Pre-treatments modulate drying characteristics, moisture removals, pigments and browning substrates, flavor, and microstructure in hot air dried daylily (Hemerocallis citrina).

INTRODUCTION: Hot air drying (HAD) of daylilies frequently results in prolonged drying. Severe browning, pigment degradation, and unacceptable colchicine retention, compromising both safety and market value. Selecting an appropriate pretreatment is key to mitigating these quality losses while enhancing drying efficiency.

METHODS: The study systematically compared five pretreatments-hot water blanching (HWB), steam blanching (SB), immersion in color-preserving solution (CP), immersion in alkaline solution for reducing toxicity (RT), and hot water blanching combined with ultrasonic treatment (UT)-in terms of drying characteristics, moisture removal, color, bioactive components, safety, microstructure, and flavor.

RESULTS: UT and CP achieved the shortest drying time (5 h), vs. 6 h for HWB and SB, and 8h for RT and CK. NMR revealed that UT accelerated the removal of free water, accounting for its fastest drying rate. CP exhibited multifunctional superiority, which preserved bright color, chlorophyll and carotenoids, while also inhibiting browning, reducing 5-hydroxymethylfurfuralformation, and retaining polyphenols. In addition, all five pretreatments can significantly reduce the colchicine content. UT preserved microstructure but caused excessive chlorophyll retention, resulting in an undesirable greenish hue that deviates from the commercial golden-yellow standard.

DISCUSSION: Considering overall quality, safety, and market acceptability, CP is recommended as the optimal pretreatment for industrial HAD of daylily, whereas UT should be applied only with additional de-greening measures. These findings provided a scientific basis for selecting pretreatment strategies that balance drying efficiency with final product quality.