Have a plan of what you want in mind before you begin. You have the burden of proof against you, though. If you go into it all pissed off and wanting shit it's going to look like you were faking it to fleece the company. This happens all the time so they resist.
Or, be as nice as pie and as to talk to their QA department. Let them know that you are not angry and nobody got hurt, but this was in you bag o' food and are worried that more might have gotten in and you don't want anyone to get hurt. If they are a good QA department, this is the best way to get something done as they hate it when things happen outside the norm.
And here's their contact us page:
http://www.bertolli.com/ca-en/contactus.aspx