Physicochemical Properties And Modification Of Buckwheat Starch

Buckwheat was well known as high nutritional value, abundant of medical functionality and health benifits. Because of the peak viscosity of buckwheat starch paste is higher, and the hot paste and cool paste are very stable, buckwheat has great potential application value. But by now it still has few reports of processing and utilization for buckwheat starch. Therefore, it is necessary to study the physicochemical properties of buckwheat starch and the key technology for buckwheat modified starch development. For the purpose of understanding the physicochemical properties and modification of buckwheat starch, 4 buckwheat varieties had been selected as the specimen, and the effects of germination on starch physicochemical properties had been analysed systematically. The enzymatic hydrolysis conditions and fungalα-amylase enzymolysis kinetics of the buckwheat starch, the influencing factors of buckwheat resistant starch preparation, the characteristics and application of the buckwheat resistant starch, and the structure and adsorption performance of buckwheat microporous starch were also studied.The main results and conclusions as follows:By combining the technology of flavonoids alcoholic extraction with alkaline protease hydrolysis during the starch extraction, the buckwheat starch content was reached to 95.21%, and more protein had been extracted. The primary factors affected flavonoids extraction in buckwheat were the ratio of material to solvent, the temperature, the alcohol concentration and the extraction time. All of the above factors had high significant or significant influences. The detail parameters for the buckwheat flavonoids extraction were, 60% alcohol concentration, 1:20 proportion of material and fluid, 50℃temperature, 3 h reaction time, and the flavonoids extraction was 0.951mg/g. The optimal condition of buckwheat protein hydrolysis with alkaline protease was, temperature 45℃, pH 11, alkaline protease 100 U/g and reaction 40 min.Buckwheat starch granules are commonly polygonical or round shaped, and the granules size are smaller and uniform. Its amylose content are about 25.82% ~ 32.67%, and the polarization cross are very clear. Its X-ray diffraction pattern are A-type, which are similar with the corn starch. The starch is hard to dissolution, meanwhile the solubility is very different between buckwheat varieties. The viscograms, including the peak viscosity, the viscosity at the beginning of holding period, the viscosity at the beginning of coolinging period, the viscosity of final cooling period, and the viscosity at the end of 50℃holding period, are higher than other starches. The gelatinization and viscosity characteristics are consistent with mung bean starch, while there is few difference between buckwheat varieties. The acid and alkali had great effects on the buckwheat starch gelatinization, and had some significant influences on the viscosity, especially the acidic treatment. Buckwheat starch paste has lower transparency, while stronger hot paste stability, cool paste stability, freezen-thawing stability and anti-retrogradation ability. Buckwheat starch gel has higher hardness, resilience and cohesiveness, and its textural properties are superior.By germination treatment, total buckwheat starch content reduced by 2.87%～4.79%, while little influence on amylose content. The starch granules still maintained the original shape, the characteristics of polarization cross and the X-ray diffraction pattern. Moderately germination increased the solubility and transparency, the peak viscosity, the viscosity at the beginning of holding period, the viscosity at the beginning of coolinging period, the viscosity of final cooling period, and the viscisity at the end of 50℃holding period, enhanced the hot paste stability and anti-retrogradation. Excessively germination reduced the transparency, accelerated the retrogradation, diminished the hardness of gel, enhanced the resilience and cohesiveness, and there was an obvious deterioration trend on starch gel texture properties.During hydrolysis, when both the mesothermal-α-amylase and fungal-α-amylase were used, the transformation ratio of the buckwheat starch hydrolysis surpassed 99.5%. It also could get a higher ratio when two kinds of enzymes mixed with different proportion. The regression equation of buckwheat starch hydrolysis with fungal-α-amylase was Y=63.31-1.80X1 +2.55X2+0.79X4-1.44X12+1.95X1X2-1.41X22-0.27X32. All the selected factors had remarkable influence to the hydrolysis of buckwheat starch with fungal-α-amylase. The effect order of the factors was pH value, temperature, the enzyme dose and the substrate concentration, and these 4 factors made 95.77% contribution to the hydrolysis degree. The actual DE value is 65.80% under the following conditions: 54℃, pH6.0, 5% substrate concentration, 130 U/g fungal-α-amylase. The initial period of fungal-α-amylase hydrolization is a first order reaction, coincided with Michaelis - Menten equation curve characteristic. The kinetics parameter Vm is 1.5873 mg/ (mL.min) and Km is 5.4699 mg/mL. The optimal temperature of kinetics model is from 303.15 K to 333.15 K, namely in 30℃~60℃.The quantity of resistant starch can be improved greatly by the processing of acidolysis,hydrothermal and low temperature storage treatment. The ideal condition of buckwheat resistant starch preparation was, 1:9 ratio of material to solvent, 2.5% diluted hydrochloric acid, 0.5 h acidolysis, 4 h gelatinization in boiling water, and storage 42h in 4℃refrigerator. There were too much diffraction peaks of resistant starch, but week intensity, and appeared the new diffraction peak when 2θat 20.64°, 26.62°. The advanced micrite quantity was low and not obvious. The primary micrite area was wide, and the micrite content was high. The crystallinity was 27.52%,and lower than the original starch obviously. The X- ray diffraction had both of V type and A type characteristics.Fungal-α-amylase had high hydrolysis activity to buckwheat raw starch, higher porosity and uniform in porous size. The prepared buckwheat microporous starch, under the conditions: 40℃, reaction temperature, pH 6.2, 14 h reaction time, 2% fungal-α-amylase dosage, had higher water-absorption and oil-absorption, and lower bulk density, it's absorption property is superior. The buckwheat microporous starch maintained the original starch crystal structure characteristic, and majority of the starch granules polarization cross still exists, and the X-ray diffraction pattern also was A-type. But the peak intensity and the position of the diffraction had some changes, and the crystallinity elevated. The methylene blue absorption kinetcs on buckwheat microporous starch followed first order absorption rate law of the Lagergren equation. In same temperature, the absorption rate under alkalinity condition (pH10.8) was 1.8~1.9 times higher than the acidic condition (pH4.8), it needed more absorption activity energy under the acidic condition than under the alkalinity condition. The thermodynamics about the buckwheat microporous starch absorbing the methylene blue follows the monolayer absorption rule of the Langmuir equation. The alkalinity condition monolayer saturated adsorption is 2.9 times higher than acidic condition. When pH value rose from 4.8 to 10.8, the negative value of absorption heats increased, absorption was spontaneous exothermic process, and that was easier to carry on at pH10.8 due to the slight adsorption electrostatic repulsion.