Food Lipids: Chemistry, Nutrition, and Biotechnology

The rate of oxygen consumption during the early stages of storage of lipids also
provides an ideal parameter for shelf life prediction. Wewala [20] used this method
for prediction of shelf life of dried whole milk and found very good correlation
between the head space oxygen content and storage time (Fig. 2).
6. Conjugated Dienes
Oxidation of polyunsaturated fatty acids is accompanied by an increase in the ultraviolet
absorption of the product. Lipids containing methylene-interrupted dienes or
polyenes show a shift in their double-bond position during oxidation due to isomerization
and conjugate formation [21]. The resulting conjugated dienes exhibit an
intense absorption at 234 nm; similarly conjugated trienes absorb at 268 nm.
Farmer and Sutton [22] indicated that the absorption increase due to the formation
of conjugated dienes and trienes is proportional to the uptake of oxygen and
formation of peroxides during the early stages of oxidation. St. Angelo et al. [23]
studied the autoxidation of peanut butter by measuring the PV and absorption increase
at 234 nm due to the formation of conjugated dienes. Shahidi et al. [24] and
Wanasundara et al. [25] found that conjugated dienes and PV of marine and vegetable
oils correlate well during their oxidation (Fig. 3). These authors concluded that the
conjugated diene method may be used as an index of stability of lipids in place of,
or in addition to, PV. However, carotenoid-containing oils may give high absorbance
values at 234–236 nm due to the presence of double bonds in the conjugated structure
of carotenoids. The conjugated diene method is faster than PV determination,
is much simpler, does not depend on chemical reactions or color development, and
requires a smaller sample size. However, presence of compounds absorbing in the
same region may interfere with such determinations.
Figure 2 Change of the headspace oxygen content of stored dried whole-milk samples.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.