Metal chelating — Fats and oils obtain metal contents from the soils where plants
are grown and later from contact during crushing, processing, and storage. Many
of the metals promote autoxidation, which results in off-flavors and odors accompanied
by color development in the finished fat and oil products. Studies have
identified copper as the most harmful metal, followed by iron, manganese, chromium,
and nickel. The effects of pro-oxidants can be diminished by using chelating
agents before and after deodorization. The most commonly used chelating agents
are citric acid, phosphoric acid, and lecithin. Citric acid is metered into the oil as
an aqueous or alcoholic solution at levels of 50 to 100 ppm. Citric acid decomposes
at 347°F (175°C), and the usual practice is to add it during the cooling stage in
deodorization. Citric acid added prior to deodorization decomposes at deodorization
temperatures but still affords a degree of protection from trace quantities of
oxygen present during preheating. Phosphoric acid, when used, is added to the
deodorizer in an aqueous solution at a concentration of no more than 10 ppm
because a slight over-addition can lead to off-flavors in some oils (e.g., watermelon
flavor in soybean oil). Lecithin has been used to chelate metals at 5 ppm.