MATERIALS AND METHODS
ARTHROSCOPE
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depends on the raw meat quality and the cooking procedure (Aaslyng et al., 2002). Other factors like concentration of glycogen could also influence the juiciness as an increased concentration of glycogen have been reported to increase the juiciness in beef with a normal pH between 5.5 and 5.75 (Immonen et al., 2000).
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perceive an off-flavour varies from person to person. Rancidity can be divided into two broad groups, which are;
A. Hydrolytic rancidity: This occurs as a result of the formation of free fatty acids and soaps (salt of free fatty acids) and is caused by either the reaction of lipid and water in the presence of a catalyst or by the action of lipase enzymes. Water and fat can exist in contact with one another for months at a time. The rate of reaction between water and fat can become significant if there is a suitable catalyst (lipase enzymes and acidic catalysts) present and the temperature is raised.
When a food is contaminated by bacteria and is subsequently heated, a hydrolytic rancidity condition occurs. In these situations, the triglycerides in the food are hydrolysed, first to diglycerides, then to monoglycerides and finally to fatty acids.
B. Oxidative rancidity: The oxidation of lipids leading to rancidity is one of the most important changes during food storage (Rosmini et al., 1996) and is a common and frequently undesirable chemical change that may impact flavour, aroma, and nutritional quality and in some cases even the texture of a product. The chemicals produced from oxidation of lipids are responsible for rancid flavours and aromas, although not all flavours derived from oxidation of lipids are undesirable, that is, give rise to unpleasant off- flavours. For instance, aldehydes with unsaturation at the 2nd positon is described as sweet and pungent at one shorter chain length and as sweet, fatty and green at longer chain lengths while aldehydes with conjugated unsaturation at the 2nd and 4th positions are noted as sweet or oily and 2, 4 dienals with chain lengths from C8 to C12 are reported to make a positive contribution to the flavour of chocolate (Dick Hamilton, 2005).
Lipid oxidation occurs from more complex lipid oxidation processes. The processes are generally considered to occur in three phases, which are initiation or induction, propagation and termination phases and in complex systems, the products of each of these phases will increase and decrease over time making it difficult to quantitatively measure lipid oxidation.
The Initiation or Induction phase involves molecular oxygen combining with unsaturated fatty acids to produce hydroperoxides and free radicals both of which are very reactive. For this phase to occur at any meaningful rate, some types of oxidative initiators must be present such as chemical oxiders, transition metals (iron or copper) or enzymes (lipoxygenases). Also, heat and light increase the rate of this phase and other phases of lipid oxidation. In this early stage,
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process of lipid oxidation, peroxides and hydroperoxides are the predominant reaction products, which continue to increase until; (a) storage conditions change, (b) one or more initiators is depleted, (c) available oxygen is consumed, or (d) the lipid substrate is exhausted. The reactive products of this initiation phase will in turn react with additional lipid molecules to form other reactive chemical species. The propagation of further oxidation by lipid oxidation products give rise to the term ‘auto oxidation’ that is often used to describe the process. In the final termination phase of lipid oxidation, relatively unreactive compounds are formed. These include hydrocarbons, aldehydes and ketones, most of which are volatile. Consequent upon their volatility, their concentration of the compounds in the product may also begin to decrease over time. The rate of decrease varies with storage conditions, packaging and fat content.
Some of the factors that can influence the rate of lipid oxidation in a product include:
The initial quality of the fat or oil used for manufacturing the product.
Conditions used to manufacture the product.
Storage conditions (heat, light, packaging).
Surface area exposed to atmospheric oxygen.
Presence of transition metals.
Concentration of active lipoxygenases.
Application of appropriate of synthetic or natural preservatives.
Presence of chemical oxidizers.
2.14 Factors affecting lipid oxidation in meat and meat products
Lipid oxidation starts immediately after slaughtering and during post slaughtering events. The rate and extent of lipid oxidation in muscle tissues appears to be dependent on degree of muscle tissues damages during pre-slaughtering events such as stress and physical damage and post-slaughtering events such as early post-mortem pH, carcass temperature, shortening and tenderization technique such as electrical stimulation (Morrissey et al., 1998). Furthermore, various processing factors can influence the rate of lipid oxidation in meat and meat products.
They include composition of raw meat, aging time, cooking or heating, size reduction processes such as grinding, flaking and emulsification, deboning, especially mechanical deboning,
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additives such as salt, nitrite, spices and antioxidants, temperature abuse during handling and distribution, oxygen availability and prolonged storage (Kanner, 1994).
Phospholipids play a critical role in the development of lipid peroxidation in raw and cooked meat. As reported by Pikul et al. (1984) phospholipid fraction contributed about 90% of the malonaldehyde measured in total fat from chicken meat. It was also reported that the polyunsaturated (PUFA) content of phospholipids was related to the development of rancidity (Igene et al., 1980). Report of the studies of Kim et al. (2002) showed that beef have a higher susceptibility to lipid peroxidation than pork and turkey breast muscle.
Oxygen availability is also one of the most important factors for the development of lipid peroxidation in raw and cooked meat. Any process causing disruption of the membranes such as size reducing processes (grinding, flaking, mincing etc), deboning and cooking result in exposure of phospholipids to oxygen and therefore accelerates development of oxidative rancidity (Ladikos and Lougovois, 1990). The level of oxygen content in modified atmosphere and vacuum packaged raw and cooked beef was proportional to that of lipid peroxidation (O’Grady et al., 2000; Smiddy et al., 2002).
To delay lipid oxidation, synthetic antioxidants have been applied extensively in food products (Ahmad, 1996). However, due to consumer preferences for natural ingredients over synthetic compounds (Ahn et al., 2002) there is a demand for discovery of new plant extracts that can reduce lipid oxidation in lipid-containing food products. Many natural plant extracts contain primarily phenolic compounds which are potent antioxidants (Wong et al., 1995). Phenols are one of the most important groups of natural antioxidants and they occur only in material plant origin and they are known to easily protect oxidizable constituents of food from oxidation.
Especially worthy of note among such plants are spices and herbs, which have been used as additives to enhance the sensory features of food (Wang et al., 1996).
For instance, grape seed and green tea extracts contain large amounts of antioxidant compounds (Fadhel and Amran, 2002). Rababah et al. (2004) investigated antioxidant activities of some plant extracts and found that grape seed and green tea extracts showed the highest antioxidant activities. Natural antioxidants such as sesamol, quercetin, rutin and rosemary have been shown to reduce thiobarbituric acid reactive substances (TBARS) value (index used to measure extent of lipid oxidation) in irradiated raw meat during storage (Chen et al., 1999). Ahn et al. (2002)
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also reported that selected natural antioxidants reduced development of warmed-over flavour and TBARS values in cooked ground beef. Studies also carried out by Devatkal and Mendiratta (2001) showed that TBARS increases during storage of different meat and meat products.