Evaluation of liquid nitrogen freeze drying and ethanol dehydration as methods to preserve partially cooked starch and masa systems
2005
Yglesias, R. | Jackson, D.S.
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 degrees C), at the top of the pasting peak (gelatinized granules, 95 degrees C), at the bottom of the trough (dispersed polymers, 95 degrees C), and a completed RVA sample stored for 120 hr at 4 degrees 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 (degrees 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.
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