Influence of immobilization of bacteria, yeasts and fungal spores on the mechanical properties of agar and alginate gels

Lactococcocus lactis bacteria, Saccharomyces cerevisiae yeasts and Trichoderma viride fungal spores were immobilized in agar and alginate gels. The mechanical properties of these gels were tested by compression using a Universal Testing Machine. Data was collected as volts versus time, and later converted to stress versus strain. Both gel systems exhibited similar behavior after incorporation of the microorganisms. In most cases addition of microorganisms up to 10⁵ CFU/ml of gel, did not change gel strength (stress at failure) and affect its deformability modulus or Hencky's strain at failure. Different behavior was observed for lactic acid bacteria in an alginate gel where no changes were detected, even after adding 10⁶ microorganisms/ml of gel. Major changes in the gel properties occurred when 10⁷–10⁹ microorganisms were immobilized per ml of gel matrix. Both agar and alginate gels suffered weakened textures (even though their mechanical properties, gelation mechanisms and immobilization methods were very different). Gel strengths and deformability moduli decreased by a factor of 1.2–4.4 depending on the type of gel, the number of microorganisms per ml and the diameter of the embedded microorganisms. Following immobilization gels became more brittle, as demonstrated by a decrease in their Hencky's strain at failure. Results are also presented in this paper as the ratio between mechanical properties of gels with or without microorganisms versus the occupied volume of the tested gel.