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Evaluation of Liquid Nitrogen Freeze Drying and Ethanol Dehydration as Methods to Preserve Partially Cooked Starch and Masa Systems¹

¹A contribution of the University of Nebraska Agriculture Research Division, Lincoln, NE 68583. Journal Series No. 14870. ²Former graduate research assistant and professor, respectively, Department of Food Science and Technology, University of Nebraska, Lincoln, NE 68583-0919. ³Corresponding author. Fax: 402-472-1693. E-mail: djackson@unlnotes.unl.edu

Preservation of starch structure/properties, including structures formed during partial or complete cooking, are important when the impact of processing conditions is being studied. Two preservation techniques used to study changes in starch during thermal-mechanical processing are commonly cited in the literature: 1) rapid freezing followed by lyophilization, and 2) a dehydration procedure using alcohols. A comparative determination on how these methods affect various starch structures has not been widely reported. Corn starch samples were collected from the Rapid Visco-Analyser (RVA) at 3 min (swollen granules, 30°C), at the top of the pasting peak (gelatinized granules, 95°C), at the bottom of the trough (dispersed polymers, 95°C), and a completed RVA sample stored for 120 hr at 4°C (retrograded starch). Samples of masa were obtained by nixtamalizing corn. Differential scanning calorimetry (DSC) endotherms of starch and masa, and X-ray diffraction (XRD) patterns of masa were evaluated after being preserved by alcohol- or freeze-drying. No significant differences (P > 0.05) between methods were found for onset, end, and peak temperatures (°C), enthalpy (J/g) and % relative crystallinity in any of the samples analyzed. Liquid nitrogen freeze-drying and ethanol dehydration are both effective methods of preserving various starch systems for structural changes detectible by DSC and XRD; freeze-drying is generally less expensive and time-consuming.