Fading lemonade challenge
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In the present challenge, lemonade is the topic. And please note that there is a prize to be won (a Springer book of your choice up to a value of €100). Please read on...
Meet the fading lemonade challenge
All this is simply cooking and indeed we should not consider culinary questions as cooks, but as chemists. Thus, in order to interpret the above experiment in the spirit of molecular gastronomy, one should note that the juice of lemon (Citrus lemon) is made of water, proteins (1%), lipids (1%), sugars (7%), organic acids (5%), with tiny quantities of various pigments, minerals, phenols, and vitamins . Among the various sugars, there are polymers such as pectins, but mainly sucrose, glucose, and fructose. The amount of solid residue obtained from citrus fruits after pressing (in order to extract the juice) accounts for half of the original mass of the whole fruit . The fibrous residue is mainly made of peels (albedo and flavedo) which make nearly one-fourth of the whole fruit mass, seeds, and fruit pulp remaining after juice and essential oil extraction .
Mint (Mentha × piperita L.)  is a plant commonly used by the pharmaceutical, cosmetic, and food industries. It has cooling properties because (−)-menthol has receptors on the trigeminal nerve as well as on the olfactory system  and it is used for its essential oil . Mint is also said to be a rich source of phenolic compounds . The various other compounds present in the mint leaf in decreasing order of magnitude are water, polysaccharides (cellulose, hemicelluloses, pectins), proteins, oligosaccharides and saccharides (including sucrose, glucose, and fructose), amino acids, organic acids (succinic, malic, or tartaric), and smaller quantities of phenolic compounds, chlorophylls, carotenoids, and fragrant compounds making up the essential oil. In the process of making the lemonade, only compounds having a low logP can be extracted and dissolved in water. As color is considered here, one has to note that chlorophylls and carotenoids are poorly soluble in water (logP ≈ 15 for beta-carotene), and only some of their thermal by-products can give color to the aqueous solutions.
Why does lemonade lose its color when squeezed lemons are added to it?
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