A Review on Vegetable Oil Refining: Process, Advances and Value Addition to Refining by-Products

Abstract

Nowadays, consumer food choices are driven by health awareness and sustainability concerns. As vegetable oil is an important component of the human diet, the source and the processing play an important role in consumer acceptability. To remove impurities that affect the color, palatability, stability, and safety of oil, crude vegetable oil must be refined. This review highlights the processes and steps used in vegetable oil refining. Depending upon the oil source type, either chemical or physical refining is employed to get the desired oil specifications. Oil refining steps are sequential, with each step removing one or more specific impurities. Refining advances aim towards minimizing chemical usage, nutrient losses, oil losses, and avoiding the formation of trans-fatty acids. The review also discusses the prospect of using the refining by-product stream for obtaining high-value products like phosphatidylcholine, tocopherols, and tocotrienols. The edible oil industry can be made more economical and sustainable through the valorization and integration of waste product streams obtained at different refining steps.

1. Introduction

Refining of crude oil is done to remove unwanted minor components that make oil unappealing to consumers. The conventional methods of oil refining mainly employ chemicals that affect the oil quality, stability, recovery and also generate high volume, low value by-products. After the extraction of oil from oil seeds, refining of this crude oil is performed to remove those impurities that impact the quality of oil. The impurities primarily are phospholipids, free fatty acids (FFA), and minor components like pigments, volatiles, and contaminants. These impurities affect the flavour, color, and stability of the refined oil. The deteriorating effects of these impurities on oil are mentioned in Table 1 [1].

Impurities		Deteriorating effects
Phospholipids		Lower oxidative stability
Free fatty acids		Lower oxidative stability, impaired functional properties
Minor Components	Color pigments (chlorophyll and carotenoids)	Lower sensory properties
	Metal salts (iron and copper compounds)	Lower oxidative stability
	Volatile aldehydes and ketones	Off-flavors

Table 1.

Impurities in oil and their deteriorating effects.

For crude edible oil, chemical (alkali) and physical refining are the standard processes used in industry. In chemical refining, FFA are removed by neutralization with alkali, while in physical refining it is removed by distillation during deodorization. Compared to chemical refining, oil losses are reduced in physical refining. The major steps involved and the components removed in refining steps are shown in Figure 1.

Figure 1.

Basic steps of edible oil refining.

2. Chemical refining

Vegetable oil refining consists of multiple steps in sequential order. Based on the oil type, either chemical or physical refining is done.

2.1 Degumming

Degumming is the first step in crude oil refining performed to remove gums. These gums consist mainly of different phosphatides, entrained oil and meal particles (Figure 2). The majority of phosphatides present in gums are hydratable. They absorb water and thus become oil-insoluble. Different types of degumming techniques practiced in the refining industry are chemical, enzymatic, and membrane degumming.

Figure 2.

Different phospholipids (gums).

2.1.1 Chemical degumming

In chemical degumming, crude oil is treated either with water, acid, ethylene diaminetetraacetic acid (EDTA), etc. Based on the chemicals used, chemical degumming can further be divided into six categories: water degumming, acid degumming, dry degumming, acid refining, organic degumming, and EDTA degumming. Amongst these, water degumming and acid degumming are widely practiced in industry. In water degumming, soft water at the same percentage (2–3%) as the total phospholipids content of crude oil is added to hot oil (70°C) and intensely mixed for 30–60 minutes. It is followed by settling or centrifugation to remove gums and phospholipids. The phosphorus content is typically lowered to 12–170 ppm [2]. In this degumming, only hydratable phospholipids are removed. In industry, acid degumming is done to convert non-hydratable phospholipids (NHP) into phasphatidic acid and calcium/magnesium biphosphate salts. In this process, crude oil is treated with phosphoric acid at a temperature of 90°C [3, 4] to settle the non-hydratable phospholipids. The super acid degumming process of Unilever is carried out at 40°C that produces oil with a phosphorus content lower than that of the standard acid degumming [5]. In the dry degumming process, concentrated acid (70–80%) is added to hot crude oil in an amount of 0.05–1.2%. The acid (phosphoric acid) decomposes the metal salts of acidic phospholipids. In the acid refining process, acid pretreated oil is neutralized with caustic. Caustic form soaps have a great emulsifying property that helps in the reduction of NHP. The process helps to achieve a phosphorus level <10 ppm [6].

The process of organic degumming involves the addition of an aqueous solution of organic acid (less than 5% w/w), usually citric acid. It involves a high shear mixing between oil and citric acid solution for less than 30 seconds and a low shear mixing of less than 15 minutes. Thus, three phases are produced; a heavy phase containing citric acid, which can be decanted and recycled; a lighter top phase containing oil; and an intermediate phase containing the gums. This process has the capability to produce oil with a phosphorus content of less than 10 ppm [7]. In the EDTA degumming process, a chelating agent like EDTA in water is added to the oil. In the soft degumming method, a detergent like sodium lauryl sulfate (SLS) is used to assist the contact between the NHP in the oil phase and the chelating agent in the water phase solution. However, there is a complexity in the separation of both phases due to the high stability of the emulsion so formed [8].

2.1.2 Enzymatic degumming

Phospholipids present in crude oil show emulsification properties that affect the oxidative stability of oil. One way to reduce the emulsification properties of phospholipids is to selectively cleave their polar and non-polar parts from one another, and these reactions can be effectively achieved through enzymes. Enzyme catalyzed reactions are selective and occur at moderate temperatures and pH. Phospholipases are the enzymes that are active on phospholipids. Several kinds of phospholipases are reported in the literature and are named based on the position of fatty acid removed from the glycerol backbone, as indicated in Table 2. The EnzyMax process was the first enzymatic degumming process adopted in the industry. The process employed the phospholipases A2 sourced from porcine pancreas for the degumming of oil rich in NHP [9]. The reaction was carried out at 50–75°C for 3–4 hours, which resulted in residual phosphorus of 3 ppm in the oil.

Types of phospholipases	Mode of action
Phospholipase A1 (PLA 1)	Remove the fatty acid from C1- carbon of glycerol backbone
Phospholipase A2 (PLA 2)	Remove the fatty acid from C2- carbon of glycerol backbone
Phospholipase B (PLB)	Acts on lysophospholipid to remove the remaining fatty acid
Phospholipase C (PLC)	Acts on phosphoro-ester group to generated diglycerides

Table 2.

Types of phospholipases.

PLA1, Lecitase® 10 L and Lecitase® novo was used for degumming of rapeseed oil where phosphorus content was reduced from 280 ppm to 10 ppm [10]. The degumming process developed with enzyme recirculation was successful. The enzyme Lecitase® novo has been used for the degumming of rice bran oil. The phosphorus content and oil losses were high in processed oil due to incomplete reaction [11]. In the enzymatic degumming process, pH is the most critical factor. The pH adjustment of the crude or water degummed oil is done with acid and caustic. The temperature of the reaction mixture is maintained at around 50–60°C and under high shear mixing, the enzyme is added, either in pure or dilute form. Enzyme dosing depends on the type of enzyme used and on the phospholipid content of the oil, but usually varies between 50 and 200 ppm. Enzyme recycling is the major constraint that limits its applicant in the commercial refining process. Enzymatic degumming produces side products that affect the oil quality. e.g. Phospholipase A1 liberates a fatty acid from the phospholipid molecule, resulting in a lysophospholipid and a FFA. Commercially available phospholipases for enzymatic degumming are mentioned in Table 3.

Enzyme trade name	Enzyme producer	Country of origin	Enzyme activity
Lecitase Ultra	Novozymes	Denmark	Phospholipase A1
Lysomax	Danisco	Denmark	Lipid acyltransferase (Type A2)
Rohalase MPL	AB Enzymes	Germany	Phospholipase A2
GumZyme	DSM	Netherlands	Phospholipase A2
Purifine	DSM-Verenium	Netherlands	Phospholipase C

Table 3.

Commercially available phospholipases for enzymatic degumming.

2.1.3 Membrane degumming

Phospholipids are amphoteric in nature, they tend to form reverse micellar structures in a given medium and possess a molar mass above 20 KDa, with a molecular size ranging from 20 to 200 nm. Ultrafiltration can, thus, be utilized to remove them from oil-hexane mixtures [12, 13]. The degumming of crude sunflower and soybean oils was made possible without the use of solvent, i.e., by using a polymeric ultrafiltration membrane with a 15 KDa molar mass cut-off, 5 bar filtration pressure, 60°C temperature and a filtration flow rate of 0.3 m³ h⁻¹. By this procedure, 77% and 73.5% of phospholipid retention for the sunflower and soybean oils, respectively, was achieved [14]. But, the use of membranes at industrial scale degumming has limitations in membrane stability in organic solvents. Membranes used in organic solvents like hexane generally have a shorter lifespan and cleaning protocols still need to be developed and optimized [13].

2.2 Neutralization

Neutralization step in chemical refining is primarily performed to remove FFA from crude edible oil. Conventionally, caustic is directly added in such an amount that the FFA in crude oil and phosphoric acid used previously during acid degumming are completely neutralized. The alkali reacts with the FFA to form soaps (Figure 3) that are further removed by centrifugal separators. But, conventional alkali neutralization has major disadvantages:

• Oil losses due to hydrolysis by alkali and by occlusion in soap stock.

• Soap stock has low commercial value. Splitting of soap stock with concentrated acid generates a heavily polluted stream.

• Water used to wash the oil after alkali neutralization needs to be treated.

Figure 3.

Neutralization reaction.

In recent advances, nano-neutralization is one of the technologies used for FFA removal. Proven industrial advantages of the nano-neutralization process are significant reductions (up to 90%) in phosphoric and citric acid consumption and a corresponding significant reduction (over 30%) in caustic usage [15].

An adsorption process using an anion exchange adsorbent for the removal of FFA from crude oil is reported [16]. But the adsorbents used for FFA removal have low adsorption capacity per unit of adsorbent, and regeneration is a major concern.

2.3 Bleaching

Bleaching is regarded as a partial or complete removal of color. Bleaching also cleans up the traces of soap, phosphatides, and pro-oxidant metals remaining after caustic neutralization and water washing that hinder filtration, darken the oil, and adversely affect the flavor of the finished oil. Another function, considered primary by many processors, is the removal of peroxides and secondary oxidation products. In conventional processes, adsorbents like neutral earth, activated earth, and activated carbon are used for bleaching. These adsorbents are added in the range of 0.15 to 3% weight of oil. Bleaching is done under vacuum for 15 to 20 minutes at 70 to 110°C [17].

Silica hydrogel has also been employed as an aid to bleaching clay to adsorb soap, residual phosphatides, and trace metals. Silica hydrogel (Sorbil® R927) absorbs soap 1.3 times faster than clay adsorbent. At low residual phosphorus levels in the oil (typically <5 ppm P), the capacity of the silica hydrogel is approximately three times that of the clay. The high adsorption capacity and affinity of silica hydrogels for phospholipids, trace metals, and soaps make these synthetic amorphous silicas ideally suited for use in edible oil refining processes. Silica hydrogel does not adsorb color bodies, such as chlorophyll and carotene, from the oil. Thus, the oil is preferably treated sequentially, first with silica hydrogel and then with bleaching clay. In the first stage, the silica adsorbs the phospholipids, trace metals, and soaps. In the second stage, sufficient bleaching earth is added to remove the color bodies [18]. In addition, use of silica seems to improve bleaching earth filterability, resulting in longer filter cycles and higher presses bleach effect [19].

2.4 Winterization

Some vegetable oils, like sunflower, maize, and rice bran, contain waxes. At low temperatures, these waxes crystallize and result in turbidity in the oil. In the winterization process, the oils are cooled in a simple way, kept at low temperatures of 10-15°C for several hours to crystallize solid-fat fractions. Then the cooled oil passes through a filter to separate waxes and clear oil is obtained [2, 17].

2.5 Deodorization

Deodorization is the last step in edible oil refining. It is basically a vacuum-steam distillation process operated at elevated temperatures to remove FFA and other volatile odoriferous components that cause the undesirable flavors and odors. Additionally, deodorization destroys carotenoid pigments, removes pesticides and cyclopropenoid fatty acids. Deodorization design is influenced by four operating variables, which include vacuum, temperature, stripping rate, and retention time. For a continuous deodorizer, the vacuum, temperature, and retention time lie within the ranges of 2–4 mbar, 200–260°C and 15–150 minutes, respectively [2, 17].

The deodorization step has some negative impact on the nutritional quality of oil. Partial loss of bioactives such as tocopherols, tocotrienols, polyphenols, sterols, and squalene is observed during this step. The high temperature of the deodorization step also results in unwanted side reactions like trans-fat formation, conjugation, and polymerization.

3. Physical refining

Physical refining consists of the same steps described in chemical refining, except for the alkali neutralization process. In this process, alkali is not used for FFA removal; rather, it is removed in the deodorization step by steam distillation. Various physical processes to remove FFAs from oil are reported, like steam refining, inert gas stripping, molecular distillation, hermetic system, and extraction with solvents. On an industrial scale, steam refining is used where superheated steam at 200–270°C is passed over degummed and bleached oil to remove FFA. The advantages of physical refining are that it reduces oil losses and minimizes liquid effluent generation.

4. Value-addition to edible oil refining by-products

Refining produces by-products like gums and deodorized distillate (DOD). Different forms of phospholipids can be obtained from gums. DOD contains compounds like FFA, tocopherols, sterols, and squalene.

4.1 Process for the extraction and purification of phospholipids

The aqueous degumming step during the oil refining process produces wet gums as a by-product. These wet gums, produced during the degumming of crude oils such as soybean, sunflower, canola, maize, etc., are processed to produce lecithin. Chemically, gums are mixtures of phospholipids (±45%), glycolipids (±10%), glycerides (±40%) and sugars (±10%). Soybean oil is the primary commercial source of vegetable lecithin. The main phospholipids (PL) present in soya are phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidic acid (PA) [20]. PL are polar lipids with hydrophilic and hydrophobic parts and have wide applications in food as emulsifiers, viscosity regulators, anti-spattering and dispersing agents. These PL are also used as nutraceuticals and/or added to animal feed for nutritional enhancement [21, 22].

Different processes have been utilized for isolation, purification or enrichment of PC from natural lecithin, such as: liquid–liquid extraction, supercritical fluid extraction, solvent fractionation, and adsorption. Based on the polarity of the head group, a solvent like ethanol is used for the fractionation of different classes of phospholipids. The PC fraction of lecithin is enriched by ethanol extraction and is very useful as an emulsifier of oil in water emulsions [23]. Lecithins modified by enzymatic hydrolysis, acetylation, and alcohol fractionation processes give a range of food grade emulsifiers with different hydrophilic–lipophilic-balance (HLB) values [24].

4.2 Purification of tocopherols (TC) and Tocotrienols

Tocopherols and tocotrienols are natural forms of vitamin E, each exhibiting four different isoforms as α, β, γ and δ. Vitamin E is an essential fat-soluble vitamin with antioxidant activity. There is growing interest in natural forms of vitamin E because they are promising compounds for maintaining a healthy cardiovascular system and blood cholesterol level [25]. α-TC is the predominant form of vitamin E in tissues, and low intake results in vitamin E deficiency-associated ataxia. Recent mechanistic studies combined with preclinical animal models indicate the tocotrienol subfamily possesses powerful neuroprotective, anticancer, and cholesterol-lowering properties [26]. Tocopherols are present in major vegetable oils, while tocotrienols are present in palm oil, rice bran oil, and annatto seed. For commercial production of natural tocopherols and tocotrienols, enrichment and purification of TC and TT from deodorizer distillate (DOD) and fractionation of palm oil are practiced in industry.

In the refining process of edible oil, deodorization is the final step carried out at high-temperature, high-vacuum steam-distillation to produce high-quality refined oil. The deodorization process removes FFAs and volatile aldehyde and ketone compounds responsible for the off-flavor and odor of oil. Along with impurities, the deodorization process also partially removes tocopherols, tocotrienols, and phytosterols. So, DOD is a complex mixture of tocopherols, tocotrienols, phytosterols, glycerides, hydrocarbons, and FFA [27]. Tocopherols and tocotrienols have been purified from DOD by a combination of molecular distillation, ethanol fractionation, chemical alcoholysis, and ion exchange chromatography [28]. Preparation of high purity concentrates of TC and TT involves a series of physical and chemical treatment steps like hydrolysis, neutralization [29], supercritical extraction with multistage counter–current column [30], trans-esterification to form methyl esters followed by fractional distillation, membrane separation, enzymatic esterification [31] and batch adsorption and desorption using silica [32].

5. Conclusion

Refining of edible oil by chemical or physical processes aims towards removing the undesirable impurities that affect oil quality and stability. Understanding the chemical nature of oil and associated impurities is important to designing refining steps. The recent refining advance has resulted in minimizing chemical usage, oil losses, and effluent generation. With the rising demand for natural emulsifiers and antioxidants, the by-product streams of gums and DOD can be utilized to obtain high-value products like phospholipids and tocopherols, respectively. There is a need for technological advances that integrate refining and valorization of refining by-products, making the edible oil industry more economical and sustainable.