| Yeast cell wall contains about 29%β-glucan, 31% mannan, 13% protein, 9% lipid and 1%–2% chitin.β-glucan and mannan receive much concern for their specific bioactivity, therefore this paper made systematical study on theβ-glucan and mannan.The fermentation conditions of Saccharomyces cerevisiae were optimized in 5L fermentor. The optimum fermentation conditions were determined and the content of cell wall polysaccharides increased significantly. The cell wall polysaccharides were prepared using Saccharomyces cerevisiae fermented in 50L fermentor. The feasible radiation modifiction conditions were determined and the water solubility ofβ-glucan increased which could enlarge its application range.In order to supply the theoretic proof for the cell wall polysaccharides products development, the rheological properties of modified glucan and mannan were studied.The fermentation conditions of Saccharomyces cerevisiae were optimized in 5L fermentor, and through investigating the influence of stirrer speed, pH value and temperature on biomass, glucan and mannan production, the optimum fermentation conditions were determined as follows: stirrer speed 400 r/min, pH value 4.00, temperature 32℃. On such fermentation conditions, the biomass, the content ofβ-glucan and mannan were 2732.00±6.38 mg/100mL, 365.16±3.89 mg/100mL, 223.6±7.23 mg/100mL separately, which increased 71.82%, 237.92% and 100.66% separately compared with those not optimized, the content ofβ-glucan increased 160.27% compared with the report .Experimental models were constructed using BP neural network to train experimental data and the BP models of biomass, sugar consumption and cell wall polysaccharides production and weight values and threshold values were determined. The predicted values of the models were compared with the experimental values. These models were proved to well simulate the change of biomass, sugar consumption and cell wall polysaccharides production, also it could make a preliminary prediction on the level of cell wall polysaccharides. According to the prediction of these BP models, the biomass,β-glucan and mannan increased rapidly in the first 10h and reach- -ed to the maximum from 48 to 54h, the biomass kept stable whereasβ-glucan and mannan decreased slowly after 54h, these trend is basically same as the experimental values.The rate of autolysis increased as the increase of the cell concentration, the highest rate reached 23.12±0.88% when the cell concentration is 30%.β-glucan and mannan were prepared , the purity and yield ofβ-glucan was 72.80±1.39% and 16.5%, the purity and yield of mannan was 78.15±1.45% and 3.9%.β-glucan was modified by radiation, non-irradiatedβ-glucan could be dissolved in water to 5±0.34 %, whereas the solubilities ofβ-glucan irradiated at 20kGy was 32.46±0.26%, the water solubility ofβ-glucan increased significantly consequent to the increase of the absorbed dose of radiation(p<0.01). The molecular weight ofβ-glucan decreased rapidly as the increase of radiation dose, the molecular weight of non-irradiatedβ-glucan was about 19.96×104, the molecular weight ofβ-glucan irradiated at 20kGy was 3.696×104, which decreased significantly(p<0.01). However, the extent of degradation of the molecular weight was not linearly proportional to the radiation dose, the degradation ofβ-glucan under low-dose range(≤10kGy) was greater than that under higher dosage. The particle size of non-irradiated and irradiatedβ-glucan were compared , the median-particle-size and bulk average size of non-irradiatedβ-glucan was 54.77μm and 69.52μm separately whereas the median-particle-size and bulk average size ofβ-glucan irradiated at 20kGy was 49.78μm and 55.58μm separately, whose bulk average size decreased significantly compared with that non-irradiatedβ-glucan(p<0.05). The particle size ofβ-glucan decreased by irradition.The viscosity ofβ-glucan decreased greatly from 5.52 Pa·s to 1.51 Pa·s as the increase of irradiation dose from 5kGy to 10kGy, the viscosity had no obvious change with the further increase of radiation dose.The non-irradiated and irradiatedβ-glucan both had a similar pattern of FTIR spectra and there were no any notable change in the functional-group status after radiation.The effects of different concentrations, shear time, temperatures, pH and ionic environments on modified glucan and mannan solutions were studied. The viscosity of modifiedβ-glucan increased to 5520 mPa·s as the increase of concentration and the viscosity of modifiedβ-glucan decreased and kept stable at 2.0 mPa·s as the increase of shear rate, which showed the characteristic pseudoplastic fluid properties. The viscosity of mannan increased to 21.90 mPa·s as the increase of concentration, the shear-thinning nature of mannan was higher when shear rate increased. The shear stress of modifiedβ-glucan decreased from more than 2.0 Pa and kept stable at 0.77 Pa as the increase of shear time. The shear stress of mannan had no obvious change as the increase of shear time. The viscosity of modified glucan decreased and kept stable at 2 mPa·s as the increase of temperature. The viscosity of mannan decreased and kept stable at 1 mPa·s as the increase of temperature. The viscosity of modifiedβ-glucan under conditions of neutral pH is 153 mPa·s, which was highest. When the pH of mannan increased from 4.00 to 10.00, the flow index increased from 0.35 to 0.53, which shifted to ideal fluid. The addition of salt ions could change the ionic environments of modifiedβ-glucan and destruct it structure. Mannan had good abilities of salt-resistant.
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