ISSN:
0006-3592
Keywords:
Chemistry
;
Biochemistry and Biotechnology
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Biology
,
Process Engineering, Biotechnology, Nutrition Technology
Notes:
A pH-stat fermentor is a continuous cultivator in which the feed rate is controlled to maintain a constant pH, i.e., end-product acid concentration. This fermentor has application to the continuous cultivation of lactic acid-producing organisms in milk-based media. The equations describing the operation of this fermentor are developed. It is shown that, where the limiting substrate is the carbon and energy source, the operation of the fermentor is essentially equivalent to that of a turbidostat. In contrast to this, where the carbon and energy source is in excess and growth is limited by another substrate, pH-state fermentation is stable both in regions of substrate excess, where D = μmax, comparable with turbidostat operation, and substrate limitation where D 〈 μmax, comparable with chemostat operation. These conditions are met in milk-based media. An analysis is presented, allowing the prediction of the degree of substrate limitation, cell density, and dilution rate in a pH-stat fermentor from batch-growth kinetics. This was confirmed using experimental data for the growth of Streptococcus thermophilus TS2 and Lactobacillus LB1 in skim milk. Stable simultaneous growth of two organisms in continuous culture occurs if their growth rates are determined by separate conditions, so that, at steady state, their growth rqtes are separately madeequal to the dilution rate. It is then predicted, and confirmed by experiment, that a mixed culture of S. thermophilus TS2 and L. bulgaricus LB1 in a pH-stat continuous fermentor in yogurt mix at pH 5.5 would be stable with the growth of L. bulgaricus being substrate unlimited and the fermentor operting with D = μmax for L. bulgaricus LB1, and the growth of S. thermophilus TS2 being substrate limited so that its growth rate is equal to the existing dilution rate. Finally, it is predicted and confirmed by experiment that if the conditions are altered so that the growth of S. thermophilus TS2 is substrate unlimited the stable association is broken down, the fermentor operates with D approaching μmax for S. thermophilus TS2, and L. bulgaricus LB1 is washed out to the level maintained by wall growth.
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bit.260210903
Permalink
