Nonhydratable phospholipids

Analysis of the phospholipid fractions can also provide useful information on the quality of oilseeds as affected by stress due to environmental conditions in the field and during storage. Mounts and Nash found that the FFA content increased and the overall phospholipid content decreased as stress was applied (49). As far as individual PL classes were concerned, the content of PC and PI decreased significantly, while the PA and PE content increased, thus giving rise to an increased nonhydratable phospholipid (NHP) content in the crude oil. The NHPs refer to the phosphatides that, during degumming of crude oils with water, do not hydrate, swell, form gel, or precipitate and hence are not removed by centrifugation. 

Nonhydratable phospholipids. According to Myers (138), about 90% of the phospholipids are removed from the oil by water degumming. Although most of the remaining phospholipids are removed by alkali neutralization, Braae et al. (139) report that soybean oil and several other types of vegetable oils often contain some phospholipids that are not removed by alkali neutralization and washing. 

The impact of enzyme activity on the nonhydratable phospholipid content of crude soybean oil was investigated by List et al. (140). Evaluation of flakes subjected to live steam and whole beans treated by microwave heating to inactivate phospholipase D suggests that heat, moisture, and enzyme activity are important factors contributing to the formation of nonhydratable phospholipids in extracted crude oils. Approximately 8-10 minutes of microwave heating is required to completely destroy enzymatic activity. 

For further information on the nonhydratable phospholipids, the reader is referred to Hvolby (154), Letan and Yaron (155), and Nielsen (114, 156). 

The crude oil with high nonhydratable phospholipids requires acid pretreatment before refining. This may increase the chlorophyll breakdown, forming pheophy-tenes, pheophorbides, and pyropheophorbides and make the finished oil more susceptible to photooxidation (44). 

Most seed oils contain 0.2-0.8% nonhydratable phospholipids (5), specifically the magnesium (Mg) and calcium (Ca) salts of phospholipids, which cannot be removed by water degumming. For many years, a common way for nonlecithin producers to degum edible oil was to treat the oil with 0.02-1% of concentrated phosphoric acid at 70-90°C, after water degumming. Then, without the removal of any precipitated solids, the oil is caustically refined. Phosphoric acid chelates the Ca and Mg in the oil so that the nonhydratable phospholipids are converted into the hydratable form. The phosphoric acid pretreatment produces a darker lecithin with lower purity (5). 

Semi-pilot plant scale trials with crude oils containing 220-580 ppm phosphorous (28) show that nonaqueous UF separations might be used to degum and partially decolorize cmde domestic oils before physical refining. No permeates from two commercially available membranes contained phosphorous, thus indicating complete removal of hydratable and nonhydratable phospholipids (28). 

Rape seed represents one of the most important oil seeds in the countries of Central Europe. The annual crop in the Czech Republic is about 1/2 milion of tons and it is the main source for the plant oil production. Rape seed oil contains approx. 0.8 - 3.5% of lecithin (1), the main components of which are phosphatidylcholine (PC), phosphati-dylethanolamine (PE) and phosphatidylinositol (PI). All phospholipids mentioned above are potential substrates for phospholipase D (E.C.3.1.4.4). The product of this enzyme reaction is phosphatidic acid ( nonhydratable phospholipid), which is not readily removed during degumming process and affects flavour of the final product. 

One to three percent of water is mixed thoroughly with the oil at 50-70 °C. The phospholipids hydrate within 1 hour to form a gum with a higher specific density than the oil. The easily hydratable phospholipids are PC, PI and LPC. PE and PA have low hydrating properties and are therefore marked as nonhydratable phospholipids (NHPs). In practice a mix of these various phospholipids is separated in the lecithin gum. After this water degumming process, sometimes enzymatic degumming is applied for hydrolysing the NHP with phospholipase-A enzymes into hydrophilic lysophosphatidylethanolamine... 

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