Biochemistry

+ FAD ------- ► R-CH = CH-COOH + H2O -FADHi p a Unsaturated fatty acid R-CHj-CHi-COOH P a Higher fatty acid +NAD 0 II 0 // > R-CH i -CH j -COOH ----- ► R-C-CHj-COOH ------- ► R-C +CO2 P a -NADH 2 P “ -CCh \ CH 3 P-cxyacid p-ketoacid Methylalkylcetone The final product of this reaction is - methylalkylcetone, gives the fat a bitter taste. The process slows down at low temperatures and stops completely at high temperatures; therefore, the fats should be stored in a refrigerator and, if necessary, stored for long periods of time, be melted. Chemical rancidity (auto-oxidation) of fats occurs upon contact with oxygen from the air. This process is slower at low temperatures, in inert gases, in a vacuum and sharply accelerated when heated (thermal oxidation). Auto-oxidation - is a self-catalyzed process, since new radicals, having a very high level of energy, stimulate the oxidation process. The primary oxidation products are hydroperoxides. Secondary - alcohols, aldehydes, ketones, acids. In addition, during auto-oxidation, changes occur in the position of double bonds in molecules of unsaturated fatty acids (an isolated system of double bonds turns into a conjugate). This accelerates the oxidation and polymerization of fat, especially when heated, and disrupts the structure of essential fatty acids. The effect of light also accelerates auto-oxidation, since ultraviolet rays supply enough energy to split the C - H bond and to decompose hydroperoxides to form free radicals. Auto-oxidation is activated by heavy metal ions (Pb, Cu, Fe, Cd) and alloys (stainless steel), which can get into the fat from the materials of dishes and equipment, especially from places prone to corrosion. Temperatures above 200 °C cause a strong decomposition (pyrolysis) of fats with the formation of highly toxic substances, in particular (during charring) carcinogenic polycyclic hydrocarbons: 3,4-benzpyrene and other (more than 200 volatile products). Thermal-oxidative degradation products were found in heated fats, moreover volatile oxidation products containe higher in oils than in solid fats. As a result of this process, volatile carboxylic acids (C2 - Cfg) are formed: monoenic (C 4 - C 18 ), diene (C 8 - C 10 ), aldehyde acids (C 8 - C14), hydroxy acids (C 6 - C ]6 ), keto acids (C 6 - C i2 ), dibasic acids (С б - Сц), as well as alkanes, alkadienes, alcohols, 208