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The effect of storage conditions on changes of pH, total soluble solids (°Brix), colour, betalains (betacyanins and betaxanthins), and 5-hydroxymethylfurfural was investigated in apple-beetroot juices stored at 2, 7 and 20 °C for 120 days. The results obtained showed that the quality of apple-beetroot juice is significantly influenced by temperature and storage duration. Increasing storage time and temperature induced an apparent rise (P < 0.05) of lightness (L*), yellowness (b*), hue angle, total colour difference (ΔE) and 5-hydroxymethylfurfural. Maximal concentration of 5-hydroxymethylfurfural reached 14.97 ± 0.03 mg/L at 20 °C; however, this concentration is under the allowable maximal level in apple juice (20 mg/L). The increase in total colour difference and 5-hydroxymethylfurfural concentration was associated with a decrease of betacyanins and betaxanthins, confirmed by strong inverse correlations (r from − 0.7991 to − 0.9813). Degradation of both betacyanins and betaxanthins followed the first-order kinetic model and showed positive correlation with redness (a*) and chroma. Higher rate constants (k) for betacyanin degradation (k ≈ 0.0064-0.0285 day −1 ) in comparison with the rate constants for betaxanthin degradation (k ≈ 0.0073-0.0471 day −1 ) demonstrate that betacyanins degraded slightly faster. The changes in all monitored characteristics were significantly greater at 20 °C (P < 0.05) than at 7 and 2 °C, except pH and °Brix. Therefore, apple-beetroot juices should be stored at lower temperatures to maintain their acceptable quality. Keywords Apple-beetroot juice quality • Storage • Colour • Betacyanins and betaxanthins • 5-hydroxymethylfurfural Abbreviations A Absorbance a* Greenness/redness AA Ascorbic acid ANOVA Tukey's HSD Analysis of variance-Tukey's honest significant difference b* Blueness/yellowness BC Betacyanins BX Betaxanthins°B rix Total soluble solids CIE L*a*b* Colour space defined by International Commission for Illumination c t=0 Initial concentration of betacyains/betaxanthins (mg/L)

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