Researchers at Shanghai Jiao Tong University have developed a method for the solid-state fermentation (SSF) of sweet sorghum stalk particles with active dry yeast.
In a study published online 23 December in the ACS journal Energy & Fuels, they describe an effective method of sweet sorghum stalk storage in dry particles for 8 months, and their experiments with SSF to clarify the effects of different variables, including temperature, particle size, yeast inoculation rate, and water content on yeast growth, CO2 and ethanol formation, and sugar use.
Solid-state fermentation, as described in the book Solid-State Fermentation Bioreactors: Fundamentals of Design and Operation
...involves the growth of microorganisms on moist solid particles, in situations in which the spaces between the particles contain a continuous gas phase and a minimum of visible water. Although droplets of water may be present between the particles, and there may be thin films of water at the particle surface, the inter-particle water phase is discontinuous and most of the inter-particle space is filled by the gas phase. The majority of the water in the system is absorbed within the moist solid-particles.
...The majority of SSF processes involve filamentous fungi, although some involve bacteria and some involve yeasts.
The Shanghai Jiao Tong researchers found that the effects of temperature and particle size on yeast growth and ethanol yield were significant. In addition, the yeast inoculation rate and water content were greatly related to yeast growth, even though they had less significant effects on ethanol yield.
The suitable ranges of temperature and particle size for yeast growth were 25-30° C and 1.6-2.5 mm, respectively, in which higher values of Ycell/sugar of 0.2681 and 0.3538 mg mg-1 were obtained.
The suitable ranges for ethanol production were 35-40° C and 0.9-1.6 mm, in which higher values of Yethanol/sugar of 0.2404 and 0.2702 mg mg-1 could be obtained.
The adequate yeast inoculation rate should be 0.250, and higher values of Yethanol/sugar and Ycell/sugar could be up to 0.2486 and 0.3017 mg mg-1, respectively.
In addition, the suitable water content for ethanol production and yeast growth was 76.47%. With this water content, the Yethanol/sugar and Ycell/sugar could reach 0.2593 and 0.4211 mg mg-1, respectively.
Fei Shen and Ronghou Liu (2008) Research on Solid-State Ethanol Fermentation Using Dry Sweet Sorghum Stalk Particles with Active Dry Yeast. Energy Fuels doi: 10.1021/ef800531e
David A. Mitchell, Marin Berovič and Nadia Krieger (Eds.) (2006) Solid-State Fermentation Bioreactors: Fundamentals of Design and Operation. Springer Berlin Heidelberg. doi: 10.1007/3-540-31286-2_1