Knowledge Management System Of Institute of process engineering,CAS
|Thesis Advisor||万印华, 王亚彪|
糖是生活中必不可少的食品添加剂，其中大部分源于甘蔗制取。在国内外现行的制糖工艺中，甘蔗混合汁清净工段主要采用碳酸法或亚硫酸法，需要添加大量化学物质，且存在产品质量不稳定、污染环境、影响食品安全等问题。近年来兴起的膜法制糖技术利用纯物理分离去除甘蔗混合汁中的杂质，绿色高效，保证了食品安全。但在砍伐甘蔗过程中，断裂处会滋生肠膜明串珠菌，这类细菌以蔗糖为碳源，发酵产生无机酸以及具有高粘度的葡聚糖，在膜法制糖过程中导致严重的生物污染，给膜法制糖工艺的长期稳定运行带来了极大的阻碍。因此，为了保障该工艺长期稳定运行、保证糖品产量和质量，本文研究了肠膜明串珠菌的生长特性、脱色膜的微生物污染行为和机理，并提出了有效的控制和清洗方法。首先，在实验室观察了料液性质对肠膜明串珠菌生长的影响，发现在pH=5~6、培养温度为30 ℃、培养液初始蔗糖浓度为10 ％的条件下，肠膜明串珠菌生长最为迅速，胞外聚合物（EPS）葡聚糖的产量最大，同时细菌分泌乳酸、乙酸，导致培养环境pH值降低。然后，在中试现场实地考察了微生物污染的具体情况，发现膜分离系统运行5 h后，料液酸化严重。对此，采取了升高温度、改变工艺顺序和投加杀菌剂等控制手段，取得了初步成效，但仍无法彻底解决问题。 接着，利用不同生物酶（葡聚糖酶、蛋白酶、溶菌酶等）针对性的对膜面污染物进行降解，以此研究纳滤膜在死端过滤过程中的生物污染机理，研究发现造成通量衰减的主要污染物是葡聚糖，此时使用碱性溶液和葡聚糖酶溶液清洗效果最佳，通量恢复率分别为95.7 ％和97.6 ％。通过多种表征实验，发现葡聚糖与其他污染物混合后，会构成更加致密的污染层，黏附于膜表面，造成难以清除的生物污染。最后，为了更接近实际生产过程，利用错流过滤装置，分别对过滤了细菌培养液和甘蔗汁（添加细菌）的原始膜、污染后的膜和清洗后的膜进行了研究。发现膜面或离心泵的剪切力会破坏细菌细胞，释放出蛋白质，与甘蔗汁中的色素一起吸附在膜上构成污染层。同时，致密的葡聚糖层可以覆盖/包裹其他污染物（例如蛋白质、色素、细胞碎片和细菌），将其粘附在膜上并保护其免受化学清洗。此时单独的碱性溶液或葡聚糖酶溶液清洗不再能够恢复膜通量，先使用葡聚糖酶清洗，降解掉覆盖/包裹其他污染物的葡聚糖，再使用十二烷基硫酸钠（SDS）碱性溶液清洗，清除其他污染物，能够有效清洗甘蔗汁纳滤脱色过程中的生物污染，膜通量恢复率可以达到98.9 ％。该研究不仅阐明了甘蔗汁纳滤脱色过程的污染机理，而且为在实际应用中控制生物污染提供了有效的策略。;Sugar is an essential food additive in our daily life, most of which is derived from sugarcane. Conventional sugar making process needs to add a large amount of chemicals, and this traditional refining method suffers from inferior and unstable product quality, serious environmental problems caused by solid waste, and potential risks in food safety. In recent years, a membrane-based sugar-making process has been developed which applies membranes to remove impurities from sugarcane juice via physical separation. However, during sugarcane cutting, L. mesenteroides subsp. dextranicum may grow on the fractured sugarcane surface. This kind of bacteria would enter into the membrane system, produce dextran and inorganic acid using sucrose as substrate. The bacteria, cell debris and metabolites result in serious biofouling in the membrane modules, which greatly hindered the long-term and stable operation of membrane sugar making process. Therefore, in order to ensure the long-term stable operation of this process and improve the yield and quality of sugar products, we herein investigated the growth of L. mesenteroides subsp. dextranicum, the fouling behaviours and biofouling mechanisms during decolorization of cane juice by nanofiltration membrane, and offered an effective cleaning strategy for biofouling control.First, the effect of feed properties on bacterial growth was observed in the laboratory. It was found that the bacterial growth in culture medium is the fastest under pH 5~6, temperature 30 °C, and 10 % of the initial sucrose concentration. And more dextran is produced at faster bacterial growth. At the same time, lactic acid and acetic acid were released from bacteria fermentation, which led to the decrease of pH value in the culture solution. Then, the specific situation of membrane biofouling was investigated in the pilot-scale tests, and it was found that the juice acidification was serious after the membrane system was operated for 5 h, indicating that the biofouling would be more serious with filtration time. In this regard, we adopted some fouling control strategies, such as increasing temperature, changing process sequence and adding bactericide, and preliminary inhibitory effect on the bacteria have been emerged, but the biofouling problem cannot be solved completely.Then, enzymatic cleaning (dextranase, trypsin, lysozyme, etc.) was applied to target specific foulant for clarifying the fouling composition and structure in the process of dead-end nanofiltration. It was found that the main foulant causing permeate flux decline was dextran. At this time, alkaline and dextranase cleaning displayed a super high permeability recovery, 95.7 % and 97.6 % respectively. Various characterization methods showed that when the dextran was mixed with other foulants, much denser fouling layer was formed. They would adsorb on the membrane, leading to a recalcitrant fouling.Finally, in order to provide more reliable reference for the real applications, the pristine, fouled and cleaned membranes after filtering culture solution and real cane juice (with bacteria addition) respectively were evaluated using a cross-flow filtration device. In the cross-flow filtration, the bacterial cells would be broken by the shearing force on the membrane or in centrifugal pump, releasing proteins to construct fouling layer with pigments in cane juice. At the same time, the compact dextran layer would cover on the adsorption fouling layer and/or enwrap other foulants such as protein, pigments, cell debris and bacteria, adhering them on the membrane and protecting them from chemical cleaning. At this time, either dextranase or alkaline cleaning could not well recover the membrane permeability. Therefore, an enzymatic (i.e. dextranase) cleaning is required to degrade the dextran layer before an effective sodium dodecyl sulfate (SDS) alkaline cleaning for removing the other organic fouling. Only in this way can the biofouling be effectively cleaned, and the membrane permeability recovery can reach 98.8％. The outcome of this work not only elucidated the fouling mechanisms during decoloration of cane juice by nanofiltration membrane, but also offered an effective cleaning strategy for biofouling control in the practical application.
|李煜堃. 膜法制糖过程中的生物污染控制及清洗[D]. 中国科学院大学,2020.|
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