預熟發芽糙米之製備及其物化性質

Abstract

預熟加工為一種水熱處理，其主要步驟包含浸漬、蒸煮及乾燥。一般而言，由於在加工過程中稻米結構改變及澱粉回凝現象發生，預熟米飯質地偏硬。另一現代穀類加工常見之方法稱為發芽，由於發芽使部分大分子物質被水解，因此其烹煮後可使質地偏軟；又因為種子生長需求，發芽過程會產生許多生物活性物質，如 γ-胺基丁酸 (GABA)。然而，發芽過程容易因環境條件控制不當，使其產生較差之烹煮品質並縮短保存期限。本篇研究目的為結合預熟與發芽之加工處理，建立預熟發芽糙米 (PGBR) 加工條件，使其提升糙米 (BR) 品質及功能性。樣品使用台稉九號糙米，將其於 37℃ 下浸漬 6 小時，再將水倒出後於同溫度下發芽 18 小時，隨後於 111℃ 下蒸煮 3 分鐘，最後於 40℃ 烘箱乾燥 5 小時可製備出預熟發芽糙米。於使用數學模型描述 PGBR 之乾燥特徵中發現，logarithmic model 在 40℃、30% 相對濕度下為最佳乾燥模型。PGBR 的表面外觀結構光滑且緻密，並有些許裂縫存在；於橫切面結構可看出靠近外圍澱粉顆粒糊化並彼此聚集，證實結構產生變化。上述改變造成其烹煮時間相較於 BR 減少約50%。在熱糊黏度分析中，PGBR 相較於發芽糙米 (GBR)，其黏度曲線具有上升趨勢，這是由於澱粉顆粒部分糊化及短時間高溫蒸煮下澱粉酶失活之綜合表現導致。部分糊化使胚乳內澱粉顆粒仍保有完整性，而酵素的失活抑制了澱粉酶的作用。透過具有抑制澱粉酶活性的硝酸銀溶液取代水作為黏度分析的溶劑，可發現 GBR 的黏度曲線與 PGBR 用水作為溶劑時之曲線相似。將糙米進行烹煮後於質地分析中可發現，因 PGBR 內部澱粉顆粒部分被水解及糊化使其在硬度表現上較預熟糙米 (PBR) 來的軟，此外，PGBR 因僅蒸煮 3 分鐘也促使其硬度提升有限。營養物質分析中，因 PGBR 包含複雜之水熱加工處理，導致維生素 B1 與 B3 略微下降。另一方面，GABA 具耐熱性，因發芽所提升約 3 倍之 GABA 含量仍能保留於 PGBR 中。以米飯進行體外消化實驗，PGBR 相比 BR 增加了抗性澱粉含量，這是由於經預熟加工後的產品可能產生具耐熱性之抗性澱粉，因此烹煮成米飯後仍然能保留抗性澱粉存在。

Parboiling is a hydrothermal treatment consisting of soaking, steaming and drying. The cooked parboiled rice generally exhibits a harder texture due to the changes in microstructure and starch retrogradation of rice. Germination is another modern process of cereal. The texture of rice becomes softer due to the partial hydrolysis of macromolecules and the physiological requirements during germination, and many bioactive substances such as γ-aminobutyric acid (GABA) are generated. However, germination has some shortcomings, such as poor cooking quality and shorter shelf life. The objective of this study was to establish a processing protocol and conditions for the manufacture of parboiled germinated brown rice (PGBR) to improve the quality and functionality of brown rice (BR). The Taiken 9 japonica brown rice was used to prepare PGBR through a series of procedures, including soaking (37℃/ 6 h), germination (37℃/ 18 h), parboiling (111℃/ 3 min), and drying (40℃/ 5 h). The logarithmic model was a suitable model for describing the drying process of PGBR at 40℃ and 30% RH. The morphology analysis illustrated that the surface of PGBR got smooth and compact; however, some fissures were observed. The starch granules near the outer layer of the PGBR were gelatinized, confirming the structural change. These physicochemical changes of PGBR resulted in a 50% reduction in cooking time as compared with BR. Compared with germinated brown rice (GBR), the viscogram of PGBR revealed an upward trend. This was due to the partial gelatinization of starch granules and the inactivation of amylase during the short steaming process. Partial gelatinization preserved the integrity of some starch granules within the endosperm, while enzymatic inactivation prevented the hydrolysis by amylase. Through inhibiting the amylase activity by replacing water with an AgNO3 solution, the viscogram of GBR was like that of PGBR with water as solvent. Texture analysis of cooked rice revealed that PGBR was softer than the parboiled brown rice (PBR) since the starch granules are partially hydrolyzed and gelatinized. The shorter steaming time processed during the preparation of PGBR might also be attributed to the softer texture compared to PBR. Vitamin B1 and B3 of PGBR were slightly reduced due to soaking and heat treatment. In GABA content, GBR yielded 3 times more GABA than BR and maintained the same level after parboiling (PGBR). The increase in resistant starch in cooked PGBR suggested that it may enhance the thermally stable resistant starch during parboiling, making it retain more than that of cooked BR after cooking.