Ethanol determination in fermented sugarcane substrates by a diffusive micro-distillation device.

Highlights

• Determination of ethanol without a distillation step.

• Analysis of a large number of samples simultaneously.

• A simplified procedure using common equipment in the laboratory and spectrophotometric determination.

Abstract

The determination of ethanol in fermented substrates is an important parameter for monitoring the production of distilled beverage samples. The correct measurement of its content has a direct impact on the profitability of the process. In this work, a diffusive micro-distillation device (DMDD) is proposed that allows the determination of ethanol directly in the fermented or distilled beverages samples. The DMDD consists of a 5 mL plastic test tube containing a reagent solution of potassium dichromate and sulfuric acid, inserted into another 50 mL polyethylene tube containing the sample. This set is heated in a water bath for 15 min at 80 °C, providing the ethanol diffusion, which reacts with the receptor solution contained in the test tube. The chromium (III) produced by the oxidation reaction, is spectrophotometrically quantified at 589 nm. The proposed procedure has a linear range between 1 and 12% (v/v) with R² = 0.999 and RSD = 3.8% and results in agreement with those obtained by the distillation-densitometry official method.

Introduction

The ethanol production from sugar derived plants such as starch (corn) or readily fermented sucrose, fructose and glucose (sugarcane or sugar beet) is an efficient biotechnological process, with sustainability and lower carbon footprint advantages. Brazil is one of the largest bioethanol producers, with production in the 2019 harvest of 31.6 billion litres, 96% of which comes from sugarcane (Conab, 2020).

The Brazilian industry produces bioethanol by batch fermentation processing using yeast (Saccharomyces cerevisae) which converts sugars from sugarcane juice and/or molasses into ethanol. The product of this fermentation is a mixture of microbial biomass, cell debris, impurities from sugarcane and ethanol, also called wine.

This fermentation process needs careful monitoring, in which premature or late withdrawal of the wine sugar wastes may result in the ethanol consumed by the yeast itself, causing reduction in production efficiency, and consequently loss of resources (Gomes et al., 2013). Methods commonly used to determine ethanol, such as the float glass densitometer or electronic densitometer, depend on the separation of ethanol from wine by distillation, before its measurement, which makes the process time-consuming and susceptible to errors (AOAC, 1990). Another method commonly used in the industrial process is gas chromatography with a flame ionization detector (FID). This method does not require the distillation process, as it allows the direct injection of wine into the equipment. It is a fast and precise technique; however, it has a high cost, in addition to requiring a specialized operator (Rattanaporn et al., 2019). Other strategies are used for the separation and determination of ethanol in complex matrices, such as those that use solid phase extraction with a cartridge to remove interferents, such as organic components, soil and oil (Chen et al., 1997); the disadvantages of this method are the high cost of the cartridges, the flow control system, as well as the long analysis time (Berrueta et al., 1995). Gaseous diffusion membranes were also used for ethanol separation and determination in flow analysis systems (Mohns and Künnecke, 1995), as well as immobilized enzyme (Fernandes and Reis, 2004; Caudy, 2017), evaporation (Mataix and Luque de Castro, 2000), and tubular diffusion and colorimetric reaction after distillation (Vicente, 2006). However, because of the long analysis time and the cost, these techniques are not used to control the process.

The monitoring of the peak ethanol concentration is essential for maximum efficiency of the production process to be achieved, thus requiring a fast, simple and precise Methodology. Colorimetric procedures are perfectly suited for this demand, as they can be processed quickly by simply mixing reagents and employ simple, low-cost instrumentation. However, this process has limitations when the sample has colour or suspended materials, such as sugarcane juice. In this sense, the introduction of a one- step procedure that allows the separation of ethanol from interferents and the simultaneous colorimetric determination becomes a good alternative.

To this end, this work aimed to create a diffusive micro-distillation device that performs the diffusion of ethanol from complex samples and enables a colorimetric chemical reaction allowing the quantification of alcohol, predominantly ethanol.