In the industry, the method of removing PL is mainly hydration. That is, a certain amount of hot water is added to crude oil so that the phospholipids in crude oil expand and are then separated after condensation to be removed, The process is generally carried out at 40-45 ºC, which is relatively energy-efficient, has a short production cycle, and reduces the acid value. The crude oil phospholipid content and temperature determine the amount of water added during degumming. For example, at high temperatures, the amount of water added is about 3-5 times that of the crude oil PL content (Dijkstra, 2017). In addition, metal ions in tap water may combine with HP in oils to form NHP, which affects the degumming effect.

The degumming process parameters and the final degumming results differ for different oil samples. The results show that the phospholipid removal rates for soybean oil, rapeseed oil, rice bran oil, sunflower oil and olive oil are 98.3, 67.1, 89.8, 80.6 and 48% respectively. The phospholipid removal rate of mixed algal oil was only 19.4% (Supansa et al., 2017). Hydration degumming can only remove hydratable phospholipids with strong hydrophilic properties, while NHP with weak polarity and hydrophobicity is difficult to remove. After degumming, the phospholipid content is still 80-200 mg/kg, which cannot guarantee low phosphorus content (Aleksandra et al., 2003). Moreover, due to the high content of NHP, it is not always the best for all kinds of oil.

The acid degumming method entails adding a certain amount of phosphoric acid or citric acid in order to dissociate NHP by adjusting the pH, and then cool the phospholipids to form crystals. The phosphorus content can be reduced to less than 30 mg/kg.

Phosphoric acid was used to refine palm oil to remove phospholipids, and the optimal dose of phosphoric acid was determined to be 0.05 wt% by adopting the wet degumming process (Harrison et al., 2022). The researchers used a 1:1 mixture of citric acid and phosphoric acid to degum sunflower oil, and the degumming effect was better and the amount of phospholipid remaining was 0.05 mg/kg (Pan et al., 2000).

Some studies have shown that phosphoric acid is used for degumming vegetable oils with high phospholipid contents, such as soybean and algal oil. As a result, the phosphorus content of crude algal oil was reduced from 530 ppm to 93 ppm in the optimal acid degumming process, with a phospholipid removal rate of 82.5%, which is much higher than that of 19.4% for water degumming (Supansa et al., 2017).

At present, acid degumming has been widely used in oil processing. The technology is simple, but closely correlated with the crude oil quality, and is suitable for low phosphorus oil such as palm oil. Some vegetable oils with high non-hydratable phosphorus contents are still not up to the requirements.

In recent years, research on the application of phospholipase in vegetable oil degumming has become more and more in-depth. Enzymatic degumming uses the biological enzymatic method to convert NHP into HP, and then the hydration method to maximize the phospholipids’ removal and recovery. There are four types of phospholipases used for vegetable oil degumming. Phospholipase A₁ (PLA₁) and phospholipase A₂ (PLA₂) remove fatty acids from positions 1 and 2, respectively, and phospholipids are hydrolyzed to free fatty acids and soluble phospholipids. Phospholipase B (PLB) eliminates fatty acids from glyceryl. Phospholipase C (PLC) hydrolyzes the bond between acylglycerol and the phosphate group to release Diacylglycerol (DAG) (Jiang et al., 2015). The site of action of phospholipase D is after the phosphate group. The action sites of four different phospholipases are shown in Figure 4.

Applying PLA₁ and PLC to eight crude oils, the researchers found that the PLA₁ degumming process significantly reduced residual phosphorus to less than 10 mg/kg and that PLC degumming improved the oil yield of most oil samples (Jiang et al., 2015). The enzyme mixture Purifine® 3G was applied to crude soybean oil to reduce the residual phosphorus content to 8.9 mg/kg (dos Passos et al., 2022). To improve the stability and reusability of phospholipase, magnetic immobilized PLA was used for degumming soybean oil. The results showed that even after 5 cycles of degumming, magnetic immobilized PLA₂ retained more than 80% of its initial activity (Qu et al., 2016). Immobilized phospholipases show excellent potential for enzymatic degumming.

Enzymatic degumming has many advantages over traditional methods, including higher degumming efficiency, lower chemical requirements, higher product yield, and less environmental pollution. Despite the obvious advantages of this approach, it has drawbacks, particularly the cost of the enzyme. Moreover, the quality of different crude oils is different, so a specific enzymatic degumming process should be selected (Jiang et al., 2015). For example, for oil samples with low NHP content and high initial phosphorus content, PLC degumming with acid pretreatment can meet the requirements of physical refining and improve the yield simultaneously. For oil samples with high NHP content and low initial phosphorus content, acid-pretreated PLA₁ degumming is the best choice.

Membrane degumming is a non-thermal process that selectively separates phospholipids based on the size or properties of the membrane and is an emerging method for efficient degumming at low energy costs. Different types of membranes are applied to the crude vegetable oil membrane degumming process, such as polyethersulfone (PES) membranes, polysulfone (PSf) membranes, polyvinylidene fluoride (PVDF) membranes and ceramic membranes have shown good permeation flow with good degumming effect.

The PVDF membrane is more stable to hexane reagent than PES and PSf membranes (de Souza Araki et al., 2010). The filtration of rice bran gross oil (CRBO) using 50 kDa PVDF membranes mixed with hexane solvent achieved 95.5% phospholipid removal (Sehn et al., 2016). It was shown that membrane degumming combined with hydration degumming and acid degumming could improve the phospholipid removal rate of crude oil. By combining conventional acid degumming with PES membrane separation, the phospholipid content of jatropha oil can be reduced to less than 20 ppm (Liu et al., 2012).

In addition to the above polymeric membranes, ceramic membranes have a highly repulsive reaction to phosphorus. In a recent study using ceramic membranes to degum CRBO, phosphorus was reduced by 95%, along with a 42-62% reduction in color. The average phospholipid retention was 95±2% using 5 kDa ceramic membranes (Abdellah et al., 2020). It has been shown that a three-stage hybrid membrane process of ultrafiltration, nanofiltration and permeation vaporization treats crude canola oil with almost complete rejection of phospholipids (> 99.9%) (Abdorrezaee et al., 2021).

The membrane filtration de-adhesive process replaces the traditional energy-intensive process. It has the advantages of low energy consumption, safety, no chemicals, good retention of nutrients, reduced wastewater production, simple operation and so on. Despite these advantages, the poor stability and low osmotic flux of some organic membranes during membrane filtration and degumming lead to problems such as scaling and clogging, requiring high operation and maintenance costs, making the process economically unfeasible and unlikely to be applied on a large scale.

In addition to the above degumming processes, some novel degumming technologies are less widely used, such as Ultrasonic Assisted Enzymatic Degumming (UAED), saturated steam degumming, cavitation reactor-assisted ethanolamine degumming and electrolyte degumming. The comparison of new degumming processes for different kinds of vegetable oils is shown in Table 4 below.

The ultrasonic treatment of crude vegetable oil can separate the phospholipids completely, which has the enhanced benefit of reducing the reaction time and processing cost. When water, acid and enzyme degumming are combined with ultrasonic degumming, phosphorus content significantly reduces, which can eliminate alkali refining neutralization, improve degumming efficiency, and reduce the use of chemical degumming reagents, temperature and processing time. Cavitation reactor-assisted degumming is used to purify crude soybean oil. The degumming treatment based on cavitation can produce high-quality oil, and reduce reaction time and energy consumption, but this process has high requirements for equipment. Compared to other methods, saturated steam degumming does not add any chemical reagent and has no residual problem, which can provide a theoretical basis for industrial production. The ethanolamine degumming can be carried out at an ambient temperature, and does not need additional energy, uses simple technology, has low production cost, and avoids energy and equipment-intensive problems. The degumming agent is cheap, highly efficient and easy to obtain. By contrast, the electrolyte degumming uses potassium chloride and sodium chloride aqueous solution to improve the degumming performance, with water, total phenol, phytosterol content and taste well retained. The recycled gelatin can be used in commercial applications, which has good potential.