Changes in antioxidant flavonoids during freeze-drying of red onions and subsequent storage M.R. Pérez-Gregorio, J. Regueiro, C. González-Barreiro, R. Rial-Otero, J. Simal-Gándara * Nutrition and Bromatology Group, Analytical and Food Chemistry Department, Faculty of Food Science and Technology, Ourense Campus, University of Vigo, E-32004 Ourense, Spain article info Article history: Received 17 September 2010 Received in revised form 11 January 2011 Accepted 18 January 2011 Keywords: Allium cepa Flavonols Anthocyanins Freeze-drying Storage Onion powder abstract The aim of this study was to elucidate the effect of the freeze-drying process and storage on onion flavonoids content. Major flavonols of a mature red onion bulb were confirmed as quercetin derivatives (3,4 0 -O-diglucoside (Qdg) and 4 0 -O-monoglucoside (Qmg)) using a combination of chromatographic comparisons, UVevisible spectra and scientific literature data. These two components account for about a 93% of the total flavonols (334  60 mg Q kg 1 , fresh weight), and are the main responsible for the IC50 antioxidant activity of 15.84  3.73 g/kg. The remaining flavonol fraction (approx. 7%) comprises up to 8 different components of which quercetin-3-O-glucoside and isorhamnetin-4-glucoside are prominent members although each contribute less than 3% of the total flavonol fraction; instead, quercetin represents less than 1% of total flavonols. In this red onion, eight anthocyanins (which represent less than 1% of total flavonols) were also found at a total level of 2.1  0.05 mg C3g kg 1 (fresh weight). Anyway, 4 of them (cyanidin 3-glucoside (C3g) > cyanidin 3-(6 00 -malonylglucoside) (C3,6mg) > cyanidin 3-(6 00 - malonyl- laminaribioside) (C3,6mlmb) > cyanidin 3-laminaribioside (C3lmb)) account for the 95% of total antho- cyanins. Freeze-drying process concludes in an increase in the extraction of flavonoids up to a 32% for flavonols and 25% for anthocyanins. Freeze-dried onion powder storage at room temperature, in dark, in air- and water-tight glass bottles for up to 6 months was keeping rather stable all flavonoids. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Onions contain high levels of non-nutrient antioxidant compounds (flavonoids) with protective effects against different degenerative pathologies (Griffiths, Trueman, Crowther, Thomas, & Smith, 2002). Flavonols and anthocyanins are the main subclasses of flavonoids present in onions, the latter being found especially in red onions. White, yellow or red onions are known to contain large amounts of flavonols, mainly quercetin, conjugated with sugars as glucose. Onions are recognized as the major dietary source of quercetin as aglycone or O-glycosylated derivatives (Hertog, Hollman, & Venema, 1992). Industry often carries out freezing and drying processes to achieve the objective of long-term storage. However, the health-promoting capacity and nutritional characteristics of plant products depend on the type of processing employed. Onions can be marketed as powder for cooking purposes after drying processes (Lee et al., 2008). There are only a few publications on potential changes of bioactive components in vegetables by preservation process technologies (Cisneros-Zevallos, 2003; Heredia & Cisneros-Zevallos, 2009; Piga, Del Caro, Pinna, & Agabbio, 2003; Robles-Sánchez, Rojas-Grau, Odriozola-Serrano, González-Aguilar, & Martín-Belloso, 2009; Tudela, Cantos, Espín, Tomás-Barberán, & Gil, 2002; Wojdy1o, Figiel, & Oszmianski, 2009). Heredia and Cisneros-Zevallos (2009) have shown that wounding intensity plays a major role in activating the synthesis and accumulation of antioxidant phenolic compounds, and the corresponding antioxidant capacity in carrot tissue. The combi- nation of wounding and hormone-stressed wounded tissue further enhanced synergistically the accumulation of phenolic compounds by triggering the phenylpropanoid metabolism through an increase in PAL activity. Similar response to wounding was reported for fresh- cut potatoes, lettuces or onions (Kang & Saltveit, 2002; Martínez, Sgroppo, Sánchez-Moreno, De Ancos, & Cano, 2005; Mateos, Ke, Cantwell, & Kader, 1993; Tudela et al., 2002). Ferreres, Gil, and Tomás-Barberán (1996), which studied the impact of perforated films on flavonoids of shredded onions, after 7 days of storage, have found that the content of all anthocyanins decreased. Ewald, Fjelkner- Modig, Johansson, Sjöholm, and Akesson (1999) found that the greatest loss of flavonoids in onion took place during the pre-pro- cessing step where the onion was peeled, trimmed, and chopped before blanching. Cooking can also affect on flavonoid onion content (Ewald et al., 1999; Lee et al., 2008; Makris & Rossiter, 2001; Martínez et al., 2005; Rodrigues, Pérez-Gregorio, García-Falcón, & Simal- * Corresponding author. Tel.: þ34 988 387000; fax: þ34 988 387001. E-mail address: jsimal@uvigo.es (J. Simal-Gándara). Contents lists available at ScienceDirect Food Control journal homepage: www.elsevier.com/locate/foodcont 0956-7135/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2011.01.006 Food Control 22 (2011) 1108e1113