Foreign microorganisms in industrial yeasts are primarily obligate beer-spoilage organisms (gram-positive lactic acid bacteria such as lactobacilli and pediococci as well as gram-negative anaerobes) and wild yeasts. If these are present in traces or inactivated in yeast, they can irreversibly damage the beer later in the process.
Process samples that contain yeast
Anaerobic cultivation, dark-field microscopy of the sample