Being an avid consumer of beverages like tea and coffee to stimulate my mind, caffeine is an important constituent of regular liquid diet. While finding another suitable ways to consume tea, I came across another staple dish of Japan known as ‘ochazuke’ which is actually a dish prepared by heating tea with boiled rice at very high temperature. I was intrigued to understand the health benefits of this particular dish and came to know that they boil tea at a very high temperature almost to dryness to get more caffeine out of it which gives a distinctive flavour and better nutritional values to the food. Thus, I had a question- Will we get more caffeine from tea if the temperature at which tea is extracted is increased? Food processing and preservation is an immensely important branch of chemistry. Extraction of naturally occurring pigments and other heterocyclic compounds like alkaloids, tarpenoids from vegetables or fruits or seeds of plants is a common practice. This process is mainly based on the principle of difference of solubility of organic compounds in different solvents and is also affected by multiple physicochemical conditions like temperature, polarity of solvent used, pH and many more. Researches have been done over time to elucidate the optimum conditions for brewing tea. This investigation is a part of that same exploration. My first objective behind this was to understand the chemical process which is involved in the extraction of caffeine from tea leaves; which I found to be simply a process of diffusion. Since middle school chemistry classes, I have studied that molecules diffuse at a faster rate at higher temperature. This investigation may allow me to illustrate or verify this understanding of mine. Moreover, specific characteristics of organic compounds and the variation they show in their properties and structure like aromatic compounds, heterocyclic compounds, alicyclic compounds were some of the terms I came across while exploring ideas to conduct this experiment and that fascinated me more towards this topic.
Caffeine is a component of coffee, tea, chocolate, soft drinks, diet pills, and analgesic making it the most widely used psychoactive drug in the world which affects the same parts of the brain as cocaine(Daly et al.).
Caffeine belongs to the family of heterocyclic compounds, purines, with 1,3,7 - Trimethylpurine - 2,6 - dione as its IUPAC name, and C8H10N4O2 as its chemical formula(E.F). Caffeine is water-soluble like most purines; however, caffeine’s solubility varies with temperature. Caffeine can be classified as an alkaloid, and like most alkaloids, caffeine has powerful physiological effects on humans such as increased focus. It exists as a white crystalline solid at room temperature. It belongs to the class of ‘methylxanthine alkaloid’(PubChem)a class of the heterocyclic aromatic family of organic compound.
Tea leaves are chemically composed of two main class of aromatic compounds other than carbohydrates-
Caffeine belongs to the category of alkaloids. This compound is a covalent non-polar compound. It is insoluble in polar solvent like water and highly soluble in non-polar solvent like chloroform or dichloromethane. Chloroform is a better solvent as it does not react with the caffeine molecules and is easily volatile. Caffeine can also dissolve easily in ethanol.
The extraction of solid caffeine from tea leaves various stages:
Tannins contains an aromatic ring with the phenolic OH group. Tannins react with sodium carbonate in the following way:
→ Ar - OH + Na2CO3 Ar - ONa + CO2 + H2O
This is an example of an acid base reaction which converts the acidic tannins into their water soluble phenolic salts. As the tannins becomes water soluble and the caffeine not, it becomes more likely for the caffeine to pass into the organic layer when a volatile organic solvent like dichloromethane or chloroform is added to the aqueous extract of caffeine. Thus in one way addition of caffeine separates the tannins from caffeine by converting the acidic tannin molecules into water soluble salts.
In a research article titled as – ‘Effect of time and water temperature on caffeine extraction from coffee’ published in the journal of Pakistan Journal of Nutrition, the effect of brewing temperature on caffeine content of two coffee grains – ‘Coffee arabica’ and ‘Coffee robusta’ was studied. The range of temperature chosen was 60.00 °C to 120.00 °C. The caffeine content was found to have increased with the increase of temperature. The correlation of caffeine with temperature was expressed as a “linear equation y = 1.7647x - 1.5023 with a R2 value of 0.9963” (Nhan and Phu)
This investigation is mainly focuses on the effect of temperature on the extent and rate at which caffeine is passing from the cells in the tea leaves to the aqueous medium in the solid-liquid extraction stage. It involves two process in sequence- degradation or decomposition of the cells in the tea leaves to release the phytochemicals it contains and diffusion of the caffeine thus released into the aqueous layer. Both of these process must occur at a faster rate as temperature increases. Thus, more caffeine must be extracted from the tea leaves if the temperature of extraction is increased. Thus, it is predicted that mass of caffeine extracted into the aqueous layer must increase as the temperature increases.
Brewing temperature in °C ; temperature to which the water was heated in which the tea leaves were soaked in.
For my experiment, I used Tetley Black Tea which has a caffeine content of 40 - 50 milligrams in a typical 8 - ounce cup(Price et al.). The standard value for caffeine content of an 8 - ounce cup is 47.4 milligrams (Cho).
Hence, Tetley Black Tea is a fair estimate for all Black Teas. Conventionally, Black Tea is consumed at a temperature of 99 °C(McNaughton et al.). However, this may not be the temperature at which caffeine content is maximised. At the same time, temperatures such as 200 °C may be impractical as they exceed the heat threshold of the human tongue. Hence, a temperature range of 60 °C to 120 °C had been chosen for this experiment. The temperature of water was varied using a Bunsen Burner. A beaker of distilled water was placed above the heater until the desired temperature was reached. The temperature was measured using a thermometer which was placed in the beaker of water.
Mass of caffeine extracted in g
The caffeine content was the dependent variable of the experiment which varied with temperature of water and hence represented the link between temperature and the caffeine content of tea. The mass of re- crystallised caffeine obtained through solvent extraction was measured using a mass balance after the solvent extraction process.
Variable Name | How it impacts? | How was it controlled? | Apparatus Used |
---|---|---|---|
Tea type and quantity | Different tea brands can have different caffeine contents which can influence the relationship between caffeine and temperature. More the mass of tea leaves taken more the mass of caffeine extracted. | Two teabags of Tetley Black Tea were used for all readings. | NA |
Brewing Time | The time taken for the dissolution of caffeine was kept constant as an increase in time would have resulted in an increase in the caffeine content of the tea. | The brewing time was fixed at 120.00 ± 0.01 seconds. | Digital stopwatch |
Volume of dichloromethane used | An increase in the volume of organic solvent- dichloromethane used would have increased the caffeine which dissolves and hence the caffeine content. More the organic solvent more the caffeine molecules that disperses to it. | To ensure that 10.00 ± 0.05 cm3 of dichloromethane was added to aqueous extract of caffeine in all trails, a graduated pipette of 10 cm3 was taken. | Graduated pipette 10 cm3 |
Mass of Na2CO3 Added | A change in the mass of Na2CO3 would have resulted in a change in the tannin content of the solution which would have created discrepancies in the caffeine reading. | 1.00 ± 0.01 g of Na2CO3 weighed using a digital mass balance was added to the aqueous extract. | Digital mass balance |
Volume of Ethanol | A change in the volume of ethanol would have caused a change in the mass of caffeine which was recrystallised by influencing its solubility. | 10.00 ± 0.05 cm3 of ethanol was added during the crystallisation process. | Graduated pipette |
Volume of Water | Water is used as the solvent in the first stage; solid-liquid extraction. More the solvent more the caffeine that dissolves and more the mass of caffeine extracted. | 100.0 ± 0.5 cm3 of distilled water was added using a graduated measuring cylinder. | Graduated measuring cylinder |
Apparatus Name | Quantity | Capacity | Least Count | Uncertainty |
---|---|---|---|---|
Burette | 1 | 50.00 cm3 | 0.10 cm3 | ±0.05cm3 |
Glass Beaker | 2 | 250.00 cm3 | - | - |
Watch Glass | 2 | - | - | - |
Temperature probe | 1 | --- | 0. 01°C | ±0.01°C |
Bunsen Burner | 1 | - | - | - |
Stopwatch | 1 | - | - | - |
Tongs | 1 | - | - | - |
Mass balance | 1 | - | 0.01g | ±0.01g |
Spatula | 1 | - | - | - |
Separatory Funnel | 1 | 50.00 cm3 | - | - |
Clamp Stand | 1 | - | - | - |
Dropper | 1 | 10.00 cm3 | - | - |
Graduated Cylinder | 1 | 100.00 cm3 | 1cm3 | ±0.5cm3 |
Material Name | Quantity | Source |
---|---|---|
Distilled Water | 1.5 L | School Laboratory |
Tea Bags | 30.00 g | School Laboratory |
Sodium Carbonate | 20.00 g | School Laboratory |
Dichloromethane | 500 cm3 | School Laboratory |
Anhydrous Sodium Sulphate | 50.00 g | School Laboratory |
Pure Ethanol | 500 cm3 | School Laboratory |
Ice | 200.00 g | School Laboratory |
No living organisms were harmed during this experiment, and all chemicals were disposed of in a very careful manner.
No harm was done to the environment, and the waste chemicals were disposed carefully.
Observation | Inferences |
---|---|
Bubbles were observed when sodium carbonate was added to the aqueous extract of caffeine. | Reaction of sodium carbonate with tannins in the aqueous tea extract released carbon dioxide. |
White lumpy solid was left in the beaker as the chloroform evaporated. | Formation of crude caffeine. |
White shinning crystals were observed after the ethanol evaporated. | Formation of crystals of caffeine. |
Two distinct layers of liquid was observed when chloroform was added to the aqueous extract. | Chloroform and water are immiscible with each other. |
Mass of caffeine crystals = (Mass of beaker + caffeine) – (Mass of beaker)
= (21.30 ± 0.01g) – (20.50 ± 0.01g) = 0.80 ± 0.02 g
\(\text{Average mass of caffeine crystals = }\frac{0.80+0.79+0.78+0.80+0.80}{5} = 0.01\)
\(\text{Standard deviation (SD) = }\frac{(0.80-0.79)^2+(0.79-0.79)^2+(0.78-0.79)^2+(0.80-0.79)^2+(0.80-0.79)^2}{5} = 0.01\)
Tempertaure ±0.010 °C | Average mass of caffeine obtained ± 0.02 g | Fractional error ± | Percentage error ± |
---|---|---|---|
60.00 | 0.79 | 0.03 | 2.53 |
70.00 | 1.43 | 0.01 | 1.40 |
80.00 | 1.66 | 0.01 | 1.20 |
90.00 | 2.19 | 0.01 | 0.91 |
100.00 | 2.64 | 0.01 | 0.76 |
110.00 | 3.06 | 0.03 | 2.53 |
120.00 | 3.47 | 0.01 | 1.40 |
Absolute error = ± 0.02
Average mass of caffeine crystals obtained = 0.79 ± 0.02 g
\(\text{Fractional error = }\frac{0.02}{0.79} \text{ = 0.0253 = 0.03 (rounded off to two decimal places)}\)
Percentage error = 0.0253 × 100 = 2.53
The average mass of caffeine increases from 0.79 ± 0.01 g to 3.47 ± 0.01 g as the temperature increases from 60.00 ± 0.01°C to 120.00 ± 0.01°C. This indicates that as the temperature of the water in which the tea leaves are soaked in the mass of caffeine extracted increases.
The data points follow a linear trend line which is y = 0.0439x – 1.77. This brings us to the claim that the increase of mass of caffeine extracted from tea leaves with temperature is linear in nature. Almost all the data points are equally spaced which again confirms that the increase is mostly gradual in nature.
At y = 0
0.0439x − 1.77 = 0
\(x = \frac{1.77}{0.0439}= 40.31\)
It means that if the trend line is extrapolated it would intersect the x axis at 40.31. This indicates that at a temperature of 40.31 ± 0.01°C, the average mass of caffeine extracted will be zero. Thus, it can be said that if the temperature of the water is brought down to or below 40.31 ± 0.010 °C, caffeine will not be extracted from the tea leaves to water. This means that the minimum temperature required for the extraction is 40.31 ± 0.01°C
The correlation value is also mentioned in the graph. The value of R2 is 0.9946. As the value is positive, it supports the fact that mass of caffeine extracted increases as temperature increases. The value of R2 is high; 0.99 which shows that the correlation between the variables is strong. Moreover, the gradient of the linear trend line is 0.0439 which again supports that the correlation is positive in nature; mass of caffeine extracted increases as temperature increases. Thus, the hypothesis predicted stands valid.
How does the mass of caffeine (in g) extracted from Tetley Black Tea leaves in its aqueous extract depends on the temperature of the water in which the tea leaves are brewed, determined using solvent extraction?
Source | How does it impact? | How can it be minimized? |
---|---|---|
Uncertainty in apparatus | Reduces the accuracy and reliability of the mass of caffeine measured. | A burette and pipette were used instead of measuring cylinder. Temperature probe was used instead of a thermometer. |
Delay in operating the stop-watch | The tea leaves gets brewed for longer leading to extraction of more caffeine than actual. | Data was collected for five times and average was calculated. |
Source of error | How does it impact? | How can it be minimized? |
---|---|---|
The tea bags may contain water within it. | Some of the caffeine may be lost with the water that is retained in the tea bag. This will gives us a lower value of the caffeine than actually obtained. | The tea bags were squeezed using a glass rod to drain out the water completely out of it. |
Although the extraction is based on the fact that caffeine is more soluble in the chloroform than water yet it cannot be ignored that caffeine is soluble in water too. | It may happen that some of the caffeine molecules may still remain in the aqueous layer even after chloroform is added to it. This will produce a lower value for the mass of caffeine extracted than the actual. | Sodium carbonate was added to make the medium basic which makes caffeine more attracted towards the organic solvent over water. |
Chloroform is used as a solvent here. It is extremely volatile and can evaporate easily. In case, it is mixed with hot water, it may evaporate. | This would make less amount of caffeine available for the caffeine to dissolve than the actually added volume. | The aqueous extract of caffeine was placed in an ice bath to cool it down before adding chloroform to it. |
There is a chance of loss of mass of caffeine crystals while transferring it from one glass apparatus to another using a spatula. | This would reduce the mass of caffeine crystals obtained. | Same beaker was used to obtain the crystals of caffeine from evaporation of chloroform and to crystallize it using ethanol. Thus it was not required to transfer the caffeine crystals from one apparatus to another. |
The methodology of extraction relies on the fact that caffeine is more soluble in organic solvent like chloroform than polar solvent like water. Thus it cannot be ensured that all caffeine molecules were transferred from the aqueous layer to the organic layer. To avert this limitation, the methodology needs to be altered. The quantity of caffeine obtained can be monitored using a UV-Visible spectrophotometer. Caffeine shows ‘strong absorbance at 205 nm and 273 nm’(CSUN).Thus, if the absorbance of the solution is measured at any of these two wavelengths and compared in reference to a calibration curve, the amount of caffeine can be calculated.
This experiment can be conducted for different types of beverages such as different types of tea, soft drinks, chocolates, and other caffeine containing items. Other factors which may influence caffeine content can also be assessed such as the pH of a solvent. Caffeine can be extracted in acidic medium instead of using drinking water. Dilute HCl solutions of various concentrations can be used for this. As the concentration of HCl changes, the pH of the medium would also change. Thus, the effect of pH on the mass of caffeine extracted can be investigated.
References has been done in MLA 8 format. As per MLA 8 guidelines the citations are listed below in alphabetical order and the in-text citations are inserted at appropriate places. Access date and time are not mentioned for research articles as per latest MLA 8 guidelines.
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CSUN. High Performance Liquid Chromatography. 10 June 2020.
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Price, Karen Overstreet, et al. “How Much Caffeine Is Too Much in Athletes?” American Journal of Health- System Pharmacy, vol. 47, no. 2, Feb. 1990, pp. 303–303. DOI.org (Crossref), doi:10.1093/ajhp/47.2.303a.
PubChem. Caffeine. https://pubchem.ncbi.nlm.nih.gov/compound/2519. Accessed 31 July 2020.