Portable Spatially Offset Raman Spectroscopy for Rapid Detection of Methanol and Ethanol in Pisco Through Sealed Containers

In this study, we introduced a portable spatially offset Raman spectroscopy (SORS) system that permits rapid, non-destructive acquisition of Raman spectra from bottled Pisco spirits, providing a practical and non-invasive solution for in situ quality control. Pisco, a Peruvian distilled spirit, was selected as a case study because methanol occurs naturally during fermentation and distillation, and the product is susceptible to adulteration. Methanol and ethanol levels in Pisco were determined using gas chromatography (GC–FID). Methanol levels in 94 authentic Pisco samples ranged from 7.4 to 67 mg/100 mL, remaining below the regulatory limits established for fruit brandies. For pure Pisco samples, the handheld SORS device demonstrated strong predictive performance for determining methanol (SEP = 0.003%, Rpre = 0.92) and ethanol (SEP = 1.25%, Rpre = 0.98) content. To further assess model applicability across a broader methanol concentration range, randomly selected Pisco samples were fortified with methanol (0.11–9.85%), resulting in a prediction model with excellent performance for methanol quantification (SEP = 0.17%; Rpre = 0.995). Overall, the SORS-based approach showed robust analytical capability, underscoring its potential as a non-contact, non-destructive technique for rapid quantification of methanol and ethanol in sealed glass containers.