Transcript Pro-oxidant behavior and effect on food quality

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 (Fe3+ and Cu2+) catalyze the reaction
■
Can occur without metal ions but very slowly
■
Chelators can induce oxidation (Fe3+ and EDTA)
AH2 + Fe3+ + O2 
H202 + Fe2+ 
A + Fe2+ + H2O2
OH + OH- + Fe3+ (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 prooxidant 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. 2011. Ascorbic acid: a
review of its chemistry and reactivity in relation to a wine environment. Critical Reviews in
Food Science and Nutrition. 51: 479-498.
■ Du, J., Cullen, J. J., Buettner, G. R. 2012. Ascorbic acid: chemistry, biology and treatment of
cancer. Biochemisty Biophysics Acta. 1826(2):443457.
■ Fennema, O. R. 2008. 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. 2015. Comparison of pro-oxidant antioxidant balance between
hospitalized patients and the healthy subjects. International Medical Journal. 22(6): 517520.
■ Yen, G., Chen, H., Peng, H. 1997. Antioxidant and pro-oxidant effects of various tea extracts.
Journal of Agriculture and Food Chemistry. 45:30-34.