Correlation Between Methods to Determine the Protein Content of Potato Tubers1

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Abstract
Tubers of potato (Solarium tuberosum L.) ranging from 6.9 to 18.6% total amino acids, on a dry weight basis, were analyzed by 6 methods which determine protein content: 1) total amino acids minus free amino acids determined by column chromatography, 2) Kjeldahl N × 5.7, 3) Kjeldahl N × 5.7 minus free amino acids determined by column chromatography, 4) Kjeldahl N × 5.7 minus ninhydrin determined free amino acids, 5) ninhydrin total minus ninhydrin free amino acids, and 6) Folin-Lowry analyses. The results of the latter 5 methods were correlated to the data of method 1 (column chromatography). All of the methods, except 4, had correlation coefficients above the 99% level of significance. When compared to the data obtained from column chromatography, methods 6 and 4 gave low, while method 3 gave approximately equal protein values. Method 2 is excellent for determining total tuber amino acids, (r = 0.96**) and is the method of choice for screening large numbers of tubers for protein, (r = 0.92**). Kjeldahl N × 5.7 correlates high with tuber protein, because there is a high correlation between tuber free and protein bound amino acids, (r = 0.74**). However, Kjeldahl N × 5.7 gives high tuber protein values. Based upon their correlation coefficients, methods 2, 5 and 6 are applicable for screening, but their individual shortcomings must be considered when interpreting results from tubers.

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