How to write category to default_content

I want to add my product category to the default content upon posting a new product so that the actual category is searchable via content.

This is what I put in my functions.php file, but it does not work:

add_filter( ‘default_content’, ‘cat_meta’ );
function cat_meta() {
global $post;

//don’t filter outside of the loop
if ( !in_the_loop() )
return $content;

$content .= mp_tag_list($post->ID);

return $content;
}

Is there another way to do this? It seems to work fine if you post actual text to the default editor, but not a function. Can someone please help with a proper function that will output the category to text and then post it to the description?

Much appreciated!