Non-microbiological factors affecting quality and safety 227 success of fast food facilities and precooked chilled meals will depend to some extent on the ability of processors to overcome the development of WoF id oxidation has long been considered to be the primary cause of woF supported by studies correlating increases in WoF determined sensorily (Love 1988) with measurements of the thiobarbituric acid (TBa)number(an indicator of lipid oxidation)(Igene et al. 1979, Igene et al. 1985, Smith et al. 1987), and identification of the volatile compounds extracted from the headspace above meat samples(St Angelo et al. 1987, Ang and Lyon 1990, Churchill et al. 1990) As with other examples of oxidative rancidity, the process of lipid oxidation results in the formation of many different compounds, some of which are more significant than others to the undesirable odour and flavour associated with rancidity. This gives rise to a less than perfect relationship between measured chemical markers and sensory assessment of rancidity The reactivity of food lipids is influenced by the degree of unsaturation of constituent fatty acids, their availability and the presence of activators or nhibitors. The composition of fats in meat reflects a number of factors including the diet of the animal and the type of fat. Lipids are most abundant as either storage depot(adipose) fats or in cell membranes as phospholipids During cooking, the unsaturated phospholipids, as opposed to the storage triglycerides, are rendered more susceptible to oxidation by disruption and dehydration of cell membranes. The higher degree of unsaturation of fatty acid in the phospholipids contributes to their more rapid rate of oxidation(Igene et al. 1981). The role of phospholipids in the formation of WoF (Igene and Pearson 1979)and TBa reactive substances( Roozen 1987, Pikul and Kummerow 1991) has been demonstrated Autoxidation of lipids is generally accepted to involve a free radical chain reaction(Fig. 9.1), which is initiated when a labile hydrogen atom is abstracted from a site on the lipid(RH with the production of lipid radicals (R) (initiation). Reaction with oxygen yields peroxyl radicals(ROO") and this followed by abstraction of another hydrogen from a lipid molecule. A hydroperoxide(rooh) and another free radical(R") which is capable of perpetuating the chain reaction, are formed (propagation). Decomposition of the hydroperoxides involves further free radical mechanisms and the formation of non-radical products including volatile aroma compounds Despite much research effort, the mechanism of initiation leading to the formation of the lipid(alkyl or allyl) radical(r") in meat is still an area of confusion and debate. The involvement of iron has been established(minotti and Aust 1987), but beyond this various mechanisms have been suggested but not supported by conclusive evidence(Ashgar et al. 1988) The rate of formation of free radicals is increased by the presence of metal atalysts. In the case of warmed-over flavour development in cooked meats, both free ferrous ions and haemoproteins, including metmyoglobin in the presence of hydrogen peroxide(Asghar et al. 1988)have been shown to have a prooxidant effect. The availability of free iron is known to increase as a result of cooking(Igene et al. 1979)as haemoproteins are broken down and release freesuccess of fast food facilities and precooked chilled meals will depend to some extent on the ability of processors to overcome the development of WOF. Lipid oxidation has long been considered to be the primary cause of WOF, supported by studies correlating increases in WOF determined sensorily (Love 1988) with measurements of the thiobarbituric acid (TBA) number (an indicator of lipid oxidation) (Igene et al. 1979, Igene et al. 1985, Smith et al. 1987), and identification of the volatile compounds extracted from the headspace above meat samples (St Angelo et al. 1987, Ang and Lyon 1990, Churchill et al. 1990). As with other examples of oxidative rancidity, the process of lipid oxidation results in the formation of many different compounds, some of which are more significant than others to the undesirable odour and flavour associated with rancidity. This gives rise to a less than perfect relationship between measured chemical markers and sensory assessment of rancidity. The reactivity of food lipids is influenced by the degree of unsaturation of constituent fatty acids, their availability and the presence of activators or inhibitors. The composition of fats in meat reflects a number of factors, including the diet of the animal and the type of fat. Lipids are most abundant as either storage depot (adipose) fats or in cell membranes as phospholipids. During cooking, the unsaturated phospholipids, as opposed to the storage triglycerides, are rendered more susceptible to oxidation by disruption and dehydration of cell membranes. The higher degree of unsaturation of fatty acids in the phospholipids contributes to their more rapid rate of oxidation (Igene et al. 1981). The role of phospholipids in the formation of WOF (Igene and Pearson 1979) and TBA reactive substances (Roozen 1987, Pikul and Kummerow 1991) has been demonstrated. Autoxidation of lipids is generally accepted to involve a free radical chain reaction (Fig. 9.1), which is initiated when a labile hydrogen atom is abstracted from a site on the lipid (RH) with the production of lipid radicals (R• ) (initiation). Reaction with oxygen yields peroxyl radicals (ROO• ) and this is followed by abstraction of another hydrogen from a lipid molecule. A hydroperoxide (ROOH) and another free radical (R• ) which is capable of perpetuating the chain reaction, are formed (propagation). Decomposition of the hydroperoxides involves further free radical mechanisms and the formation of non-radical products including volatile aroma compounds. Despite much research effort, the mechanism of initiation leading to the formation of the lipid (alkyl or allyl) radical (R• ) in meat is still an area of confusion and debate. The involvement of iron has been established (Minotti and Aust 1987), but beyond this various mechanisms have been suggested but not supported by conclusive evidence (Ashgar et al. 1988). The rate of formation of free radicals is increased by the presence of metal catalysts. In the case of warmed-over flavour development in cooked meats, both free ferrous ions and haemoproteins, including metmyoglobin in the presence of hydrogen peroxide (Asghar et al. 1988) have been shown to have a prooxidant effect. The availability of free iron is known to increase as a result of cooking (Igene et al. 1979) as haemoproteins are broken down and release free Non-microbiological factors affecting quality and safety 227