Lachenmeier Dirk W, Godelmann Rolf, Steiner Markus, Ansay Bob, Weigel Jürgen, Krieg Gunther
Chemisches und Veterinäruntersuchungsamt (CVUA) Karlsruhe, Karlsruhe, Germany.
Chem Cent J. 2010 Mar 23;4:5. doi: 10.1186/1752-153X-4-5.
Ever since Gay-Lussac's time, the alcoholic strength by volume (% vol) has been determined by using densimetric measurements. The typical reference procedure involves distillation followed by pycnometry, which is comparably labour-intensive and therefore expensive. At present, infrared (IR) spectroscopy in combination with multivariate regression is widely applied as a screening procedure, which allows one to determine alcoholic strength in less than 2 min without any sample preparation. The disadvantage is the relatively large investment for Fourier transform (FT) IR or near-IR instruments, and the need for matrix-dependent calibration. In this study, we apply a much simpler device consisting of a patented multiple-beam infrared sensor in combination with a flow-through cell for automated alcohol analysis, which is available in a portable version that allows for on-site measurements.
During method validation, the precision of the infrared sensor was found to be equal to or better than densimetric or FTIR methods. For example, the average repeatability, as determined in 6 different wine samples, was 0.05% vol and the relative standard deviation was below 0.2%. Accuracy was ensured by analyzing 260 different alcoholic beverages in comparison to densimetric or FTIR results. The correlation was linear over the entire range from alcohol-free beers up to high-proof spirits, and the results were in substantial agreement (R = 0.99981, p < 0.0001, RMSE = 0.279% vol). The applicability of the device was further proven for the analysis of wines during fermentation, and for the determination of unrecorded alcohol (i.e. non-commercial or illicit products).
The flow-through infrared device is much easier to handle than typical reference procedures, while time-consuming sample preparation steps such as distillation are not necessary. Therefore, the alcoholic strength can be economically and quickly controlled (requiring less than 60 s per sample). The device also gives the opportunity for mobile on-site control in the context of labelling control of wine, beer and spirits, the process monitoring of fermentations, or the evaluation of unrecorded alcohols.
自盖-吕萨克时代以来，体积酒精度（% vol）一直通过密度测量法来测定。典型的参考方法包括蒸馏后进行比重测定，这种方法劳动强度较大，因此成本较高。目前，红外（IR）光谱结合多元回归被广泛用作一种筛选方法，该方法能够在无需任何样品制备的情况下，在不到2分钟内测定酒精度。其缺点是傅里叶变换（FT）红外或近红外仪器的投资相对较大，且需要进行基于基质的校准。在本研究中，我们应用了一种更为简单的设备，它由一个专利多光束红外传感器与一个流通池组成，用于自动酒精分析，该设备有便携式版本，可进行现场测量。
在方法验证过程中，发现红外传感器的精密度等于或优于密度测量法或傅里叶变换红外光谱法。例如，在6种不同葡萄酒样品中测定的平均重复性为0.05% vol，相对标准偏差低于0.2%。通过与密度测量法或傅里叶变换红外光谱法的结果对比分析260种不同酒精饮料，确保了准确性。从无醇啤酒到高度烈酒的整个范围内，相关性呈线性，结果基本一致（R = 0.99981，p < 0.0001，均方根误差 = 0.279% vol）。该设备的适用性在葡萄酒发酵过程分析以及未记录酒精（即非商业或非法产品）的测定中得到了进一步验证。
流通式红外设备比典型的参考方法更易于操作，同时无需蒸馏这类耗时样品制备步骤。因此，可以经济快速地控制酒精度（每个样品所需时间不到60秒）。该设备还为葡萄酒、啤酒和烈酒标签控制、发酵过程监测或未记录酒精评估中的移动现场控制提供了机会。
