2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 221
compiled in Table 3.43 indicate that heating of
oil causes reactions involving double bonds. This
will result in a decrease in iodine number. As can
be deduced from changes in the composition of
fatty acids (Table 3.43), in the case of soybean
oil, linoleic and linolenic acid are the most affected.
Peroxides formed at elevated temperatures
fragment immediately with formation of hydroxy
compounds thus increasing the hydroxyl number
(Table 3.43). Therefore, determination of peroxide
values to evaluate the quality of fat or oil in
deep frying is not appropriate.
Unsaturated TG polymerize during heating
thus increasing the viscosity of the fat. Di- and
trimeric TG are formed. The increase of these
components can be monitored by means of gel
permeation chromatography (GPC) (Fig. 3.36).
Before or after methanolysis of the oil sample,
GPC is a valuable first tool to analyze
the great number of reaction products formed
during deep frying. Monomeric methyl esters
are further fractionated via the urea adducts,
while the cyclic fatty acids enrich themselves in
the supernatant. Dimeric methyl esters can be
preseparated by RP-HPLC and further analyzed
by GC/MS after silylation of the OH-groups.
A great number of volatile and nonvolatile
products are obtained during deep frying of oil
or fat. The types of reactions involved in and
responsible for changes in oil and fat structures
are compiled in Table 3.44. Some of the
reactions presented will be outlined in more
detail.
Table 3.44. A review of reactions occurring in heat
treated fats and oils
Fat/oil Reaction Products
heating
1. Deep frying Autoxidation Volatile acids
without food Isomerization aldehydes
Polymerization
esters
alcohols
Epoxides
Branched chain
fatty acids
Dimeric fatty acids
Mono- and
bicyclic compounds
Aromatic compounds
Compounds with
trans double bonds
Hydrogen, CO 2
2. Deep frying As under 1. As under 1. and in
with food and addition free fatty
added in addition acids, mono- and
hydrolysis
diacylglycerols and
glycerol
3.7.4.1 Autoxidation of Saturated Acyl Lipids
The selectivity of autoxidation decreases above
60 ◦ C since the hydroperoxides formed are subjected
to homolysis giving hydroxy and alkoxy
radicals (Reaction RS-4 in Fig. 3.19) which, due
to their high reactivity, can abstract H-atoms even
from saturated fatty acids.
Numerous compounds result from these reactions.
For example, Table 3.45 lists a series of
aldehydes and methyl ketones derived preferentially
from tristearin. Both classes of compounds
are also formed by thermal degradation of
free fatty acids. These acids are formed by
triglyceride hydrolysis or by the oxidation of
aldehydes.
Fig. 3.36. Gel permeation chromatography of heated
soybean oil (according to Rojo and Perkins, 1987).
Oil samples (composition and heating conditions see
Table 3.41) were analyzed immediately (I), as well
as after 8 h (II), 24 h (III), 48 h (IV) and 80 h (V),
1 Trimeric TG, 2 Dimeric TG, 3 TG, 4 DG, 5 free fatty
acids