Temperature and pH Stability of Anthraquinones from Native Aloe vera Gel, Spray-Dried and Freeze-Dried Aloe vera Powders during Storage

Abstract

The present study explored the stability of extracted anthraquinones (aloin, aloe-emodin and rhein) from whole-leaf Aloe vera gel (WLAG), its freeze-dried powder (FDP) and spray-dried powder (SDP) under varying pH and temperature conditions during storage. Each anthraquinone behaved differently under different processing parameters. The amount of anthraquinones present in the gel was higher than in FDP and SDP. The aloin contents decreased by more than 50% at 50 °C and 70 °C, while at 25 °C and 4 °C, the decrease was moderate. A substantial reduction in aloin concentration was noticed at pH 6.7, whereas it remained unaffected at pH 3.5. The temperature and pH had no significant effect on the stability of aloe-emodin. Interestingly, a small quantity of rhein was detected during storage due to the oxidative degradation of aloin into aloe-emodin and rhein. These findings can provide significant insight into retaining anthraquinones during processing while developing functional foods and nutraceuticals to obtain maximum health benefits.

Keywords: Aloe vera; aloe-emodin; aloin; anthraquinones; freeze drying; pH; processing temperature; rhein; spray drying; stability.

Figure 1

HPLC chromatographs of (A) reference standards of aloin, aloe-emodin and rhein and (B) WLAG extract containing aloin, aloe-emodin and rhein.

Figure 2

FTIR spectra of (A) anthraquinone extracted from whole leaf Aloe vera gel (WLAG) at various temperatures, (B) anthraquinone extracted from whole leaf Aloe vera gel at various pH conditions and (C) anthraquinone extracted from the whole leaf Aloe vera gel (WLAG), freeze-dried powder (FDP) and spray-dried powder (SDP) of Aloe vera.

Figure 3

Plots of aloin in whole-leaf Aloe vera gel (a), aloin in freeze-dried powder of Aloe vera (b), aloin in spray-dried powder of Aloe vera (c) and plots of aloe-emodin (d) and rhein (e) in whole-leaf Aloe vera gel, vs. time under various temperature conditions. (Values are from triplicates; the mean and standard deviation are shown. Values significantly different from control to various temperature treatments in a two-way ANOVA with Dunnett’s test analysis indicated with asterisks: * p< 0.01, ** p < 0.001, *** p < 0.0003, **** p < 0.0001).

Figure 4

Plots of aloin in whole-leaf Aloe vera gel (a), aloin in freeze-dried powder of Aloe vera (b), aloin in spray-dried powder (c) of Aloe vera and plots of aloe-emodin (d) and rhein (e) in whole-leaf Aloe vera gel, vs. time under various pH conditions. (Values are from triplicates; the mean and standard deviation are shown. Values significantly different from control to various pH treatments in a two-way ANOVA with Dunnett’s test analysis are indicated with asterisks: * p < 0.01, ** p < 0.001, *** p < 0.0003, **** p < 0.0001).

Figure 4

Plots of aloin in whole-leaf Aloe vera gel (a), aloin in freeze-dried powder of Aloe vera (b), aloin in spray-dried powder (c) of Aloe vera and plots of aloe-emodin (d) and rhein (e) in whole-leaf Aloe vera gel, vs. time under various pH conditions. (Values are from triplicates; the mean and standard deviation are shown. Values significantly different from control to various pH treatments in a two-way ANOVA with Dunnett’s test analysis are indicated with asterisks: * p < 0.01, ** p < 0.001, *** p < 0.0003, **** p < 0.0001).

Figure 5

Hydrolysis of aloin into aloe-emodin-9-anthrone and subsequent oxidation to aloe-emodin, a free anthraquinone.