2009_Book_FoodChemistry

3.8 Unsaponifiable Constituents 229
Table 3.51. Odor-active C 19 -steroids
Compound
5α-Androst-16-en-3-one 0.6
5α-Androst-16-en-3α-ol 0.9
5α-Androst-16-en-3β-ol 1.2
4,16-Androstadien-3-one 7.8
5,16-Androstadien-3β-ol 8.9
3.8.2.2.2 Vitamin D
Odor threshold
(mg/kg ; oil)
Cholecalciferol (vitamin D 3 ) is formed by photolysis
of 7-dehydrocholesterol, a precursor in
cholesterol biosynthesis. As shown in Fig. 3.42,
UV radiation opens the B-ring. The precalciferol
formed is then isomerized to vitamin D 3 by
a rearrangement of the double bond which is influenced
by temperature. Side-products, such as
lumi- and tachisterol, have no vitamin D activity.
Cholecalciferol is converted into the active
hormone, 1,25-dihydroxy-cholecalciferol, by hydroxylation
reactions in liver and kidney.
7-Dehydrocholesterol, the largest part of which is
supplied by food intake and which accumulates
in human skin, is transformed by UV light into
vitamin D 3 . The occurrence and the physiological
significance of the D vitamins are covered in
Sect. 6.2.2.
Ergosterol (ergosta-5,7,22-trien-3β-ol), which
occurs in yeast, moulds and algae, is provitamin
D 2 . It can serve as an indicator for fungal
contamination. A tolerance limit of 15 mg/kg
solid has been proposed for tomato products.
3.8.2.3 Plant Steroids (Phytosterols)
The sterols and stanols (hydrogenation products
of sterols) occurring in plants are known as phytosterols.
The best known representatives are the
desmethylsterols shown in 3.8.2.3.1.
The phytosterols are of interest from a nutritional
and physiological point of view because they
lower the concentration of cholesterol and LDL in
the blood plasma (cf. 3.5.1.2). The absorption of
cholesterol is inhibited, a significant effect being
reached with an intake of 1 g/day of phytosterol.
Since the normal dietary intake amounts to only
200–400 mg/day of phytosterol, margarines are
enriched with phytosterols. However, as the free
sterols are only poorly soluble in the fat phase,
sterol esters are used in the production of margarine.
Sterol esters are hydrolysed in the digestive
tract. The starting material for the extraction
of phytosterols is plant oils and tall oil (Swedish
“tall” = pine), which accumulates as a by-product
in the production of paper and pulp. Tall oil is
rich in phytostanols, mainly β-sitostanol.
3.8.2.3.1 Desmethylsterols
Cholesterol, long considered to be an indicator of
the presence of animal fat, also occurs in small
amounts in plants (Table 3.52). Campe-, stigmaand
sitosterol, which are predominant in the sterol
fraction of some plant oils, are structurally related
to cholesterol; only the side chain on C-17 is
changed. The following structural segments (only
Table 3.52. Average sterol composition of plant oils a
Component Sun- Peanut Soya Cotton- Corn Olive Palm
flower
seed
Cholesterol 0.5 6.2 0.5 0.5 0.5 0.5 0.5
Brassicasterol 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Campesterol 242 278 563 276 2655 19 88
Stigmasterol 236 145 564 17 499 0.5 42
β-Sitosterol 1961 1145 1317 3348 9187 732 252
5 -Avenasterol 163 253 46 85 682 78 0.5
7 -Stigmasterol 298 0.5 92 0.5 96 0.5 51
7 -Avenasterol 99 34 63 18 102 30 0.5
24-Methylene- 204 0.5 53 0.5 425 580 0.5
cycloartenol
a Values in mg/kg.