Citral encapsulation in plasmolyzed yeast cells increases loading and reduces initial burst release while extending observed repellency against Haemaphysalis longicornis.

BACKGROUND

Citral repels Haemaphysalis longicornis nymphs, but its volatility limits persistence. This study compared citral-loaded Saccharomyces cerevisiae formulations prepared with or without plasmolysis-associated pretreatment for recoverable citral-equivalent content, empirical release behavior, and repellency.

RESULTS

Yeast was incubated in 10% (w/v) NaCl at 50 °C for 48 h and heated at 80-85 °C for 20 min before citral loading. Plasmolyzed microcapsules (P-C) contained more UV-recoverable citral-equivalent material than non-plasmolyzed microcapsules (NP-C) (35.5 ± 3.3% versus 24.3 ± 2.4%; n = 3, P < 0.05). Confocal images and line scans showed stronger, more uniform cell-associated Nile red fluorescence in P-C, but localization was qualitative and did not distinguish intracellular from surface-associated material. Fourier transform infrared spectra retained citral-related bands, although matrix overlap precluded compound-specific confirmation and exclusion of minor oxidized species. In 30:70 v/v water/ethanol, NP-C was better fitted by a burst-adjusted first-order model, whereas P-C was better fitted by a modified Higuchi-type model; therefore, no common or diffusion-controlled mechanism was established. The modified Higuchi apparent burst parameter was lower for P-C than for NP-C (0.119 versus 0.324). In silicone membrane choice assays, pure citral was significantly repellent at 3 h but not at 4 or 5 h, whereas P-C remained significantly repellent at 5 h.

CONCLUSION

P-C showed higher recoverable citral-equivalent content, more uniform cell-associated fluorescence, lower apparent early release, and significant 5-h repellency. However, activity beyond 5 h, residual citral at 5 h, vapor-phase exposure, and compound-specific integrity were not determined. Interpretation was conservative because UV detection was not chromatographically validated for specificity. © 2026 Society of Chemical Industry.