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Chapter 15
Effect of Different Heating Processes on Cytotoxic and Free Radical Scavenging Properties of Onion Powder 1
Hui-Yin Fu and Tzou-Chi Huang
2
1
Department of Food Science and Technology, Tajen Institute of Technology, 907, Pingtung, Taiwan, Republic of China Department of Food Science and Technology, National Pingtung University of Science and Technology, 912, Pingtung, Taiwan, Republic of China
2
Drying method affects the quercetin composition of onion subsequently and significantly affecting the antioxidative activity. Onion powder dried by hot-air (60 °C), vacuum (35 °C) and lyophilization (35 °C) were used to prepare methanol extract. DPPH radical and peroxide radical scavenging activities of crude extracts from dried onion powders were evaluated. The results showed that hot air dried onion had higher radical scavenging activities in both DPPH and peroxide radicals than that of the lyophilized and vacuum dried onion. HPLC analyses showed that lyophilized and vacuum dried onion contained more quercetin glycosides, whereas hot air dried onion dominated in aglycone. The inhibitory effects of onion on the growth of human leukemia cell lines CCFF-CEM, U937, K562, P3H-1 and Raji in vitro were investigated. A strong cell proliferation inhibition activity in hot-air dried onion was observed for cell lines CEM and U937, whereas freeze dried and vacuum dried onion gave comparatively moderate inhibition. Low growth reduction was seen in cell lines K562, Raji, and no inhibition was observed in P3H-1 for both onion samples.
© 2003 American Chemical Society In Oriental Foods and Herbs; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Introduction Onion (Allium cepa L.) is a major source of flavonoids which are recognized as bioactive substances with potential health effects (/). Previous pharmacological testing showed that onion could be used as an anti-asthmatic, anti-thrombotic, anti-hypertensive, anti-hyperglycemic, anti-hyperlipemic and anti-tumor agent (2,3). A simultaneous case-referent study of a high epidemic area in Jiangsu Province, China, along with the results of nine out of 11 epidemiological studies, including those of a cohort study, have suggested a decreased risk of both stomach and esophageal cancer with increased consumption of onion, garlic or related Allium vegetables (4). Dehydrated onion has great commercial value due to its culinary and medicinal properties as a nutraceutical (5). In modern times, onion powder is sold as a nutraceutical and as a dietary supplement. Several experiments have been conducted to investigate the effect of processing on the composition of quercetins in onion. During cooking we found an overall loss for quercetin-3,4'diglucoside (Qdg) and quercetin-4'-monoglucoside (Qmg) of 11.3% and 2.6%, respectively. A significant proportion, 18% of quercetin-3,4'-diglucoside and 19% of quercetin-4'-monoglucoside, of each conjugates was leached, unchanged, from the onion tissue into the cooking water (6). Hirota et al. (7) found similar results and attributed the decrease of quercetin conjugates to the release of quercetin-3,4'-diglucoside and quercetin-4-monoglucoside into cooking water and their oxidation by peroxidase. In studying the effect of processing on major flavonoids in processed onions, Ewald et al. (8) reported that the greatest loss of flavonoids in onions took place during the pre-processing step where the onion was peeled, trimmed and chopped before blanching. Boiling for 60 min caused overall flavonol losses of 20.6 % in onion (9). Very few papers have been published concerning the effect of drying on the nutraceutical changes in onion. Although, according to Polish recommendations, sliced onions should be dried at 58-60° C (10), a vacuum dryer (Lab-Lone Duo-Vac oven) and a freeze dryer (Labconco Freezone Plus 6) were utilized to dehydrate onion slices at 30°C to study the effect of drying temperature on nutraceutical properties of onion. The fresh onions harvested in February, 2001 from a local farm (Herchung, Pingtung) were used in this study. The onion bulbs were hand-peeled and mechanically sliced into 3-3.5 mm thick slices, and dehydrated to a final moisture content of approximately 5%, moisture-free basis or less, as follows: Freeze-drying: freezing temperature -30° C, drying temperature 35° C, vacuum 0.15-0.20 mm Hg, drying time 16 h. Vacuum dryer: drying temperature 35+2° C, vacuum 510 mm Hg, and total drying time 16 h. Hot air shelf-drying: drying temperature 60 +2 °, and total drying time 20 h.
In Oriental Foods and Herbs; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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217 Moisture sorption isotherms were determined by the method described by Hutchinson and Otten (//). Sorption isotherm curves of these samples are shown in Figure 1. The isotherms had sigmoidal shapes, typical for dehydrated vegetables. This is in accordance with the results of Debnath et al. (5) for dehydrated onions. Moisture content at equilibrium was slightly higher in the hot air-dried sample than that of either thefreeze-driedor cold air-dried ones. Some of the propectic substance was transformed into water soluble pectin. The isotherm curves for bothfreeze-dried(30 °C) and vacuum-dried (30 °C) onion are the typical sigmoidal shape. They indicate that equilibrium moisture increases very slowly, with an increase in Aw up to 0.43, beyond which there is a steep rise in moisture in all three samples. Moisture-sorption data showed that onion powder belongs to a class of food powders that is highly hygroscopic by nature. Polyuronides, with a high level of the galactan side chain, were characterized in dietary fiber of onion tissue (12). These chains bring about the solubility of pectin. This data indicates that enzymatic autolysis may occur during the hot air-drying process rather than in either vacuum or freeze-drying.
WATER ACTIVITY
Figure 1. Moisture sorptions of onion powder.
The levels of the individual quercetin glucosides and quercetin aglycone were determined for the ethanol extracts from all the three onion powder samples. Changes in the composition of the quercetin conjugates due to drying
In Oriental Foods and Herbs; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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218 are shown in Table I. The proportion of Qdg, Qmg and quercetin in the hot airdried onion powder was 47%, 39% and 12%, respectively. Both freeze-drying and vacuum-drying at low temperature (35° C) showed a lesser extend of transformation of Qdg into Qmg or aglycone quercetin as compared with that in hot air-dried onion powder. Flavonoid composition in the edible portion of the onion bulb is made up primarily of two quercetin conjugates, quercetin-3,4'diglucoside and quercetin-4'-monoglucoside (13). The quercetin conjugates present in the onion bulb are resistant to degradation during low temperature drying operations and there is only a small amount of free quercetin present in the ethanol extract. Peeling, trimming and chopping processes may lead to some extend of autolysis in the broken onion tissues.
Table 1. Changes of quercetin glycosides in onion powder by different heating treatments Heating process Freeze-dried Vacuum-dried Hot air-dried
Concentration (Mg/g dry weight) (%) Total Quercetin Qdg Qmg 4574° 2834" (62%) 1375" (30%) 365" (8%) 4466" 352" (8%) 2770" (63%) 1294" (29%) 4474" 539" (12%) 2112" (47%) 1723" (39%)
Qdg = quercetin-3,4'-diglucoside, Qmg = quercetin-4'-monogIucoside; Data indicate is means of triplicate trials; ***Within the same column, means with no letter in common are significantly different (p
