Simple string replace not working.

[snugRugBug]

{{$category_title = str_replace(' and ' , ' and<br/>', $category->_title);}}

In one of my templates, I want to insert a break after every occurrence of the word 'and' in my category title. The line above should do it, but when the code below is used, I see no effect.

<h4>{$category_title}</h4>

How do I circumvent this?