Storage stability of freeze-dried Lactobacillus acidophilus (La-5) in relation to water activity and presence of oxygen and ascorbate.

Storage stability of freeze-dried Lactobacillus acidophilus was found to depend on water activity (0.11-0.43), oxygen level (atmospheric oxygen level and <4% oxygen compared) and presence of sodium ascorbate (0% and 10% (w/w)). Increasing water activities decreased bacterial survival, and a reduced oxygen level (<4% oxygen) improved the storage stability, which strongly indicates a connection between oxidative reactions and bacterial instability. The detrimental effect of atmospheric oxygen was reduced by including ascorbate in the freeze drying medium. However, when ascorbate was present a pink/red colour was observed on the surface of the dried samples increasing with the water activity and oxygen level. Increased water activity lead to increased browning also for samples without ascorbate. Free radicals were detected in the dried bacteria by ESR spectroscopy (broad single-peak ESR spectra), where the shape and the g-value was found to depend on the presence of ascorbate and the extent of browning. For increasing water activities the content of radicals increased to a certain level, after which it levelled off and/or decreased. The highest concentrations of radicals were detected in the dried bacteria with highest survival for a given water activity, i.e. low oxygen level and presence of ascorbate, pointing towards a role of semi-stable ascorbyl radicals as a "dead end" for otherwise detrimental free radical reactions.