command line conversion and font in epub

Hi,

When I am converting in Calibre some format to epub I can click options and select a font to force and/or embed. Sans Serif fonts are easier for me to read so I always choose such a font (which is also present on my reader).

But I want to make some bulk conversion using "ebook-convert" command line script in Linux terminal. How can I force the font that I want? The default font is Serif and I don't like it at all...

Any ideas?