Kia does this sometimes except she'll growl and sometimes show teeth. If she doesn't spit it out when I say firmly, "Drop It!". Then I go fishing for what I can get. She'll usually then try to swallow it whole so she doesn't have to open her mouth to chew.

Once I get anything out or if she succeeds in eating it, I tell she's been a bad dog and I give her the silent treament for the rest of the day.

She was way worse when I first got her but she's gotten better over time with dropping it. Once in awhile she'll be stubborn if it's something really "juicy".

Good luck!