How does the mass of caffeine (in g) extracted from Tetley Black Tea leaves in its aqueous extract depend on the temperature of the water in which the tea leaves are brewed, determined using solvent extraction?

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-

• Alkaloids
• Flavonoids or tannins.

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:

• Solid-liquid extraction - In this step, caffeine molecules are extracted from tea leaves in the aqueous layer. Soaking the tea leaves are soaked in hot water. In a more sophisticated analytical procedure, ‘Soxhlet extractor’ can also be used. This step is an example of a biological process of active transport mechanism where the caffeine molecules in the cells of tea leaves degrades down to release the phytochemicals inside it like tannins and flavonoids. These molecules then spread into the aqueous layer through the process of diffusion.
• Acid-base liquid-liquid extraction: In this step, first anhydrous sodium carbonate is added to the aqueous extract and then an organic solvent like dichloromethane or chloroform is added to the same. Caffeine is transferred from the aqueous layer to the organic layer because of its high affinity towards the organic solvent. This step involves an acid base reaction between the base sodium carbonate and the acidic tannin molecules. It is termed as the liquid-liquid extraction as the caffeine gets transferred from one liquid layer (water) to another liquid layer (chloroform/dichloromethane). Chloroform and dichloromethane are both highly volatile and inert in nature; so they are the best choice as solvents for this step.
• Liquid-Solid extraction: In this step, the organic layer is separated from the aqueous layer through a separating funnel. The solvent is the allowed to evaporate leaving behind the crude crystals of caffeine. This is then crystallised using ethanol as solvent.

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.

• A 250 cm³ glass beaker was taken and filled with 50 cm³ of drinking water using a graduated measuring cylinder.
• The beaker was placed on the tripod stand with the Bunsen burner below it to heat the water.
• A temperature probe was inserted into the water to check its temperature and a Vernier Logger pro was connected to the temperature probe.
• The water was heated until the temperature reaches 60.00 ± 0.01°C.
• Two tea bags were taken and dipped into the water and the stop-watch was started.
• The tea bags were dipped until the stop-watch reads 120.00 ± 0.01 s.
• The tea bags were then squeezed completely using a glass rod to drain out all water inside it.
• The tea bags were taken out and discarded.
• The tea extract was allowed to cool down. It was kept in an ice bath for this.
• The temperature of the tea extract was measured using the temperature probe and the cooling process was continued until the temperature of the tea extract reaches to or below 20.00 ± 0.01°C.
• 10.00 ± 0.05 cm³ of chloroform was added to the tea extract using a graduated pipette keeping the tea extract in the ice water bath.
• The extract was then transferred into a separating funnel.
• A 50.00 cm³ glass beaker was taken and its mass was recorded using a digital mass balance.
• The organic layer was extracted out in a 50.00 cm³ glass beaker.
• The beaker was covered with a filter paper. The filter paper was tied with a rubber band and small holes were pricked in the filter paper using a sharp pointed pencil.
• The beaker was placed in a safe place and the solvent was allowed to evaporate.
• In the following day, crude crystals of caffeine were observed in the beaker.
• The crystals were dissolved in 10.00 ± 0.05 cm³ of ethanol added to the same beaker using a graduated pipette.
• The beaker was tied with a filter paper and tied with a rubber band as done earlier. Small holes were pricked in the filter paper allowing the solvent(ethanol) to evaporate.
• In the following day, pure shinning white crystals of caffeine was obtained in the beaker and the mass of the beaker was recorded again using a mass balance.
• Steps 1 - 20 were re-iterated for four more times to collect data in five trial sets.
• All of the above steps were repeated for other temperature values - 70.00 ± 0.01°C, 80.00 ± 0.01°C, 90.00 ± 0.01°C, 100.00 ± 0.01°C, 110.00 ± 0.01°C and 120.00 ± 0.01°C

• During the heating processes, gloves, lab coat, facemask and eye-goggles were worn to ensure that human skin does not come in contact with high temperatures and consequently burn.
• The aforementioned pieces of equipment were also worn during the addition of chemicals such as 100% Ethanol, which is highly flammable; Sodium Carbonate, which is a strong skin irritant; Dichloromethane, which is highly toxic; and Anhydrous Sodium Sulphate, which can severely dehydrate your skin.
• The facemask was particularly important during the addition of these substances, and during the transfer of caffeine into the vial, considering the fact that caffeine can even be lethal in a very small amount if consumed.
• To further reduce the risk of burns, metallic tongs were used for the movement of apparatus to and away from the heat source.
• Whilst any substance was heating, vision was entirely focussed on the heating process to minimise the chance of any mishaps at school.
• Hands were cleaned with water and soap after every trial rather than sanitiser which is flammable.
• The experiment was conducted in a fairly secluded space with minimal movement which improved the overall safety of it.

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.

Mass of caffeine crystals = (Mass of beaker + caffeine) – (Mass of beaker)

= (21.30 ± 0.01g) – (20.50 ± 0.01g) = 0.80 ± 0.02 g

\(\text{Average mass of caffeine crystals =  }\frac{0.80+0.79+0.78+0.80+0.80}{5} = 0.01\)

\(\text{Standard deviation (SD) = }\frac{(0.80-0.79)^2+(0.79-0.79)^2+(0.78-0.79)^2+(0.80-0.79)^2+(0.80-0.79)^2}{5} = 0.01\)