Impact of probiotic cell inclusion and metabolic activity on the rheological properties of low-methoxyl pectin solutions and gels

The rheological behavior of low-methoxyl (LM) pectin solutions and gels was characterized to evaluate the impact of the inclusion of probiotic cells (Lactobacillus rhamnosus GG and L. casei) and its metabolites. Considering that probiotic cells metabolize macro and/or micro-nutrients in growth media and/or can modulate pectinCa2+ interactions occupying the free volume in the gel, two scenarios were tested: 1) A 2% pectin in freecarbohydrate basal medium (P-MRS-B) (control sample) was mixed with probiotic cells (-107 cells/mL) and incubated at 37 degrees C for 48 h (fermented samples); 2) A probiotic suspension in P-MRS-B was prepared as described above and used immediately to measure rheological properties (in situ samples). All gel-forming solutions presented Newtonian behavior and their corresponding gels showed an intermediate behavior between weak-gel and strong-gel. Mean viscosity (eta) values ranged between 15.9 and 20.1 mPa s and no significant differences (p = 0.643) were observed between the samples. Mean elastic modulus at 1 rad/s (G ' 1) values ranged between 447 and 499 Pa, with no significant differences (p = 0.975) between the samples. These results suggest that: 1) The volume fraction of probiotics was so low that its presence did not have a significant effect on the viscosity and on the elastic modulus, and neither on the gelation mechanism of the LM-pectin; 2) Metabolic products of LM-pectin fermentation did not have a significant effect on the viscosity of the solutions, nor on the elastic modulus of the gels and their gelation mechanism.