Extracting the aroma of Sichuan pepper begins before its peel enters the still. If the raw material is first treated with another liquid, how do the amount and composition of the recovered essential oil change?
A study published by Yadong Zhu and colleagues in 2025 compared pretreatment with five mixed solvents using dried peel of Zanthoxylum bungeanum, a Sichuan pepper used as a spice in China. Beyond efficient oil extraction, the researchers aimed to investigate components with potential value in food preservation and their effects on bacteria.
Treating the peel with five liquids before distillation
The researchers used peel from the cultivar “Fengjiao,” harvested in Shaanxi Province, China. The peel was dried and ground, then pretreated before water was added for three hours of distillation. Samples without mixed-solvent pretreatment served as the comparison.
The pretreatment solvents are known as NADES. They combine several components, using interactions between them to form a liquid solvent. This study used five combinations of choline chloride with either lactic acid, malic acid, citric acid, glycerol, or glucose. The component mixing ratio was held at a single setting.
Such solvents are expected to help release components from plants. However, calling a solvent “naturally derived” does not by itself establish the safety of the resulting essential oil or its uses.
The lactic-acid treatment gave the highest yield
Without pretreatment, the recovered essential oil amounted to 4.67% of the dry peel weight. Treatment with the lactic-acid combination raised this to 6.42%, the highest yield among the conditions tested. The malic-acid treatment also increased yield, to 5.63%.
By contrast, the citric-acid combination gave a yield of 3.81%, lower than without pretreatment. Even within this class of mixed solvents, the outcome depended on which components were combined. Adding a pretreatment did not invariably increase the amount of oil.
The authors suggest that acidity, effects on plant tissue, and interactions between the solvent and aroma compounds influence release and recovery. Their interpretation calls for considering both effects that make compounds easier to extract and effects that retain or transform them in the solvent.
Component proportions changed too
GC–MS analysis showed that the oil obtained without pretreatment was rich in components including limonene, linalool, and linalyl acetate. The relative proportion of α-terpineol was higher with the three acid-containing treatments.
For example, α-terpineol accounted for about 5.8% of the measured signal area without pretreatment and about 15.0% with the citric-acid treatment. This is a proportion among the analyzed components, not a direct measure of the absolute amount of α-terpineol recovered from the peel.
The authors suggest that acidic conditions may stabilize some compounds or promote conversion of linalool into α-terpineol. However, they did not use a label to track particular molecules as they changed into others. What the study directly demonstrated was a difference in the composition of the oils recovered after treatment.
The highest-yielding oil was not the most active against bacteria
The researchers next tested each oil on plates carrying cultures of Escherichia coli and Staphylococcus aureus. They measured the diameter of any zone around the oil in which bacteria did not grow.
The largest zones in this test were produced by oil from the citric-acid pretreatment. This differed from the lactic-acid treatment, which gave the most oil. The preferred pretreatment therefore depends on whether the priority is yield, particular components, or activity in a culture assay.
Because samples with a higher proportion of α-terpineol produced larger growth-inhibition zones, the researchers focused on this compound. They also used commercially purchased α-terpineol reagent to test its activity on its own. The tests on the essential oils and those on the purchased compound were conducted separately.
Tests of the isolated component showed changes in bacterial membranes
Adding α-terpineol inhibited growth in broth cultures of both bacteria. Under these test conditions, the minimum inhibitory concentrations were 0.70 milligrams per milliliter for S. aureus and 1.41 milligrams per milliliter for E. coli.
Electron microscopy revealed wrinkling and damage on cell surfaces, while other measurements indicated leakage of intracellular components. Fluorescence signals used to examine the electrical state of the cell membrane also changed. The authors interpret disruption of membrane structure and function as contributing to growth inhibition.
Tests of DNA extracted from treated bacteria also showed changes, including fainter bands on gels, which the authors regard as evidence of DNA damage. This alone does not identify a direct molecular target. In addition, monitoring broth turbidity in a growth assay differs from counting viable bacteria to confirm complete killing.
The study did not remove only α-terpineol from the essential oil and then add it back to compare activity. Activity of the component alone does not prove that this single compound explains all of the oil’s effects.
Summary
Pretreatment before distillation changed both the amount and combination of components in Sichuan pepper essential oil. In this study, the lactic-acid treatment gave the highest yield, whereas the citric-acid treatment produced the largest zones of bacterial growth inhibition in culture. The authors conclude that choosing pretreatment according to desired components and applications can help improve extraction methods.
Application to food preservation remains a further goal. These tests did not investigate whether shelf life could be extended in real foods, how taste and aroma would change, or whether the concentrations people would consume would be safe. Both extraction and use conditions require further investigation, including the solvent mixing ratio held fixed in this study.
References
Zhu, Y., Zhang, C., Liu, J., Xue, Z., Zhang, B., & Zhao, H. (2025). Proper NADES treatment improved Zanthoxylum bungeanum essential oil yield and boosted α-terpineol antibacterial efficacy. npj Science of Food, 9, 73. DOI: 10.1038/s41538-025-00437-4 / Full text.
The research received partial support from a science and technology program on sustainable development in Chengde, China. The authors declared no competing interests. Original article: © 2025 The Author(s), CC BY-NC-ND 4.0.
