Phụ gia chống oxi hóa

Phụ gia chống oxi hóaThS. Đặng Bùi KhuêTypes of RancidityTypes of RancidityLipolytic rancidity is mainly due to lipases (triacylglycerol acyl hydrolases, EC 3.1.1.3), which are enzymes catalyzing the cleavage of triacylglycerols (triglycerides) into free fatty acids and partial glycerol esters—monoacylglycerols (monoglycerides), diacylglycerols (diglycerides), and glyceroltriacylglycerolTypes of RancidityFlavor reversion is a type of rancidity, typical for soybean oil. It is connected with minute absorption of oxygen by oilTypes of RancidityKetonic rancidity, with a characteristic floral off-flavor, is sometimes observed during the storage of foods containing short- or medium-chain fatty acids (4–10 carbon atoms), such as those containing milk fat or coconut oil  It is caused by microbial degradation of medium-chain fatty acids into the respective alkan-2-ones or methyl ketonesmethyl ketonesMechanism of Oxidative Ranciditythree main phasesinitiationPropagationterminationInitiation Reactionspolyenoic (essential) fatty acidsmethylene group adjacent to two double-bonds (-CH=CH–CH2-CH=CH-) is the primary site of oxygen attackconverted into the respective free radical: R-H ⇒ R* + H*Monoenoic fatty acidsfree radicals are formed by cleavage of a hydrogen atom on either one side of the double bond CH2-CH-CH-CH2⇒C*H-CH=CH-CH2 or CH2-CH=CH-C*HInitiation Reactionsactivation energy of this reaction is higherMost often, free radicals are formed by cleavage of a hydroperoxide moleculePropagation Reactions (Primary Reactions)forming a peroxy radicalabstracts a hydrogen atom from another molecule of a polyunsaturated fatty acid, forming a hydroperoxide and an alkyl free radicalformation of a free peroxy or alkoxy radicalpolyunsaturated fatty acid is usually isomerized into a more stable conjugated dienoic systemTermination Reactionsrecombination of two free radicalsDefinitions of Antioxidants and Antioxidant TypesMechanism of Action of Antioxidants(1) Antioxidants react with peroxy radicals produced in oxidized lipids, forming a hydroperoxide molecule and a free radical of the antioxidant Mechanism of Action of Antioxidants(2) alkoxy free radical formed during the decomposition of Hydroperoxides(3,4) free antioxidant radicals react with a peroxy or an alkoxy radical forming a copolymer Mechanism of Action of Antioxidants(5) free antioxidant radicals react with another antioxidant free radical(6) free antioxidant radical is con-verted into an antioxidant peroxy radicalMechanism of Action of Antioxidants(7) Free antioxidant radical can also react with some labile compounds, such as terpenes, which form free radicals Some antioxidants Synthetic Antioxidants Added Directly to FoodAnoxomerButylated HydroxyanisoleButylated HydroxytolueneEthoxyquin4-Hydroxymethyl-2,6-di-tert-butylphenol2-(1,1-Dimethylethyl)-1,4-Benzenediol2,4,5-TrihydroxybutyrophenoneAnoxomer polymeric antioxidant that is prepared by condensation polymerization of divinylbenzene (m- and p-) with tert-butylhydroquinone, tert-butyl-phenol, hydroxyanisole,p-cresol, and 4,4′-isopropylidenediphenoldivinylbenzene tert-butylhydroquinonep-cresol4,4′-isopropylidenediphenolAnoxomernot less than 98.0% puritytotal monomers, dimers, and trimers below MW 500 not to exceed 1%phenol content not less than 3.2 meq/g and not more than 3.8 meq/gheavy metals, not more than 10 ppm lead, 3 ppm arsenic or 1 ppm mercuryin food at a level of not more than 5000 ppmButylated Hydroxyanisolemixture of 2-tert-butyl-4-methoxyphenol and 3-tert-butyl-4-methoxoyphenol3-isomer being 90% or more‘‘hindered’’ phenol, and the tert-butyl group ortho or meta to the hydroxyl group serves to suppress antioxidant activity2-tert-butyl-4-methoxyphenol3-tert-butyl-4-methoxoyphenolButylated HydroxyanisoleButylated HydroxyanisoleThe steric hindrance is probably responsible for the relative ineffectiveness of BHA in vegetable oils because the tertiary butyl group interferes with the antioxidant activity of the phenolic structureBHA is commonly used in combination with other primary antioxidants, such as gallatesgallatesButylated Hydroxyanisolestrong phenolic odor (high temperatures)BHA effectively controls the oxidation of animal fats, but is a relatively ineffective antioxidant in most vegetable oilsButylated Hydroxyanisolesynergism with acids, BHT, propyl gallate, hydroquinone, methionine, lecithin, thiodipropionic acidAMIF-72 mixture: contains 20% BHA, 6% propyl gallate, and 4% citric acid in propylene glycolthiodipropionic acidlecithinButylated HydroxyanisoleTotal BHA must assay at 98.5% minimum, with a minimum melting point of 48°CFood dehydrated potato shreds 50ppmactive dry yeast 1000 BHA onlybeverages and desserts prepared from dry mixes (2 BHA only)dry breakfast cereals (50)Butylated Hydroxyanisoledry diced glazed fruit (32 BHA only)dry mixes for beverages and desserts (90 BHA only)Emulsion stabilizers for shortenings (200)potato flakes (50)potato granules (10)sweet potato flakes (50)Butylated Hydroxytoluene2,6-di-tert-butyl-p-cresol; 2,6-bis(1,1-dimethylethyl)-4-methylphenolwater-insoluble, white, crystalline solid antioxidantmore soluble in food oils and fats than is BHAButylated Hydroxytolueneeffective in animal fatsnot as effective in vegetable oilsBHT is frequently used in combination with BHA in foodsButylated HydroxytolueneBHT is noted for its high-temperature stabilityless effective than BHA because of the greater steric hindrance presented by two tert-butyl groups surrounding the hydroxyl groupButylated HydroxytolueneA food antioxidantSoluble in glyceridesInsoluble in waterSusceptible to loss by volatilizationNegative aspect of BHT, is that it may give a yellow coloration due to the formation of stilbenequinone in the presence of iron