On-line size measurement of yeast aggregates using image analysis

Alcohol fermentation productivity can be strongly improved using a floccula tion-based yeast recycle. However, the efficiency of the biomass retention system depends strongly on the yeast particle size. Accordingly, the monito ring and control of yeast floe diameter are of primary importance. The on-l ine measurement of mean floe diameter has been achieved using on-line image analysis, based on the evaluation of image texture. The texture analysis m ethod consisted in the building of a co-occurrence matrix from which the so -called "Energy parameter" was extracted. While image texture is usually us ed for classification purposes, it has been used here as a quantitative des criptor: a correlation has been found between this statistical image featur e and off-line manual floe-size determinations. In the floc-size range inve stigated ((null set) 0.5-4.3 mm), the evaluated mean diameter was in good a greement with the actual particle size, with a determination coefficient eq ual to 0.980. In contrast with manual measurements, slow and tedious, this method gave the value of the mean particle diameter in real-time, without s ampling. This novel too] has been used to investigate the behavior of yeast aggregates as a function of fermentation conditions. While biomass concent ration was kept constant, step increases of the feed rate led to a decrease of the mean floe diameter. Image analysis showed that the particle-size re duction could occur within a few minutes after modification of the medium d ilution rate, demonstrating the disruptive effect of the CO2 eff lux. The k inetic of aggregate formation was dependent on the gas-phase composition. I nstead of recycling fermentation gas, sparging the fermentor with nitrogen, to reduce dissolved CO, concentration, increased the rate of floc-size gro wth. (C) 2001 John Wiley & Sons, Inc.