Quality Of Fats
Oxidative rancidity occurs due to the oxidation of double bonds in unsaturated fatty acids, forming peroxides or hydro-peroxides that later polymerise and decompose with the production of aldehydes, ketones and low molecular weight acids. One of the major cause of loss of quality of the ingredient is the process of oxidative rancidity. This process will affect the flavour, aroma, colour and texture of the ingredient.
Oils and fats that is high in unsatured fatty acids are particularly prone to attacks by free radicals. A free radical is formed when the chemical structure allows the removal of a hydrogen atom from a CH2 group of the carbonic chain, starting a lipidic peroxidation process. The combination of several free radicals of fatty acids produces a huge variety of stable final products such as hydrocarbons, aldehydes, ketones, alcohols and organic acids.
Several analysis can be used to determine the oxidative profile of ingredients and rations. Quality of fats and oils is evaluated by parameters such as: moisture, impurities, percentage of free fatty acids, unsaponifiables, saponification value, fatty acid profile. Methods specific to evaluate oxidative stability are initial peroxide values, active oxygen method (AOM), oil stability index (OSI), iodine values and thiobarbituric acid reactive substances (TBARS).
Moisture is determined by the weight percentage of the lipidic source after drying at 105°C for 4 hours. Maxiumum accepted values are usually between 0.5 and 1.0 %. This is because moisture percentage directly affects the energy content.
Impurity is calculated as the percentage of the insoluble fraction of the lipid in petroleum ether at temperatures between 40 - 60°C. Impurity contents should be less than 1%.
Percentage of free fatty acids or acidity
Studies show that for each 1% of increase in acidity, 10kcal of metabolizable energy is lost per kg of diet/ingredient.
Unsaponifiable matter is comprise of steroids, pigments and hydrocarbons that form soaps when mixed with caustic soda. These substances are indigestible and are soluble in common solvents for oils. Hence, the higher the percentage of these substances, the lower the energetic value of the oil or fat. The unsaponifiable matter percentage should be below 1%.
The saponification value is obtained by the quantity of KOH (mg) needed to saponify 1g of oil or fat. The saponification value is higher when triglyceride chains are shorter. Each type of lipid has its own reference value.
Fatty Acid Profile
The composition of the fatty acids of lipids is determined by the separation of the methyl esters of the fatty acids using gas chromatography. The fatty acid profile is very important to the quality of the utilized lipid and to the absorption of these lipids by dairy cattle or birds.
Peroxide value is used to evaluate the quality of oils, fat and sub-products with high percentage of fat.
Active Oxygen Method (AOM)
The maximum acceptable is 20 meq of peroxide per kilogram of fat. AOM results might be related to the stability of the fat or the ration. One of the advantages of this test is the ability to assess the relative effectiveness of different antioxidants.
This test measures the induction period, which is the time needed to degrade organic acids derived from aldehyde and ketones, as well as the time needed to concentrate peroxide products in the solution.
This method is frequently used to measure fat stability. The iodine value estimates the number of double bonds present in the fat or oil. Consequently, it is necessary to previously know the nature of the lipid and the number of double bonds that might be present in the lipid. The higher the extension of the oxidation, the lower is the iodine value of the sample.
Analysis of the thiobarbituric acid (TBARS)
This analysis quantifies the concentration of malonaldehyde formed by the oxidation of a triglyceride. Malonaldehyde is a product of the oxidation of the peroxides that are formed initially.