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Fats and oils are composed mainly of triglycerides, three molecules of fatty acids joined to a glycerol molecule. The chain length of the fatty acids and their organization on the glycerol backbone vary greatly, although most of the edible oils consist of those with 16 and 18 carbons. Fatty acids which make up triglycerides vary from oil to oil.

The table lists fatty acid levels for the primary commodity oils. These levels will vary somewhat with an animal's diet or, in the case of vegetable oils, the growing conditions. The table illustrates that fats and oils are a combination of fatty acids, both saturated (C14:0, 16:0, etc.) and unsaturated (C 18:1, 18:2, 18:3). There is no natural fat or oil which is either saturated or unsaturated. Some fats, such as lard, palm and coconut oils, have higher concentrations of saturated fatty acids than other oils. They are referred to as saturated fats, even though they contain percentages of unsaturated fatty acids. One way to describe these "saturated" fats is to say they are solid at room temperatures.
Stearic, oleic, linoleic and linolenic are just four of many kinds of C18 fatty acids. Stearic acid contains no double bonds anywhere on its carbon chain backbone. Oleic acid has one double bond and is therefore monounsaturated. Linoleic and linolenic acids, with two and three double bonds respectively, are referred to as polyunsaturated. An oil high in saturated fat is very stable in a wide variety of food manufacturing applications, including baking and frying. Mono- and polyunsaturated oils are naturally less stable but are often rendered stable through a process called hydrogenation.

All oils may be modified to varying degrees during processing to enhance stability, increase solidity, or improve clarity or functionality. This means that for each oil, there are an enumerable variety of potential finished products. In the future, manufacturers can look forward to an increasing number of new oils with the advent of new and improved crop breeding programs and genetic engineering. Many experiments, including work on cottonseed, soybean, canola, palm and sunflower oils, will genetically engineer and breed in the desired characteristics now achieved through special processing.

Vegetable oils contain other components as well, the most important being tocopherols or Vitamin E. These materials act as antioxidants to protect the oil from rancidity. The table summarizes the tocopherol content of vegetable oils.

SOURCES OF EDIBLE OILS

The processing parameters for each type of oil seed or source may vary slightly, but the general process for all oils is very similar. The variation is a result of the type of seed or source, but the ultimate goal is to yield a clean and stable product which is 96 percent or higher pure triglyceride. Animal fats are rendered from tissue using dry heat or steam and are processed in facilities which are under USDA jurisdiction. Vegetable oils are obtained through expression (pressing) or solvent extraction from the source. The fats and oils obtained directly from rendering, expression or extraction are called crude oils. These crude oils contain varying levels of non-triglyceride materials, the majority of which would be considered impurities in most finished oils. This is not to say this is always bad. The most expensive olive oils are those from the first press. There also are processors who market specialty oils which are only mechanically pressed. However, the great majority of the oils which are sold to food processors and at the retail level are fully refined, that is, they have been processed to remove most impurities or non-triglyceride materials.

Refining of Crude Oil

Degumming - The first step in the refining process of many oils is degumming. Oils are degummed by mixing them with water to hydrate phosphatides, which are then removed by centrifuging. Degumming may be enhanced by adding phosphoric or citric acid or silica gel. Degumming removes valuable emulsifiers such as lecithin. Cottonseed oils are not degummed, but the process is necessary for such oils as soybean and canola.

Alkali Refining - The degummed oil is then treated with an alkali to remove free fatty acids, glycerol, carbohydrates, resins, metals, phosphatides and protein meal. The oil and alkali are mixed allowing free fatty acids and alkali to form a soap. The resulting soapstock is removed through centrifuging. Residual soaps are removed with hot water washings. Cottonseed oil is also refined using a process called miscella refining. This process allows oil to be refined in the miscella stage at the solvent extraction plant prior to removal of the solvent. The oil produced using this method has higher yields and has what some consider a lighter, more desirable color.

Bleaching - During the bleaching process, trace metals, color bodies such as chlorophyll, soaps and oxidation products are removed using bleaching clays, which adsorb the impurities. Bleached oils are nearly colorless and have a peroxide value of near zero. Depending on the desired finished product, oils are then subjected to one or more processes.

Winterization (Fractionation) - Oils destined for use as salad oils, or oils that are to be stored in cool places undergo a process called winterization so that they will not become cloudy when chilled. The refined, deodorized oils are chilled with gentle agitation, which causes higher melting fractions to precipitate. The fraction which settles out is called stearin. Soybean oil does not require winterization, but canola, corn, cottonseed, sunflower, safflower and peanut oils must be to be clear at cool temperatures.

Hydrogenation - Treatment of fats and oils with hydrogen gas in the presence of a catalyst results in the addition of hydrogen to the carbon-carbon double bond. Hydrogenation produces oil with the mouth feel, stability, melting point and lubricating qualities necessary to meet the needs of many manufacturers. It is important to note that hydrogenation is a selective process that can be controlled to produce various levels of hardening, from very slight to almost solid.

Deodorization - Deodorization is a steam distillation process carried out under a vacuum, which removes volatile compounds from the oil. This may be a batch or continuous process. The end product is a bland oil with a low level of free fatty acids and a zero peroxide value. This step also removes any residual pesticides or metabolites that might be present, which are more volatile than the triglycerides in the oil. Some manufacturers favor cottonseed oil as it can be deodorized at lower temperatures, which results in more tocopherols (natural antioxidants) being retained. Deodorization produces some of the purest food products available to consumers. Few other products are so thoroughly clean as refined, bleached and deodorized oil.

Interesterification - This process allows fatty acids to be rearranged or redistributed on the glycerol back-bone. This is most often accomplished by catalytic methods at low temperatures. The oil is heated, agitated and mixed with the catalyst at 90癈. There also are enzymatic systems which may be used for interesterification. It does not change the degree of saturation or isomeric state of the fatty acids, but can improve the functional properties of the oil.

OIL SPECIFICATIONS AND SELECTION

The computer industry created the saying "Garbage In, Garbage Out." The same can be applied to food processing. The use of poor quality materials can compromise product quality. One pitfall is the purchase of materials based solely on cost. If the "great deal" does not have the right functional characteristics, the oil's performance in process may be compromised and the finished product may not even be salable. Purchases should never be based on cost alone, although the seasonality of the various oil source harvests, and the supply's effect on price, does affect purchasing decisions. Changing oil prices are one reason why federal law allows labels of fried snacks and prepared foods to read "Made with one or more of the following oils?."

While food processors work with their suppliers or contract packers in a multitude of fashions, many have initiated programs to validate the companies that supply them.

Five Steps Toward Validation

1. Management from both supplier and buyer establish performance characteristics or specifications. The buyer must be specific in communicating to the supplier his expectations about what the product or service should be. When reviewing suppliers, be sure to check:

Production capabilities
Record keeping
Sanitation/good manufacturing practice compliance
Food safety and quality assurance programs
Laboratory/quality staff
Overall company commitment to quality
2. Review analytical methods to determine the necessary data to be submitted and the format it should take. Many processors provide their suppliers with data acquisition or collection sheets. This makes the processor's life easier, especially when dealing with many suppliers.

3. Measure key attributes and conduct collaborative studies to assure validation and correlation between the methods used by the supplier and buyer laboratories.

4. Run tests of the material(s) in the presence of the buyer and the supplier at the manufacturing plant. This provides both parties with a greater understanding of how the operation will work.

5. The buyer and supplier should agree on a mutually acceptable quality control system such as Statistical Process Control, Total Quality Management, or Hazard Analysis Critical Control Point. (From Stier, R.F. and M.M. Blumenthal, "Is Vendor Certification Really Worth the Hassle?", 1994).

Packaging of Fats and Oils

Manufacturers of fried foods (snacks, coated products, etc.), salad dressings, many formulated foods and a range of other products use vast quantities of fats and oils in the production of these products. In general, transport packaging is designed to inhibit oxidation; methods include minimizing exposure to ultraviolet light and nitrogen gas blanketing. Bulk deliveries of oil are made via:

Rail cars and/or tanker trucks: To assure the quality of oils delivered in bulk, the containers must be thoroughly cleaned and drained prior to loading. Residual water, cleaning compounds or other contaminants which are allowed to remain in the tanks may compromise the entire tanker or car. All seals and valves should be properly secured and sealed to protect the product and provide an indicator of tampering. When loading or unloading bulk tanks, hoses and pumps must be cleaned. When receiving oils, many operators and refiners of crude pass the oils through a screen or filter to ensure that any contaminants are removed.

Collapsible or foldable containers: Manufactured from metal or plastic and designed to be used with a plastic or laminate inliner. The advantage of these containers is they fold up, increasing the efficiency of the backhaul operations. Also, if a panel is damaged, it, and not the whole container, may be replaced. These bins are now being used in Europe for oils for small bakery operations and for foodservice and catering operations.

Rigid plastic reusable container: Must be cleaned after each use. They usually contain a built-in spigot to allow the containers to drain or to which a pump may be attached. Oil must be fluid in this package.

Plastic containers: Encased or contained in a steel mesh to protect and support the plastic, these containers are meant to be reused and must be washed before refill.

Reinforced fiber containers: In which the oil is filled into a single-use plastic or laminate inliner. The fiber may be reused, but once it gets wet or oily, it must be discarded.

Bag-in-box: Another relatively new package used for frying and salad oils. Used in Europe, the package has yet to catch on in the United States. As the bag empties and collapses, headspace is minimized, which can increase the shelf life of the oil. The container was adopted to reduce waste in the foodservice industry.

Other packaging: Smaller quantities, including one- gallon jugs to 35-lb. jugs in a box, are available for foodservice applications and small processing operations.


QUALITY EVALUATION

There are a number of tests used to monitor oil quality. These include chemical, physical and sensory tests. There also are several rapid tests which can be used as quality tools. The quality of the fats and oils used in manufacturing directly affects the finished product. Standards are published by the American Oil Chemists Society (AOCS).

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.

AOCS Cc 3-25

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