The effects of photoperiod on physical and chemical properties of broiler muscle

A series of experiments were conducted on broiler males to study the effects of photoperiod, fasting, time post-mortem and exercise on the time required for the development of rigor mortis, tenderness of breast and thigh muscle, muscle glycogen levels, and histopatho-logical characteristics of muscle.

Shear values of breast muscle of birds reared on photoperiods of 24L:0D and 16L:8D were not significantly different when tested at either 2 or 20 hrs. post mortem. Excising the muscle sample from the bone immediately following slaughter resulted in higher shear values than those of muscle allowed to remain on the bone prior to cooking. This effect was not as great with respect to the thigh muscle as it was for the breast muscle and especially so for birds reared on 24L:0D.

The light treatment of 24L:0D resulted in a very rapid onset of rigor mortis approximately one-half the time required by birds reared on 16L:8D. Exercising for 1 min. prior to slaughter reduced rigor time significantly in the 24L:0D group but not in the 16L:0D birds. Fasting for 24 hours prior to slaughter significantly reduced rigor time in both light treatment groups. The combination of the 24L:0D photoperiod, exercise and fasting were additive in their effects and resulted in extremely rapid onset of rigor mortis. These birds evidenced rigor on an average of 3.5 min. post slaughter.

No significant effect of the two photoperiods on muscle glycogen levels could be established. Muscle glycogen was found to decrease rapidly following slaughter of the birds in both treatments. The glycogen level of muscle excised from anesthetized but living birds averaged 3-4 x of that from slaughtered birds.

The myosin ATPase reaction revealed no information regarding the effects of photoperiod. Evidence of vacuolar degeneration or necrosis was present in sections of breast muscle from the 24L:0D birds but not in the 16L:8D birds. This difference was thought to be possibly related to the anaerobic glycolytic metabolism of white muscle as opposed to the oxidative aerobic metabolism of the red thigh muscle.