To investigate how cell physiological functions can predict the stability of freeze-dried probiotics. In addition, the effect of the fermentation pH on the stability of probiotics was investigated. Fermenter-grown (pH 5.8 or 5.0) Lactobacillus rhamnosus cells were freeze-dried and their survival was evaluated during storage at 37 degrees C, in apple juice and during acid [hydrochloric acid (HCl) and malic acid] and bile exposure. Cells grown at pH 5.0 were generally coping better with acid-stress than cells grown at pH 5.8. Cells were more sensitive to malic acid compared with HCl. Short-term stability results of Lact. rhamnosus cells in malic acid correlated well with the long-term stability results in apple juice, whereas the results of cell membrane integrity studies were in accordance with bile exposure results. Malic acid exposure can prove useful in evaluating the long-term stability of probiotic preparations in apple juice. Fermentation at reduced pH may ensure a better performance of Lact. rhamnosus cells during the subsequent acid-stress. The beneficial effect of lowered fermentation pH to Lact. rhamnosus stability during storage in apple juice and the usefulness of malic acid test in predicting the stability were shown.