Abstract
Methanol occurs naturally in most alcoholic distillates. Yet, suitable detectors to check liquor adherence to legal limits and, most importantly, monitor in situ methanol content during distillation are not available. Usually, distillers rely on error-prone human olfaction while “gold standard” liquid or gas chromatography (GC) are rarely used being off-line, time-consuming, and expensive. Here, we explore monitoring the methanol concentration during industrial distillation of cherry, apple, plum, and herb liquor (196 samples) with a low-cost and hand-held detector combining a Pd-doped SnO2 sensor with a packed bed separation column of Tenax TA. Therein, individual methanol concentrations (0.1–1.25 vol % or 153–3266 g methanol per hectoliter of pure ethanol) are quantified rapidly (within 2 min), bias-free and with high precision (i.e., 0.082 vol %) by headspace analysis, as confirmed by GC. Most importantly, methanol levels above E.U. and U.S. legal limits were recognized reliably without interference by much higher ethanol contents (5–90 vol %) and aromas. Also, the detector worked well even with viscous and inhomogeneous mash samples containing fruit pulp. As a result, this device can help consumers, legal authorities, and distillers to check product safety, guide distillation, and monitor even fermentation to possibly prevent occupational methanol exposure.
Highlights
Methanol originates naturally from the degradation of pectins during fermentation (Figure 1).[1]
Thereby, apples, pears, plums, and oranges are prone to high methanol concentrations, in contrast to spirits from wheats, roots or molasses such as whisky, vodka, or rum.[2]
Lower ones apply for brandy (i.e., 200 g/hL), vodka from agricultural alcohol (30 g/hL), and London gin (5 g/hL)
Summary
Methanol originates naturally from the degradation of pectins during fermentation (Figure 1).[1]. Chemoresistive Ag-LaFeO3,18 Pt/WN particles,[19] or electrochemical cells[20] had been proposed These feature rather high detection limits (e.g., 4 vol % for fluorescence sensors),[21] cannot distinguish methanol from ethanol (chemoresistive sensors),[18,19] or require bulky instrumentation (e.g., photoluminescence spectrometer).[15,17] none has been validated on the different fractions of distillation, where high volatility compounds in the “head” might interfere with the sensors. The portable device combines a separation column with a chemoresistive gas sensor.[22] It has quantified methanol selectively in artificially spiked exhaled human breath[23] for medical diagnostics,[24] as well as in the headspace of liquors[25] and hand sanitizers.[26] Here, naturally occurring methanol at challenging contents (0.1−1.25 vol %) from all stages of distillation of cherry, plum, apple, and herb spirits are traced and compared to GC measurements. Adherence to U.S and E.U. limits is assessed by simultaneous detection of ethanol
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