The decolorization stage of vegetable oils requires significant inputs, including energy, adsorbents, and filter aids, which account for a large portion of the oil refining cost. Optimizing the process is a decisive factor in enhancing the competitiveness of vegetable oil refineries. Optimizing the decolorization process is crucial.
Oils and fats extracted from oilseeds, fruits, and kernels can yield oils with good flavor and few impurities, or oils with high impurity content and poor flavor. In this sense, the purpose of refining is to eliminate these impurities and improve their physical, chemical, and sensory properties, thereby making the resulting oil suitable for human consumption.
The final stage in obtaining edible vegetable oils is crude oil refining, which involves several physical and chemical steps aimed at removing impurities. One of the most subtle steps is decolorization, removing impurities from soybeans through adsorption processes. Bleaching or decolorization is traditionally used to describe pigment reduction processes, such as adsorbing chlorophyll and carotenoids from vegetable oils in adsorbents called bleaching clay or decolorizing clay. After mixing and stirring the vegetable oil with the adsorbent and controlling the temperature to promote pigment removal, the adsorbent is removed from the vegetable oil through a filtration process, resulting in a cleaner and more stable oil. The decolorization process is a crucial step in refining edible oils and can be considered one of the most expensive steps.
The
bleaching process occurs through several different mechanisms, including various chemical actions.
These mechanisms include: 1. Adsorption, the mechanism of adsorbing impurities, which occurs in three different ways: physical, chemical (through chemical absorption, forming electrochemical bonds with the bleaching clay surface); and through molecular sieves (during filtration, molecular sieves retain impurities in the bleaching clay under pressure).
2. Filtration, a mechanism that primarily removes suspended solids or contaminants through physical means. Simultaneously, the physical action of removing suspended bleaching clay through filtration removes tiny contaminants absorbed by the bleaching clay particles. Filters used in the decolorization process include process filters (vertical and horizontal flat plate filters) and polishing filters (cartridges, flat plates).
3. Catalysis, a mechanism that removes contaminants through interaction with the bleaching clay surface. For example, peroxides are effectively reduced through the interaction between bleaching clay and oil (which polymerizes or decomposes into volatile products). The adsorbents used in the vegetable oil decolorization stage are typically aluminum silicate, bentonite, activated carbon, or montmorillonite, which can be treated by acidification to increase the specific surface area of contact, thereby increasing the material's adsorption capacity. Natural bleaching clay, known as fullerite, contains hydrated aluminum silicate. The most common natural bleaching adsorbent is montmorillonite. Currently, acid-activated (sulfuric acid and hydrochloric acid) decolorizing adsorbents are used instead of natural bleaching clay. Studies show that natural bleaching clay absorbs 20-25% w/w of oil, while activated bleaching clay absorbs 35-40% w/w.
Impurities present in the unbleached oil are adsorbed by van der Waals forces at the active sites of the adsorbent. The adsorption force depends on factors such as the electrostatic force between the impurities and the active sites, the particle size of the impurities, the stirring of the mixing medium between the adsorbent and the oil to be decolorized, the porosity of the adsorbent, and the specific surface area of the adsorbent. Traditional decolorization processes using activated clay remove pigments such as chlorophyll and other components unsuitable for human consumption, whether naturally occurring in the oil or formed during refining. Bioactive compounds like tocopherols and sterols can cause loss of oxidative stability in oils and increase free fatty acids. Conventional decolorization processes for vegetable oils often lead to problems such as oil loss carried by spent adsorbents, environmental issues, and high wastewater treatment costs. Optimizing the decolorization process depends on factors such as temperature, adsorbent dosage, reaction time, and the type of oil to be decolorized. Key parameters for
decolorization include: process, adsorbent dosage, adsorbent type, temperature, time, moisture, and filtration. In addition to these parameters, the concentration of soaps and phospholipids in the oil to be decolorized, the formation of a uniform pre-layer on the filter plates, the filtration area, and the spacing between filter plates are all control parameters for an effective decolorization process.
