By Teresa Fan 樊銘姍
The recent trend of long-distance running has caused a lifestyle revolution of many Hongkongers. In 2018, more than 130 long-distance races were held in Hong Kong . Nevertheless, some environmentalists concern whether the plastic bottles being disposed of in such sporting events place a burden on our planet, for example, about 920,000 plastic bottles were generated in the 2018 London Marathon [2, 3]. It is worth noting that recycling facilities are not always available in large-scale events like the marathon. Apart from the production of plastic waste, Lucy Ashe, a British distance runner, suggested that the amount of water provided by a plastic bottle is much more than sufficient to keep them hydrated during the race . So, are there any better choices?
The edible water bottle can provide a perfect solution to the problems. First invented by a Spanish chef for the use in molecular gastronomy, scientists have adopted this technique, called spherification, to produce the biodegradable and fully edible water bottle. The water bottle can be made from the two edible chemicals, sodium alginate and calcium chloride (CaCl2). As both chemicals are soluble in water, we can first dissolve them in two bowls of water separately. By adding drops of sodium alginate solution into the calcium chloride solution, caviar-like beads form – that’s the “edible water droplet”!
Nevertheless, that is still far from being a “water bottle” – the droplet is too small that the runners probably won’t find it satisfying. To make a bigger water pouch, we can first freeze the drinking water so that it becomes an ice ball comparable to the size of a tennis ball. By dipping the ice ball into the calcium chloride solution and the warm sodium alginate solution sequentially, a chemical reaction starts, and a soft membrane form around the ice ball. After the ice thaws, it becomes the “water bottle” with the ideal size.
What magic has been done by the solutions? Let’s explore the chemistry behind!
It is clear that the membrane forms whenever the solutions of sodium alginate and calcium chloride come into contact. Sodium alginate, usually derived from seaweed, is the sodium salt of alginic acid. Alginic acid is a long-chain polysaccharide (a carbohydrate), which contains many carboxyl groups (–COO–H+). In sodium alginate, sodium ions (Na+) with a single positive charge neutralize the negative charge of the anionic groups (–COO–) instead of hydrogen ions (H+) – forming a kind of sodium carboxylate (–COO–Na+).
How about replacing the sodium ions (Na+) with calcium ions (Ca2+), which have two positive charges? Then, each calcium ion will bind to two anionic groups (–COO–) at a time instead of one. That means, the two “hands” of each calcium ion (Ca2+) can now hold two long alginate chains of different molecules simultaneously and join them together, which is technically called the formation of crosslink. An extensive network of molecules is therefore formed, and it is not surprising that this large structure is not soluble in water anymore – this forms the membrane of the water bottle.
The biodegradable alginate membrane will not burden the landfill sites even if you choose to dispose of it, instead of swallowing it. While a plastic bottle takes up to 450 years to decompose , the film naturally breaks down within 4 to 6 weeks . Therefore, the edible water bottle can possibly replace plastic bottles in major sporting events and reduce disposable waste.
However, the novel container does have some intrinsic weaknesses comparing to its conventional counterpart. For instance, the membrane is too fragile that it can be pierced easily, making the pouches difficult to tolerate the rigors during transportation. In addition, the gel-like membrane degrades over time, restricting its replacement of the plastic water bottle on store shelves.
Recently, the edible sachets have carved out another niche in restaurants, mainly for carrying the sauces for takeout orders . Let’s be open to innovation, and remember that reducing plastic waste demands our efforts!
 Sportsoho. (n.d.). Saishi Rili [Event Calendar]. Retrieved from https://www.sportsoho.com/mod/sh_calendar/calendar-month.php
 Kottasová, I. (2019, April 27). Seaweed pouches will replace thousands of plastic bottles at the London marathon. Retrieved from https://edition.cnn.com/2019/04/26/business/london-marathon-seaweed-water-bottles/index.html
 Nace, T. (2019, April 29). London Marathon Runners Were Handed Seaweed Pouches Instead Of Plastic Bottles. Retrieved from https://www.forbes.com/sites/trevornace/2019/04/29/london-marathon-runners-were-handed-seaweed-pouches-instead-of-plastic-bottles/#22fef922ba20
 BBC News. (2018, September 16). Harrow Half Marathon to use edible water bottles [Video file]. Retrieved from https://www.bbc.com/news/av/uk-england-london-45520944/harrow-half-marathon-to-use-edible-water-bottles
 Wright, M., Kirk, A., Molloy, M., & Mills, E. (2018, January 10). The stark truth about how long your plastic footprint will last on the planet. The Telegraph. Retrieved from https://www.telegraph.co.uk/news/2018/01/10/stark-truth-long-plastic-footprint-will-last-planet/
 Skipping Rocks Lab. (n.d.). Products – Notpla. Retrieved from https://www.notpla.com/products/
 Baguley, R, & McDonald, C. (2015, July 14). Appliance Science: Edible water bottles and the strange chemistry of spherification. Retrieved from https://www.cnet.com/news/appliance-science-edible-water-bottles-and-the-strange-chemistry-of-spherification/
 BBC Science Focus Magazine. (n.d.). How do edible water bottles work? Retrieved from https://www.sciencefocus.com/future-technology/how-do-edible-water-bottles-work/