| Yeast is the only one of the microorganisms whose annual production exceeds one million tons in the world,which is widely used in food,medicine,cosmetics and feed industries and so on.Saccharomyces cerevisiae is the first yeat species used by people,and it was mainly applied in the fermentation for ethanol producing.In traditional ethanol fermentation,low concentration of microorganisms and low fermentation efficiency are the main constraint on ethanol industry development.So far,a lot of researches have been conducted to investigate the methods to accelerate fermentation process.Some research results indicated that appropriate ultrasound treatment can effectively promote the microbial fermentation process.In this study,the influence of ultrasound on Saccharomyces cerevisiae cell proliferation,morphology,membrane permeability and ethanol tolerance in seed solution was explored.In addition,the influence of ultrasound,conducted in the process of the ethanol fermentation,on the growth rate,mortality rate,metabolites and their contents was also investigated.Furthermore,the kinetic model of ethanol fermentation assisted by ultrasound in a 5 litre fermentation tank was built.The main results are as follows:(1)The optimum ultrasonic treatment conditions were determined based on the biomass increment of Saccharomyces cerevisiae through the single factor experiment.The results revealed that the biomass increased by 127.03% under the conditions of frequency 28 kHz,power 140 W/L and ultrasonic time 1 hour when Saccharomyces cerevisiae cultured to the latent anaphase compared with control group.(2)Effects of ultrasound on Saccharomyces cerevisiae in cell morphology,cell membrance permeability and ethanol tolerance were studied.The inverted fluorescence microscope exhibited the visible changes in the cell morphology.The number of Saccharomyces cerevisiae ultrasonic treated was higher than that of the control.And it also be observed that there were some daughter cells around the mother cells due to the budding reproduction of Saccharomyces cerevisiae,revealing that the cells were in a rapid growth period after stimulated by ultrasound irradiation.After sonication treatment under the optimal ultrasonic conditions,the contents of extracellular protein,nucleic acid and fructose-1,6-diphosphate(FDP)increased by19.6%,15.08% and 220%,respectively.This result indicated that ultrasound treatment enhanced the membrane permeability of Saccharomyces cerevisiae,resulted in the promotion of intracellular and extracellular material exchange.In addition,sonication could accelerate the damage of Saccharomyces cerevisiae in high concentration alcohol although the ethanol tolerance of Saccharomyces cerevisiae was not affected significantly by ultrasound.(3)Effects of ultrasonic models on growth rate,mortality rate of Saccharomyces cerevisiae as well as the yield of ethanol,β-phenethyl alcohol and other volatile metabolites were determined when multiple frequencies scanning slit ultrasound equipment was employed during fermentation.The results showed that both fixed frequency ultrasound and scanning frequency ultrasound could accelerate the growth of Saccharomyces cerevisiae,but a high mortality rate was observed by using scanning frequency ultrasound.When fermentation liquor was processed with ultrasound under the conditions of frequency 28 kHz and fermentation time 40 h,respectively,the ethanol concentration increased to 12.05%(v/v)and β-phenethyl alcohol increased to the highest content.Moreover,according to GC-MS determination results,the contents of volatile components such as alcohols and esters in fermentation broth all increased.(4)Effects of ultrasonic frequency on growth rate,mortality rate and the yield of ethanol,β-phenethyl alcohol and other volatile metabolites of Saccharomyces cerevisiae were analyzed under the optimal ultrasonic model.The results indicated that the enhancement of growth of Saccharomyces cerevisiae decreased when frequency was above 33 kHz,and the mortality rate of Saccharomyces cerevisiae in fermentation broth increased with the increasing of ultrasonic frequency.And the yield of ethanol increased by 19.33% over the control when ultrasound frequency of23 kHz was employed and fermented for 48 h,the content of β-phenethyl alcohol and other volatile metabolites such as esters increased also at the same time.(5)Effects of ultrasound under the optimal conditions described as above onSaccharomyces cerevisiae fermentation in a 5 L fermentantion tank to produce ethanol were investigated by evaluation of cell concentration,sugar content and the yield of ethanol.The results revealed that growth rate and cell density of Saccharomyces cerevisiae cultured in a 5 L fermentantion tank were all higher than that in a flask,while the consumption rate of sugar in broth was low,accordingly,the eventually yield of ethanol was only 10.48%(v/v).Logistic and Gompertz models were all employed to simulate the process of ethanol fermentation based on the experimental data.The results showed that Logistic model generated satisfactory mathematical model that accurately explained the behavior of the system with a coefficient of 0.995,allowing one to predict ethanol fermentation process with Saccharomyces cerevisiae. |
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