Save node when content with reference to it is created
in rules (sorry the terminology is not exact) :


action : node save 
condition : type = review 
select the bussiness from the review 
resave that business (you will have to force it probably)


or the same in a hook (node_api, operation save, nodetype review)

the best would probably be creating an agregated view end embedding that into your node (or attaching with a block, many possibilities)