Health-related compounds in broccoli (Brassica oleracea L. var. italica) as affected by postharvest temperature, light and UV-B irradiation

Summary

Broccoli (Brassica oleracea L. var. italica) belongs to Brassica vegetables, consumption of which has been shown to reduce the risk of several types of cancer and cardiovascular disease mortality in epidemiological studies. The health-promoting properties of broccoli have been attributed to high levels of bioactive compounds including vitamin C, flavonols and glucosinolates (GLS) in this vegetable. The contents of these compounds in broccoli at harvest are not only determined by pre-harvest factors but are further influenced by various postharvest factors on the way from producer to consumer. However, the effect of storage temperature on flavonols and GLS in particular is still not clear and little is known about how postharvest light influences vitamin C and GLS contents. Moreover, UV irradiation is considered as a tool for enhancing contents of bioactive compounds in fresh fruits and vegetables after harvest. However, little is known how temperature influences the outcome of postharvest light or UV-B radiation treatment. Therefore, the main purpose of the present study was to evaluate the effects of postharvest temperature, light and UV-B irradiation on the vitamin C, L-ascorbic acid (AA), dehydroascorbic acid (DHA), flavonols and GLS levels in broccoli flower buds in an experiment with multi-level design simulating mapped commercial storage conditions from harvest until consumer purchase. The contents of these compounds were examined in flower buds during pre-storage and storage of broccoli heads, representing refrigerated transport with wholesale distribution and retail, respectively. In addition, the GLS analysis was conducted for floret stalks of chosen samples and epidermal flavonols in flower buds of all broccoli heads were monitored during storage using non-destructive repeated measurements. Broccoli heads were pre-stored for 4 or 7 d at 0 or 4 °C in the dark and then stored for 3 d at 10 or 18 °C. During storage, the broccoli heads were exposed for 12 h per day to three different levels of visible light (13, 19 or 25 μmol m-2 s-1) or a combination of visible light (19 μmol m-2 s-1) and UV-B irradiation (20 kJ m-2 d-1), or they were stored in the dark. The vitamin C, AA, DHA, flavonols and GLS levels in broccoli flower buds were not significantly changed during pre-storage at both temperatures 0 °C or 4 °C for both 4 and 7 d. Moreover, the contents of flavonols and GLS were found to be in general stable, while vitamin C levels degraded as a function of increasing pre-storage period (PP), pre-storage temperature (PT) and storage temperature (ST) during storage at higher temperatures (10 °C or 18 °C). However, the levels of epidermal flavonols, monitored with non-destructive repeated measurements of the broccoli heads during storage, were observed to increase with prolonged PP and storage period. In addition, general linear model analysis revealed that PP and ST significantly influenced almost all GLS levels, except for 4-methoxyglucobrassicin in broccoli flower buds during storage. Higher levels of these compounds were found in broccoli pre-stored for 7 d than in those pre-stored for 4 d. The 4-hydroxyglucobrassicin content was increased after storage at 18 °C, while the levels of all other GLS were higher in flower buds of broccoli after storage at 10 °C than at 18 °C. Moreover, the effect of ST on aliphatic GLS in broccoli flower buds depended on PT and it was negligible for broccoli heads pre-stored at 4 °C but significant for those pre-stored at 0 °C. For flavonols the PP × PT interaction was found significant, with higher contents after storage observed for broccoli pre-stored shorter (4 d) at 4 °C and pre-stored longer (7 d) at 0 °C. In addition, the PP × ST interaction was found significant for vitamin C and DHA and to a minor extent for AA, indicating that the difference in contents of these compounds in flower buds of broccoli stored at 10 °C and at 18 °C depended on PP and was higher for broccoli pre-stored for 4 d. The DHA level and DHA/AA ratio were stable in flower buds of broccoli pre-stored for 7 d during storage but were increased in those pre-stored for 4 d. These findings suggest a shift in the ascorbate metabolism in broccoli flower buds during storage with its higher rate observed for broccoli pre-stored for shorter time. The radiation treatments significantly affected both quercetin content and epidermal flavonol levels in broccoli flower buds during storage, with the highest levels observed after a combination of visible light and UV-B irradiation treatment. However, the outcome of this postharvest treatment was influenced by a combination of factors including PP, PT and ST. The accumulation of epidermal flavonols upon UV-B exposure was initiated earlier in broccoli heads stored at higher temperature (18 °C), that is, already during the first day of storage. However, no effect of radiation treatments was found for vitamin C and GLS contents in broccoli flower buds during storage. In conclusion, the present study has shown that PP and PT influence the contents of vitamin C, flavonols and GLS during storage, which emphasise the importance of pre-storage treatment for both postharvest research and commercial industry. Moreover, the potential of UV-B irradiation combined with visible light to increase flavonol contents in broccoli flower buds during storage has also been demonstrated. These findings will hopefully contribute to improved postharvest handling procedures in order to preserve high contents of health-related compounds in broccoli.

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NVA : hdl.handle.net/11250/2448099