Growth studies on the lactic streptococci: IV. Some observations on redox potential
Summary Redox potential changes at 30°C in anaerobic reconstituted skim-milk cultures of Streptococcus cremoris AM1 and AM2 and Streptococcus lactis ML8 have been investigated in batch cultures and continuous cultures maintained at constant pH by the addition of fresh medium. Str. lactis ML8 was more strongly reducing than either Str. cremoris AM1 or AM2. The positive Eh values which can arise in dense continuous cultures of lactic streptococci are not likely to affect either the lactic acid formation or proteolytic activity of the resultant bacterial concentrate. An electrolytic reduction cell is described which allows variations in redox potential in cultures to be controlled simply and effectively.
Characterisation of lactic starters based on acidification and reduction activities
The acidification and reduction activities of lactic starters have been followed by continu- ous 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 Lactococcussp., 6 strains of Streptococcus thermophilus and 5 strains of Lactobacillus helveticuswere 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. Acidification / reduction / Lactococcus / Streptococcus thermophilus / Lactobacillus helveticus Resume - Caracterisation de levains lactiques sur la base de leurs activites acidifiantes et re- ductrices. Le suivi de l'acidification et de la reduction du lait par des levains lactiques a ete realise par l'enregistrement en continu du pH et du Eh. Les valeurs mesurees ont permis de calculer les vitesses maximales d'acidification et de reduction, et le temps, le pH et le Eh auxquels ces maxima sont obte- nus. L'etude a porte sur 9 souches de Lactococcussp., 6 souches de Streptococcus thermophilus ,e t 5 souches de Lactobacillus helveticus. D'une maniere generale, la vitesse maximale de reduction des lactocoques etait six fois plus grande que celle des streptocoques et des lactobacilles. Par contre, les streptocoques et lactobacilles acidifiaient avec une vitesse maximale superieure a celle des lactoco- ques. En consequence, toutes les cultures avec les lactocoques atteignaient leur Eh final avant la fin de la fermentation lactique, alors que l'acidification avec les streptocoques et les lactobacilles se
Continuous and Automatic Propagation of Dairy Cultures
Summary A small fermentation system was devised for continuous automatic propagation of dairy cultures. The fermentor is a circulatory, 1-in. diameter, stainless steel pipe assembly of 1,000-ml. capacity, in which skimmilk is continuously and slowly circulated. Acidity is controlled with a pH meter and meter-relay which activates a solenoid valve controlling the skimmilk inflow. The fermentation system has been operated for 12 wk., but not without contamination and undesirable fementations. Successful trials free from contamination have been conducted for 3 wk. These have provided information regarding the throughput rate at various temperatures between 50 and 90° F. At 90° F. the retention time or growth rate was 30 min. The critical pH level was 5.3, below which the rate of growth was reduced. Dairy cultures can be propagated continuously at maximum growth rates without impairment of flavor or activity, with constant temperature and pH (above 5.3 for maximum rate) and in continuous motion.