Food Chemistry

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 af-
fected. Peroxides formed at elevated temperatures
fragment immediately with formation of hydroxy
compounds thus increasing the hydroxyl number
(Table 3.43). Therefore, determination of perox-
ide 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 sam-
ple, 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 struc-
tures are compiled in Table 3.44. Some of the
reactions presented will be outlined in more
detail.

3.7.4.1 AutoxidationofSaturatedAcylLipids......................

The selectivity of autoxidation decreases above
60 ◦C since the hydroperoxides formed are sub-
jected 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 reac-
tions. For example, Table 3.45 lists a series of
aldehydes and methyl ketones derived preferen-
tially 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.

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

Fig. 3.36.Gel permeation chromatography of heated

soybean oil (according toRojoandPerkins, 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