2009_Book_FoodChemistry

14.5 Analysis 663
Table 14.20. Determination of the fat content of canned corned beef
Analytical method Fat content Fatty acid composition
(%) a (g/100 g)
Saturated
acids 18:1 (9) 18:2 (9,12) 18:3 (9,12,15)
1. Dried sample is extracted
with ethyl ether 7.9 3.98 2.06 0.05 0.08
2. Sample is homogenized in 95%
ethanol and then extracted
with ether 15.8 4.0 2.60 0.77 0.32
3. Sample is hydrolyzed with
4mol/1 HC1(at60 ◦ C for 30 min),
then extracted with ether 12.3 5.66 3.94 0.95 0.71
4. Sample is hydrolyzed with conc.
HC1 (at 100 ◦ C for 1 h), methanol
added and then extracted
with carbon tetrachloride 13.9 2.45 1.68 0.34 0.21
5. Sample is homogenized in chloroform
methanol mixture (2:1 v/v),
washed with water and then
the chloroform phase recovered 11.2 4.89 3.31 0.85 0.39
a The fat is determined gravimetrically after the solvent is evaporated.
A rapid and accurate determination of fats or oils
in food is achieved by IR- (cf. 15.3.1) and 1 H-
NMR spectrometry. The method is based on the
fact that hydrogen nuclei in fluids respond to substantially
higher magnetic resonance effects than
do immobilized hydrogen atoms of solid substances.
Thus, the 1 H-NMR signal of a fluid, such
as an oil, differs from that of a nonoil matrix, such
as carbohydrate, protein or firmly-bound water.
The intensity of the signal is directly proportional
to the oil content. This method is also of great
value in oilseed selection or breeding research,
since it permits determination of the oil content
in a single kernel without damaging it by grinding
or drying, i. e. retaining its ability to germinate.
The proportion of solid to fluid triacylglycerols in
fat can also be determined using 1 H-NMR spectrometry.
14.5.2 Identification of Fat
14.5.2.1 Characteristic Values
For both, the identification and the determination
of the quality of a fat or oil, the older lipid
chemistry defines a series of characteristic values
in which the reagent uptake is used to quantitatively
estimate the selected functional groups
or calculate the constituents of a fat or oil. The
introduction of new analytical methods, such as
gas chromatography of fatty acids and the HPLC
of triacylglycerols (cf. 3.3.1.4), has made many
of these measures obsolete. The values which are
still used to differentiate fats or oils are:
Saponification Value (SV). This is the weight of
KOH (in mg) needed to hydrolyze 1 g of fat or oil
under standardized conditions. The higher the SV,
the lower the average molecular weight of the
fatty acids in the triacylglycerols (for examples,
see Table 14.21).
Acid Value (AV). This value is important for a first
quick characterization of the quality of a fat. It
is the number of milligrams of KOH needed to
neutralize the organic acids present in 1 g of fat.
Iodine Value (IV). This number is the number
of grams of halogen, calculated as iodine, which
bind to 100 g fat (cf. 3.2.3.2.1). The halogen uptake
by fat or oil is affected by the contents of
oleic (IV: 89.9), linoleic (IV: 181) and linolenic
(IV: 273) acids. Examples of iodine numbers are
provided in Table 14.21.