1. Reactive Oxygen Species in Foods
ROS
Reactive Oxygen Species in Foods
Ariela Thomas

2. Overview Introduction ROS Formation in foods
Electron reduction potentials
Reaction mechanisms of ROS with food components
Soybeans reversion rancid flavor
Milk sunlight flavor formation

3. Introduction Radicals Non-radicals Hydroxy OH· Hydrogen Peroxide H2O2
ROS includes oxygen radicals and non-radical derivatives of oxygen.
Mainly responsible for initiation of oxidation in foods
Undesirable volatile compounds
Destroy essential FA, amino acids and vitamins
Produce carcinogens
Vitamins
Lipids
Proteins
Sugars
Radicals
Hydroxy OH·
Superoxide O·̄2
Peroxy ROO ·
Alkoxy RO ·
Hydroperoxy HOO ·
Non-radicals
Hydrogen Peroxide H2O2
Singlet Oxygen 1O2
Ozone O3
Hydroxy radical
Singlet oxygen

4. Factors affecting food oxidation
Oxygen, free radicals
Fatty acid compositions
Prooxidants
Antioxidants
Processing conditions (Irradiation, cooking, grinding, cutting, mixing, restructuring etc.)
Storage: time and conditions

5. ROS Formation in Foods
Can be formed enzymatically or chemically from triplet oxygen
Most stable and abundant form of oxygen
Has two electrons occupying two molecular orbitals of equal energy
Dirradical  can not react with food components
Triplet Oxygen
Superoxide anion O·̄2
Unless the food component becomes a radical itself
Enzymatically  Xanthine oxidase, NADPH oxidase
Photoactivation
Gamma irradiation, pulsed electric field, microwave

6. ROS Formation in Foods Singlet Oxygen 1O2 Hydroxy Radical OH·
Major pathway  photosensitization
Ground stat sensitizer is exposed to light of a specific wavelength  becomes an exited singlet state
From water: high energy radiation of γ-rays
From H2O2
UV-induced fission of the molecule
Decomposition in presence of transition metal complexes

7. ROS Formation in Foods Proteins 1O2 Tyrosine Tryptophan Histidine
Methionine
Cysteine
Lipids
Unsaturated phospholipids
Cholesterol
Carbohydrates
None
Singlet Oxygen 1O2
Photosensitizer +
Light
1O2

8. In general….
ROS formation in foods during storage and processing is closely interrelated
Controlling the formation of ROS is important

9. Standard Electron Reduction Potentials of ROS
Tendency for a chemical species to be reduced
↑ reduction potential,
↑ the oxidizing capability
Hydroxy radical OH·  strong oxidizing agent and a powerful electrophilic radical

10. Reaction Mechanisms of ROS in Foods
Lipids
H2O2  precursor of OH ·
O·̄2  reduction potential is not strong enough to abstract H from UFA
HOO · is a strong lipid oxidation initiator
HOO · + UFA  lipid hydroperoxide (strong oxidizing agents )
1O2  attack double bond of UFA
can abstract H from other lipid molecules, forming peroxy radicals

11. Reaction Mechanisms of ROS in Foods
Proteins and Amino Acids
Mostly hydroxy and singlet oxygen
Superoxide and hydroperoxy  lower rates
Main problem: formation of crosslinked products
OH · abstract H from proteins  protein radicals
Protein radicals +
protein radicals ↓
crosslinked products
PTN radicals abstract H from other PTN ↓
oxidizing that PTN as well

12. Reaction Mechanisms of ROS in Foods
Vitamins
β-Carotene
Peroxy, Hydroxy and Singlet Oxygen
β-Carotene can donate an e- to ROO ·  Car radical + peroxy anion
Car radical  relatively stable due to resonance

13. Reaction Mechanisms of ROS in Foods
Vitamins
Ascorbic Acid
Superoxide
Hydroxy
Singlet oxygen
↓ pH  decreases reaction rate
Singlet Oxygen  produces hydroperoxides of AA
Superoxide + AA produces dehydroascorbic acid and H2O2

14. Soybean Reversion Rancid Flavor
Soybean Oil
Inexpensive
Nutritious
Widely available
Reversion flavor
Caused by reaction of linolenic acid with singlet oxygen 1O2 producing 2-pentenylfuran
formed in soy-bean oil containing chlorophyll in presence of light
Chlorophyll can produce singlet oxygen under light in the presence of triplet oxygen.
linolenic acid
2-pentenylfuran

15. Milk Sunlight Flavor Described as sulfur flavor
Methional and dimethyl sulfide  responsible for the flavor
1O2 is formed from triplet oxygen in the presence of riboflavin in milk under sunlight
1O2 reacts with sulfur group in methionine to form hydroperoxide
hydroperoxide is decomposed to methional and thiomethyl radicals

16. Reaction Rates + Summary 2
Hydroxy is the most reactive ROS, followed by singlet oxygen.
Superoxide and hydroperoxy are much less reactive.
Amino acids and vitamins are more liable to 1O2 then lipids and sugars. 1

17. References
Choe, Eunok, and David B. Min. "Chemistry and reactions of reactive oxygen species in foods." Journal of Food Science 70.9 (2005): R142-R159.
Smouse, Thomas H. "A review of soybean oil reversion flavor." Journal of the American Oil Chemists’ Society 56.11 (1979): 747A-751A.
Addis, P. B. "Occurrence of lipid oxidation products in foods." Food and Chemical Toxicology  (1986):

18. THANK YOU!
Any questions?