Page 290 - Natural Antioxidants, Applications in Foods of Animal Origin

Application of Natural Antioxidants in Dairy Foods 269
VetBooks.ir Photosensitizers are the substances that absorb light and become excited to

one or more higher energy-rich state(s). They promote the photo-oxidation of
diverse substrates, when foods are exposed to visible light (Dalsgaard et al.,
2007). Photosensitizers are reported to have two excited states: singlet and
triplet. The triplet-excited state has a longer lifetime and initiates the oxida-
tion. Photo-oxidation by a photosensitizer can proceeds through two types of
reactions, that is, either type I or type II. In type I reaction, the excited sensi-
tizer (Sen*) undergoes internal reactions that ultimately results in the oxida-
tive alteration of a second molecule primarily by free radical mechanism on
the exposure of the primary substrate to UV radiation. In type I reactions,
transfer of hydrogen atoms or electrons occurs via interaction of the triplet
excited state of the sensitizer with the target, while in the type II reactions,
the excited triplet sensitizer reacts with ground state oxygen to produce O
1
2
by energy transfer. O is a strong oxidant because of its higher reactivity and
1
2
has the potential to damage proteins, lipids, and so forth (Airado-Rodríguez
et al., 2011). The two reactions can occur simultaneously, in a competitive
mode (Spikes, 1988) and the same has been observed in milk (Lee & Min,
2009). However, at low oxygen concentrations, type I reactions are most
common. After photodegradation, riboflavin is reported to break down to
lumiflavin (under alkaline condition) or lumichrome (under acidic condi-
tions) (Ahmad et al., 2006) and probably formylmethylflavin. Among these,
lumichrome is reported to be a strong photosensitizer (Parks & Allen, 1977).
Riboflavin has three absorption bands. The band with maxima between 430
and 460 nm is the main band responsible for the photo-oxidation of food,
especially milk and dairy products (O’Connor & O’Brien, 2006). Wold et al.
(2006) reported the presence of five photosensitizers in butter other than ribo-
flavin: protoporphyrin, hematoporphyrin, a chlorophyll a-like molecule, and
two unidentified tetrapyrroles. Chlorophyll and porphyrin molecules absorb
light in the UV and violet region with absorption peaks of ~410 nm (the
soret band) along with the absorption of light in the red above 600 nm, and
therefore, they may be responsible for the formation of off-flavors in dairy
products when exposed to light having wavelengths longer than 500 nm
(Wold et al., 2006). Chlorophyllic compounds have also been suggested to
contribute prominently to the major part of photo-oxidation in cow’s milk
(Airado-Rodríguez et al., 2011).
Proteins are very complex molecules organized in large structures and
oxidation of proteins may have severe consequences on product quality,
their functionality and nutritional qualities like loss of essential amino acids.
Structurally, protein oxidation may lead to a number of modifications either
on its side chains or on the backbone, including amino acid changes, protein