Energy-size relationship and starch modification in planetary ball milling of quinoa

Abstract

Quinoa flours were obtained using a planetary ball mill (PBM). The effects of speed (250-450 rpm) and time (10-50 min) on milling energy (E), particle size distribution (PSD), crystallinity degree (CD), damaged starch (DS), microstructure, hydration and pasting properties of flours were investigated based on a Doehlert experimental design. Traditional milled flour was adopted as control. Ball milled flours showed polydispersed PSD with a shift towards smaller size and greater dispersion (Span: 3.0 - 5.2) in comparison with control. At 350 rpm-30 min (central point) a main peak (417 um) and lower peaks at 40 um and 2 um were observed. SEM images of modified flours showed particles with rounded edges and polished surfaces in contrast with control. The relationship E - D50 were satisfactorily predicted by the Walker´s equation. A significant correlation between DS and CD was found (r= -0.83, p<0.01). The hydration tests revealed a complex flour behavior as function of temperature and milling conditions due to differences in PSD, composition, and DS content. The increasing milling energy caused a decrease in peak viscosity (PV), trough viscosity (TV) and final viscosity (FV) of up to 36%, 29%, and 25%, respectively. In addition, the significant and positive correlations between viscosities and D50 (PV-D50, r = 0.92, p<0.01; TV-D50, r = 0.94, p<0.01; FV-D50, r = 0.90, p<0.01) denoted the increase of starch degradation as milling severity rises. These results can be used to improve the manufacture and the selection criteria of quinoa flour with specific functional attributes.