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chloroplatinate method and throw a still greater error on the sodium as determined by difference. Moreover, the chloroplatinate method is liable to error through numerous conditions not affecting the indirect method.
Vol. 19, No. 1
Except where potassium alone is sought, the indirect method is therefore concluded to be equal to the chloroplatinate method in accuracy, and it is superior in all cases from the standpoint of economy of time and cost of reagents.
Susceptibility of Fats to Autoxidation‘ By Geo. E. Holm, Geo. R. Greenbank, and E. F. Deysher RWEARCH LABORATORIEX~, BUREAUOF DAIRY INDUSTRY, WASHINGTON, D. C.
T
HE deterioration of most fats and oils through reac- of practically no absorption. The nature of this reaction tions involved in oxygen absorption usually proceeds is shown in Figure 2. Further study of this phase of the reaction has not been rapidly once active absorption has begun. At this point, however, the reactions are relatively far advanced, attempted, but the following explanation seems plausible. and tests dependent upon the products of the reactions give Studies upon the nature of the compounds formed in the little or no information concerning the ability of the product oxidation of unsaturated compounds seem to indicate that to withstand oxidation. oxides and peroxides are not the first compounds formed in The Kreis test, the photographic plate image test, and the the reaction.6 To these unisolable products the term peroxidase test are very delicate, but so far have proved LLmoloxides” has been applied. The initial a b s o r p t i o n of no great value in deternoted in a number of cases is mining the susceptibility of a fat to autoxidation. The perhaps due to the formation Evidence is presented of the existence of loosely of such compounds, which last two give evidence of bound oxygen compounds in butter oil. These subbeing directly connected may be looked upon as loose stances, termed “moloxides,” are undoubtedly reoxygen c o m b i n a t i o n s of with certain changes that sponsible for oxidation in uacuo. The powerful repotential oxidizing ability. occur early in the oxidation tarding action of the OH group in several compounds s t a g e a n d therefore may The exact conditions under has been studied. Data obtained show that the suswhich this initial oxygen abhave some value in deterceptibility of cottonseed oil to autoxidation varies with sorption occurs and may be mining this factor. the commercial treatment. The effect of ultra-violet noted are not known. The curve which illuslight upon the autoxidation of cottonseed oil is shown. T h e foregoing explanatrates autocatalytic oxidation, based w o n the idea tion is logarithmic for most that loosely combined oxyfats (Figure 1). The length of the period preceding the phase of rapid absorption, known gen compounds are formed, receives support also from the as the induction period, varies, however, with the type and following observations: Certain butter oils were always quality of the fat or oil. improved when steam was passed through them for a short In view of the fact that deterioration is rapid after it has times6 Other samples did not respond so readily. A become perceptible, it is most important to know to what sample of a fat that had been steamed and a sample of the stage in the induction period a certain fat belongs. A measure same fat left untreated were sealed in glass containers under of the length of this period under specified conditions there- high vacuum (applied for 30 minutes at 40” C.) and were fore gives a relative quantitative measure of the ability of a placed in sunlight. The untreated sample bleached rapidly; the steamed sample remained unbleached for months. A product to withstand oxidation. The Bailey rancidity test2 is dependent upon autocatalysis treated sample, which, sealed in vacuo, had been kept in the and therefore gives values for susceptibility. It is, however, direct sunlight for 3 months, showed a negative Kreis test open to the objection that catalytic products are progressively after it had been stored at room temperatures for 3 years. A source of oxygen must be postulated to account for the removed during its operation. These products also affect the indicator used to determine the end point. The authors oxidation a t the double bond of the oleic acid at relatively have adopted the principle of measuring the length of the in- low temperatures. That it comes from the normal comduction period under more constant conditions, thereby ob- pounds is improbable. The oxidation of the unsteamed viating losses in the system. The apparatus now used in the sample may be explained by assuming that enough loosely laboratories of the Bureau of Dairy Industry is described in a combined oxygen to cause perceptible oxidation is present. The lack of oxidizing action in the steamed sample indicates publication already i ~ s u e d . ~ During experiments with butter oil certain irregularities a lack of free or loosely combined oxygen. An explanation in the curve within the induction heriod were noted. There of the mechanism of the reaction involved in the oxidation seemed to be a slight initial absorption some time before the in vacuo might perhaps be based upon the observations of a rapid absorption began. Closer study of this absorption number of investigators that hydrogen peroxide is found showed that a small amount of oxygen was actually taken up when oxidation occurs. The explanation of the formation by the fat, after which there was a comparatively short period of hydrogen peroxide assumes the formation of peroxides as the initial step in the reaction, subsequent formation of 1 Presented at the symposium on Cotton and Its Products, and Vegeactive oxygen, and the union of this oxygen with water. table Oils before the Division of Agricultural and Food Chemistry at the 71st Meeting of the American Chemical Society, Tulsa, Okla., April 5 to 9, It is evident that an interpketation of the foregoing observa1926. tions on milk fat, in the light of this theory, calls for a reaction I Cotton Oil Press, 7, 8,35 (1923). upon which data are too conflicting and too meager to sup8 Proc. World’s Dairy Congress, 2, 1253 (1923); J. Dairy Sci., 8, 515 (1925). 4
6
Tars JOURNAL, 17, 625 (1925).
6
Staudinger, Ber., W B , 1075 (1925). Greenbank and Holm, THIS JOURNAL, 16, 598 (1924).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January, 1927
port even a tentative hypothesis. That the reaction occurs in a t least two stages, in the building up of compounds of high oxidizing potential and in the actual oxidation itself, seems to be confirmed. Browne’s’ experiments upon the spontaneous decomposition of butterfat indicate that the absorption of oxygen is periodic, as shown by the fluctuations in weight of the samples used.
I
I
7
2 3 4 5 6 7 Time (hours) . , Figure 1-General Nature of an Autoxidation Reaction
Various writers have claimed that the presence of air is not necessary for the oxidation of certain fats. Perhaps the fats used were not entirely free from loosely bound oxygen. Results of previous experiments in the Bureau of Dairy Industry* have shown that the Kreis test has no direct quantitative relation to the degree of rancidity, though in the oxidation of oleic and linoleic acids there are definite relationships between the intensity of the Kreis test and the amount of oxygen absorbed by the fat. Linoleic acid failed to show this relationship under the conditions of the experiment. It has been shown alsog that of the unsaturated acids oleic acid is mainly concerned in the production of tallowy odors and flavors. Cottonseed oil upon oxidation gives very little odor in proportion to the amount of oxygen absorbed. It is probable that the unsaturated acids other than oleic are largely involved. (Intramolecular rearrangements may also be concerned.) Results indicate further that the course of the oxidation where two double bonds exist is not analogous to that where one double bond exists. I n the presence of moisture, the compounds which form the tallowy odor show a tendency t o decrease in quantity.
Further work has shown that a high vapor pressure tends to retard oxidation, and once it begins its rate is rapid. By careful adjustment of this factor, however, it has been found that the principle can be used in regulating the rate of changes in certain products, such as milk powders and cereals. The finding that acids act as catalysts in the oxidation of fats and oils seems to hold true in the oxidation of oleic acid. In butter oils the addition of acids in quantities barely detectable by titration materially changes the susceptibility. The quantitative effect was not so great when fats containing principally linoleic were used (cottonseed oil). Here again it seems that the course of the oxygen absorption reaction is not analogous to that when only one double bond is involved. The nature of the action of acids is not clear, and more work is necessary t o determine the relation of this factor to structure from the standpoint of susceptibility. Although fats and oils which contain highly unsaturated acids oxidize readily, castor oil is a slow oxidizer, in spite of its fairly high iodine value (83). In addition to having one double bond, wherein it resembles oleic acid, it contains an OH group which occupies the same relative position as the second double bond of linoleic acid. Oleic CH,ICH,) ,CH=CH(CHP) COOH Linoleic CH;(CH&CH=CHCH,CH=CH( CH2),COOH Ricinoleic CBH&H(OH)CH~CH=CH(CH~) ,COOH
The rates of oxidation of oleic, linoleic, and ricinoleic acids were determined. Linoleic was found to be a rapidly oxidizing fat, whereas ricinoleic oxidized very slowly. The results in Figure 3 indicate that a hydroxy group in as close proximity to the unsaturated bond as it is in ricinoleic acid has a marked retarding effect upon oxidation.
Figure 3-Rate
Hours Figure 2-Nature of Absor t i o n w i t h i n Induction P e r i o f
A dry atmosphere favors their production. This phenomenon has been explained by assuming that in the absence of moisture-the oxidation proceeds t o the aldehyde stage, whereas in the presence of moisture acids of little or no tallowy odor are the final products. It should be remembered, however, that the presence of water is conducive to the formation of acids through hydrolysis, and these have been shown to be catalysts in the-reaction. 7 THIS~JOURNAL,
17,44 (1925).
* Holm and Greenbank, I b i d . , 16, 1051 (1923). 9
I b i d . , 16, 518 (1924).
157
Time (hours) of Oxygen Absorption b y Various Unsaturated Acids
In view of the work of Moureu and his collaborators upon the anticatalytic substances (compounds containing OH groups, such as phenol, resorcinol, and benzyl alcohol) lo it was decided to determine the relative value of OH groups attached to molecules other than the molecule oxidized with respect both to rate of oxygen absorption and to susceptibility. A number of compounds were added to linseed oil, and the induction periods upon these mixtures were determined. The rate of absorption subsequent to the induction period was also followed. The results are shown in Figure 4. The fact that in the oxidation of cottonseed oil, compounds which give a tallowy odor and those that give the Kreis test are not formed to any great extent indicates that aldehydes are not stable in this reaction. The close proximity of a second unsaturation seems to alter the mechanism of the reaction. Other compounds which may be intermediate in the reaction and which are catalysts are also perhaps unstable. Therefore the rate of oxidation of this oil is somewhat slower than that of many other fats. 10
Comfit. rend.
SOL.
bid., 86, 321 (1922); Compt. rend., 174, 258 (1922).
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158
In addition to the constitutional factor, the method of treatment, such as heating and neutralization, materially changes the susceptibility of fats and oils. Through the courtesy of David Wesson a number of samples of cottonseed oils, which differed in the treatment received in the manufacturing process, and in the value of some of their physical constants, were obtained. Complete data on the conditions of manufacture were not available. The susceptibilities of these samples were determined (Table I). T a b l e I-SUSCeDtibilitV
of C o t t o n s e e d Oils
this connection, tests were made to find the quantitative effect of ultra-violet light upon the susceptibility of cottonseed oil. A cottonseed oil with an induction period of 4 hours was irradiated for various lengths of time with a Cooper-Hewitt lamp (160 volts across terminals), at a distance of 24 inches. A current of air directed over the surface of the oil kept the sample a t its initial temperature. Figure 5 shows the effect of ultra-violet light upon the susceptibility of cottonseed oil to oxidation. These results are in accord with those obtained upon a product where oleic acid only is involved. Conclusions
INDUCTION
IODINE FREEFATTY PERIOD
SAMPLE
No.
1 Choice crude cottonseed from refining kettle Yellow oil after refining with NaOH and drying 3 Same as No. 2 after bleaching and filtering with fuller's earth 4 Average bleached same a s No. 3 Same as No. 4 after deodorizing 5 with steam and vacuum Same a s No. 5 after removal of 6 stearin 7 Same a s No. 6 after filtration and deodorization 8 Sample especially prepared for the market 9 Cottonseed stearin from sample No. 6 10 Hydrogenated oil: 2
A
B
C SDecial from off market
90' c. Minutes
X'O.
ACIDA S OLEIC Per cent
108
0.7
390
109.2
0,025
Lost
107.7 108.8
0.02 0.025
95 95
108.3
0.02
165
112.3
0.045
95
113.1
0.045
125
AT
112,o
0.015
90
89.1
0.023
150
... ...
...
35 120 105 240
,.. , . .
...
... ..
The first noticeable point wherein the results in Table I are at variance with the data thus far presented is the fact that crude cottonseed oil higher in acidity than any of the other samples shows the greatest resistance to oxidation. Although no attempt is made to give reasons for this fact it is well to remember that neutralization is usually a part of the refining process. Neutralization with strong alkalies always involves some hydrolysis, and this is detrimental to keeping quality. Hydrolysis that may occur and the temperature to which oil is subjected in the treatment are conducive to polymerization. 30 grams linseed oil oxidized a t 95' C.
VOl. 19, No. 1
Under some conditions, especially in the butter oils, certain loosely combined oxygen compounds are the first compounds formed in the autoxidation process. When the oil is acted upon by light, or when it is stored for a long time, these compounds of potential oxidizing capacity affect oxidation. They are undoubtedly responsible for the oxidation of a sweet oil or fat sealed in vacuo. Inert gases or vacuum therefore are not absolute safeguards against oxidation of some fats. The relatively great susceptibility of old oils and fats which show no signs of having undergone oxidation might also be attributed to these w compounds. T h e relatively slight effect of acids upon the suscepti- 3 bility of cottonseed e oils and the fact that I this oil gives little 3 tallowy odor upon oxidation i n d i c a t e g that the oxygen ab- 'f sorption reaction in t h i s case (presum- cI ably largely linoleic 60 acid radical) is not e n t i r e l y analogous to the reactions involved in autoxida* tion of the oleic acid radical. Time of exposure (minutes)
;.
The rates
Of Oxy-
Figure &Effect of Ultra-Violet L i g h t u p o n S u s c e p t i b i l i t y of C o t t o n s e e d Oil t o
Ren a b s o r p t i o n of A u t o x i d a t i o n oleic, linoleic, and ricinoleic acids indicate a powerful retarding action of an OH group in a position analogous to that of the second unsaturated bond of linoleic acid. Hydroxy groups attached to molecules other than the one containing the unsaturated bond have retarding effects, but once absorption begins the rate seems to be that of the original oil or fat used. Apparently the retarding effect of the hydroxy groups is due to a reaction with some intermediate oxidizing compounds. The susceptibility of cottonseed oils to oxidation seems to depend largely upon the conditions to which they are subjected in the manufacturing process rather than upon their constitution. Acidity appears to play a small role as compared to variations in the oil brought about by the refining processes. Ultra-violet light has a powerful effect upon the susceptibility of cottonseed oils to oxidation. ~
Time (minutes) F i g u r e 4-Effect
of A n t i c a t a l y s t s upon A u t o x i d a t i o n of L i n s e e d Oil
Although the figures in Table I show no direct relationship between susceptibility and iodine number and acidity, they indicate that the method of treatment is a major factor in the stability of cottonseed oil toward oxidation. They also indicate the possibility that a constitutional factor may be involved. Results previously reported in this paper show that the presence of any compounds containing an hydroxy group tends to retard the oxidation reaction. Sunlight has often been mentioned as a catalyst in the oxidation of fats and oils. Though not of great interest in
Research Fund for Stearic Acid Makers-The National Association of Stearic Acid Manufacturers has approved an appropriation of $50,000 for research. The research will be directed toward reducing the cost of rubber specialties, face creams, and other articles of manufacture in which stearic acid is utilized.
