S. cerevisiae var. diastaticus contamination is a growing concern for craft breweries worldwide. This S. cerevisiae variant can efficiently ferment residual starch due to production of extracellular glucoamylases. The presence of S. cerevisiae var. diastaticus in the brewing process and after packaging can cause a variety of problems due to the fermentation of residual sugars, including altered flavour, increased ethanol, and increased CO2. As the source of this contaminating yeast is not known, the only control measures focus on cleaning and sanitation. However, little is known about how contamination of beer with S. cerevisiae var. diastaticus occurs, both in terms of the evolutionary history of S. cerevisiae var. diastaticus, and in the physical source of contamination in breweries. We detected the presence of S. cerevisiae var. diastaticus-like phenotypes in a selection of commercially available yeasts. We then used DIA/SWATH-MS to identify and measure secreted proteins from these yeasts that are associated with the digestion of residual carbohydrates in finished beer. Together, this provides additional insights to the environmental impact on gene expression and growth patterns of S. cerevisiae var. diastaticus.