Types of oxidation and factors affecting it.

oxidation is an important chemical reaction especially in fat rich products. to improve the shelf life of a fatty products it is neccessary to control the oxidation process by the addition of antioxidants. there are so many types of oxidation is occurs in food when it is exposed with oxygen like thermal oxidation, auto oxidation etc...

1. TYPES OF OXIDATION TOPIC - 2 R.KARTHIKA PHD (DAIRY CHEMISTRY)

2. Definition : Fats, when in contact with air with oxygen producing products with undesirable characteristics. Loss of electrons. Loss of hydrogen. Gain of oxygen.

3. Types Auto-oxidation Photo-oxidation Enzymatic oxidation Thermal oxidation

4. Thermal oxidation: Occur - exposed to heat. Frying temperature - 160ºC to 195ºC. 120 to 270ºC – hydrogenation, physical refining and deodorization. Chemical changes : Randomization of the glyceride structure, dimer formation, cis -trans isomerization formation of conjugated fatty acids. Prevent – vacuum.

5. Thermal oxidation can result Oxidized monomeric , dimeric , and oligomeric triglycerides Volatile compounds - aldehydes , ketones , alcohols, and hydrocarbons. Oxidized sterols may be formed. Cyclic monomers with a cyclopentenyl ring have been isolated from heated sunflower oil The loss of polyunsaturated fatty aids during deep frying

6. PHOTO OXIDATION: Light induced oxidation Results from the reactivity of an excited state of oxygen – singlet oxygen. Activation energy - normal oxygen with an unsaturated fatty acid is very high(146–273 kJ/mol). When normal oxygen converts to the singlet state, energy is taken up amounting to 92 kJ/mol. Production of singlet oxygen requires the presence of a sensitizer. Type 1 and type 2 sensitizer.

7. Singlet oxygen is produced from a photo sensitizer molecule reacting with ground state oxygen.

8. Molecular triplet states are chemically reactive due to their long decay times and the presence of unpaired valence electrons . Reactivity with ground state oxygen ( 3 O 2 ) will yield singlet oxygen ( 1 O 2 ). Singlet oxygen has an emission spectrum peak around 1270 nm, which results in photons emitted from a triplet state (phosphorescence). The lifetime of singlet oxygen is very long.

9. Photo sensitizer Photosensitizers  are light absorbers that alter the course of a photochemical reaction. They usually are catalysts. They can function by many mechanisms, sometimes they donate an electron to the substrate, sometimes they abstract a hydrogen atom from the substrate. At the end of this process, the photosensitizer returns to its ground state.

10. Type I In Type I photosensitized reactions, the photosensitizer is excited by a light source into a triplet state. The excited, triplet state photosensitizer then reacts with a substrate molecule which is not molecular oxygen to both form a product and reform the photosensitizer . 

11. Type II In Type II photosensitized reactions, the photosensitizer is excited by a light source into a triplet state. The excited photosensitizer then reacts with a ground state, triplet oxygen molecule. This excites the oxygen molecule into the singlet state, making it a reactive oxygen species. Upon excitation, the singlet oxygen molecule reacts with a substrate to form a product. Type II photosensitized reaction result in the photosensitizer being quenched by a ground state oxygen molecule which then goes on to react with a substrate to form a product

12. Food components as sensitizer(chlorophyll, myoglobin , riboflavin, and heavy metals) Activated by light Produce singlet oxygen The singlet oxygen reacts directly with the double bond No induction period Reactions stopped by quenchers( carotenoids )

13. Carotenoids are lipophilic molecules with a tendency to accrue in membrane or lipoproteins. Milk fat globule membrane is considered as the most volatile site for auto-oxidation. β-carotene is regarded as preventive antioxidant, it can quench singlet oxygen and one molecule of β-carotene can quench 250 to 1000 molecules of singlet oxygen. Among the various antioxidant systems in milk, carotenoids act a scavenger of singlet oxygen and peroxyl radicals. 

14. Dairy lipids may suffer from oxidation, which leads to the negative impact on quality and sensory characteristics of finished products. Auto-oxidation and light induced oxidation is affected by a complex interaction of pro and antioxidants. Photo-oxidation is predominantly inhibited by β-carotene, it absorbs light that would otherwise be absorbed by riboflavin, which may give rise to quality related issues . β-carotene absorbs light in a concentration dependent manner.

15. The reactivity of singlet oxygen is 1500 times greater than that of normal oxygen. Oils : Ascorbyl palmitate - quencher virgin olive oil pheophytin A - sensitizer β-carotene - quencher

16. Type II, which is less specific for free linoleic acid, acts as a general autoxidation catalyst. Type I reacts only on free fatty acids after they have been released from triacylglycerols by lipase. Type II will act directly on triacylglycerols .

17. Enzymatic oxidation: In animal systems: Oxidative transformation of arachidonic acid to prostaglandins, thromboxanes , and leukotrienes . Enzyme - cyclooxygenase . In plant systems: Enzyme - lipoxygenase Major - legumes, soybeans, other beans, and peas Lesser - peanuts, wheat, potatoes, and radishes Metalloprotein with an iron atom in its active center

18. Auto oxidation: Unsaturated bond – react with oxygen. Formation of primary, secondary, and tertiary oxidation products that may make the fat or fat-containing food unsuitable for consumption. The process of autoxidation and the resulting determination in the flavor of fats and fatty foods – rancidity Flavor reversion - objectionable flavors that develop in oils containing linolenic acid such as soybean oil

19. Autoxidation behavior is different between complex lipids and simple lipids e.g 30°C in the dark , phospholipid is the most oxidatively stable molecular form of the fatty acids, followed by triglycerides, then fatty acid methylesters , and then free fatty acids. In addition, the position of fatty acids on triglycerides plays a role in determining the relative rate of autoxidation .

20. The autoxidation of unsaturated compounds may be initiated by reactions with  singlet oxygen or environmental pollutants such as  ozone and NO 2. Saturated polymers , such as  polyolefins  would be expected to resist autoxidation , they contain hydroperoxides formed by thermal oxidation during their high temperature moulding and casting, which can act as initiators. In biological systems  reactive oxygen species are important. For industrial reactions a radical initiator, such as  benzoyl peroxide , will be intentionally added All of these processes lead to the generation of carbon centred radicals on the polymer chain (R•)

21. Auto catalysed free radical chain reaction Three stages: Initiation, Propagation, Termination. The initial step involves abstracting a hydrogen atom from a fatty acid, forming a fatty acid (FA) free radical. Saturated fatty acids may lose a H• and undergo oxidation Sensitive - methylene , –CH2– ( polar lipids are more sensitive than neutral lipids).

22. The overall mechanism of lipid oxidation consists of three phases: Initiation, the formation of free radicals; Propagation, the free-radical chain reactions; and Termination, the formation of nonradical products.

23. RH is any unsaturated fatty acid R· is a free radical formed by removing a labile hydrogen from a carbon atom adjacent to a double bond ROOH is a hydroperoxide , one of the major initial oxidation products that decompose to form compounds responsible for off-flavors and odors

24. Factors : Amount of oxygen present, Degree of unsaturation , Presence of antioxidants, Presence of prooxidants (especially copper and some organic compounds such as heme -containing molecules and lipoxidase ), Storage temperature, Exposure to light.

25. Oxygen Lipid oxidation requires the presence of oxygen . Products with a large surface area or with a porous structure is more exposed to oxygen. Reducing the oxygen content in the package headspace will minimize oxidation rate . Oxygen has greater solubility in nonpolar than in polar solvents and, hence, is more soluble in liquid milk fat than in whole raw milk. Kinetically, ground-state (triplet- 3 O 2 ) oxygen is not very reactive, 3 O 2 requires ‘‘activation’’ to facilitate oxidative reactions. singlet oxygen is very electrophilic and readily reacts with unsaturated lipids with the formation of hydroperoxides .

26. Light The water-soluble vitamin, riboflavin, present in milk acts as a potent photosensitizer Riboflavin has three absorption bands. The band with a maximum between 430–460 nm is the main band responsible for the photooxidation of food, especially milk and dairy products. After photodegradation , riboflavin breaks down to lumichrome and formyl-methylflavin . Lumichrome is also a strong photosensitizer . Potential oxidation-induced off- flavors in butter may be reduced by light-barrier packaging, such as aluminium foil.

27. TEMPERATURE: With increasing temperature, oxidative stability of milk and milk products decreases. Storage, in air, at 2°C inhibited the development of oxidized flavor in dry whole milk when compared with control samples held at 38°C. Oxidative deterioration of UHT cream occurred two to three times more rapidly at 18°C than at 10°C, while little or no oxidation occurred at 4°C. In a study on butteroil held at a temperature ranging from -10 to +50°C, oxidation rate increased with increasing temperature

28. ANTI OXIDANTS: Tocopherols : The principal factor influencing the tocopherol content of milk is the feed of the cow. Summer milk produced on green pasture feed typically has a higher α- tocopherol content than winter milk produced on dry-lot feeding. Ascorbic acid : At the concentrations normally found in milk (20 mg/L), ascorbic acid acts as a pro-oxidant. Above this concentration, it has been reported to have anti-oxidant effect. Carotenoids : β-Carotene is one of the most potent quenchers of 1 O 2 , with one molecule estimated to quench 250–1000 molecules of 1 O 2 . The rate of quenching is influenced by the number of conjugated double bonds present.