B. Kristiansen

The influence of glucose concentration on citric acid production and morphology of Aspergillus niger in batch and culture

Studies in conventional batch culture confirmed that initial glucose concentration in the fermentation medium affected both the rate of citric acid fermentation by Aspergillus niger and the morphology of the producer micro-organism. A series of fed-batch (glucostat) experiments, in which the concentration of glucose was kept constant whereas all other conditions were allowed to change, were used to eliminate the effects of decreasing sugar concentration found during conventional batch experiments. The level of glucose had a marked effect on production rates; the specific rate of citric acid formation increased with increasing initial glucose concentration (batch culture) or glucose levels (glucostat culture), and in both culture methods, the specific growth rate increased with decreasing glucose concentration for the first 48 h of fermentation. The reduction observed in the mean length of filaments at low glucose levels could be explained by increased branching frequency as a result of the increasing specific growth rate at the early stages of fermentation. The size of the mycelial clumps was reduced at low glucose levels and their shape was also affected as the ratio between the perimeter of the clump and the perimeter of its core increased as glucose concentration decreased. Very low glucose levels during fermentation in glucostat culture had a striking effect on the metabolism of the micro-organism because the specific production rate decreased in favor of the specific growth rate and the morphology changed with a doubling of the size of clumps and pellet formation. The main morphological observations in both batch and glucostat cultures seemed to be related to the growth rate, which is restricted by the glucose concentration, and, thus, indirectly related to the concentration of glucose in the fermentation media. The glucose levels in the medium were shown to lead to a significant noncatalysed entry into the mycelium, which accounted for the citric acid productivity and growth rate changes.

Citric acid biotechnology

Citric acid biotechnology is the subject of both industrial and academic interest. For a long time the production of citric acid was one of the driving forces in industrial biotechnology, lying in the interface between old and new technology. Now, not only is citric acid biotechnology of great industrial and economic importance, it is also used to illustrate biotechnological principles at undergraduate level. This comprehensive book covers subjects such as mycology redox reactions mass and energy balances downstream processing fermentation substrates and design of industrial plants. All the authors are still active in the field and come from both industrial and academic backgrounds. Undergraduates and postgraduates studying industrial microbiology, biotechnology or biochemical engineering, or those wishing to pursue careers in the biotechnological industry will find this a comprehensive, accessible and definitive resource. It will also provide a useful reference for researchers and managers in the industry.

Effect of ammonium ion concentration on polysaccharide production by Aureobasidium pullulans in batch culture

SummaryExperiments have been carried out to study the production of polysaccharides by Aerobasidium pullulans in a 6L fermenter. In batch culture, a lag in polysaccharide production but not biomass was experienced. This lag increased with increasing initial nitrogen concentration. Polysaccharide production commenced on reaching nitrogen limiting conditions. At high nitrogen levels, the production of polysaccharides was reduced considerably.

Morphology and citric acid production of Aspergillus niger PM 1

SummaryAspergillus niger PM 1 was grown in a tubular loop and a stirred tank bioreactor. Batch fermentations were performed under various agitation conditions and pH. Citric acid, oxalic acid, extracellular polysaccharides and proteins were assayed. The following morphological parameters were measured: mean perimeter of clumps, mean perimeter of the central core of clumps, mean length of filaments and mean diameter of filaments. Citric acid production and morphology in both reactors were dependent on agitation intensity and pH. The length of the filaments was shown to be the only parameter that could be related to citric acid production in both reactors: the shorter the filaments the more citric acid was produced. However, for the same amount of citric acid produced the morphology of the organism grown in the stirred tank differed considerably from that grown in the loop reactor.

HYPHAL VACUOLATION AND FRAGMENTATION IN BATCH AND FED-BATCH CULTURE OF ASPERGILLUS NIGER AND ITS RELATION TO CITRIC ACID PRODUCTION

Abstract The relationship between hyphal vacuolation, fragmentation and citric acid production by Aspergillus niger, was investigated in batch and fed-batch culture. Quantitative information on morphology and vacuolation was obtained by image analysis. Time profiles of the morphology parameters — mean perimeters of clumps, P, lengths of filaments, L — and vacuoles, together with, specific growth and production rates, were used to establish a link between vacuolation, fragmentation and product formation under various agitation conditions and glucose levels. Under intensive agitation conditions and during the early fermentation stages, the characteristics observed were the increased specific growth rates and hyphal branching, along with low vacuolation levels. These were followed by fragmentation of the highly vacuolated parts of filaments and regrowth at later stages, a process that maintained that the mycelium grew old and highly vacuolated, with limited renewal, conditions that did not favour increased biosynthetic activity. Increased vacuolation and low specific production rates were observed at low glucose levels in fed-batch culture. The results indicate that vacuolation weakened the hyphae and low glucose levels created the conditions that favoured fragmentation and made the mycelium more susceptible to it when exposed to increased agitation.

Morphology of Aureobasidium pullulans during polysaccharide elaboration

In batch culture, the production of exocellular polysaccharide by Aureobasidium pullulons did not coincide with any major transformation in the organisms morphology. Increasing the level of ammonium sulphate resulted in a more filamentous growth form and less polysaccharide production but continuous culture studies questioned the direct influence of nitrogen level on culture morphology. Medium pH appeared more important, and several experiments carried out under steady-state conditions at various pH values raised doubts that a unicellular growth form was a prerequisite for polysaccharide production.

Design of a Tubular Loop Bioreactor for Scale-up and Scale-down of Fermentation Processes

Microorganisms traveling through circulation loops in large‐scale bioreactors experience variations in their environment such as dissolved oxygen concentration and pH gradients. The same changes are not experienced in small bioreactors, and it is suggested that herein lies one of the major reasons for the problems encountered when translating fermentation data from one scale to another. One approach to study this problem is to look at the circulation loop itself. The present work concerns an attempt to simulate the circulation loops inside stirred tank reactors, using a tubular loop reactor specially constructed for the purpose. The reactor carries a number of ports and probes along its length for the determination of concentration gradients within. The broth is circulated around the loop by the use of peristaltic pumps, and the circulation time (tc, s) is used as a measure of simulated reactor size. The reactor system has been evaluated using the citric acid fermentation by Aspergillus niger as a test process. Acid production and fungal morphology, in terms of the mean convex perimeter of mycelial clumps quantified by image analysis, were used as the parameters of evaluation for the two systems in comparison. From comparative experiments carried out in 10 and 200 L stirred tank bioreactors, it appears that the loop reactor simulates the corresponding stirred tank representing a valuable tool in scaling up and scaling down of fermentation process.

Substrate inhibition kinetics of Saccharomyces cerevisiae in fed-batch cultures operated at constant glucose and maltose concentration levels

Fed-batch culture is the mode of operation of choice in industrial baker’s yeast fermentation. The particular mode of culture, operated at stable glucose and maltose concentration levels, was employed in this work in order to estimate important kinetic parameters in a process mostly described in the literature as batch or continuous culture. This way, the effects of a continuously falling sugar level during a batch process were avoided and therefore the effects of various (stable) sugar levels on growth kinetics were evaluated. Comparing the kinetics of growth and the inhibition by the substrate in cultures grown on glucose, which is the preferential sugar source for Saccharomyces cerevisiae, and maltose, the most common sugar source in industrial media for baker’s yeast production, a milder inhibition effect by the substrate in maltose-grown cells was observed, as well as a higher yield coefficient. The observed sugar inhibition effect in glucostat cultures was taken into account in modeling substrate inhibition kinetics. The inhibition coefficient Ki increased with increasing sugar concentration levels, but it appeared to be unaffected by the type of substrate and almost equal for both substrates at elevated concentration levels.

Citrate transport during the citric acid fermentation by Aspergillus niger

SummaryAspergillus niger SZ was grown under citric acid accumulation conditions in a 1l bioreactor. Radioactive citric acid was added during the fermentation to the culture medium. The accumulation of radioactive citric acid in the mycellium was measure by measuring radioactive CO2 in the offgas. Evidence was obtained of significant turnover and metabolism of exogenous citric acid even during the phase of maximum accumulation into the medium.

Extracellular esterase activity from Bacillus stearothermophilus

Bacillus stearothermophilus has been reported to produce an extracellular esterase with molecular weight of 42–47 kDa. Extracellular esterase activity in Bacillus stearothermophilus (NCIB 13335) was found to reside in protein with a molecular weight less than 10 kDa. This small esterase was responsible for all the esterase activity observed in this strain under the conditions studied.