The results presented indicate that the production of diacetyl in butter cultures and pure cultures of citric acid fermenting streptococci can be greatly increased by growing the cultures under 30 pounds or more of air pressure per square inch, provided the cultures are agitated by aeration or stirring. This increase frequently amounts to several hundred percent. In some cases the yield of acetylmethylcarbinol was increased by aeration under pressure, and in other cases it was not. Occasionally, the increase was relatively large.

Pressure without agitation was not effective in increasing the diketone content. The cultures under pressure must be agitaed by aeration or stirring in order to bring about an increased formation of diacetyl.

Pressures exceeding 15 pounds per square inch were necessary for substantially increased yields of diacetyl. Although the highest yields of diacetyl were found in cultures ripened under 60 pounds pressure, intermediate pressures produced satisfactorily large amounts and caused less difficulty in operation.

It is probable that agitation under pressure changes the normal anaerobic dissimilation of citric acid by the citric acid fermenting streptococci rather than that the acid is more rapidly fermented.

The effect of the high diacetyl content of butter cultures is to give a high flavor and aroma. Butter churned with such cultures uniformly possesses a high flavor and aroma.

The results obtained in this investigation suggest new methods in butter making practice. High flavor cultures may benefit the dairy industry in either of two ways. Such cultures may aid in the production of butter with an unusually high flavor and aroma, or less culture may be used to obtain the same flavor and aroma as normal cultures impart to butter.