On-line size measurement of yeast aggregates using image analysis

Alcohol fermentation productivity can be strongly improved using a flocculation-based yeast recycle. However, the efficiency of the biomass retention system depends strongly on the yeast particle size. Accordingly, the monitoring and control of yeast floc diameter are of primary importance. The on-line measurement of mean floc diameter has been achieved using on-line image analysis, based on the evaluation of image texture. The texture analysis method consisted in the building of a co-occurrence matrix from which the so-called "Energy parameter" was extracted. While image texture is usually used for classification purposes, it has been used here as a quantitative descriptor: a correlation has been found between this statistical image feature and off-line manual floc-size determinations. In the floc size range investigated (0.5-4.3 mm), the evaluated mean diameter was in good agreement with the actual particle size, with a determination coefficient equal 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 sampling. This novel tool has been used to investigate the behavior of yeast aggregates as a function of fermentation conditions. While biomass concentration was kept constant, step increases of the feed rate led to a decrease of the mean floc diameter. Image analysis showed that the particle-size reduction could occur within a few minutes after modification of the medium dilution rate, demonstrating the disruptive effect of the CO2 efflux. The kinetic of aggregate formation was dependent on the gas-phase composition. Instead of recycling fermentation gas, sparging the fermentor with nitrogen, to reduce dissolved CO2 concentration, increased the rate of floc-size growth.