3.4.1. Antioxidant Capacity

The results of the antioxidant capacity of the *Aloe vera* extracts are presented in Table 7. For the OxHLIA assay, data were given as IC50 values, corresponding to the extract concentration capable of protecting 50% of the erythrocyte population from oxidative haemolysis for 60 min; whereas, for the TBARS and β-CBI assays, data were expressed as EC50 values, meaning the extract concentration capable of providing 50% of antioxidant activity. In both cases, the lower the EC50 or IC50 values, the higher the antioxidant capacity.

The mucilage extract (at 47 ± 2 μg/mL) was the most effective in inhibiting the formation of TBARS. This cell-based assay allowed for the measuring the extract capacity to inhibit the formation of malondialdehyde and other reactive substances, which are generated from the *ex vivo* decomposition of lipid peroxidation products resulting from the oxidation of the porcine brain cell membranes.

The rind extract provided the best protection against oxidative haemolysis (IC50 value, 56 ± 4 μg/mL) and β-carotene bleaching (EC50 value, 51 ± 4 μg/mL). In the OxHLIA assay, the erythrocytes were exposed to the haemolytic action of hydrophilic radicals that resulted from the thermal decomposition of the peroxyl radical initiator AAPH and, subsequently, to the action of lipophilic radicals generated through a lipid peroxidation phenomenon as a result of the initial attack [57]. In the β-CBI assay, β-carotene underwent discoloration in the absence of antioxidant extract, which results in a reduction in the absorbance of the test solution with increasing reaction time. The presence of antioxidants hindered the bleaching extension by neutralizing the linoleic hydroperoxyl radicals formed in the reaction emulsion [58]. These results are consistent with those of Lucini et al. [59], which concluded that the green rind is more antioxidant than the inner parenchyma.

Pearson's analysis indicated a strong correlation between the antihaemolytic and β-carotene bleaching inhibition capabilities and the flavonoids content (*r* = −0.778, *p* = 0.003, and *r* = −0.865, *p* < 0.001, respectively). In fact, the higher IC50 and EC50 values obtained with these two *in vitro* assays, respectively, were achieved with fillet extract (Table 7), in which no flavonoids were detected (Table 6). On the other hand, TBARS inhibition was strongly correlated with malic acid contents (*r* = −0.946, *p* <0.001) and moderately with the levels of anthrones (*r* = −0.667, *p* = 0.018) and chromones (*r* = −0.676, *p* = 0.016). The results of a previous work [60] suggested that the antioxidant capacity of *Aloe vera* gel extract might be ascribed to a synergistic action of bioactive compounds. The same study also evidenced that the gel extract is able to protect the erythrocyte membrane from AAPH-induced oxidative injury and partially restore its normal protein profiles. This observation may support the fact that the best results of the antioxidant activity of the present study were achieved with the OxHLIA assay because the extracts had IC50 values closer to those of trolox.


**Table 7.** Antioxidant, anti-tyrosinase, and antimicrobial capacities of *Aloe vera* leaf (fillet, mucilage, and rind) and flower extracts and positive controls.

na: no activity; MIC: minimum inhibitory concentration (mg/mL); MBC: minimum bactericidal concentration (mg/mL); MFC: minimum fungicidal concentration (mg/mL). <sup>1</sup> Statistics for antioxidant activity: (a) homoscedasticity was tested by the Levene's test: *p* = 0.130 for OxHLIA (homoscedastic); *p* = 0.010 for TBARS (heteroscedastic); and *p* = 0.305 for β-CBI (homoscedastic); and (b) Statistically significant differences (*p* < 0.05) were assessed by a one-way ANOVA (and indicated by different letters) using Tukey's honestly significant difference (HSD) or Tamhane's T2 multiple comparison tests, when homoscedasticity was verified or not, respectively: *p* < 0.001 in all cases. <sup>2</sup> Inhibition percentage of tyrosinase activity at 8 mg/mL.
