Improved consistency in DNPH-mediated pyruvic acid analysis of onion juice by modifying the sample processing order

Onion pungency is commonly measured on absorbency of the wine pink color that results from adding NaOH to the heated mixture of dinitrophenylhydrazine (DNPH) and onion juice. However, significant variation exists among several modifications of the original Schwimmer and Weston (SW) method. We observed differences in pyruvic acid concentrations of 20%-30% between our automated method and a batch method with manual absorbency readings. To determine the source of the differences, we examined the heating time and waiting time of the sample-DNPH mixtures and found no differences. The differences were caused by differential color degradation between the pyruvic acid standards and onion juice samples. These differences could be minimized by reading the absorbency within 1 min of NaOH addition. Using this information, we devised the one-by-one method to control the reading time at 30 s. We compared 5 different analysis methods of 40 onion samples representing 4 onion colors. The automated, high-performance liquid chromatography, and SW methods had similar results, with only about a 5% difference. However, the batch method resulted in differences of approximately 24% as compared to the automated method. The one-by-one method produced very comparable results, within 5%, to the automated method. By modifying the procedure to ensure a uniform and fast reading time, we increased the consistency between the pungency analysis methods. Therefore, fast and uniform absorbency reading time is essential for an accurate measurement of pungency in undiluted onion juice.