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Bleaching in Edible Oil Refining

Modern oil and fat bleaching processes remove a variety of impurities, including various pigments. Removing impurities from finished oils requires advanced processes that use a combination of different bleaching agents to agglomerate specific impurities, which are then filtered out along with the bleaching agents. Efficient bleaching can remove many pigments, such as:

Carotenoids and chlorophyll

Oxidation products (through decomposition and partial removal)

Contaminants, such as soap and trace metals

Trace phospholipids

Polycyclic aromatic hydrocarbons

Other contaminants

Removal of these substances is essential to ensure the commercial viability of the product, as they can negatively impact the quality and stability of the final product and affect the performance of subsequent processes, such as phospholipids used in double-distilled water (DDW).

Oil Bleaching Process

Typically, bleaching involves mixing the oil with the bleaching agents and then separating them using filters. Once saturated, the original filter becomes regenerated, and a new filter takes over the filtration duties. Saturated filters contain clay (a bleaching agent), which still has some bleaching capacity. Prefiltration involves passing the raw oil through a saturated filter before mixing it with fresh clay. This technique can reduce clay consumption. However, prefiltration is only suitable for drying oils; therefore, it may not always be possible.

Two-Stage Bleaching of Edible Fats and Oils

Using two-stage oil bleaching techniques (for example, mixing the oil with different reagents under different conditions, such as temperature and humidity), the bleaching process can often be optimized, resulting in the deepest extraction of components such as chlorophyll. Two-stage bleaching, typically with two filtration steps, is used to achieve the best results for refined oils (such as coconut or grapeseed) with minimal color and/or the highest purity. To optimize investment costs, two-stage bleaching can be combined with single-stage filtration, saving significant amounts of bleaching clay in equipment used in processes such as palm oil or rapeseed oil processing.

Commonly used bleaching agents include:

Natural bleaching earth, particularly useful for treating animal fats, although its color reduction is limited; acid-activated bleaching earth, widely used to remove various types of impurities; activated carbon for the removal of polycyclic aromatic hydrocarbons and various specific contaminants; and synthetic amorphous silica, primarily used in wet bleaching, with a focus on the selective removal of phospholipids, trace metals, and soap stock.