This experiment introduces the students to an immobilized cell fermentor. Yeast cells will be entrapped in calcium alginate gels by using the similar techniques as in enzyme immobilization. Other cell entrapment media that have been previously attempted include polyacrylamide, gelatin, chitosan, and k-carrageenan gels.
Due to the constraint in the available equipment to carry out the immobilization procedure aseptically, the experiment will be conducted without autoclaving. The immobilized cell reactor will be employed to convert glucose into ethanol anaerobically. The reasons for choosing this system of microorganism and product are many folds. First, the anaerobic condition will eliminate the need for aeration, which causes many technical problems. Secondly, the lack of oxygen will prevent the uncontrolled growth of aerobic contaminants in an unsterilized fermentor. The presence of high levels of ethanol should also discourage most microorganisms from taking over the fermentor. To reduce further the chance of contamination by bacteria, the pH of the fermentor will be kept low; a value of 4.0 should drastically slow down the growth of most bacteria but only slightly affect the yeast's ethanol producing capacity.
The production of ethanol in an immobilized bioreactor is a relatively well studied process. As high as 95% of the theoretical yield of alcohol based on glucose (8.5 % ethanol from 14% glucose) has been reported. A high space velocity, defined as the volume of nutrient feed per hour per gel volume, of 0.4-0.5 hr-1 is commonly used to maximize the ethanol productivity. An ethanol productivity of 20 g/l-hr can be achieved.
Both the steady state response and the transient approach to the steady state will be studied in this experiment .