Starch gelatinization: an X-ray diffraction study.

Cereal Chem. 65(6):443-446 Starch gelatinization was defined as the melting of starch crystallites, as granule fatty acids. Formation of these complexes partly explains determined by X-ray diffraction, in which the complete destruction of differences reported for heat of gelatinization values of amylose/lipidcrystallite integrity was observed as a function of moisture content and bearing starches because of competing melting and crystallizing effects. temperature. Melting occurred over a range of temperatures, and melting X-ray analysis failed to confirm differential scanning calorimetry evidence temperatures increased as starch moisture content decreased. For moisture of starch melting in samples at moisture contents lower than about 30% and contents of 30% and higher, endothermic transitions determined by heated to temperatures as high as 1750 C. A glass transition was postulated calorimetry were confirmed as reflecting loss of starch crystalline structure. to account for reports of apparent differential scanning calorimetry The wide range of melting temperatures was attributed to different degrees endotherms in such low-moisture starches. In the absence of an of crystallite perfection within the granule and to their respective intermediate amorphous melt, the B to A or C structure changes observed interaction with granule moisture content. For normal maize starch, in heat and moisture treated potato starches were interpreted as solid-state heating caused formation of crystalline V-type complexes of amylose with transitions. There is widespread interest in the process of starch gelatinization, in which the organized granule structure is disrupted, often the first step in starch applications (Zobel 1984). Although the phenomena of gelatinization are readily apparent, the various ways (optical, Theological, calorimetric) used to determine gelatinization have made it difficult to formulate a precise and common definition. In addition, gelatinization thermograms obtained by differential scanning calorimetry (DSC) have been interpreted differently (Donovan 1979, Eberstein et al 1980, Biliaderis et al 1980, Evans and Haisman 1982, Zobel 1984, Maurice et al 1985, Hoseney et al 1986). On the other hand, the observed first-order endotherms are generally interpreted to mean that gelatinization is a melting process. Identification of the (DSC) transitions by X-ray diffraction, however, is critical to a comprehensive understanding of starch thermograms. Kugimiya et al (1980), for example, used complementary X-ray data to show that their 110120°C endotherms were caused by the melting of crystalline V-type amylose-lipid complexes. Application of the term "melting" to starch gelatinization is based on X-ray and synthetic polymer technologies. Thus, from diffraction studies, crystalline specimens yield reflections from crystal planes. After melting, these reflections disappear and a broad halo appears, indicating a change from a crystalline to an amorphous (molten) state. Accordingly, the material is no longer a rigid solid but rather exhibits the properties of a liquid. In this state, polymeric materials may show low fluidity, and as such, fit into the classic definition of a melt. High molecular weight materials, however, impart high viscosity or rubberlike qualities to the molten-liquid state (Mandelkern 1964). In this report, X-ray diffraction was used to characterize transitions that produce endothermic DSC peaks when starches are heated after having been equilibrated to different moisture contents (Donovan 1979, Eberstein et al 1980, Eliasson 1980). Potato starch was used to illustrate typical starch-water interactions because of available DSC data and its purity with respect to granule lipids. Some of the data reported here are unique in that the X-ray diffraction patterns were recorded as the starch was maintained at its gelatinization (melt) temperature. This differs from past studies in which samples were gelatinized and then cooled before X-ray analysis. As a consequence of the cooling process, moist gels could display crystalline artifacts. In this study, samples that were first heated, cooled, and then subjected to X-ray analysis were in sealed tubes and were limited to 20% or less moisture to avoid recrystallization. 'Presented at the AACC 7 1st Annual Meeting, October 1986, Toronto, ON, Canada. 2 CPC International, Box 345, Argo/Summit, IL 60501. 'Pennsylvania State University, University Park 16802. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1988. Maize (corn), potato, and waxy maize starches were investigated to cover granules with a range of crystal structure, amylose/amylopectin ratio, and lipid content. Maize has an Atype structure, 27% amylose, and polar lipids that can complex with the amylose fraction to form V crystals. These V structures are formed from collapsed amylose helices in which chemical adjuncts are trapped inside. Complex formation has been established by X-ray studies and investigated by DSC; the effect on thermograms of starch, however, has been subjected to speculation (Hellman et al 1954, Kugimiya et al 1980, Stute and Konieczny-Janda 1983, Zobel 1984, Biliaderis et al 1985). Waxy maize also has an A structure and lipids similar to those in maize, but amylose is absent for complex formation. Potato starch has a B structure and 22% amylose but lacks a lipid fraction for complex formation. The C form is native to certain root and tuber starches. From the standpoint of this study, C is formed when potato starch is heat and moisture treated (Sair 1967). X-ray patterns of starch structures are diagrammed in Figure 1 (Zobel 1964, 1988a). MATERIALS AND METHODS X-ray patterns were taken primarily with a Guinier-Simon (G-S) circular cassette camera in which sample temperatures were Fig. 1. Starch X-ray pattern designations. Vol. 65, No. 6,1988 443 II