GB2031290A - Refining edible oils and fats - Google Patents

Abstract

Edible oils and fats of improved characteristics are obtained by the removal of partial glycerides. This removal may be carried out during refining by subjecting the oil or fat to molecular distillation prior to the deodorising step.

Description

SPECIFICATION Improvements in or relating to the refining of oil and fat This invention relates to a process for refining oil and fat and, more particularly but not exclusively, is concerned with improving the quality of edible refined oils and fats.

Edible animal or vegetable oils and fats are often used for frying. During frying, food materials are usually subjected to a heating treatment at temperatures of about 1 800 C. Under such severe conditions, the oils and fats used for frying gradually exhibit phenomena such as increasing acid value, lowering smoke point, increasing viscosity, coloration, lowering of iodine value, increasing carbonyl value and degrading of the flavour. Of these phenomena, the decrease in smoke point and the increase in acid value cause difficulties in frying because of the formation of volatile materials and evaporation and coloration imparts undesirable color to the fried foods and also has the economically undesirable result of requiring the oil or fat to be discarded and replaced by fresh oil or fat after a relatively short time.On the other hand, the decrease in iodine value and the increase in carbonyl value are the phenomena caused by polymerisation or decomposition of the oil and fat, and these are undesirable from the point of view of nutrition. Further, the degradation of the flavour of the oil and fat imparts and unpleasant odor to the fried foods and this makes them less appetising.

It is an object of the present invention to improve the quality of edible oil and fat by improving the frying characteristics, e.g. by minmising the lowering of the smoke point during frying, the formation of color and the lowering of the iodine value, and by improving the flavour of the fried foods and the flavour-stability of the oil or fat Conventionally, crude oil or fat is refined by subjecting it to various treatments such as degumming, deacidifying, decoloring and deodorizing and the oil or fat is often also subjected to oil or fat-processing steps such as hydrogenation and factionation.

It has been found that the quality of edible oil or fat can be improved by distilling the oil or fat so as to obtain a small amount of distillate mainly comprising partial glycerides. This may be effected by including a molecular distilling step just before the deodorizing treatment during the refining procedure.

The molecular distillation is advantageously carried out using a wiped film or centrifugal technique at a vacuum of about 10-2 to 10-3 mm Hg and a temperature of about 190 to 2300 C. Optionally the refining procedure may be combined with processing steps such as hydrogenation or fractionation. In this way a refined oil or fat specifically suitable for frying can be obtained. Furthermore, since the oil or fat produced according to the present invention has improved flavour stability, it can also be used to improve the quality of ordinary processed foods other than fried products.

Conventionally the refining of crude oil and fat comprises the following treatments. Firstly, the crude material is degummed so as to remove impurities. This is effected by the use of an inorganic or organic acid. Then the material is deacidified by treatment with an aqueous alkali solution such as caustic soda. This removes the free fatty acid. The material is then decolored by use of an adsorbing agent such as activated clay or active carbon. Finally the material is de-odorised to remove the odorous components therefrom. Recently, there has widely been used for refining palm oil or the like a process where the use of alkali to deacidify is dispensed with. Instead, the free fatty acid is simultaneously distilled out during the deodorising treatment. This is called steam refining.The present invention is applicable both in the case where deacidification is effected by the use of alkali and in the case where the steam refining technique is used. Further, the molecular distilling step can also be used to simultaneously distil out and remove the free fatty acid.

As palm oil is an inexpensive oil and has an excellent oxidation-stability even without particularly subjecting it to a hydrogenating process, its use has increased noticeably in recent times and it is expected to be used even more frequently in the future. Despite its merits, crude palm oil ordinarily has a hydroxyl value of from 4 to 12 (usually 6 to 8) and contains a large amount of carotenoid pigments.

Even though it can be produced as a pale-colored and odorless oil by ordinary refining, it is apt to develop flavour reversion, and its falvour stability is poor. Further, when used for frying, it develops particularly noticeable flavour reversion, its smoke point is lower than other liquid oils and the lowering of it's smoke point with the lapse of time is marked. In spite of the increase in production and use of plam oil, the solutions to these defects have not hitherto been satisfactory.

The process of the present invention is particularly suitable for improving the quality of plam oil (and hydrogenated plam oil) although it is also useful for other animal or vegetable fats and oils.

Crude palm oil is a solid fat having the following physical properties: acid value 2 to 15; saponification value 196 to 202; iodine value 48 to 56; hydroxyl value 4 to 12; melting point 38 to 450C. ("Progress in the Chemistry of Fats and other Lipids" 15 5 (1977)). It is extracted from the fruit of the oil palm, but it is hydrolyzed by the action of the enzyme that is present in a large amount in the fruit and thereby the free fatty acid content is rapidly increased and, as a consequence, the acid value is also rapidly elevated. When the free fatty acid content of the oil or fat is thus increased the amount of partial glyceride such as monoglyceride, diglyceride, etc. is increased and hence the hydroxyl value increases.

For this reason, attempts have been made to prevent such increases in acid value and hydroxyl value by shortening the time required for extracting the oil from the fruit. These have not yet been fully satisfactory.

Refined palm oil produced by the aforementioned conventional refining process, contains partial glycerides and thus the hydroxyl value cannot be lowered. Hence during frying, the oil is readily subjected to hydrolysis which promotes degradation. We have discovered that the frying characteristics of the oil can be remarkably improved by reducing the content of partial glycerides and at the same time the hydroxyl value by, for example, molecular distillation.

Accordingly, the present invention enables an oil or fat to be improved by including in the course of the refining process a molecular distilling step just before the deodorizing treatment. Such molecular distillation not only effectively removes partial glycerides as mentioned above (and thereby lowers the hydroxyl value) but also has the effect of increasing, and minimising the lowering of, the smoke point.

However, the reason why the refined palm oil obtained using a refining process in accordance with the invention is noticeably improved in its frying characteristic cannot be explained merely because of the reduction of the partial glyceride content. That is to say, it is impossible to explain fully such affects as the prevention of coloring, the minimising of iodine value reduction, and the prevention of flavour degradation of the fried foods solely on the basis that hydrolysis is prevented because of the removal of partial glyceride. On the other hand, the substantial reduction of the hydroxyl value as a result of molecular distillation in accordance with the present invention can serve as an indication that the molecular distillation has been conducted effectively.As the plam oil has a hydroxyl value of 4 to 12 (usually 6 to 8) as mentioned above, the process of the present invention can result in a lowering of the hydroxyl value of the refined plam oil to less than 3. The fact that refined palm oil having a hydroxyl value of less than 3 as a result of molecular distillation has the characteristics mentioned above has been confirmed by our experiments though the reasons therefor still remain uncertain.

Generally speaking, animal and vegetable crude oils and fats have various hydroxyl values and there are some that have hydroxyl values as low as less than 3, even without being subjected to molecular distillation. Nevertheless it is desirable, for improving their frying characteristics, for the hydroxyl values to be reduced as far as possible through molecular distillation. It has been confirmed through our experiments that even in the case of oils and fats having hydroxyl values less than 3, various affects, including the improvement of frying characteristics, can be obtained when the hydroxyl value is reduced to less than 1.

With regard to the improvement of the flavour stability of oils or fats, this is required when the oil or fat is to be used for ordinary processed foods other than fried products. Conventional refined palm oil and the processed foods produced using the same generally give out various odours during the preservation period which cause degradation of the flavour and lower the commercial value of the food.

The odour that is developed in the initial stage is called flavour reversion. This is the odour developed when the peroxide content of the oil or fat is still small, and is considered to be due to the formation of oxidised products and decomposed unsaturated acids, such as linoleic acid, particularly as a result of the formation of various carbonyl compounds. As a method for evaluating the flavour-stability, there has often been practised an oven-test, according to which oil or fat or the products thereof are stored at 630C and thereby the degree of oxidization is evaluated. It is expressed by the number of days required until the flavour reversion is sensed or the peroxide content reaches a definite value.But such qualitiative tests include several uncertainties, and recently a method of directly measuring quantitatively the amount of decadienal, the odorous component, in the oil and fat by gas chromatography has been reported. We have therefore demonstrated the improvement of the flavour-stability achieved by the present invention by quantitatively measuring the amount of decadienal (which is said to be the substance that directly affects the flavour) as well as by reference to more qualitative tests.

Crude palm oil contains a large amount of carotenoid pigments as already mentioned and has a red color. Conventionally these pigments are partially removed by decoloring and hydrogenating steps, and they undergo a change (e.g. by decomposition or the like) during the deodorizing treatment whereby a pale-colored refined oil is obtained. The fresh refined oil has a pale color and a good flavour, but when it is subjected to the oven test, it readily exhibits the characteristic flavour reversion. One of the causes of this is the linoleic acid present in the palm oil However it is also believed that the modified carotene substances which remain in the refined oil are readily oxidized and produce the characteristic flavour reversion.On the other hand, refined palm oil obtained in accordance with the present invention in which the molecular distilling step is included, exhibits very little flavour reversion. Thus, it has been found that the present invention prevents flavour reversion and it is believed that this is due to the flavour reversion-causing substance being removed or reduced during the molecular distillation.

Although it is possible to delay the increase in peroxide content of the palm oil by reducing the linoleic acid content by hydrogenation, it is still difficult to control the development of the flavour reversion. On the other hand, refined and hydrogenated palm oil obtained in accordance with the present invention develops flavour reversion only with difficulty.

As mentioned above, the hydroxyl value of crude palm oil is ordinarily 4 to 12 (usually 6 to 8), and this hydroxyl value of the palm oil can hardly be changed by means of the deodorizing step in the conventional refining process. On the other hand, the hydroxyl value of refined palm oil obtained according to the process of the present invention in which molecular distillation is included, can easily be lowered to less than 3. Though there are differences, depending upon the apparatus used, ordinarily 2 to 15% of the oil is removed in the moleculardistilling step as the distillate, and the hydroxyl value of the distillate is about 40 to 70. The residual oil obtained by the molecular distillation develops a slight odour which can be deodorized by conventional treatment.

As compared to conventional refined palm oils, refined palm oil obtained according to the present invention develops flavour reversion only slowly and has excellent oxidization-stability. That is to say, when the amount of decadienal formed (which is an indication of the extent of the flavour reversion) is measured by means of gas chromatography, the oil obtained by the present invention shows much less development of flavour reversion than oil obtained by the conventional method.

In the case of the refined palm oil obtained according to the process of the present invention, the emanation of irritating odour is much less in a heating test at 1 800C than is the case for conventionally refined palm oil. That is to say, when the amount of decadienal formed was measured after heating for 15 minutes, nothing was detected in the case of refined palm oil obtained according to the process of the present invention whereas from 2 to 5 ppm were detected in the case of conventionally refined palm oil.

In a frying test using palm oil refined in accordance with the present invention, doughnuts, French fried potatoes and like fried products had a light flavour and showed less changes in acid value, iodine value, carbonyl value, viscosity, smoke point and colour (which changes are used as the indication of the degradation of frying oils) and had a good flavour. Thus the oil had desirable characteristics as a frying oil.

The process of the present invention can be utilized not only for palm oil but for other animal and vegetable oils and fats. Various vegetable oils have been hydrogenated and used for the production of various confectioneries, non-dairy foods and fried foods, etc. Hydrogenated vegetable oils are desirable oils for improving the oxidization-stability of food products to be preserved and for treatment under severe conditions such as frying because of the reduction in unsaturated acid content. Further hydrogenation leads to improved oxidization stability because of the conversion of the linoleic acid, linolenic acid and the like into oleic acid.Nevertheless, when the hydroxyl value of hydrogenated vegetable oil is reduced to less than 1 by the inclusion of a molecular distilling step just before the deodorizing step during refining in accordance with the present invention, the oxidization-stability and frying characteristics are improved just as in the case of the palm oil. Example 4 shows an example of this. In the case where the present invention is utilized for animal fats, lowering of the iodine value and of the smoke point can be prevented. Example 5 shows an example using lard.

The following Examples illustrated the invention.

EXAMPLE 1 Five kilograms of alkali-refined, decolored palm oil (acid value 0.35; hydroxyl value 6.5; iodine value 54.2) was subjected to molecular distillation by means of a wiped-film still at a temperature of 205+5 C and an av. degree of vacuum of 2 x 10-3 mm Hg. 4.75 kg of residual oil were obtained. In this instance, the distilled oil had an acid value of 7.9 and a hydroxyl value of 59.2 whereas the residual oil had an acid value of 0.07; an iodine value of 54.4; and an hydroxyl value of 2.5.When the residual oil thus obtained was subjected to a deodorizing operation for 120 minutes while blowing steam into the still at a temperature of 245+50C. and under a reduced pressure of 2 to 3 mm Hg, a refined oil was obtained having an acid value of 0.05; iodine value of 54.4; a hydroxyl value of 2.5; a carbonyl value of 5.2 m.e/kg; Lovibond colorimeter 5 1/4 inch cell color 1.2 R - 12.0 Y; a smoke point of 2430C; and a viscosity of 45.2 cst (400C). The following Tables 1 and 2 show the results of an oven test on the thus obtained refined oil at 630C and of a heating test at 1 800C. As is clearly apparent from these Tables, the refined oil of the present invention more slowly develops flavour reversion as compared to the oil of the comparison Examples, and the results of the heating test are favorable.

1.0 kg of the refined oil of the invention was put in a beaker made of stainless steel having a capacity of 2 litres, and the temperature was controlled to 180+20C. Then frozen potato pieces for French-frying were fried, each in 30 g for 4 minutes at intervals of 10 minutes. The test was continued for 10 hours. The acid value, iodine value, crbonyl value, smoke point, kinematic viscosity, and Lovibond (Registered Trade Mark) colorimeter color of the residual oil used for the frying are shown in the following Table 3. It is apparent from this Table that the changes in the respective items of the above oil are smaller and the heat-stability thereof is better than in the case of Comparison Example 1.

EXAMPLE 2 Ten kilograms of a degummed, decolored palm olein (acid value 10.5; iodine value 58.0; hydroxyl value 5.5; Lovibond colorimeter 5 1/4 in. cell color 6.OR - 45.0Y; metling point 22.60C.) was subjected to molecular distillation in a wiped-film still at a temperature of210+50C. and an av. degree of vacuum of 5 x 1013 mm Hg. Thereby 8.3 kg of residual oil were obtained. The residual oil thus obtained was subjected to a deodorizing operation for 120 minutes while blowing steam into the still at a temperature of 245+5 OC. and a reduced pressure of 2 to 3 mm Hg.There was obtained a refined oil having the physical properties: acid value 0.05; iodine value 57.8; hydroxyl value 2.6; carbonyl value 6.1 m.e./kg; melting point 230C.; smoke point 245 0 C.; viscosity 45.2 cst; and Lovibond colorimeter 5 1/4 in. cell color 12 R; 12.0Y. The following Tables 1 and 2 show the results of an oven test on the refined oil at 630C. and a heating test at 1 800C. As is clearly apparent, the refined oil more slowly exhibited flavour reversion as compared to the material of Comparision Example 2 and the results of the heating test were favourable.

1.0 Kg of the refined oil of the invention was put in a beaker made of stainless steel having a capacity of 2 litres, and the temperature thereof was controlled to 1 80+20C. by means of an electric heater. Doughnuts were fried in 40 g of the oil for three minutes each at intervals of 10 minutes. The test was continued for 10 hours, and Table 3 shows the acid value, iodine vlue, carbonyl value, smoke point, viscosity, and Lovibond colorimeter color of the oil used.

As compared with Comparison Example 2, the changes in properties of the oil were smaller. The refined oil of the invention had excellent heat stability and was desirable as a frying oil.

EXAMPLE 3 A degummed, declored and fractionated palm olein (acid value 8.5; iodine value 57.5; hydroxyl value 6.5; and melting point 23.00 C) was subjected to steam refining for 120 minutes while blowing steam at a temperature 245+50C. and under a reduced pressure of 2 to 3 mm Hg. An oil having an acid value of 0.15; an iodine value of 57.6; a hydroxyl value of 6.2; and a melting pointof23.20C. was obtained.The oil was then subjected to hydrogenation by using a nickel catalyst supported by kieselguhr (content of nickel 20.0%) a-hydrogen pressure of 5 kg/cm2, and a temperature of 1 40-1 7O0C. The hydrogenated oil was subjected to a decoloration treatment by using activated clay 1.0% according to a conventional method, and thereby an oil having an acid value of 0.20; an iodine value of 51.2; a hydroxyl value of 6.1; and a melting point of 34.00C was obtained. 100 kg of this oil were subjected to molecular distillation in a centrifugal still at a temperature of21O+50C. and an av.

degree of vacuum of 2 x 10-3 mm Hg. 94.3 kg of residual oil were obtained. This residual oil was subjected to a deodorizing teatment for 120 minutes while blowing steam into the still at a temperture of245+50C. and under a reduced pressure of 2 to 3 mm Hg. A refined oil having an acid value of 0.08; an iodine value of 51.3; a hydroxyl value of 2.3; a carbonyl value of 4.7 m.e./kg; a melting point of 33.90C; a smoke point of 245 C; a kinematic viscosity of 45.9 cst (400 C); and a Lovibond colorimeter 5 1/4 in. cell color of 0.9R-9.OY, was obtained. Tables 1 and 2 show the results of an oven test at 630C. and a heating test at 1 800C.As is clearly apparent from these Tables, the refined oil, as compared with the oil of Comparison Example 3, more slowly develops flavour reversion and the results of the heating test are favourable.

1.0 kg of the refined oil of the invention was put in a beaker made of stainless steel and having a capacity of 2 litres, and the temperature was controlled to 1 80+20C. by means of an electric heater.

Frozen potato pieces for French frying were fried, each in 30 g of oil, for 4 minutes at intervals of 10 minutes. The test was continued for 10 hours. The acid value, iodine value, carbonyl value, smoke point, kinematic viscosity, and Lovibond colorimeter color of the residual oil used for frying are shown in Table 3.

It will be apparent that, as compared to the oil of Comparison Example 3, the changes in properties are smaller, and the oil has desirable physical properties as a frying oil.

COMPARISON EXAMPLE 1 An alkali-refined, decolored palm oil as used in Example 1 was subjected to deodorizing operation for 120 minutes while blowing steam at a temperature of245+50C. and under a reduced pressure of 2 to 3 mm Hg. The refined oil obtained had an acid value of 0.06; an iodine value of 54.1; a hydroxyl value of 6.4; a carbonyl value of 5.5 m.e./kg; a kinematic viscosity of 45.0 cst (400C); a Lovibond colorimeter 5 1/4 in. cell color 1 .2R - 1 2.OY; and a smoke point of 228 OC. The results of an oven test at 630C. and a heating test at 1 8O0C. are shown in Tables 1 and 2.As shown in these Tables, the oil, as compared with'that of Example 1, more readily developed flavour reversion, and in the heating test, the development of an irritating odour was noticeable. Table 3 shows the results of the frying test under the same conditions as were employed in Example 1. As compared to the oil of Example 1 , the changes in the properties were greater.

COMPARISON EXAMPLE 2 A degummed, declored palm olein identical to that of Example 2 was subjected to a deacidifying and deodorizing treatment for 1 50 minutes while blowing steam at a temperature of 245+50C. and under a reduced pressure of 2 to 3 mm Hg. A refined oil was obtained having an acid value of 0.10; an iodine value of 58.0; a hydroxyl value of 5.2; a carbonyl value of 6.0 m.e./kg; a melting point of 22.7 OC.; a smoke point of 2250C; a kinematic viscosity of 45.0 cst; and a Lovibond colorimeter 5 1/4 in. cell color of 1 .2R - 1 2.OY. Tables 1 and 2 show the results of an oven test at 630C. and a heating test at 1 800 C. on the refined oil. As shown in these Tables, the refined oil, as compared with the refined oil obtained in Example 2, more readily develops flavour reversion and, in the heating test, the development of irritating odour was noticeable. Table 3 shows the results of a frying test of this refined oil using the same conditions as employed in Example 2. It will be noted that the changes in properties are greater than in the case of Example 2.

COMPARISON EXAMPLE 3 A hydrogenated oil identical to that used in Example 3 was subjected to a deodorizing treatment for 120 minutes while blowing steam at a temperature of 245+50C. and under a reduced pressure of 2 to 3 mm Hg. A refined oil was obtained having an acid value of 0.09; an iodine value of 51.2; a hydroxyl value of 6.0; a carbonyl value of 4.5 m.e./kg; a melting point of 34.1 OC; a smoke point of 2260C.; a kinematic viscosity of 46.0 cst: and a Lovibond colorimeter 5 1/4 in. cell color of 0.9 R - 8.0 Y. Tables 1 and 2 show the result of an oven test at 630C. and a heating test at 1 800 C. on this refined oil.From these results, it is apparent that the refined oil, as compared with the oil of Example 3, more readily develops flavour reversion, and, in the heating test, an irritating odour was developed. Table 3 shows the results of the frying test of this refined oil under the same conditions as were employed in Example 3. As compared to the oil of Example 3, the changes in properties are greater.

Table 1 Results of Oven Test at 630C Decadienal Amount Peroxide Value (ppm) Flavour Example 1 Refined oil 6.8 10.5 Rather good Comparison Example 1 ,, 7.0 50.2 Flavour reversion Example 2 ,, 9.3 12.5 Rather good Comparison Example 2 12.5 66.5 Flavour reversion Example 3 ,, 2.0 0 Good Comparison Example 3 ,, 6.2 43.8 Flavour reversion Testing Method: The sample oil,100 g, was put in a beaker of a capacity 300 ml, covered with an aluminum foil, then preserved for ten days in a chamber at a constant temperature 63i0.5C., and the respective items were measured.

Peroxide Value: Measured according to the Official and Tentative Methods of the Japan Oil Chemists' Society (2.4.12-71).

Decadienal Amount; Measured by gas chromatography (Agri. Biol. Che. 2485 (1976)).

Table 2 Results of Heating Test at 1 800 C.

Decadienal Amount (ppm) Odour Example 1 Refined oil 0 moderate odour Comparison Example 1 ,, 4.0 irritating odour Example 2 ,, 0 moderate odour Comparison Example 2 ,, 5.0 irritating odour Example 3 ,, 0 moderate odour Comparison Example 3 ,, 2.3 irritating odour Testing Method: The sample oil, 100g, was put in a beaker of a capacity 300 ml then it was placed in a oil bath whose temperature was controlled at 80+O.50C., and 15 minutes after it reached 1 800C. the respective items were measured.

Table 3 Analyzed Values of Residual Oil after Frying Carbonyl Value Acid Value Iodine Value m.e./kg Example 1 Residual Oil after frying 0.50 (+0.45) 54.3 (-0.1) 8.8(+3.6) Comparison Example 1 ,, 0.75 (+0.69) 52.2 (-1.9) 40.2 (+34.7) Example 2 ,, 0.27 (+0.18) 57.6 (~0.2) 8.2 (+2.1) Comparison Example 2 0.51 (+0.41) 558(-2.2) 31.5 (+25.5) Example " 0.49 (+0.41) 51.2(-O.1) 7.8 (+3.1) Comparison Example 3 ,, 0.86 (+0.77) 49.8 (-1.4) 35.6 (+30.8) Remark: The + and - put before the numerical value in the parentheses show the increased and decreased value.

Method for Measuring the Respective Items: Acid Value: According to the Official and Tentative Methods of the Japan Oil Chemists' Society (2.4.1-71).

Iodine Value: The same as above (2.4.5.1-71).

Carbonyl Value: According to the method disclosed in YUKAGAKU, 14, 1 67 (1965).

Smoke Point: According to the Official and Tentative Methods of the Japan Oil Chemists' Society (2.3.10.1-71).

Kinematic Viscosity: The same as above (2.3.9.1-71). (Temp. at the measuring 400C.).

Lovibond Color: The same as above (2.3.1.1B-71) Kinematic Lovibond colorimeter Smoke Point (OC) Viscosity (cst) 2in. cell Color 220(-23) 48.5(+3.3) 2.9R-29.OY 198(--30) 52.3(+7.3) 5.3R--51.OY 226(-19) 47.2 (+2) 1.9R-18.OY 200(--25) 50.9(+5.9) 3.3R-34.0Y 218(-27) 48.8(+2.9) 1.1R-9.0Y 183(-43) 52.4(+6.4) 4.8R-48.OY EXAMPLE 4 By hydrogenation of a deacidified and declored soybean oil obtained by a process identical to that employed in Example 3, a hydrogenated soybean oil was obtained having an acid value of 0.26; an iodine value of 74.2; a hydroxyl value of 1.2; a melting point of 33.3 C; and a Lovibond colorimeter 5 1/4 in. cell color of 0.6R - 6.0Y. Ten kilograms of this hydrogenated oil were subjected to molecular distillation in a wiped-film still at 21 0+50C. and an average degree of vacuum of 5x 10-3 mm Hg. 9.7 kg of residual oil were obtained. This oil was then subjected to deodorization at 245+ 50C. under a reduced pressure of 2 to 3 mm Hg for 120 minutes while blowing steam into the still.There was obtained a refined and hydrogenated soybean oil having the following physical properties : acid value 0.05 : iodine value 74.4; hydroxyl value 0.80; carbonyl value 4.0 m.e./kg; melting point 33.20C; smoke point 2440 C; kinematic viscosity 50.8 cst (400 C); and Lovibond colorimeter 5 1/4 in. cell color 0.8 R - 8.0 Y. The results obtained in an oven test at 630C. and a frying test under the same conditions as were employed in Example 1 are shown in Tables 4 and 5.

COMPARISON EXAMPLE 4 A hydrogenated soybean oil identical to that used in Example 4 was subjected to a deodorizing treatment while blowing steam into the still at 245*50C and under a reduced pressure of 2 to 3 mm Hg for 120 minutes. There was obtained a refined oil having the physical properties: acid 'value 0.06; iodine value 74.1; melting point 33.60C; hydroxyl value 1.2; carbonyl value 3.8 m.e./kg; smoke point 2400 C; kinetic viscosity 50.9 cst (400C); and Lovibond colorimeter 5 1/4 in. cell color 0.7R - 7.5Y.

The results of an oven test and a frying test on this refined oil are shown in Tables 4 and 5. As compared with Example 4, deterioration was observed.

EXAMPLE 5 Ten kg. of an alkali frefined, decolored lard (acid value 0.17; iodine value 67.1; hydroxyl value 2.1) was subjected to molecular distillation in a wiped-film still at 200+50C. in a vacuum of 5 x 10-3 mm Hg. 9.6 kg of a residual oil were obtained. The distilled oil had an acid value of 3.14; an iodine value of 59.0; a hydroxyl value of 24.4; and a melting point of 30.70C. After decloring the residual oil, it was deodorized under the same conditions as employed in Example 1. This refined oil had an acid value of 0.03; an iodine value of 67.4; a hydroxyl value of 0.97; a carbonyl value of 5.1 m.e./kg; a melting point of 33.5 or; a Lovibond colorimeter color of 0.8R - 7.6Y; a kinematic viscosity of 44.0 cst; and a smoke point of 243 OC.

1.0 Kg of this refined oil was put in a beaker made of stainless steel and having a capacity of 2 litres. The temperature of the oil was controlle.d to 1 45+50C and then Chinese noodles were fried in it, 1 5 g each for 4 minutes at intervals of 10 minutes. The test was continued for 10 hours, and the analyzed values of the residual oil used for frying are shown in Table 5.

COMPARISON EXAMPLE 5 A decolored lard identical to that used in Example 5 was deodorized under the conditions as employed in Example 1. The physical properties of the refined oil were: acid value 0.04; iodine value 67.0; hydroxyl value 2.2; arbonyl value 5.0 m.eikg; kinematic viscosity 44.1 cst (400 C); melting point 34.50C; smoke point 2400C; and Lovibond colorimeter 5 1/4 in. cell color 0.8R - 7.4Y. The results obtained in a frying test carried out in the manner practised in Example 5 are shown in Table 5. As compared to Example 5, there was a noticeable lowering of the smoke point and the iodine value.

Table 4 Results of Oven Test at 630C.

Decadienal Amount Peroxide Value (ppm) Flavour Example 4 Refined oil 70 0 Good Comparison Example 4 7.5 4.8 Odour of hydrogenated oil Testing method: The same as Table 1. The preserved days were 20 days.

Table 5 Analyzed Values of Residual Oil used for Frying Carbonyl Smoke Lovibond value point Kinematic colorimeter color Acid value Iodine value m.e./kg ( C} viscosity (5 1/4" cell) Example 4 refined oil 0.23(+0.18) 74.3(0.1) 8.5(+4.5) 236(-8) 53.0(+2.2) 1.2R- 12.OY Comparison Example 4 ,, 0.45(+0.39) 71.4(--2.7) 1 5.2(+1 1.4) 204(-36) 57.2(+6.3) 2.4R 24.OY Example 5 ,, 0.26(+0.23) 67.3(0.1) 1 6.7(+1 1.6) 237(-6) 45.3(+1.3) 1 .2R 1 2.0Y Comparison ,, 0.29(+0.25) 65.1 (-1.9) 1 8.0(+1 3.0) 228(-1 2) 46.0(+1 .9) 1 .3R - 1 2.0Y Remark: The + and - put before the numerical value.in the parentheses show the increased and decreased value.

The analyzing methods employed for the respective items are the same as in Table 3.

Claims (17)

1. A process for improving the quality of an edible oil or fat which comprises the step of removing partial glycerides from the oil or fat.

2. A process according to claim 1 wherein the partial glycerides are removed by molecular distillation.

3. In a process for refining an edible oil or fat which includes a deodorising step, the improvement which comprises removing partial glycerides by means of a molecular distillation step prior to the deodorising step.

4. A refining process according to claim 3 which comprises degumming, deacidifiying with alkali, and decloring the oil or fat prior to carrying out the molecular distillation.

5. A refining process according to claim 3 which comprises degumming, decoloring and steam refining the oil or fat prior to carrying out the molecular distillation.

6. A refining process according to claim 3 which comprises degumming and decoloring the oil or fat and thereafter carrying out the molecular distillation step so as to remove fatty acid as well as partial glycerides.

7. A process according to any one of the preceding claims and including the additional step of subjecting the oil or fat to an oil- or fat-processing step.

8. A process as claimed in claim 7 wherein the oil- or fat-processing step includes a hydrogenation step.

9. A process as claimed in claim 7 or 8 wherein the oil- or fat-processing step includes a fractionation step.

10. A process as claimed in any one of the preceding claims wherein the oil or fat initially has a hydroxyl value of from 4 to 12 and the process is effected such that the hydroxyl value is reduced to less than 3.

11. A process as claimed in any one of the preceding claims wherein the oil is palm oil.

12. A process as claimed in any one of claims 1 to 9 wherein the oil or fat initially has a hydroxyl value of less than 3 and the process.is effected such that the hydroxyl value is reduceçLto less than 1.

13. A process according to claim 2 or 3 or to any one of claims 4 to 12 when appendant to claim 2 or 3 wherein the molecular distillation is effected using a high vacuum of from 10-2 to 1 0-3mm Hg and a temperature of from 1 to 2300C.

14. A process according to claim 2, 3 or 13 or to any one of claims 4 to 12 when appendant to claim 2 or 3 wherein the molecular distillation is effected using a centrifugal or wiped film technique.

i 5. A process according to claim 1 substantially as hereinbefore described in any one of Examples 1 to 5.

1 6. An oil or fat whenever it has been subjected to the process claimed in any one.of claims 1 to 15.

17. The use or an oil or fat as claimed in claim 16 for the purpose of frying food.