New sensor allowing continuous water activity measurement of submerged or solid-substrate fermentations

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Abstract
Water activity of the substrate is an important and acute factor for growth as well as for metabolic production of microorganisms or for biocatalyst systems. A sensor has been designed in order to control this parameter on‐line during submerged and solid‐substrate fermentations. This sterilizable sensor allows the measurement of the relative humidity of the atmosphere in a small chamber by means of a capacitive element separated from the medium by a thin ethylenepolytetrafluoride membrane. For high aw values (>0.90) a sequential circulation of a dried gas prevents the sensor saturation. Measurements are rapid and accurate, and control of the water activity of a fermentation medium has been carried out for 5 days using this sensor.