感謝素食英雄熱心解惑，並提供建議

後來我真的寫信到COSTCO美國總公司去問

幾番波折得到結論：不含任何動物的成分

所以奶蛋素可以吃喔。原文如下

Dear Ms. Cheng:

Thank you for contacting Costco. 

We reviewed the ingredients of the natural flavoring in #744110 KS unsalted sweet cream butter and confirmed it does not contain any animal source ingredients. 

Once again, thank you for your email and for shopping at Costco. Your opinions and concerns are of great value to us.