Susceptibility of Waxy Starch Granules to Mechanical Damage

Cereal Chem. 77(6):750–753 Starch samples isolated from wheat flour that represented four possible waxy states (0, 1, 2, and 3-gene waxy) were subjected to crushing loads under both dry and wet conditions. Calibrated loads of 0.5–20 kg were applied to the starch samples and the percentage of damaged granules was visually determined. Under dry crushing conditions, starches containing amylose (0, 1, and 2-gene waxy) had between 1% (5-kg load) to 3% (15and 20-kg load) damaged granules, whereas waxy starch (3gene waxy; <1% amylose) began rupturing at 0.5-kg load (3.5% damaged granules) and had 13% damaged granules when ≥10-kg load was applied. Under wet crushing conditions, normal and partial waxy starch (0, 1, and 2-gene waxy) showed little difference in percentage of damaged granules when compared to the results of dry crushing. Waxy starch (3-gene waxy), however, showed substantially increased numbers of damaged granules: 12% damaged granules at 0.5-kg load, rising to 55% damaged granules at 15-kg load. The results indicate that waxy starch granules are less resistant to mechanical damage than normal starch granules. Furthermore, blends of normal and waxy wheats or wheat flours intended to have a particular amylose-amylopectin ratio will be a complex system with unique processing and formulation considerations and opportunities. Starch is the primary component of wheat flour (Triticum aestvum, L.) and plays an important role in many temperature-dependent interactions with water, including gelatinization, pasting, and retrogradation (Atwell et al 1988). Starch is comprised of amylopectin and variable amounts of amylose (Zeng et al 1997). The amylose component is synthesized by granule-bound starch synthase I (GBSS, EC 24.1.21). Due to the allohexaploid nature of wheat, various combinations of the three homoeologous GBSS or waxy genes may occur. When all three GBSS genes are present and functional, the grain is referred to as having normal starch. When only one or two GBSS genes are functional, the grains have intermediate levels of amylose and are referred to as partially waxy. Generally, 0-gene waxy (normal) wheat has ≈22–23% apparent amylose; 1gene waxy wheat has ≈19–20% apparent amylose; 2-gene waxy wheat has ≈18% apparent amylose. When all three genes are absent (3-gene waxy wheat), the starch is essentially comprised of <1% apparent amylose (>99% amylopectin), and the grain is referred to as being completely or fully waxy. In this manner, amylose content roughly corresponds to GBSS gene dosage (Nakamura et al 1995, Zeng et al 1997). Many functional aspects of starch and flour, and many end-use qualities are affected by the amylose content of starch. Texture and quality of white salted (udon) noodles is better in partial waxy wheat as compared with normal types (Wang and Seib 1996, Batey et al 1997, Briney et al 1997). In bread, the rate of starch retrogradation and staling may be manipulated by adjusting the amylose content relative to amylopectin (Schoch 1965). Bread with higher amylopectin content may be more prone to staling and thus should be avoided. Extruded, expanded snack foods are also dependent on the content of amylose and amylopectin. Amylose forms films and produces a stronger, crunchy texture. Amylopectin produces products with greater volume and crispiness (Wang 1997). Although waxy-type grains have been long known to exist in other crops such as maize (Zea mays), it has only been in recent times that fully waxy wheat has become available (Nakamura et al 1995). The availability of wheat starch (and flour) comprised of essentially 100% amylopectin presents challenges and opportunities to the milling and food industries. Differences in rheological properties among normal, partial waxy and fully waxy wheat and other cereal starches suggest that granule structure and molecular organization differ significantly. Pasting as measured by the Rapid Visco Analyser (RVA) or viscoamylograph indicates that partial waxy samples gelatinize at temperatures similar to those of normal starch, but that fully waxy granules are less stable and gelatinize at lower temperatures (Hayakawa et al 1997, Morris et al 1998). Partial waxy starches tend to create higher viscosity pastes (Tester and Morrison 1992, Hayakawa et al 1997, Zeng et al 1997, Morris et al 1998), indicating that the amylopectin molecules interact with more water. Greater starch swelling, when heated in excess water, is also associated with reduced amylose (Crosbie 1991, Morris et al 1997), as is the manner in which lipids associate with the starch molecules (Morrison et al 1993). In theory, the final amylose content of any product or process could be manipulated between 0% (using waxy starch) and ≈25% (using normal starch) on a starch basis. However, such binary mixtures may not be expected to necessarily perform like partial waxy starches, where all granules share more or less the same amylose content. To better predict how mixtures of starch types will perform, the physical and chemical characteristics of fully waxy wheat starch need to be defined. The physicochemical, structural integrity of waxy starch granules is one aspect that will affect utilization. The structural and organizational differences among cereal starches that differ in amylose content suggested by the pasting and rheological studies also may impart differences in resistance to mechanical stress. Fractured and cracked granules contribute to greater water absorption and therefore affect dough handling and rheology. In bread, a modest level of starch damage is beneficial because a greater amount of water may be added to a dough while maintaining adequate handling properties. Furthermore, damaged granules are also more susceptible to attack and hydrolysis by α-amylase when compared with intact granules; some starch hydrolysis is desirable to provide substrate for yeast respiration in panary fermentation. However, too much damaged starch leads to sticky doughs that are difficult to handle and have reduced waterholding ability (Gibson et al 1992). Crumb structure deteriorates, and the sticky texture interferes with slicing. In noodles, increased starch damage leads to undesirably firmer and duller colored noodles (Oh et al 1985, Elbers et al 1997). Dombrink-Kurtzman and Knutson (1997) hinted that differences in susceptibility of maize starch granules to physical damage might be due to differences in amylose and amylopectin contents. Reduced amylose content in maize starch granules was associated with greater susceptibility to physical damage by an electron microscope’s beam; the resistance of granules to wrinkling and fracturing with exposure to the electron beam appeared to decrease with decreasing amylose. This 1 USDA-ARS Western Wheat Quality Laboratory, Washington State University, Pullman, WA 99164-6394. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 Department of Plant Sciences, Montana State University, Bozeman, MT 59717-3150. 3 Corresponding author. Phone: 509-335-4062. Fax: 509-335-8573. E-mail: