Development of a grapefruit-flavoured spirit with the opalescence properties of pastis (2008)

In the form of a potable spirit, an extract of grapefruit skin has been found to develop an attractive opalescence when diluted to below about 38 % ethanol (v/v). This phenomenon is analogous to the pastis effect, called louching, common in many drinks popular in some countries bordering the Mediterranean. The main objective of this research was to develop spirit liquor with commercial potential from grapefruit skin, largely a waste commercial product, as the dominant if not exclusive ingredient other than alcohol and water. This would require making extracts by distillation of undried skins, which would develop a dense opalescence significantly below 40 % v/v ethanol, the common alcoholic strength of spirits as sold in New Zealand. The product concept was thus a clear liquid which when poured over ice for example, would yield a cool opalescent drink with a characteristic grapefruit flavour. In the case of citrus, the chemical basis of louching is the greater solubility of citrus skin terpenes, principally limonene, in ethanol than in water. The louch point is synonymous with the chemical expression critical micelle concentration, detected here by light scattering at the arbitrary wavelength of 450 nm. Early results with an obvious opalescence showed that the alcohol concentration at which the terpenes ceased to be soluble in the grapefruit distillate was about 38 % (v/v). This point was similar to that for pastis (Pernod brand), where the principle louchable ingredient is anethole. However, the light scattering was much greater for pastis. Thus, a grapefruit spirit sold at 40 % ethanol with the louching intensity of pastis should require increasing the solubility of limonene in solutions with a lower ethanol concentration. This in turn should allow higher concentrations of limonene in true solution in 40 % ethanol, theoretically resulting in a more intense louch in the final drink. Thus, a broad range of hydrocolloids and surfactants was tested in an attempt to increase the solubility of limonene in lower ethanol concentrations. None of hydrocolloids or surfactants lowered the louch point of a standard limonene concentration in ethanol/water. The informal flavour assessment showed that the spirit flavour from grapefruit zest alone lacked intensity. It was then thought that better flavour might be obtained by using the whole grapefruit (zest, pith, and juice) rather than zest alone. The additional of pith or pith plus juice caused no significant change in the louch point or light scattering. However, the final flavour was informally judged to be more intense than that derived from zest alone. The problem still remained that the light scattering of grapefruit distillate was never as high as that achieved by pastis, even though it was found (by gas chromatography) that the concentration of limonene (w/w) in the grapefruit spirit was at least as high as that of anethole in pastis. The cause was due to the fact that as a louched compound, anethole was a solid with a flat crystal structure because the melting point (21.4C) is well above that of a cool drink (Handbook of Chemistry and Physics, 1947). By contrast, limonene remains a liquid even in an iced drink because its melting point is far below 0C. Flat crystals would obviously scatter light far more than would a micelle containing a liquid, in this case limonene. Although such a grapefruit distillate did not louch as well as pastis, it could still have market potential on the basis that it would be made from the distinctive New Zealand cultivar of grapefruit. Thus a formal sensory assessment was conducted, using a focus group. The grapefruit distillates at 40 % ethanol were perceived by most panellists as refreshing, clean, fruity, and citrusy in aroma, but somewhat deficient in grapefruit flavour, and there was a common perception of strong chemical finish. At this stage of development a commercial proposition cannot be sensibly made.