For the base-catalyzed interesterification reaction carried out at low (<100°C) temperatures, a number of mechanisms have been proposed in the literature. As these mechanisms are not in accordance with experimental observations, a novel mechanism will be proposed instead. This novel mechanism assumes that the reaction of a base (such as sodium methanolate) with the oil will eventually lead to the abstraction of an α-hydrogen from a fatty acid moiety and that the enolate anion thus formed acts as the catalytic intermediate. This enolate can re-abstract a proton from the hydroxyl group of a partial glyceride, whereupon the resulting alcoholate attacks the carbonyl group. This leads to a new ester and a glycerolate anion that then regenerates a new enolate anion. If the enolate anion reacts with methanol, this will lead to the formation of a fatty acid methyl ester and a glycerolate anion that again regenerates an enolate anion. Reaction with water leads to catalyst inactivation by converting the enolate anion to an unreactive fatty acid moiety (free fatty acid or soap) and a partial glyceride. Thermal inactivation of the enolate intermediate is assumed to be through the formation of catalytically inactive β-keto esters. The accelerating role of acetone is explained by assuming this compound to act as a highly mobile hydrogen transfer agent that facilitates the reaction between the glycerolate anion and the α-hydrogen atoms in fatty acid moieties. The above assumptions are independently supported by the observation that the addition of acetone-d6 to an interesterifying reaction mixture leads to the almost quantitative incorporation of deuterium into the α-position of fatty acid moieties. Theoretical calculations on the enolate-alcohol system at PM3 level are also in agreement with the enolate mechanism.
- Enolate anion
- Ester interchange
ASJC Scopus subject areas
- Food Science
- Industrial and Manufacturing Engineering