DISTILLERY ANALYSES Part Two
Written by PAUL HUGHES, PH.D.
Enhanced Analyses
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a)
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Spectroscopy and chromatography
T
here are occasions where there is a need or a desire to know more about processes and products. This can be from a product performance perspective in the market (e.g. consumer insights), sensory properties, or chemical composition. For the purposes of this chapter, the discussion will be restricted to the latter. To gain specific compositional data of a product or work-inprogress, the traditional approach was to develop specific analyses for individual or groups of similar molecules. One example is the measurement of diacetyl colorimetrically by its reaction with 1,2-diaminobenzene to create a quinoxaline with molar absorption coefficients over 10,000 from 190–380 nm (Scheme 1). O
H2N
N
H2N
N
+ O
Scheme 1. Sensitive colorimetric reaction for the specific determination of vicinal diketones, such as diacetyl.
Today, diacetyl is most conveniently measured using gas chromatography with electron capture detection (ECD), a method also effective at detecting contaminating halogenated compounds. Hopefully these latter are absent. The use of colorimetry is an example of the application of spectroscopy to derive data and information from the absorption and/ or the emission of energy from chemical or biological systems. Spectroscopic methods are diverse, with cost of instrumentation ranging from around $4,000 to $200,000 or more, depending on the technique applied. Common spectroscopic methods are summarized in Table 1. The simplest technique is UV-visible spectroscopy. Here the sample of interest is presented as a solution, and the absorption
Fig. 1. Idealized paper chromatography of black ink. a) Starting point, solvent at base of paper. b) Solvent has moved up the paper by capillary action separating the ink components according to the relative affinity of these components for the paper and solvent. characteristics of the sample are determined, anywhere from 190 nm, far into the UV spectrum, to around 800 nm, at the edge of visible and infrared radiation. For substances that bear one or more chromophores (i.e. molecular properties that allow for light to excite electrons), light is absorbed, which can then be detected. Additionally, in the case of turbidity, use of visible light in the range of 580–620 nm can be used to assess scatter and therefore assess cell density. Fluorescence spectroscopy is less common and is often more commonly used as a liquid chromatography detector (see below). It is known as a selective detector as it relies on specific UV and/or visible wavelengths for both excitation and measured emission. The use of infrared spectroscopy usually relies on the detection of specific functional groups in molecules. It is diagnostic for many functional groups, such as carbonyls, carbon-halogen bonds, alcohols, and carboxylic acids. It is included here for completeness, but it does not have many uses in the distillery beyond evaluation of pure compounds. Near infrared (NIR) spectroscopy, however, does have distillery-relevant applications. Unlike infrared spectroscopy, it does not obviously pick out specific molecular properties. Rather, the relatively unstructured spectra vary in a subtle way that can, with careful measurements of example substances, be calibrated to be able to indirectly derive analytical data. For instance whole cereals at intake can be assessed for nitrogen content in a matter of minutes by NIR,
Table 1. Common spectroscopic methods relevant to the distilled spirits industry.
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DETECTOR
SUITABLE SAMPLES
EXAMPLE APPLICATIONS
UV/visible
Compounds with appreciable absorption from 190–800 nm
Color, turbidity measurement, wood extractives such as tannins
Fluorescence
Selective based on excitation and emission wavelengths
Certain mycotoxins, tannins such as scopoletin
Infrared
Molecules bearing polarized chemical bonds
Generally confined to appraising pure chemical species
Near infrared
Potentially rapid method for screening wide range of materials
Rapid alcohol measure-ment in obscured liquids, and raw material properties W W W . ARTISANSPIRITMAG . C O M