WATER RELATIONS OF SOLID-STATE FERMENTATION

The hydration level of the culture medium affects drastically the phys iology of microorganisms. Thermodynamic properties of water in a solid -state medium such as water potential or water activity, and dynamic p roperties of water such as water diffusion are examined. The mechanism s of water transfer between intra and extracellular media are discusse d, and different results on the incidence of the water-activity level on the survival, growth and metabolism of microorganisms are reported. Optimal macroscopic and microscopic growth, or metabolic production b y fungi occurs in media with water activity values lower than those of classical cultivation media (approximate to 1). In solid state media, diffusional factors combined with low water content can also limit th e growth of filamentous fungi. The main parameter involved in the reac tion of microorganisms to the hydration level of the medium seems to b e the plasma membrane, acting as an osmotic cellular sensor.