To decide if the application runs in production or development mode, it checks the NODE_ENV environment variable, which is a variable that you'll set in your shell and that will be read from node.
If you want to run grunt in production mode, use:
$ NODE_ENV=production grunt
which will set the variable only for this execution.
If you want to permanently set NODE_ENV, use export:
$ export NODE_ENV=production
$ grunt
On your production server, you can then edit .bashrc with this line to permanently set NODE_ENV.
For further information, you can read this blog post.
Edit: On Windows, use set NODE_ENV=production on the command line. See this relevant question to know how to permanently set this variable.