Tea or coffee?

Tea and coffee are the two most widely consumed hot beverages, with around six million tonnes of tea and nine million tonnes of coffee produced each year. This results in a lot of waste material, in the form of spent coffee grains (SCGs) and used tea leaves, both of which could potentially make a handy biofuel feedstock. Unfortunately, when scientists have looked into this possibility, they’ve discovered that transforming SCGs and used tea leaves into biofuel is not as straightforward as they’d hoped.

Although SCGs contain up to 20% lipids, which can be extracted with a solvent such as hexane and then converted into biodiesel, this process is currently too expensive to be economically viable. Used tea leaves, on the other hand, contain cellulose that can be converted into glucose for fermentation into ethanol. As an added bonus, tea leaves contain fairy low concentrations of lignin, allowing the cellulose to be released and converted into glucose by simply treating the leaves with dilute acid. Unfortunately, tea leaves contain more hemicellulose (40% dry weight) than cellulose (24%), meaning this dilute acid treatment produces more xylose and arabinose than glucose, and xylose and arabinose can’t be utilized by the yeast used for commercial bioethanol production (Saccharomyces cerevisiae).

These problems have now inspired two research groups to explore alternate uses for SCGs and used tea leaves. Izuru Kawamura and his colleagues at Yokohama National University in Japan have tried using SCGs as a source of cellulose nanofibers, while Jianzhong Huang and his colleagues at Fujian Normal University in China have tried employing used tea leaves as a growth medium for lipid-producing microbes.

CNFs are becoming an increasingly important resource, as scientist find they can be used to produce a wide range of products and materials, including filters, battery electrodes and optical fibers (see Cutting-edge cellulose). Most of these CNFs are derived from wood, often waste pulp from the timber industry, but scientists have been exploring various other potential sources of CNFs, including bamboo, banana peel and now SCGs.

Cellulose accounts for around 10% of the dry weight of SCGs. To extract this cellulose, Kawamura and his colleagues treated the SCGs with an oxidant called 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), which releases the cellulose as clumps of CNFs. As Kawamura and his colleagues report in a paper in Cellulose, they then simply immersed these clumps in water and blasted them with ultrasound to break them apart into individual nanofibers around 25nm wide.

One major potential application for CNFs is as a reinforcing material in plastic composites, replacing carbon fibers. Demonstrating that their SCG-derived CNFs could be utilized for this purpose, Kawamura and his colleagues integrated them with poly(vinyl alcohol) to form a composite film.

Used tea leaves actually contain double the amount of cellulose as SCGs, meaning they could potentially also be used as a source of CNFs. But Huang and his colleagues wanted to test their potential as a microbial growth medium, for which application SCGs have also been considered. Because although S. cerevisiae can’t make use of xylose or arabinose, other microbes, including other species of yeast, can utilize them perfectly happily, especially with a bit of mutation thrown in. 

Huang and his colleagues wanted to develop a strain of a yeast known as Rhodosporidium toruloides that would grow on the xylose, arabinose and glucose released when used tea leaves are treated with dilute acid. They chose this species of yeast because it is known to produce lipids and several carotenoids with medicinal properties, all of which could generate revenue. 

As they report in a paper in Biotechnology for Biofuels, Huang and his colleagues developed such a strain by using a technique called plasma mutagenesis to create a range of mutants, which they then grew on tea leaf hydrolysate to find those that produced the most lipids and carotenoids. In this way, they were able to develop strains that could produce lipids and carotenoids when grown on tea leaf hydrolysate at sufficiently high concentrations for industrial production. Perhaps using the same technique, they could next develop strains that prefer coffee.

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