Omid Pourali

称号及び氏名博士（工学）

学位授与の日付

2010 年 3 月 31 日

論

Production of Valuable Materials from Rice Bran Biomass
Using Subcritical Water

文

名論文審査委員

主査

吉田弘之

副査

小西康裕

副査

白井正充

Summary
The worldwide demand for fossil fuels continues to rise at a rapid pace while supplies are finite.
As developing nations increase their needs for the fossil resources and hydrocarbon products,
supplies will become tighter. Furthermore, due to excessive use of fossil resources in recent decades,
environmental issues such as global warming have become considerably serious in the world.
Therefore, it is urgently required to look for alternatives to fossil resources to have sustainableeconomic growth. Biomass is the most abundant renewable resource in the world. As an annual
production is up to 1.7-2.0 × 1011 ton, it has been identified as an important source for alternative
fuels and valuable chemicals. However, only 6 × 109 ton of biomass is currently used for food andnon-food applications; therefore, it is indispensable to develop the environmentally protection
technologies, such as biorefinery, to bring the lowest impact to the environment. Subcritical water isone of the biorefinery techniques which have been utilized in various fields of green engineeringand material cycling.
The present study has been devoted to develop subcritical water treatment technique to obtain
valuable and edible substances from rice bran which is inexpensive and abundant biomass. It
contains oil, carbohydrates, proteins, vitamins, enzymes, phenolic compounds, and dietary minerals.
The effect of several parameters of subcritical water was investigated in detail. This study had threemain objectives. The first objective was to develop the possibility of rice bran decomposition andhydrolysis for production and/or extraction of valuable substances such as organic acids, aminoacids, and water-soluble sugars. The second objective was utilization of subcritical water asextractive medium for extraction of rice bran oil simultaneous with the inactivation of lipase
enzyme of bran in order to produce edible oil. The third objective was to clarify the feasibility ofphenolic compounds production by hydrothermal degradation of lignin/phenolics-carbohydratecomplexes of rice bran under subcritical water conditions. The major and important results of thisthesis are summarized as follows:
In chapter one, the general introduction and importance of the present work as well as review
of previous works were presented.
In chapter two, application of subcritical water as green medium for hydrolysis anddecomposition of rice bran was investigated in order to obtain value-added bio-based substances. A
batch type subcritical water reaction system was used to perform the decomposition of rice branover the whole temperature range of subcritical water at the reaction time of 5 min. It was observedthat hydrolysis reaction was effectively carried out without utilization of any organic solvent, acid,base, and/or enzyme.
Significant increases of total nitrogen (TN) and total organic carbon (TOC) in the water phaseproved that bio-macro polymers of rice bran such as proteins and polysaccharides were hydrolyzed
using subcritical water; they showed peaks at around 553 and 505 K, respectively. Decompositionof proteins under subcritical water conditions produced several amino acids; eight essential aminoacids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, and valine) andsix non- and/or conditionally essential amino acids (alanine, asparatic acid, glutamic acid, glycine,serine, and tyrosine) were identified in water phase which generally showed highest yield at 400 K.
It was found that lysine and glutamic acid had the highest yields among essential and non-essential
amino acids. Besides amino acids, organic acids were also identified in water phase after subcriticalwater hydrolysis reaction; the identified organic acids were acetic, citric, formic, glycolic, andlevulinic acids. They were produced with decomposition of carbohydrates and amino acids attemperatures higher than 463 K.
Production of acidic organic substances like organic acids and amino acids duringdecomposition process may autocatalyze hydrolysis reaction of rice bran and increase production of
useful materials.
Results indicated that cellulosic parts of rice bran could be successfully decomposed towater-soluble sugars using subcritical water treatment; fructose, glucose, glyceraldehyde, andsucrose were four sugars identified in the water phase. Furthermore, total soluble sugars of theaqueous phase (including mixtures of poly-, oligo-, di-, and mono saccharides) were investigated.
The optimum condition for the best yield of total soluble sugars (nearly 20% of initial dry matter)
was achieved at temperature of 463 K.
It was understood that hexane-soluble compounds (mainly rice bran oil) and acetonesoluble
substances (attributed to tar, carbonized biomass, and in general water- and hexane-insolublecompounds) were produced during hydrolysis reaction, and could be isolated after subcritical water
reaction.
In chapter three, application of subcritical water for extraction of rice bran oil in order toproduce edible oil was investigated. In the first part of this chapter, effect of lipase enzyme of branon the rancidity of the rice bran oil during storage period was studied. Then, inactivation of enzyme
was studied using subcritical water. Experiments were performed at temperature range of 393 to513 K and reaction time of 10 and/or 20 min. The quality of the obtained oil was evaluated withrespect to its total free fatty acids concentration over a 12 week period, and compared with the oil
obtained by the conventional extraction methods.
Without any treatment of the oil obtained by the conventional extraction methods and due tothe enzyme activity, it was realized that total free fatty acids concentration in untreated sample
increased sharply from 5.6% to 36.0% over storage period. On the other words, in less than one
week from the date of rice milling, rice bran oil deteriorated (total free fatty acids concentration was
higher than 10.0% in untreated oil) and became unfit for human consumption.
Experimental evidence showed that the enzymatic problem could be completely andirreversibly overcome using subcritical water treatment; total free fatty acids concentration
remained constant in the stabilized oil during 12 weeks and no increase was observed. Total freefatty acids concentration in the treated oils was a function of subcritical water temperature andreaction time; the lowest and highest concentration of total free fatty acids were obtained at 433 K
for 10 min and 473 K for 20 min, respectively. It was also observed that the conventional
solid-solvent extraction methods, such as using hexane at ambient temperature, could noteffectively inactivate enzyme, and total free fatty acids concentration increased in the course of oil
storage.

Furthermore, the kinetic of free fatty acids formation in untreated sample was investigated.
Based on the experimental data obtained from free fatty acids formation, a kinetic model wasdeveloped. Theoretical line matched well with the experimental formation curve of total free fatty
acids. The kinetic model could accurately predict the rate of free fatty acids formation; therefore,
this model can be used in the food industries to process the oil from rice bran prior to its
deterioration. In the second part of this chapter, extraction of rice bran oil in subcritical water
medium was investigated. Subcritical water could successfully extract oil in a short reaction time of
10 min simultaneous with oil stabilization. It was found that the temperature of subcritical water
affected the oil extraction considerably; the higher the temperature was, the greater was the yield ofoil extraction. Approximately 94% of total oil of rice bran was successfully extracted using
subcritical water technique.
In chapter four, the possibility of phenolic compounds production as well as other useful
substances from rice bran under subcritical water conditions was studied. Decomposition of ricebran was investigated at different temperatures ranging from 373 to 633 K at the reaction time of 10min, and also at 493 K at reaction time of 2 to 30 min. Decomposition oflignin/phenolics-carbohydrate complexes of this biomass produced several phenolic compounds.
The main products were found to be caffeic, ferulic, gallic, gentisic, p-coumaric, p-hydroxybenzoic,protocatechuic, sinapic, syringic, vanillic acids, and vanillin which showed antioxidative properties.
Protocatechuic and vanillic acids were the major phenolic compounds among the identified ones.
Results demonstrated that phenolic compounds were mainly produced from decomposition oflignin/phenolics-carbohydrate complexes of rice bran and not from rice bran oil. In addition,
production of these valuable materials was influenced by subcritical water temperature and reaction
time. It was found that the phenolic compounds could be selectively and optimally produced withtemperature and reaction time variations. From reaction time point of view, production of phenolic
compounds could be rapidly achieved in subcritical water medium which was much less than thosereported in conventional methods that increased the economic feasibility of this method.
Subcritical water could also efficiently decompose the released carbohydrate macromoleculesof rice bran to water-soluble sugars. Significant amount of the produced soluble sugars can be used
as an inexpensive feed stock for bioethanol production with vast food and industrial applications.
In chapter five, general conclusions of the present work were given.
From the results obtained in this thesis, it is concluded that the decomposition, hydrolysis,
and treatment of rice bran, which is a low-cost and abundant biomass, to valuable and usefulbio-based compounds using subcritical water as cheap, safe, clean, and environmentally friendly
technique is a feasible process. In addition, it seems that the production of valuable compounds
from rice bran in subcritical water medium can be scaled up to the industrial level. The author
believes that the technical feasibility and other significant results of this thesis can eventuallycontribute to the further progress in the treatment of not only rice bran but also other biomass torecover and produce several valuable bio-based substances in order to replace fossil derivechemicals.
審査結果の要旨

米ぬかには、食用として最良と言われている脂質、炭水化物、蛋白質、セルロース、ビタミン、酵素、フェーノール系の薬効成分、ミネラルなど人体に有効な成分が多く含まれているが、これらの多くは利用されていない。
本論文は、亜臨界水の強力な加水分解力を用いて、米ぬかの蛋白質、でんぷん、セルロースを分解し、有価物が生成することを明らかにした。蛋白質からは多数のアミノ酸が生成した。特にリジンとグルタミン酸の生成量が多く、400Ｋ付近で最大値を示すことを明らかにした。また、米ぬかのセルロースが分解して、大量の糖を生成させることにも成功した。463Ｋ付近で原料の 20％が糖に転換できることを示しており、今後エタノール発酵の原料としての適用の可能性を示した。
米は良質の脂質（米ぬか油）を表面付近に含有している。この米ぬか油は、玄米の間はほとんど劣化しないが、精米と同時に急速に劣化する。これは、玄米表面付近に存在する加水分解酵素リパーゼが精米と同時に活性化し短期間に劣化させてしまうためである。本論文では、亜臨界水により米ぬかから約 10 分という短時間で 94％という高い収率で米ぬか油を抽出できること、さらに、亜臨界水が油を抽出している間に、リパーゼも分解し完全に不活化できることを明らかにした。亜臨界水により抽出した油の劣化を12 週間にわたって追跡したが、全く劣化が見られないことを示している。すなわち、亜臨界水処理という高速で安価な方法により、劣化しない良質の米ぬか油を生産できることが明らかとなり、今後の米ぬか油の食用としての利用促進に大きく道を開いた。
米ぬかには、フェノール系の多くの薬効成分が含まれている。亜臨界水により、これらが効率よく生産できることを示した。さらに、得られたフェノール系薬効成分が、単なる亜臨界水による抽出ではなく、リグニンと炭水化物の亜臨界水加水分解反応により生成することを実験的に証明した。