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 degreesC and 8-9 degreesC 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.