WO1998003461A1

WO1998003461A1 - Purification of esterified propoxylated glycerin fat substitutes

Info

Publication number: WO1998003461A1
Application number: PCT/EP1997/003726
Authority: WO
Inventors: Charles Cooper
Original assignee: Arco Chemical Technology, L.P.; Arco Chemie Technologie Nederland B.V.
Priority date: 1996-07-18
Filing date: 1997-07-12
Publication date: 1998-01-29
Also published as: AU3623297A

Abstract

The free fatty acid remaining in a reaction product obtained by esterification of a propoxylated glycerin with excess fatty acid may be reduced to low levels by treating the reaction product with a solid basic compound, preferably one having a small particle size and high surface area, and then filtering. The purified composition thus obtained contains little or no residual basic compound and is suitable for use as a reduced calorie fat substitute in the preparation of food products.

Description

PURIFICATION OF ESTERIFIED PROPOXYLATED GLYCERIN FAT SUBSTITUTES

FIELD OF THE INVENTION This invention relates to methods whereby esterified propoxylated glycerin compositions useful as reduced calorie fat substitutes may be

conveniently and economically prepared. The invention provides a process wherein excess unreacted fatty acid may be removed from an esterification reaction product.

BACKGROUND OF THE INVENTION A wide variety of substances have been proposed for use as fat substitutes in food compositions. The chemical structures of such substances are selected such that they are more resistant to breakdown by the metabolic

processes of the human digestive system which normally occur upon ingestion of conventional triglyceride lipids. Because of their increased resistance to digestion and absorption, the number of calories per gram available from the fat substitutes is considerably reduced as compared to common vegetable oil, animal fats, and other lipids. The use of such substances thus enables the preparation of reduced calorie food compositions useful in the control of body

weight.

U.S. Patent No. 4,861 ,613 (incorporated herein by reference in its entirety) describes one class of particularly useful fat substitutes wherein a polyol such as glycerin is alkoxylated with an epoxide such as propylene oxide and then esterified with any of a number of fatty acids or fatty acid derivatives to form an esterified alkoxylated polyol. These substances have the physical and organoleptic properties of conventional triglyceride lipids, yet are

significantly lower in available calories owing to their pronounced resistance towards absorption and pancreatic lipase-catalyzed hydrolysis. The thermal and oxidative stability of the esterified alkoxylated polyols renders them especially suitable for use in the preparation of reduced calorie food

compositions requiring exposure to high temperatures such as fried or baked foods.

The preparation of esterified propoxylated glycerin fat substitutes from free fatty acids is described in more detail in U.S. Pat. No. 4,983,329. A propoxylated glycerin is reacted with an excess of C₁₀-C₂₄ fatty acid at an elevated temperature and the resultant product purified by methanol extraction or steam stripping and neutralization of excess fatty acid. The excess fatty acid which remains at the completion of esterification must be substantially removed prior to formulation of the fat substitute into a food composition. Residual fatty acid may adversely affect taste, odor, and stability. Generally, it will be highly desirable to attain a final fatty acid level comparable to or less

than that of refined edible vegetable oils (about 0.3 mg KOH/g acidity).

According to U.S. Pat. No. 4,983,329, the excess fatty acid remaining after methanol extraction or steam stripping may be removed by adding an

aqueous solution of potassium hydroxide or sodium hydroxide to precipitate the

fatty acid as the alkali metal salt. The precipitated fatty acid salt is then removed by filtration. While this purification method effectively removes most of the excess fatty acid, it tends to introduce unacceptably high levels of alkali

metal into the esterified propoxylated glycerin. Although the residual alkali metal level can be reduced by treatment with an adsorbent such as magnesium silicate, such further purification steps increase the batch time and cost of the process. Water washing, which is commonly used to remove alkali metal from

triglyceride fats and oils, is not practicable due to the tendency of esterified propoxylated glycerin to form persistent emulsions. It would thus be highly desirable to develop an efficient method of lowering the fatty acid content of an esterified propoxylated glycerin to a satisfactorily low level without such

complications.

SUMMARY OF THE INVENTION This invention provides a method for purifying a crude esterified

propoxylated glycerin containing free fatty acid impurities, said method comprising the steps of:

(a) contacting the crude esterified propoxylated glycerin with a basic compound in finely divided particulate form for a time and at a temperature effective to convert at least a portion of the free fatty acids to insoluble form and to form a purified esterified propoxylated glycerin having a reduced level of free fatty acids;

and

(b) separating the insolubilized free fatty acids from the purified esterified propoxylated glycerin. DETAILED DESCRIPTION OF THE INVENTION To obtain crude esterified propoxylated glycerin suitable for purification in accordance with the present invention, a propoxylated glycerin may be reacted with one or more fatty acids. Methods of preparing propoxylated

glycerin are well-known in the art and include, for example, the base-catalyzed reaction of glycerin with propylene oxide. Typically, from 2 to 20 moles of propylene oxide are employed per mole of glycerin. In one preferred embodiment, the propoxylated glycerin has the structure

O - ( - oxypropylene - )_x - H

O - ( - oxypropylene - )_y - H O - ( - oxypropylene - )_z - H wherein x, y and z are the same or different, x + y + z is from 2 to 20, and

not more than one of x, y, or z is 0. The oxypropylene units may have the structure

CH₃ CH₃

I I

- CH₂ - CH - O - , - CH - CH₂ - O - , or combinations thereof.

The fatty acid to be reacted with the propoxylated glycerin may be a single fatty acid or mixture of two or more different fatty acids, including saturated, unsaturated (including polyunsaturated), linear and branched fatty acids. Suitable fatty acids include, but are not limited to lauric acid, myristic acid, palmitic acid, stearic acid, eicosanioc (arachidic) acid, docosanoic

(behenic) acid, tetracosanic (lignoceric) acid, caproic acid, caprylic acid,

pelargonic acid, capric acid, myristoleic acid, palmitoleic acid, oleic acid, elaidic acid, vaccenic acid, gadoleic acid, cetoleic acid, erucic acid, linoleic acid, linolenic acid, and the like and mixtures thereof. Such fatty acids may be

synthetically prepared using known methods or obtained from natural sources such as triglycerides. The splitting of triglycerides from naturally occurring lipids to yield fatty acids and glycerin is well known. Unsaturated fatty acids

may be fully or partially hydrogenated prior to or subsequent to such splitting. Lipids suitable for use as fatty acid sources include, for example, high erucic rapeseed oil, canola oil, corn oil, soybean oil, sunflower seed oil, safflower seed oil, peanut oil, cottonseed oil, sesame oil, tallow, lard, butter fat, coconut oil,

palm oil, palm kernel oil, cocoa butter, and the like and mixtures thereof.

Although the esterification process may be carried out using the fatty acid(s) and the propoxylated glycerin in any proportion, it will generally be desirable where the esterified propoxylated glycerin is to be utilized as a fat

substitute to accomplish substantially complete esterification of the propoxylated glycerin. That is, at least 67% and more preferably at least 90% of the hydroxyl groups of the propoxylated glycerin are preferably transformed

into ester groups. Typically, a 5 to 40% stoichiometric excess of the fatty acid relative to the desired degree of esterification to be achieved is utilized. For example, if 80% esterification of one mole of a propoxylated glycerin is desired, the amount of fatty acid employed is preferably from about 2.5 to 3.4

moles.

The excess fatty acid serves to self-catalyze the esterification process, thus eliminating the need to employ additional acidic or metallic catalysts. If desired, however, any conventional esterification catalyst could be used. The esterification reaction may be readily monitored by standard means such as hydroxyl number and the reaction halted when the target degree of esterification is realized.

The temperature at which the propoxylated glycerin is reacted with the fatty acid is not critical, but should be sufficient to accomplish the desired degree of esterification within a practically short period of time (typically, 0.5 to 18 hours) while avoiding substantial decomposition or by-product formation.

The optimum temperature thus will vary greatly depending upon the reactants used and their relative proportions, among other factors, but typically temperatures in the range of from 150 ^° C to 300 ^° C (more preferably, 200 ° C

to 275 ° C) will be effective where the reaction is self-catalyzed. The esterification rate can be suitably enhanced by providing a means for removing or binding the water generated during esterification so as to drive the reaction to completion or near completion. For example, a reduced pressure of from about 0.01 mm up to atmospheric pressure (more preferably, from 1 to 50 mm) may be utilized to take the water overhead. An inert gas such as nitrogen, helium, an aliphatic hydrocarbon, carbon dioxide or the like may be sparged or passed through the reaction mixture in order to remove the water as it is

formed. Azeotropic distillation of the water with a suitable azeotropic agent (entrainer) such as an aliphatic or aromatic hydrocarbon will also be effective for this purpose. The use of molecular sieves or other water absorbing or

reactive substances may also be helpful in reducing the reaction time required

to achieve a high degree of hydroxy group conversion. The conditions for water removal are selected such that a minimum amount of fatty acid is taken overhead.

Depending upon the level of unreacted fatty acid remaining after esterification, it may be desirable to remove a portion of the unreacted fatty acid using steam stripping or other such technique prior to treatment with a basic compound in accordance with the present invention. Steam stripping of

esterified propoxylated glycerin to remove fatty acids is well-known and is described, for example, in U.S. Pat. No. 4,983,329 (incorporated herein by reference in its entirety) . Typically, steam stripping is performed under vacuum

at an elevated temperature (e.g., 180 ° C to 220 ^° C) where steam or water is slowly introduced beneath the surface of the liquefied esterified propoxylated glycerin. The steam assists in stripping out the fatty acid, which may be taken overhead and subsequently recycled in esterification if so desired. While steam

stripping can be effective in removing most excess fatty acid from an esterified propoxylated glycerin reaction product, for reasons which are not well understood it is exceedingly difficult to reduce the free fatty acid level below about 0.2 weight percent (calculated as oieic acid) by steam stripping. In one

embodiment of the invention, steam stripping is utilized to lower the free fatty acid content from its initial level (typically, 1 to 20 weight %) to the 0.2 to 1.0 weight % range, followed by treatment with a basic compound in particulate form to attain the final desired free fatty acid concentration in the esterified

propoxylated glycerin. The final free fatty acid concentration is generally less than 0.2 weight percent, more preferably less than 0.05 weight percent, most preferably less than 0.02 weight percent (calculated as oleic acid).

The crude esterified propoxylated glycerin is treated with a basic compound in finely divided particulate form for a time and at a temperature to convert at least a portion of remaining free fatty acids to insoluble form. The

basic compound is preferably a strong base (i.e., a material which when dissolved in water provides a solution having a pH of at least 12), but in any event should be sufficiently basic so as to react with the free fatty acid. Without wishing to be bound by theory, it is believed that the basic compound

converts the fatty acid into an insoluble salt but remains in particulate (solid) form rather than becoming solubilized or dispersed in the esterified propoxylated glycerin. As discussed earlier, when an aqueous solution of alkali metal hydroxide is added to an esterified propoxylated glycerin in order to

neutralize fatty acid, as has been proposed in the prior art, some of the alkali metal in the product obtained is present in a form which cannot be readily removed by filtration means. In the process of the present invention, however, alkali metal levels following filtration are exceedingly low, e.g., 5 ppm or less. The need for further treatment with an adsorbent such as magnesium silicate is thus avoided.

The amount of basic compound employed is not critical, but should be sufficient to effect the desired reduction in free fatty acid concentration.

Generally speaking, at least one equivalent of basic compound per equivalent of free fatty acid in the crude esterified propoxylated glycerin is utilized. The

optimum quantity of basic compound will vary depending upon a number of factors such as the basicity and surface area of the basic compound, contact

temperature, free fatty acid concentration, contact time and so forth. Typically, however, from 0.05 to 2 parts by weight basic compound per 100 parts by weight crude esterified propoxylated glycerin will be suitable.

Preferred basic compounds for use include alkali metal hydroxides (e.g., sodium hydroxide, potassium hydroxide), alkaline earth metal hydroxides (e.g., calcium hydroxide, barium hydroxide, magnesium hydroxide), alkali metal oxides (e.g., sodium oxide, potassium oxide), and alkaline earth metal oxides (e.g., calcium oxide, barium oxide). Generally speaking, the use of strong bases is preferred. Other suitable basic compounds include but are not limited to, alkali metal and alkaline earth metal salts of carbonic acid and phosphoric acid such as sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, sodium phosphate (monobasic, dibasic, tribasic),

potassium phosphate (monobasic, dibasic, tribasic), sodium pyrophosphate, potassium pyrophosphate, calcium carbonate, calcium phosphate (monobasic, dibasic, tribasic), magnesium phosphate (monobasic, dibasic, tribasic), magnesium carbonate, and the like and mixtures thereof. The basic compound

selected must be insoluble in the esterified propoxylated glycerin at the treatment temperature. For this reason, the use of inorganic basic compounds is preferred. Additionally, the basic compound should be in finely divided particulate form such that a relatively high surface area is available for reaction

with the excess fatty acid. The particle size of the basic compound should, however, be sufficiently large to permit facile separation from the esterified propoxylated glycerin following the contacting step. Preferably, the basic compound has an average particle size of less than 500 microns and a surface

area of at least 10 m²/g.

In one embodiment of the invention, the basic compound is impregnated or otherwise incorporated into a support, preferably a support which is porous and/or has a high surface area. Such support materials are well-known and include, for example, inert refractory oxides such as silica, alumina, titania, zirconia and the like, activated carbon, fire brick, clay, bauxite, bentonite,

kieselguhr, diatomaceous earth, silicon carbide, pumice, molecular sieves (including zeolites), and the like. The support, in effect, assists in dispersing the basic compound in finely divided form so as to create a higher surface area available for reaction with the excess fatty acid. At the same time, the support

may be formed into relatively large granules, pellets, aggregates, pills, spheres, cylinders, rings or the like which can be more readily handled than when the

basic compound in finely divided form is utilized in unsupported form. The support also improves performance by lowering the pressure drop through the filter bed or fixed bed and by facilitating heat and mass transfer. The amount of basic compound is preferably from about 1 to 15 weight percent of the

support.

Treatment with the basic compound is preferably performed under

substantially anhydrous conditions, as the presence of water tends to result in high levels of solubiiized residual basic compound in the esterified propoxylated

glycerin and/or emulsion formation. The amount of water present should be

sufficiently low so as to avoid solubilization of the basic compound and to

maintain a single liquid phase during the treatment step (i.e., separation of an aqueous phase from the esterified propoxylated glycerin phase is avoided).

Typically, no more than about 5000 ppm water is present. Organic solvents

may be used to dilute the esterified propoxylated glycerin, but are preferably avoided since the amount of solvent present must subsequently be reduced to a level acceptable for food purposes,

The crude esterified propoxylated glycerin should be contacted with the basic compound at a temperature sufficient to result in the desired conversion

of the free fatty acid impurities to insoluble form within a practicable period of time. The contact temperature can vary considerably, depending upon a number of factors such as the surface area and reactivity of the basic compound, but typically is in the range of from about 20 ° C to 200° C. At a

minimum, the temperature should be such that the esterified propoxylated glycerin is melted or in liquid form so as to facilitate and promote contact between the basic compound and the esterified propoxylated glycerin. Higher temperatures will, in general, increase the rate at which the fatty acid is

converted to insoluble form and thus shorten the processing time. Excessively high temperatures should be avoided, however, in order to minimize potential problems with discoloration and generation of undesired byproducts.

Following treatment with the basic compound for the desired period of

time, the esterified propoxylated glycerin is separated from the remaining particulate basic compound (if any) and the insolubilized excess fatty acid. This separation may be accomplished by any of the methods known in the art for separating solid particles from a liquid matrix such as decantation,

centrifugation, or, most preferably, filtration. The use of a filter-aid such as diatomaceous earth or the like may be advantageous in order to assist in the

desired separation.

In one desirable embodiment of the invention, the crude esterified propoxylated glycerin is passed through a fixed bed of the basic compound. For example, the basic compound may be impregnated in a porous support as

previously described and the impregnated support placed in a column or other such tubular reactor. The crude esterified propoxylated glycerin is introduced

into one end of the packed column, permitted to flow over the impregnated support at a temperature effective to cause the desired removal of free fatty

acid, and then withdrawn in purified form at the opposite end of the column. The effluent may be continuously recycled through the bed until the desired reduction in free fatty acid content is achieved.

Following separation of the basic compound and insolubilized fatty acids,

the resulting esterified propoxylated glycerin is frequently sufficiently pure to utilize directly as a food ingredient. In particular, the levels of residual basic

compound and derivative products thereof in the esterified propoxylated glycerin are typically quite low. For instance, where the basic compound used is an alkali metal hydroxide or oxide, the total concentration of alkali metal (Na, K, etc.) in the product is generally 5 ppm or less.

If desired, however, the esterified propoxylated glycerin may be subjected to further processing or purification steps known in the art such as, for example, deodorization, hydrogenation, bleaching, decolorization,

stabilization, or the like.

The recovered basic compound could, if so desired, be regenerated for

reuse in the process of the invention, particularly where the basic compound is impregnated in a porous support. Such regeneration may include heating at a high temperature in the presence of air or the like in order to decompose or burn off any fatty acid or other organic substances associated therewith. Reimpregnation of the recovered support with basic compound following regeneration may be desirable.

Examples

Example 1

An esterified propoxylated glycerin sample (100 parts by weight) having a melting point of approximately 40 ° C and an acidity level of about 0.3 weight % (calculated as oleic acid) was heated to above the melting point in order to

liquefy the sample. Pelletized sodium hydroxide (0.2 parts by weight) was ground to a powder and combined with the liquefied sample. The resulting mixture was stirred at a temperature somewhat above the melting point for 30 minutes and then filtered through a bed of "Celite" diatomaceous earth filter-

aid. The filtrate contained less than 0.01 weight % acidity (calculated as oleic acid) as measured by titration with base.

Example 2

Pelletized sodium hydroxide (1 part by weight) was mixed and mulled with a liquid esterified propoxylated glycerin sample ( 1 part by weight; initial acidity ca. 0.3 weight %) having a melting point less than 25 " C. A portion (0.5 parts by weight) of the mulled mixture was mixed with an additional quantity ( 100 parts by weight) of the esterified propoxylated glycerin and stirred at 30-40 ° C for 30 minutes. After filtration, the acidity was found to be less than 0.01 weight % (calculated as oleic acid).

Example 3 - Comparative

Liquid esterified propoxylated glycerin (approx. 100g) having an acid value of 0.3 weight % was poured through a column of solid sodium hydroxide

pellets

approximately 0.25 inches in diameter in a 100 mL burette. The acidity of the eluent was still relatively high (0.2 weight %), indicating the criticality of having the basic compound in finely divided particulate form.

Example 4

A quantity (approx. 0.5L) of the purified esterified propoxylated glycerin prepared in Example 1 was placed in a small commercial deep fat fryer and

heated to 175 ° C. Blanched sliced potatoes were added in batches over a

period of approximately 5 hours. No smoking of the esterified propoxylated glycerin was observed, foaming was extremely low, and the fried potatoes had excellent taste and mouth feel.

Examples 5-7

To demonstrate that low levels of residual alkali metal may be attained using the process of the invention even without post-treatment with magnesium silicate adsorbent, samples of esterified propoxylated glycerin (100 parts by weight) were each treated with sodium hydroxide in powder form (0.2 parts by weight). After stirring for approximately one hour at about 90 ° C, the

mixtures were filtered through a bed of "Celite" diatomaceous earth. In

Examples 5 and 6, the quantity of "Magnesol" magnesium silicate shown in the following table (based on esterified propoxylated glycerin weight) was added

to the filtrate and the mixtures heated for another hour at 90 ° C before filtering a second time. In each case, the level of residual sodium was satisfactorily low (0.2 ppm), even where no post-treatment with adsorbent was performed.

Example Magnesium Silicate, wt.% Residual Sodium,

ppm

5 1 0.2

6 0.5 0.2

7 0 0.2

Claims

Claims:

1. A method for purifying a crude esterified propoxylated glycerin containing free fatty acid impurities, said method comprising the steps of:

(a) contacting the crude esterified propoxylated glycerin with

a basic compound in finely divided particulate form for a

time and at a temperature effective to convert at least a portion of the free fatty acids to insoluble form and to form a purified esterified propoxylated glycerin having a reduced level of free fatty acids; and

(b) separating the insolubilized free fatty acids from the purified esterified propoxylated glycerin.

2. The method of claim 1 wherein the basic compound is an inorganic strong base.

3. The method of claim 1 performed in the substantial absence of water and organic solvent.

4. The method of claim 1 wherein the crude esterified propoxylated glycerin is obtained by the esterification of a propoxylated glycerin with excess

fatty acid.

5. The method of claim 1 wherein the temperature is from 20 ° C to

200 ° C.

6. The method of claim 1 wherein the basic compound is selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal oxides, alkaline earth metal oxides, and mixtures thereof.

7. The method of claim 1 wherein the free fatty acid impurity level of the crude esterified propoxylated glycerin is from 0.2 to 1 weight percent calculated as of oleic acid.

8. The method of claim 1 wherein the level of free fatty acid in the

purified esterified propoxylated glycerin is less than 0.05 weight percent calculated as oleic acid.

9. The method of claim 1 wherein the basic compound is impregnated in a porous support.

10. The method of claim 9 wherein said contacting is accomplished by passing the crude esterified propoxylated glycerin through a bed of the porous support impregnated with the basic compound.

11. The method of claim 1 wherein from 0.05 to 5 parts by weight of the basic compound per 100 parts by weight of the crude esterified propoxylated glycerin is utilized.

12. The method of claim 1 wherein the basic compound has a surface

area of at least 10 m²/g.

13. A method for purifying a crude esterified propoxylated glycerin containing free fatty acid impurities and obtained by esterification of a propoxylated glycerin with excess free fatty acid, said method comprising the

steps of:

(a) contacting the crude esterified propoxylated glycerin with a strong inorganic base selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal oxides, alkaline earth metal oxides, and

mixtures thereof in finely divided particulate form in the

substantial absence of water and organic solvent for a time and at a temperature effective to convert at least a portion of the free fatty acids to insoluble form and to form purified

esterified propoxylated glycerin having a reduced level of free fatty acid; and

(b) filtering to separate the insolublilized fatty acids from the purified esterified propoxylated glycerin.

14. The method of claim 13 wherein the strong inorganic base has a surface area of at least 10 m /g.

15. The method of claim 13 wherein the strong inorganic base is impregnated in a porous support.

16. The method of claim 13 wherein the propoxylated glycerin contains from 3 to 16 oxypropylene units per equivalent of glycerin.

17. The method of claim 13 wherein prior to step (a) a portion of the excess fatty acid is removed by vacuum steam stripping.

18. The method of claim 13 wherein the crude esterified propoxylated glycerin comprises from 0.2 to 1 weight percent free fatty acid calculated as

oleic acid.

19. The method of claim 13 wherein the purified esterified

propoxylated glycerin has an alkali metal content of 5 ppm or less.

20. The method of claim 13 wherein the strong inorganic base is an alkali metal hydroxide selected from sodium hydroxide, potassium hydroxide,

and mixtures thereof.