Beyond guaiacol and halophenols: Unravelling isobutyric and isovaleric acids as new culprits in off-flavour spoilage by Alicyclobacillus spp.

Industries that produce or use fruit-based products have faced several spoilage events, resulting in economic losses caused by product recalls and loss of consumer confidence. Some of these events correlate to the presence of Alicyclobacillus (ACB) in food products since they can produce off-flavours and odours in the final products. Guaiacol (2-methoxyphenol) and halophenols (2,6-dichlorophenol and 2,6-dibromophenol) have been widely explored as the primary culprits of off-flavour spoilage by ACB. However, different compounds might be correlated with these spoilage events. In this work, volatile metabolites produced by distinct ACB species (Alicyclobacillus acidoterrestris, Alicyclobacillus acidocaldarius, and Alicyclobacillus cycloheptanicus) in laboratory medium and fruit juices were identified by HS-SPME-GC-MS and investigated as potential spoilage-related compounds. Isobutyric acid (2-methylpropanoic acid) and isovaleric acid (3-methylbutanoic acid) were revealed to be produced by all three ACB species at concentrations that surpass the odour threshold. These cheesy, sweaty, and sour compounds were responsible for dissonant odours in peach, orange, and tomato juice, harming fruit-based products' quality. More importantly, this work suggests that ACB species previously identified as non-spoilage bacteria, based on a lack of ability to produce guaiacol and halophenols, can also threaten the juice, beverage, and dairy industries. As such, identification methods currently used in industries for ACB control in final products should be revised.