【作者】 鲁战会；

【导师】 李里特；

【作者基本信息】 中国农业大学 ， 食品科学， 2002， 博士

【摘要】 我国稻米总产量占世界总产的35％，占粮食总产量的2／5左右，约55％的大米作为口粮直接消费，加工用米仅占总产的6％。其中份额最大的加工品米粉仅占总产的0.48％，每年约33％的大米积压在农民手中或粮库，导致大量劣变及损耗。积压稻谷中80％是早籼稻。研究表明早籼稻是米粉生产的优质原料，显然对米粉生产技术的深入研究是解决积压问题的关键。传统发酵米粉以其滑爽筋道的口感深受消费者的喜爱，本文首次对其加工工艺进行了科学整理，研究了发酵微生物的种类及变化规律；自然发酵主要作用微生物菌种的分离筛选及单菌种实验；微生物发酵对淀粉结构的作用模式；发酵对淀粉改性机理以及米粉凝胶形成的机理。从而奠定利用早籼米生产高品质米粉的理论基础。主要研究内容与结果如下： 1．发酵显著地改变了大米的组成成份，大米总蛋白的33％、总脂肪的61％为微生物降解，总灰分的62％以上由于发酵而溶出，从而起到纯化淀粉的作用。发酵利用大米中的小分子糖产生了大量有机酸，HPLC分析表明90％以上为乳酸。发酵对总淀粉含量影响不大，但改变了淀粉的结构和组成，直链淀粉含量增加3.28％。除发酵的作用外，浸泡也有一定作用。 2．采用凝胶过滤层析法研究了发酵对淀粉分子结构的改性作用，结果表明大米淀粉依分子大小可为分两区：FrⅠ为大分子，FrⅡ为小分子。发酵使大分子区减少，小分子区及中间分子增加。大分子区DPn从对照的12597.6 GLU降至发酵后的12051.0 GLU，而小分子区从3061.5 GLU增加到3423.1 GLU。β-淀粉酶水解率测定发现淀粉分子分支化程度降低，表明支链淀粉发生了断链与脱支。进一步将支链淀粉脱支分析其微观结构，支链淀粉又可分为两区，发酵使分子链较长的第一区平均链KCL从55.6 GLU缩短到52.9 GLU；而较短的第二区平均链长增加2.7个葡萄糖苷单位。说明断链作用主要发生在支链淀粉的主链或长链上。 3．利用X-衍射、FTIR、DSC及RVA研究了发酵对淀粉颗粒结构及热特性的影响，发现淀粉结晶类型未变，但晶区比例增大；DSC糊化温度降低，但糊化所需时间延长，糊化焓增大；RVA最高粘度降低。结果一致表明发酵主要作用于淀粉颗粒的无定形区，对晶区的结构影响不大。淀粉颗粒显微观察表明发酵没有破坏淀粉颗粒外形，发酵的改性作用发生在淀粉颗粒内部。 4．微生物菌群及数量的演变研究发现发酵以厌氧发酵为主，乳酸菌在整个发酵过程中为优势菌群，最高可达10⁹～10¹¹cfu／mL，占细菌总数的90％以上。其次是酵母菌，约10⁴～10⁶cfu／mL。发酵微生物主要来自于大米本身，大米原料的初发菌数在10⁵cfu／mL左右，主要为乳酸菌与酵母菌。进一步分离筛选获得5株细菌、2株乳酸菌、3株酵母和3株霉菌。经过工艺试验确定主要作用菌种为乳酸菌和酵母菌。流变性测定表明，菌株的产酸量越大，对米粉凝胶流变性改善作用越明显。因此发酵米粉的流变性与淀粉酸变性有密切的关系。 5．微生物发酵代谢产物中乳酸的作用效果最明显。通过不同pH乳酸溶液的浸泡实验验证，表明pH4左右最佳，与自然发酵有类似效果。酶类以蛋白酶、果胶酶及脂酶作用效果为好。小分子糖类与乙醇的存在对米粉拉伸性有负面作用，但这些物质在发酵及洗米过程中溶出而流失，不会影响发酵米粉的凝胶性。 6．对发酵大米淀粉的糊化老化特性研究表明，发酵样品的糊化速度及老化速度快，老化后再糊化的速度慢，表现为成糊快，米粉耐煮，溶出少。添加预糊化淀粉可以改善米粉的拉伸性。 通过以上全方位研究，阐述了微生物发酵对淀粉的改性机理和米粉凝胶形成的机理。分析认为，发酵米粉特殊的流变性质是微生物产酸、酶作用于大米淀粉颗粒对淀粉改性和纯化的结果。乳酸对大米淀粉凝胶性质的改变起着主要作用。乳酸作用于淀粉颗粒的无定形区引起支链淀粉长链断链，使淀粉的分子量分布趋于均匀，降低了单个支链淀粉分子的分支化程度，减弱了再结晶性。中间分子及直链淀粉分子比例增加，老化趋势增强。同时低pH值促进快速聚合老化，降低了直链淀粉与支链淀粉产生相分离形成各自富集区的程度。直链淀粉迅速聚合或与支链淀粉的外侧链形成结合点，然后聚合不断形成连续相凝胶网络结构，将老化慢的支链淀粉分散相包埋进来，支链淀粉的内链被封闭，包埋较多的水分，构成凝胶的粘性，而连续相直链淀粉老化后难再糊化，构成凝胶的弹性，共同构成了发酵米粉的粘弹性。另外发酵微生物所产酶类降解了蛋白、脂肪等，起到纯化淀粉的作用而对米粉的流变性也有重要贡献。更多还原

【Abstract】 China accounts for 35% of global rice production and 2/5 of total domestic grain production. About 55% of rice yield is consumed directly as cooked rice. Only 6% is used for processing in China. Consequently, there is as much as 33% of total rice yield overstocked in farmers or storehouses, and about 80% of the overstocked rice is Indica, which causes large quantity of deterioration every year. Rice cultivars Indica is widely planted with its high yield and good quality for producing rice noodles in South of China. Identification of new uses for Indica rice is necessary to generate great demand for it.Traditional fermented rice noodles are very popular because of its favorite mouth-feel and taste in China. The further theoretical research is needed for its industrialized production. In this study, the processing technology of fermented rice noodles was thoroughly investigated; the evolution of microbial population were tracked in time course; the varieties of fermentation microorganisms were counted, isolated, and screened; fermentation effect and modification mechanism on starch granule structure and molecular structure were determined, simulated, and validated. The main contents and results are as follows.1. Chemical components of rice flour were significantly modified by whole rice granule fermentation. About 33% of total protein, 61% of total lipid and 62% of total ash content were lost during fermentation period. The purity of rice starch was improved. A large quantity of organic acid was produced from low weight molecular sugars. More than 90% of total acid is lactic acid by Ion Chromatography analysis. The rice starch content remained almost constant, but the starch structure and constituents were changed by fermentation. The amylose content had a 3.28% increase.2. Starch modification was analyzed by using GPC (Gel Permeation Chromatography, equipped with two consecutive columns: Sepharose CL-2B and Sephadex G-50 respectively). Starch molecular profile could be divided into two distinct fractions (Fr I and Fr II). Fr I is the larger molecular and FrII is the smaller one. The area of FrII fraction increased but Fr I fraction decreased because the larger molecular starch was degraded by fermentation. The number of average molecular polymerization was changed from 12597.6 GLU to 12051.0 GLU for Fr I and from 3061.5 GLU to 3423.1 GLU for FrII after fermentation. The (3-hydrolysis ratio was increased. It showed that starch chain was broken and debranched. Amylopectin fraction (Fr I ) could further be fractionated into two distinct fractions after debranched by isoamylase and pullulanase treatment. The average chain length of first fraction decreased from 55.6 GLU to 52.9 GLU and second fraction increased by 2.7 GLU after fermentation. These results indicated that the chain breaking occurred mainly on the long chains of the amylopectin molecules.3. Effects of fermentation on starch granule structure and thermal properties were determined by using X-ray diffraction, FTIR, DSC and RVA. Results indicated that fermentation had little effect on starch crystal structure, but the ratio of crystal area to amorphous area increased; the gelatinization temperature (Tp) of rice starch decreased, starch gelatinization time prolonged, and the gelatinization enthalpy (AH) increased; RVA peak viscosity decreased. Results proved that fermentation mainly affected the amorphous region of the starch granule structure. The micrography of starch granule showed that the shape of starch granule remained unchanged. It also indicated that starch modification by fermentation took place inside the granule.4. The varieties and populations of the natural fermentation microorganisms were inspected throughout the fermentation period. The main source of microorganisms was from rice itself with initial bacterial count up to 105cfu/mL, mainly consisting of lacto bacilli and yeasts. Results showed that the fermentation of whole rice granules took place under an anaerobic condition. Lacto bacilli were the domi更多还原