Abstract: Apparatus, methods, and systems for in-line detection of chemical compounds in beer are provided. In these apparatus, methods, and systems, a small sample of wort is removed from a fermentation vessel, heated, analyzed by infrared attenuated total reflectance (IR-ATR) spectroscopy, and returned to the fermentation vessel. The concentrations of one or more chemical compounds in the wort can be ascertained directly from IR-ATR spectroscopy data, or indirectly from applying an algorithm to the IR-ATR spectroscopy data based on known reaction kinetics and stoichiometry. The apparatus, methods, and systems do not destroy the wort sample or contaminate the fermenting wort and so can be employed continuously during fermentation, and are rapid, accurate, and inexpensive.