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In this study of the nitrosation of morpholine in the presence of ascorbic acid, the amount of ascorbate required to inhibit completely the formation of nitrosomorpholine depended on whether oxygen was present in the system. The nitric oxide (produced during the oxidation of ascorbate by nitrous acid) might have reacted with oxygen to yield additional oxidizing equivalents, or oxygen might have directly oxidized the ascorbate semiquinone intermediate produced in the initial step of oxidation reaction. A pH- dependent induction period observed during the nitrosation of morpholine in the presence of ascorbate and excess nitrite was accounted for by the kinetics of the reactions.

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The reaction of nitrite and morpholine as a model of a nitrosation reaction has been studied in frozen model systems and in frozen milk. The rate was considerably enhanced in frozen systems compared to that expected for a supercooled solution at the same temperature. The degree of enhancement appeared to be a function of overall solute concentration, which in turn controls the amount of unfrozen solution at any temperature. Because this type of nitrosation reaction is third-order, a threefold concentration of reactants leads to a 27-fold increase in reaction rate. A second type of rate enhancement was caused by selective precipitation of sodium phosphate buffers, leading to an increase in the acidity of the solvent and a higher reactant concentration. pH changes under these conditions were determined through the use of indicator dyes. Experiments in frozen milk show the results to be applicable to foods.Many reactions continue in frozen solutions, sometimes proceeding at a faster rate than in the supercooled liquid state at the same temperature (Pincock, 1969). This does not mean the reaction in solid state proceeds faster than