Absolute Ethanol

Pure, sometimes called “absolute” ethanol, is 100% ethanol. Because water and ethanol form an azeotropic mixture, the highest concentration of ethanol that can be achieved through distillation is 95.6%. In non-food grade ethanol, the azeotrope can be disrupted using denaturants, but for food grade or other high-purity applications, the remaining water must be removed using adsorbents such as activated carbon. Adsorption is also important for the removal of other fermentation byproducts, which cannot be separated during distillation because the boiling point is too similar to that of ethanol. Examples of byproducts could be toxic compounds like phenol, benzoic acid, or styrene; or aromatic compounds such as short chain fatty acids, aldehydes, and esters, which may impart undesirable aromas. Absolute ethanol is hygroscopic, which means it will absorb water from the air if not properly sealed and stored. Purity of food grade ethanol is regulated by the FCC (Food Chemical Codex) under the USP (US Pharmacopeia), and verified using chromatography-mass spectrometry techniques.

Absolute food grade ethanol is preferred for many common techniques in food research and development, including food grade extraction and fraction isolation, or food grade synthesis. Liquid chromatography and flash chromatography methods may require pure solvents for the mobile phase because the solvent system in combination with the stationary column dictates the characteristic retention time of the analytes. Absolute ethanol for direct extraction (also called solid-liquid extraction, SLE) may increase recovery yields by reducing the polarity of the extracting solvent. Solid-phase extraction may require absolute ethanol as the eluting solvent. For food grade synthesis, the presence of water could result in undesired byproducts. For example, many volatile organic compounds are synthesized in ethanol, and can be added to food to manipulate flavor and aroma. The use of food grade solvents in all of these applications ensures the safety and purity of food and food standards for consumers. 

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