EFFECTS OF PRETREATMENTS AND DRYING METHODS ON TOTAL PHENOLIC CONTENT AND INSTRUMENTAL COLOR OF YACON ROOTS (SMALLANTHUS SONCHIFOLIUS)

Abstract: Yacon (Smallanthus sonchifolius) is an Andean tuberous root recognized as a functional food due to its high fructooligosaccharide content and phenolic compounds with antioxidant properties. However, its industrial use is limited by high perishability and susceptibility to enzymatic browning. This study evaluated the effects of citric acid, potassium metabisulfite, and steam pretreatments combined with convective or microwave drying on moisture, total phenolic content, and instrumental color of yacon roots. The experiment followed a completely randomized design with three replicates. Data were subjected to analysis of variance, and means were compared using Tukey’s test (p < 0.05). Roots were sanitized in chlorinated water containing 100 ppm available chlorine for 15 min and subjected to the following treatments: untreated control; immersion in 2% citric acid for 10 min; immersion in 0.3% potassium metabisulfite for 10 min; or steam blanching for 5 min followed by cooling in an ice bath. Drying was performed in a forced-air dehydrator at 55 °C or in a microwave oven at 630 W, with periodic weighing until constant mass. Initial moisture was 87.69%, decreasing to 13.25–24.11% after drying. Citric acid-treated samples had the highest final moisture with both drying methods (p < 0.05). Total phenolic contents were expressed on a dry-matter basis. The fresh sample, used as a reference, contained 3,272.85 mg gallic acid equivalents (GAE)/100 g dry matter. Among convectively dried samples, steam pretreatment resulted in the highest total phenolic content (213.43 mg GAE/100 g dry matter), followed by potassium metabisulfite (161.49), the untreated control (133.23), and citric acid (72.50), with significant differences among treatments. Microwave-dried samples showed absorbances below the quantifiable range of the calibration curve and were excluded from the statistical analysis of phenolics. Potassium metabisulfite produced the highest lightness during convective drying (L* = 86.79), but also the highest yellowness (b* = 23.92) and total color difference. Steam resulted in the lowest color difference relative to the untreated dried control (ΔE* = 4.79 ± 0.60), although it did not increase lightness. Citric acid produced a more balanced light and yellow color, particularly after microwave drying. No pretreatment simultaneously provided the best phenolic preservation and instrumental color. Steam-assisted convective drying favored phenolic preservation, citric acid maintained a lighter appearance, and potassium metabisulfite provided intermediate phenolic preservation and the highest lightness but intensified yellowness. Thus, pretreatment selection should balance bioactive compound preservation and desired visual characteristics.

Acknowledgments: The authors thank the Research Support Foundation of the State of Rio Grande do Sul (FAPERGS) for financial support through a research scholarship and the Federal Institute of Education, Science and Technology Farroupilha (IFFar) for providing the infrastructure and materials required for this study.