不同糖/鹽溶液對紫玉甘藷澱粉理化性質之影響

Abstract

本實驗旨在探討糖類和鹽類對台農73號紫玉甘藷澱粉理化特性之影響。生鮮甘藷含水份約74 %，乾物中粗蛋白約佔0.94 %、粗脂約佔0.21 %、灰份約佔0.63 % 以及粗纖維約佔0.9 %，其餘則為澱粉，其中直鏈澱粉約占總澱粉的31 %，破損澱粉約占總澱粉的1.52 %。澱粉顆粒表面光滑，偏圓狀多角形，大小約5-25 μm，屬Ca型澱粉，結晶度約為38 %，膨潤力和溶解度均隨溫度增加而上升。以快速黏度分析儀(RVA)和示差掃描熱分析儀(DSC)觀察其糊化間差異，並以動態流變儀觀察澱粉膠黏彈性質的改變。結果顯示，紫玉甘藷澱粉之糊化溫度、尖峰黏度和最終黏度皆隨著糖濃度的增加(5-40 %)而提高，且提高的效果依序為蔗糖>麥芽糖>葡萄糖>果糖>木糖。而由回升黏度值的結果可知，延緩凝膠及回凝的效果以木糖最明顯，蔗糖最弱；另外糖濃度愈高，裂解黏度值也增加，但若達臨界濃度後(雙醣30%、單糖40%)，裂解黏度值反而下降，顯示提升澱粉對熱及剪切外力破壞的忍受力，雙醣的作用比單糖要強。熱分析的結果顯示，糊化起始、尖峰及終止溫度(TO、TP、TC)均和糖濃度呈正相關，顯示糖能穩定澱粉結晶區結構；黏彈性質的結果顯示，甘藷澱粉屬弱膠結構，糖的添加與否並不影響其弱膠的黏彈行為，貯存模量會隨溫度降低或糖的添加而增加，顯示適量糖濃度對澱粉分子間凝膠之網狀結構，有正面的貢獻。適量之鹽濃度(0.5-10 %)同樣也可提高澱粉系統的糊化溫度，但濃度達20 %時，糊化溫度反而下降，且雙價鹽影響力大於單價鹽(鎂離子>鈣離子>鈉離子>鉀離子)。裂解黏度值則因鹽類的添加而降低，但單價離子添加濃度愈高，裂解值會依序降低；若添加雙價離子，裂解值則有先降後升的趨勢，數值高於控制組，以鈣離子組別上升程度最顯著，顯示鈣離子穩定澱粉糊結構的能力最弱；熱分析的結果顯示，除添加20 %氯化鈣的組別外，TO、TP、TC隨鹽濃度增加(0.5-10 %)而上升，然當濃度達20 %時，TO、TP、TC反而降低；加鹽的膠體也趨向弱膠，頻率掃描所得之G’值隨鹽濃度增加會先升後降，正切值均小於1，高鹽環境(20 %)時，正切值會明顯增加，顯示高鹽系統對澱粉黏性性質有相當程度之貢獻，並以雙價離子之影響較明顯。

The aim of this study is to characterize the physicochemical properties of starch isolated from Purple Jade sweet potato. Effect of sugar or salt solutions would be investigated as well. It was found that fresh sweet potato contains approximately 74.7 % moisture, 0.94 % crude protein, 0.21 % crude fat, 0.63 % curde ash and 0.9 % crude fiber. The amylose and damaged starch content of Purple Jade sweet potato starch were 31 and 1.52 %, respectively. Starch granules isolated from Purple Jade sweet potato appeared to be round polygonal shape with smooth surface and the granule size ranged from 5-25 μm. X-ray diffraction pattern revealed that the crystallinity type belonged to Ca-type with approximately 38% crystallinity. Swelling power and solubility of starch increased exponentially with increasing temperature. The gelatinization behavior of sweet potato starch were further studied by using RVA and DSC. The visco-elastic properties were then studied by using dynamic rheological measurement. The pasting temperature, peak viscosity and final viscosity increased with increasing sugar concentration in the following order: sucrose> maltose> glucose>fructose> xylose. As indicated by the setback values, xylose showed strongest impact on inhibiting gelation and retrogradation reactions. In constrast, sucrose showed the weakest impact. Breakdown values generally increased with the sugar concentration to a certain extent, then decreased with further increase in sugar concentration (30% and 40% for disaccharide and monosaccharide, respectively). Such results implied that disaccharide could enhance the resistance of starch to high temperature or shear force more as compared to monosaccharide. DSC results revealed that the onset, peak and conclusion temperature (TO, TP, TC) for starch gelatinization had positive correlation with sugar concentration, suggesting that sugar could stabilize the crystalline structure of starch. Dynamic rheological results indicated the weak gel structure of starch paste with or without sugar addition. The storage modulus (G') of starch paste increased with increasing sugar concentration, suggesting that sugars had a positive contribution on the gel network structure. The pasting temperature also increased with increasing salt concentration to a certain extent(0.5-10 %) in the following order : Mg2+ >Ca2+ > Na+ >K+, then decreased with further increase in salt concentration (20 %). In contrast, the breakdown value decreased by salt addition. It generally decreased with increasing monovalent salt concentration, but decreased with increasing divalent salt concentration only to a certain extent, then increased with further increase in divalent salt concentration, particularly for calcium ions. Such results implied that calcium ions had the least capabilities to stabilize starch gel structure. DSC results revealed the TO,TP, TC for starch gelatinization also increased with increasing salt concentration up to 10%, then decreased with further increase in salt concentration to 20 %. Starch paste with salt addition exhibited a weak gel structure as characterized by dynamic rheological evaluation. The G' generally increased with salt addition to a certain extent, then dropped with further increase in salt concentration. The significant increase in tanδ with high salt concentration revealed the viscous contribution of high concentration of salt , particularly for divalent salts.