Flours of different botanical sources are composed of mainly starch granules (i.e., >90%) plus other minor components such as lipids and proteins. Depending on their botanical sources, starch granules can have varying morphological characteristics and can contain varying ratio of amylose (AM) and amylopectin (AP), as well (Table1) (1)(2). Amylose and amylopectin are both polysaccharides made up of alpha-glucans (sugar unit) but differ primarily in that amylose is essentially linear, whereas amylopectin has a branched structure. Starches of most botanical sources generally contain about 30% of amylose and 70% of amylopectin, but this can differ according to cultivars. It is the ratio of AM/AP and their morphological characteristics and molecular weights plus the presence of other minor components such organic phosphorus that greatly influence the rheology, pasting properties and retrogradation of starch upon cooking or thermal treatment.
During thermal treatment of flour in excess water, the intermolecular bonds of starch are broken down, followed by water uptake by amylose and amylopectin, resulting in the swelling of starch granules to a point when this leads to the leaking of amylose and amylopectin into the surrounding water. This process is also known as gelatinization. Upon cooling, molecules that were released during gelatinization begin to aggregate to form gel-like structure, in a process known as retrogradation. Starches with higher percentage of amylose such as wheat flour will tend to form stiffed-gel as the linear amylose molecules can be packed more closely than can the branched amylopectin. Higher amylose starches also tend to inhibit extensive granule swelling, thereby lowering the paste viscosity (1). Contrary, starches containing higher amylopectin such as the waxy varieties of different flours including glutinous rice which contains predominantly amylopectin [>90% (1)(4)], can have 1 – 1.5 times higher water retention capacity and therefore, higher paste viscosity upon cooking (1). Another factor that can impact the properties of cooked starch is the unit chain-length of distribution of amylopectin, measured as degree of polymerization (DP) (3). In general, amylose can have DP between 100 – 10,000 glycosyl units, whereas amylopectin can have 10 to 100 times higher DP values. For starches that have higher amylopectin such as glutinous rice which is used in making Chinese dumpling, increased branched chain of the amylopectin (higher DP) can result in increase in gelatinization temperature and extent of retrogradation (3), yielding products with very different rheological properties, which in the case of Chinese dumpling, it is known for its stickiness mouthfeel.
It is because of the inherent differences in primarily AM/AP ratio (Table 1) which leads to the varying rheological properties of cooked starch, that different flours are being used in the manufacturing and making of different types of starch-based food products. Apart from botanical sources, there are a number of other factors that can affect the AM/AP ratio in different starches, and these include types of cultivars or genotypes, as well as cultural practices and environmental factors, all of which can lead to the modification of starch morphological structures and characteristics (5). Therefore, not all cultivars of the same botanical species can produce flours that are suitable to be used in making a certain type of product. Because of that, for kuih-making, flours of different botanical sources are sometimes mixed together in order to achieve the right proportion of AM/AP ratio, with an aim to give rise to finished products with desired quality, texture and mouthfeel.
1. M. Schirmer, A. Höchstötter, M. Jekle, E. Arendt, T. Becker, Physicochemical and morphological characterization of different starches with variable amylose/amylopectin ratio. Food Hydrocoll. 32, 52 – 63 (2013).
2. N. Singh, J. Singh, L. Kaur, N. Singh Sodhi, B. Singh Gill, Morphological, thermal and rheological properties of starches from different botanical sources. Food Chem. 81, 219 – 231 (2003).
3. S. SRICHUWONG, T. SUNARTI, T. MISHIMA, N. ISONO, M. HISAMATSU, Starches from different botanical sources I: Contribution of amylopectin fine structure to thermal properties and enzyme digestibility. Carbohydr. Polym. 60, 529 – 538 (2005).
4. S. N. Moorthy, L. Andersson, A.-C. Eliasson, S. Santacruz, J. Ruales, Determination of Amylose Content in Different Starches Using Modulated Differential Scanning Calorimetry. Starch – Stärke. 58, 209 – 214 (2006).
5. A. Rolland-Sabaté et al., Structural characterization of novel cassava starches with low and high-amylose contents in comparison with other commercial sources. Food Hydrocoll. 27, 161 – 174 (2012).