Effects of Preparation Temperature on Gelation Properties and Molecular Structure of High-Amylose Maize Starch

Cereal Chem. 78(4):442–446 In this study, 3% aqueous high-amylose maize starch (Hylon VII) dispersions were heated to temperatures of 140–165°C. The onset and rate of gel formation was observed using a small-strain oscillation rheometer as a function of temperature from 90 to 25°C. The gel formation clearly began earlier in high-amylose starch paste preheated at lower temperatures, but the rate of gelation was slower and the resulting gel was weaker in comparison with starch pastes preheated at higher temperatures. In addition, the structure of the final gels was studied using large deformation compression measurements. The most rigid gel structure on the basis of small and large deformation tests was obtained for high-amylose starch gel preheated to 150–152°C, depending on the type of measurement. The rate of gelation was also fastest in that temperature range. High-amylose gels heated to higher temperatures lost their rigidity. The molecular weight distribution of starch molecules was measured by size-exclusion chromatography. Heating caused extensive degradation of amylopectin, which had a great effect on amylose gel formation and the final gel properties of high-amylose maize starch. Micrographs of Hylon VII gels showed that phase separation of starch components visible in light microscopy occurred on heating to higher temperatures. Amylose and amylopectin are two macromolecular components of starch granules. The ratio of these components varies according to the source plant. Normal maize starch consists of ≈75% branched amylopectin; the remaining 25% is linear amylose. Since the 1950s, it has been possible to alter the ratio of amylose to amylopectin by genetic hybridization. Clear changes have been detected in granule architecture when the amylose content of starch is increased. Dry granules with a high amylose content possessed an amylopectin center surrounded by amylose, which in turn was encapsulated by an amylopectin surface (Atkin et al 1999). Today, high-amylose starches are commonly used in the confectionery industry because they have excellent gelling and film-forming properties (Jane 1997; Case et al 1998). Starch components are dissolved by heat and shear from granules, forming a molecular dispersion of amylose and amylopectin. The gelatinization temperatures of high-amylose starches are higher than those of normal and waxy cornstarches (Jane 1997; Case et al 1998; Edwards et al 1998; Shi et al 1998; Boltz and Thompson 1999). According to the manufacturer (National Starch and Chemical), the temperature typically used for heating Hylon VII high-amylose maize starches is 154–171°C, depending on the soluble solids level of the desired formulation. Different properties can be obtained for the starch gels depending on the amount and type of material solubilized during gelatinization (Ring et al 1987; Gidley 1989; Leloup et al 1992; Durrani and Donald 1995). The ratio of amylose to amylopectin can influence the formation of a network system (Doublier and Llamas 1993; Miles et al 1985; Leloup et al 1991; Klucinec and Thompson 1999). Strong gels are formed above an amylose-to-amylopectin ratio close to 15:85. At higher ratios of amylose to amylopectin, amylose forms a continuous network structure. Using dynamic viscoelastic measurements, Parovuori et al (1997) showed that the gel formation of amylose was highly dependent on both the ratio of amylose to amylopectin and the molecular weights of amylose and amylopectin. Medium size α-dextrins substantially weakened the gel formation of amylose. The aim of this work was to study the effects of heating temperature on the gelation properties and the consistency of highamylose maize starch gel. The focus was on ascertaining at which temperature high-amylose starch adequately loses its granule structure to be able to form a firm gel. Gel formation properties and the structure of formed gels were investigated by measuring smallstrain oscillation and backward extrusion during compression. The gel microstructure and network organization were examined by light microscopy. In addition, the effect of the heating temperature on the molecular weight distribution of high-amylose maize starch heated at various temperatures was studied by size-exclusion chromatography. MATERIALS AND METHODS Starch High-amylose maize starch (Hylon VII) from National Starch and Chemical Company was used in this study. The dry matter content of the product was 90.4%. The amylose content of Hylon VII starch was 70% as determined by the potentiometric iodine method, as well as by size-exclusion chromatography; the remaining 30% was amylopectin. Preparation of Samples Solutions of 3% (w/w, dry matter basis) Hylon VII starch were heated in a special pressure vessel (VTT Automation, Espoo, Finland) using a mixing blade under elevated pressure at 140–165°C. The detector element of the thermometer was located in the jacket of the pressure vessel. The desired temperature in the pressure vessel was reached after 30 min. It was then allowed to cool for 1 hr to 100°C. The gelatinized solution was collected in a preheated vacuum bottle before rheological measurements were taken. For compression tests, the heated solution (50 g) was poured into cylindrical cups (50 mm, i.d.) and sealed carefully; then the gels were allowed to set overnight at room temperature before measurements were taken. In addition, the resulting starch gels were freeze-dried as part of the gel permeation chromatography measurement process. Rheological Measurements A controlled stress rheometer (StressTech, ReoLogica Instruments AB, Lund, Sweden) was used to perform small-strain oscillatory measurements using a concentric cylinder measuring geometry (CC 25 CCE). The rheometer was preheated to 90°C. After the gelatinized solution was stabilized for 5 min in the rheometer, the oscillation measurement was started at 90°C and continued down to 25°C at a cooling rate of 1.5°C/min. The oscillation frequency was fixed at 1 Hz and the first rheological measurements were made using the smallest possible stress (0.025 Pa) to avoid any breakage of the forming network. The second and third measurements were recorded with a constant strain of 0.003. No significant differences between measurements were detected. 1 VTT Biotechnology, P.O. Box 1500, FIN-02044 VTT, Finland. 2 Corresponding author: Phone: +358 9 456 5175, Fax: +358 9 455 2103, E-mail: