Molecular changes during deep-frying of potato and wheat starch determined by temperature-controlled time domain 1H NMR are linked to the microstructure

Molecular changes during deep-frying of potato and wheat starch determined by temperature-controlled time domain 1H NMR are linked to the micro structural changes evaluated by 4D X-ray micro computed tomography

Isabella. M. Riley1*, Ujjwal Verma2, Pieter Verboven2, Bart Nicolai2, Jan A. Delcour1

1Laboratory of Food Chemistry and Biochemistry and Leuven Food Science and NutritionResearch Centre (LFoRCe), KU Leuven, Leuven, Belgium;2Division BIOSYST-MeBioS, KU Leuven, Leuven, Belgium

*isabellamaria.riley@kuleuven

 

Starch exhibits various functional properties in many food systems including its gelling,

texturizing, and thickening capacity. When it is heated in sufficient water, starch granules

lose their molecular order and gelatinize, which is accompanied by starch swelling, leaching

of amylose, and melting of amylopectin crystals. During deep-frying of starch-based

products, the starch constituent undergoes physical transformations (e.g., starch

gelatinization) that contribute to the structural properties (e.g., expansion/collapse, pore

formation, oil absorption) of the resultant food. However, the transformations of starch as

related to its interactions with water and microstructure development during deep-frying

remain largely unexplored. Moreover, when studied earlier, analyses were performed after

deep-frying and cooling wherein additional physical changes had potentially occurred (e.g.,

amylose crystallization). The goal of this work was to characterize the physical

transformations of potato and wheat starch during simulated deep-frying conditions by

utilizing temperature-controlled time domain (TD) 1H NMR. These starches were chosen

because of differences in their granule sizes and chemical structures. Measurements were

performed on a 0.47 T (20 MHz for 1H) Bruker Minispec mq20 with a variable temperature

probe accessory and Bruker BVT3000 tempering unit. The probe head temperature was set to

175 °C and free induction decay (FID) and Carr-Purcell-Meiboom-Gill (CPMG) pulse

sequences were applied every 12 and 27 s, respectively, for approx. 180 s. Complimentary

analyses (ex-situ) assessing starch swelling properties, crystallinity [differential scanning

calorimetry (DSC)], and morphology [scanning electron microscopy (SEM)] were performed.

The changes in proton distributions, as determined by TD 1H NMR, could be linked to the

microstructural changes during deep-frying, evaluated by 4D (time-resolved) X-ray

microcomputed tomography (European Synchrotron Radiation Facility). Temperature-

controlled TD 1H NMR provided evidence for the timings of starch swelling, gelatinization,

and the glass transition during deep-frying which related to the microstructural changes in

both starches. This work demonstrates that variable temperature TD 1H NMR is a valuable

technique to assess changes in starch structure during deep-frying.

Acknowledgments: The authors sincerely thank the Research Foundation – Flanders for

financial support for this project (grant G090319N). The European Synchrotron Radiation

Facility (ESRF) is thanked for providing beamtime at beamline ID19 under the proposal ‘LS-

3126’.