Extraction and gelatinization characteristics of Chenopodium quinoa Willd starch based on ridge analysis

Quinoa (Chenopodium quinoa Willd.), which is a pseudocereal or pseudograin, has been recognized as a complete food due to high nutritional value. Recently, quinoa has attracted more and more attention. Starch is main nutritional constituent of quinoa. In order to understand the gelatinization characteristics of starch, quinoas were collected from Xinzhou City in Shanxi Province. The granule morphology, functional group, swelling power, transparencies, freezing thawing stability, texture and rheological characteristics of starch granules were investigated in this study. Firstly, the quinoa starches were obtained by alkali extraction method. The cardinal parameters of extracting technique have been studied including solid-liquid ratio, sodium hydroxide (NaOH) concentration and extraction time. The ridge analysis was used to find the condition on a set of design variables that maximize or minimize an estimated second-order correlation function on spheres of varying radius. According to the ridge analysis, the optimum parameters were as follows: solid-liquid ratio of 1:5 (g/mL), NaOH concentration of 0.2% and extraction time of 5.5 h. Under this condition, the extraction rate of starch was 98.94%±0.26%. The shape and chemical structure of starch granules were examined by the scanning microscope and Fourier transform infrared spectrometer. The starch granules of quinoa had irregular shape. The functional groups of quinoa starch contained -OH, -CH2, -CHO, C-O-C and pyran ring, and quinoa starch had typical molecular structure of native starch. And the 1047 cm-1/1022 cm-1 absorption peak ratio was 0.77. The gelatinization characteristics of starch were also studied, including swelling property, transmittance, freeze-thaw stability and retrogradation. The swelling property of quinoa starch was related to pasting temperature, and increased with the increasing of temperature. The swelling was divided into 2 stages: the initial swelling period and rapid swelling period. So, the quinoa starch was restricted swelling starch. And its highest swelling degree was 15.581 g/g at 95℃. The transmittance and retrogradation were closely related to standing time. The initial transmittance rate of starch was 1.7, which decreased with retention time. However, the retrogradation volumes increased with the time, which remained basically unchanged after 12 h. The syneresis rate was 10.50% during the first freeze-thaw cycle. The syneresis rate increased with the number of freeze-thaw cycles and entered stable value (33.01%) after 3 cycles. The texture properties of starch gelatinization were analyzed by TMS-Pro texture analyzer. The texture indices, which contained hardness, cohesion, elasticity, adhesiveness and chewiness, increased with the concentration of starch. Through rotational rheometer, the result showed that quinoa starch belonged to pseudoplastic fluid. When angular frequency range was 0-100 rad/s, loss modulus was higher than storage modulus. Thus, the starch paste was a compact cross-linked structure. As a main chemical component of quinoa, the physicochemical properties of starch would directly affect food processing and product quality. In this paper, the structure and physicochemical properties of quinoa starch have been studied systematically by various experimental methods, and the present result will offer a basic theoretical support for expanding the applications of quinoa starch. © 2016, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.