Part III. Extracting Analytes From Samples
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Brewing coffee is nothing more than a simple solvent extraction, which makes it a useful and a familiar model for considering how a solvent extraction works. There are a variety of methods for brewing coffee that differ in how the solvent and the coffee are brought together. Investigate at least five of the following methods for preparing coffee: Turkish, French Press, Aeropress, Chemex, Pour Over, Stovetop, Vacuum Pot, Espresso, and Cold Brew. In what ways are these methods similar to each other and in what ways are they different from each other? What variables in the extraction process are most important in terms of their ability to extract caffeine, essential oils, and fragrances from coffee?
The intention of this investigation is to place solvent extraction in a context more familiar to students. The various methods for brewing coffee generally fall into four groups based on how the coffee grounds and water are brought together: boiling (or decoction), steeping (or infusion), gravity filtration, and pressure.
Whatever the method, there is general agreement that the ideal extraction yield (the percentage, by weight, of the coffee grounds solubilized during brewing) is approximately 20% and that the ideal strength (the amount of dissolved coffee solids per unit volume) varies by geographic region, but is approximately 1.25 mg per 100 mL in the United States. Extraction yields and strength depend on the ratio of coffee and water, the coarseness of the coffee’s grind, the brew temperature, and the brew time. Methods relying on courser grounds, such as French Press, require longer brew times; drip filtration methods use a finer grind and require shorter brew times. Extraction yields that are too high result in bitter-tasting coffee and extraction yields that are too small result in a more acidic-tasting coffee. The greater the strength, the darker, thicker, and oilier the brew.
Why might a combination of high temperature, a lengthy extraction time, and the need for two extractions be undesirable when working with a medicinal plant such as Danshen?
An extraction at a high temperature runs the risk of destroying some of Danshen’s analytes through thermal degradation; this is a more significant problem at higher temperatures, particularly when using a longer extraction time. The concentration of analytes in the final sample is smaller if we must combine two (or more) extracts of equal volume; if an analyte already is present at a low concentration in Danshen, then its concentration as analyzed may be too small to detect without first concentrating the extract.
What variables might we choose to control if we want to maximize the microwave extraction of Danshen’s constituent compounds? For each variable you identify, predict how a change in the variable’s value will affect the ability to extract from Danshen a hydrophilic compound, such as rosmarinic acid, and a lipophilic compound, such as tanshinone I.
The intention of this investigation is to have students begin considering how experimental conditions will affect the extraction of hydrophilic and lipophilic analytes from Danshen. As the investigations that follow demonstrate, the variables explored here are not independent of each other, which makes impossible accurate predictions; of course, this is why method development is necessary! The comments below outline important considerations for five possible variables: the solvent; the solvent-to-solid ratio; the extraction temperature; the extraction time; and the microwave’s power.
The choice of solvent must meet two conditions: the analytes of interest must be soluble in the solvent, and the solvent must be able to absorb microwave radiation and convert it to heat. All three options for the solvent included in this study—methanol, ethanol, and water—are effective at absorbing microwave radiation and converting it to heat, although water is better than methanol and ethanol at absorbing microwave radiation and methanol is better than ethanol and water at converting absorbed microwave radiation into heat. In terms of solubility, we cannot predict easily the relative trends in solubility for either the hydrophilic or the lipophilic analytes when using methanol or ethanol as a solvent; however, we expect that the lipophilic analytes will not extract into water. Although the lipophilic analytes may be more soluble in a non-polar solvent, such as hexane, a non-polar solvent cannot absorb microwave radiation.
In general, we expect that increasing the solvent-to-solid ratio will increase extraction efficiency for all analytes; this certainly is the case with conventional extractions. For some microwave extractions, and for reasons that are not always clear, increasing the solvent-to-solid ratio beyond an optimum value decrease extraction efficiency.
For all analytes, extraction efficiency generally increases at higher temperatures for a variety of reasons, including the easier penetration of a solvent into the sample’s matrix as a result of a decrease in the solvent’s viscosity and surface tension. This increase in extraction efficiency with increasing temperature is offset if the analytes are not thermally stable. It is important to note, as well, that for an open-vessel atmospheric pressure microwave extraction, the method used here, the highest possible temperature is the solvent’s boiling point.
In general, we expect that extraction efficiency for all analytes will increase with longer extraction times. As is the case with temperature, however, the increase in extraction efficiency at longer times is offset if the analytes are not thermally stable.
For all analytes, the relationship between microwave power and extraction efficiency is not intuitive. An increase in microwave power results in greater localized heating. In some extractions, the increased localized heating helps break down the sample matrix, increasing extraction efficiency; in other cases, extraction efficiency decreases because the increase in localized heating results in more thermal degradation of the analytes. For other extractions, a change in microwave power has little effect on extraction efficiency.