EP0189049B2 - Stabilization of palm kernel oil fatty-acid methyl esters for their processing into subsequent reaction products with a neutral colour - Google Patents

Description

Today, coconut oil and palm kernel oil are important natural raw materials for the production of fat chemical secondary products on an industrial scale. The chemical preparation of this natural starting material is generally preceded by the conversion of the triglyceride obtained to the corresponding fatty acid esters with lower monofunctional alcohols, in particular to the corresponding methyl esters. Here, the methyl ester or the fatty acid methyl ester mixture can be obtained by transesterifying the triglyceride directly with methanol or via the triglyceride cleavage to fatty acid and glycerol and subsequent esterification of the fatty acids or fatty acid mixtures to give the alkyl esters.

A comparison of the quality - determined in particular in terms of storage behavior, oxidation stability and stabilization options and in particular with regard to the influence on the color values of subsequent reaction products from these stored feedstocks - shows that coconut oil or the reaction products obtained from coconut oil behave much better than palm kernel oil and its reaction follow-up products. Coconut oil is much more stable to oxidation than palm kernel oil. The same applies to the methyl esters or methyl ester mixtures produced therefrom. The reason for this can only partially be the slightly increased iodine numbers of palm kernel oil compared to the iodine numbers of coconut oil. Both fat raw materials contain natural antioxidants, essentially in the form of α-tocotrienol. At 21 ppm, coconut oil contains significantly more antioxidant than palm kernel oil (3 ppm), compare H. Behringer et al. in Fette, Seifen Anstrichmittel 78 (1976) pp. 228-231. Ullmann's Encyclopedia of Industrial Chemistry, 4th edition, volume 8, 1974, Verlag Chemie GmbH, Weinheim / Bergstrasse, pp. 26-32, discloses the use of a number of phenolic compounds as antioxidants for fats and oils. Di-3,5-tert-butyl-4-hydroxy-toluene is on p. 25 as an antioxidant and others. described for animal and vegetable fats and oils.

While coconut oil and coconut fatty acid methyl ester are usually stable against autoxidation for months at room temperature, this is not the case for palm kernel oil and palm kernel fatty acid methyl ester. The applicant was able to demonstrate that palm kernel oil and palm kernel fatty acid methyl ester take up oxygen significantly faster than coconut oil and cocofatty acid methyl ester. Corresponding studies have already shown that palm kernel oil itself has a 10- to 25-fold greater oxygen uptake than coconut oil. In the stricter aging test, in which the oxygen uptake of natural fats and oils and fat and oil derivatives is measured in a manometer device according to Warburg as a measure of the oxidation behavior, the induction time of the palm kernel oils, after which a strong oxygen uptake occurs, is around 80 ° C 10 to 20 hours, while in comparative studies with coconut oil after oxygen measuring times of 50 or 90 hours, no oxygen absorption can be determined.

These quality differences between palm kernel oil and its derivatives on the one hand and coconut oil and its derivatives on the other hand mean that the respective starting materials and / or their reaction products are damaged differently during storage, transport and / or processing and accordingly result in considerable quality differences.

The invention is based on the object of eliminating the undesirable instability of palm kernel oil or the reaction products obtained therefrom. However, the obvious idea to compensate for the deficiency of palm kernel oil in natural antioxidants by the addition of known antioxidants led to the surprising finding that satisfactory reaction products cannot easily be obtained with this concept.

Important figures for the usability of a fat raw material on a natural basis are color number values, which are measured on soap solution - obtained by alkaline saponification of the corresponding fatty acid methyl esters or fatty acid methyl ester mixtures - or on fatty acid diethanolamides - obtained by amidating cleavage of the fatty acid methyl ester starting materials. These color numbers are decisive parameters for the usability of both the oils originally used and the methyl esters made from them.

The object of the invention in the narrower sense is accordingly the stabilization of palm kernel fatty acid methyl esters against harmful influences during storage in the presence of air in such a way that reaction products of such color neutrality can be obtained in the subsequent reaction of the stored oleochemical starting materials that no undesirable color deteriorations have to be accepted. The term "color neutrality" here and in the following description of the invention means a color stability within the scope of the individual reaction stages, which approximately corresponds to the color stability of coconut oil of normal quality and its secondary products known and accepted in practice. The invention thus aims to make the palm kernel fatty acid methyl esters obtainable from palm kernel oil comparable in practical handling and processing to the corresponding materials based on coconut oil.

The investigations carried out to solve this task have surprisingly shown that the selection of two very specific - known per se - oxidation stabilizers ensures the desired success. The use of other oxidation stabilizers, which due to general chemical knowledge can be regarded as equivalent, does not lead to the result of the previously defined color neutrality in the reaction secondary products from palm kernel fatty acid methyl esters.

The invention accordingly relates to the use of di-3,5-tert-butyl-4-hydroxytoluene in amounts of 10-100 ppm as the exclusive stabilizing agent in palm kernel fatty acid methyl esters of the C₁₂-C₁₈ fatty acid range for their processing into color-neutral reaction products.

It has been shown that even the addition of very small amounts of BHT, palm kernel fatty acid methyl ester at room temperature can stabilize for a long time, for example up to 2 months, in such a way that alkanolamides or soaps produced therefrom have the desired good color values. Further information on the desired color values can be found in the example section.

Said palm kernel fatty acid methyl esters of different provenance can be stabilized by adding small amounts of BHT according to the invention to standard-compliant starting materials. It is advisable to add the stabilizing additives used according to the invention to the fatty acid methyl ester or fatty acid methyl ester mixture immediately after production - or to clean an insufficient fatty acid methyl ester mixture by preferably multiple distillation and to stabilize the distillate immediately.

If other known antioxidants are used instead of the stabilizers used according to the invention, surprisingly, the required color neutrality is not obtained in the reaction secondary products of the palm kernel fatty acid methyl ester. For example, the use of 2-tert-butylhydroquinone or the use of butylated hydroxyanisole and the use of propyl gallic acid do not show the stabilization effect aimed for according to the invention. High color number values are obtained for the saponification products and for the diethanolamides both in the case of the tert-butylhydroquinone derivative and in the case of the propyl gallate. The use of the butylated hydroxyanisol initially leads to undesirably high saponification color numbers with higher additions, for example in amounts above 500 ppm, but after prolonged storage, undesirably high diethanol amide color numbers are also obtained.

BHT can deliver satisfactory results in the sense of the task according to the invention even in amounts of 10 to 100 ppm and in particular in the range of about 20 to 50 ppm. When stored in metal containers, for example in steel tanks, it can be advantageous to add complexing agents for metal ions to the material to be stabilized, for example citric acid, ascorbic acid or ascorbic acid fatty alcohol ester, in particular ascorbic acid palmitate.

The palm kernel fatty acid methyl esters to be stabilized according to the invention are obtained by known processes either by fat cleavage of the palm kernel oil and subsequent esterification of the fatty acid mixture obtained with methanol or by direct transesterification of the palm kernel oil with methanol.

It is desirable that the palm kernel fatty acid alkyl esters intended for chemical processing contain only fatty acids with 12 to 18 carbon atoms as the acid component. From this point of view, the so-called leading fatty acids with 6 to 10 carbon atoms are separated from the mixtures of the free fatty acids by fractional distillation in the direct esterification. If the palm kernel fatty acid alkyl esters are obtained by transesterification, the undesired esters of C₆-C₁₂ fatty acids are removed from the ester mixture by distillation after the transesterification. The invention accordingly relates in a particular embodiment to palm kernel fatty acid methyl ester mixtures of the C₁₂-C₁₈ fatty acid range, prepared by esterification of palm kernel fatty acids having 12 to 18 carbon atoms with methanol or by transesterification of palm kernel oil with methanol and isolation of the C₁₂-C₁₈ fatty acid ester cut, exclusively as a stabilizing agent added di-3,5-tert-butyl-4-hydroxytoluene in amounts of 10 to 100 ppm for the production of color-neutral reaction products.

Examples

In the following examples, the following key figures are used as a measure of the stability of the palm kernel fatty acid esters: peroxide number (POZ), carbonyl number (COZ), saponification color numbers (VFZ) and diethanolamide color number (DEAFZ). These key figures were determined as follows:

Peroxide number (POZ)

Hierbei bedeuten:
   a = Verbrauch an Natriumthiosulfat-Maßlösung (ml)
   N = Normalität der Natriumthiosulfat-Maßlösung
   E = Gewicht der Probe (g)The POZ was determined using the Wheeler method - see DGF standard methods C-VI 6a (84), Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart (1950-1984) reacted a sample of the ester in a mixture of chloroform and glacial acetic acid with potassium iodide in the cold and the iodine formed by the peroxide present was determined titrimetrically using starch solution as an indicator using starch solution. The POZ in mVal O₂ / kg is calculated using the formula $$\text{POZ =}\frac{\text{a · N,000}}{\text{E}}$$

Here mean:
a = consumption of sodium thiosulfate standard solution (ml)
N = normality of the standard sodium thiosulfate solution
E = weight of the sample (g)

Carbonyl number (COZ)

   a = Verbrauch an 0.5n Natriumhydroxidlösung (ml)
   E = Gewicht der Probe (g)The COZ was also determined using a DGF standard method. See loc. cit. CV 18 (58). A sample of the ester is heated to reflux with a 0.5 N aqueous alcoholic hydroxylamine chlorohydrate solution for a long time. The released hydrogen chloride in the cooled mixture is then titrated with 0.5 N alcoholic sodium hydroxide solution using a pyridine / bromophenol blue indicator. The COZ is calculated using the formula $$\text{COZ =}\frac{\text{a14}}{\text{E}}$$

a = consumption of 0.5n sodium hydroxide solution (ml)
E = weight of the sample (g)

Saponification color numbers (VFZ)

To determine the VFZ, 10 g of fatty acid ester are heated to reflux with 100 ml of methanolic 1N potassium hydroxide solution for 1 hour. The VFZ of the cooled sample are measured using a 5 1/4 '' cuvette in a Lovibond tintometer. For good quality esters the values red <1.0 and yellow <5 should be observed.

Diethanolamide color numbers (DEAFZ)

To determine the DEAFZ, equivalent amounts of fatty acid ester and diethanolamine in the equivalent of 1 percent by weight, based on the total amount, of sodium methylate at 85 ° C. are reacted with one another over a period of 5 hours. The methanol released is continuously distilled off. The DEAFZ are measured on the cooled reaction product using a 5 1/4 '' cuvette in a Lovibond tintometer. The color of the diethanolamide can be described as satisfactory for red <15 and yellow <15 (corresponding to a Gardner color number of <3).

Examples 1 and 2

In an autoclave, palm kernel oil was split with water at 220 ° C. After washing out the glycerol, the C₆₋₁₀ fatty acids were largely distilled off from the fatty acid mixture obtained, so that a mixture consisting essentially of C₁₂₋₁₈ fatty acids receded.

The C₁₂₋₁₈ palm kernel fatty acid mixture obtained was reacted in an autoclave with methanol in a weight ratio of 1: 1 at 220 ° C. under autogenous pressure over the course of 2 hours. Methanol and water of reaction were distilled off from the resulting mixture. By repeated reaction with excess methanol and distillation, a C₁₂₋₁₈ palm kernel fatty acid methyl ester mixture was obtained, which had the following characteristics: acid number 0.3; Saponification number approx. 238; Iodine number 18.

POZ, COZ, VFZ and DEAFZ were determined on the freshly distilled ester. Samples of the ester were mixed with 10 ppm of 3,5-di-tert-butyl-4-hydroxytoluene (BHT; commercial product) as a stabilizer. These samples and a stabilizer-free sample (approx. 500 ml each) were stored in 500 ml wide-mouth bottles with a loosened screw cap at approx. 20 ° C in the dark for 6 weeks. After that, the above-mentioned key figures were determined again.

In the case of Example 2, a C₁₂₋₁₈ palm kernel fatty acid methyl ester prepared by the above method was stabilized with 10 ppm BHT.

The results found are shown in Table I below.

Example 3

C₁₂₋₁₈ palm kernel fatty acid methyl ester, which had been stored for more than 6 weeks and showed POZ 40 and COZ 0.6, was distilled twice, with a total of 3.7 percent by weight residue being obtained. The distillate had the key figures: acid number 0.3; Saponification number 241; Iodine number 14.

POZ, COZ, VFZ and DEAFZ were measured on the freshly distilled ester. Samples of the ester were treated with 10 or 50 ppm of 3,5-di-tert-butyl-4-hydoxytoluene (BHT; commercial product), as substances to be used according to the invention and 50 ppm of 2-tert-butylhydroquinone (TBHQ; commercial product) and 50 ppm 2-tert-butylhydroxyanisole BHA; Commercial product) added as a reference substance. These samples and a stabilizer-free sample (approx. 500 ml each) were stored in 500 ml wide-mouth bottles with a loosened screw cap at approx. 20 ° C in the dark for 4 weeks. Then the above-mentioned key figures were determined again. The results found are shown in Table II below.

From Table II it can be seen that a methyl kernel fatty acid ester already damaged by oxidation can be effectively stabilized after renewed distillation with the help of the substances to be used according to the invention, so that after longer storage with the stabilized ester, secondary products of better color quality can be obtained than with the unstabilized esters.

Example 4

• A) Palmkernfettsäuremethylester ohne Zusatz,
• B) Palmkernfettsäuremethylester + 10 ppm BHT,
• C) Palmkernfettsäuremethylester + 50 ppm BHT,
• D) Kokosfettsäuremethylester ohne Zusatz.

In a manometer device according to Warburg (see laboratory technology for biochemists, published by B. Keil and Z. Sormova, Academic Publishing Company Geest & Portig KG. Leipzig 1965, pp. 247-249), samples of the C₁₂₋₁₈-palm kernel fatty acid methyl ester described in Example 1 were also included and without stabilizer additive and, as a comparative substance, a sample of a C₁₂₋₁₈ coconut fatty acid methyl ester (acid number 0.3; saponification number 240; iodine number 11) is subjected to the more severe aging test. The samples (each 3g sample weight were exposed to oxygen at 80 ° C and the oxygen uptake was continuously monitored. The following samples were examined in detail:

• A) palm kernel fatty acid methyl ester without addition,
• B) palm kernel fatty acid methyl ester + 10 ppm BHT,
• C) palm kernel fatty acid methyl ester + 50 ppm BHT,
• D) Coconut fatty acid methyl ester without additive.

The test results found are shown graphically in FIG. 1. It can be seen from this that an addition of 50 ppm BHT was sufficient to give the palm kernel fatty acid methyl ester the same stability as that of a comparable coconut fatty acid methyl ester.

Claims (3)

1. The use of di-3,5-tert.-butyl-4-hydroxytoluene in quantities of 10 to 100 ppm as sole stabilizer in palm kernel oil fatty acid methyl esters of the C₁₂₋₁₈ fatty acid range for the processing thereof to neutral coloured secondary reaction products.
2. The use claimed in claim 1, characterized in that palm kernel oil fatty acid methyl esters obtained by transesterification of palm kernel oil with methanol are present as the material to be stabilized.
3. Palm kernel oil fatty acid methyl ester mixtures of the C₁₂₋₁₈ fatty acid range - produced by esterification of palm kernel oil fatty acids containing 12 to 18 carbon atoms with methanol or by transesterification of palm kernel oil with methanol and isolation of the C₁₂₋₁₈ fatty acid ester cut - containing added di-3,5-tert.-butyl-4-hydroxytoluene in quantities of 10 to 100 ppm as sole stabilizer
   for the production of neutral coloured secondary reaction products.