Scope and method of study. The influence of growth medium and pH on survival, storage stability, and fatty acid composition after freeze-drying and storage of strains of lactobacilli was studied. Lactobacillus acidophilus (L-1, L-23, O-16, and 381-IL-28), L. casei (E-5 and E-10), and L. reuteri X-18 were grown in MRS broth supplemented with different concentrations of Tween 80 or cultivated in broth at different pH levels. The cultures were freeze dried, sealed under vacuum, and stored at 5 °C for 21 days. At regular intervals during storage, viable cells were enumerated and viability was determined. Fatty acid was extracted and analyzed by gas chromatography.

Findings and conclusions. Tween 80 improved growth of most of the cultures but did not improve viability after freeze-drying and storage except in L. reuteri X-18. Significantly higher C18:1 and C19:0cyc11 concentrations in cell membrane of X-18 grown with Tween 80 could have contributed to better resistance during freeze-drying and storage. In general, cells grown in more acidic pH (pH 4.5, 5.0, and 5.5) had higher survival after freeze-drying and subsequent storage; however, relationship between viability and fatty acid was not clearly elucidated due to high variability in the data. Stability of cultures during subsequent storage was not affected by pH or Tween 80 since they exhibited similar trends in viability throughout the storage period regardless of Tween 80 concentration or growth pH. It was attributed to minimal exposure to oxygen since the vials were sealed under vacuum, as well as low temperature and absence of light during refrigerated storage. The present study provided evidence that different cultures and strains of lactobacilli exhibited different responses to changes in growth conditions. It will be interesting to investigate the combined effect of Tween 80 and pH on growth and survival to determine optimum conditions that would be beneficial in the selection of appropriate additives and growth pH during the production and preservation of concentrated cultures.