ASCORBIC ACID AND POLYPHENOLS: PRO-OXIDANT BEHAVIOR AND EFFECT ON FOOD QUALITY By: Maritza Ashton Sirven

Overview ■Definitions ■Vitamin C o General info o Oxidation (pro-oxidant activity) o Cross-Over Effect o Anaerobic Degradation o Browning ■Example with polyphenols (tea flavonoids)

DEFINITIONS

Definitions ■Pro-oxidant o Promotes oxidation o A highly reactive molecule that can damage other molecules (biological) ■Antioxidant o Promotes reduction o Scavenges free radicals ■Oxidation o Loss of an electron ■Browning (in foods) o Pigments that form due to oxidation products, causes issues with food quality

VITAMIN C

Ascorbic Acid (Vitamin C) ■Water soluble vitamin, prevents scurvy ■Naturally found in fruits and vegetables, produced from glucose ■Biological - Reduces metal ions within enzymes to ferrous state so that they are active ■Helps increase absorption of Iron (marinate meat with OJ) ■Added to foods as an antioxidant in hopes that it will prevent browning ■However, can promote browning of foods!

Mechanism of Ascorbic Acid Oxidation Because food systems tend to be acidic in nature, there is a considerable amount of ascorbic acid present as ascorbate in food systems (pka = 4 according to other sources) Oxidation is proportional to concentration of ascorbate (pH dependent) Loss of two electrons result in dehydroascorbic acid

Mechanism of Ascorbic Acid Oxidation ■Metal ions (Fe 3+ and Cu 2+ ) catalyze the reaction ■Can occur without metal ions but very slowly ■Chelators can induce oxidation (Fe 3+ and EDTA) AH 2 + Fe 3+ + O 2  A + Fe 2+ + H 2 O 2 H Fe 2+  OH  + OH - + Fe 3+ (Fenton Reaction)  What is acting as the pro-oxidant in the reaction?

Ascorbic Acid Crossover Effect ■At higher concentrations (in relation to metal ions) believed that ascorbic acid acts as an antioxidant - More ascorbic acid present to scavenge any free radicals formed, preventing them from reacting with other species (shorter chain length) ■Lower concentrations believed to act as a pro-oxidant - Less ascorbic acid present to scavenge free radicals that are produced (longer chain length)

Ascorbic Acid Crossover Effect High Concentration Low Concentration

Ascorbic Acid Browning ■Anaerobic degradation can also occur, forming degradation products that polymerize into brown pigments o Occurs low pH (3-4) o Researches argue if this can occur at all in foods ■Dehydroascorbic acid can react with amino acids to form yellow and red pigments ■Browning is not oxidation, browning is caused by polymerized compounds that were a result of oxidation  Why does ascorbic acid prevent brown color formation in some products and not others (coconut water versus apple juice)?

POLYPHENOLS

Polyphenols ■Diets high in polyphenols are thought to be associated with reduced risk of cardiovascular disease, ageing, and even cancer ■Long believed that antioxidant activity is main mechanism attributed to health benefits ■Some reports of pro-oxidant activity, leading to damage of DNA and proteins (In vitro) ■Similar if not the same mechanism as ascorbic acid

Pro-oxidant Activity of Flavonoids in Tea Polyphenols also exhibit the cross over effect Oxidative damage to deoxyribose increased when ascorbic acid (100 uM) was added to tea Black tea (the most heavily fermented tea of the samples) did not exhibit as high pro- oxidant activity as the other tea varieties – concentration not standardized? Could pro-oxidant activity of polyphenols have a negative impact on health in vivo?

Conclusion (Take Home Points) 1.Ascorbic acid has shown to have pro-oxidant activity resulting from the reduction of metal ions and the Fenton Reaction. 2.In relation to food quality, another side effect of the oxidation of ascorbic acid is the formation of brown pigments; brown pigments do not occur simply because of the oxidation of ascorbic acid/polyphenols, they occur because polymerization of large brown pigments. 3.Ascorbic acid can cause quality issues in foods (browning) aerobically and anaerobically (although anaerobic pathway not well understood). 4.Polyphenols have shown to have pro-oxidant activity in vitro through a similar mechanism to that of ascorbic acid, but there is no definite study confirming their mechanism of action in vivo.

References ■Bradshaw, M. P., Barril C., Clark, A. C., Prenzler, P. D., Scollary, G. R Ascorbic acid: a review of its chemistry and reactivity in relation to a wine environment. Critical Reviews in Food Science and Nutrition. 51: ■Du, J., Cullen, J. J., Buettner, G. R Ascorbic acid: chemistry, biology and treatment of cancer. Biochemisty Biophysics Acta. 1826(2): ■Fennema, O. R Fennema’s Food Chemistry. Taylor and Francis Group: Boca Raton, FL. ■Nouri, M, Sadeghian, R., Nematy, M., Hosseini, G., Mostafavi-Toroghi, H., Tavallaie, S., Ghayour-Mobarhan, M Comparison of pro-oxidant antioxidant balance between hospitalized patients and the healthy subjects. International Medical Journal. 22(6): ■Yen, G., Chen, H., Peng, H Antioxidant and pro-oxidant effects of various tea extracts. Journal of Agriculture and Food Chemistry. 45:30-34.