Preparation of edible neem oil
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This invention relates to a new process for refining and deodorizing the oil from the seeds of the neem tree (Azadirachta indica Juss.) to provide edible neem oil.
The neem (or nim) tree is a subtropical tree which is native to the arid regions of India, Pakistan, Sri Lanka and parts of Southeast Asia and Western Africa. Once or twice a year, it bears a yellow, bitter fruit. The fruit contains a seed composed of a kernel and a husk. The kernel contains about 40 to 60% by weight neem oil. This oil can be isolated by standard procedures used in vegetable oil industry that involve expelling the oil in a cooker-prepress followed by extraction of the residual oil with hexane in a solvent extractor.
Neem oil has all the typical features of an edible, vegetable oil. It is composed of edible triglycerides and non-edible or undesirable impurities such as phospholipids (gums), fatty acids, soaps, colored impurities, for example carotenoids and chlorophyll, and a host of other molecules. Such impurities are routinely removed in a typical refining process comprising the steps:
- 1. Degumming with water or an acid such as citric or phosphoric acid to remove phospholipids.
- 2. Caustic refining with sodium hydroxide to remove fatty acids and soaps.
- 3. Absorption on bleaching earth to remove colored materials.
- 4. Steam distillation to remove low molecular weight materials.
The first two steps of the refining process are most commonly carried out on raw oils stripped of hexane. However, for some oils, such as cotton oil, it is preferable to refine hexane solutions of the oil, for example 65% oil in hexane. As part of the refining process, the oils are commonly hydrogenated in order to improve their thermal and storage stability.
In order to be considered an edible product, the oil has to pass certain criteria. Some of these are chemical tests while others such as taste and smell are more arbitrary and are dependent on local cultures and conditions. The chemical tests include analysis of the fatty acid content of triglycerides. Erucic acid (C₂₀) and saturated acids such as palmitic (C₁₆) and stearic (C₁₈) are undesirable. Trace metals, chlorophyll, free fatty acids and phosphorous are also undesirable. Peroxide value, indicating stability towards oxidants, should be close to zero. Oils should be pale and their colour is expressed by a standardized colour index. In addition to these analyses, rape, mustard and canola oils are routinely assayed for sulfur which indicates the presence of glucosinolates. These compounds, occurring only in wild varieties, are catalyst poisons and therefore undesirable from the viewpoint of the refiners. More importantly, however, they lend the oil a specific odour and taste which is not popular with many consumers.
Analysis of crude neem oil suggests that it has the characteristics of a standard edible oil with regard to its fatty acid composition. Most of its impurities are of the standard, expected type except for sulfur, a very important exception. Two methods are in use for sulfur analysis: the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP) analytical procedure and the Raney Nickel Reduction (Ra(Ni)) method. The ICP method is very rapid and convenient, but the industry standard in North America, <1 part per million (ppm) for canola, is based on the older and rather cumbersome Ra(Ni) method which routinely gives values ten times lower than those obtained using the ICP method. The ICP and Ra(Ni) analytical procedures are described in "Sulfur Levels in Canola Oils from Canadian Crushing Plants: Analysis by Raney Nickel Reduction and Inductively Coupled Plasma Atomic Emission Spectroscopy," Proceedings of the GCIRC Eighth International Rapeseed Congress, July 9, 1991, Saskatoon, Sask., Canada, Vol. 5, page 1396, D. I. McGregor, ed., published by the Organizing Committee of the Eighth International Rapeseed Congress under the Auspices of the Groupe Consultatif International de Recherche sur le Colza (GCIRC) and the Canola Council of Canada. The sulfur content in neem oil is very high, 2200 ppm, using the ICP analytical procedure, and consequently imparts a very disagreeable odour and taste to the neem oil.
All sulfur values reported in this specification were obtained using the ICP method for sulfur analysis unless otherwise indicated.
Standard vegetable oil refining steps do not reduce the sulfur content down to an acceptable level in neem oil, although the other typical impurities are removed. Hydrogenation, to improve oil stability, is not possible due to the poisoning of the hydrogenation catalyst by the sulfur. This failure of the standard refining methodology may account for the lack of interest by food producers in neem oil.
C. Rukmini, Food Chemistry 26, 119-124 (1987) discloses a debitterized neem oil. However, the details for obtaining such debitterized neem oil are not revealed.
Thus there is a need for a process for preparing a grade of neem oil that is substantially odour-free and which possesses a sulfur content sufficiently low for the oil to be regarded as edible. Such a sulfur content is below about 1550ppm. Such a process also needs to be easily employable in existing standard refining procedures used by the producers of vegetable oils.
Accordingly the present invention provides in one aspect a process for the preparation of neem oil containing less than 1550 ppm of sulfur which comprises:
- a) dissolving crude neem oil in an alkane, preferably hexane,
- b) treating the alkane solution with an alkaline solution of hydrogen peroxide,
- c) isolating the alkane solution,
- d) removing the alkane to obtain crude neem oil,
- e) purifying the crude neem oil, and optionally including the further step(s) of
- f) hydrogenating, bleaching and/or further purification.
Although this process is suitable for preparing neem oils having a sulfur content below about 1550ppm, it will be appreciated that it can be used to reduce the sulfur content to any desired level higher than that if required. A further aspect of the invention comprises the use of the above process to reduce the sulfur content of neem oil.
The Table below summarises the standard procedure for refining oils in hexane, and compares it with a preferred embodiment of the present invention.
- 1. Caustic refining to remove free fatty acids.
- 2. Water wash to remove soaps and gums.
- 3. Hexane strip.
- 4. Hydrogenation for oil stabilization.
- 5. Bleaching to remove coloured materials using bleaching earth with activated carbon.
- 6. Deodorization to remove lower molecular weight materials by steam distillation.
- 1. Dissolve crude neem oil in an alkane.
- 2. Treat the alkane/neem oil solution with hydrogen peroxide/caustic/alcohol refining to remove free fatty acids and most sulfurous materials.
- 3. Water wash or filtration through silica of the alkane solution to remove soaps and gums.
- 4. Alkane strip.
- 5. Bleaching to remove colored materials using bleaching earth with activated carbon.
- 6. Purifying by distillation or chromatography to remove lower molecular weight impurities including sulfurous materials, preferably by steam distillation.
- 7. Hydrogenation for oil stabilization.
- 8. Deodorization to remove lower molecular weight materials by steam distillation.
Although the steps following the hydrogen peroxide/caustic/ alcohol refining step of the present invention process are listed in the particular order as shown in Table 1, other sequences of steps would also yield edible neem oil. For example, the water wash or filtration through silica of step 3 may be performed after the alkane strip of step 4. In another example, the bleaching process of step 5 may be performed after the hydrogenation of step 7: or the purification of step 6 may be performed prior to the bleaching process of step 5.
The choice of an alkaline solution of hydrogen peroxide for the removal of sulfur containing compounds from neem oil is based on the theory that the majority of such compounds would have a disulfide linkage and therefore could be oxidized to water soluble sulfates. The presence of an alcohol allows the sulfur containing compounds to be solubilized in the aqueous phase. This can be shown, using sodium hydroxide as the base, as follows:
The neem oil may be derived from either unrefined neem oil or neem oil which has been previously caustic refined. The solution of hydrogen peroxide generally comprises hydrogen peroxide, a base, for example sodium hydroxide, potassium hydroxide or a mixture thereof, and, optionally, an alcohol, for example methanol or, more preferably, ethanol. The hydrogen peroxide solution may be made up from about one part of aqueous 35% hydrogen peroxide solution and about ten parts of an alcoholic solution of an alkaline material on a volume/volume (v/v) basis. Such an alkaline material may comprise about 1% on a w/w basis of the alcoholic solution of the alkaline material.
Various proportions of neem oil and alkane may be employed. A preferred amount of neem oil in the alkane solution is from about 50% to about 80%, more preferably about 65%, neem oil on a w/w basis.
In a preferred embodiment of this invention, 65 parts of unrefined or previously caustic refined neem oil in 35 parts of hexane on a w/w basis is treated with a solution comprising hydrogen peroxide, sodium hydroxide and ethanol. The concentration of hydrogen peroxide in the solution is from about 0.1% to about 15% on a w/w basis; the concentration of sodium hydroxide in the solution is from about 0.05% to about 5%, more preferably from about 0.5% to about 3%, and even more preferably from about 1.0% to about 3.0% on a w/w basis; and the concentration of ethanol is from 0% to about 75%, more preferably from about 25% to about 70%, and even more preferably from about 50% to about 70% on a w/w basis. The neem oil which resulted from this treatment, after a water wash to remove soaps and gums followed by a hexane strip, possessed not more than 1550 ppm of sulfur, more preferably not more than 800 ppm of sulfur, and even more preferably not more than 500 ppm of sulfur.
The neem oil which results from the hydrogen peroxide treatment may be further processed by subjecting it to a steam distillation to remove lower molecular weight materials, including sulfur containing materials, to provide a neem oil containing less than 200 ppm of sulfur. This deodorized neem oil may be partially hydrogenated in order to enhance its stability and to provide a neem oil containing less than 100 ppm of sulfur. If desired, this partially hydrogenated oil may be subjected to a bleaching step, using bleaching earth with activated carbon in order to remove coloured impurities, and a deodorization step to remove residual lower molecular weight materials in order to provide a neem oil containing not more than 50 ppm of sulfur.
Preferably, the process of the invention comprises the following:
- 1. dissolving crude neem oil in an alkane such as heptane, hexane and the like;
- 2. treating the alkane solution with from about 10 milliliters (mL) to about 100 mL, preferably from about 40 mL to about 90 mL, of an alkaline solution of hydrogen peroxide optionally in the presence of an alcohol, such as a sodium hydroxide solution of hydrogen peroxide in ethanol, per 100 grams (g) of the alkane solution at a temperature range of from about 30° C to about 70° C, preferably in the range of from about 45° C to about 60° C;
- 3. isolating the alkane solution and washing or filtering or both if desired;
- 4. removing the alkane to obtain low sulfur containing crude neem oil;
- 5. purifying the crude neem oil by distillation, preferably steam distillation, or by chromatographic methods; followed by, if desired,
- 6. hydrogenating, bleaching and further purification.
Examples 1-12 were experiments run on 100 g of a caustic refined, 65% by weight solution of neem oil, containing 1650 ppm of sulfur, in hexane. Examples 13 and 14 were experiments run on 100 g of an unrefined, 65% by weight solution of neem oil, containing 2200 ppm of sulfur, in hexane. The alkaline solutions of hydrogen peroxide were prepared from aqueous 35% hydrogen peroxide, aqueous 95% ethanol (C₂H₅OH) and aqueous 2%, 4%, 8% or 16% sodium hydroxide (NaOH) solutions in the proportions listed in Table 2. The hexane solution of neem oil and the alkaline solution of hydrogen peroxide (H₂O₂) were stirred together for 30 minutes at 50-55°C, the phases separated by centrifugation, and the hexane solution of neem oil phase stripped of hexane and analyzed for sulfur.
A solution made up of 65% by weight of caustic refined neem oil in hexane, 2000 g total, was treated with 1600 ml of alkaline hydrogen peroxide solution according to the conditions of Example 11. The total weight of sodium hydroxide present in the alkaline hydrogen peroxide solution was 32 g. The weight concentrations of the NaOH, C₂H₅OH and H₂O₂ components of the alkaline hydrogen peroxide reagent were 2.0%, 62.5% and 8.75%, respectively, as in Example 11. After work up according to the procedure of Example 11, the neem oil was analyzed for sulfur and found to contain 268 ppm of sulfur.
The neem oil resulting from the alkaline hydrogen peroxide treatment of Example 15 was filtered through silica to remove soaps. This was followed by bleaching on bleaching earth mixed with activated carbon at 110°C and reduced pressure (28 inches of Hg). The bleached and filtered neem oil was analyzed and found to contain 174 ppm of sulfur.
The bleached and filtered oil was next subjected to a two hour deodorization step with 3% by weight of steam per hour at 255° C and 4 mm of Hg pressure. The once deodorized neem oil was analyzed and found to contain 77 ppm of sulfur.
The once deodorized neem oil was hydrogenated for 30 minutes using a 2% by weight standard nickel on kiselghur catalyst. The resulting partially hydrogenated neem oil had a melting point of 43° C and a sulfur content of 45 ppm.
The partially hydrogenated neem oil was subjected to a second deodorization step using the conditions noted above for the first deodorization step. The resulting completely treated neem oil was analyzed for sulfur and found to contain 44 ppm using the ICP analytical method and 4 ppm using the Ra(Ni) analytical method and was colourless and odourless.
One part, by weight, of an unrefined 65% (w/w) neem oil in hexane solution was treated with one part of an alkaline solution of H₂O₂ prepared from 1 part of aqueous 35% H₂O₂ and 10 parts of methanolic 1% NaOH (v/v) for 30 minutes at 45-55° C. Analysis for sulfur indicated that the sulfur content of the neem oil was reduced from an initial value of 2200 ppm of sulfur to 95 ppm of sulfur.
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