Postmortem Metabolism And Quality Characteristics In Chicken Breast Meat
Substantial economic losses in the poultry industry is on the increase due to high environmental temperatures affecting the growth and survival of broilers. Heat stress due to high environmental temperature can induce decreased broiler productivity and quality deterioration of chicken meat. Heat stress results from a negative balance between the net amount of energy flowing from the animal to its surrounding environment and the amount of heat energy produced by the animal. Acute heat stress refers to exposure to short and sudden periods of extremely high temperatures while chronic heat stress refers to exposure to extended periods of high temperatures. High temperature is closely linked to relative humidity and this has an effect on bird performance. Rajkumar et al. , (2011) also indicated that air saturated with water vapor prevents loss of heat from birds through panting.
Mack, Felver-Gant, Dennis, & Cheng (2013) indicated that birds subjected to heat stress conditions spend less time feeding, moving or walking and more time drinking, panting, resting and with their wings elevated. Geraert, Guillaumin, & LeClerq (1993) explained that broiler chickens are more sensitive to high ambient temperatures because of their normally elevated body temperature, rapid metabolism and lack of sweat glands compared to other animals. Broilers are also highly susceptible to heat stress because they have a narrower critical temperature range for growth and survival (16-26 ºC) and homeostatic imbalance due to intensive selections for fast growth rate and breast muscle mass.
Tang et al. (2013) also stated that modern broiler breeds are more susceptible to heat stress than earlier genotypes. Heat stress negatively affects the welfare and productivity of broilers and laying hens. Feed intake, body weight gain as well as on carcass yield and mortality rates are greatly affected by thermal stress. Heat-stressed poultry tend to reduce their heat production by limiting feed intake, leading to decreased body weight, feed efficiency, egg production and quality. Heat stress, chronic or acute, is one of the primary causes of stress during preslaughter activities. Aberle et al. (2001) stated that stress-susceptible animals have unusually high temperatures, rapid glycolysis (pH drop), and early postmortem onset of rigor mortis (stiffening) in their muscles.
A study by Zhang et al. , (2012) concluded that chronic heat stress significantly increased lactate production, reduced meat pH value by accelerating meat glycolysis, and eventually reduced meat quality. Heat stress is a major cause of Pale Soft Exudative (PSE)-like incidences in chicken breast meat. PSE-like meat is the result of accelerated postmortem glycolysis, which results in a rapid postmortem decline in pH while carcass temperatures are still high. This combination can lead to protein denaturation and resulting in a PSE-like breast generally characterized by a pale color, soft texture, and an exudative surface due to poor water-holding capacity. PSE-like chicken meat may occur as a result of an excessive release of Calcium ions (Ca2+), due to genetic mutation of ryanodine receptors found in the sarcoplasmic reticulum of skeletal muscle cells. Wilhelm et al. (2010) also indicated that protease activity is enhanced by excessive Ca2+ which causes protein denaturation in meat and ultimately affecting the functional properties of PSE-like meat. The undesirable characteristics of PSE-like meat has caused a huge economic loss in the poultry industry. The annual economic loss is estimated at over $200 million in the US broiler industry alone. Consumer demands for chicken has rocketed, leading to considerable increase in the loss of broilers. Oxidative stress is induced by heat stress in live broilers and occurs when the balance of formation of oxidants exceeds the ability of antioxidant systems to remove reactive oxygen species (ROS). High temperature may increase ROS and cause a deregulation of intracellular Ca2+ levels, resulting in metabolic changes that eventually affect meat quality. Molecular changes in DNA, proteins, lipids and other biological molecules may also occur.
The results from a study by Lin et al. (2006) also implied that elevated body temperature can induce metabolic changes that are involved in the induction of oxidative stress. The supplementation of the diet with antioxidants has the potential to improve the health and production performances of farm animals by reversing the negative effects of oxidative stress. Celi and Chauhan (2014) found that during the production cycle, supplementation with antioxidants is necessary because poultry can be exposed to environmental stressors that might result in oxidative stress. Vitamin E is the first line of defense against cellular damage by peroxidation. It is an essential nutrient and hence should be provided in the diet. Hashizawa et al. (2013) concluded in their study that supplementation of broiler feed by Vitamin E should effectively prevent deterioration of meat quality by chronic heat stress. Other studies showed that 200 mg/kg Vitamin E inhibited development of PSE and reduced lipid oxidation and drip loss.
Heat-stressed induced PSE-like incidence in chicken breast meat will continuously rise because of increase in environmental temperature. It is important to determine the mechanism involved in the heat stress induced quality deterioration, especially PSE-like incidence, in broiler breast meat in order to prevent huge economic losses to the poultry industry.
