Abstract: This invention provides a method for killing cells in fermentation mixtures in order to prepare the fermentation mixture for processing to recover or extract a desired product from the fermentation mixture. A preferred method of this invention comprises in either order, adjusting the pH of the fermentation mixture to a value equal to or less than about two pH units below the pK.sub.a of the compatible organic acid using a mineral acid, and adding a sufficient amount of a compatible organic acid and/or organic acid salt to the mixture to effect a substantially complete cell kill. The method of this invention is useful for killing microorganisms such as yeast, bacteria or fungi in any culture or fermentation mixture and is particularly useful in systems where it is desired to kill the cells without lysing them.

Abstract: This invention provides a method for killing fungal cells without lysing in fermentation processes in order to prepare the fermentation mixture for processing to recover or extract an extracellularly expressed enzyme from the fermentation mixture. A preferred method of this invention comprises adjusting the pH of the fermentation mixture to less than 2.79 using a mineral acid, then adding sufficient acetic acid to the mixture to affect a substantially complete cell kill in mixture. A salt of the acetic acid can be used. The organic acid or salt can be added, then the pH adjusted to the desired level. Other organic acids can be used, in which case the pH of the mixture is adjusted to the pK.sub.a of the selected organic acid before the organic acid is added to the mixture.