Effect of Rheological Property of Batter on Baking Quality

Abstract

Pure rice bread is possible when viscoelasticity of rice batter is suitable to keep foam structure produced in fermentation. One way to adjust viscoelasticity of rice batter for bread baking is to use amorphous rice flour that acts as a thickening agent. In this study, instead, without using amorphous rice flour, we try to find a type of rice flour that is suitable for bread baking. The molecular architecture and composition of rice starch depend on the type of rice. They affect the viscosity of rice batter. The main component of rice is starch, which is composed of amylose and amylopectin. Connected with glycosidic bonds, amylose is a linear polymer and amylopectin is a branched polymer. Fraction and branch architecture of amylopectin depend on the type of rice. The purpose of this study is to clarify how baking quality and rheological property depend on the type of rice. Names of rice type used in the present study were #4019, e1, Nipponbare, Milky Queen and Himenomochi. Type # 4019 is ultra-high amylose rice and has a molecular architecture with few branches and long double helices. Type e1 is high amylose rice and has a molecular architecture with many branches and shorter side chains. Type Nipponbare is medium-amylose rice. Type Milky Queen is low-amylose rice. Type Himenomochi is waxy rice. These three types are considered to have familiar molecular architectures for general rice starch. We baked pure rice bread from each type of rice. For bread baking, batter was composed of rice flour, water, sugar, dry yeast and salt. These materials were stirred using a mixer (KitchenAid KSM90WW; FMI Co., Ltd., Tokyo, Japan) for 10 min. After mixing, we measured 100g of the batter and put it in a loaf pan. Then we fermented in an electric fermentor (SK-15; Taisho Electric Co., Ltd., Kusatsu, Japan) for 30 min at 40°C. The batter was then baked for 15 min at 180oC in a gas oven (OZ100BOEC; Ozaki Co., Ltd., Tokyo, Japan). For rheology measurements, rice batter samples comprising only rice flour/water 50/50 (wt%) were prepared by stirring in the mixer for 10 min. We measured storage modulus G´ as a rheology data using a rotational rheometer (Physica MCR 301; Anton Paar Co., Ltd., Graz, Austria). During the measurement of G´, the temperature was increased from 25oC to 35oC at 0.3oC/min (corresponding to the temperature increase during fermentation), and from 35oC to 90oC at 2.5oC/min (corresponding to baking). The angular velocity and strain were 10 rad/s and 0.1%, respectively. A coaxial cylinder with an inner diameter of 24 mm and an outer diameter of 26 mm was used. Figure 1 shows cross-section images of baked bread samples. Batter made from high-amylose content rice showed good swelling. Bread made from ultra-high amylose rice contained large bubbles. Storage modulus G' of low-amylose rice was too low to keep bubbles for bread crumb. Figure 2 shows the storage modulus G´ obtained for rising temperature. The initial storage modulus G´ in the fermentation temperature range increased as the amylose content increased. At room temperature, linear amylose dissolves in water more easily than does branched crystalline amylopectin. We expected that amylose dissolved in water increased the viscoelasticity of the batter, causing increase of G´ of the sample. The G´ of rice batter typically increases gradually due to gelatinization up to their peak temperatures. We inferred that gelatinization occurs when water enters double helices of amylopectin. The peak value of G' showed a larger value as amylose content increased. Storage modulus G' of # 4019 showed a modest increase compared to other types. We consider that this modest increase was caused by the fact that #4019 has longer double helices than other types. Longer double helix needs longer time to completion of gelatinize. The decrease of G´above the peak temperature is caused by disintegration of starch granules. Around the peak of G’, bubbles stop growing and be stabilized. Stabilization of type # 4019 was slower than other types, so bubbles got bigger. In conclusion, difference of rice type caused difference in rheological properties of rice flour batter. When initial G ' is high, bubbles are kept. Stabilization of bubbles is faster when gelatinized temperature is lower. Figure 1