Evaluation of the extracts of purple basil (Ocimum basilicum L.) as natural pH-indicator dyes anticipated to be utilised in intelligent food packaging—an optimisation study

Purple basil (Ocimum basilicum L.) extract has significant potential as a natural colorimetric pH-indicator material owing to its abundance in anthocyanins. This contribution is the first to optimise the ultrasound-assisted extraction of purple basil by response surface methodology in terms of maximum total monomeric anthocyanin (TMA) content and pH-dependent increase in green intensity (IGI) using a three-level, three-factor Box–Behnken design. The latter was considered as a measure of the pH-indicator ability of the extracts. The independent variables were ethanol concentration, solvent/solid ratio, and extraction time for two types of solvents [aqueous ethanol with 0.1% (v/v) acetic acid (Ac) or hydrochloric acid]. Optimum conditions were determined for single and combined responses. The extracts were further characterised according to total phenolic content (TPC), percentage polymeric colour (PPC), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) antioxidant, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities. All models explained more than 93% of the variability. The combined response model for the Ac-acidified system gave more favourable optimal conditions [55.25% (v/v) ethanol, 30 mL/g solvent/solid ratio and 39.24 min] regarding lower ethanol consumption and shorter times. Validation experiments verified the model successfully predicted the responses. The corresponding TMA, IGI, PPC, TPC, ABTS, and DPPH values were determined as 4.83 ± 0.18 mg cyanidin-3-glucoside equivalent, 2.29 ± 0.05, 68.07 ± 3.17, 28.79 ± 1.54 mg gallic acid equivalent, 55.85 ± 1.71 mg Trolox equivalent, and 16.80 ± 1.08 mg ascorbic acid equivalent per gram dry weight, respectively.