PURPOSE: To obtain a natural flavor, by adding citric acid (salt) to animal milk or whey, inoculating and culturing two specific kinds of lactobacilli in the medium, adding citric acid (salt) to the cultured product, subjecting to steam- distillation, adding an oxidizing agent to the distillate and subjecting again to steam-distillation.

CONSTITUTION: Citric acid (salt) is added to animal milk or whey. Two kinds of lactobacilli belonging to genus Streptococcus cremoris and genus Streptococcus diacetilactis are inoculated to and cultured in the above medium. The cultured produce is added with citric acid (salt) and subjected to steam-distillation. The obtained distillate is added with an oxidizing agent (e.g., ferric chloride) and subjected again to steam-distillation to obtain a natural flavor having high diacetyl content and suitable for foods such as butter.