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Effects of high-dose postharvest melatonin on weight loss, physical quality and bioactive compounds of 'Keppler' highbush blueberry (Vaccinium corymbosum L.) during cold storage
Abstract
Melatonin has been proposed as a natural postharvest treatment for fresh fruits; however, its effective concentration range for blueberry has not been established. This study evaluated the effects of postharvest melatonin applied at 0, 500, 1000 and 1500 ppm (0, 2.15, 4.31 and 6.46 mmol L⁻¹) on the quality of 'Keppler' highbush blueberry (Vaccinium corymbosum L.) during 20 days of cold storage at 4°C. Weight loss, total soluble solids (TSS), fruit firmness, surface colour (L*, a*, b*), titratable acidity (TA), vitamin C, total phenolic content (TPC), total anthocyanin content (TAC) and DPPH radical scavenging activity were determined at 5-day intervals. Storage period was the dominant source of variation for all traits except colour, and the treatment × storage interaction was significant for TSS, weight loss, firmness and all biochemical traits. The 1000 ppm dose was the only treatment that reduced weight loss (by 48% and 36% relative to the control at days 15 and 20, respectively) and limited the rise in TSS; however, this response was not dose-dependent and was variable among replicate packages, so it requires confirmation. Melatonin did not affect fruit colour, and firmness was improved only by 1500 ppm at day 15. Melatonin transiently increased TPC and TAC during the first 5–10 days of storage; however, from day 15 onward, control fruit retained significantly higher TA, vitamin C, TPC, TAC and DPPH activity than all melatonin-treated fruit. Pearson correlation and principal component analysis showed that TA, vitamin C and DPPH activity were almost perfectly correlated with each other (r = 0.99–1.00) and that TPC was strongly correlated with DPPH activity (r = 0.90); these associations largely reflected the parallel decline of these traits during storage, and PC1 (63.5%) described a storage-related quality-loss gradient. The applied doses (about 2.2–6.5 mmol L⁻¹) were well above the range effective in previous blueberry studies, suggesting a supra-optimal response. Lower melatonin concentrations should be tested to combine reduced water loss with better retention of nutritional quality in blueberry.
