Lemon juice, the ultimate anti-foaming agent?

Question

As an amateur soda maker, I have noticed an odd phenomenon: lemon juice sodas do not foam. Virtually every juice I have tried foams to some degree when mixed with carbonated water. Lemon juice does not. I have read "folk" explanations for this as being due to the "acidity" of the lemon juice, but this is not correct because other acidic juices like lime and orange juice foam. Why is it that lemon juice has an anti-foaming action?

Further information: it should be noted that "pasteurized" (ie, boiled) lemon juice does foam. Only fresh-squozen lemon juice is anti-foaming. Therefore, what is doing the anti-foaming may likely be some volatile compound.

My understanding of anti-foaming is that it is caused primarily by localized depression of surface tension, and that liquids having heterogeneous species promote anti-foaming because of the surface tension differential between those species at the surface. Therefore, one possible theory is that the lemon juice has some volatile oil which populates the surface of the water only in spots and is causing the surface tension breakage. Even if this theory is correct, it remains to determine what this mysterious volatile oil is that is the cause.

Answer 1

A lemon has an oil that is present mostly in the skin. For instance if you squeeze lemon into hot tea in a styrofoam cup and let the tea sit, then you'll see a surface roughness at the liquid level. The oil actually dissolves the styrofoam. The oil acts a defoamer. So with a styrofoam cup you're suppose to use "commercial" lemon juice which has no oil (lest the cup dissolve and spill the liquid).

Answer 2

If you squeeze a fresh lemon and then use a strainer that retains any pulp and then add the juice to carbonated water it will foam - I think this indicates that it's the Limonene in the pulp that prevents foaming

Answer 3

If you imagine a bubble as being composed of walls of thin films, then anything which weakens those walls can lead to collapse. (note that no pure liquid will foam, foam is proof that there is surface active agents present.) Anyway, there are several mechanisms to cause bubble collapse: addition of a volatile, addition of a viscosity lowering agent, addition of a particulate which disrupts the film, addition of an agent which lowers the surface tension...Those are all I can think of off the top of my head. Foam has an internal structure but in liquids is usually not stable, due to drainage. Obviously, faster drainage (lower viscosity) tends to more rapidly thin the film (hence disrupt it). Increased evaporation both thins the film (weakening it) and increases stresses on it (shrinkage). There's no way for me to determine which if any of these factors are contributing in your case. One interesting experiment is to store the liquid and see if the amount of foaming changes with time (in a well-sealed (ie glass) bottle). The most effective general defoamers I am familiar with are silicone dispersions which are colloidal and partially soluble. Their slow dissolution disrupts the films, while their low density tends to float them (and the silicone oligiomers are surface active). These types of materials once they are at equilibrium concentration, are much,much less effective. Similarly, once any volatiles reach equilibrium (in the head-space) their contribution will be quite limited. So, while it's really difficult to be definitive with natural products (they tend to oxidize or even become food for microorganisms) a liquid which continues to be non-foaming (with the addition of component X) after hours and days is likely due to something floating on its surface.