2009_Book_FoodChemistry

666 14 Edible Fats and Oils
The detection of adulteration of oils and fats has
been improved further by coupled HPLC and GC
of the minor constituents. The saponification of
the sample is not required, free and esterified
compounds being detected separately.
An example is the differentiation between the
olive oil qualities “extra vierge” and“lampante”.
After esterification of the free OH-groups with
pivalic acid, the free fat alcohols, wax esters,
free acids, triterpene alcohols and esters are
eluted in a relatively narrow fraction in HPLC
and separated from the triacylglycerides. The
eluate is transferred to a gas chromatograph
and analyzed on an apolar capillary column. As
shown in Fig. 14.7, a clear distinction is made
between “lampante” oils and “extra vierge”
oils because the former have high contents of
wax and sterol esters (sitosterol, 24-methylene
cycloartenol) (cf. 14.3.2.1.1).
14.5.2.5 Melting Points
In addition to specific density, index of refraction,
color and viscosity, the melting properties can be
used to identify fats and oils.
The composition and the crystalline forms
(cf. 3.3.1.2) of triacylglycerols present in fat
determine the melting points and the temperature
range over which melting occurs. The onset, flow
point and end point of melting are of interest.
They are determined by standardized procedures.
The melting properties of fat are more accurately
determined by differential thermal analysis. The
temperature difference is measured between the
fat sample and a blank, i. e. a thermally inert
substance, as a function of the heating temperature
(Fig. 14.8). In this way the temperatures at
which polymorphic transitions of fat occur are detectable.
In addition, the content of solid triacylglycerols
can be assessed from the heat absorbed
during melting at various temperatures. Thus, the
solid triacylglycerol (TG) portion of coconut oil
at −3 ◦ C can be calculated using data from the
recorded curve (Fig. 14.8) and the following formula:
% Coconut (solid TG)=
Area (BCDE)
Area (AEDA) · 100
(14.11)
The solid: liquid ratio of acylglycerols is of importance
in fat hydrogenation and interesterification
processes (cf. 14.4.3). This ratio can also be
assessed using the Solid Fat Index by measuring
the expansion of the fat, i. e. the volume increase
of a fat during its transition from solid to liquid
and by 1 H-NMR spectroscopy (cf. 14.5.1).
Fig. 14.7. On-line HPLC-GC of sterol and wax fractions
of olive oils. a “Extra vierge” oil, b “lampante”
oil. Peak 1: sitosterol, peak 2: 24-methylene cycloartenol,
peak group 3: wax esters, peak 4: sitosterol
ester, peak 5: 24-methylene cycloartenolester (according
to Grob et al., 1991)
14.5.2.6 Chemometry
To solve difficult problems in food chemistry,
e. g., the detection of the authenticity of olive