* Corresponding author

Financial support: Goverment Research Center from Department of Agriculture and Agri-Food Canada.

Lactic acid bacteria (LAB) are commonly used in the production of cheese, yoghurt, dry sausages, wine, sourdough breads and sauerkraut. In many processes, a commercial culture is added directly to the food matrix, or in culture media for the production of starters. The commercial cultures available to the processors are marketed mostly in two forms: frozen and dried. The process used for the drying of food-related LAB is freeze-drying (FD), which is an expensive and slow method of drying. Bakers' yeast, on the other hand, are dried by fluid-bed technology, and the application of such a convection air-drying (CAD) technology to LAB would be of interest to the producers of LAB starters, particularly in the emerging field of probiotic cultures.

Many compounds have been shown to increase survival of LAB to freeze-drying, but few in CAD. The presence of compatible solutes in the medium, such as betaine or trehalose, has been shown to increase survival of microorganisms to air-drying. The aim of this study was to evaluate the effect of various compounds in the growth medium as well as in the protective solutions on the survival of Streptococcus thermophilus cultures to CAD and FD.

It was demonstrated that the protective effect of trehalose to CAD drying was increased if this carbohydrate was found both on the internal and external sides of the cytoplasmic membrane. Thus, the selection of a trehalose-positive strain was initiated in the hope that, in addition to the culture's ability to metabolize trehalose, this strain would accumulate trehalose internally and show better survival to CAD. Most of the Streptococcus thermophilus cultures also showed the typical trehalose-negative except for strain Y12S. In order to study strains of the same species, it was thus decided to use the strains Y12S and Y24S of S. thermophilus.

Streptococcus thermophilus cultures were grown either on trehalose or lactose M-17 media. Gel beads were prepared by adding a concentrated cell suspension to a sodium alginate solution, and by adding droplets of this mixture to a CaCl₂ bath. This produced gel beads in which the bacteria were entrapped. Prior to drying, the beads were dipped into peptone, lactose, trehalose or whey/sucrose protective solutions for 1 h at 23ºC, and their post-dipping pH values were respectively of 6.48, 5.25, 5.87 and 6.22; this must be kept in mind when examining the survival levels to the drying processes. For the air-drying treatment (CAD), the beads were spread on a filter paper under a running laminar flow hood 16 h at 24ºC. For the freeze-drying (FD) process, beads were initially frozen at -70ºC and placed into freeze-dryer for 36 h with a drying temperature of 23ºC. Freeze-dried powders of LAB generally have between 1 and 4% residual moisture, but the FD products obtained in this study contained between 4.1 and 8.6% moisture. Therefore, after the initial CAD or FD, the beads were placed in a dessicator containing an over-saturated solution of LiCl₂ in order to bring all dried cultures to a similar water activity (a_w) level. Although the a_w of the chamber was of 0.10, final a_w values of the cultures were between 0.27 and 0.34.

Immobilized cultures dipped in the 0.1% peptone solution did not show good survival to CAD or FD, as mortality was over 99% (Table 1). The calculations of mortality rates take into account the humidity of the dried cultures as well as the total solids concentration factors (because of the variable solids of the protective solutions). For example, the biomass that was dried following the dipping of the 100 g of beads in the peptone solution was concentrated in only 2.5 g of powder, while the same population in presence of whey-sucrose was found in 16 to 21 g of powder. There was no significant difference in mortality levels, in both methods of drying, when lactose or trehalose were used as protective ingredients. The highest survival levels (50 to 98%) were with a whey-sucrose protective medium, but this was potentially related to a higher pH and solids of the solution. The whey-based medium should reflect what could be expected in commercial formulations.

With both strains, mortality levels were higher in FD, and paired t tests using the data of Table 1 showed this difference to be significant. It was our concern that this was related to the fact that the FD powders contained less water and had thus been submitted to a more extensive drying treatment. The incubation in the dessicator, carried out to stabilize a_w values, resulted in lower viable counts in the dried powders, but the populations were still higher in the samples dried by CAD (Table 1). This suggests that the higher survival levels obtained in the CAD treatments were not related to the slightly higher residual moisture values.

Strains Y12S and Y24S did not show different mortality patterns to drying. As a whole, the effect of the protective medium was the most important on survival of the streptococci to CAD or freeze-drying. Cells grown on lactose had slightly higher survival rates to drying than those obtained from CAD. Trehalose-positive and trehalose-negative cultures of S. thermophilus did not show different mortality patterns to CAD or FD.

The high survival rates of S. thermophilus to CAD suggest that fluid-bed drying, used commercially for Saccharomyces cerevisiae, could successfully be applied to some lactic cultures. However, fluid-bed drying could only be applied to particulate concentrated cultures. Immobilization in alginate gels is instrumental in providing the solid support that is required for fluid bed drying. The advantage of alginate as a support is that it can easily be used for the production of concentrated suspensions of lactic cultures, and that the solid matrix can be dissolved upon rehydration, with citrates or phosphates, if the industrial application requires the use of the culture in a liquid form.

There did not seem to be a difference between trehalose-positive and trehalose-negative strains with respect to survival to drying in general. Thus the capacity of LAB to metabolize trehalose does not seem to confer greater resistance to drying, but this needs to be examined more closely. In particular, strains could be selected not only for their capacity to absorb trehalose, but also for their ability to accumulate this carbohydrate, as do some yeasts.

Although this study shows that CAD can be better than FD with respect to survival to the drying process itself, the stability to storage remains to be examined. The higher moisture content of the CAD products could translate into reduced stability during storage. This aspect will be crucial for the commercial exploitation of the technology, and studies are currently under way to examine this feature of the products obtained.

The technical support of R. Gera and L. Charlebois is gratefully acknowledged.