Inactivation of Endogenous Rice Flour β-Glucanase by Microwave Radiation and Impact on Physico-chemical Properties of the Treated Flour

Abstract

The apparent reduction of β-glucan (BG) molecular weight in rice-based gluten-free (GF) breads fortified with cereal BG concentrates reveals the presence of β-glucanase activity in rice flour. Inactivation of endogenous β-glucanase in rice flour thus seems to be a necessary step when developing GF breads enriched with BG of high molecular weight. The aim of this work was to study the thermal inactivation of endogenous β-glucanase in rice flour by means of microwave (MW) processing; rice flours preconditioned at four different moisture levels (13, 16, 19, 25 %) were treated by MW radiation at 900 W and five MW treatment times (ranging from 40 s to 8 min, applied stepwise at 20-s intervals). The effects of microwaves on starch crystallinity, pasting, and thermal properties of MW-treated rice flours were also explored. The β-glucanase activity in rice flours was assessed by the rate of decrease in specific viscosity of a dilute solution of a purified β-glucan preparation, upon addition of flour extracts. MW proved to be a useful alternative for thermal inactivation of endogenous β-glucanase in rice flours when applied to moistened samples. The inactivation process followed a first-order kinetic response and the apparent rate constant of thermal inactivation increased exponentially with the moisture content of the flour, M, according to the equation 0.0146·exp (0.212· M) (R 2 = 0.97). The MW time required for complete β-glucanase inactivation was only 4 min when the initial flour moisture increased to 25 %. Following MW treatment, the starch crystallinity was unaffected (p > 0.05) and the side effects of the treatment on flour pasting and thermal properties were rather negligible.

Microwave absorption capacity of rice flour. Impact of the radiation on rice flour microstructure, thermal and viscometric properties

“…In the first 20s of treatment, the flours with 20% and 30% water content reached 46ºC and 60ºC, respectively. The higher microwave absorption capacity could explain the higher temperature as was seen in our previous work (Pérez-Quirce et al, 2016). However, between 20s and 80s both samples showed similar temperature profile probably due to the fact that the higher energy absorption was compensated with the higher specific heat that makes this flour need more absorbed heat to get a similar change in temperature.…”

“…The final long plateau of constant temperature probably corresponded to a period in which the heat absorbed by the sample (not too much as the water amount was really low) was equal to the heat lost by it towards the surrounding that increased with the temperature of the sample. In our previous work, where the sample was treated inside a hermetic container, a single and long temperature plateau was obtained below 100ºC, with water acting as 'protector' of the flour constituents, which was used to explain the low impact of the MWT in such conditions on flour functional properties (Pérez-Quirce et al, 2016), differently to what we have observed in this case.…”

The Mutual Correlation of Glucose, Starch, and Beta-Glucan Release During Microwave Heating and Antioxidant Activity of Oat Water Extracts

“…Such environment was found as highly efficient in various food industry processes (Guo et al 2017) and as medium supporting extraction (Ekezie et al 2017). In cereals, processing the microwave treatment was used for inactivation of undesired enzymatic activity like betaglucanolytic (Pérez-Quirce et al 2016) or lipolytic (Qian et al 2009), to change the breadmaking properties of gluten-free flours (Pérez-Quirce et al 2017), to assess the structural characteristic of beta-glucan (Ahmad et al 2016), or to measure of antioxidant activity of cereal fraction (Dar and Sharma 2011;Stevenson et al 2008).…”

Mejora nutricional y funcional de panes sin gluten: adición de beta-glucanos de diferentes orígenes y pesos moleculares atendiendo a las declaraciones de salud aprobadas por la EFSA

“…A particular attention was paid to achieve a uniform temperature and constant humidity during the MIWA process. The moisture contents of the flour before and after the microwave treatment were determined following the AACC 44-19 method, and the water required to adjust it to a certain value was calculated as in a previous work (Perez-Quirce et al 2016); the tempering procedure of flours before MIWA treatment was also described in the latter study.…”

“…The inactivation of flour enzymes and the use of relatively short processing time have been proposed as effective means to minimize β-glucan degradation . Some methods previously used for the β-glucanase inactivation were autoclaving, scalding, oven heating, microwave heating and ethanol refluxing Perez-Quirce et al 2016). Among these options, the process with the greatest potential on the inactivation of endogenous β-glucanase rice flour seemed to be microwave heating since it has been previously found that a treatment for just 4 min after tempering of the flour to 25% moisture content resulted in enzyme Therefore, in the present study gluten-free breads fortified with a high molecular weight β-glucan concentrate were made from rice flours pretreated by microwave (MIWA) radiation at different times and moisture levels aiming at the retention of βglucan molecular size in breads and hence at maximizing their physiological functionality.…”