Cryo-milling of starch granules leads to differential effects on molecular size and conformation

Milling of starch granules is important for many food applications and involves a combination of mechanical and thermal energy. In order to understand the effects of mechanical force alone, four commercial starches including maize starch (MS), potato starch (PS), and two high amylose maize starches (HAMS) (Gelose 50 and Gelose 80) were cryo-milled for 20 min under the same conditions. The structural and conformational changes of the starches after cryo-milling were evaluated using X-ray diffraction, NMR spectroscopy, IR and Raman spectroscopy, and size exclusion chromatography (SEC). The cryo-milled starches had less crystallinity (15–35%) and 35–50% less ordered structure (double and single helices) than the native starch counterparts. The gelatinisation temperatures of the starches were not significantly altered by cryo-milling, but the gelatinisation enthalpies were significantly reduced in line with the reductions in the amount of double helices. Although, all four starches showed similar extent of degradation of crystalline/ordered structure, SEC results showed a greater degradation of amylopectin molecule in MS and PS than in HAMS. Increased amylose content in starch seemed to reduce the molecular degradation during milling, which is consistent with a role for amylose as a mechanical plasticiser in starch granules. It is concluded that (i) cryo-milling has differential effects on molecular size and conformation depending on starch granule type, and (ii) deterioration of starch crystalline and molecular order by mechanical treatment is not necessarily linked with the reduction in molecular size. The implication from the results is that the mechanical forces acting during cryo-milling are capable of disrupting helical and crystalline structures without breaking covalent bonds of starch molecules.