How to specify in crontab by what user to run script?

I have few crontab jobs that run under root, but that gives me some problems. For example all folders created in process of that cron job are under user root and group root. How can i make it to run under user www-data and group www-data so when i run scripts from my website i can manipulate those folders and files? My server runs on Ubuntu. Current crontab job is: */1 * * * * php5 /var/www/web/includes/crontab/queue_process.php >> /var/www/web/includes/crontab/queue.log 2>&1