Chemical Tests
Active Oxygen Method (AOM, AOCS Cd 12-57). Measures oxidative stability. Air is bubbled through an oil or fat which is held at 97.8癋. Oil samples are withdrawn at regular intervals and the peroxide value (PV) is determined. The AOM is expressed in hours and is the length of time needed for the PV to reach a certain level. AOM is used as a specification for fats and oils. AOM hours tend to increase with the degree of saturation or hardness. This method, though popular, is being replaced with the Oil Stability Index.
Alkaline Soaps (AOCS Cc 17-95) Alkaline soaps, formed by the reaction of metals and free fatty acids in the presence of water, are a chemical marker of oil degradation. They are most commonly formed as a result of reaction with residual caustic cleaners and, during deep-fat frying, from coatings, breadings and from animal blood and bone cells. This test helps in part to predict food quality and frying oil performance.
Anisidine Value (AOCS Cd 18-90) Aldehydes are products of the decomposition of peroxidized fatty acids. The Anisidine Value measures aldehyde levels, using them as a marker to determine how much peroxidized material has already broken down. In conjunction with current peroxide levels, the past and future degradation profile of an oil can be mapped out-especially for oils processed twice to reduce free fatty acids and reheated frying oils.
Fatty Acid Methyl Esters (FAME, AOCS Ce 1-62). Used to determine the fatty acid composition of fats and oils. Triglycerides are converted to methyl esters and then analyzed using gas-liquid chromatography. With new food regulations and the new oils being produced through breeding and genetic engineering, it is essential these values be known.
Free Fatty Acids (FFA, AOCS Ca 5a-40). Using a titration procedure, FFA is a measure of the amount of fatty acid chains hydrolyzed off the triglyceride backbone. It can be a useful marker for the degraded oil on the surface of a fried food, but is considered by many to be a poor indicator of frying oil quality. Results are reported as %FFA, calculated as oleic acid.
Iodine Value (AOCS Cd 1-25). This test measures the degree of unsaturation in fats and is used as a finished product specification for fresh oils. Elemental iodine is added to the double bonds of unsaturated fatty acids and measured. Results are expressed as grams of iodine absorbed per 100 grams of fat.
Oil Stability Index (OSI, AOCS Cd 12b-92). This automatic test measures the rate at which an oil oxidizes when air is bubbled through it. A breakdown product, formic acid, is conveyed into distilled water contained in a cell. The instrument continuously monitors conductivity in the water. The time at which the conductivity rises sharply is the endpoint.
Peroxide Value (PV, AOCS Cd 8b-90). This is the classic test for measuring oxidation in fresh oils, but has limited value for frying oils as the test is highly sensitive to temperature. Peroxides are unstable radicals formed from triglycerides. Processors extract oil from a food to measure the PV; a PV over 2 is an indicator that the product has a high rancidity potential and could fail on the shelf.
Polar Materials (TPM, AOCS Cd 20-91). Many manufacturers consider polar material measurement to be the single most important test for degrading oil. Polar materials are all non-triglyceride materials soluble in, emulsified in, or suspended in the frying oil. Once an oil is exposed to frying temperatures and food, a portion of the triglycerides are converted into myriad degradation products. Since they also include conversion products, %TPM measures cumulative degradation of the oil.
Polymers (AOCS Cd 22-91). Usually the largest single class of degradation products in frying oil, polymers include dimers, trimers, tetramers, etc., and can be formed through oxidative and thermal reactions. The dark "shellacs", which form on fryer walls, heater tubes and belts, are polymeric materials. The official method to test polymer levels uses high-pressure liquid chromatography. They are an excellent chemical marker of oil degradation
Thiobarbituric Acid (TBA, AOCS Cd 19-90). This test is an excellent indicator of fatty acid oxidation products and detects the onset of rancidity reactions. The addition of TBA results in colored pigments when it reacts with aldehydes and other oxidative breakdown products. Absorption is read at 450 nm for the yellow pigments; 530 nm for red.
Physical Tests
Melting Point. Refers to the point at which a pure compound changes from a solid to a liquid (including the ranges of temperature in which fats will melt in the mouth to produce the desired mouthfeel). Commercial oil products do not melt at one sharp point, but rather over a range of temperatures. Among the methods for determining melting points are Complete Melting Point (AOCS Cc 1-25); Wiley Melting Point (AOCS Cc 2-38); Dropping Point (AOCS Cc 18-80); and Slip Point (AOCS Cc 3-25, 3b-92).
Oil Color (Lovibond, AOCS Cc 13c-92). Color is used as a quality index for frying and as a specification in finished oils. The range of oil colors varies, but if an oil from a refiner is darker than expected, it could indicate abuse or improper refining. Measure Lovibond red, yellow and blue when evaluating a frying system and developing quality standards.
Smoke, Flash, Fire Points (AOCS Cc 9a-48). By heating an oil in a cup under strong light, the temperature at which the oil begins to smoke is observed. With continued heating and the use of a small flame, the flash and fire points are determined. These points are key for oils used in deep-fat frying and griddle cooking.
Solid Fat Index/Content (SFI, AOCS Cd 10-57, SFC, AOCS 16b-93). These measurements describe the percentage of a product that is solid at different, defined temperatures. The creation of this curve gives an understanding of properties and performance of oils over a range of temperatures, information that is essential to create basestocks for blending to produce margarines or shortenings. SFI is determined using dilatometry, a technique that measures the changes in volume that occur when a solid goes to liquid. Magnetic resonance imaging is used to determine SFC. It measures the amounts of liquid and solid fat in a sample, based on relaxation of protons after the sample has been pulsed. The SFC test is faster than the SFI test, but is more expensive.
Sensory Tests
Sensory Analysis (AOCS Cg 2-83). This method is a flavor evaluation of vegetable oils. The oils are placed in covered glass beakers, which are then placed in an aluminum block. The block is heated in the dark. Covering the beakers allows volatiles to build up and the samples are evaluated for flavor and odor. The table lists various descriptors for fats and oils, all of which can be related to the presence of one or more specific compounds.