Formation of guaiacol in chocolate milk by the psychrotrophic bacterium Rahnella aquatilis.

AIMS The aim of this study was to identify the causative agent of a smoky/phenolic taint in refrigerated full cream chocolate milk. METHODS AND RESULTS Microbiological examination of spoiled and unspoiled milk samples from the same processor showed high numbers of the psychrotrophic coliform Rahnella aquatilis in the spoiled samples only. Gas chromatography/mass spectrometry (GC/MS) was used to identify and quantify the taint compound as guaiacol (2-methoxyphenol) in the spoiled milk. Challenge studies in UHT chocolate and white milks inoculated with the isolate and incubated at 4-5 degrees C and 8-9 degrees C for 6 d showed the production of guaiacol in chocolate milk only, which was confirmed and quantified by GC/MS. CONCLUSIONS The results indicate that if present in refrigerated chocolate milk, Rah. aquatilis can produce guaiacol within the expected shelf-life of the product, even without temperature abuse. SIGNIFICANCE AND IMPACT OF THE STUDY This is the first report that the coliform Rah. aquatilis can produce guaiacol in refrigerated chocolate milk products.