Formulación del Problema

Crude oil contains relatively small and variable amounts of nonglyceride impurities.

The quality and yield of finished oil are affected by some of the undesirable impu-rities that are not properly removed during processing.

The oil-insoluble impurities consist of seed fragments, excess moisture, meal fines, and a waxy substance. These oil-insoluble impurities are normally and readily removed by filtration. However, the oil-soluble impurities such as free FAs, phos-phatides, gummy or mucilaginous substances, color bodies, proteins, hydrocarbons, ketones, and aldehydes are more difficult to remove.

A series of unit operations are required to remove objectionable impurities with the least detrimental effect on finished oil quality and minimum oil loss.

6.3.2 R^ECEIVING

Receiving, sampling, drying, storage, and cleaning are the typical operations of oil-bearing materials prior to oil processing. The moisture content of the raw material is one of the prime factors for extended storage and final product quality. High moisture of oil-bearing materials results in reduced oil and protein content and darker color and increased refining loss of the extracted oil. For proper storage and subse-quent processing, the contaminants must be removed and the grains or seeds must be dried to around 12–13% water content prior to storage. During storage, it is a routine practice to monitor the temperature of grains or seeds. If heating is occurring, the grains or seeds must be processed immediately. Otherwise, rotation of the grains or seeds is required to avoid severe heating and damage.

6.3.3 P^REPARATION

Proper preparation of grains or seeds is required for extraction of the oil, either by solvent or mechanical methods. The unit operations typically involve scaling, cleaning, cracking, conditioning, and flaking. The grains or seeds are scaled and cleaned to remove contaminants. For soybeans, cracking separates the hulls from the meats and the hulls are then removed by aspiration. After dehulling, the meats are conditioned to soften the cracks that are pliable for the subsequent flaking. During flaking, the meats are generally passed to the flaking rolls to squeeze the meats into flakes of approximately 0.30 mm in thickness. For oil-bearing materials with high oil content, such as sunflower, canola, peanut, or sesame, the purpose of conditioning or cooking is to break down the oil cell walls to the point where the oil is available to be expelled.

In addition, protein coagulation in the meal, adjustment of the moisture content of the meal, and reduction in oil viscosity for proper pressing are also included.

6.3.4 E^XTRACTION

Mechanical extraction is used to press the oil-bearing materials with high oil content.

Oil from a mechanical pressing operation contains meal fines in high concentrations,

which are removed by filtration. The filtered oil can be used either for direct consumption or for subsequent refining. Sesame and peanut oils, as the typical pressed oils with unique flavors, are widely consumed in Oriental countries. Olive oil, an important product in the Mediterranean area, is also obtained by mechanical extraction from the fruit of the Olea europaea L. tree.

For oleaginous materials having a low oil content (18–20%), such as soybean and rice bran, solvent extraction is often applied for oil recovery. Hexane is widely accepted as the most effective solvent used today. Most of the extractors currently used are designed as countercurrent flow devices. The solid material flows in an opposite direction of solvent-oil miscella with an increasing oil concentration. The miscella containing around 25–30% oil after extraction is subjected to solvent distillation to recover the oil. The extracted solid material, commonly known as white flakes, is also conveyed to the desolventizing process.

A combination of mechanical and solvent extraction is often applied to oilseeds with high oil content, e.g., sunflower, safflower, corn germ, and canola. The most efficient method of extracting the oil is mechanically expelling about 60% of the oil and then using solvent extraction of the remaining oil.

6.3.5 R^EFINING 6.3.5.1 Degumming

Degumming is an optional process and is used to remove phosphatides and foreign materials that are present in crude oil. Phosphatides are an excellent emulsifier that interfere with the oil–water separation in the acidulation process and cause neutral oil loss. Water degumming is effective for water-hydratable phosphatides.

The phosphatides contained in soybean oil from good-quality beans are 90%

hydratable and can be removed by water hydration. Severely damaged beans contain increased amounts of nonhydratable phosphatides (NHPs), mainly calcium and magnesium salts of phosphatidic acids. For the removal of NHPs, several acid treatments are used to produce a lower phosphorus degummed oil. Pretreatment of oil with phosphoric acid, citric acid, or other agent with proper time, temper-ature, and agitation conditions, followed by water hydration, is effective in remov-ing NHPs from the oil.

6.3.5.2 Chemical Refining

The purpose of chemical or physical refining is to remove nonglyceride impurities that consist of free FAs and mucilaginous substances, phosphatides, chlorophyll, and color bodies. Alkali refining is associated with the proper choice of alkalies, amounts of alkalies, and refining practice to produce refined oil without excessive saponification of neutral oil. The concentration and amount of caustic alkali solution to be used for refining the crude oil varies with the content of FAs in the oil. If excess caustic alkali solution is used, prolonged heating will result in saponification of the oil and neutral oil losses.

6.3.5.3 Physical Refining

Traditional alkali refining is replaced by physical refining, in which the use of chemicals is reduced. The most widely used method is steam refining. The crude oil quality is very important in order to obtain high-quality refined oil. The oil before physical refining should be efficiently degummed to remove phospholipids, as well as heavy metals, and bleached to remove pigments. The phospholipid content of the oil must be sufficiently low ⎯ less than 5 mg/kg phosphorus before steam stripping and less than 20 mg/kg phosphorus before bleaching. By applying superheated steam under low pressure and at a temperature higher than 220°C, both FAs and undesirable volatiles are removed. The quality of physically refined oil is close to that of alkali-refined oils, but losses of neutral oil are lower and the environment is less polluted (Cmolik and Pokorny, 2000).

6.3.5.4 Bleaching

The bleaching process is used to remove color bodies and other minor impurities.

The bleaching adsorbent, usually a clay product, removes residual soap from alkali refining, aldehydes and ketones from decomposed peroxides, and color bodies. The color of bleached oil is widely measured by the Lovibond tintometer color scale.

6.3.5.5 Deodorizing

The deodorizing process is used to improve the taste, odor, color, and stability of the oils by the removal of FAs; various flavor and odor compounds classified as aldehydes, ketones, alcohols, and hydrocarbons; and oxidation products and pigments. Deodorization is primarily a high-temperature, high-vacuum, steam distillation process. High-temperature treatment bleaches the oil by destruction of the carotenoids. Some minor compounds, including tocopherols and phy-tosterols, are partially removed by deodorization. Typical conditions for deodor-ization of vegetable oil in semicontinuous deodorizers are: a temperature of 245–260°C, a holding time of 15–40 min, an absolute pressure of 3–6 mm of Hg, 3–8% stripping and sparge steam based on oil throughput, and oil cooled to 60°C in the deodorizer.

6.3.5.6 Dewaxing

In some vegetable oils such as sunflower, safflower, corn, rice bran, and canola oil, waxy materials may appear as sediments during storage at lower tempera-tures. To avoid a hazy appearance of oils, dewaxing is usually done by chilling the oils in a continuous heat exchanger to about 0–5°C for 4–16 h to complete the growth of the wax crystals. After stabilization, the temperature is normally increased to about 15°C, and the proper amount of a filter aid is metered into the chilled oil to facilitate filtration. The wax content of the filtered oil should be reduced to a level of about 10 µg/g in order to obtain an oil with a good cold storage stability.

6.3.6 MODIFICATION

6.3.6.1 Hydrogenation

Hydrogenation is the reaction of oils and fats with hydrogen gas in the presence of a catalyst. A nickel catalyst is normally used in the edible oil industry. Hydrogenation is designed to saturate the double bonds in the TAGs. Isomerization of the cis orientation to the trans position of double bonds also occurs during hydrogenation.

Both the FA composition and physical properties of the oils and fats are modified.

The modified oils and fats can be used for specific applications such as the manu-facture of margarines, bakery and confectionary fats, and shortenings.

6.3.6.2 Interesterification

Interesterification is a process to prepare functional plastic fats by exchanging FA within and between TAGs. Chemical and enzymatic methods are the two types of interesterification presently in use. The most commonly used catalyst for chemical interesterification is sodium methoxide. In order to maintain the catalyst activity, the water content of the oils and fats should be less than 0.01% (w/w), and the levels of free FAs and peroxides should be as low as possible, preferably less than 0.05%

(w/w). Lipase-catalyzed interesterifications are classified into random (no regiospec-ificity) and specific (1,3-regiospecific) categories. Random lipases include those from Candida rugosa, Geotrichum candidum, and Staphylococcus aureus. Specific lipases include pancreatic lipase and the enzymes from Mucor miehei, Aspergillus niger, Pseudomonas fluorescens, and Rhizopus arrhizus.

6.3.6.3 Fractionation

This process involves partial crystallization under controlled conditions and separa-tion of the remaining liquid from the solidified part. Dry, solvent, and detergent fractionations are normally used in this system. The first system is the simplest separation; it involves cooling the oil to a desired end temperature and then filtering the liquid oil on a vacuum filter or in a membrane press filter. The latter two systems, involving solvent or detergent separation of the crystallized phase from the liquid phase, are not widely used, due to their high production costs, capital investments, and contamination.