Pisco is technically a brandy that originated in Peru, but made its name in the United States in the days of the California Gold Rush, brought by traders from Peru and immortalized by the likes of Mark Twain and Rudyard Kipling. In recent years it has enjoyed a reprise in the form of artisan cocktails. In Peru, however, pisco is much more. It is the national drink, a key export, and is important enough to Peruvian culture to merit its own holiday each July.
Pisco may be distilled from a single aromatic grape varietal or it may be a mixture. These blends, called pisco acholado, make use of the more common Quebranta grape and are made from closely guarded recipes, much like the blended whiskies of Scotland. Each Pisco carries its own Protected Denomination of Origin (PDO), which lays claim to the grape(s) and purity of its composition. Since assessment of the authenticity of PDO claims must be performed by tasting experts, pisco is very prone to adulteration, particularly for the more expensive pure aromatics. Inexpensive blends may be passed off as pure, or a less prestigious grape may be substituted. No analytical or reference method has been established to validate the grape varietal, leaving a leaving a gap in efforts to deter counterfeiting.
A group at The Ohio State University is working to close this gap for Pisco by using 1064 nm Raman spectroscopy to distinguish between pure varieties and mixtures of Pisco, and even identifying the specific grape varietal. In addition to offering superior discrimination over UV-VIS, Raman also allowed quantification of both ethanol and methanol content in pisco, making it a well-rounded tool for quality assessment of spirits.
How validation was performed
Spectroscopy has become an indispensable tool for testing both quality and authenticity of foods and beverages. PhD student Ahmed Menevseoglu at The Ohio State University tested the applicability of UV-VIS and Raman spectroscopy to validating the PDO of pisco. The study included three grape varietals:
- Aromatic Italia (n=9)
- Torontel (n=9)
- Non-aromatic Quebranta (n=16)
Pisco samples originating from the Ica and Lima regions of Peru were collected for each grape, courtesy of the Universidad Nacional Agraria in Lima, Peru and several local markets. Samples decanted into cuvette were measured using direct UV-VIS transmittance on a benchtop dual-beam spectrometer (200-400 nm), followed by Raman spectroscopy with 1064 nm excitation using a Wasatch Photonics integrated Raman system with quick-fit cuvette holder (WP 1064-L).
The Raman system reported spectra for 250-1850 cm-1 with 11 cm-1 resolution (50 Âµm slit), and measurements were made at 2000 ms integration time with 3 averages. â€We tried 830 nm Raman first,â€ says Ahmed, â€but it didn´t give good results. We changed to 1064 nm and optimized the probe distance and how we took the background â€" it made a big difference in the results.â€
Spectra were then analyzed using pattern recognition by Soft Independent Modeling of Class Analogies (SIMCA) in an effort to classify the pisco samples, with a focus on the 990-1530 cm-1 spectral region for Raman.
Each method â€" Raman and UV-VIS â€" was able to accurately discriminate aromatic from non-aromatic varieties, but Raman provided the clearest discrimination between aromatics, non-aromatics, and mixtures by far. Raman was also the only method to provide clear discrimination between the two aromatic varietals, allowing pisco made from Torontel grapes to be identified versus Italia grapes.
Quantifying ethanol & methanol content
As a second phase, the study explored the ability of Raman spectroscopy to determine ethanol and methanol content in Pisco. The PDO requirements on pisco mandate 38-48% alcohol content, which could tempt less scrupulous producers or suppliers to supplement weaker products with methanol. While low levels of methanol are present naturally, and even allowed up to 150 ppm, higher levels can cause a variety of serious symptoms including vomiting, decreased consciousness, and loss of sight.
Rapid, direct analytical measurement of both ethanol and methanol in the field would allow authorities to ensure both product authenticity and safety for consumers. A Partial Least Squares Regression (PLSR) model was developed to predict the ethanol and methanol content based on spectral differences within the sample set and levels measured using a reference GC method, and was found to have excellent correlation with the known contents. Correlation of the predicted alcohol content was very good with the measured values, showing excellent promise for prediction of ethanol content from 44% to 24% and methanol from 0.5% to not detectable using 1064 nm Raman spectroscopy.
Protecting a unique spirit
Raman spectroscopy is a powerful tool for the authentication and analysis of foods and beverages, and its performance for authenticating protected denomination of origin (PDO) of Pisco is no exception. By using the optimum excitation wavelength for this material (1064 nm), Raman spectroscopy was able to rapidly and effectively discriminate between grape varietals in Pisco at all levels: aromatic vs non-aromatic, and even the specific grape varietal. Furthermore, Raman spectroscopy demonstrated excellent predictive ability for the measurement of ethanol content for regulatory purposes, and of methanol content for protection of consumer safety. These capabilities combined make it a very promising tool for rapid authentication of pisco in the field for the protection of this unique spirit, as well as for those who enjoy it.