Evaluation of colour stability of clear red pitaya juice treated by thermosonication

The impact of thermosonication (TS) processing on colour and associated properties of clear red pitaya juice was evaluated. Although a longer treatment duration and higher temperature led to a distinct colour change, the results indicated potential colour protection by TS relative to thermal treatment (83 °C, 1.5 min). Increased in the colour change during TS mainly due to decreasing a* and b* values (p < 0.05). The results of UV–visible spectroscopy, spectrophotometry, and ultra-performance liquid chromatography were consistent with a decrease in a* value, indicating decomposition of betacyanins. Further analysis revealed that the degradation and isomerisation of betanin and phyllocactin were involved in the decomposition of betacyanins by TS primarily, and which was differed with thermal processing. Besides that, an increase in 5-hydroxymethylfurfural and the absorbance at 420 nm following Millard reactions also contributed to colour change. The effective inactivation of polyphenol oxidase, and >92.97% retention of polyphenols in clear red pitaya juice was observed, suggesting that polyphenol oxidase catalysed browning was not a major contributor to colour instable during TS. Overall, results obtained in the current study suggested a potential of colour protection, and bioactive compounds retention capacity of fruit juice by TS.