The solid-state stability of aspartame hemihydrate (APM) sweetener is important information for the food industry.

Transcription

1

2 The solid-state stability of aspartame hemihydrate (APM) sweetener is important information for the food industry. This study uses FTIR microspectroscopy coupled with DSC to induce and simultaneously determine thermallydependent formation of impurities in solid-state APM.

3

4 The apparatus used was a DSC-910 from TA Instruments. APM crystals were sealed between two pieces of KBr pellets by a hydraulic press. The compressed KBr disc was inserted in a DSC microscope cell, which was placed on the FTIR microscope stage. The FTIR system was operated in the transmission mode. The position and focus of the sample was adjusted using the microscope.

5 The temperature of the DSC microscopy cell was monitored. Heating rates were 3 and 10 o C/min starting from ambient temperature. The sample disc was equilibrated at the starting temperature (25 o C) for 3 min, then heated from 25 to 300 o C. Thermally-modified IR spectra were recorded while the sample disc was heated on the DSC microscope stage. TGA was performed at the same heating rate, but in an open system.

6 DSC: 3 endothermic peaks observed. Positions changed slightly when using a faster temperature gradient due to lack of equilibration time. These DSC plots show that some APM is in the hemihydrate form. 3 o C/min 10 o C/min

7 TGA: first mass loss assigned to adsorbed water (humidity). Second TGA mass loss from 110 o C (DSC peak at 123 o C) is due to dehydration of the hemihydrate form. The TGA curve from 153 o C (DSC peak at 181 o C) is due to loss of CH 3 OH methanol by intramolecular cyclization of APM to form DKP.

8 DKP DKP: 3-carboxymethyl-6-benzyl-2,5-diketopiperazine.

9 The third DSC peak at 240 o C is assigned to the melting of APM. The presence of DKP, as an impurity of APM, contributes to broaden the melting point range of dehydrated APM. Beyond 240 o C, APM starts to degrade.

10 TGA curve: the weight loss is 1.69% within o C (evaporation of free water adsorbed) The weight loss within o C is due to the dehydration of APM hemihydrate. The weight loss within o C is due to the intramolecular cyclization in the APM molecule to liberate methanol. APM melts at 240 o C, and beyond that it degrades.

11

12 DKP DKP: 3-carboxymethyl-6-benzyl-2,5-diketopiperazine.

13 Noticeable changes are seen from 50, 110 or 153 o C, attributed respectively to the onset temperature of water evaporation (humidity), dehydration and cyclization processes. As an impurity, DKP is produced from solid state APM by intramolecular cyclization and liberation of methanol.

14 wave number (cm -1 ) 3315 NH stretch 1738 C=O stretch, ester 1662 C=O stretch, amide I 1585 asymmetric stretch, COO NH bending, amide II 1377 CH symmetric bending, methyl 1367 symmetric stretch, COO C-O stretch, ester

15 FTIR band intensities started to change while approaching the DSC temperatures of 50 o C (water desorption), 153 o C (dehydration) and 223 o C (cyclization). The IR spectra anticipated the changes faster than DSC.

16 Initially: IR peaks at 3315, 1585 and 1367 cm 1 are due to H-bonded NH. Asymmetric/symmetric carboxylate bands (1585, 1367 cm -1 ) are due to the H-bonding among water, NH 3 and COO -. 3-D plot of T-dependent changes in IR spectra of aspartame hemihydrate from 25 to 120 C

17 Beyond 50 o C, the peaks at 3315, 1585 and 1367 cm -1 gradually disappeared due to elimination of water. Peaks at 3340 (free NH stretching of amine) and 1627 cm -1 (free water, probably from KBr) appeared stepwise instead.

18 3-D plot of temperature-dependent changes in IR spectra of aspartame hemihydrate. Temperatures between 120 and 200 o C.

19 The peak at 3338 cm -1 (asymmetric stretching, free NH 2 ) disappears beyond 174 o C. Two new peaks at 3203 and 3088 cm -1 (NH stretch, DKP) appear. The peak at 1736 cm -1 (ester carbonyl) disappears gradually beyond 153 o C. A new peak at 1718 cm -1 (C=O, carboxyl) gradually increases. A transitional state is noticeable at 165 o C. The peak at 1666 cm -1 (amide C=O of APM) shifts to 1670 cm -1 (C=O of DKP) with twice the intensity to a diketone in DKP. With the outgassing of methanol from APM, the peaks at 1377 and 1225 cm -1 (methyl, ester C=O) disappear. A CN peak for DKP (1283 cm -1 ) gradually appears.

20 An impurity was created and detected in solid-state APM while heating the hemihydrate. FT-IR microspectroscopy equipped with a thermal DSC system was used. The structural rearrangement of APM molecule during thermal treatment was observed stepwise in the IR spectra.