1. Introduction
Refined edible oils with less than 1 wt% free fatty acids (FFA) are mainly used as feedstocks for industrial biodiesel production and the feedstock price is one of the most significant factors affecting the economic viability of fuel production [1], [2], [3] and [4]. The byproducts, such as soapstock, acid oils and deodorized distillates, obtained during the refining of edible oils have been suggested as alternative and cheaper feedstocks for biodiesel [5], [6] and [7]. Especially, the crude oils of palm and rice bran have a high content of FFA and the amount of acid oils discharged during the refining of these edible oils, is large, at ca. 10–20 wt% refined oil output, e.g. 5.6–11 million tons for palm. The main components of acid oils are FFA, remainder comprising triglycerides, and hence the oils are almost solid at room temperature owing to the higher melting points of FFA. Such oils with a high FFA content could not be used as feedstocks using conventional processing methods, using homogeneous alkali catalysts.