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Abstract
This study investigated the effects of constant and cyclic heat stress on muscle metabolism
and meat quality of broiler breast fillet and thigh meat from 4 to 6 wk of age. Male
Arbor Acres (AA) broilers (n = 270, 4 wk old) were raised under different temperature
conditions: standard (temperature was 23°C); constant high temperature (temperature
was 34°C); and cyclic high temperature (temperature was 36°C from 1000 h to 1600 h
and 23°C from 1600 h to 1000 h). On d 42, broilers were stunned and sampled. The results
showed that chronic high temperature significantly decreased the proportion of breast
muscle and significantly increased the proportion of thigh muscle (P < 0.05). The
moisture concentration was significantly higher in the breast muscle of the birds
exposed to constant high temperature (P < 0.05), whereas the protein content was significantly
lower (P < 0.05) and fat deposition was significantly higher (P < 0.05) in the breast
muscle of the birds exposed to constant or diurnal cyclic high temperature than those
grown under standard temperature. The breast and thigh muscle of the birds grown under
constant high temperature had significantly higher lightness, cook loss, and shear
force (P < 0.05) and significantly lower initial pH (pH(i)), ultimate pH (pH(u)),
and redness compared with those grown under standard temperature (P < 0.05). The pH(i),
pH(u), and redness were significantly lower (P < 0.05) while the lightness and shear
force were significantly higher for the breast muscle of the chickens raised under
diurnal cyclic high temperature (P < 0.05) than those grown under standard temperature.
In contrast, lightness and yellowness of thigh muscle were significantly higher (P
< 0.05) in the chickens grown under diurnal cyclic high temperature than under standard
temperature. Breast and thigh muscle of broilers exposed to constant high temperature
produced higher (P < 0.05) lactic acid and pyruvate kinase activities than those exposed
to the standard temperature. These results indicated that chronic heat stress significantly
increased lactate production, reduced meat pH value by accelerating meat glycolysis,
and eventually reduced meat quality.