Alginate bubbles are useful in chemistry lessons as well as in molecular gastronomy.
Molecular gastronomy is a new trend in haute cuisine, with chefs providing their guests with novel and strange culinary experiences using liquid nitrogen, gels and foams. One of the techniques that is becoming more well known is the use of alginate spheres containing different fruit juices or flavours. Even if you don’t frequent Michelin-starred restaurants, you may have come across these spheres in bubble tea.
Bubble tea, originally invented in Taiwan in the 1980s, spilled over from Eastern Asia to Western countries some years ago. It consists of a tea-based drink that also contains fruit jellies, tapioca or alginate spheres, filled with fruit juice or syrup.
Making and examining the behaviour of alginate bubbles can be fascinating and can be used in inquiry-based learning in the sciences.
In this article, we suggest how alginate bubbles can be used to teach various scientific concepts, presenting scientific phenomena in an aesthetic fashion. We introduce how to make alginate bubbles and present three example experiments, each of which can be performed in a one-hour lesson: an acid-base reaction, chemo-luminescence with redox chemistry, and thermal convection with a thermochromic effect.
Alginate bubbles are formed when an aqueous alginate solution (figure 1) comes into contact with a solution containing calcium ions, creating a membrane of calcium alginate where the two solutions meet (figure 2). Alginate is a long polysaccharide that becomes cross-linked in the presence of a divalent cation, such as calcium, to make a water-insoluble gel.
When the liquids come into contact, gelatinous calcium alginate is formed, encapsulating the alginate solution in spheres. If other compounds are also added to the alginate solution, such as flavours, colouring agents, or indicators, they are also encapsulated.
Although the indicator solution is inside the bubbles, the alginate membrane can exchange hydroxide or hydronium ions (hydrated protons) between the bubbles’ contents and the surrounding liquid. Changing the pH value of the surrounding liquid by adding an acid or a base will therefore change the pH of the liquid inside the spheres, and so the indicator will change colour.
While technical indicators can be used in the classroom, pH-sensitive extracts of red cabbage or garden radish peel could be used at home.
Alginate bubbles can be used to illustrate the phenomenon of luminescence by simply adding a luminescent compound to the alginate solution before the bubbles are formed. One easy way to do this is to use riboflavin (vitamin B2), which fluoresces under UV light. Although you can use pure riboflavin, you can also extract it from a food such as an instant custard powder that contains it.
Extracting riboflavin (optional):
Making luminescent bubbles:
Adding a thermochromic ink to the alginate solution can help to illustrate the phenomenon of convection. In Japan, a special thermo ink based on a lactone of crystal violet (and not to be confused with the thermo inks used in thermo printers) is sold to illustrate heat-related phenomena in physics.
The alginate bubbles will move to show convection: rising as they become less dense when heated, and then cooling and sinking back down as they become more dense again. At the same time, the alginate bubbles will change colour, showing that convection is associated with a change in temperature.
Part of this work was funded by the Teaching Enquiry with Mysteries Incorporated (TEMI) project (Peleg et al., 2015), supported by the European Union under the 7th Framework Programme for Research Funding "Science in Society" under Grant Agreement No. 321403.