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The following is an extract from a letter I received from Tillamook County
Creamery Association:

Dear Ms. Collier:

Thank you for letting us know how pleased you are to find we list the
modified food starch in our sour cream as being specifically corn starch.
You will be happy to hear that all of our cheese, yogurt, and butter are
also gluten free, as are most of our Ice Cream flavors.

Ice cream flavors listing any ingredients such as pie, cookie dough, cake,
etc. will contain gluten. The following is a list of Tillamook Ice cream
flavors which do contain gluten: Chocolate Chip Cookie Dough, German
Chocolate Cake, Marionberry Pie, Mom's Apple Pie and Super Mix.

...

Sincerely,
Kathy Holstad
Marketing Admin. Manager

They also sent me a coupon for free sour cream.