Chemical refining is the traditional method used in past centuries. The main purpose of chemical refining is to saponify the FFA by an alkaline solution and dilute the resulting soaps in a water phase. These soaps are removed by separators. The neutral oils are subsequently bleached and deodorised. This chemical refining can be used for reliably refining virtually all crude oils, including oils of low quality, with the exception of castor oil. In addition to the removal of FFA, other undesirable non-glyceride materials are also removed.
These are mainly:
Metal ions (e.g. iron, copper)
Colour pigments (e.g. gossypol)
Insoluble impurities (e.g. meal fines)
The classical alkaline refining method usually comprises the following steps:
Degumming with water (wet degumming) to remove the easily hydratable phospholipids and metals.
Addition of a small amount of phosphoric or citric acid to convert the remaining non hydratable phospholipids (Ca, Mg salts) into hydratable phospholipids.
Neutralising of the free fatty acids with a slight excess of sodium hydroxide solution, followed by the washing out of soaps and hydrated phospholipids.
Bleaching with natural or acid-activated clay minerals to adsorb colouring components and to decompose hydro peroxides.
Deodorising to remove volatile components, mainly aldehydes and ketones, with low threshold values for detection by taste or smell. Deodorisation is essentially a steam distillation process carried out at low pressures (2-6 mbar) and elevated temperatures (180-220°C).
For some oils, such as sunflower oil or rice bran oil, a clear table product is obtained by a dewaxing step or crystallization of the wax esters at low temperature, followed by filtration or centrifugation.
Chemical Refining By-Products
The two major by-products from the refining processes are soapstock from chemical refining and hydrated gums from the degumming process prior to caustic refining or the physical refining pretreatment stage.
Soapstock from alkali refining is a source of fatty acids, but it also presents a handling, storage, and disposal problem. Originally, many years ago, the caustic refining by-products were merely discarded. Then, it became a valuable source of fatty acids for the soapmaker and the fatty acid distiller. Soapstock was shipped from the refiner in the raw form as it was separated from the neutral oil. The growth of synthetic detergents over soaps reduced this market for soapstock considerably and, in the fatty acid field, soapstock utilization was replaced with tall oil, a by-product of the paper industry. These changes turned edible-oil refiners to soapstock acidulation to produce acid oil. Acid oil is essentially the fatty portion of soapstock with the moisture content reduced to 1.0 or 2.0%. It is traded on a total fatty acid (TFA) basis of 95%, and shipments can be rejected if the TFA falls below 85%. The impurities originally in the crude oil, such as phosphatides, carbohydrates, proteins, pigments, sterols, or heavy metals, are transferred in part or in full to the soapstock during refining and then to the acid oil with acidulation. Acid oil is used mainly as a high-energy ingredient in feeds. They are high-energy ingredients that provide nine calories per gram when metabolized as compared to four calories per gram from starch or protein. Acid oils act as carriers and protectors for several fat-soluble vitamins and antioxidants and are an excellent source of polyunsaturates in most cases. The main competition for the formulated animal feeds is corn, which provides 3.5 calories per gram.
The problems associated with acidulation of soapstock are mainly the corrosive nature of the process and the fact that the separation of the acid oil phase from the acid water phase is often relatively poor, which leads to high fat losses and wastewater contamination with fatty material. Soapstock should be processed as soon as possible after it is produced to minimize fermentation and emulsification.
Lecithin is the preferred outlet for the hydrated gums recovered from water degumming. Commercial lecithin is one of the most important byproducts of the edible-oil processing industry because of its functionality and wide application in food systems and industrial utility.
Lecithin is the commercial name for a naturally occurring surface-active agent made up of a mixture of phospholipids. It can be obtained from a number of vegetable oils, but the major source is soybean oil phospholipids or gums, which provide excellent emulsification properties with good flavour and colour.
Lecithin production starts with degumming the crude oil with approximately 2% steam or water added during slow agitation to hydrate the lecithin. The hydrated gums are separated from the crude oil and dried carefully to below 1.0% moisture to avoid damaging the colour. After cooling, ingredients are added to the lecithin to meet the desired specification limits. Soybean oil and fatty acid additions are used to control acetone-insoluble matter, acid value, and viscosity. Lecithin can be chemically bleached with hydrogen peroxide, either before or after drying, to control the colour. The National Soybean Processors Association rules define six common grades of lecithin. In addition, a variety of modified lecithins can be produced for specialty uses.
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