When bleaching an-edible oil or fat with acid-activated clay, the acidic sites of the clay donate a proton to the pigment molecules and form a positively charged organic cation. The pigment molecule is held to the surface of the clay by electrostatic bonding. In addition to this, there is a chemisorption process which occurs on the surface and can also produce or change the coloured pigment - for example beta-carotene – this turns the acid-activated clay to a green or blue-green colour during processing. The initial raw clay has a surface area of some 60m² per gram and this is increased to approxiametly 300m² per gram after the activation process has taken place. It is important to have the correct pore size and pore volumes present on the surface area of the clay. An optimum particle size distribution is required in order to facilitate efficient filtration after the bleaching process has taken place During the bleaching process, the active sites of the bleaching clay compete for removal of pigment, primary oxidation products, trace metals and residual phospholipids. Where required, activated carbon may be added or blended with the activated bleaching earth to facilitate the adsorption of polycyclic aromatic hydro-carbons (PAH) from the oil to within specification.
Trace metals which may be present in the oil are effeftively removed by the bleaching earth. Bleaching earth always reduces or removes trace metals and does not add trace metals so in this area the oil stability is always increased. The removal of trace metals along with the destruction of primary oxidation products greatly improves oxidative stability and shelf live.
The photosynthetic pigment chlorophyll deserves a separate discussion with regards to the bleaching process. Chlorophyll removal during the bleaching step requires a heavier dose of acid-activated clay. Its removal is desirable in oils with the exception of virgin olive oil since in addition to promoting an unwanted green colour, it can be responsible in promoting promoting photo-oxidation, which in turn lowers the flavour stability of the oil. When chlorophyll is present in the edible oil, it is important to reduce the phospholipids to the lowest levels possible since the phospholipids compete with chlorophyll for the active sites within the bleaching earth.
The highest performance adsorbents, and therefore those that offer true cost performance, can only be manufactured by the rigorous attention to pore size, pore volume, particle size distribution and surface area. If the acid-activation process is carried out correctly, both pore size and pore volumes will be optimised to the best effect possible leading directly to minimum dosage and maximum filtration rates. Please view the website "edible oil bleaching video" for more detailed information.
Further insight into the absorption of chlorophyll pigment is reported in the following paper
Co-written by this author (Davies M E, P R Shanks, D S Anderson and R S Taylor, Investigations into the Measurement of Chlorophyll Derivatives from Edible Oils, Laporte Industries Research & Development, Widnes, UK. Also published in Edible Fats and Oils Processing: Basic Principles and Modern Practices edited by D R Ericson CK, American Oil Chemists Society, Champaign IL 1989).