Phụ gia chống oxi hóa ThS Đặng Bùi Kh • Types of Rancidity • Types of Rancidity – Lipolytic 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 glycerol triacylglycerol • Types of Rancidity – Flavor reversion is a type of rancidity, typical for soybean oil It is connected with minute absorption of oxygen by oil • Types of Rancidity – Ketonic 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 mediumchain fatty acids into the respective alkan-2-ones or methyl ketones methyl ketones • Mechanism of Oxidative Rancidity • three main phases – – – initiation Propagation termination • Initiation Reactions – polyenoic (essential) fatty acids • methylene group adjacent to two double-bonds • (-CH=CH–CH2-CH=CH-) is the primary site of oxygen attack converted into the respective free radical: R-H ⇒ R* + H* – Monoenoic fatty acids • free 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*H • Initiation Reactions – activation energy of this reaction is higher – Most often, free radicals are formed by cleavage of a hydroperoxide molecule • Propagation Reactions (Primary Reactions) – forming a peroxy radical – abstracts a hydrogen atom from another molecule of a polyunsaturated fatty acid, forming a hydroperoxide and an alkyl free radical – formation of a free peroxy or alkoxy radical – polyunsaturated fatty acid is usually isomerized into a more stable conjugated dienoic system • Termination Reactions – recombination of two free radicals • Mechanism 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 Food – Anoxomer – Butylated Hydroxyanisole – Butylated Hydroxytoluene – Ethoxyquin – 4-Hydroxymethyl-2,6-di-tert-butylphenol – 2-(1,1-Dimethylethyl)-1,4-Benzenediol – 2,4,5-Trihydroxybutyrophenone • Anoxomer – 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′-isopropylidenediphenol divinylbenzene tert-butylhydroquinone p-cresol 4,4′-isopropylidenediphenol • Anoxomer – not less than 98.0% purity – total 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/g – heavy metals, not more than 10 ppm lead, ppm arsenic or ppm mercury – in food at a level of not more than 5000 ppm • Butylated Hydroxyanisole – mixture of 2-tert-butyl-4-methoxyphenol and 3-tert-butyl-4-methoxoyphenol – 3-isomer being 90% or more – ‘‘hindered’’ phenol, and the tert-butyl group ortho or meta to the hydroxyl group serves to suppress antioxidant activity Butylated Hydroxyanisole 3-tert-butyl-4-methoxoyphenol 2-tert-butyl-4-methoxyphenol • Butylated Hydroxyanisole – The 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 structure – BHA is commonly used in combination with other primary antioxidants, such as gallates gallates • Butylated Hydroxyanisole – strong phenolic odor (high temperatures) – BHA effectively controls the oxidation of animal fats, but is a relatively ineffective antioxidant in most vegetable oils • Butylated Hydroxyanisole – synergism with acids, BHT, propyl gallate, hydroquinone, methionine, lecithin, thiodipropionic acid… – AMIF-72 mixture: contains 20% BHA, 6% propyl gallate, and 4% citric acid in propylene glycol lecithin thiodipropionic acid • Butylated Hydroxyanisole – Total BHA must assay at 98.5% minimum, with a minimum melting point of 48°C – Food • dehydrated potato shreds 50ppm • active dry yeast 1000 BHA only • beverages and desserts prepared from dry mixes (2 BHA only) • dry breakfast cereals (50) • Butylated Hydroxyanisole • dry 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 Hydroxytoluene – 2,6-di-tert-butyl-p-cresol; 2,6-bis(1,1-dimethylethyl)-4-methylphenol – water-insoluble, white, crystalline solid antioxidant – more soluble in food oils and fats than is BHA • Butylated Hydroxytoluene – effective in animal fats – not as effective in vegetable oils – BHT is frequently used in combination with BHA in foods • Butylated Hydroxytoluene – BHT is noted for its high-temperature stability – less effective than BHA because of the greater steric hindrance presented by two tert-butyl groups surrounding the hydroxyl group • Butylated Hydroxytoluene – A food antioxidant • Soluble in glycerides • Insoluble in water • Susceptible to loss by volatilization – Negative aspect of BHT, is that it may give a yellow coloration due to the formation of stilbenequinone in the presence of iron ... Definitions of Antioxidants and Antioxidant Types • Mechanism of Action of Antioxidants – (1) Antioxidants react with peroxy radicals produced in oxidized lipids, forming a hydroperoxide molecule... 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 radical •... 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