Determination of glycoalkaloids, phenolic acids and protease inhibitors in selected cultivated potato (solanum tuberosum l.) varieties

Abstract

Potato (Solanum tuberosum L.) synthesizes a variety of potentially toxic and antinutritional compounds during growth and post harvest as a defense mechanism against pathogens, insects and other pests. As part of a program to improve its safety and nutritional quality, the concentration of glycoalkaloids (GAs), phenolic acids (PAs) and protease inhibitors (PIs) in the leaves and tubers of five field-grown potato varieties; Tigoni, Asante, Kenya Karibu, Desiree and Dutch Robijn were determined at 40, 55, 95 and 125 days after planting. Glycoalkaloid concentration was determined by means of high performance liquid chromatography (HPLC). Methanolic and ethanolic extracts of total phenolics (TP) and chlorogenic acid (CGA) were quantified using Folin-Ciocalteu’s method and UV spectrophotometry, respectively. The inhibitory activities of potato Tris-HCl extracts were used to determine the concentration of PIs. The recovery values were high (85.9-93.5%) indicating the validity of the extraction methods. Foliar total glycoalkaloids (TGA) significantly (p<0.001) varied from 60.61 to 88.71 mg/100g fresh weight (Fwt) in vars. Dutch Robijn and Tigoni, respectively, with tuber values of 6.80 and 10.56 mg/100g Fwt, respectively which displayed a significant (p<0.001) reduction from the time of initiation to maturity. The foliar CGA content in the tested varieties significantly differed at p=0.05 and was highest (252.93 mg/100g Fwt) and lowest (244.02 mg/100g Fwt) in vars. Asante and Desiree, respectively. Tuber CGA content in the extracts was significantly (p<0.001) different. The vars. Ti and Dutch Robijn recorded the highest and lowest average tuber CGA concentrations of 58.04 mg/100g Fwt and 46.39 mg/100g Fwt, respectively. The highest TP contents of 603.4 mg CGA/g Fwt and 192.5 mg CGA/g Fwt were observed in the leaf and tubers of vars. Kenya Karibu and Tigoni, respectively, and the lowest amounts were detected in vars. Asante (435.9) and Desiree (127.1), respectively. Phenolic contents in the test varieties decreased significantly (p<0.05) during growth. The protease inhibitors (PIs) content (Units/mg) in the foliar and tuber extracts significantly (p<0.001) differed among the potato varieties. The chymotrypsin inhibitor (CI) and trypsin inhibitor (TI) content varied from 257.49 to 912.71 Units/mg and from 877.30 to 1646.56 Units/mg, respectively, with var. Desiree recording the highest levels. The foliar CI content was highest at 40 days after planting and declined with maturity, but peaked at maturity in tubers. These results demonstrate that the concentration of GAs, phenolic acids (PAs) and PIs was variety dependent and was strongly influenced by the stage of growth and growing season with higher levels in leaves than tubers. Light induced the synthesis of glycoalkaloids (GAs) and phenolic acids but not of protease inhibitors. The synthesis of total glycoalkaloids and total phenolics was higher in tubers that were stored under fluorescent light (FL) as compared to sunlight. The exposure to FL stimulated the synthesis of GAs to potentially toxic levels in var. Tigoni suggesting that the period between harvest and consumption of potatoes should be minimized. Routine monitoring of GAs, PAs and PIs in tubers of potato varieties at different growth stages in the field and during storage will guarantee the consumers of eating safe and good-quality diet.