Reactive Oxygen Species in Foods · · · · · ROS · · · Reactive Oxygen Species in Foods Ariela Thomas

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 24.10-11 (1986): 1021-1030.

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