eq. 1

where C and C₀are the concentrations of aspartame at time t and at time zero, respectively, and k is the apparent first-order degradation rate constant.

Equation 1 gives us the value of the degradation rate constant, which is equal to 3.47 ×10^–3min^–1.

The shelf life of aspartame was defined as the time by which the aspartame concentration had decreased by 10% from the initial concentration. The predic- ted shelf life obtained from the Arrhenius equation was 43.6 min.

Photodegradation of aspartame 525

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The present study has improved aspartame stability knowledge. The mole- cule appears to be photodegradable at pH 5. However, it is more photostable than acesulfame-K (COIFFARDet al., 1999).

Received 2^ndMarch 2000, accepted 26 June 2000.

REFERENCES

526 Sci. Aliments 20(4/5), 2000 C. Couteau et al.

Time (min)

0 50 100 150 200 250

C/C0

0,1 1

Figure 1

Kinetic diagram for the photodegradation during irradiation of the aspartame aqueous solution (3.4 ×10^–3M) at pH 5. Data are the average of three determinations

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