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Cooking oil consists of edible vegetable oils derived from olives, peanuts, and safflowers, to name just a few of the many plants that are used. Liquid at room temperature, cooking oils are sometimes added during the preparation of processed foods. They are also used to fry foods and to make salad dressing.
People in many regions began to process vegetable oils thousands of years ago, utilizing whatever food stuffs they had on hand to obtain oils for a variety of cooking purposes. Early peoples learned to use the sun, a fire, or an oven to heat oily plant products until the plants exuded oil that could then be collected. The Chinese and Japanese produced soy oil as early as 2000 B.C., while southern Europeans had begun to produce olive oil by 3000 B.C. In Mexico and North America, peanuts and sunflower seeds were roasted and beaten into a paste before being boiled in water; the oil that rose to the surface was then skimmed off. Africans also grated and beat palm kernels and coconut meat and then boiled the resulting pulp, skimming the hot oil off the water. Some oils have become available only recently, as extraction technology has improved. Com oil first became available in the 1960s. Cotton oil, watermelon seed oil, grape seed oil, and others are now being considered as ways to make use of seeds that were, until recently, considered waste.
The first efforts to increase output were under-taken independently in China, Egypt, Greece, and Rome, among other places. Using a spherical or conical stone mortar and pestle, vertical or horizontal millstones, or simply their feet, people began to crush vegetable matter to increase its available surface area. The ground material would subsequently be placed in sieves such as shallow, flat wicker baskets that were stacked, sometimes as many as 50 high. The matter was then pressed using lever or wedge presses. The Greeks and Romans improved this process by introducing edge runners to grind and a winch or screw to operate a lever press. Their method was used throughout the Middle Ages.
Refinements of this approach included a stamper press that was invented in Holland in the 1600s and used until the 1800s to extract oil, a roll mill invented by English engineer John Smeaton in 1750 to crush vegetable matter more efficiently, and the hydraulic press, invented by Joseph Bramah in England. The first improved screw press was invented by V. D. Anderson in the United States in 1876. His Expeller (a trade name) continuously operated a cage press. When vegetable matter was placed in Anderson’s closed press, the resultant oil drained out of slots in the side. A screw increased the pressure through the cage toward a restricted opening.
Enhancements in grinding and pressing plant matter were followed by improvements in extracting the oil. In 1856, Deiss of England obtained the first patent for extraction of oil using solvents, following experiments by Jesse Fisher in 1843. At first, solvents such as benzene were pumped through the material and drained through false perforated bottoms. Later, Bollman and Hildebrandt of Germany independently developed continuous systems that sprayed the material with solvent. Both methods were eventually improved, and today solvent extraction is standard in the vegetable oil industry.
(Cooking oil manufacture involves cleaning the seeds, grinding them, pressing, and extracting the oil from them. In extracting, a volatile hydrocarbon such as hexane is used as a solvent. After extracting, the oil is refined, mixed with an alkaline substance, and washed in a centrifuge. Further washing and refining follows, and then the oil is filtered and/or distilled. It is then ready for packaging.)
Over time extracting vegetable oils has become more and more efficient. The very earliest methods of pressing the vegetable matter probably obtained, at best, 10 percent of the oil available. On the other hand, more modem methods involving solvent extraction can extract all but .5 to 2 percent of the oil.
The average bottle of cooking oil contains vegetable oil, with no additives, preservatives, or special flavorings. The oil comes from various parts of plants, in most cases from what are commonly called seeds (including sunflower, palm kernel, safflower, cotton, sesame, and grape seed oils) or nuts (including peanut, soybean, almond, and walnut oils). A few special cases involve merely squeezing the oil from the flesh of the fruit of the plant. For example, coconut oil comes from the coconut’s white meat, palm oil from the pulp of the palm fruit, and olive oil from the flesh of fresh olives. Atypically, com oil is derived from the germ (embryo) of the kernel.
The Manufacturing Process
Some vegetable oils, such as olive, peanut, and some coconut and sunflower oils, are cold-pressed. This method, which entails minimal processing, produces a light, flavorful oil suitable for some cooking needs. Most oil sources, however, are not suitable for cold pressing, because it would leave many undesirable trace elements in the oil, causing it to be odiferous, bitter tasting, or dark. These oils undergo many steps beyond mere extraction to produce a bland, clear, and consistent oil.
Cleaning and grinding
1 Incoming oil seeds are passed over magnets to remove any trace metal before being dehulled, deskinned, or otherwise stripped of all extraneous material. In the case of cotton, the ginned seeds must be stripped of their lint as well as dehulled. In the case of corn, the kernel must undergo milling to separate the germ.
2 The stripped seeds or nuts are then ground into coarse meal to provide more surface area to be pressed. Mechanized grooved rollers or hammer mills crush the material to the proper consistency. The meal is then heated to facilitate the extraction of the oil. While the procedure allows more oil to be pressed out, more impurities are also pressed out with the oil, and these must be removed before the oil can be deemed edible. Cooking oil manufacture involves cleaning the seeds, grinding them, pressing, and extracting the oil from them. In extracting, a volatile hydrocarbon such as hexane is used as a solvent.
After extracting, the oil is refined, mixed with an alkaline substance, and washed in a centrifuge. Further washing and refining follows, and then the oil is filtered and/or distilled. It is then ready for packaging.
3 The heated meal is then fed continuously into a screw press, which increases the pressure progressively as the meal passes through a slotted barrel. Pressure generally increases from 68,950 to 20,6850 kilopascals as the oil is squeezed out from the slots in the barrel, where it can be recovered.
Extracting additional oil with solvents
4 Soybeans are usually not pressed at all before solvent extraction, because they have relatively little oil, but most oil seeds with more oil are pressed and solvent- treated. After the initial oil has been recovered from the screw press, the oil cake remaining in the press is processed by solvent extraction to attain the maximum yield. A volatile hydrocarbon (most commonly hexane) dissolves the oil out of the oil cake, which is then recovered by distilling the light solvent out. The Blaw-Knox Rotocell is used to meet the demands of the United States soybean oil industry. In using this machine, flakes of meal are sent through wedge-shaped cells of a cylindrical vessel. The solvent then passes through the matter to be collected at the bottom. Also still in use by a significant number of manufacturers is the Bollman or HansaMuhle unit, in which oilseed flakes are placed in perforated baskets that circulate continuously. The solvent percolates through the matter which is periodically dumped and replaced.
Removing solvent traces
5 Ninety percent of the solvent remaining in the extracted oil simply evaporates, and, as it does, it is collected for reuse. The rest is retrieved with the use of a stripping column. The oil is boiled by steam, and the lighter hexane floats upward. As it condenses, it, too, is collected.
Refining the oil
6 The oil is next refined to remove color, odor, and bitterness. Refining consists of heating the oil to between 107 and 188 degrees Fahrenheit (40 and 85 degrees Celsius) and mixing an alkaline substance such as sodium hydroxide or sodium carbonate with it. Soap forms from the undesired fatty acids and the alkaline additive, and it is usually removed by centrifuge. The oil is further washed to remove traces of soap and then dried.
7 Oils are also degummed at this time by treating them with water heated to between 188 and 206 degrees Fahrenheit (85 and 95 degrees Celsius), steam, or water with acid. The gums, most of which are phosphatides, precipitate out, and the dregs are removed by centrifuge.
8 Oil that will be heated (for use in cooking) is then bleached by filtering it through fuller’s earth, activated carbon, or activated clays that absorb certain pigmented material from the oil. By contrast, oil that will undergo refrigeration (because it is intended for salad dressing, for example) is winterized—rapidly chilled and filtered to remove waxes. This procedure ensures that the oil will not partially solidify in the refrigerator.
9 Finally, the oil is deodorized. In this process, steam is passed over hot oil in a vacuum at between 440 and 485 degrees Fahrenheit (225 and 250 degrees Celsius), thus allowing the volatile taste and odor components to distill from the oil. Typically, citric acid at .01 percent is also added to oil after deodorization to inactivate trace metals that might promote oxidation within the oil and hence shorten its shelf-life.
Packaging the oil
The completely processed oil is then measured and poured into clean containers, usually plastic bottles for domestic oils to be sold in supermarkets, glass bottles for imports or domestic oils to be sold in specialty stores, or cans for imports (usually olive oil).
By products/ Waste
The most obvious byproduct of the oil making process is oil seed cake. Most kinds of seed cake are used to make animal feed and low-grade fertilizer; others are simply disposed of. In the case of cotton, the lint on the seed is used to make yam and cellulose that go into such products as mattresses, rayon, and lacquer. Coconut oil generates several byproducts, with various uses: desiccated coconut meat (copra) is used in the confectionery industry; coconut milk can be consumed; and coir, the fiber from the outer coat, is used to make mats and rope. Since com oil is derived from a small portion of the entire kernel, it creates corn meal and hominy if it is dry milled, and com starch and com syrup if it is wet milled.
Lecithin is a byproduct of the degumming process used in making soybean oil. This industrially valuable product is used to make animal feed, chocolate, cosmetics, soap, paint, and plastics—to name just a few of its diverse uses. Recent research has focused on utilizing the residual oil seed cake. The cake is high in protein and other nutrients, and researchers are working to develop methods of processing it into a palatable food that can be distributed in areas where people lack sufficient protein in their diets. This goal requires ridding (through additional processing) the oil seed cake of various undesirable toxins (such as gossypol in cotton seed, or aflatoxin in peanut meal). Initial results are promising.
The nuts and seeds used to make oil are inspected and graded after harvest by licensed inspectors in accordance with the United States Grain Standards Act, and the fat content of the incoming seeds is measured. For the best oil, the seeds should not be stored at all, or for a only very short time, since storage increases the chance of deterioration due to mold, loss of nutrients, and rancidity. The seeds should be stored in well-ventilated warehouses with a constantly maintained low temperature and humidity. Pests should be eradicated, and mold growth should be kept to a minimum. Seeds to be stored must have a low moisture content (around 10 percent), or they should be dried until it reaches this level (dryer seeds are less likely to encourage the growth of mold).
Processed oil should be consistent in all aspects such as color, taste, and viscosity. Color is tested using the Lovibund Tintometer or a similar method in which an experienced observer compares an oil’s color against the shading of standard colored glasses. Experienced tasters also check the flavor of the oil, and its viscosity is measured using a viscometer. To use this device, oil is poured into a tube that has a bulb at one end set off by two marks. The oil is then drained, and the time required for the bulb to empty is measured and compared to a chart to determine viscosity.
In addition, the oil should be free of impurities and meet the demands placed upon it for use in cooking. To ensure this, the product is tested under controlled conditions to see at what temperature it begins to smoke (the smoke point), flash, and catch on fire; warnings are issued appropriately. To allow its safe use in baking and frying, an oil should have a smoke point of between 402 and 503 degrees Fahrenheit (204 and 260 degrees Celsius). The temperature is then lowered to test the oil’s cloud point. This is ascertained by chilling 120 milliliters of salad oil to a temperature of 35 degrees Fahrenheit (zero degrees Celsius) for five and a half hours, during which period acceptable salad oil will not cloud.
Before being filled, the bottles that hold the oil are cleaned and electronically inspected for foreign material. To prevent oxidation of the oil (and therefore its tendency to go rancid), the inert (nonreactive) gas nitrogen is used to fill up the space remaining at the top of the bottle.
Where To Learn More
Hoffman, G. The Chemistry & Technology of Edible Oils & Fats & Their High Fat Products. Academic Press, Inc., 1989.
Kirschenbauer, H. G. Fats and Oils. Reinhold Publishing, 1960.
Lawson, Harry W. Standards for Fats and Oils. Avi Publishing Company, 1985.
Salunkhe, D. K. World Oilseeds: Chemistry, Technology, and Utilization. Van Nostrand Reinhold, 1992.
Toussaint-Samat, Maguelonne. A History of Food. Blackwell Publishers, 1992.
“A Cook’s Tour of Cooking Oils,” Changing Times, October 1990, pp. 90-1.