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

Abstract

ABSTRACTPreservation 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.