A long lasting and stable foam head is a key quality indicator for beer around the world. The distribution of beer with inferior foaming properties can lead to substantial financial losses and damage to the brand reputation. The determination of beer foam stability throughout the brewing process is essential for breweries as a key quality indicator. A stable foam head is achieved through the interplay of various proteins, metal ions and small molecules1. Proteinase A, an aspartic yeast proteinase, was shown to degrade foam promoting proteins and may be responsible for inferior foaming properties2. Here, we demonstrate the applicability of liquid chromatography mass spectrometry (LC-MS) methods to quantitate Proteinase A in commercial beer samples. Label-free quantitation was performed on beer samples to determine the relative amount of Proteinase A in beer and its associated peptides. A targeted LC-MS/MS method was developed for the quantification of Proteinase A in commercial beer types. The targeted assay enables breweries to assess beer foam stability and improve current quality control processes.