SPME optimisation hits the sweet spot for sugar cane spirits

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  • Published: May 1, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: Laboratory Informatics / Chemometrics & Informatics
thumbnail image: SPME optimisation hits the sweet spot for sugar cane spirits

Analysis of sugar cane spirit volatiles is a challenge

Sugar cane spirit, also known as cachaça and a variety of other names, is a popular alcoholic drink in Brazil. Apart from ethanol, many other volatile compounds are present in the spirits, giving the drink its particular flavour. The analysis of these volatile components is an important measure of quality and can be used to determine, in doubtful instances, whether bottles have been processed in a particular manner, such as being aged in wooden barrels. Sensitive detection methods are needed to give a good picture of the many volatiles present, some of which can be difficult to detect.

One effective method of analysis is solid-phase microextraction (SPME) coupled with GC-MS. The Brazilian researchers wished to optimise the efficiency of the extraction, so they examined the influence of the main SPME parameters using a factorial central composite design.

SPME optimised for sugar cane spirit volatiles

Equal portions of liquor were diluted with water and then subjected to headspace analysis/SPME, using a Supelco DVB/CAR/PDMS fibre. GC-MS was carried out with a Shimadzu QP2010 plus GC-MS instrument, fitted with a Shimadzu AOC-5000 automatic injector and an SLBTM column. The oven temperature was ramped from 35 to 240 °C. The mass spectrometer was run in electron impact mode with a solvent cut-off of 1.75 minutes. The peaks of the volatiles were identified using Automated Mass Spectral Deconvolution and Identification System (AMDIS) software.

The investigators examined three SPME variables in a central composite designed experiment: extraction temperature (30–55 °C), extraction time (13–47 minutes) and desorption time (19–221 seconds). Fourteen volatiles (not counting ethanol) were detected by the GC-MS system, mainly higher alcohols and esters. Sixteen experimental runs were carried out, with the total area of the fourteen volatiles being used as the experimental output. Chemoface version 1.4 was used for the experimental design and the analysis of the results.

All the three variables examined (extraction temperature, extraction time and desorption time) along with their three products (i.e. extraction temperature × extraction time, extraction temperature × desorption time, and extraction time × desorption time) were calculated to be statistically significant. Displaying the results for the magnitude of these variables on a Pareto plot (i.e. a bar chart with the variables plotted in descending order of magnitude) enabled their importance to be clearly visualised. As might be expected, extraction time gave the largest effect, the total area of volatiles increasing with increased extraction time. Two of the interaction terms (extraction time × desorption time and extraction temperature × extraction time) gave quite large (negative) effects. Surprisingly, the desorption time on its own showed a small negative effect.

Three response surfaces, i.e. three-dimensional plots of the variables and output, were plotted, each showing the output along with two of the input variables. These surfaces enabled the optimum extraction conditions to be defined: an extraction temperature of 45 °C, an extraction time of 50 minutes and a desorption time of 100 seconds.

Experimental design enabled SPME process to be improved

The experimental design allowed the parameters of the SPME process to be carefully optimised, giving the best conditions for the detection of volatiles. Not all the results were obvious, or would have been likely to be picked up by the equivalent number of ‘changing one variable at a time’ runs. This type of optimisation can be a powerful tool for many other processes, not just SPME.

Related Links

Journal of the Institute of Brewing, 2017, Early View paper. Zacaroni et al. Response surface optimization of SPME extraction conditions for the analysis of volatile compounds in Brazilian sugar cane spirits by HS-SPME-GC–MS.

NIST/SEMATECH e-Handbook of Statistical Methods, Central Composite Designs (CCD)

Wikipedia, Cachaça

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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