| Summary: | Phenolic acids including chlorogenic acids are major polyphenolic compounds found in Jerusalem artichoke (<i>Helianthus tuberosus</i> L.). The plant itself is an emerging biorefinery crop due to the inulin-rich tubers, a bioethanol feedstock, but the aerial parts represent a rich source of bioactive compounds. We have determined the level of major phenolic acids in extracts of four plant organs: tuber, leaf, flower, and stem. Employing three heating conditions (20 °C, 60 °C, and microwaving), corrected total phenolic content (TPC) was highest in the leaves (4.5−5.7 mg gallic acid equivalents g<sup>−1</sup> dry substance), followed by flower (2.1−2.9), tuber (0.9−1.4), and lowest in stem extracts (0.1−0.2). A previously overlooked interference of the Folin−Ciocalteu assay, namely a signal contribution from ascorbic acid, caused overestimation of TPC in various organs ranging from 65% to 94%. Radical scavenging activity of extracts correlated significantly with TPC, both on corrected (R<sup>2</sup> = 0.841) and uncorrected (R<sup>2</sup> = 0.884) values. Out of the identified phenolic acids determined by quantitative HPLC-UV analysis, chlorogenic and dicaffeoylquinic acids accounted for 72−82% of corrected TPC in leaf and tuber extracts. Optimization of leaf extraction was tested in a 2<sup>3</sup>-factorial Central Composite Face (CCF) design. Temperature was the most important model term, and a solvent strength of less than 50% ethanol promoted the highest TPC yields. Further developments in extraction processing of crop residues may open avenues for improving the utilization of Jerusalem artichoke in valuable products.
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