One of the compounds most biologically active in vegetable oils is squalene, which is a terpenoid hydrocarbon that in plants is synthesized as a biochemical intermediate of the phytosterol biosynthetic pathway. Squalene is a high‐value compound with some applications in the pharmaceutical and cosmetics industries (Dunford 2004). Virgin olive oil and crude sunflower are rich sources of this compound (Grompone 2005).
Other common compounds in vegetable oils are pigments as carotenoids and chlorophylls. There are two types of carotenoids, xanthophyll and carotenes, where the differences is the presence of oxygen in the molecule or not, respectively. In sunflower oil xanthophyll and lutein are the majority pigments. In sunflower oil β‐carotene is added to increase its oxidation stability because of a synergistic effect with tocopherols (Yanishlieva et al. 2001).
Phospholipids are the major constituents of biological membranes. They have great antioxidant effects due to their activity as metal scavengers, their synergistic action with tocopherols, and their capacity to decompose hydroperoxides (Smouse 1995; Carelli et al. 1997). To maintain the fluidity of membranes, phospholipids have a high degree of fatty acid unsaturation, which make them susceptible to oxidation. Crude sunflower has high levels of phospholipids as phosphatidylcholines, phosphatidylethanolamines, phosphatidylinositols, and phosphatidic acids (Gupta et al. 2002).
2.2.5 Antinutritional Factors
“Antinutritional factors are present in different food substances in varying amounts, depending on the kind of food, mode of its propagation, chemicals used in growing the crop as well as those chemicals used in storage and preservation of the food substances” (Inuwa et al. 2011). Oxalates, phytates, tannins, trypsin inhibitors, and hemagglutinins are examples of antinutritional compounds present in vegetable oils. For example, palm and soybean oil have oxalate content of 4.95 and 1.16 g/kg, respectively, and the lethal dose established for oxalates is 2–5 g/kg. The content of alkaloids in palm oil is 0.16/100 mg and in soybean oils is 16.3/100 mg, where the lethal dose for alkaloids is 20/100 mg (Inuwa et al. 2011; Chatepa et al. 2019).
Trypsin inhibitor can bind with trypsin to inhibit the enzyme. The phytic acid or inositol could form insoluble compounds in the form of stable phytates inducing the suppression of living bodies. This compound is a strong chelating agent affecting to the bio‐availability of some health‐related minerals including zinc, calcium, magnesium, phosphorus, and iron. It has been demonstrated in some studies conducting in rats fed with soy products that the bio‐availability of zinc was low. Interactions of diets rich in oilseed crops are still under investigation as many factors could be of influence, such as food processing conditions or digestibility of the foods (Wang 2016).
Tannins not only could provide a bitter taste but also can react with trypsin and amylase or their substrates by the hydroxide radicals and reduce the utilization rate of protein or carbohydrate. They could also interfere with the absorption of some minerals such as iron ions and form insoluble complexes on the intestinal mucosal surface damaging the intestinal wall. Similarly, the combination of tannin with metal ions including calcium, iron, and zinc could form complexes with vitamin B12 and negatively affect their absorption (Wang 2016).
2.3 Factors Affecting Oil Yield
Oil yield depends on a number of interrelated factors. The potential yield is defined as a yield of a cultivar when grown in environments to which it is adapted; with non‐limiting nutrients and water; and with pests, diseases, weeds, lodging, and other stresses being effectively controlled (Evans and Fischer 1999). There is a gap between the potential yield and the actual yield of a crop. Once the type, variety, and location are determined, climatic conditions and agronomic practices have an impact on the amount of oil that can be obtained. The potential yield is primarily determined by the type of crop, as each has its characteristic fruit or seed density, and those have an average amount of percentage oil in them. Most oilseeds typically accumulate 20–50% of the dry weight of their seed tissue as storage oil, usually as triacylglycerol. However, there are some oilseed species, such as candlenut or sesame, which contain as much as 60–76% oil in their seeds (Walker 2008). Among the cultivated oil sources around the world, Palm fruit has the highest yield, followed by soybeans, olives, rapeseed, and sunflower, whereas poppy seed and sesame have the lowest yields (FAOSTAT 2018).
According to the definition, the potential yield is calculated in the absence of stresses, but there exist some abiotic factors – drought, extreme temperatures, and salinity – or biotic factors – diseases – that severely affect the growth and yield of the crops. Climatic periods of drought may affect crops around the world, and according to the studies in climatic changes, such extreme periods might