Crude fat containing non glyceride components like mucilages, gums, hydrocarbons, free fatty acids, pigments, sterols along with oil are prone to rancidity. Therefore, impurities should be removed prior to storage and distribution of fats and oil. Purification steps normally involves
Not all non-glyceride components are deleterious. Tocopherol protect oil against autoxidation and provide vitamin E activity while β – carotene provide vitamin A activity. Other phenolic compounds such as sesamolin act as natural antioxidant. Unfortunately, some of the refining operations may remove some beneficial components along with undesirable ones.
Refining of oil refers to removal of free fatty acids, gums and mucilages from oil. Gums are usually removed by mixing a small amount of aqueous solution of phosphoric acid in the oil. Free fatty acid is removed by treatment with caustic soda (NaOH) of desired strength (usually 14.24 %). Refining process includes degumming and neutralization (alkali refining).
Degumming is a water washing process to remove phosphatides. Unless removed, phosphatides can spontaneously hydrate from moisture in the air during storage in the headspace. Degumming can be carried out either as separate operation or simultaneously with neutralization.
In the case of oil rich in phosphatides, such as soybean and canola oil, degumming is usually a separate operation. Hydration makes phosphatides insoluble in oil and they precipitate yielding unattractive oil because of sludge and gums. Phosphatides can degrade and cause dark colors when the oil is heated as in deodorization step.
All soybean oil for the export trade is degummed. Phosphatides are also surfactants and if present in frying oil cause dangerous foaming. When hot oil foams up and spills over the rim of working vessel may burn the user. If the foaming oil contacts a flame, it will catch fire.
There are both hydratable and non hydratable phosphatides. Of the 1 – 3 % phosphatides in soybean oil, 0.2 – 0.8 % are non-hydratable. The phosphatides are composed of phosphatidyl choline, phosphatidyl inositol, phosphatidyl ethanolamine and phosphpatidic acid. The first two are hydratable but later two can complex with divalent cations (metal ions) and are non-hydratable.
Acid degumming and super degumming make phosphatides more hydratable and insoluble. Non hydratable phosphatides remains oil soluble and arise from enzymatic reaction of phospholipases when cellular structure of oil seed is damaged. Non- hydratable phosphatides are particularly problematic in soybean oil. Lipase in the presence of moisture, air hydrolyze lipid to make rancid.
Acid degumming: it is usually practiced in soybean oil processing industries. Small amount of (0.05 – 0.2 %) conc. Phosphoric acid (75%) added to warm oil (70°C) followed by stirring or agitation for 5 to 30 minutes.
Phosphoric acid is added to make phosphatides more hydratable by binding calcium and magnesium ions. Phosphoric acid also partially removes chlorophyll from the oil.
Super degumming: recently super degumming process have been developed in which more of the phosphatides are rendered hydratable. A strong solution of citric acid is added to warm oil (70°C) and the mixture is stirred and cooled to 25°C to precondition the gums. This process causes phosphatides to from liquid. Phospholipid crystals are then easily removed by centrifugation.
Dry degumming: it is occasionally applied to refining of oil with relatively low phosphatides content ex. Palm oil, coconut oil, peanut oil etc. The oil is treated with conc. acid to agglomerate. Gums and then separated from oil by being adsorbed to bleaching earth during subsequent steps of bleaching and filtering.
Neutralization (Alkali Refining):
It is the most important operation in refining edible oil. An improperly neutralized oil will present problem in subsequent refining steps of bleaching and deodorizing and in conversion operation of hydrogenation and interesterification. Neutralization is achieved by reacting the free fatty acid with caustic soda (NaOH) to form soap, referred to as soapstock. Saponification refers to reaction between glycerides (mono, di, tri) and sodium hydroxide to form soap.
The amount of NaOH used depend on amount of free fatty acid (FFA) present in the oil. Nearly all oil other than soybean and rapeseed are simultaneously degummed and neutralized. The amount of NaOH used is termed as ‘treat’. The proper treat produces adequately refined oil with the lowest refining loss. Excessive treat can saponify triglycerides and reduce the yield of refined oil. The proper treat is determined by titrating the oil to determine free fatty acid content.
Prior to neutralizing, oil is treated with 0.02 to 0.5 % phosphoric acid at 60 – 90°C for 15 to 30 minutes, making the phosphatides less soluble in the oil and more easily removed. The proper amount of caustic is proportionally metered into warm oil stream with good mixing for 5 to 10 minutes. The emulsion is then thermally shocked by heating to about 75°C to break out the soap stock. Soapstock is removed from oil by using continuous disc type centrifuges. Refined oil is then washed with soft water (10 – 20%) at 90°C and reinforced to remove most of the soap. The remaining soap is removed during bleaching.
Miscella refining: Alkali refining in the presence of hexane is known as miscella refining. Usually the oil content of miscella is concentrated to 40 – 60 % oil. The oil is mixed with NaOH with high shear mixture. The mixture is heated to 65°C to melt the soapstock and then cooled to 45°C and the aqueous oil phases are separated by centrifugation. Water washing is not required in miscella refining. The neutralized oil miscella must then be evaporated and the oil stripped dried and cooled. Miscella refining produces oil with better color.
Drying: The water saturation level in edible fat is about 0.8 % but oil should contain less than 0.3%. Drying is accomplished by spraying the hot oil (115°C) into vacuum tower (15mmHg). The moisture content of degummed and neutralized oil is reduced to less than 0.1%.
The primary purpose of bleaching is to improve oil color by removing pigments with neutral clays, activated earths, synthetic silicates, silica gel and carbon black. Other benefits of bleaching are breakdown of peroxides and clean- up of residual traces of soap and phosphatides. The primary pigments of concern are those that give red, brown color like carotenoids, xanthophyll, gossypol etc. or green colors (chlorophyll).
The process is generally done under vacuum because the usual bleaching clays can catalyze oxidation in presence of air. Adsorbents is mixed with hot oil (80 – 110°C) for 15 to 30 minutes to form a slurry. The pigments are adsorbed on the surfaces of various clays or earth. Even sometimes activated carbon and solids are removed by filtration. Activated earths are made from certain bentonites specifically “montmorillonite”.
About 0.2 to 2% bleaching clay is usually sed. The precise amount depending on the amount of pigments present. 0.2 to 0.4 % is used for soybean oil while rice bran oil requires 3 -5 %. In addition to removing pigments and residual soap, bleaching takes out trace metals and some oxidation products.
Filters are usually pre-coated with diatomaceous earth to enhance removal of bleaching earth by leaf filters. Removal of earth is very important to oil stability because the earth acts as pro-oxidant. Refined edible oil is pale yellow in color and color is measured by lovibond tintometer usually in red and yellow terms. Most finished edible oil are less than 10 yellow and 2.5 red.
Dewaxing: waxes can harm the appearance of bottle or pouched oil by causing unsightly cloudiness or sediments. Corn, rice bran, safflower, sesame and sunflower seed oil are notorious for problematic high wax content 0.2 – 3 % and must undergo dewaxing. Waxes can be removed by cooling the oil to 6 – 8°C followed by centrifuging at cold temperature. To get wax crystals large enough to ease separation, cooling must be done slowly over four hours and the crystals should be allowed to mature for another six hours. The oil is then carefully heated to 18°C and filtered.
The final step in refining fat and oil is deodorization. The primary objective of deodorization is to remove compounds responsible for undesirable odors such as residual free fatty acids, aldehydes, ketones and alcohol. It also removes peroxides decomposition products. Freshly deodorized oil should have peroxide have of 0 and free fatty acid less than 0.03%. these compounds are more volatile than triglycerides and are easily removed.
Deodorization is a steam distillation performed at high temperature (180 – 270°C) and under high vacuum (3 – 8 mmHg) absolute pressure. Steam is sprayed to carry away the volatiles and to provide agitation by using steam injectors. Vacuum is usually provided by 3 – 5 stages of steam ejectors connected in series. Because deodorization is a mass transfer process, deodorizers are designed to provide large surface area and shallow oil depth. The amount of stripping stream ranges from 10 – 15 kg of steam per 100 kg of oil.
- Steam chest: The path through which propelling steam is administered.
- Jet nozzle: Jet steam nozzle allows propelling steam expand and convert its pressure energy into kinetic energy. Thus pressure gradient is created.
- Solution chamber: The air chamber through which air vapor or gas to be evaporated enters and distributes itself around the steam jet nozzle.
- Diffusion inlet and throat outlet: The diffusion area through which the steam is compressed and discharged at pressure higher than suction.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.