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Effects of storage temperature and time on the quality of commercially canned grapefruit juice were investigated. The most noteworthy and reproducible trend observed by HPLC was a significant increase in concentration of 5-hydro~methylfurfural and furfural with increased storage temperature. During storage over 50% of sucrose was hydrolyzed after 6 wk at 3O"C, and the reducing sugars increased in accordance with hydrolysis of sucrose. In the juice stored at 5O"C, more than 98% of the ascorbic acid, and about 9% of total sugars were lost. Lightness value of juice decreased continuously during storage.

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Model systems were employed to determine the relative reactivity of sucrose, glucose, and fructose in the formation of 5‐hydroxymethylfurfural (5‐HMF) at pH 3.5. Fructose was the major reactant for formation of 5‐HMF. Fructose was 31.2 times faster than glucose, whereas sucrose was 18.5 times faster than glucose in the rate of 5‐HMF formation when averaged over three different sugar‐catalyst systems. Accelerating effects of citric acid, minerals (calcium, magnesium, and potassium), and amino acids (alanine, aspartic acid, and γ‐aminobutyric acid) in the formation of 5‐HMF from sugars were evaluated. With fructose as the substrate, 5‐HMF formed 5 times faster in the presence of citric acid and minerals than in the presence of HCl. Varying catalytic effects were noted with the three amino acids. Rates of 5‐HMF formation from glucose and sucrose showed slight enhancement in the presence of the amino acids, whereas virtually no enhancement occurred when fructose was the substrate.