糜子澱粉及穀粉理化性質之研究

Abstract

本研究旨在探討由中國不同地區來源的39種黃米(proso millet)品種之澱粉及全穀粉的理化性質。首先，分別以酸法及鹼法分離黃米澱粉，經由蛋白質含量測定、高效能分子篩層析儀(high-performance size-exclusion chromatography, HPSEC)及高效能陰離子層析儀(high-performance anion exchange chromatography, HPAEC)測定其純度、澱粉分子量與其支鏈澱粉微結構，並以掃描式電子顯微鏡(scanning electron microscopy, SEM)觀察澱粉粒大小與形態。結果顯示，酸法分離黃米澱粉過程可能會造成澱粉分子降解，而以0.1% NaOH浸泡穀粉24 h接續過濾及離心等步驟之鹼法分離之澱粉有較低蛋白質含量及較高收率，且分離出澱粉粒的外觀及澱粉分子變化不大，因此為較佳的澱粉分離方法。第二部分為將13個品種之糯性黃米及23個品種之稉性黃米依穀粉性質分群，包括以碘呈色方法測定直鏈澱粉含量及以快速糊液黏度測定儀(Rapid Visco Analyser, RVA)分析穀粉於水中及硝酸銀中之糊液黏度性質，以評估穀粉中酵素活性之影響。據此，將黃米樣品分類為糯性7組及稉性10組，每組中挑選一代表樣品，進一步進行澱粉及全穀粉之物化性質分析以及性質間的相關性分析。結果顯示，直鏈澱粉含量為影響黃米澱粉及全穀粉性質最重要的參數，在糯性黃米品種中，直鏈澱粉含量與糊液黏度性質及膨潤性質有顯著相關；稉性黃米之直鏈澱粉含量則與熱性質、糊液黏度性質、膨潤性質及消化性質參數有顯著相關。支鏈澱粉鏈長分佈也會影響黃米性質，糯性黃米品種中，以示差掃描熱分析儀(differential scanning calorimetry, DSC)測得澱粉糊化熱焓值(ΔH)與支鏈澱粉短鏈(DP 6-12)之含量呈負相關(r = -0.827)，與長鏈(DP > 37)含量呈正相關(r = 0.814)。與精白稻米澱粉相比，糯性黃米與稉性黃米澱粉有較低的快速消化性澱粉(rapid digestible starch, RDS)。烹煮過的黃米與稻米穀粉的推估升糖指數(estimated glycemic index, eGI)則無顯著差異。主成分分析(Principle component analysis, PCA)結果顯示，七組糯性黃米代表樣品之穀粉及澱粉性質有明顯差異；在稉性黃米樣品中，Ningmi11 (no. 14)及Jinshu6 (no. 34)與其他品種間性質差異較大。在PCA score圖中，相同地區來源的穀粉樣品有相近的分布關係，如Ningmi14 (no. 10)和Ningmi11 (no. 14)，顯示PCA分析可能可作為一個辨別黃米樣品地理起源的方式。

The purpose of this study was to investigate the physicochemical properties of proso millet starch and flour of 39 varieties collected from different regions in China. First, a suitable method for starch isolation was needed to be established. Thus, both acidic and alkaline methods were tested and protein content, morphology, molecular weight and side-chain length distribution of isolated starches were determined by using Kjedahl method, scanning electron microscopy (SEM), high-performance size-exclusion chromatography (HPSEC) and high-performance anion exchange chromatography (HPAEC), respectively. The results indicated that the acidic hydrolysis of proso millet starch molecules during isolation process was suspected. Steeping the proso millet flour in 0.1% NaOH followed by isolating its starch with filtration and centrifuration steps was chosen because the low protein content, high yield and insignificant changes on the morphology and molecular characters of isolated starches could be obtained. In the second part of this study, 13 varieties of waxy proso millet and 23 varieties of nonwaxy proso millet were differenciated by their amylose content and pasting properties of flours. Pasting properties of flour samples were analyzed in both water and AgNO3 (aq), the later was used to evaluate the pasting properties with limited influences of endogeneous enzymes in flour. Seven groups of waxy proso millets and 10 groups of nonwaxy proso millets were classified according to the viscographic patterns of flour. The physicochemical properties of isolated starch and the ground flour of one representative sample from every group were determined. The results indicated that the amylose content has the highest correlation to the pasting properties and swelling power of waxy proso millet starch, and the amylose content was correlated to the thermal, pasting, swelling and digestible properties of nonwaxy proso millet starch. The enthalphy value (ΔH) determined by differential scanning calorimetry (DSC) was negatively correlated to the amount of short chains (DP 6-12) of amylopectin (r = -0.827) of waxy proso millet starch but positively correlated to the amount of long chains (DP > 37) of amylopectin. (r = 0.814). The amount of rapid digestible starch (RDS) in proso millet starch was less than in rice starch. There was no significant difference in the estimated glycemic index (eGI) between the cooked rice and the cooked proso millet flour. Principle component analysis (PCA) showed that there were disdinguished properties among the 7 varieties of waxy proso millets selected from each group. Ningmi11 (no. 14) and Jinshu6 (no. 34), two nonwaxy proso millets had different PCA score were different from other cultivars. Proso millet flours milled from the varieties collected from the same origin might have similar score in PCA analysis indicating that PCA analysis may be a useful tool to identify the geographic origins of proso millet samples.