BLEACHING EARTHS Bleaching earths are well known for their pigment removal properties, hence the name bleaching earth. They are also an important absorbent for many other impurities in oils including primary and secondary oxidation products, residual gums, soaps and metals. Each of these impurities, if not removed, would adversely affect the properties of the oil. This article focuses on the importance of bleaching earths in the removal of metals from edible oils. The presence of metals in fully refined oils leads to colour reversion and oxidation of the oil during storage and use, resulting in the development of off-odours, off-flavours, rancidity and a shorter shelf life. Some metals, if accumulated in the body, may also be toxic.
Metal sources
There are two main sources of metals in edible oils – endogenous and exogenous. Endogenous sources of metals are from where the plant grows, such as the soil, water, pesticides and fertiliser. Exogenous sources of metals are from the handling and processing of the oil crop, including transport, storage, crushing, extraction, refining and hydrogenation processes. Endogenous sources are responsible for the presence of those metals that are beneficial to the growth and development of plants and humans, such as sodium, potassium, calcium, magnesium, iron, manganese, zinc and copper. However, some potentially harmful heavy metals such as lead, cadmium, chromium, cobalt, nickel and copper may also be present as pollutants in the environment, such as in ground waters. Cadmium and lead contaminants accumulate in the body as they have long half-lives and can be present from fuel and industrial emissions. Exogenous sources of iron and copper are present due to corrosion and erosion of processing and handling equipment. Rust from mild steel and copper from brass or bronze fittings are common sources. Iron and copper are catalysts for the oxidation of the oil. Iron catalyses the formation of hydroperoxides, and copper catalyses the decomposition of the hydroperoxides to secondary oxidation products. Iron and copper together synergically promote rapid oxidation of the oil. It should be noted that no copper or copper containing metals should come into contact with the oil at any stage, from harvesting until final use. Residual sodium in the form of soap may be present in chemically neutralised oil. Transition metals, such as nickel, copper and chromium, are present in post 28 OFI – JUNE 2020
metal and bleaching earths.indd 2
Removal of m The presence of metals in fully refined oils leads to colour reversion and oxidation, resulting in off-odours, off-flavours, rancidity and a shorter shelf life. Bleaching earths play an important role in removing metal impurities from edible oils Patrick Howes refined hydrogenated oils, resulting from the use of hydrogenation catalyst.
Removal and reduction of metals All metals present in the oil, regardless of their source, need to be removed or reduced, in order to produce wholesome and stable refined oil. The reduction of the concentration of metals take place at various stages in the refining of the crude oil. Washing of the oil will remove some of the metals. In chemical refining, the caustic neutralisation and washing steps will help greatly in the removal of metals. Degumming also helps reduce the concentration of metals. In physical refining, the caustic treatment and washing stages are eliminated, and the degummed oil normally goes directly to the bleacher.
The demands on the bleaching stage in physical refining are therefore much higher than for chemical refining. Therefore, the bleaching earth consumption is higher for physical refining compared with chemical refining. As the degummed and optionally caustic-treated and washed oil proceeds to the bleacher, there remains traces of metals that need to be removed. The metals are present in several inorganic and organic forms, such as as gums and soaps. Some pigments carry metals, such as chlorophyll which contains magnesium. There are various mechanisms involved in the removal of the different forms of the metals present.
Metals in different forms
Metals such as calcium and magnesium can be present as salts of phosphatidic
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11/06/2020 15:03:21
