A. M. Gaddis

Carbonyls in oxidizing fat. V. The composition of neutral volatile monocarbonyl compounds from autoxidized oleate, linoleate, linolenate esters, and fats

The steam volatile monocarbonyl compounds in mildly autoxidized esters of oleic, linoleic, linolenic acids, and animal and vegetable fats were quantitatively estimated. The major aldehydes in oleate and linoleate were those that might be expected from the scission of reported monomeric hydroperoxide isomers. The predominance of hept-2,4-dienal and propanal in linolenate suggested that the major monomeric hydroperoxides were 12-and 16-hydroperoxy conjugated dienoic isomers. The number of minor aldehydes increased with degree of unsaturation of the fatty acid. The amounts of monocarbonyl compounds in the fats examined generally agreed with their average fatty acid composition. Appreciable amounts of heptanal in lamb and beef fat and heptanal and decanal in butterfat, under the conditions of oxidation, could not have come from the three unsaturated acids. Heating at 165°C. in all samples increased the proportions of the most unsaturated major aldehydes.

Precursors of alk-2,4-dienals in autoxidized lard

Evidence suggested that the low proportions of C10 alk-2,4-dienal found in mildly autoxidized linoleate or lard was basically due to a selective scission mechanism of the 13 and 9 conjugated linoleate hydroperoxide isomers. Spontaneous scission tended to be at the carbon linkage between a double bond and the hydroperoxide group. Thus the 13 hydroperoxide isomer produced the typically predominant amounts of hexanal. The 9 hydroperoxide isomer formed little C10 dienal and scission between the 9 and 10 carbons possibly led to some C9 alk-2-enal. Earlier studies of free aldehyde formation by autoxidized oleate, linoleate, linolenate and arachidonate support such a scission mechanism. Autoxidized linoleate was decomposed by dilute acid to C6 alkanal, some C9 enal and no dienal. Under different stress, such as heat, alkaline conditions and Cu++, large proportions of C10 dienals were formed. This appeared due to a shift in the point of scission to the saturated side of the hydroperoxide group.

Carbonyls in oxidizing fat: XII. The isolation of free aldehydes from autoxidized triolein, trilinolein and trilinolenin

Methods of aldehyde isolation have been com-pared in the separation of free aldehydes from autoxidizing triglycerides of pure unsaturated fatty acids. Vacuum distillation was the mildest method of aldehyde isolation. Amounts of decomposed monocarbonyl precursors were directly related to acidity of extraction conditions. Analysis of autoxidizing trilinolenin showed, after attainment of an early maximum, a decrease in monocarbonyl values from vacuum distillation and Girard T reagent methods. Known aldehydes isolated by the acid celite methods increased progressively. These results were considered to be due to a decline in monomeric hydroperoxides as related to free aldehydes, and decomposition of not only monomeric peroxides but also polymers by the acid celite methods. Amounts of decomposed monocarbonyl precursors (broken down) were directly related to acidity of extraction conditions. Analysis of autoxidizing trilinolenin showed a decrease in monocarbonyl values from vacuum distillation and Girard T reagent values and an increase in these values with (those of) the acid celite reaction methods. This was considered to be due to a decline in monomeric hydroperoxides and decomposition of polymers by the acid celite methods. Studies with autoxidizing lard demonstrated several factors having a strong influence on the amount of aldehydes isolated by the phosphoric acid celite reaction method. Passage through the column decreased hydroperoxide values 10−15%. Amount of monocarbonyl compounds was directly related to the time of exposure on the column. The yield of aldehydes decreased with increase in the amount of oxidized material applied to the 2,4-dinitrophenylhydrazine reaction column.

Carbonyls in oxidizing fat. VII. A comparison of methods of isolating monocarbonyl compounds

Comparison of quantitative methods for the isolation of aldehydes from lard oxidized at 23C showed an extreme sensitivity of precursors to isolation conditions. Differences existed in the quantity and class compositions of the monocarbonyl compounds isolated. Acid conditions caused considerable breakdown of alkanal and alk-2-enal precursors. Steam distillation (100C) released alkanals, alk-2-enals, and alk-2,4-dienals. The mild Girard T method apparently produced a small amt of alk-2-enals from precursors, but except in the initial stages isolated the lowest amounts of aldehydes. Complete evaluation of the method for isolation of free aldehydes was not possible because alkanals were released from precursors during the vacuum distillation procedure (50C). The Schwartz (dilute phosphoric acid), Girard T, and vacuum distillation methods isolated similar amounts of alk-2,4-dienals. Curious changes in relationships, possibly indicative of oxidation stages, were observed as autoxidation progressed.

Carbonyls in oxidizing fat. X. Quantitative differences in individual aldehydes isolated from autoxidized lard by mild methods of extraction

Major aldehydes characteristic of the oxidation of the unsaturated fatty acids present in lard differed somewhat in proportions isolated by the mild methods of separation. This indicated an extra sensitivity of specific fatty acid primary oxidation products to conditions, and some deviation from the normal in the breakdown caused by the methods. Linolenate primary oxidation products showed the greatest sensitivity to separation conditions and the stage of oxidation. The mild methods differed in the amounts of individual aldehydes separated to a degree dependent on the stage of oxidation. These results favor the Girard T method for the isolation of free aldehydes from autoxidized lard.

Effect of transesterification of lard on stability, antioxidant-synergist efficiency, and rancidity development

Glyceride rearrangement of lard did not affect its resistance to oxidation or alter the efficiency of antioxidants and synergists. Changes in stability were due to the decomposition of tocopherol and the formation of reducing substances. The position of unsaturated fatty acids in the glyceride may influence the free volatile carbonyl compounds present in autoxidized lard and in rancidity development, Interesterification under vacuum produced an odorless and colorless randomized lard with natural stability in the range of the parent lard.

Metal ion catalysis of autoxidation in a lard gel

The prooxidant effects of transition metal compounds upon lard triglycerides in an aqueous heterogeneous model system were examined. Catalytic activity of a metal salt depended on its identity, the phase in which it is dissolved, the buffering salts and pH of the system. The iron and copper inorganic salts dissolved in the aqueous phase were powerful catalysts. Other transition metal salts had little positive effect on autoxidation at acid and neutral pH values. Manganous chloride had an inhibiting action. Freeze-drying reversed catalysis patterns, and Mn++ and Co++ became the strongest prooxidants. Cobaltous and manganese stearates were the most active of such compounds. Reversals in catalytic activity were due to the formation or disappearance of aquo ions.

Carbonyls in oxidizing fat. VIII. Effect of the Pool and Klose method on monocarbonyl precursors in autoxidized lard

Monocarbonyl compounds isolated by the Pool and Klose method greatly exceeded those obtained by a mild quantitative vacuum distillation method. As indicated by amounts of individual aldehydes, the Pool and Klose method of reaction with 2,4-dinitrophenylhydrazine on hydrated alumina partially decomposed hydroperoxides of unsaturated fatty acid glycerides and appeared to shift the normal balance of isomeric hydroperoxides. High proportions of unsaturated aldehydes were separated, and a new, highly unsaturated class in substantial amounts was present. Evidence, based on yield and type of aldehyde, suggests that in polyunsaturated fatty acids the hydroperoxide may preferentially form at the appropriate carbon furthest removed from the carboxyl group in the pentadiene system. This might partially explain the low proportions of alk-2,4-dienals in the free aldehydes of lard oxidized under mild conditions.