Oils account for less than 40%. Their biosynthesis

Oils
and fats are mainly composed of triacylglycerols as major components (95 to
98%) and complex mixtures of minor compounds (2 to 5%). Minor compounds include
diacylglycerols, monoacylglycerols, phospholipids, free fatty acids, sterols
and sterol esters, waxes, tocopherols, phenolic compounds, undesirable
components, pigments such as chlorophylls and carotenoids(1, 2). The oil composition can be divided into two groups
of saponifiable and unsaponifiable materials. Glycerolipids,
including triacylglycerol, diacylglycerols, monoacylglycerols, free fatty
acids, phospholipids, galactolipids, sphingolipids and waxes comprise the
saponifiable fraction of vegetable oils. The main components of unsaponifiable fraction
consist of phytosterols, alcohols, terpenoids, tocopherols, and hydrocarbons (3).

Plant
sterols and plant stanols, collectively known as Phytosterols (PS), are generally
a major portion of unsaponifiable components. They are naturally occurring
compounds that structurally and functionally resemble cholesterol in mammals (4).
Cholesterol (27 carbon sterol) is the most well-known animal sterol and present
in relatively high abundance. There is a widespread misconception that plant
tissues 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., camelina
oil (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 saponins
and glycoalkaloids (7).
The normal value of 20–50 ppm cholesterol in vegetable oils is much lower than
the levels reported for animal fats (up to 1000 ppm), fish oils (up to 7000
ppm), dairy fats (2000–3000 ppm) and egg yolk (12500 ppm) (8).
Mammals are unable to synthesize PS and therefore can only obtain them from
their diet (4).

Higher
plants contain a complex mixture of sterols. 24-Ethyl sterols (sitosterol and
stigmasterol) account for more than 60% and 24-methylsterols account for less
than 40%. Their biosynthesis is complex and involves at least 25 steps from
isopentenyl diphosphate, the committed precursor of all isoprenoids, to end product
sterols (9).There
are about 10 common PS and more than one hundred total PS. Most plant sterols
contain 28 or 29 carbons, and most are unsaturated, and contain at least one
carbon-carbon double bond, but some sterols are completely saturated, and
contain no carbon-carbon double bonds, which are known as stanols (10).

Consumption
of PS through diet has beneficial effects on human health (11).
Since the mid-1990s, PS fortified products have been commercialized and
introduced to the market with the ability of lowering the blood cholesterol
level (12).
PS consumption is known to decrease low-density lipoprotein (LDL) cholesterol levels
by 5–15%, and thus lower cardiovascular disease risk. Plant sterols and stanols
absorption 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-enriched
margarine, spreads and yogurts. Typically, the daily intake of plant sterols
and stanols in human diet is estimated on 300 mg and 20 mg, respectively (4).

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PS
are industrially isolated from the distillates obtained from the deodorization
or steam distillation step of vegetable oils. PS can be hydrogenated to obtain
phytostanols. PS and phytostanols are high melting point compounds (14).
The solubility of free, crystalline plant sterols and stanols in edible oils
and fats is low, limiting their use especially in fat containing food. However,
PS with fat-like properties are obtained by esterifying them, e.g., plant
stanols esterified with fatty acids (15).
PS aqueous solubility is about ? 2.0 ?g/L which is lower than that of
cholesterol. This is partly due to PS larger molecular size (16, 17). Phytostanol and PS esters are
chemically stable materials, having comparable chemical and physical properties
to edible fats and oils which are insoluble in water, but soluble in non-polar
solvents, such as hexane, iso-octane and 2-propanol (14).

Analysis
of these minor constituents is essential as they are used as a reference for
edible oil regulation and for the analytical assessment of oil quality, its
origin, the extraction method, the refining procedures used, and possible
adulteration of the oils. The PS profile is used as a means of differentiating
between vegetable oils or detecting their authenticity (18).