Interesterification reaction activity, fatty acid composition and selectivity ratio of soybean oil

Se han llevado a cabo reacciones de interesterificación mediante la adición de ácido oleico a aceite de soja en la relación 1:2 w/w bajo diferentes condiciones de temperatura, agitación y porcentaje de catalizador. La evaluación de la interesterificación de los aceites se realizó por determinación del índice de saponificación, el índice de iodo y la composición en ácidos grasos. Este estudio mostró que el ácido linolenico, que es responsable de la inestabilidad del flavor del aceite de soja y considerado como factor primario que contribuye a la deterioración de este aceite, podría ser reducido a cantidades menores o iguales al 3%.


INTRODUCTION
Interesterification of edible oils is an important process for the modification of physical and functional properties as are hydrogénation and fractionation (1).It is usually involves at least two oils that have different fatty acids composition (2).The classical interesterification is characterized by a randomization in the distribution of acyl moitiés in the triacylglycerol molecules by applying a chemical catalyst such as sodium alkoxide, sodium potassium alloy, metalic sodium and sodium hydroxide (3,4).Interesterification is an old process whereby fats and olis can be randomized to improve plasticity, crystal habit or functional properties (5).
Chemical interesterification causes a statistical randomization of fatty acid distribution that leads to modification in triacylglycerols composition and, consequently, in physical behavior (6).
The increase in stability was attributed to a decrease in the amount of linolenic acid in soybean oil (7).The lack of frying stability and the poor frying oil performance of soybean and canola oils have been well documented (8,9).Linolenic acid has been identified by various researchers as the primary factor contributing to deterioration of these oils during high-temperature use (10).

MATERIALS AND METHODS
Soybean oil and oleic acid were obtained from Cairo Company of Fats and Soap.The interesterification reaction carried out by adding oleic acid to soybean oil with ratio 1:2 w/w and 0.2% Nickel catalyst at 60°C with stirring for 2 hours in oil bath.The experiment was repeated for 4 and 6 hours respectively, under the same conditions of temperature and catalyst percentage.The experiment was re-done at temperatures of 90 and 120°C under same conditions of catalyst percentage, temperature and stirring times reported before.The experiment giving the optimum results was repeated with 0.4% and 0.6% Nickel catalyst at 90°C with stirring for 4 hours.
Analysis of fatty acids was done by gas liquid chromatography (13) using Varian 3700 and flame ionization detector where the conditions are: -Column package 20 % diethylene glycol succinate (DEGS) on chromosorb W (60 -80 mesh) column length 6 feet with internal diameter of 0.25 inch.-Carrier gas (He) flow rate is 30ml/min and hydrogen flow rate is 30ml/min also, but air flow rate is 300ml/min.

RESULTS AND DISCUSSION
Concerning this study, the chemical characteristics of interesterified soy oil such as iodine and saponification values are registered in tables I and II.Table I shows the results of chemical characteristics of interesterification process at 60, 90 and 120°C for 2, 4 and 6 hours of stirring using 0.2% Nickel catalyst.On the other hand table II reports the chemical characteristics of soy oil which was interesterified at 90°C for 4 hours of stirring using 0.4 and 0.6 % Nickel catalyst.Since iodine value reached a low comparable with that of soy oil before earring out interesterification process (control) while saponification values moved just a little after interesterification.The iodine number represents true unsaturation of fats and fatty acids only when the double bonds are unconjugated and the structure is not severely hindered, i.e during interesterification process some of the double bonds going to be broken and iodine value moves to a low level.An interesterification at 90°C for two hours using 0.2% of nickel catalyst was markedly more active where activity of interesterification process equals droping in iodine value/time.Interesterification reaction using 0.2 % of nickel catalyst at the other temperatures and times of stirring had activity in the range of 1.43 -4.08 table III whereas the activity of interesterification process using 0.4 and 0.6 % of nickel catalyst at 90°C for 4 hours stirring were 3.45 and 2.57 respectively.We may trace that iodine values of the interesterified soy oil met linoleic and linolenic acid percentages.Also decreasing or increasing percentages of fatty acids are proportional to their molecular weights and the total molecular weights of individual fatty acids related to saponification value of the oil.Where IV = Droping in iodine value IV/time = Activity of interesterification process.
Tables IV-VII shows fatty acids composition of soy oil after doing interesterification process at different temperature and several times of stirring using various percentages of nickel catalyst comparable with that of soy oil before interesterification process.The results appear decreasing of linoleic and linolenic acids while increasing of oleic and stearic acids percentages.
From the changes in fatty acids composition during interesterification, the selectivity ratio (SR) was calculated using Allen's method (14) as original proposed by Albright (15).
[SR is defined as K2/K3, where K2 = l-U, K3 = S-So (Lo and So) represent the linoleic and stearic acid contents in the original oil (control) and L and S in the interesterified sample (table VIII).In the same manner the selectivity ratio (SR) regarding linolenic acid would be calculated and recorded in table IX.The highest values of selectivity ratio of interesterification reaction are 0.08 and 0.04 for linoleic and linolenic acid concerning the reaction using 0.2 % nickel catalyst at 60°C for 2 hours of stirring.The selectivity ratio during interesterification reaction using 0.4 and 0.6 % of nickel catalyst at 90°C for 4 hours stirring were 0.03 and 0.03 respectively for linoleic acid and 0.03 and 0.02 respectively for linolenic acid.

Figure 1 3 MB 2hr «Figure 2
Figure 1Relation between temperature and dropping in iodine value for 2, 4 and 6 hrs using 0.2 % nickel catalyst