Fermentation by lactic acid bacteria at two temperatures of pre-heated reconstituted milk. I - Behaviour of proteins and minerals

Abstract

The pH-induced physico chemical changes in a reconstituted heated (90 °C - 10 min) skim milk were studied during fermentation by lactic acid bacteria at two temperatures (30 °C and 42 °C). Different variables were examined during acidification: the water content of ultracentrifugation pellets (150000g for 70 min at 30 or 42 °C), the partition of proteins into pelleted and unpelleted fractions (analysed by RP-HPLC on supernatants) and the dissociation of colloidal salts (determined by atomic absorption spectrophotometry on the ultrafillrates). The proportion of unpelleted protein was found to be pH-dependent with a gradual linear decrease during acidification. All the caseins pelleted below pH 5.5. As the pH was reduced, calcium was progressively released from the micelles. In contradiction with a number of previous findings, fermentation induced a shift toward acid pH values of the salt dissociation compared with a glucono-delta-lactone acidification. The partition of casein between the pellet and the supernatant due to acidification by fermentation was temperature-dependent, as was the solvation of the pellet. Both were greater at 30 °C compared with 42 °C. The temperature had no effect on the salt distribution, The results are discussed in terms of transfer phenomena between the colloidal and the solvent fractions and diffusion-limited processes.