CASTOR OIL EXTRACTION
Castor oil can be extracted from castor beans by either mechanical pressing, solvent extraction, or a combination of pressing and extraction.74 After harvesting, the seeds are allowed to dry so that the seed hull will split open, releasing the seed inside. The extraction process begins with the removal of the hull from the seeds. This can be accomplished mechanically with the aid of a castor bean dehuller or manually with the hands. When economically feasible, the use of a machine to aid in the dehulling process is more preferable.
After the hull is removed from the seed, the seeds are then cleaned to remove any foreign materials such as sticks, stems, leaves, sand, or dirt. These materials can usually be removed using a series of revolving screens or reels. Magnets used above the conveyer belts can remove iron. The seeds can then be heated to harden the interior of the seeds for extraction. In this process, the seeds are warmed in a steam-jacketed press to remove moisture, and this hardening process will aid in extraction. The cooked seeds are then dried before the extraction process begins. A continuous screw or hydraulic press is used to crush the castor oil seeds to facilitate removal of the oil. The first part of this extraction phase is called prepressing. Prepressing usually involves using a screw press called an oil expeller. The oil expeller is a high-pressure continuous screw press to extract the oil.
Although this process can be done at a low temperature, mechanical pressing leads to only about 45% recovery of oil from the castor beans. Higher temperatures can increase the efficiency of the extraction. Yields of up to 80% of the available oil can be obtained by using high-temperature hydraulic pressing in the extraction process. The extraction temperature can be controlled by circulating cold water through a pressing machine responsible for cold pressing of the seeds. Cold-pressed castor oil has lower acid and iodine content and is lighter in color than solvent-extracted castor oil.
Following extraction, the oil is collected and filtered and the filtered material is combined back with new, fresh seeds for repeat extraction. In this way, the bulk filtered material keeps getting collected and runs through several extraction cycles combining with new bulk material as the process gets repeated. This material is finally ejected from the press and is known as castor cake. The castor cake from the press contains up to approximately 10% castor oil content.75 After crushing and extracting oil from the bulk of the castor oil seeds, further extraction of oil from the leftover castor cake material can be accomplished by crushing the castor cake and by using solvent extraction methods. A Soxhlet or commercial solvent extractor is used for extracting oil from the castor cake. Use of organic solvents such as hexane, heptane, or a petroleum ether as a solvent in the extraction process then results in removal of most of the residual oil still inaccessible in the remaining seed bulk.
CASTRO OIL FILTERATION
Following extraction of the oil through the use of a press, there still remain impurities in the extracted oil. To aid in the removal of the remaining impurities, filtration systems are usually employed. The filtration systems are able to remove large and small size particulates, any dissolved gases, acids, and even water from the oil.75 The filtration system equipment normally used for this task is the filter press. Crude castor seed oil is pale yellow or straw colored but can be made colorless or near colorless following refining and bleaching. The crude oil also has a distinct odor but can also be deodorized during the refining process.
CASTOR OIL REFINING
After filtration, the crude or unrefined oil is sent to a refinery for processing. During the refining process, impurities such as colloidal matter, phospholipids, excess free fatty acids (FFAs), and coloring agents are removed from the oil. Removal of these impurities facilitates the oil not to deteriorate during extended storage. The refining process steps include degumming, neutralization, bleaching, and deodorization. The oil is degummed by adding hot water to the oil, allowing the mixture to sit, and finally the aqueous layer is removed. This process can be repeated. Following the degumming step, a strong base such as sodium hydroxide is added for neutralization. The base is then removed using hot water and separation between the aqueous layer and oil allows for removal of the water layer. Neutralization is followed by bleaching to remove color, remaining phospholipids, and any leftover oxidation products. The castor oil is then deodorized to remove any odor from the oil. The refined castor oil typically has a long shelf life about 12 months as long as it is not subjected to excessive heat. The steps involved in crude castor oil refining are further discussed in the next section.
CRUDE CASTOR OIL REFINING
While the previous section briefly discussed the general overview involved in a castor oil refining step, this section thoroughly explains each of the processes involved in it. Unrefined castor oil leads to rapid degradation due to the presence of impurities as mentioned in “Castor oil refining” section, making it less suitable for most applications.1 Hence, a refining process has to be conducted prior to the derivatization of the oil. The order of the steps performed in the refining process, which includes degumming, neutralization, bleaching, deodorization, and sometimes winterization, should be taken into consideration for efficient oil refining (Fig. 6) and are described extensively and specifically in a castor oil industry setting in “Degumming”, “Neutralization”, “Bleaching”, “Deodorization”, and “Winterization” sections.
The first step in the castor oil refining process, called degumming, is used to reduce the phosphatides and the metal content of the crude oil. The phosphatides present in crude castor oil can be found in the form of lecithin, cephalin, and phosphatidic acids.76 These phosphatides can be classified into two different types: hydratable and nonhydratable,77 and accordingly, a suitable degumming procedure (water degumming, acid degumming, and enzymatic degumming) has to be performed for efficient removal of these phosphatides. In general, crude vegetable oil contains about 10% of nonhydratable phosphatides.77 However, the amount may vary significantly depending on various factors such as the type of seed, quality of seed, and conditions applied during the milling operation. While hydratable phosphatides can be removed in most part by water degumming, nonhydratable phosphatides can only be removed by means of acid or enzymatic degumming procedures
Good quality castor seeds stored under controlled conditions produce only low FFA content of approximately 0.3%.82 Occasionally, oil seeds that are old or stored for more than 12 months with high moisture content produce a high FFA content of about 5% level.83 This excess FFA present in the castor oil does not provide the same functionality as the neutral oil and has the ability to alter its reactivity with different substances. Hence, it is highly essential to remove the high FFA content so as to produce a high-quality castor oil. This process of removal of FFA from the degummed oil is referred to as neutralization.82
In general, the refining process can be divided into two methods: chemical and physical refining. Physical refining is usually done by maintaining a high temperature above 200°C with a low vacuum pressure. Under these processing conditions, the low boiling point FFA is vacuum distilled from the high boiling point triglycerides. However, physical refining is not recommended in the case of castor oil, due to its sensitivity to heat as it normally starts disintegrating above 150°C, which can result in the hydrolysis of the hydroxyl groups. On the other hand, chemical refining is based on the solubility principle of triglycerides and soaps of fatty acids.82 FFAs (acid) react with alkali (strong base) to form soaps of fatty acids . The formed soap is generally insoluble in the oil and, hence, can be easily separated from the oil based on the difference in specific gravity between the soap and triglycerides. The specific gravity of soap is higher than that of triglycerides and therefore tends to settle at the bottom of the reactor. Most of the modern refineries use high-speed centrifuges to separate soap and oil mixture.
Bleaching of castor oil can be done under vacuum at around 100°C while constantly stirring the oil with an appropriate amount of activated earths and carbon.78 The bleaching process requires around 2% bleaching earth and carbon to produce a desirable light colored oil. Under these processing conditions, colored bodies, soap, and phosphatides adsorb onto the activated earth and carbon. The activated earth and carbon are removed by using a commercial filter. The spent earth-carbon, thus obtained, retains around 20%–25% oil content. Bleaching castor oil containing higher phosphatide and soap content often leads to high retention of oil due to the large amount of activated earth used and thus causes filtration issues. Although this retained oil on the spent earth can be recovered by boiling the spent earth in water or by a solvent extraction method, the recovered oil from the spent earth is highly colored with high FFA and high peroxide content, normally greater than 10 mg KOH/g and 20 meq/kg, respectively
Deodorization is simply a vacuum steam distillation process that removes the relatively volatile components that give rise to undesirable flavors, colors, and odors in fats and oils. Unlike other vegetable oils, castor oil requires limited or no deodorization, as it is a nonedible oil where slight pungent odor is not an issue for most of its applications, with the only exception being pharmaceutical grade castor oil.89,90 Deodorization is usually done under high vacuum and at high temperature above 250°C to remove undesirable odors caused by ketones, aldehydes, sterols, triterpene alcohols, and short-chain fatty acids.85 Pharmaceutical grade castor oil is deodorized at low temperatures, approximately 150°C–170°C under high vacuum for 8–10 hours to avoid hydrolysis of hydroxy group of RA
The majority of vegetable oils contain high concentrations of waxes, fatty acids, and lipids. Hence, it is subjected to the process of winterization before its final use. Winterization of oil is a process, whereby waxes are crystallized and removed by a filtering process to avoid clouding of the liquid fraction at cooler temperatures. Kieselguhr is the generally used filter aid and the filter cake obtained at the end can be recycled to a feed ingredient. In certain cases, a similar process called “dewaxing” can also be utilized as a means to clarify oil when the amount of cloudiness persists
Castor oil is a promising commodity that has a variety of applications in the coming years, particularly as a renewable energy source.
Essential to the production and marketing of castor oil is the scientific investigation of the processing parameters needed to improve oil yield. In the recent years, machine learning predictive modeling algorithms and calculations were performed and implemented in the prediction and optimization of any process parameters in castor oil production. Utilization of an artificial neural network (ANN) coupled with genetic algorithm (GA) and central composite design (CCD) experiments were able to develop a statistical model for optimization of multiple variables predicting the best performance conditions with minimum number of experiments and high castor oil production.93 In a separate study by Mbah et al,17 a multilevel factorial design using Minitab software was used to determine the conditions, leading to the optimum yield of castor oil extraction through a solvent extraction method. This study found that optimum conditions that included leaching time of two hours, leaching temperature of 50°C, and solute:solvent ratio of 2 g:40 mL garnered optimum yield of castor oil extraction. Such mathematical experimental design and methodology can prove to be useful in the analysis of the effects and interactions of many experimental factors involved in castor oil production.