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 (150000 g 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 ultrafiltrates). 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. Milk / yoghurt / pH / mineral / micellar solvation / caseinRésumé -Fermentation du lait reconstitué traité thermiquement par des bactéries lactiques à deux températures. I -Protéines et minéraux. L'effet du pH sur les évolutions physicochimiques de lait écrémé reconstitué et traité thermiquement à 90°C, 10 min a été étudié au cours d'une acidification par fermentation lactique à deux températures (30°C et 42°C). La quantité d'eau a été évaluée par séchage des culots d'ultracentrifugation (150000 g pendant 70 min à 30 ou 42°C). La distribution des protéines entre le culot et le surnageant a été analysée par chromatographie RP-HPLC sur les surnageants avec ou sans dithiothreitol. La dissociation des sels colloïdaux a été déterminée par spectrométrie d'absorption atomique dans les ultrafiltrats de lait. La proportion de protéines dans le surnageant était dépendante du pH avec une diminution graduelle au cours de

Article

“…At the final equilibrium (in the static condition), the diffusion of H + and CCP is complete and the concentration of CCP released from the casein micelle is maximum at a given pH, leading to a higher T2 value. In the previous paper of this series [23], we found a shift toward more acid values of the soluble concentrations of calcium with the bacterial acidification compared with either GDL or HCl addition. It is well known that the dissociation of minerals from milk under acidification is dependent on the rate of acidification [19].…”

“…However, changing the temperature changes not only the rate of fermentation, but also some characteristics of the micelles, as observed for solvation data [23], the rate of aggregation and thermal motion. We chose to change the temperature rather than the inoculum level to obtain a low or high acidification rate because, as already shown by Mietton et al [33], varying the inoculum level changes the lag time but has no effect on the acidification rate between pH 6 and 4.6.…”

“…The skim milk was reconstituted to 11% solid content from a low-heat skim milk powder and was heated at 90°C for 10 min as described by Laligant et al [23].…”

“…In the hypothesis of a diffusion-limited gel formation by fermentation, changing the rate of acid production will modify the diffusion processes. In a previous paper [23], the composition of the heated milk and static measurements during acidification were presented. Dynamic measurements such as rheological characterisations, frontface fluorescence of intrinsic tryptophan (TRP) and 1 H-NMR are studied in this one.…”

Quality indices of the set-yoghurt prepared from bovine milk treated with horseradish peroxidase

“…Texture of set-yoghurt is an important quality criteria (Lee and Lucey 2004;Lucey 2004), which is influenced by milk composition (Marafon et al 2011), dry matter (Wu et al 2009), type of starter culture (Amatayakul et al 2006b), incubation temperature (Laligant et al 2003), processing conditions (Rekha et al 2011), storage time (Ünal et al 2003) and other factors. Textural defects as roughness (irregularities in yoghurt matrix) and syneresis are undesirable to the consumers.…”