Excessive Levels Of Nutrients Research Articles

Effect of feed zone in fed‐batch fermentations of Saccharomyces cerevisiae

In production-scale, fed-batch fermentations, feed is often added to a single point at the top of the fermentor, which, combined with poor mixing, results in formation of a "feed zone" rich in nutrients. Frequent exposure of the culture to high concentrations of nutrients in the feed zone for sufficient duration can produce unexpected effects on its performance. The effect of the feed zone was evaluated by conducting aerobic fed-batch fermentations of Saccharomyces cerevisiae with both complex and defined media. The broth was recirculated between a recycle loop and a bench-scale fermentor, and feed was intermittently added into the recycle loop to simulate the circulation of cells through the feed zone. Experiments were carried out for a range of residence times in the recycle loop from 0.5 to 12 min. Biomass yields from the complex-media fermentations were not affected by exposure to high nutrient levels in the recycle loop for residence times up to 12 min. Ethanol consumption was reduced by as much as 50% for residence time in the loop up to 3 min. Very long exposure of yeast cells to excess nutrient levels (12 min) gave acetic acid formation. In a defined medium, the simulated feed zone effect increased biomass yield by up to 10%, but had no effect on ethanol levels. This study indicates that the feed zone effect on biomass yield in yeast fermentation, using complex substrates, will be negligible under fully aerobic conditions.

Induction of a manganese-containing superoxide dismutase in leaves of Pisum sativum L. by high nutrient levels of zinc and manganese

The effect of excess nutrient levels of Zn and Mn on the activity of the leaf metalloenzymes catalase and the SOD isozyme system, involved in the cell protection assembly against O 2-derived toxicity, was studied in pea plants. Moderately high nutrient levels of Zn (16.2 μM) and, to a lesser extent of Mn (90.1 μM), induced the appearance of a new manganese-containing superoxide dismutase isozyme in leaves of Pisum sativum L. plants grown under controlled conditions. The maximum inhibition of plant growth by sublethal levels of Mn and Zn was accompanied by a simultaneous increase of catalase and Mn-SOD activity in plant leaves, suggesting the existence of a functional link between both metalloenzymes in the plant cell to remove the possible toxic effects of H 2C 2 and O 2 -. The possibility that the nutritional induction of the Mn-SOD isozyme could take place as a result of interactions between micronutrients in the plant cell is discussed.