Oilsand fats are mainly composed of triacylglycerols as major components (95 to98%) and complex mixtures of minor compounds (2 to 5%). Minor compounds includediacylglycerols, monoacylglycerols, phospholipids, free fatty acids, sterolsand sterol esters, waxes, tocopherols, phenolic compounds, undesirablecomponents, pigments such as chlorophylls and carotenoids(1, 2). The oil composition can be divided into two groupsof saponifiable and unsaponifiable materials. Glycerolipids,including triacylglycerol, diacylglycerols, monoacylglycerols, free fattyacids, phospholipids, galactolipids, sphingolipids and waxes comprise thesaponifiable fraction of vegetable oils.
The main components of unsaponifiable fractionconsist of phytosterols, alcohols, terpenoids, tocopherols, and hydrocarbons (3).Plantsterols and plant stanols, collectively known as Phytosterols (PS), are generallya major portion of unsaponifiable components. They are naturally occurringcompounds that structurally and functionally resemble cholesterol in mammals (4).Cholesterol (27 carbon sterol) is the most well-known animal sterol and presentin relatively high abundance.
There is a widespread misconception that planttissues do not have cholesterol, but this sterol often accounts for 1–2%of the total plant sterols with very few exceptions (5).Some tropical oils have relatively large amounts of cholesterol, e.g., camelinaoil (188 ppm), cocoa butter (59 ppm), linseed oil (42 ppm), palm kernel oil(39.6 ppm), palm oil (26 ppm), and coconut oil (23 ppm) (6).
It amounts to 30 to 40% in members of some families (e.g. Solanaceae,Liliaceae, and Scrophylariaceae), where it serves as a precursor of steroidal saponinsand glycoalkaloids (7).The normal value of 20–50 ppm cholesterol in vegetable oils is much lower thanthe levels reported for animal fats (up to 1000 ppm), fish oils (up to 7000ppm), dairy fats (2000–3000 ppm) and egg yolk (12500 ppm) (8).Mammals are unable to synthesize PS and therefore can only obtain them fromtheir diet (4).Higherplants contain a complex mixture of sterols. 24-Ethyl sterols (sitosterol andstigmasterol) account for more than 60% and 24-methylsterols account for lessthan 40%.
Their biosynthesis is complex and involves at least 25 steps fromisopentenyl diphosphate, the committed precursor of all isoprenoids, to end productsterols (9).Thereare about 10 common PS and more than one hundred total PS. Most plant sterolscontain 28 or 29 carbons, and most are unsaturated, and contain at least onecarbon-carbon double bond, but some sterols are completely saturated, andcontain no carbon-carbon double bonds, which are known as stanols (10).Consumptionof PS through diet has beneficial effects on human health (11).Since the mid-1990s, PS fortified products have been commercialized andintroduced to the market with the ability of lowering the blood cholesterollevel (12).
PS consumption is known to decrease low-density lipoprotein (LDL) cholesterol levelsby 5–15%, and thus lower cardiovascular disease risk. Plant sterols and stanolsabsorption in humans is considerably less (2-5%) than that of cholesterol (60%)(13).Food rich in PS include vegetables, fruits, nuts, cereals, vegetable oils, and PS-enrichedmargarine, spreads and yogurts. Typically, the daily intake of plant sterolsand stanols in human diet is estimated on 300 mg and 20 mg, respectively (4).PSare industrially isolated from the distillates obtained from the deodorizationor steam distillation step of vegetable oils. PS can be hydrogenated to obtainphytostanols.
PS and phytostanols are high melting point compounds (14).The solubility of free, crystalline plant sterols and stanols in edible oilsand fats is low, limiting their use especially in fat containing food. However,PS with fat-like properties are obtained by esterifying them, e.g., plantstanols esterified with fatty acids (15).
PS aqueous solubility is about ? 2.0 ?g/L which is lower than that ofcholesterol. This is partly due to PS larger molecular size (16, 17).
Phytostanol and PS esters arechemically stable materials, having comparable chemical and physical propertiesto edible fats and oils which are insoluble in water, but soluble in non-polarsolvents, such as hexane, iso-octane and 2-propanol (14).Analysisof these minor constituents is essential as they are used as a reference foredible oil regulation and for the analytical assessment of oil quality, itsorigin, the extraction method, the refining procedures used, and possibleadulteration of the oils. The PS profile is used as a means of differentiatingbetween vegetable oils or detecting their authenticity (18).