Effect of Thermal Processing on the Nutritional Quality of Processed Vegetables

EFFECT OF THERMAL PROCESSING ON THE NUTRITIONAL QUALITY OF PROCESSED VEGETABLES

Sponsoring Institution

National Institute of Food and Agriculture

Project Status

COMPLETE

Funding Source

HATCH

Reporting Frequency

Annual

Accession No.

0193802

Grant No.

(N/A)

Cumulative Award Amt.

(N/A)

Proposal No.

(N/A)

Multistate No.

(N/A)

Project Start Date

Oct 1, 2002

Project End Date

Sep 30, 2005

Grant Year

(N/A)

Program Code

[(N/A)]- (N/A)

Recipient Organization
CORNELL UNIVERSITY
(N/A)
ITHACA,NY 14853

Performing Department
FOOD SCIENCE

Non Technical Summary
Processed vegetables have been considered to have lower nutritional value than fresh based on the loss of vitamin C. Our research found antioxidant activity of apples came mainly from phytochemicals. This suggests processed vegetables may retain antioxidant activity. Our hypothesis is that heat processing elevates antioxidant activity of vegetables. The objective of the proposal is to determine the effect of heat processing on nutritional quality of vegetables.

Animal Health Component

35%

Research Effort Categories

Basic

50%

Applied

35%

Developmental

15%

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
501	1499	1010	40%
502	1499	1010	30%
701	1499	1010	15%
702	1499	1010	15%

Knowledge Area
702 - Requirements and Function of Nutrients and Other Food Components; 501 - New and Improved Food Processing Technologies; 502 - New and Improved Food Products; 701 - Nutrient Composition of Food;

Subject Of Investigation
1499 - Vegetables, general/other;

Field Of Science
1010 - Nutrition and metabolism;

Keywords

Goals / Objectives
Vitamin C has been routinely used to evaluate the nutritional value of fruits and vegetables because of its antioxidant activity. Processed fruits and vegetables have been long considered to have lower nutritional value than their fresh commodities based on the loss of vitamin C during processing. Our research group found vitamin C in apples contributed less than 0.4 percent of total antioxidant activity indicating most of the activity comes from the natural combination of phytochemicals (Eberhardt, 2000). This suggests that processed fruits and vegetables may retain their antioxidant activity in spite of the loss of vitamin C. Therefore, we propose to analyze the contents of phytochemicals and antioxidant activity of 5 major processed vegetables in the U.S. such as tomatoes, sweet corn, beets, green beans, and green peas. The overall objective of this proposal is to determine the effect of heat processing on nutritional quality of vegetables (tomatoes, sweet corn, beets, green beans, and green peas). This study will demonstrate the effect of processing on the phytochemicals content (phenolic acids and carotenoids) and the total antioxidant activity in common vegetables in the American daily diet. Our hypothesis is that thermal processing may break down bound phytochemicals as glycosides to the cell matrix and insoluble fibers, and elevate the bioaccessible phytochemicals and the antioxidant activity in processed vegetables in spite of loss of vitamin C content. The specific objectives are: 1) To determine if thermal processing affects the levels of bio-accessible phytochemicals content (total phenolics, carotenoids, lycopene, vitamin C) in processed vegetables (such as tomatoes, sweet corn, beets, green beans, and green peas). 2) To determine if the thermal processing increases the total antioxidant activity of processed vegetables (tomatoes, sweet corn, beets, green beans, and green peas) by total oxyradical scavenging capacity (TOSC) assay. 3) To determine the effect of thermal processing on anti-cancer activity of vegetables by studying the inhibition capability on tumor cell proliferation.

Project Methods
1) To determine if thermal processing affects the levels of phytochemicals content (total phenolics, carotenoids and vitamin C) in processed vegetables such as tomatoes, sweet corn, beets, green beans, and green peas. Sample preparation and thermal processing: Selected vegetables (tomatoes, sweet corn, beets, green beans, and green peas) will be processed in our pilot plan at different heating times or temperatures based on the industry processing methods. Extraction: Phytochemicals will be extracted from vegetables by the methods developed at this laboratory. The final extracts will be stored at -70 C until use. Total phenolics assay: The total phenolics of the extracts will be determined by the method previously described (Singleton, et al 1999). Lycopene and other carotenoids assay: The lycopene and other carotenoids will be determined by the method previously described (Stahl and Sies, 1992). This method is routinely used in P.I.'s laboratory. Vitamin C assay: Total L-ascorbic acid content will be determined using 2,6-dicholorophenol (DIP) titrimetric method adapted from the Official Methods of Analysis of the Assocation of Official Analytical Chemists (1990). 2) To determine if the thermal processing increases the total antioxidant activity of raw and heat processed tomatoes by total oxyradical scavenging capacity (TOSC) assay, which is routinely used in P.I.'s laboratory. 3) To determine the effect of thermal processing on anti-cancer activity of vegetables by studying the inhibition capability on tumor cell proliferation in a colon cancer cell culture model and a liver cancer cell culture model in vitro. Cell culture: Colon cancer Caco-2 cells are maintained in DMEM supplemented with 10 mM Hepes and 10% fetal bovine serum (FBS) at 37oC and 5 percent CO2. HepG2 liver cancer cells are maintained in WME supplemented with 10 mM Hepes and 5 percent fetal bovine serum (FBS) at 37oC and 5 percent CO2. Measurement of cell proliferation: 4 x 105 of colon cancer Caco-2 cells in DMEM or HepG2 cells in WME will be placed into each well of 96 well flatbottom plates. The phytochemical extracts from 0, 5, 10, 20, 30, 40 and 50 mg vegetables will be added directly into the medium. Control cultures will receive the same amount of solvent as treated samples but without the phytochemicals. Proliferation will be assessed using the MTT assay (Mosmann and Fong, 1989). Cytotoxicity: Cytotoxicity will be determined by the inhibition of the uptake of neutral red dye by cells grown in 96-well plates. The cells will be treated with tested compounds in 0.2 ml medium/well. The phytochemical extracts from 0, 5, 10, 20, 30, 40 and 50 mg corn will be assayed in triplicates. CC50 value (50% cytotoxic concentration: drug concentration which induces a 50 percent inhibition of dye uptake vs control cultures) will be calculated by linear regression analysis.

Progress 10/01/02 to 09/30/05

Outputs
Processed fruits and vegetables have been long considered to have lower nutritional value than their fresh produce based on the loss of vitamin C during processing. Our research group found vitamin C in apples contributed less than 0.4 percent of total antioxidant activity indicating most of the activity comes from the natural combination of phytochemicals. This suggests that processed fruits and vegetables may retain their antioxidant activity despite the loss of vitamin C. We found that thermal processing elevated total antioxidant activity and bioaccessible lycopene content in tomatoes, and had no significant changes in the total phenolics and total flavonoids content although loss of vitamin C was observed. Thermal processing also elevated total antioxidant activity of tomatoes. We found that thermal processing significantly elevated total antioxidant activity of sweet corn and increased bioaccessible phytochemical content such as ferulic acid and total phenolics although 25 percent vitamin C loss was observed. Antioxidant activity of beets processed under typical commercial processing conditions remained constant despite an 8 percent loss of vitamin C, a 60 percent loss of color, and 30 percent loss of dietary folate. There was a slight but significant 5 percent increase in phenolic content of processed beets. In contrast, vitamin C and dietary folate content of green beans remained constant, whereas a 32 percent reduction in phenolic compounds occurred after typical commercial processing conditions. The antioxidant activity of green beans was reduced by 20 percent. These findings along with previous works suggest that the effects of thermal processing vary with the respective produce crop type. It also reinforces the concept that optimal health benefits may be achieved when a wide variety of plant foods (fruits, vegetables and whole grains) and preparation methods are incorporated into the diet. Epidemiological studies have shown that consumption of carotenoid-rich fruits and vegetables is associated with a reduced risk of developing chronic diseases. However, little is known about the bioavailability of carotenoids from whole foods. We characterized the intestinal uptake performance of carotenoids using monolayers of differentiated Caco-2 human intestinal cancer cells and mimicked human digestion to assess carotenoid absorption from carrots and corn. Results showed that Caco-2 cellular uptake of beta-carotene and zeaxanthin was higher than that of lutein. Uptake performance of pure carotenoids and carotenoids from whole foods by Caco-2 cells was both curvilinear, reaching saturated levels after 4 hours of incubation. The time kinetics and dose response of carotenoid uptake presented a similar pattern in Caco-2 cells after plating for 2 days and 14 days. Furthermore, we verified the applicability of this new model with whole grain-corn, showing that cooked corn grain significantly enhanced carotenoid bioavailability. These results support the feasibility of the in vitro digestion cell model for assessing carotenoid absorption from whole foods as a suitable and cost-effective physiological alternative to current methodologies.

Impacts
This work provides direct supportive evidence for the Five-a-Day program and suggests that consumers obtain their antioxidants from fruits and vegetables for health improvement and disease prevention. This research has a critical economic impact on US fruits and vegetables growers and processing industry by increasing consumption of processed fruits and vegetables.

Publications

He, X., Lobkovsky, E., and Liu, R.H. 2006. Benzoic Acid beta-D-Glucopyranoside. Acta Crystallographica E61:, 2005 (2S*,3S*,4R*,5R*)-3,4,5-Trihydroxy-6-(hydroxymethyl)-3,4,5,6-tetrahyd ro-2H-pyran-2-yl benzoate. Acta Cryst. E62: O471-O472.
Sun, J. and Liu, R.H. 2005. Cranberry phytochemical extracts induce cell cycle arrest and apoptosis in human MCF-7 breast cancer cells. Cancer Letters (published on line 12/27/05).
He, X.J., Lobkovsky, E., and Liu, R.H. 2005. (R*)-Methyl 3-carboxy-2-hydroxypropanoate. Acta Cryst. E61: O4104-O4106.
Adom, K.K. and Liu, R.H. 2005. A rapid peroxylradical scavenging capacity (PSC) assay for assessing both hydrophilic and lipophilic antioxidants. J. Agric. Food Chem. 53 (17): 6572-6580.
Chu, Y-F., and Liu, R.H. 2005. Cranberries inhibit LDL oxidation and induce LDL receptor expression in hepatocytes. Life Science 77 (15): 1892-1901.
Liu, R.H. and Finely, J. 2005. Potential cell culture models for antioxidant research. J. Agric. Food Chem. 53 (10): 4311-4314.
Liu, R.H., Liu, J., and Chen, B. 2005. Apples prevent mammary tumors in rats. J. Agric. Food Chem. 53:2341-2343.
Adom, K.K., Sorrells, M.E., and Liu, R.H. 2005. Phytochemicals and antioxidant activity of milled fractions of different wheat varieties. J. Agric. Food Chem. 53: 2297-2306.
Liu, R.H. 2005. Phytochemicals in Cancer Prevention: Mechanism of Action. Symposium of Division of Agricultural and Food Chemistry: Chemistry and Biochemistry of Antioxidative Phytochemicals. PACIFICHEM 2005, Honolulu, Hawaii, December 15-20, 2005.
Liu, R.H. 2005. Potential Role of Phytochemicals in Cardiovascular Disease Prevention. Symposium of Division of Agricultural and Food Chemistry: Chemistry and Biochemistry of Antioxidative Phytochemicals. PACIFICHEM 2005, Honolulu, Hawaii, December 15-20, 2005.
Liu, R.H. 2005. Potential role of phytochemicals in cancer prevention: Mechanisms of action. Symposium of Division of Agricultural and Food Chemistry: The Potential Health Benefits of Antioxidants. The 230th American Chemical Society National Meeting, Washington, D.C., August 28-September 1, 2005.
Adom, K.K. and Liu, R.H. 2005. A rapid peroxylradical scavenging capacity (PSC) assay for assessing both hydrophilic and lipophilic antioxidants. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.
Boyer, J., Brown, D., and Liu, R.H. 2005. In vitro digestion and lactase treatment influence uptake of quercetin and quercetin glucoside by the Caco-2 cell monolayer. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.
Chu, Y.-F. and Liu, R. H. 2005. Apple phytochemicals inhibit human LDL oxidation and induce hepatic LDL receptor expression. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.
Liu, C-S. and Liu, R.H. 2005. Investigation of factors affecting the bioavailability of carotenoids in whole foods by a Caco-2 cell culture model coupled with an in vitro digestion. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.
Liu, J. and Liu, R.H. 2005. beta-Ionone inhibits rat mammary carcinogenesis. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.
Sun, J. and Liu, R.H. 2005. Inhibition of cranberry extracts on cyclooxygenase-2 expression and activity in human pharyngeal cancer cells. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.
Yang, J., Halim, L., and Liu, R.H. 2005. Antioxidant and antiproliferative activities of common nuts. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.
Yoon, H. and Liu, R.H. 2005. Effect of selected phytochemicals and apple extracts on PARP activity in oxidatively-stressed MCF-7 breast cancer cells. IFT 2005 Annual Meeting, New Orleans, LA, July 16-20, 2005.

Progress 01/01/04 to 12/31/04

Outputs
It has been shown that thermal processing of tomatoes and sweet corns results in increased antioxidant activities despite the loss of vitamin C. Until now, it is unclear whether this positive effect of thermal processing occurs with all crop produce. Therefore, analysis of a root vegetable (beets) and of a legume (green beans) was undertaken to address this question. Antioxidant activity of beets processed under typical commercial processing conditions remained constant despite an 8 pct loss of vitamin C, a 60 pct loss of color, and 30 pct loss of dietary folate. There was a slight but significant 5 pct increase in phenolic content of processed beets. In contrast, vitamin C and dietary folate content of green beans remained constant, whereas a 32 pct reduction in phenolic compounds occurred after typical commercial processing conditions. The antioxidant activity of green beans was reduced by 20 pct. These findings along with previous works suggest that the effects of thermal processing vary with the respective produce crop type. It also reinforces the concept that optimal health benefits may be achieved when a wide variety of plant foods (fruits, vegetables and whole grains) and preparation methods are incorporated into the diet. Epidemiological studies have shown that consumption of carotenoid-rich fruits and vegetables is associated with a reduced risk of developing chronic diseases. Beta-carotene, Alpha-carotene and Beta-cryptoxanthin are precursors of vitamin A, a nutrient essential for human health. However, little is known about the bioavailability of carotenoids from whole foods. This study characterized the intestinal uptake performance of carotenoids using monolayers of differentiated Caco-2 human intestinal cancer cells and mimicked human digestion to assess carotenoid absorption from carrots and corn. Results showed that Caco-2 cellular uptake of B-carotene and zeaxanthin was higher than that of lutein. Uptake performance of pure carotenoids and carotenoids from whole foods by Caco-2 cells was both curvilinear, reaching saturated levels after 4 hours of incubation. The time kinetics and dose response of carotenoid uptake presented a similar pattern in Caco-2 cells after plating for 2 days and 14 days. Furthermore, we verified the applicability of this new model with whole grain-corn, showing that cooked corn grain significantly enhanced carotenoid bioavailability. These results support the feasibility of the in vitro digestion cell model for assessing carotenoid absorption from whole foods as a suitable and cost-effective physiological alternative to current methodologies.

Impacts
This work provides direct supportive evidence for the Five-a-Day program and suggests that consumers obtain their antioxidants from fruits and vegetables for health improvement and disease prevention. This research has a critical economic impact on US fruits and vegetables growers and processing industry by increasing consumption of processed fruits and vegetables.

Publications

Yang, J., Meyers, K.J., van der Heide, J. and Liu, R.H. 2004. Varietal differences in phenolic content, and antioxidant and antiproliferative activities of onions. J. Agric. Food Chem. 52 (22): 6787-6793.
Liu, C-S, Glahn, R. P., and Liu. R.H. 2004. Assessment of carotenoid bioavailability of whole foods using a Caco-2 cell culture model coupled with an in vitro digestion. J. Agric. Food Chem. 52: 4330-4337.
Boyer, J. and Liu, R.H. 2004. Apple phytochemicals and their health benefits. Nutrition Journal 3:5.
Jiratanan, T. and Liu, R.H. 2004. Antioxidant activity of processed table beets (Beta vulgaris var conditiva) and green beans (Phaseolus vulgaris L.). J. Agric. Food Chem. 52 (9): 2659-2670.
Liu, R.H. 2004. Potential synergy of phytochemicals in cancer prevention: mechanism of action. J. Nutr. 134: 3479S-3485S.
Boyer, J., Brown, D., and Liu, R.H. 2004. Uptake of quercetin and quercetin-3-glucoside from whole onions and apple peels by Caco-2 cell monolayers. J. Agric. Food Chem. 52 (23):7172-7179.
Chu, Y-F. and Liu, R. H. 2004. Novel low-density lipoprotein (LDL) oxidation model: Antioxidant capacity for the inhibition of LDL. J. Agric. Food Chem. 52 (22): 6818-6823.
Boyer, J., Brown, D., and Liu, R.H. 2005. In vitro digestion and lactase treatment influence uptake of quercetin and quercetin glucoside by the Caco-2 cell monolayer. Nutritional Journal 4:1.

Progress 01/01/03 to 12/31/03

Outputs
It has been shown that thermal processing of tomatoes and sweet corns results in increased antioxidant activities despite the loss of vitamin C. Until now, it is unclear whether this positive effect of thermal processing occurs with all crop produce. Therefore, analysis of a root vegetable (beets) and of a legume (green beans) was undertaken to address this question. Antioxidant activity of beets processed under typical commercial processing conditions remained constant despite an 8 percent loss of vitamin C, a 60 percent loss of color, and 30 percent loss of dietary folate. There was a slight but significant 5 percent increase in phenolic content of processed beets. In contrast, vitamin C and dietary folate content of green beans remained constant, whereas a 32 percent reduction in phenolic compounds occurred after typical commercial processing conditions. The antioxidant activity of green beans was reduced by 20 percent. These findings along with previous works suggest that the effects of thermal processing vary with the respective produce crop type. It also reinforces the concept that optimal health benefits may be achieved when a wide variety of plant foods (fruits, vegetables and whole grains) and preparation methods are incorporated into the diet. Our laboratory also found that phytochemical extracts from apples, grapes, and raspberries have strong antioxidant and anticancer activities, and that the major part of total antioxidant activity is from the combination of phytochemicals.

Impacts
This work provides direct supportive evidence for the Five-a-Day program and suggests that consumers obtain their antioxidants from fruits and vegetables for health improvement and disease prevention. This research has a critical economic impact on US fruits and vegetables growers and processing industry by increasing consumption of processed fruits and vegetables.

Publications

Sun, J. and Liu, R.H. 2003. Antiproliferative activity of apples. Institute of Food Technologists, IFT Annual Meeting, Chicago, IL, July 12-16, 2003.
Adom, K.K., Sorrells. M. and Liu, R.H. 2003. Phytochemical profiles and total antioxidant activity of wheat varieties. Institute of Food Technologists, IFT Annual Meeting, Chicago, IL, July 12-16, 2003.
Jiratanan, T. and Liu, R.H. 2004. Antioxidant Activity of Processed Table Beets (Beta vulgaris var conditiva) and Green Beans (Phaseolus vulgaris L.). (submitted 2004)
Adom, K.K., Sorrells, M.E., and Liu, R.H. 2003. Phytochemicals and antioxidant activity of wheat varieties. J. Agric. Food Chem. 51 (26): 7825-7834.
Liu, R.H. 2003. Health benefits of fruits and vegetables are from additive and synergistic combination of phytochemicals. Am. J. Clin. Nutr. 78: 517S-520S.
Liu, R.H. 2003. Protective role of phytochemicals in whole foods: Implications for chronic disease prevention. Applied Biotechnology, Food Science and Policy 1(1): 39-46.
Wolfe, K. and Liu, R.H. 2003. Apple peels as a value-added food ingredient. J. Agric. Food Chem. 51 (6): 1676-1683.
Yeung , C.K., Glahn, R.P., Wu, X., Liu, R.H., and Miller, D.D. 2003. Iron bioavailability and antioxidant activity of raisins. J Food Sci 65 (2): 701-705.
Wolfe, K., Wu, X., and Liu, R.H. 2003. Antioxidant activity of apple peels. J. Agric. Food Chem. 51 (3): 609-614.
Boyer, J. and Liu, R.H. 2003. Antioxidants of apples. New York Fruit Quarterly 11(4): 11-15.
Liu, R.H. and Wolfe, K.L. 2003. Role of plant-based whole foods in the prevention of chronic diseases. Foods & Food Ingredients Journal of Japan. 208 (6): 465-490.
Liu, R.H. 2004. Health Benefits of Fruits, Vegetables and Whole Grains: Mechanism of Action. Nutrition Division Symposium: An Update on Antioxidants in Fruits, Vegetables and Whole Grains. IFT 2004 Annual Meeting, Las Vegas, Nevada, July 12-16, 2004.
Liu, R.H. 2004. Effects of processing on the antioxidant activity and phytochemical bioavailability in fruits and vegetables. Fruit and Vegetable Products Division Symposium: Practical Applications of Antioxidant Measurements for Fresh and Processed Fruits and Vegetables. IFT 2004 Annual Meeting, Las Vegas, Nevada, July 12-16, 2004.
Boyer, J., Brown, D. and Liu, R.H. 2004. Assessment of quercetin and quercetin-3-glucoside bioavailability from onions and apple peels using Caco-2 cell model. IFT 2004 Annual Meeting, Las Vegas, Nevada, July 12-16, 2004.
Liu, C-S., Liu, R.H. 2004. Assessment of carotenoid bioavailability of whole foods using a Caco-2 cell culture model coupled with an in vitro digestion. IFT 2004 Annual Meeting, Las Vegas, Nevada, July 12-16, 2004.
Adom, K.K., Sorrells, M.E., and Liu, R.H. 2004. Phytochemical profiles of milled fractions of wheat varieties. IFT 2004 Annual Meeting, Las Vegas, Nevada, July 12-16, 2004.
Liu, R.H. 2003. Health benefits of phytochemicals of fruits and vegetables. American Aging Association Meeting: Nutritional Modulation of Aging and Age-Related Diseases, Baltimore, MD, June 7-9, 2003
Wolfe, K.L. and Liu, R.H. 2003. Apple peel powder-a potential value-added food ingredient for functional foods. Institute of Food Technologists, IFT Annual Meeting, Chicago, IL, July 12-16, 2003.

Progress 01/01/02 to 12/31/02

Outputs
Processed fruits and vegetables have been long considered to have lower nutritional value than their fresh produce based on the loss of vitamin C during processing. Our research group found vitamin C in apples contributed less than 0.4 percent of total antioxidant activity indicating most of the activity comes from the natural combination of phytochemicals. This suggests that processed fruits and vegetables may retain their antioxidant activity despite the loss of vitamin C. We found that thermal processing at 115 C for 25 min significantly elevated total antioxidant activity of sweet corn by 44 percent and increased phytochemical content such as ferulic acid by 550 percent and total phenolics by 54 percent although 25 percent vitamin C loss was observed. Processed sweet corn has increased antioxidant activity equivalent to 210 mg vitamin C per 100 g corn compared to the remaining 3.2 mg vitamin C in the sample that contributed only 1.5 percent of its total antioxidant activity. Our findings do not support the notion that processed fruits and vegetables have lower nutritional value than fresh produce. This information may have a significant impact on consumers' food selection by increasing their consumption of fruits and vegetables to reduce the risk of chronic diseases. Our laboratory also found that phytochemical extracts from apples, grapes, and raspberries have strong antioxidant and anticancer activities, and that the major part of total antioxidant activity is from the combination of phytochemicals. Our found that the combination of phytochemicals in apples is critical to its potent antioxidant activity and antiproliferative activity. Apple with skin displayed higher antioxidant and antiproliferative activities than apple without skin. Additionally, the minimal contribution of vitamin C to the antioxidant activity of apples further supports the proposal the other phytochemicals, such as phenolic acids and flavonoids, significantly contribute to the in vitro antioxidant activity of apples.

Impacts
This work provides direct supportive evidence for the Five-a-Day program and suggests that consumers obtain their antioxidants from fruits and vegetables for health improvement and disease prevention. This research has a critical economic impact on US fruits and vegetables growers and processing industry by increasing consumption of processed fruits and vegetables.

Publications

Wolfe, K. and Liu, R. H. 2003. Apple peels as a value-added food ingredient. J. Agric. Food Chem. 51 (7).
Wolfe, K., Wu, X. and Liu, R. H. 2003. Antioxidant activity of apple peels. J. Agric. Food Chem. 51 (3): 609-614.
Dewanto, V., Wu, X., and Liu, R. H. 2002. Processed sweet corn has higher antioxidant activity. J. Agric. Food Chem. 50 (17): 4959-4964.
Dewanto, V., Wu, X., Adom, K. K., and Liu, R. H. 2002. Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity. J. Agric. Food Chem. 50 (10): 3010-3014.
Abstract Dewanto, V., Wu, X., Adom, K. K., and Liu, R. H. 2002. Effect of thermal processing on the nutritional value of tomatoes. Institute of Food Technologists, IFT Annual Meeting, Anaheim, California, June 15-19, 2002.