Characterisation of lactic starters based on acidification and reduction activities

Abstract

-The acidification and reduction activities of lactic starters have been followed by continuous measurement of the pH and the Eh during the course of milk fermentation. These measurements allowed the calculation of the maximum acidification and reduction rates, and the time, pH and Eh at which these values occurred. Nine strains of Lactococcus sp., 6 strains of Streptococcus thermophilus and 5 strains of Lactobacillus helveticus were studied. In general, the maximum reduction rate of the lactococci was six-fold higher than those of the streptococci and lactobacilli. On the other hand, the streptococci and the lactobacilli acidified with a higher maximum acidifying rate than those of lactococci. Consequently, it was observed that all the cultures with lactococci reached their final Eh before the end of the lactic acid fermentation, while acidifications with the streptococci or the lactobacilli finished before the end of the reduction phase. A principal components analysis clearly differentiates the three species on the basis of their aptitudes for acidification and reduction. This new approach might be used to select adequate starters for the manufacture of fermented dairy products. terminait avant la fin de la phase de réduction. L'analyse en composante principale différencie clairement les 3 espèces sur la base de leurs aptitudes acidifiantes et réductrices. Ces nouveaux paramètres pourraient être associés à ceux classiquement utilisés pour sélectionner les levains les plus adéquats pour la fabrication de produits laitiers fermentés. Acidification / réduction / Lactococcus / Streptococcus thermophilus / Lactobacillus helveticus

NoxE NADH Oxidase and the Electron Transport Chain Are Responsible for the Ability of Lactococcus lactis To Decrease the Redox Potential of Milk

“…A few studies have reported that the reducing activity of lactic acid bacteria is species dependent and that L. lactis is one of the most reducing lactic acid bacteria (5,8). Also, Jeanson et al (21) showed that the E h of milk drops only after all of the dissolved oxygen is consumed by L. lactis.…”

“…The AhpR system may also participate in reduction of oxygen to H 2 O. However, the E h of oxygen-free milk (saturated with nitrogen) is still positive (21), while the E h of milk fermented with L. lactis is close to Ϫ220 mV (8,21). This suggests that, in addition to oxygen, L. lactis reduces other oxidizing compounds present in milk.…”

“…Its major role is to acidify milk, but it is also one of the most reducing lactic acid bacteria and produces a very negative redox potential (E h ) value in milk (5,8). The E h is indicative of the ability of a system to donate or accept electrons and is strongly dependent on levels of dissolved oxygen (39).…”

“…Acidification data were expressed as the ⌬pH, which is the difference (in pH units) between the initial pH (pH 6.6) and the pH measured at each time. As described by Cachon et al (8), the measured redox potential (E m ) was converted to E h , with reference to the hydrogen electrode, using the following formula: E h ϭ E m ϩ E r , where E r is 204 mV at 30°C for our Ag/AgCl electrodes. In addition, because redox potential values are pH dependent (19), E h was transformed to the redox potential at pH 7 (E h7 ) with the Leistner-Mirna equation: E h7 ϭ E h Ϫ ␣ ϫ (7 Ϫ pH) (25).…”

Redox Potential: Monitoring and Role in Development of Aroma Compounds, Rheological Properties and Survival of Oxygen Sensitive Strains During the Manufacture of Fermented Dairy Products

“…This value may vary according to the nature of the oxido-reducing molecules in the media. For example, the Nernst factor is 40 mV/pH unit in milk [6].…”