One way would be to add an "Other" option. I'm not sure this really gives you exactly what you want though so I had a little play around and found a different option: you can add a choice in your list of food that has the name "default" and the label "Don't Know" and then you can use a choice filter like so to filter on the previous answer and include the default option:
selected(${previous_question}, name) or name='default'
If you want to then not include the default in other questions you can just do:
name!='default'