Diffusion of lactose in k‐carrageenan/locust bean gum gel beads with or without entrapped growing lactic acid bacteria

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Abstract
Effective diffusion coefficients (De) of lactose in κ‐carrageenan (2.75% wt/wt)/locust bean gum (0.25% wt/wt) (LBG) gel beads (1.5–2.0‐mm diameter)with or without entrapped lactic acid bacteria (LAB) were determined at 40°C. The effects of lactose concentration, bacteria strain (Streptococcus salivarius subsp. thermophilus and Lactobacillus casei subsp. casei) and cell content at various steps of the fermentation process (after immobilization, pre‐incubation of the beads and successive fermentations) were measured on De as a first step for process modelling. Results were obtained from transiend concentration changes n well‐stirred lactose solutions in which the beads were suspended. A mathematical model of unsteady‐state diffusion in a sphere was used, and De was obtained from the best fit of the experimental data. Diffusivity of lactose in cell‐tree beads was significantly lower than in pure water mainly because of the obstruction effect of the polymer chains and the hydration region. Furthermore, effective diffusivity and equilibrium partition factor were independent of lactose concentration in the range from 12.5 to 50 g/L. No significant difference was found for De (effective diffusivity) and Kp (partition) coefficients between beads entrapping S. thermophilus (approximately 5 × 109 CFU/mL) and cell‐free beads. On the other hand higher cell counts obtained with L. casei (close to 1.8 × 1011 CFU/mL) increased mass transfer resistance resulting in lower effective diffusivities and Kp. Finally, the effects of the type of bacteria and their distribution in the beads on the diffusivity were also discussed.