2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 191
Table 3.25. Purified potato acyl hydrolase: substrate
specificity
Substrate Relative Substrate Relative
activity
activity
(%) (%)
Monolein 100 Lecithin 13
Diolein 21 Monogalactosyl-
Triolein 0.2 diacylglycerol 31
Methyloleate 28 Digalactosyl-
Lysolecithin 72 diacylglycerol 17
3.7.2 Peroxidation of Unsaturated
Acyl Lipids
Acyl lipid constituents, such as oleic, linoleic
and linolenic acids, have one or more allyl
groups within the fatty acid molecule (cf. Table
3.7) and thus are readily oxidized to
hydroperoxides. The latter, after subsequent
degradation reaction, yield a great number of
other compounds. Therefore, under the usual
conditions of food storage, unsaturated acyl
lipids cannot be considered as stable food
constituents.
Autoxidation should be distinguished from
lipoxygenase catalysis in the process denoted
as lipid peroxidation. Both oxidations provide
hydroperoxides, but the latter occurs only in the
presence of the enzyme.
Lipid peroxidation provides numerous volatile
and nonvolatile compounds. Since some of
the volatiles are exceptionally odorous compounds,
lipid peroxidation is detected even
in food with unsaturated acyl lipids present
as minor constituents, or in food in which
only a small portion of lipid was subjected to
oxidation.
Induced changes in food aroma are frequently assessed
by consumers as objectionable, for example,
as rancid, fishy, metallic or cardboardlike, or
as an undefined old or stale flavor. On the other
hand, the fact that some volatile compounds, at
a level below their off-flavor threshold values,
contribute to the pleasant aroma of many fruits
and vegetables and to rounding-off the aroma of
many fator oil-containing foods should not be neglected.
3.7.2.1 Autoxidation
Autoxidation is quite complex and involves
a great number of interrelated reactions of intermediates.
Hence, autoxidation of food is usually
imitated by the study of a model system in which,
for example, changes of one unsaturated fatty
acid or one of its intermediary oxidation products
are recorded in the presence of oxygen under
controlled experimental conditions.
Model system studies have revealed that the rate
of autoxidation is affected by fatty acid composition,
degree of unsaturation, the presence and activity
of pro- and antioxidants, partial pressure of
oxygen, the nature of the surface being exposed to
oxygen and the storage conditions (temperature,
light exposure, moisture content, etc.) of fat/oilcontaining
food. The position of the unsaturated
fatty acid in the triacylglyceride molecule also influences
the rate of autoxidation. TGs with an unsaturated
fatty acid in the 1- or 3-position oxidize
faster than TGs with an unsaturated acyl residue
in the more protected 2-position.
The oxygen uptake of an unsaturated fatty acid as
a function of time is shown in Fig. 3.18. Studying
this figure helps in the understanding of the
elementary steps involved in autoxidation. The
extreme case 1 demonstrates what has invariably
been found in food: the initial oxidation products
are detectable only after a certain elapsed storage
time. When this induction period, which is typical
for a given autoxidation process, has expired,
a steep rise occurs in the reaction rate. The proox-
Fig. 3.18. Autoxidation of unsaturated acyl lipids.
Prooxidant concentration: 1 low, 2 high