Logistic模型在不同还原糖初始浓度乙醇发酵中的应用

酿酒酵母(Saccharomyces cerevisiae)利用可发酵性糖生成乙醇的过程与效率受多种环境因素的影响。研究乙醇形成的动力学有利于理性认识乙醇形成机理,为放大与优化工艺条件提供理论指导。对Logistic模型方程重新参数化,将酵母生长动力学方程类比乙醇生成动力学,拟合性很好,对不同初始浓度还原糖的乙醇同步糖化发酵过程进行Logistic模型方程模拟,给出了乙醇浓度的显式函数模型,用310g/L玉米粉物料浓度的同步糖化发酵工艺,首次揭示在酿酒酵母工业菌株能够承受的一定的糖化醪还原糖浓度范围内,尽可能提高还原糖浓度至上限,可以缩短发酵周期,提高可发酵性糖转化为乙醇的转化率。这对于优化乙醇工业生产工艺条件、提高生产效率具有较重要的实践指导意义与应用价值。     

基于近红外光谱实时监测乙醇发酵过程多组分参数

生物量、葡萄糖浓度和乙醇浓度是乙醇发酵过程的重要参数,传统的方法通常对发酵液取样作离线测量,不仅需要采用多种仪器进行测试分析,而且耗时耗力,成为实时过程调控和优化的障碍。文中针对这些重要过程参数提出了一个基于近红外光谱技术的原位实时检测方法。通过采用浸入式近红外光谱仪对发酵溶液进行原位测量,基于多输出最小二乘支持向量机回归(MLS-SVR)方法建立了利用近红外光谱同时分析葡萄糖浓度、生物量和乙醇浓度的多输出预测模型。实验结果表明,该方法能实时准确地检测乙醇发酵过程中的葡萄糖浓度、生物量和乙醇浓度,而且相对于现有的偏最小二乘法(PLS)分别对各组分建模和预测,能明显提高测量准确性和可靠性。     

自絮凝颗粒酵母乙醇连续发酵耦合酵母回用工艺的研究

模拟现有酒精发酵行业普遍采用的多级发酵罐串联系统,建立了一套由三级串联操作的搅拌式发酵罐和两个沉降罐组成的反应器系统,以脱胚脱皮玉米粉双酶法制备的糖化液为发酵底物,培养基初始还原糖浓度为220g/L,添加(NH4)2HPO41.5g/L和KH2PO42.5g/L,以0.057h-1的恒定稀释速率流加,将自沉降浓缩后的酵母乳先后经活化和不活化两种方式处理并循环至第一级发酵罐,系统在两种操作条件下分别达到了拟稳态。实验结果表明活化处理对改善发酵工艺技术指标方面发挥了显著的作用,发酵终点乙醇浓度达到101g/L,还原糖和残总糖分别在3.2和7.7g/L左右,发酵系统的设备生产强度指标为5.77g/(L.h),与无酵母回用的搅拌式反应器系统中自絮凝颗粒酵母乙醇发酵工艺相比,提高了70%。     

利用中温蒸煮工艺进行高浓度酒精发酵

用中温蒸煮工艺生产高浓度酒精,首先利用耐高温α－淀粉酶在9s～97℃下同时糊化和液化淀粉．接着在60℃下加高转化率的糖化酶进行糖化,最后在30℃下加酵母菌悬液进行发酵。酵母菌w4在60h内可以产生18．3％的乙醇,在成熟发酵醪中的残还原糖和总糖分别为1.2％和4．1％,细胞存活率为68．5％。如果在发酵培养基中添加一定量的硫酸铵,可进一步改进这一工艺,使成熟发酵醪的乙醇浓度提高到18.9%,发酵周期缩短到50h,残还原糖和总糖分别减少到0.27％和3.1％。     

利用功能基因组学方法研究酿酒酵母乙醇生物合成的调控机理

酿酒酵母在发酵生产乙醇的过程中存在的主要问题是前期高浓度底物葡萄糖的抑制和后期高浓度产物乙醇的抑制。功能基因组学技术的发展为从基因组水平上系统研究酿酒酵母乙醇生物合成的调控机理提供可能。本研究模拟工业发酵的条件,对酿酒酵母实验菌株BY4743为遗传背景的116个单基因缺失菌株进行了乙醇发酵试验,以发现基因和乙醇发酵的关系。结果表明乙醇对菌体得率系数高于平均值30%以上的基因缺失株有20株,其中高于50%以上基因缺失株有5株;低于平均值30%以上的基因缺失株有11株,其中低于45%以上的有5株。本研究为从整个酿酒酵母基因组水平上系统研究乙醇生物合成的调控机理建立了研究方法,提供了可行性验证。     

燃料乙醇发酵技术研究进展

在分析木质纤维素类生物质制备燃料乙醇原理基础上,重点对燃料乙醇转化过程的发酵工艺进行了论述。目前乙醇发酵工艺主要包括直接发酵、分步糖化发酵、同步糖化发酵、同步糖化共发酵和联合生物加工技术等,对这几种技术的研究现状进行了分析并对其发展趋势进行了展望,通过基因工程构建高效发酵菌种的联合生物加工技术将是未来高效发酵工艺的发展趋势,旨在为有效提高发酵菌株的底物代谢能力,获得高的乙醇产量提供重要参考。     

酿酒酵母发酵高浓度乳清粉生产燃料乙醇

利用酿酒酵母NL22对乳清粉进行分步糖化发酵(SHF)和同步糖化发酵(SSF),对其生产燃料乙醇的条件进行比较,同时考察pH、温度和底物浓度对SHF和SSF过程的影响。结果表明:SHF工艺和SSF工艺都可以实现酿酒酵母NL22对高浓度乳清粉的发酵,但SSF工艺可明显缩短生产周期,提高生产效率。在pH 6和30℃的条件下进行补料同步糖化发酵,最终乙醇质量浓度为118.52 g/L,产率为1.74 g/(L·h)。     

自絮凝颗粒酵母乙醇连续发酵耦合废液全循环工艺的研究

在以CO2为驱动力的单级悬浮床生物反应器中,进行了自絮凝颗粒酵母乙醇连续发酵耦合废液全循环实验研究。以双酶法制备的玉米粉糖化液为底物,系统连续运行了28 d,每隔5 d将收集到的发酵液集中精馏处理,得到的废糟液直接用于玉米粉调浆制糖。实验数据表明,在稀释率为0.05 h-1条件下,发酵液中乙醇、残还原糖、残总糖质量浓度基本稳定,其平均值为82.97,30.02和40.87 g.L-1。对废液循环工艺过程中,发酵液中的8种高沸点有机酸进行了定量分析,发现发酵液中仅丙酮酸有明显积累,并最终在0.1~0.3 mol.L-1范围内波动。实验结果表明:系统中存在的高沸点副产物不对乙醇发酵产生明显抑制作用,乙醇连续发酵能够在废液全循环条件下良好运行。     

纤维素乙醇高温发酵的研究进展与展望

利用高温细菌发酵,纤维素乙醇生产有望实现“生物质降解-乙醇发酵-乙醇蒸馏”过程的同步化,从而最大限度地降低纤维素乙醇的生产成本;这是一个目标更高、道路更远、科学性更强的可再生能源发展策略.纤维素乙醇高温发酵研究已经取得了重要进展,目前面临的主要挑战包括发酵乙醇的高温细菌的遗传转化系统不够稳定、缺少内源的高活性和耐热性纤维素酶,以及乙醇代谢调控机理有待进一步解析.这些科技难题将会在DNA生物合成和进化技术、细胞生物学技术,以及合成生物学技术的发展中得到解决.     

多种通气策略下的高浓度乙醇生产

氧气在环境胁迫强的高浓度乙醇发酵中具有重要作用.考察了自絮凝酿酒酵母在多种通气策略下的乙醇发酵及絮凝状况.使用氧化还原电位(ORP)检测发酵液中氧浓度并划分了厌氧、微氧和好氧状态.厌氧条件下的终点乙醇浓度最低(119±1.5 g/L);微氧条件下使用ORP精密控制氧气供给取得较高的乙醇浓度(131±1.8和125±1.7 g/L);在通气量0.2 vvm的好氧条件下,生物量、甘油量和乙醇损失皆最大,与最优收率相比较乙醇收率降低了12.2％.高通气量增强了细胞的絮凝能力,增大了絮凝体粒径.绘制雷达图进行综合评价,恒定通气0.05 vvm的过程在乙醇生产和絮凝各方面表现均突出.     

Stillage backset and its impact on ethanol fermentation by the flocculating yeast

The second largest cost in fuel ethanol production is from energy consumption with ethanol distillation and stillage treatment, particularly when stillage is treated by the multi-evaporation process. Therefore, stillage backset is the most economically competitive strategy for reducing discharge and saving energy consumption. In this article, continuous ethanol fermentation was performed by the flocculating yeast under stillage backset conditions. Compared to regular yeast, immobilized yeast within the fermentor through flocculation reduced byproducts formation in the stillage, since heat lysis of yeast during ethanol distillation was prevented, and many side reactions were thus eliminated, making more stillage backset within the fermentation system possible. Although pyruvic acid, succinic acid, citric acid, α-ketoglutaric acid, fumaric acid and glycerol from yeast metabolism, furfural and 5-hydroxymethyl furfural from process operations, and acetic acid and lactic acid from slight contamination were accumulated with the stillage backset, they had no significant impact on yeast growth and ethanol fermentation due to low concentrations accumulated within the fermentation system. However, propionic acid that was generated mainly during hydrolysate sterilization and distillation of the fermentation broth was detected as the major inhibitor, but this byproduct would be significantly reduced under industrial conditions without hydrolysate sterilization, making the stillage backset more reliable for industrial application.     

New technologies in value addition to the thin stillage from corn-to-ethanol process

Grain-to-ethanol production has increased steadily in the United States in the past few decades, which resulted in remarkable records in the availability of co-products. Dry-grind is the most common method of ethanol production worldwide, which concentrates the corn and yeast nutrients in the downstream operations. The ethanol co-products have traditionally been a commodity for livestock feed as they contain desirable nutrients, mostly sold as distiller’s grains. The liquid fraction produced after the centrifugation of the bottoms of the ethanol rectifying and distilling operations is named thin stillage, produced at volumes several times greater than those of ethanol. A portion of thin stillage is normally recycled as backset water, while the remaining goes through a series of evaporations. Evaporating a large amount of water from thin stillage is an energy-consuming process and recycling the thin stillage may lead to the accumulation of nutrients in co-products in distiller’s grains. There are several other industrial processes to utilize thin stillage, such as oil extraction, anaerobic digestion, and secondary fermentation. Recently, promising results have been reported on the production of important commodity chemicals, extracting high-value products, and energy production from thin stillage. This review provides an overview on the new processes and products via valorization of thin stillage by innovative technologies that are being currently developed. The new applications of thin stillage discussed in this review could open new opportunities for the ethanol plants and ethanol researchers by increasing the revenue and simultaneously reducing negative environmental impacts of ethanol production.     

Fate of Fumonisin B(1) in Naturally Contaminated Corn during Ethanol Fermentation

Two lots of corn naturally contaminated with fumonisin B(1) (15 and 36 ppm) and a control lot (no fumonisin B(1) detected) were used as substrates for ethanol production in replicate 8.5-liter yeast fermentations. Ethanol yields were 8.8% for both the control and low-fumonisin corn, while the high-fumonisin corn contained less starch and produced 7.2% ethanol. Little degradation of fumonisin occurred during fermentation, and most was recovered in the distillers' grains, thin stillage, and distillers' solubles fractions. No toxin was detected in the distilled alcohol or centrifuge solids. Ethanol fermentation of fumonisin-contaminated corn coupled with effective detoxification of distillers' grains and aqueous stillage is suggested as a practical process strategy for salvaging contaminated corn.     

Grain sorghum stillage recycling: Effect on ethanol yield and stillage quality

Stillage obtained from ethanol production of grain sorghum was separated into two fractions: thin stillage and wet solids. A portion of the thin stillage was recycled as cooking water in subsequent fermentation runs using both bench- and full-scale ethanol production plants. When thin stillage replaced 50-75% of the cooking water, large increases occurred in solids content, COD, and EC of the resulting thin stillage. It was found that while the volume of thin stillage requiring treatment or disposal was reduced, there was little reduction in the total pollutant load. Stillage rcycling had little effect on the quality of the stillage wet solids fraction. At the high levels of stillage recycle used, ethanol yield was reduced after three to five runs of consecutive recycling.     

Fate of Fumonisin B1 in Naturally Contaminated Corn during Ethanol Fermentation

Two lots of corn naturally contaminated with fumonisin B₁ (15 and 36 ppm) and a control lot (no fumonisin B₁ detected) were used as substrates for ethanol production in replicate 8.5-liter yeast fermentations. Ethanol yields were 8.8% for both the control and low-fumonisin corn, while the high-fumonisin corn contained less starch and produced 7.2% ethanol. Little degradation of fumonisin occurred during fermentation, and most was recovered in the distillers' grains, thin stillage, and distillers' solubles fractions. No toxin was detected in the distilled alcohol or centrifuge solids. Ethanol fermentation of fumonisin-contaminated corn coupled with effective detoxification of distillers' grains and aqueous stillage is suggested as a practical process strategy for salvaging contaminated corn.     

Anaerobic digestion of thin stillage for energy recovery and water reuse in corn-ethanol plants

Recycling of anaerobically-digested thin stillage within a corn-ethanol plant may result in the accumulation of nutrients of environmental concern in animal feed coproducts and inhibitory organic materials in the fermentation tank. Our focus is on anaerobic digestion of treated (centrifugation and lime addition) thin stillage. Suitability of digestate from anaerobic treatment for reuse as process water was also investigated. Experiments conducted at various inoculum-to-substrate ratios (ISRs) revealed that alkalinity is a critical parameter limiting digestibility of thin stillage. An ISR level of 2 appeared optimal based on high biogas production level (763 mL biogas/g volatile solids added) and organic matter removal (80.6% COD removal). The digester supernatant at this ISR level was found to contain both organic and inorganic constituents at levels that would cause no inhibition to ethanol fermentation. Anaerobic digestion of treated-thin stillage can be expected to improve the water and energy efficiencies of dry grind corn-ethanol plants.     

Detoxification of acidic catalyzed hydrolysate of Kappaphycus alvarezii (cottonii)

Red seaweed, Kappaphycus alvarezii, holds great promise for use in biofuel production due to its high carbohydrate content. In this study, we investigated the effect of fermentation inhibitors to the K. alvarezii hydrolysate on cell growth and ethanol fermentation. In addition, detoxification of fermentation inhibitors was performed to decrease the fermentation inhibitory effect. 5-Hydroxymethylfurfural and levulinic acid, which are liberated from acidic hydrolysis, was also observed in the hydrolysate of K. alvarezii. These compounds inhibited ethanol fermentation. In order to remove these inhibitors, activated charcoal and calcium hydroxide were introduced. The efficiency of activated charcoals was examined and over-liming was used to remove the inhibitors. Activated charcoal was found to be more effective than calcium hydroxide to remove the inhibitors. Detoxification by activated charcoal strongly improved the fermentability of dilute acid hydrolysate in the production of bioethanol from K. alvarezii with Saccharomyces cerevisiae. The optimal detoxifying conditions were found to be below an activated charcoal concentration of 5%.     

Feed component inhibition in ethanolic fermentation by Saccharomyces cerevisiae

Inhibition by secondary feed components can limit productivity and restrict process options for the production of ethanol by fermentation. New fermentation processes (such as vacuum or extractive fermentation), while selectively removing ethanol, can concentrate nonmetabolized feed components in the remaining broth. Stillage recycle to reduce stillage waste treatment results in the buildup of nonmetabolized feed components. Continuous culture experiments are presented establishing an inhibition order: CaCl(2), (NH(4))(2)xSO(4) > NaCl, NH(4)Cl > KH(2)PO(4) > xylose, MgCl(2) > MgSO(4) > KCl. Reduction of the water activity alone is not an adequate predictor of the variation in inhibitory concentration among the different components tested. As a general trend, specific ethanol productivity increases and cell production decreases as inhibitors are added at higher concentration. We postulate that these results can be interpreted in terms of an increase in energy requirements for cell maintenance under hypertonic (stressed) conditions. Ion and carbohydrate transport and specific toxic effects are reviewed as they relate to the postulated inhibition mechanism. Glycerol production increases under hypertonic conditions and glycerol is postulated to function as a nontoxic osmoregulator. Calcium was the most inhibitory component tested, causing an 80%decline in cell mass production at 0.23 mol Ca(2+)/L and calcium is present at substantial concentration in many carbohydrate sources. For a typical final cane molasses feed, stillage recycle must be limited to less than onethird of the feed rate; otherwise inhibitory effects will be observed.     

Production of ethanol and biomass from thin stillage by Neurospora intermedia: A pilot study for process diversification

Dry mill ethanol processes produce ethanol and animal feed from whole grains, where the wastewater after the distillation and separation of solid materials is called “thin stillage.” In this work, similar production of ethanol (3.5 g/L) and biomass (5 g/L) from thin stillage was obtained during batch cultivation of the edible fungus Neurospora intermedia in a 2‐m high airlift reactor and bubble column. The fungal biomass, containing 50% w/w protein and 12% w/w lipids, was rich in essential amino acids and omega‐3 and ‐6 fatty acids. In a continuous mode of fermentation, dilution rates of up to 0.2 h^?1 could be applied without cell washout in the bubble column at 0.5 vvm. At 0.1 h^?1, around 5 g/L of ethanol and 4 g/L of biomass containing ca. 50% w/w protein were produced. The fungus was able to assimilate saccharides in the liquid fraction as well as sugar backbones such as xylan and arabinan in the solid fraction. The inclusion of the current process could potentially lead to the production of 11 000 m³ of ethanol (5.5% improvement vs. normal industrial process) and around 6300 tons of high‐quality biomass for animal feed at a typical facility producing 200 000 m³ ethanol per year.     

Enhanced bioenergy recovery from rapeseed plant in a biorefinery concept

The present study investigated the utilization of the whole rapeseed plant (seed and straw) for multi-biofuels production in a biorefinery concept. Results showed that bioethanol production from straw was technically feasible with ethanol yield of 0.15 g ethanol/g dry straw after combined alkaline peroxide and stream pretreatment. The byproducts (rapeseed cake, glycerol, hydrolysate and stillage) were evaluated for hydrogen and methane production. In batch experiments, the energy yields from each feedstock for, either methane production alone or for both hydrogen and methane, were similar. However, results from continuous experiments demonstrated that the two-stage hydrogen and methane fermentation process could work stably at organic loading rate up to 4.5 gVS/(Ld), while the single-stage methane production process failed. The energy recovery efficiency from rapeseed plant increased from 20% in the conventional biodiesel process to 60% in the biorefinery concept, by utilization of the whole rapeseed plant for biodiesel, bioethanol, biohydrogen and methane production.