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.