Effect of Hydrogenation on oil and Isomerization

Hydrogenation brings changes on physical and functional properties of fats and oil. it  raises the melting point and reduces the iodine value (IV) of triglycerides. Completely hydrogenated fats (IV<1) are solid and brittle at room temperature. An advantage of hydrogenation is that a wide range of physical properties can be achieved. Functional properties of hydrogenated fat can be determined by solid fat index (SFI) and nuclear magnetic resonance (NMR). The parameters describe the relative amount of solid fat and liquid oil at fixed temperature. NMR gives value according to atomic structure and molecular weight of the substances.

There are some terminologies that need to be understood in hydrogenation process. They are as below.

Catalyst poisoning: Phosphorous and Sulphur compounds, free fatty acids and residual soap (due to inadequate washing after neutralization) have a poisoning effect on nickel catalyst. So, it is necessary to ensure that the oil is free from these substances prior to hydrogenation.

Churning: It is done for proper mixing of catalyst, hydrogen, bleaching earth (1.25 %) and charcoal (0.125 %).Mixing of vitamin A and D premix, sesame oil and distribution of heat with the help of agitator or impeller and baffle.

Mixing: Vitamin A and D premix and sesame oil is mixed at 70°C. Here, small amount of sesame oil is added in vanaspati ghee. It shows positive boudin test (due to sesamolin). This test is also positive for adulterated dairy ghee with vanaspati ghee.


Two types of isomerization generally occurs during hydrogenation.

  1. Geometrical isomerization
  2. Positional isomerism

Geometrical isomerization: this refers to conversion of ‘cis’ double bond to ‘trans’ double bond. Most un-hydrogenated plant oil have only cis double bond in their fatty acid constituents. However, milk and animal depot fat have modest amount of natural trans fatty acid.




Positional isomerization: this refers to the shift of the double bond position within the chain. When the catalyst have sufficient hydrogen atom, another hydrogen is added such that the original double bond is converted to saturated single bond between carbons. When there are not sufficient hydrogen atoms to cover the catalyst a hydrogen may be removed from either side by partially saturated bond. This produces a new double bond which may form in its original position or move one carbon away.

When double bond refers, it may take either the cis or trans configuration. If hydrogenation is carried out at high pressure and low temperature, high agitation, high gassing rate and low catalyst level, then catalyst is more saturated or covered with pre hydrogen atom and there is little geometrical and positional isomerization.

Trans isomerization: if the purity of catalyst is low or there is more use of spent catalyst, there is more chance of formation of trans isomers. This type of isomerization also depends on amount of hydrogen molecule, temperature and pressure during hydrogenation.