Edible oil refining

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Edible oil refining is a set of processes or treatments necessary to turn vegetable raw oil into edible oil.

Contents

Raw vegetable oil, obtained from seeds by pressing, solvent extraction, contains free fatty acids and other components such as phospholipids, waxes, peroxides, aldehydes, and ketones, which contribute to undesirable flavor, odor, and appearance; [1] for these reasons, all the oil has to be refined. [2]

Steps

General block scheme of the refining process of edible oil Edible oil refining scheme.png
General block scheme of the refining process of edible oil

Degumming

Vegetable oil contains lecithins, phospholipids, and metals, which are generally called, because of their appearance, mucilaginous gums or simply gum. The process of elimination of the gums is called degumming. These gums are natural emulsifiers, that can cause an increase in viscosity which is an important parameter for the final product. [1] Because of this physical problem, it is important to perform this first step since the high viscosity could create difficulties during the subsequents steps of filtrations. [2]

Depending on the raw oil, these substances are more or less common, so degumming is not an obligatory step: it much depends on the value of lecithins of the source and the concentration of gums in the raw oil. For example, this step is more common in soybean and rapeseed oils, which contain much more gums, than sunflower oil. [3] Because of that, different degumming processes are developed.

Dry degumming

This process is generally used when low phospholipids content is present, such as in coconut and in palm oil. Raw oil is mixed with a solution of citric acid, in order to coordinate metals and phospholipids. Usually, the mixture is sent directly to the bleaching treatment. [3]

Soft degumming

In this process, the raw oil is treated with a water solution of a chelating agent. Similarly to the previous treatment, the solution coordinates metals and phospholipids, however the chelating agent used, such as EDTA, is able to remove more than 90% of phosphorus content in the matrix such as rapeseed oil. [3]

Total degumming process

Also abbreviated as TOD, this is the traditional process where raw oil is treated with acid water and then treated with base, either in this step or during the neutralization. [3] [4]

Membrane degumming

In this process, it is used the ability of gums to form micelles, which are then separated using a process similar to ultrafiltration. Using this principle, it is possible to separate not only gums, but also other impurities, such as fatty acids, to limit further processes of the oil and reduce waste and energy to purify it. However, it is required the use of hexane to form the micelles, and in the processes in which is possible to avoid it, the flux is low, so it is not feasible on an industrial scale. [3]

Enzymatic degumming

In this process enzymes, that are able to hydrolyze phospholipids, are added to the raw oil. With this method it is possible to avoid the use of high temperature and of acidic agents. However, the cost of enzymes and the necessity of different methods of separation represent big drawbacks of the process. [2] [1]

Neutralization

Neutralization consists in the removal of free fatty acids, which come from the partial natural hydrolysis of triglycerides. This is done or by steam stripping in the case of short fatty acids, such as in palm or coconut oil. [4] Otherwise, pH is increased by adding a dilute solution sodium hydroxide or lime. The fatty acids are transformed in soaps, which are separated from the basic water phase, and submitted to a second treatment with hot water at 65-90 °C. [3] [1] The byproduct of this step is called soapstock .

Alternative processes

In order to minimize soapstock, and the energy and base used in the process, different attempts were tried to improve the process. In membrane degumming, it was demonstrated that it is possible to separate some free fatty acids.

Another technology has evaluated the esterification of free fatty acids with glycerol, in order to re-obtain vegetable oil, both with metal catalysis or by using microorganisms. However, all these attempts resulted to be less convenient respect than the original neutralization process. [3]

Winterization or dewaxing

Winterization or dewaxing is a process in which oil is separated from waxes, tocopherols, and residual phospholipids, which may cause turbidity issues in the oil if they remain present in the final product. In a standard process, room temperature oil is blandly stirred while it slowly cooled down, in order to crystalize all the high melting temperature substances. The solids are then separated either by decantation, filtration, centrifugation, or other solid liquid separation techniques. [5]

Since crystallization is a difficult process, and natural oils show different composition and botanical sources, different variants were developed, in which different temperatures, residence times are used and the presence or introductions of surfactants, phospholipids or organic solvents are employed to optimize the separation. [5] [3]

Bleaching and filtration

Raw oil contains various pigments such as chlorophylls, carotenoids, xanthophyll, etc., which can cause problems with subsequent treatments or can color the final product during storage. This process aims to remove them using bleaching earth, which is a class of acidic clay that is capable to absorb oil's pigments and also metals. This process can be performed after or before winterization.

Oil is mixed with this earth in 0.2-2% weight ratio, then vacuum is applied and the suspension is heated at 70-140 °C to both improve the decomposition of fatty acid peroxides and the absorption of the pigment. [3] [4] After that, the oil is passed through a filter press. [6] It is also possible to treat oil with silica to improve purification and metals removal. [4]

Deodorization

During the last steps of refining process, the oil is stripped with vapor at a high temperature to remove all of the remaining unpleasant odors and flavors present in it. This step removes residual fatty acids, sterols, and other unsaponifiable substances. Usually, 270 °C vapor is used, but an inferior temperature could be applied if the oil has low odor content and if vacuum is applied. No alternative processes are considered currently. [3] [4]

Polishing

As a final step before conservation, another filtration is performed, similar to the one performed on wine. This is done to remove final residual impurities in the oil and to improve the final appearance. [7]

Waste valorization

Soapstock and exhausted bleanched earth from oil refining process, Politecninco di Milano, 2022 Soapstock and exausted bleanched earth from oil refining process.jpg
Soapstock and exhausted bleanched earth from oil refining process, Politecninco di Milano, 2022

During the last decade, the advance in organic chemistry, enzymatic, metalorganic, and organo catalysis, made it possible not only to improve the process but also to recover some of the waste in order to implement a circular economy process and develop new chemical reactions for biorefinery.

Gums

Gums coming from the degumming process are usually purified and used as emulsifiers. [4] If the lecithins are not valuable they can be disposed of in a bio-digestion process.

In a recent work was demonstred that it is possible to isolate and characterize the different phospholipids present in the gum discard, from the mixture they were able to obtain either phosphatidic acid, with enzymatic catalysis, and to fermatate other interesting phosphatidic ester. [8]

Soapstock

Soapstock comes from the neutralization step and it contains alkaline water, sodium salts of fatty acid, residual tri -, di- and Monoglycerides, and other minor components. It represents 6% volume of the original mass. Usually, it is neutralized, separated to obtain oleins, and sent to bio-digest to recover energy. [1]

Recently, oleic acid, recovered from high-oleic sunflower oil soapstock by enzymatic treatment, was submitted to a chemo-enzymatic oxidative cleavege, in order to produce pelargonic acid, and azelaic acid. [9]

In another application it was possible to recover oleic acid, linoleic acid and linolenic acid and fermented, using a safe-to-eat probiotic Lacticaseibacillus rhamnosus , to (R)-10-hydroxystearic acid, (S)-(12Z)-10-hydroxy-octadecenoic acid, and (S)-(12Z,15Z)-10-hydroxyoctadecadienoic acid, respectively; these compounds can be use as intermediate coumpound to produce flavor. [10]

Exhausted sorbents

Exhausted bleaching earths are the waste of bleaching process. They are constituted by the earth and residual oil that represent 30-40% of the weight of the waste. They are considered dangerous because it can spontaneously catch fire. Because of this risk, typically this waste needs to be treated properly before disposal into landfills. However, this represents an environmental issue. Therefore, there are attempts to both regenerate the earth [6] and reuse the remaining oil for the synthesis of biodiesel. [11]

Winterization oil cake

The solid waste coming from winterization is often called winterization oil cake or just filter cake. It consist of 50-60% of oil, and the other parts of solid waxes. In sunflower oil, for example, waxes composition ranges from C36 to C60. [5] The residual cake is usually disposed. However, it has been recently shown that waxes can be submitted to solid state fermentation, using Starmerella bombicola in combination with sources of sugar, such as beetroot molasses, and it can be exploited to produce surfactants. [12]

Related Research Articles

<span class="mw-page-title-main">Fatty acid</span> Carboxylic acid

In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28. Fatty acids are a major component of the lipids in some species such as microalgae but in some other organisms are not found in their standalone form, but instead exist as three main classes of esters: triglycerides, phospholipids, and cholesteryl esters. In any of these forms, fatty acids are both important dietary sources of fuel for animals and important structural components for cells.

<span class="mw-page-title-main">Wax</span> Class of organic compounds which are malleable at room temperature

Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to give low viscosity liquids. Waxes are insoluble in water but soluble in nonpolar organic solvents such as hexane, benzene and chloroform. Natural waxes of different types are produced by plants and animals and occur in petroleum.

<span class="mw-page-title-main">Sunflower seed</span> Seed of the sunflower (Helianthus annuus)

A sunflower seed is a seed from a sunflower. There are three types of commonly used sunflower seeds: linoleic, high oleic, and sunflower oil seeds. Each variety has its own unique levels of monounsaturated, saturated, and polyunsaturated fats. The information in this article refers mainly to the linoleic variety.

<span class="mw-page-title-main">Oleic acid</span> Monounsaturated omega-9 fatty acid

Oleic acid is a fatty acid that occurs naturally in various animal and vegetable fats and oils. It is an odorless, colorless oil, although commercial samples may be yellowish. In chemical terms, oleic acid is classified as a monounsaturated omega-9 fatty acid, abbreviated with a lipid number of 18:1 cis-9, and a main product of Δ9-desaturase. It has the formula CH3−(CH2)7−CH=CH−(CH2)7−COOH. The name derives from the Latin word oleum, which means oil. It is the most common fatty acid in nature. The salts and esters of oleic acid are called oleates. It is a common component of oils, and thus occurs in many types of food, as well as in soap.

Biodiesel production is the process of producing the biofuel, biodiesel, through the chemical reactions of transesterification and esterification. This involves vegetable or animal fats and oils being reacted with short-chain alcohols. The alcohols used should be of low molecular weight. Ethanol is the most used because of its low cost, however, greater conversions into biodiesel can be reached using methanol. Although the transesterification reaction can be catalyzed by either acids or bases, the base-catalyzed reaction is more common. This path has lower reaction times and catalyst cost than those acid catalysis. However, alkaline catalysis has the disadvantage of high sensitivity to both water and free fatty acids present in the oils. August 10 is international biodiesel day

<span class="mw-page-title-main">Sesame oil</span> Edible oil from sesame seed

Sesame oil is an edible vegetable oil derived from sesame seeds. The oil is one of the earliest-known crop-based oils. Worldwide mass modern production is limited due to the inefficient manual harvesting process required to extract the oil. Oil made from raw seeds, which may or may not be cold-pressed, is used as a cooking oil. Oil made from toasted seeds is used for its distinctive nutty aroma and taste, although it may be unsuitable for frying, which makes it taste burnt and bitter.

In biochemistry and nutrition, a monounsaturated fat is a fat that contains a monounsaturated fatty acid (MUFA), a subclass of fatty acid characterized by having a double bond in the fatty acid chain with all of the remaining carbon atoms being single-bonded. By contrast, polyunsaturated fatty acids (PUFAs) have more than one double bond.

<span class="mw-page-title-main">Saponification value</span> Milligrams of a base required to saponify 1g of fat

Saponification value or saponification number represents the number of milligrams of potassium hydroxide (KOH) or sodium hydroxide (NaOH) required to saponify one gram of fat under the conditions specified. It is a measure of the average molecular weight of all the fatty acids present in the sample in form of triglycerides. The higher the saponification value, the lower the fatty acids average length, the lighter the mean molecular weight of triglycerides and vice versa. Practically, fats or oils with high saponification value are more suitable for soap making.

<span class="mw-page-title-main">Shea butter</span> Fat from the nut of the African shea tree

Shea butter is a fat extracted from the nut of the African shea tree. It is ivory in color when raw and commonly dyed yellow with borututu root or palm oil. It is widely used in cosmetics as a moisturizer, salve or lotion. It is edible and is used in food preparation in some African countries. It is occasionally mixed with other oils as a substitute for cocoa butter, although the taste is noticeably different.

In chemistry, acid value is a number used to quantify the acidity of a given chemical substance. It is the quantity of base, expressed as milligrams of KOH required to neutralize the acidic constituents in 1 gram of a sample. The acid value measures the acidity of water-insoluble substances like oils, fats, waxes and resins, which do not have a pH value.

<span class="mw-page-title-main">Corn oil</span> Oil from the seeds of corn

Corn oil or maize oil (British) is oil extracted from the germ of corn (maize). Its main use is in cooking, where its high smoke point makes refined corn oil a valuable frying oil. It is also a key ingredient in some margarines. Corn oil is generally less expensive than most other types of vegetable oils.

Rice bran wax is the vegetable wax extracted from the bran oil of rice.

<span class="mw-page-title-main">Sunflower oil</span> Oil pressed from the seed of Helianthus annuus

Sunflower oil is the non-volatile oil pressed from the seeds of the sunflower. Sunflower oil is commonly used in food as a frying oil, and in cosmetic formulations as an emollient.

Omega-9 fatty acids are a family of unsaturated fatty acids which have in common a final carbon–carbon double bond in the omega−9 position; that is, the ninth bond from the methyl end of the fatty acid.

Winterization of oil is a process that uses a solvent and cold temperatures to separate lipids and other desired oil compounds from waxes. Winterization is a type of fractionation, the general process of separating the triglycerides found in fats and oils, using the difference in their melting points, solubility, and volatility.

A lysophosphatidylethanolamine (LPE) is a chemical compound derived from a phosphatidylethanolamine, which is typical of cell membranes. LPE results from partial hydrolysis of phosphatidylethanolamine, which removes one of the fatty acid groups. The hydrolysis is generally the result of the enzymatic action of phospholipase A2. LPE can be used in agricultural use to regulate plant growth such as color increase, sugar content increase, plant health increase, and storability increase without side effect.

<span class="mw-page-title-main">Diglyceride</span> Type of fat derived from glycerol and two fatty acids

A diglyceride, or diacylglycerol (DAG), is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages. Two possible forms exist, 1,2-diacylglycerols and 1,3-diacylglycerols. Diglycerides are natural components of food fats, though minor in comparison to triglycerides. DAGs can act as surfactants and are commonly used as emulsifiers in processed foods. DAG-enriched oil has been investigated extensively as a fat substitute due to its ability to suppress the accumulation of body fat; with total annual sales of approximately USD 200 million in Japan since its introduction in the late 1990s till 2009.

Palm stearin is the solid fraction of palm oil that is produced by partial crystallization at controlled temperature. It is a stearin in the sense of stearins and oleins being the solid and liquid fractions respectively of fats and oils; not in the sense of glyceryl tristearate.

<span class="mw-page-title-main">Cooking oil</span> Oil consumed by humans, of vegetable or animal origin

Cooking oil is a plant or animal liquid fat used in frying, baking, and other types of cooking. Oil allows higher cooking temperatures than water, making cooking faster and more flavorful, while likewise distributing heat, reducing burning and uneven cooking. It sometimes imparts its own flavor. Cooking oil is also used in food preparation and flavoring not involving heat, such as salad dressings and bread dips.

References

  1. 1 2 3 4 5 Beatrice Casali; Elisabetta Brenna; Fabio Parmeggiani; Davide Tessaro; Francesca Tentori (3 February 2021), Enzymatic Methods for the Manipulation and Valorization of Soapstock from Vegetable Oil Refining Processes, doi: 10.3390/SUSCHEM2010006 , Wikidata   Q112790819
  2. 1 2 3 Wim De Greyt (4 June 2013), Edible Oil Refining: Current and Future Technologies, pp. 127–151, doi:10.1002/9781118535202.CH5, Wikidata   Q112793141
  3. 1 2 3 4 5 6 7 8 9 10 Sook Chin Chew; Kar Lin Nyam (2020). "Refining of edible oils". book : 213–241. doi:10.1016/B978-0-12-817105-9.00006-9. Wikidata   Q112812698.
  4. 1 2 3 4 5 6 D. Gordon Dorrell; Brady A. Vick (1 January 1997), Properties and Processing of Oilseed Sunflower, pp. 709–745, doi:10.2134/AGRONMONOGR35.C15, Wikidata   Q112795888
  5. 1 2 3 Mayra C. Chalapu; , Erica R. Baümler; Amalia A. Carelli (28 April 2016). "Characterization of waxes and residual oil recovered from sunflower oil winterization waste". European Journal of Lipid Science and Technology. 119: 1500608. doi:10.1002/EJLT.201500608. ISSN   1438-7697. Wikidata   Q112795430.
  6. 1 2 Pieter J. A. Maes; Albert J. Dijkstra (1992), Process for regenerating spent bleaching earth, Wikidata   Q112840070
  7. ↑ Kenneth S. Watson; Curt H. Meierhoefer (1976). "Use or disposal of by-products and spent material from the vegetable oil processing industry in the U.S.". Journal of the American Oil Chemists' Society . 53: 437–442. doi:10.1007/BF02605740. ISSN   0003-021X. Wikidata   Q112841265.
  8. ↑ Chiara Allegretti; Andrea Bono; Paola D’Arrigo; et al. (21 July 2020). "Valorization of Corn Seed Oil Acid DegummingWaste for Phospholipids Preparation byPhospholipase D-Mediated Processe". MDPI . 10: 809. doi: 10.3390/CATAL10070809 . Wikidata   Q112818902.
  9. ↑ Beatrice Casali; Elisabetta Brenna; Fabio Parmeggiani; Francesca Tentori; Davide Tessaro (4 February 2022). "Multi-step chemo-enzymatic synthesis of azelaic and pelargonic acids from the soapstock of high-oleic sunflower oil refinement". Green Chemistry . 24: 2082. doi:10.1039/D1GC03553C. ISSN   1463-9262. Wikidata   Q112820081.
  10. ↑ Stefano Serra; Davide De Simeis (9 March 2018). "Use of Lactobacillus rhamnosus (ATCC 53103) as Whole-Cell Biocatalyst for the Regio- and Stereoselective Hydration of Oleic, Linoleic, and Linolenic Acid". MDPI . 8: 109. doi: 10.3390/CATAL8030109 . Wikidata   Q112819981.
  11. ↑ K. Suppalakpanya; S.B. Ratanawilai; C. Tongurai (6 January 2010). "Production of ethyl ester from esterified crude palm oil by microwavewith dry washing by bleaching earth". Applied Energy (87): 2356–2359. doi:10.1016/J.APENERGY.2009.12.006. ISSN   0306-2619. Wikidata   Q112841041.
  12. ↑ Pedro Jiménez-Peñalver; Teresa Gea; Antoni Sánchez; Xavier Font (24 August 2016). "Production of sophorolipids from winterization oil cake by solid-state fermentation: Optimization, monitoring and effect of mixing". Biochemical Engineering Journal (115): 93–100. doi:10.1016/J.BEJ.2016.08.006. ISSN   1369-703X. Wikidata   Q112820169.
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