Density and or Specific Gravity Measurements for Alcohol Determination
-This article is written by Mr. Gary Spedding, Ph.D., Alcohol Beverage Chemist-
Historically, alcohol measurements were grounded in physical measurements of mass and volume through density or mass per unit volume intensive properties. Through density and specific gravity relationships, instruments and devices such as density bottles, hydrometers, densitometers, refractometers and pycnometers were used to establish a recognized and officially accepted body of work. This extensive research effort culminated in the derivation of algorithms and tables which define the relationships between density values and specific gravity readings and alcohol by weight and by volume.
Alcohol Measurements Using Spectroscopy
Infrared (IR) spectroscopy is a method which utilizes the energies of the infrared light espectrum to promote transitions within the various functional groups of molecules. Within certain regions of the electromagnetic spectrum, chemical compounds may absorb the infrared radiation and specific vibrations may be measured. The infra-red spectra of molecules are absolutely specific – certain bands which regularly appear near the same spectral wavelength of energy may be assigned to specific molecular groupings. By measuring the vibrations of atoms and bond stretching, those functional groups can be determined. The frequencies and intensities of the infrared bands exhibited by a chemical compound uniquely characterize the material – generating a fingerprint – and thus, infrared spectra can be used to not only identify a particular substance in an unknown sample but can also be used to quantify that substance. As such, the use of the energies associated with the mid-range and near range of the infrared spectrum may be used to determine the content of alcohol in beverages. Moreover, it is noted that IR spectroscopy in both the mid-infrared (MIR) and near-infrared (NIR) regions is gaining popularity both qualitatively and quantitatively as an analytical technique with potential in other areas of alcoholic beverage and beverage raw materials testing; near- infrared spectroscopy techniques have been implemented for example in malting and brewing since the early 1990’s.
Instruments require calibration, but once set up, several components in samples can be measured simultaneously with little to no sample preparation needed. However, a major limitation of NIR spectroscopy in alcohol beverage and food analysis is its dependence on less-precise reference methods. Also, once again noting the need to obtain independently the density values of samples toper form subsequent calculations. This measurement of density, alongside the alcohol by volume determination, is covered in brief notes below.
Infrared devices are also finding application within in – line measurements in the brewery and other alcohol beverage production facilities. Most significant to the present discussion is that highly accurate NIR spectrometers are now on the market that can measure the alcohol content of beer and malt beverages in the range of 0-12 percent v/v. These units are often used today along with coupled density meters.
Off Coupled Oscillating Density Meters and NIR Alcohol Meters As seen above some coupled instruments today rely on two fundamental properties of alcohol; namely its density and its absorption intensity in the Near-Infrared region of the spectrum (often a carbon-hydrogen bond stretching vibration near 1200 nanometers in the energy spectrum). In principle the alcohol by volume at 20°C is determined by the NIR instruments based on a specific function of the absorption intensity of the NIR line of alcohol (see above). The specific absorbance of energy being dependent (proportional) to the ethanol concentration. The coupled instrument’s software programs’ reference the OIML (or other) tables for solving for the percentage of alcohol by weight. The coupled instruments can then determine the ethanol concentration (weight and volume), the specific gravity of the sample, and then, via calculation, the original extract for the alcohol containing sample.
The calculation for weight of alcohol relies on the density value for pure alcohol at 20°C (taken as 0.78924 g/cm3) and the density of the sample measured in the density meter. [The NIR – Alcolyzer method has been extended recently to cover a wider range of alcohol content with high accuracy and precision but the details have not yet been presented for public viewing. The manufacturer, Anton-Paarin Austria might be consulted for the details or to the date and citation when published.]
Nuclear Magnetic Resonance Spectroscopy (NMR)
Another sophisticated method, as yet only available in a few facilities (typically in academic research settings), that can also accurately determine alcohol is nuclear magnetic resonance (NMR). This is a technique for detecting atoms which have nuclei that possess a magnetic moment such as 1H, 13C, 31P, 23Na, 15N, etc. The nuclear magnetic moments of these atoms interact with the magnetic component of electromagnetic radio-waves giving rise to the phenomenon known as nuclear magnetic resonance. Most studies are conducted using the lightest isotope of hydrogen, 1H (thus the term proton magnetic resonance or p.m.r. may also be used). This method is being increasingly used to analyze commercial products. In very simple terms the method can chemically fingerprint the bulk solution of an intact beverage with minimal sample preparation. Unlike GC and HPLC methods it is not a chromatographic technique which gives it its own differential way of determining, for example, the true ethanol concentration in a beverage. In 1H-NMR each chemical component has a unique spectrum based on the different hydrogen chemistry on the molecule.
Thus protons in CH, CH2, and CH3 groups present in different chemical groups such as olefins, aromatics, organic acids, alcohols, esters, carbohydrates etc., may be detected by the occurrence of particular peaks and multiplets at specific chemical shifts in the NMR spectrum. In addition, the relative number of 1H atoms in each type of chemical group within the sample is indicated by the relative intensities of the appropriate peaks; that is by the areas under those peaks. A peak unique to a component of interest is chosen and the ratio of its signal intensity to that of an internal standard is determined. The signal intensities are divided by the number of protons they represent in order to obtain a signal intensity on a molar basis. With a knowledge of the molecular weights of the standard and the component, the weight of the standard present and the sample volume, the concentration in milligrams per liter for the component of interest can be determined along with all other components simultaneously. From there the alcohol by weight and by volume can be computed. Thus, NMR is a very powerful and growing method for product analysis.
Stay tuned for Part V of this informative series, to be continued next