Effect of post-harvest practices on flavonoid content of red and white onion cultivars

Abstract

Onions are major sources of flavonoids in the human diet. However, little information is available regarding the effects of long-storage or exposure to specific stress conditions on flavonoids content of onions. The aim of this work was to assess the effect of different post-harvest treatments on the flavonoid composition of two Portuguese landrace varieties of onions (‘Branca da Póvoa’ and ‘Vermelha da Póvoa’). The evolution of the content of some major flavonols and anthocyanins was measured in red and white onion bulbs (from 2005 and 2006 harvests) during 7 months of storage, under refrigerated and under traditional bulk storage in the field. Total flavonols increased up to 64% after 6 or 7 months of storage. This increase was especially important during the first 3 months of storage (58% increase). In red onions, with the largest concentrations in flavonols, bulbs stored in the field reached higher levels of flavonoids (64% maximum) than refrigerated onions (40% maximum). For red onions, the increase after 6-months storage usually has place when the flavonol post-harvest levels are low (40–64% increase), whereas for white onions the increase after 6-months storage is important for onions with higher levels after harvest (44–60% increase). These results suggest that storage at fluctuating ambient temperatures can positively affect flavonol metabolism, while keeping the flavonols profile. There were no significant modifications of the total levels of anthocyanin pigments after 6 months of storage of red bulbs, but after 7 months total anthocyanin content was reduced between 40% and 60%. Post-harvest UV (40 kJ/m², 1 week storage) and ethylene (100 μL/L for 24 h, 2 months storage) treatments did only affect the flavonol content of the edible portion of onions with a profitable increase.

Introduction

Onions (Allium cepa L.) are one of the world’s oldest cultivated vegetables and are the second most important horticultural crop after tomatoes in Portugal. Onions contain high levels of flavonoids, a major class of non-nutrient antioxidants. The major classes of flavonoids present in onions are flavonols (quercetin glycosides) and anthocyanins (cyaniding glycosides). These compounds are effective scavengers of free radicals that are thought to induce DNA damage and tumour promotion. Flavonoids, in general, also have preventive effects on a number of degenerative pathologies such as cardiovascular and neurological diseases, and other dysfunctions related to oxidative stresses (Griffiths, Trueman, Crowther, Thomas, & Smith, 2002).

Plant phenolic compounds are synthesized via the phenylpropanoid pathway and play a role in plant defence mechanisms against biotic and abiotic stresses. The induction of phenylpropanoid metabolism can be achieved artificially by treatments with elicitors or exposure to specific stress conditions (Matern & Grimmig, 1994). Interest in the role of antioxidants in human health lead to an effort to evaluate antioxidant properties of fruits and vegetables and to determine whether these properties can be maintained or improved through crop breeding, cultural practices, post-harvest storage and emergent technologies such as UV-C (Arcas et al., 2000, Cantos et al., 2003, Hagen et al., 2007, Higashio et al., 2005, Mahdavian et al., 2008) or ethylene treatments (Bellincontro et al., 2006, Heredia and Cisneros-Zevallos, 2009).

Several studies have addressed the changes in the phenolic compounds during storage of different fruits and vegetables (Gennaro et al., 2002, Leja et al., 2003, Price et al., 1997, Zhang et al., 2008; Kevers, Falkowski, Tabart, Defraigne, Dommes, & Pincemail, 2007). Generally, these results show that antioxidant activity and concentration of phenolics often increases during storage (Kevers et al., 2007, Leja et al., 2003, Zhang et al., 2008), although a few studies report constant or decreasing levels during storage (Gennaro et al., 2002, Kevers et al., 2007, Price et al., 1997). UV radiation, used in post-harvest as a sanitizing treatment, can induce biological stress in plants with the consequent production of phytoalexin compounds such us flavonoids or stilbenes. A number of studies have reported increased levels of flavonoids in fruits and vegetables treated with UV-C (Arcas et al., 2000, Cantos et al., 2003, Hagen et al., 2007, Higashio et al., 2005). Similar effects have been observed with exogenous applications of ethylene to several fresh fruits and vegetables (Bellincontro et al., 2006, Heredia and Cisneros-Zevallos, 2009).

Little information is available regarding the effects of long-term storage or exposure to specific stress conditions on changes of the flavonoids content in whole unpeeled onions (Benkeblia, 2000, Gennaro et al., 2002, Price et al., 1997). Therefore, the aim of this work was to assess the effect of post-harvest UV-C and ethylene treatments on the flavonoid composition of two landrace varieties of Portuguese onions (‘Branca da Póvoa’ and ‘Vermelha da Póvoa’). The evolution of the content of some major flavonols and anthocyanins was measured in red and white onion bulbs (from 2004 and 2005 harvests) during 7 months of storage, under refrigeration and also under environmental conditions (a traditional Portuguese storage). The effect of post-harvest irradiation and ethylene treatment on the accumulation of flavonoids in bulbs was also studied for the white variety from the 2005 harvest.