Determination and source tracing of toluene in spices and spice extracts: focus on thermal conversion of α-pinene to toluene during processing.

This study investigated the presence and origins of toluene in spices and their extracts. A headspace gas chromatography-mass spectrometry method quantified six benzene series compounds, with limits of detection of 0.10 mg/kg, limits of quantification of 0.30 mg/kg, and recoveries of 72.2-109.4% (RSD <10%). Toluene was the major contaminant in multiple spice extracts, reaching up to 109.53 mg/kg. Traceability analysis indicated contamination primarily from thermal processing, with a small proportion from raw materials. Specifically, extraction temperatures exceeding 90 °C triggered a sharp increase in toluene levels in ginger and black pepper essential oils. Controlled heating experiments with pure standards confirmed that α-pinene undergoes thermal conversion to toluene at elevated temperatures. Finally, vacuum treatment removed over 99.50% of residual toluene from the essential oils and was proven highly effective. These findings provide critical mechanistic insights and practical mitigation strategies for quality control in the botanical extract industry.