Why does a pitcher with lemon juice have foam, while one with pure water does not?

Question

Whenever I pour water into lemon juice (pouring directly from the tap into the pitcher, not quietly along its edge) I get a foam on top:

The same pitcher with water (same water tap, pitcher, time between the water poured and the picture, temperature):

Another answer discusses the formation of foam when pouring water but I did not find any mentions of lemon juice being particularly surface active (for instance Kitchen Mysteries: Revealing the Science of Cooking only mentions that lemon juice adds water to various sauces and modifies the surface active elements like oil, but does not mention lemon juice's own surface active properties).

Which substances in lemon juice could help to form such a persistent foam (it lasts at least 10 minutes)?

Answer 1

I too have seen this effect with pure, unsweetened lemon juice.

To form a foam, (1) a surfactant is needed to lower the host liquid's surface tension (2) one needs to do mechanical work on the liquid to swell the surface area of the bubbles/foam and (3) the foam needs to be made faster than it breaks down. The Foam Wikipedia Page has a good summary of this.

The pouring of water and the pouring of lemon juice both supply the required work. So the only difference must be the presence of surfactants in the lemon juice. Citrus fruits contain significant amounts of oils and lipids, particularly in the skin and the matrix that makes up the cells of liquid in the lemon's flesh. All kinds of weird things- phenols for example, are found in lemons as well. So some of these compounds are clearly lowering the water-air surface tension to make foam as described in the Wiki page. I'm speculating that most of them come from the skin/flesh matrix, but this hypothesis would be very hard to test (as you'd need to extract the lemon juice without crushing the fruit, thus contaminating the juice with the skin).

Answer 2

Manufactured 'lemon juices' such as cordials typically contain emulsifiers which can act as surfactants to lower the surface tension of the liquid. This would result in a foam forming more easily when air becomes 'trapped' during pouring water into the jug.

Even homemade lemon squash which often has honey added as a sweetener will produce a foam because the honey can have a similar effect (as an emulsifying agent).

If it is pure lemon juice producing the foam, my hypothesis would be that naturally occurring amphiphilic molecules in lemon juice act as the emulsifying agent.

In fact, orange, lemon and grapefruit juices naturally contain a class of amphiphilic molecules called phospholipids, including phosphatidylethanolamine and phosphatidylchlorine, which have surfactant properties and help give them a 'cloudy' appearance. This would have something to do with the foaming observed when you pour water into a jug containing the juice as air becomes trapped beneath the surface.

Answer 3

This is a chemical reaction that occurs when the strong citric acid in lemon juice reacts with the water(water acts as a base). To form methane among other gases.

$$C_6H_8O_7 (acid) + H_2O(base) = CH_4 + others$$ (Not balanced). The reason why it does not foam as much whenever you pour slowly/closely is because you are not supplying enough energy for a larger reaction.