Page 8 - Science Focus (Issue 016)

The recent trend of long-distance running has What magic has been done by the solutions? Let’s
caused a lifestyle revolution of many Hongkongers. In explore the chemistry behind!
2018, more than 130 long-distance races were held in It is clear that the membrane forms whenever the
Hong Kong [1]. Nevertheless, some environmentalists solutions of sodium alginate and calcium chloride
concern whether the plastic bottles being disposed come into contact. Sodium alginate, usually derived
of in such sporting events place a burden on our from seaweed, is the sodium salt of alginic acid.
planet, for example, about 920,000 plastic bottles Alginic acid is a long-chain polysaccharide (a
were generated in the 2018 London Marathon [2, 3]. It carbohydrate), which contains many carboxyl groups
is worth noting that recycling facilities are not always (–COO H ). In sodium alginate, sodium ions (Na )
+
+
-
available in large-scale events like the marathon. with a single positive charge neutralize the negative
Apart from the production of plastic waste, Lucy Ashe, charge of the anionic groups (–COO ) instead
-
a British distance runner, suggested that the amount of hydrogen ions (H ) – forming a kind of sodium
+
of water provided by a plastic bottle is much more carboxylate (–COO Na ).
+
-
than sufficient to keep them hydrated during the race
[4]. So, are there any better choices? How about replacing the sodium ions (Na ) with
+
calcium ions (Ca ), which have two positive charges?
2+
The edible water bottle can provide a perfect Then, each calcium ion will bind to two anionic
solution to the problems. First invented by a Spanish groups (–COO ) at a time instead of one. That means,
-
chef for the use in molecular gastronomy, scientists the two “hands” of each calcium ion (Ca ) can now
2+
have adopted this technique, called spherification, hold two long alginate chains of different molecules
to produce the biodegradable and fully edible water simultaneously and join them together, which is
bottle. The water bottle can be made from the two technically called the formation of crosslink. An
edible chemicals, sodium alginate and calcium extensive network of molecules is therefore formed,
chloride (CaCl ). As both chemicals are soluble in and it is not surprising that this large structure is not
2
water, we can first dissolve them in two bowls of soluble in water anymore – this forms the membrane
water separately. By adding drops of sodium alginate of the water bottle.
solution into the calcium chloride solution, caviar-like
beads form – that’s the “edible water droplet”! The biodegradable alginate membrane will
not burden the landfill sites even if you choose to
Nevertheless, that is still far from being a “water dispose of it, instead of swallowing it. While a plastic
bottle” – the droplet is too small that the runners bottle takes up to 450 years to decompose [5],
probably won’t find it satisfying. To make a bigger the film naturally breaks down within 4 to 6 weeks
water pouch, we can first freeze the drinking water so [3]. Therefore, the edible water bottle can possibly
that it becomes an ice ball comparable to the size of replace plastic bottles in major sporting events and
a tennis ball. By dipping the ice ball into the calcium reduce disposable waste.
chloride solution and the warm sodium alginate
solution sequentially, a chemical reaction starts, and a However, the novel container does have some
soft membrane form around the ice ball. After the ice intrinsic weaknesses comparing to its
thaws, it becomes the “water bottle” with conventional counterpart.
the ideal size.

可以食用的

Water Bottle

「 水樽」 Edible By Teresa Fan 樊銘姍