In order to enable this feature you will have edit
# Specifies if WebSphere Portal Server writes to the WebSphere Application
# Server log files or uses its own log file.
# Default: true
useAppServerLog = true
# The name of the WebSphere Portal Server's log file.
# This setting is ignored if the WebSphere Application Server's log files are
# The following tokens are replaced by the corresponding values:
# $APPSERVER_NAME The name of the WAS node, this should be used for vertical
# clusters to enforce that the different nodes write into
# different files or directories
# $CREATE_TIME The time a file was created
# Default: log/wps_$CREATE_TIME.log
logFileName = log/wps_$CREATE_TIME.log
In this file value of useAppServerLog property defines if you want portal to use different log file. By default it is set to true so portal server writes to trace.log file. If you set it to false(Means use differnet log file) then portal server will read value of logFileName property to find the location of log file where the trace messages should be written.
Once you make changes in log.properties file you will have to restart the server for these changes to take affect. After you restar the server the log generated by portal component such as PUMA log will go to seperate file.