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Tissue disruption caused by freezing and thawing may contribute to rapid enzymatic breakdown of dhurrin [p-hydroxy-(S)mandelonitrile-β-D-glucoside], the cyanogenic glucoside of sorghum [Sorghum bicolor (L.) Moench], but published reports are not in agreement as to the effects of freezing on the hydrocyanic acid potential (HCN-p) of field-grown sorghum leaves. These effects were investigated in a series of experiments in the fan of 1983, a period during which weather conditions at Lincoln, NE, permitted repeated sampling of frozen and non-frozen leaves of field-grown sorghum tillers. Assays of these samples, both spectrophotometrically and with the cyanide electrode, indicated that the HCN-p of KS8 and N32 tiller leaves decreased after the tissue was frozen, but potentially dangerous levels remained in N32 tillers for at least 6 or 7 days after the leaves were frozen. Levels of HCN-p in KS8 tiller leaves were much lower than those in N32 leaves. p-Hydroxybenzoic acid, identified by its ultraviolet absorbance spectrum and chromatographic behavior, was found in extracts of frozen N32 tillers. Freezing and thawing of the tillers evidently led to partial breakdown of dhurrin to p-hydroxybenzaldehyde and subsequent oxidation of this aldehyde to p-hydroxybenzoic acid.