Rheology of pure and mixed kappa-carrageenan gels in lactic acid fermentation conditions

Texture profile analysis and rupture tests, respectively, at a deformation of 5 and 80%, were carried out on commercial kappa-carrageenan gel cylinders (3% w/v concentration) and on mixed gel cylinders (kappa-carrageenan/iota-carrageenan or locust bean gum [LBG], in different proportions), to select the best gel composition as an immobilization matrix for continuous lactic acid-fermentations of dairy media. Mixed gel cylinders were soaked in a model system for lactic fermentations consisting of 36 dairy media: milk, whey and whey permeate, with 4 lactate concentrations (5, 20, 35 and 50 g/L of medium), combined with 3 pH values (6.5, 6.0, 5.5). GENUGEL X-0909 n degrees 5180850 was the most resistant of all commercial kappa-carrageenans available; mixing this kappa-carrageenan with LEG in the respective proportions of 2.75%/0.25% gave the best rheological properties to the resulting mixed gel. Our study of the impact of fermentation parameters showed that pH has a significant effect on hardness, cohesion and percentage of deformation at rupture in milk (but not in the other 2 media). A pH drop of milk from 6.5 to 5.5 resulted in more brittle gels which may be explained by an increase in the amount of soluble calcium cations. Generally, soaking in milk produced the most brittle gels due to its calcium content, whey produced the hardest gels and whey permeate, the softest. An increase in the ammonium lactate concentration significantly improved hardness, rupture force and percentage of deformation at rupture, while reducing cohesion and resilience: NH4+ cations would account for this effect. The differences in rheological behaviour of immobilization gel beads during continuous lactic fermentations of milk and whey permeate would be due to the cation content and more specifically to the higher calcium concentration in milk.