Effects of food formulation and thermal processing on flavones in celery and chamomile
Introduction
Flavones are a class of flavonoids found in a variety of fruits and vegetables and are most abundant in artichoke heads, kumquats, parsley, and celery (Azzini et al., 2007, Justesen et al., 1998, Sakakibara et al., 2003). The flavone apigenin exhibits toxicity to cancer cells in vitro (Engelmann et al., 2002, Gupta et al., 2002, Mak et al., 2006, Manthey and Guthrie, 2002, Piantelli et al., 2006) and animal studies with flavones demonstrate the ability to attenuate the inflammatory response (Nicholas et al., 2007, Ueda et al., 2004). However, human trials with flavone rich foods such as parsley and celery show limited bioavailability of these compounds, with maximum plasma concentrations of <1 μM (Cao et al., 2010, Meyer et al., 2006). The time of maximum plasma concentration after eating these foods was over 7 h (Cao et al., 2010, Meyer et al., 2006), indicating that the flavones are likely absorbed in the colon rather than the small intestine. Future use of flavone-rich foods for health benefits requires a better understanding of the forms that are most readily absorbed and their stability during food processing.
Both parsley and celery are rich in flavone glycosides, particularly apigenin 7-O-apiosylglucoside (apiin) (Lechtenberg et al., 2007, Lin et al., 2007) and malonyl apiin, which can be converted by endogenous esterases to apiin (Hostetler, Riedl, & Schwartz, 2012). Because intestinal absorption of flavonoid glycosides varies according to the aglycone core and the sugars and other functional groups attached, the glycosylation of flavones may determine their site and efficiency of absorption. Similar to other flavonoids, flavone glucosides can be hydrolysed by intestinal β-glucosidase (Németh et al., 2003) and absorbed in the small intestine. In contrast, flavonoid glycosides with disaccharide and malonyl moieties are more resistant to intestinal β-glucosidase than their simple glucoside counterparts, potentially limiting their bioavailability (Németh et al., 2003). Caco-2 models show that flavone aglycones were more readily absorbed into intestinal cells than flavone glucosides, with 10 times more apigenin or luteolin transported across the intestinal epithelium than apigenin 6-C-glucoside or luteolin 7-O-glucoside (Tian, Yang, Yang, & Wang, 2009).
Flavone glycosides in foods may be modified slightly by endogenous enzymes such as malonyl esterases (e.g. conversion from malonylapiin to apiin) (Hostetler et al., 2012, Matern, 1983), but remain as glycosides after processing such as shredding lettuce (DuPont, Mondin, Williamson, & Price, 2000), juicing artichoke heads (Schütz, Kammerer, Carle, & Schieber, 2004), and heating orange juice (Gil-Izquierdo, Gil, & Ferreres, 2002). Rhamnosidase has been used to convert hesperetin rhamnosyl glucoside to hesperetin glucoside, thereby improving bioavailability 4-fold (Nielsen et al., 2006). The β glycoside linkage of flavonoid glycosides can also be cleaved by β-glucosidase to release flavonoid aglycones (Riedl, Zhang, Schwartz, & Vodovotz, 2005), and β-glucosidase-rich ingredients such as almond (Ducret, Trani, & Lortie, 2006), flax seeds (Fan & Conn, 1985), and chickpeas (Hösel, 1976) can potentially be used for this purpose in food processing. In an effort to develop a functional food rich in flavone aglycones and ultimately to improve the bioavailability of these compounds, several processing parameters were tested for their effects on flavone glycosylation in celery. In addition, the thermal stability of purified flavone derivatives from celery (Fig. 1) was evaluated to better understand processing effects on flavone-rich foods.
Section snippets
Materials
Chinese celery (Apium graveolens L., Apiaceae), raw almonds (Prunus dulcis Mill., Rosaceae), flax seeds (Linum usitatissimum L., Linaceae), and chickpea flour (Cicer arietinum L., Fabaceae) were purchased from local grocery stores in Columbus, Ohio. Flavo-Natin tablets with chamomile extract were obtained from Koehler Pharma (Alsbach-Haehnlein, Germany). Formic acid, apigenin, and luteolin were from Sigma (St. Louis, MO), and apigenin 7-O-glucoside was from Chromadex (Irvine, CA). Ammonium
Thermal stability of purified flavones
Several flavone derivatives were tested for their thermal stability. Apiin, apigenin 7-O-glucoside, apigenin, luteolin, and chrysoeriol were heated at 100 °C for up to 300 min at pH 3, 5, or 7. Apiin was relatively stable to thermal processing at pH 5 and 7, but its concentration fell rapidly after heating 1 h at pH 3 (Fig. 2A). The primary hydrolysis product of apiin at pH 3 was apigenin 7-O-glucoside (data not shown).
Of all flavones tested, apigenin 7-O-glucoside was the most stable to thermal
Discussion
Flavonoid glycosides vary in their thermal stability, and the results shown here were most similar to anthocyanins rather than flavonol glycosides or other flavone glycosides. The flavone diglycoside in the current study, apiin, hydrolysed progressively at pH 3 to afford apigenin 7-O-glucoside (Fig. 2A). Apigenin 7-O-glucoside was very stable at 100 °C regardless of pH (Fig. 2B). A similar pattern has been shown with cyanidin 3-glycosides at pH 3.5, where those with a terminal glucose moiety are
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