Proteins extracted from chicken muscle during post-mortem aging in the cold were examined by starch gel electrophoresis. Myofibrillar proteins, extracted and analyzed in concentrated urea, revealed no detectable, consistent change during the two-day aging period. Myogen proteins, extracted by vigorous homogenization in dilute buffer, also remained unchanged in white muscles, but an additional electropboretic component, possibly derived from myglobin, slowly appeared in red muscle extracts; the delay in its development suggests a secondary relation with tenderizing processes. Important constituents of myogen were lacking in “sacroplasmic” proteins extracted from breast muscle, pre-rigor or in-rigor, with gentle homogenization in 0.44M sucrose solution. Such “sarcoplasmic” preparations could not be obtained effectively by the same method from red muscle with higher content of stroma. Some of the myogen components absent from “sarcoplasm” gradually reappeared as tenderization proceeded; the time required to achieve rigor and to complete tenderization varied with the bird, but the observed changes were consistent. Yields of total and protein nitrogen from fresh, rigor, and aged breast musele were in agreement with the electrophoretic data.

It is suggested that the additional components obtained in “sarcoplasm” of tenderized muscle reflect soluble proteins escaping into the extract because of the breakdown of intracellular barriers or subcellular particles. These components may include enzymes instrumental in initiating changes of the myofibril, ultimately evident in tenderization.