Dept. of Food Science, Purdue Univ., 745 Agriculture Mall Drive, West Lafayette, IN, 47907, U.S.A.
J Food Sci. 2019 Mar;84(3):507-523. doi: 10.1111/1750-3841.14472. Epub 2019 Feb 19.
Water plays a significant role in the gelatinization and retrogradation (crystallization) of starch. Amylopectin crystalline regions can adopt several hydrated polymorphic forms; however, reports differ on the migration of water during retrogradation. The objectives of this study were to determine the moisture sorption patterns of gelatinized starch lyophiles during retrogradation in controlled relative humidity (RH) environments and document the amylopectin polymorph(s) formed. Starches from different botanical sources containing A-type and B-type amylopectin polymorphs were studied. Suspensions of starch were heated and then frozen and freeze-dried to make primarily amorphous matrices. Moisture sorption profiles of the dried samples were collected from 5% RH to 95% RH at 25 °C. To capture the retrogradation event, sample masses were also monitored at constant RHs over time (95%, 92.5%, and 90% RH). Powder X-ray diffraction was used to document the physical state of the samples, including the amylopectin polymorph formed upon retrogradation, and differential scanning calorimetry was used to determine glass transition temperatures (T s). In all lyophiles, water was first absorbed (mass gain), and if a critical water content was reached (at ≥92.5%RH), sample T s dropped below room temperature and concurrent retrogradation and water expulsion (mass loss) occurred, regardless of starch botanical source and whether A- or B-type polymorphs were formed. Overall, retrogradation and water expulsion increased as storage RH increased. These results offer further knowledge into the role of water in amylopectin retrogradation and the relationship among starch type, environmental RH, moisture sorption prior to retrogradation, and water redistribution during retrogradation. PRACTICAL APPLICATION: Starch gelatinization and retrogradation require molecular mobility, which is facilitated by water. Limited retrogradation occurred in lyophiles in the glassy state (90% RH, 25 °C), but increasing the storage RH (to ≥92.5% RH) resulted in increasing amylopectin retrogradation (note: many baked products have water activities in this range). Regardless of starch type (botanical source and amylose content), when the storage RH was high enough, the starch lyophiles first absorbed water, which depressed the T below the storage temperature, and then exhibited concomitant retrogradation and water expulsion. The water expelled during amylopectin retrogradation was not (fully) retained in the amorphous starch fraction, which is why samples lost weight. Water leaving the starch matrix during retrogradation could pose challenges for quality, texture, and shelf-life of starch-based products.
水在淀粉的胶化和回生（结晶）中起着重要作用。支链淀粉晶体区域可以采用几种水合的多晶型形式；然而，关于回生过程中水的迁移，报告结果存在差异。本研究的目的是确定在受控相对湿度（RH）环境中回生过程中胶凝淀粉冻干物的水分吸附模式，并记录形成的支链淀粉多晶型体。研究了来自不同植物来源的含有 A 型和 B 型支链淀粉多晶型体的淀粉。淀粉悬浮液被加热，然后冷冻和冻干，以形成主要无定形基质。在 25°C 下，从 5%RH 到 95%RH 收集干燥样品的水分吸附曲线。为了捕捉回生事件，还在恒 RH 下随时间监测样品质量（95%、92.5%和 90%RH）。粉末 X 射线衍射用于记录样品的物理状态，包括回生时形成的支链淀粉多晶型体，差示扫描量热法用于确定玻璃化转变温度（T s）。在所有冻干物中，水首先被吸收（质量增加），如果达到临界含水量（≥92.5%RH），则样品 T s 降至室温以下，同时发生回生和水的排出（质量损失），而与淀粉的植物来源以及是否形成 A 型或 B 型多晶型体无关。总体而言，随着储存 RH 的增加，回生和水的排出增加。这些结果进一步揭示了水在支链淀粉回生中的作用以及淀粉类型、环境 RH、回生前的水分吸附以及回生过程中水的再分配之间的关系。 实际应用：淀粉的胶化和回生需要分子流动性，这是由水促进的。在玻璃态（90%RH，25°C）下，冻干物中的回生有限，但增加储存 RH（≥92.5%RH）会导致支链淀粉回生增加（注意：许多烘焙产品的水分活度在此范围内）。无论淀粉类型（植物来源和直链淀粉含量如何），当储存 RH 足够高时，淀粉冻干物首先吸收水分，使 T 降低到储存温度以下，然后同时发生回生和水的排出。在支链淀粉回生过程中排出的水没有（完全）保留在无定形淀粉部分，这就是为什么样品减重的原因。回生过程中离开淀粉基质的水可能会对淀粉基产品的质量、质地和保质期构成挑战。
