Chemistry Reference
In-Depth Information
al., 2010). Carotenoids may further protect cholesterol in the product against
singlet-oxygen, which otherwise adds to the single double bond of cholesterol to
yield a specific 5-hydroperoxide often used as an indicator of involvement of
singlet oxygen in oxidative deterioration of food lipids. Cholesterol is only
present in food of animal origin. Dairy products with added plant oils are
becoming increasingly popular and present a special challenge as cholesterol in
such products are becoming exposed to lipids typically of lower oxidative
stability than the milk fat with the risk of increased cholesterol oxidation
through coupling with autoxidation of more highly unsaturated plant lipids.
1.5 Aqueous phase oxidations
Lipid oxidation is recognized as an important cause of quality deterioration of
many foods like edible oils, muscle-based foods like fish and meat, and milk and
dairy products. Most foods have besides the lipid phase also an aqueous phase
with oxido-reductase activity, with redox-active metal ions present together with
a steady formation of ROS and RNS. For buttermilk made by churning of milk
from cows of two different feeding regimes to yield milk fat of varying
unsaturation, it was found, not very surprisingly, that the buttermilk with the
more saturated fat had a better resistance against oxidative rancidity during chill
storage measured as formation of both lipid hydroperoxides and hexanal
(Kristensen et al., 2004). However, for both products, lipophilic antioxidants did
not decrease during storage, while antioxidative capacity of the aqueous phase
decreased significantly and with similar rates for the two products, as measured
by ESR spectroscopy, indicating that radical formation as the early event of lipid
oxidation occurs in the aqueous phase and independent of the degree of
unsaturation of the lipids.
For meat systems, lipid oxidation seems also to be initiated in the aqueous
phase. Myoglobins are moderate catalysts for lipid oxidation although their
hypervalent forms may abstract allylic hydrogen from lipids (MbFe(V)0) or
cleave lipid hydroperoxides (MbFe(IV)0, see eq. 1.19). Partly proteolyzed
myoglobins show a dramatic increase in catalytic activity for lipid oxidation in
heterogeneous systems (Fig. 1.9), and hydrolytic degradation of heme pigments
during digestion may accordingly initiate oxidative damage in the digestive tract
(Carlsen and Skibsted, 2004). During heat treatment of meat, iron seems to leak
from metalloproteins opening up for simple Fe(II)/Fe(III) catalysis of lipid
oxidation leading to warmed-over-flavour upon reheating (Tims and Watts,
1958).
For the aqueous phase, urate is important as an antioxidant in milk, in meat
reducing co-factors like NADH determines the redox status of meat pigments,
while in fruit products ascorbate and plant phenols together seems more
important for the redox potential. Proteins may oxidize, and for the myofibrillar
protein of meat, oxidation leads to cross-linking affecting water binding capacity
and tenderness negatively (Decker et al., 1993). Cystein side-chains of the
Search WWH ::
Custom Search