Fermentation is a metabolic process where organisms convert sugars into acids, gases, or alcohol in the absence of oxygen. This anaerobic pathway generates energy when the environment lacks sufficient oxygen for aerobic respiration. A respirometer is a laboratory device designed to measure the rate of gas exchange. It is the standard tool used to precisely quantify the speed of this anaerobic process by measuring the volume of gas produced over time.
The Chemical Principle of Measurement
The respirometer works for fermentation because the anaerobic breakdown of sugar reliably yields a gaseous byproduct. For common alcoholic fermentation, such as that performed by yeast, one molecule of glucose is ultimately converted into two molecules of ethanol and two molecules of carbon dioxide (\(\text{CO}_2\)). The chemical equation \(\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\) shows that carbon dioxide is a direct and proportional product of the reaction.
This consistent production of gas allows the respirometer to serve as a proxy for metabolic activity. Since the ethanol remains in the liquid solution, the only component that escapes as a gas is the carbon dioxide. By physically collecting and measuring the volume of this gas, the rate of fermentation can be accurately inferred.
Components and Preparation of the Apparatus
The apparatus used to measure fermentation is a form of simple respirometer, often constructed using a fermentation tube or two test tubes inverted over one another. Essential components include the biological material, typically a suspension of baker’s yeast (Saccharomyces cerevisiae), and a substrate solution, such as glucose or sucrose. These two components are mixed together in the smaller tube.
The apparatus must be assembled to create a sealed environment that excludes atmospheric oxygen, establishing the necessary anaerobic conditions. This is often achieved by completely filling the small tube with the yeast-sugar mixture and then carefully inverting a larger tube over it, displacing any initial air. Alternatively, a layer of non-toxic mineral oil or paraffin can be floated on the surface to create an airtight seal. Placing the assembled respirometer in a temperature-regulated water bath, often set to a constant \(37^\circ\text{C}\), ensures that temperature fluctuations do not influence the gas volume measurements.
Data Collection and Rate Calculation
The core of using the respirometer involves tracking the accumulating volume of carbon dioxide gas over a specific time period. After the respirometer is prepared, the initial volume or height of the gas bubble trapped at the closed end of the inverted tube is recorded at time zero. As the yeast ferments the sugar, the gaseous \(\text{CO}_2\) bubbles out of the solution and collects at the top of the tube, increasing the bubble’s volume.
Measurements are then taken at regular intervals, such as every five or ten minutes, by recording the new volume or height of the collected gas. This step-by-step process allows for the creation of a dataset that links elapsed time to the total volume of \(\text{CO}_2\) produced. The physical measurement of the gas bubble’s size is the direct evidence of fermentation activity.
To determine the fermentation rate, the change in the gas volume is divided by the time interval over which that change occurred. This quantitative rate, often expressed in \(\text{mL}\) of \(\text{CO}_2\) produced per minute, provides a precise metric for comparing metabolic efficiency under different conditions, such as varying sugar types or temperatures. For example, if the gas volume increases by 10 millimeters in 5 minutes, the rate is \(2\text{ mm/minute}\).
To validate the results, control respirometers are typically run simultaneously. These controls, such as a tube containing yeast but no sugar or a tube with sugar but no yeast, ensure that the measured gas production is solely due to the fermentation process being studied.