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Studies on Cause of Color Reversion of Edible Soybean Oil and its PreventionMamoru Komodaa, Noriji Ōnukia & Ichiro Haradab

a Sugiyama Chemical Research Institute, Mitaka-shib Hohnen Oil Co., Ltd., Shimizu Plant, Shimizu-shiPublished online: 09 Sep 2014.

To cite this article: Mamoru Komoda, Noriji Ōnuki & Ichiro Harada (1966) Studies on Cause of Color Reversion of Edible Soybean Oil and its Prevention, Agricultural and Biological Chemistry, 30:9, 906-912

To link to this article: http://dx.doi.org/10.1080/00021369.1966.10858698

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[Agr. BioI. Chem. Vol. 30, No.9, p. 906~912, 1966]

Studies on Cause of Color Reversion of Edible Soybean Oil and its Prevention

Part I. Relation between the Moisture of Soybeans and the Quantity of Tocopherol in them

By Mamoru KOMODA, Noriji ONUKI and Ichiro HARADA*

Sugiyama Chemical Research Institute, Mitaka-shi; *Hohnen Oil Co., Ltd., Shimizu Plant, Shimizu-shi;

Received March 4, 1966

Properties and minor constituents were examined for soybean oils obtained from soy­beans with various moisture contents. Yields of crude oils by extraction process of experi­mental scale anotocopherol content in the extracted oil were found to be correlated to the.

, . moisture content in soybeans (moisture range, 8_18%). When moistened raw soybeans (moisture 18%) are dried (moisture 8%), the tocopherol content in the extracted crude oil from the dried beans increased. This phenomenon can be observed only in the case of drying whole beans and not in the case of drying crushed raw soybeans. When soybeans with ordinary moisture (moisture 13%, tocopherol 1.25 mg/g) are dried to reduce the moisture to 1.9%, the tocopherol content in the extracted crude oil increases (1.90 mg/g), but it decreases (0.33 mg/g) again when the dried soybeans are moistened (moisture 18%).

INTRODUCTION Soybean oils just after deodorization are

quite pale but become darker during storage in the air. The degree of color change de­pends on the following conditions: nature of raw soybeans, refining and deodorization con­dition of oil, duration of storage, temperature, intensity of light and contact surface with the air during storage, etc. With some oils it takes only several hours to reach the maximum of color reversion, while others take several months. In both cases, the color becomes paler after it reaches the peak.

We call the phenomenon of the color of the oil getting darker during storage "the color reversion of oil." As to the coloring of oil during storage, it is hitherto known that the oxidized-polymerized unsaturated acids affect the coloration, but this coloring appears only in the rancid oils and is clearly distinguished

This research has been financed by a grant made by the United States Department of Agriculture under P. L. 480.

..'

from what we call the color reversion, which occurs at a very early stage of oxidation.

As to cottonseed oil, the coloration by gos­sypol was studied.1.2) Swift3) proved that the phenomenon of refined cottonseed oil becom­ing reddish is due to the 2,7,8-trimethyl-2-(4',8',12'-trimethyltridecyl-I-)-chroman-5,6-qui­none derived by the oxidation of 2,7,8-tri­methyl-2-(4', 8', 12'-trimethyltridecyl)-6-chro· manol (r-tocopherol) in the oil.

There are a few papers which treat the color reversion of refined and deodorized soy­bean oil. It is reported by Beal4 ) that phos· phorus content of refined and deodorized soybean oil is closely related to the color and

, the oxidation stability of oil. <

We recognized that the color reversioR.,is •

1) J. M. Dechariy, R. P. Kupperman, F. H. Thurber and R. T. O'conner, J. Am. Chern. Soc., 31, 420 (1954). . .

2) V. P. Rzhekin, Masloboino·ZhirotJaya Pro., 19, No. 28 (1954). <

3) C. E. Swift, G. E. Mann and G. S. Fisher, Oil & Soap, 21, 317 (1944). , .

4) R. E. Beal, E. B. Lancaster and D. E. Brakke, 'J. Am. Chern. Soc., 33, 619 (1956).

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Studies on Cause of Color Reversion of Edible SObyean Oil and its Prevention. Part I 907

characteristic to refined and deodorized soy­bean oil and wrote a number of papers on this phenomenon.5- I2) We happened to find that the color reversion was far slighter in the refined and deodorized oil prepared from the crude oil obtained by extraction process from the 1963 crop of Illinois soybeans than in the refined and deodorized oils hitherto manufac­tured. We knew by the reports from U.S.A. that the 1963 crop soybeans in Illinois had experienced a draught in the growing and harvesting season, which led us to deduce that the low moisture might be a cause of the slighter color reversion. Therefore, we have studied1S) the relation between the moisture content of soybeans and the color reversion of the refined and deodorized oils obtained from them. From the experiments described in the previous paper,13) we found that the moisture of soybeans is closely related to the color reversion of refined and deodorized oil.

Now, we have found further that the mois­ture of soybeans affects the quantity of toco­pherol in crude oil. The mechanism involved in the changes of the tocopherol in soybeans is not yet clear but we have found several new phenomena about the behavior of tocopherol in soybeans.

EXPERIMENT AND RESULT

1) Properties of crude oils extracted from soy­beans with various moisture contents.

Raw soybeans used in this experiment were the 1963 crop Illinois soybeans with 12% moisture. From them soybeans with the mois­ture content of 7.8, 10.1, 14.8 and 17.6% were prepared by the same process described al­ready.IS)

5) M. Nakamura, S. Tomita and M. Komoda, Yukagaku, 9, 319 (1960).

6-8) I. Harada, Y. Saratani and M. Ishikawa, ,. AgT. Chem. Soc. lapan, 34, 545, 551, 558 (1960).

9i"'12) I. Harada, Y. Saratani and M. Ishikawa, ibid., 35,668, 674, 800, 807 (1960).

13) S. Tomita, M. Kornoda, S. Enomoto and N.Onuki Yuka­gaku, 13, 530 (1964).

Five kinds of moisture-controlled soybeans were crushed and extracted with n-hexane. Yield of crude oil and the analytical results of the meal are shown in Table I.

TABLE I. YIELD OF CRUDE OILS AND ANALYTICAL

RESULTS OF SOYBEAN MEALS

Moisture of Soybeans (Xylene Method)

(%) 7.8

10.1 12.1 14.8 17.6

Extracted Crude Oil (Dry Basis)

(%) 20.2 21.5 21.8 22.4 23.2

Soybean Meals •

Moisture

(%) 6.99 8.03 9.95

llAO 13.30

- , Oil

(Dry Basis) (%) 1.67 1.26 1.05 0.72 0.50

TABLE II. PROPERTIES OF CRUDE OIL

Moisture of Acid

Soybeans Value (%)

Iodine Value (Wijs)

7.8 0.99 130.8 10.1 1.22 129.6 12.1 1.29 128.3 14.8 1.28 129.6 17.6 1.29 127.7

Con-jugated Diene (%) 0.27 0.35 0.41 0.43 0.83

Lipoid

(%) 0.42 0.96 1.75 2.33 2.53

Total Toco­pherol (mg/g)

1045 1.28 0.89 0.71 0.57

The percent of the yield of crude oil is proportional to the moisture content of soy­beans. The crude oil obtained from soybeans with less moisture (moisture 7.8, 10.1, 12.1%) is orange in color and transparent. The color of crude oil becomes darker and the crude oil has more sludge as the moisture of the raw soybeans increases (moisture 14.8, 17.6%). The less the moisture of soybeans, the more is the residual oil in the meal and the darker is the extracted matter.

Figs. 1 and 2 indicate the ultraviolet absorp­tion spectrum of each extracted oil. The amount of chlorophyll which has maximum absorption at 670 lllJL decreases as the moisture of soybeans decreases.

The crude oil from soybeans with high moisture (14.8, 17.6%) has distinct absorption maximum at 350 lllJL and has weak absorption

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908 Mamoru KOMODA, Noriji ONUKI and Ichiro HARADA

_El ,0

"' ..... ~

'" 0 -

FIG. 1.

!

0-

-1

10%

- / 2

-3 8%

10% - I 4

51--

200 300 400 500 600 700

Wave length (mp)

Absorption Curves of Crude Soybean Oils

from Dried Soybeans.

o

-1

200 300 400 500 600 700

Wave length (mp)

FIG. 2. Absorption Curves of Crude Soybean Oils

from Raw Beans and Moistened Soybeans.

maximum at 400-500 mfL characteristic to carotenoid pigments. The carotenoid pig­ments are most extractable at 10-12% mois­ture of soybeans.

As to the minor constituents of crude soy­bean oil, changes in the yield of phosphatides

and tocopherol are most noticeable. Phospha­tides increase and tocopherol decreases as the moisture of soybeans increases.

2) Quantitative estimation of total tocopherol. • •

Emmerie-Engel method14) was usecl. for the quantitative estimation of total tocopherol: As crude soybean oil contains various kinds of minor constituents such as carotenoid' pig­ments and sterols that interfere in the absorp­tion spectrum of tocopherol analysis,. thes~ minor constituents were removed by Green's method.15) .•

In the check test in which dl·a-tocophe~@ '. ',;,}"

(E~:~ 292 m,a max.=73.2, ethyl alcohol) was . , \ ' ".'

used as a standard sample, the recovered I","·

amounts were satisfactory as shown,' in Table III.

• •

TABLE III. RATE OF RECOVERY OF TOTAL TOCOPHEROl.

No.

1

2 3 4

4-3

dl-lX-Tocopherol

Added (mgjg) 0.071 0.143

0.071

Soybean Oil

(g)

0.2 0.2

Total Tocopherol

(mgjg)

0.070 0_144 0.218 0.290 0.072

Rec0very

(%) 97.9

•

100.7 .

-100.6 • . '

3) Residual tocopherol in soybean meal. "

The tocopherol content in extracted crude oil decreases as the moisture of soybeans • Increases.

•

There is some doubt that the decrease of tocopherol may partly be attributed to' the decrease in the solubility of tocopherol in n-hexane due to the moisture from the soy' beans. The following experiment was carried out to clarify this question.

Extraction of meal first with n-hexane and the»: with ethyl ether.

Raw soybeans (moisture 12%) and moisture-

14) A. Emmerie and C. Engel, Nature, 142,!873' (1938); RIC,' Trav. Chim., 57, 1351 (1938); ibid., 58, 283 (1939). I ' ': ;'1' 'i5) Vitamine E Panel, Analytical Methods Committee 1969,

Analyst, 84, 356 (1959). .

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Studies on Cause of Color Reversion of Edible Sobyean Oil and its Prevention. Part I 909

TABLE IV. YIELD OF CRUDE OIL ANn TOTAL TOCOPHEROL

Moisture of Soybeans (Xylene Method)

(%) 8.1

12.1 17.7

Extd. with n-Hexane .~ __ A~ __ _

Yield of Oil (%)

18.34 19.40 18.74

Total Tocopherol (mg/g) (A)

1.66 0.96 0.09

controlled soybeans (moisture 8, 18%) are all crushed to pass a 700fL sieve. Five hundred grams of pulverized beans were extracted with 1500 ml of n-hexane at 40°C. After extrac­tion, the meal was cooled to 25°C and ex­tracted with 1500 ml of ethyl ether. The residual solvent was completely distilled under the reduced pressure. The yield of the crude oil and the total tocopherol are shown in Table IV.

Extraction of pulverized and then dried soybeans.

Four kinds of sovbeans with various mois-•

ture contents were crushed and dried at 90°C in a vacuum dryer until the moisture is re­duced to 1-2%.

The pulverized and dried soybeans were extracted by Soxleht extractor with n-hexane and the tocopherol in the crude oil was de­tenllined. Among the four kinds of the above soybeans, those with 16% and 18% moisture

TABLE V. YIFLD OF CRUDE OIL AND

TOTAL TOCOPHEROL

Moisture of Soybeans

• , , Before Before

Pulveriz- Extrac-• • mg tlOn (%) (%) 7.9 1.56

11.2 1.17 16.0 2.08 17.8 1.67 8.0 1.36

7.7 2.32

Oil (Dried Basis)

(%) 20.80 21.37 22.04 21.65 21.56

21.71

Total Toco­pherol

(mg/g) 1.54 1.63 0.27 0.25 1.67

1.36

Note

18% moistened soybeans dried in the sun 16% moistened soybeans dried in the sun

Extd. with Ethyl Ether '~--''----,

Yield of Total Oil Tocopherol (%) (mg/g) (B) 0.92 2.28 0.60 0.80 0.42 0.90

Total Tocopherol

(A) + (B) (mg/g) on Dried Basis

of Raw Soybeans 1.8 1.1 0.1

were dried in the sun in the form of raw beans until the moisture decreased to about 8% and the tocopherol in the crude oil from these dried beans was determined. The results are shown in Table V.

Ethyl ether is hydrophilic and will gradually dissolve the residual moisture in the meal, so that this solvent dissolves almost all of the tocopherol in the meal. But the tocopherol in the ethyl ether extract was small contrary to expectation.

From the above experiments it may be con­cluded that the tocopherol is transformed to non-reducing substance in soybeans with high moisture level.

We have already described in 1) that the tocopherol in extracted crude oil increases when raw soybeans are dried, crushed and then extracted. By this experiment we have further confirmed that the tocopherol in ex­tracted crude oil does not increase when mois­tened soybeans are crushed, dried and then extracted.

As moistened soybeans in this case are crushed and then dried before the extraction of oil, the moisture in the crushed beans is considered to give no influence on the extrac­tion of oil. Table V indicates that the toco­pherol in crude oil is less in quantity when soybeans have more moisture before the pro­cess of pulverizing.

4) Changes in the quantity of tocopherol during the storage of moistened soybeans. _

The changes of the tocopherol content in soybeans with 15, 18 and 20% moisture, respec­tively, were determined during the storage

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910 Mamoru KOMODA, Noriji ONUKI and Ichiro HARADA

1.0

~

0.8 ~ ~

"" El \ ~

t -8 " ... ' \ '" \ 8 \ 18% 15% 0 , \ ... --_L <ii ~\.- ---- . ... • • 0 -\",' •

E-< 0.2 20%

Storage (day)

FIG. 3. Changes in the Quantity of Tocopherol during the Storage of Moistened Soybeans.

of the soybeans at the room temperature and in the diffused light as shown in Fig. 3.

The 1964 crop U.S. soybeans with 13% moisture (Xylene method) were used in this experiment.

The tocopherol decreased rapidly after the soybeans were stored for one day and only slight changes could be observed afterward. In the case of soybeans with ordinary moisture (13%), the quantity of tocopherol was almost constant during the storage, while it decreased sharply in the case of beans kept with 15% moisture.

This result indicates that there exists a close relation between the quantity of moisture of beans and the quantity of tocopherol.

5) Increase of tocopherol by drying moisteneIl. soybeans.

In the experiment in 3) and 4), we have observed that tocopherol in moistened soy· beans is less than in soybeans with ordinary moisture but increases again when soybeans are dried.

We supposed that the changes of the quanti. ty of tocopherol might be accelerated by sunlight and the enzyme action might be involved in the phenomenon. The following experiments were carried out in order to

make clear these points. . '.

Changes of tocopherol content by drying moisten~d: •

soybeans in the sun. . .

Soybeans with 18% moisture (total toco. pherol 0.28 mg/g) were used as sample. They were dried in the sun on a fine day," The degree of moisture was determined by the weight loss of the soybeans. This experiment was carried out in summer and the drying was very fast. But it was impossible to reduce the moisture below 7% by drying in the sun.

One hundred and fifty grams of soybeans with 18% moisture were put in a fiat glass basin and were dried in strong sunlight for 8 hrs, the maximum atmospheric temperature being 33°C, and the decrease of moisture and the increase of tocopherol were determined

•

as shown in Fig. 4. The close relation between the moisture of

soybeans and the quantity of tocopherol as pointed in experiment 4) has now been clear· ly observed in Fig. 4. The increase of toco· pherol is slow when the moisture of soybeans

1.5f-

~

'" ..... "" El ~ -0 LO ... ., ... '" 0 () 0 ...

<ii ... 0

E-<

0.5

18 16 14 12 10 8 6

Moisture of soybeans (%)

FIG. 4. Changes of Tocopherol Content by Drvmg Moistened Soybeans in the Sun and in the Dark

•

at 40°C.

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Studies on Cause of Color Reversion of Edible Sobyean Oil and its Prevention. Part I 911

is from 18% to 15%, but it increases very ,

rapidly when the moisture is less than 15%. When the moisture of beans falls below 11 % the curve becomes flat again. Mark EB on the curve in Fig. 4 indicates the position of the soybeans (moisture 13%) before moistening.

Changes in the quantity of tocopherol by drying moistened soybeans in the dark at 40°C.

Changes in the quantity of tocopherol dur­ing the drying the moistened soybeans in the .dark were determined in order to make clear whether the increase of tocopherol during the drying of moistened soybeans in the sun is accelerated by sunlight.

Soybeans with 18% moisture (total toco­pherol 0.28 mgjg) were put in a completely dark were determined in order to make clear the thermostat were stirred by a fan. The changes of the tocopherol content by the dry­ing are shown in Table VI.

TABLE VI. CHANGES OF TOCOPHEROL CONTENT

BY DRYING MOISTENED SOYBEANS

IN THE DARK AT 40°C

Moisture of Oil Total Soybeans Tocopherol Drying (Dried (Xylene in Crude Conditions Basis) Method) Oil

(%) (%) (mgjg) 40'C Dark 7 hrs. 12.0 22.3 1.32

" 23 hu. 8.4 21.6 1.53

" 47 hrs. 7.4 21.6 1.57

As shown in Fig. 4 both curves are identical when the moisture of soybeans is less than 12% and the increase of tocopherol in the case of drying in the dark is a little less than in the case of drying in the sun, from which it can be concluded that sunlight gives practically no influence on the increase of tocopherol.

Changes in the quantity of tocopherol by heat drying of soybeans.

The increase or decrease in the quantity of tocopherol in dried or moistened soybeans is supposed to be due to the biochemical re­action. If it is caused by enzyme action, it

must be stopped when the enzyme becomes inactive by heating.

Therefore, the following two severe heating conditions were chozen in this experiment.

Soybeans with 18% moisture (total tocophe­rol 0.28 mgjg) were placed for 7 hrs in a vacu­um dryer kept at 90°C under 15-20 mmHg and cooled under reduced pressure for a night. In the other experiment, the same soybeans were heated at 120°C for an hour. The analysis of the dried soybeans is shown in Table VII.

TABLE VII. CHANGES OF TOCOPHEROL CONTENT BY

HEAT DRYING OF MOISTENED SOYBEANS

Moisture of Oil Total

Soybeans Tocopherol Drying (Dried (Xylene in Crude Conditions Basis) MethOd) Oil (%) (%) (mgjg)

90°C, Vacuum, 2.6 20.8 1.52 7 hrs.

120°C, Ordinary 9.2 21.7 1.50 Pressure, 1 hr.

The values of increase of tocopherol are nearly equal to those described before, in which case moistened soybeans were dried in the sun. It is expected that the heating of soybeans at 90°C for 7 hrs or at 120°C for 1 hr will make the enzyme inactive, but the values of increase of tocopherol in either case are nearly equal to those described in 3) in which moistened soybeans were dried in the sun.

This fact shows that the enzymes in soybeans concerned with the transformation of toco­pherol are relatively resistant to heat.

Changes in tocopherol by repeated treatment of moistening and drying of soybeans.

Raw soybeans (moisture 13%) and soybeans with 18% moisture were dried in a vacuum dryer kept at 85°C for 15 hrs until the weight became almost constant. These dried soy­beans were moistened again by the same pro­cess described already. Table VIII shows the analysis of soybeans at each stage of drying

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912 Mamoru KOMODA, Noriji ONUKI and Ichiro HARADA

TABLE VIII. CHANGES IN TOCOPHEROL IN REPEATED DRYING AND MOISTENING OF SOYBEANS

Moisture of Soybeans (Xylene Method)

(%) A Raw Soybeans 13.0 B A-moistened 18.2 C A -dried 1. 9 DB-dried 1. 9 E C-moistened 18.0 F D-moistened 18.3

and moistening.

Oil (Dried Basis)

(%) 22.0 22.0 20.5 20.7

21.9

22.1

Total Tocopherol

in Crude Oil

(mgjg) 1.25 0.28 1.90 1.87

0.33 0.34

It was already pointed that the tocopherol in raw soybeans with ordinary moisture (13%) increased when they were dried, and decreased when they were moistened.

The noticeable and interesting point in the results of this experiment is that the quantity of tocopherol changes according to the mois­ture of soybeans during the repeated drying and moistening treatment of soybeans and that it shows a certain fixed value at a certain moisture level of the beans.

DISCUSSION

The tocopherol in raw soybeans increases or decreases according to the moisture of beans, especially in the moisture range from II to 15%. Hitherto, nothing has been re­ported as to the relation between the quantity of tocopherol and the moisture of soybeans.

M. Roche16) observed that the loss of toco­pherol was least when wheat germ or flour was stored at a low temperature in the atmo-

16) M. Roche, W. Feldheim and B. Thomas, Ernahrungs­forschung, 3, 386 (1958).

sphere of high moisture, ahd that the loss increased when the moisture exceeded. ,12%. M. Roche reported also that the mechanism for the above phenomenon was not clear and that the loss of tocopherol during the storage of wheat germ was not caused by the oxida­tive enzymes. We observed that the decrease of tocopherol in soybeans became sharp when the moisture of beans exceeded 12%.

In the case of soybeans, the decrease of tocopherol appeared rapidly when soybeans were moistened, while, in the case of wheat flour, the loss of tocopherol occurred very slowly during a long storage, from which it is supposed that the loss of tocopherol in both cases is caused by different functions.

The tocopherol content in moistened soy­beans is small but it increases again when the beans are dried. Moreover, this phenomenon is only observed in raw whole soybeans and not in crushed and dried soybeans.

Therefore, the changes in the quantity of tocopherol according to the moisture of the soybeans may not be the simple oxidation of tocopherol by air but are supposed to be due to the biochemical reaction.

To prove this, drying of soybeans at 90°C or at 120°C was attempted, but the increase of tocopherol in each case was nearly equal to the case when beans were dried at 40°C or dried in the sun.

From this we have learned that the enzymes in soybeans concerned with the transformation of tocopherol are relatively resistant to heat. The roles of enzymes in soybeans in the trans­formation of tocopherol will be made cleat by further studies.

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