According to the United States Depa

According to the United States Department of Agriculture, 168.85
million metric tons of vegetable oils are estimated to be produced
globally at the end of 2013-2014 season [1]. World vegetable oil
production has increased over the past decades, especially production
of palm oil, soybean oil, rapeseed oil (canola) and sunflower oil (Table
1). Vegetable oils are regarded as the healthier choice relative to
animal fats in view of their unsaturated fatty acid and cholesterolfree
contents. In this fast-paced society, frying remains as one of the
popular methods in food preparation. Consumption of ready-made
deep-fried food is high, especially in developing countries. Highly
oxidized fatty acids are consumed through intake of these fried
foods. Edible vegetable oil is the major ingredient in these fried food
products. Therefore, the cost of the oil becomes the most important
factor to be considered in terms of economy. As a result, vegetable oil
is often to be repeatedly heated to ensure cost effectiveness. The oil is
thus reused until it is discarded and replaced with fresh oil.