Cider sickness is a spoilage disorder affecting sweet, low-acid ciders and perries, characterized by haze formation, gas production, fruity aroma development, and eventual harsh flavors with aldehyde-tannin precipitate formation. Nancy Millis isolated the causative organism from infected cider using anaerobic incubation on apple juice medium containing yeast extract and Actidione (a yeast inhibitor) at pH 4.5. The isolate was a Gram-negative motile rod identified as a new variety of Zymomonas anaerobia, named Z. anaerobia var. pomaceae. This organism was compared with two related bacteria: Termobacterium mobile (Pseudomonas lindneri) and Achromobacter anaerobium (Saccharomonas anaerobia). All three organisms share the distinctive ability to ferment glucose to nearly theoretical yields of ethanol and carbon dioxide. The cider-sickness bacillus resembled Z. anaerobia most closely, differing mainly in its inability to ferment sucrose and its specific effects on cider flavor. The study clarified the taxonomic relationships of these ethanol-producing bacteria and established that the organism grows vigorously at pH 3.7-8.0, with growth threshold around pH 3.4-3.6, explaining the susceptibility of low-acid ciders to this disorder.

Key findings

• The cider-sickness bacillus is a Gram-negative motile rod identified as Zymomonas anaerobia var. pomaceae, closely related to two previously described organisms with similar fermentation properties
• The organism ferments glucose to almost theoretical yields of ethanol (approximately 49-50% conversion), comparable to yeast fermentation but via a different biochemical mechanism
• Growth occurs vigorously at pH 3.7-8.0, with the organism unable to grow below pH 3.4, explaining why low-acid sweet ciders are particularly susceptible to spoilage
• The organism tolerates legally permissible sulfur dioxide concentrations (up to 200 μg/ml), as accumulated aldehyde rapidly inactivates the preservative