Grappa rectification: a question of head, heart (and tail)
Rectification is the most crucial technical step in obtaining an artisanal grappa. The master distiller must skillfully adjust the temperatures of the still and the timing of distilling the pomace in order to separate out the substances that reduce the quality of the grappa and thereby rendering a unique spirit, full of character.
There are numerous volatile compounds present in the grape pomace, the most important of which are alcohol and water. The term distillation describes the physical process of separating the various substances and then transforming certain substances into steam which is then condensed into a spirit. This process occurs by changing the state of the liquid into steam through heat and then back into a liquid with cold.
Pomace, as was previously mentioned, contains hundreds of various volatile compounds besides alcohol and water, which when masterfully distilled into the grappa produce a nuanced flavour and finesse. However, if those compounds are over distilled, they depreciate the taste of the spirit and can even lead to a product that can cause physiological harm if consumed. Rectification is the process through which the distiller is able to separate the valuable compounds from the base compounds. It functions by fractioning the grappa into three parts: the head, the heart and the tail.
The head is removed in the first division. It is made up of compounds that have a boiling point lower than ethyl alcohol and is always the first fraction of liquid that passes through the cooling tubes. By using a system of discontinuous distillation, it is possible to separate out the head at the sacrifice of only a small portion of ethyl alcohol. This eliminates methyl alcohol – which is harmful to health if consumed – as well as ethyl acetate, which is responsible for an acetone odor.
A difference of just 4 °C between the boiling point of ethyl acetate and ethyl alcohol explains why so many poorly crafted distillates have a strong acetone smell and why it is important to avoid acetic fermentation of the pomace. As long as the pomace is distilled as fresh as possible, with little oxidation, these components (alcohols, esters and aldehydes) become present in the final grappa in limited qualities and provide it’s typical structure and characteristics.
The body or “heart” of the grappa is the central fraction, containing the majority of ethyl alcohol and the lowest percentage of impurities. The heart is made up of all the components that boil between 78.4 and 100 °C.
The tail is formed of the compounds that boil over 100 °C and are collected as the last fraction of the distillate. It is very rare to reach extremely high temperatures in distillation, however, many of these compounds are partially soluble in the hot alcohol vapours and pass through to the final condensate. Often times when the pomace is very rich in substances and compounds, the tail is found in larger part in the grappa.
Acids, and in particular acetic acid, give the grappa a pungent taste and must be separated with considerable care as they are able to pass through to the final distillate even if none of the compounds have a boiling point under 100 °C