Mechanically and Thermally Induced Degradation and Modification of Cereal Biopolymers during Grinding

Sabina Paulik,Mario Jekle,Thomas Becker

doi:10.3390/polym11030448

Sabina Paulik, Mario Jekle + Show 1 more

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https://doi.org/10.3390/polym11030448

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Journal: Polymers	Publication Date: Mar 8, 2019
Citations: 9	License type: CC BY 4.0

Affiliation: Technical University of Munich

Abstract

It is presumed that structural and functional alterations of biopolymers, which occur during grinding, are caused by a mechanical modification of polymers. As a result, thermally induced changes of flours are neglected. In this study, the impact of thermo-mechanical stress (TMS), as occurring during general grinding procedures, was further differentiated into thermal stress (TS) and mechanical stress (MS). For TS, native wheat flour, as well as the purified polymers of wheat—starch and gluten—were heated without water addition up to 110 °C. Isolated MS was applied in a temperature-controlled ultra-centrifugal grinder (UCG), whereby thermal and mechanical treatment (TMS) was simultaneously performed in a non-cooled UCG. TS starch (110 °C) and reference starch did not show differences in starch modification degree (2.53 ± 0.24 g/100 g and 2.73 ± 0.15 g/100 g, AACC 76-31), gelatinization onset (52.44 ± 0.14 °C and 52.73 ± 0.27 °C, differential scanning calorimetry (DSC)) and hydration properties (68.9 ± 0.8% dm and 75.8 ± 3.0%, AACC 56-11), respectively. However, TS led to an elevated gelatinization onset and a rise of water absorption of flours (Z-kneader) affecting the processing of cereal-based dough. No differences were visible between MS and TMS up to 18,000 rpm regarding hydration properties (65.0 ± 2.0% dm and 66.5 ± 0.3% dm, respectively). Consequently, mechanical forces are the main factor controlling the structural modification and functional properties of flours during grinding.

Highlights

Wheat flours display unique interactions of water, gluten proteins and starch fractions during dough preparation and bread making, which enable the production of porous foods such as white bread and cakes [1]
Particle size distribution (PSD) of flours provides important knowledge to predict the functional behavior of modified flours
An isolated thermal treatment of wheat flour or pure starch did not lead to alterations of the mean volume particle size of the samples

Summary

Introduction

Wheat flours display unique interactions of water, gluten proteins and starch fractions during dough preparation and bread making, which enable the production of porous foods such as white bread and cakes [1]. Wheat flour quality is often described by the degree of starch damage (hereinafter called the starch modification degree (SMD)) [2,3,4]. The term starch damage comprises poorly defined structural as well as functional modifications of flours, such as the water retention capacity [5], that occur during grinding. (Post)-Grinding or extrusion of cereals resulted in enhanced hydration properties of physically modified flours, which can lead to desired higher bread weight [6], and to an adverse decrease in loaf volume [7,8] by creating altered dough properties [9]. A detailed elucidation of underlying mechanisms has not been completed yet, since different types of physical forces take place simultaneously during the modification process and affect biopolymer modification on the molecular, nanoscopic and microscopic scale.

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