Foam is a natural by-product of boiling rice and in no way an indication that something has gone awry. That said, eruptions of gluey mousse over the stove top are rarely welcome. A few simple tweaks help you keep the foam at bay.
Starch Content
Just like beans, rice forms a head of foam as the starches build volume in boiling water. Some types of rice are starchier than others, particularly those high in amylopectin. As a general rule, long-grain rice such as basmati produces less foam than medium- or short-grain varieties, such as jasmine. To get the sticky rice favored in many Asian dishes, be prepared to put up with a greater level of foam.
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Rinse It Clean
Running the rice under cold water and rubbing the grains together will draw out much of the excess starch, which can then be discarded.
Swirl and rub the rice until the water runs clear. It may take two or three passages until the water is no longer cloudy.
Likewise, soaking the rice beforehand breaks down the starches. While many varieties of commercial rice are prewashed, any amount of extra soaking will still draw out starch. For long-grain or tough brown rice, soaking is a worthwhile step in any case, to soften the grains.
Add Oil
Oil naturally subdues foaming, so add a dash of olive oil or a knob of butter to the rice before boiling. Either add the oil to the rice and water, or stir it into the dry rice and lightly toast it, which creates a thicker rice in the style of pilaf.
Perfect Measures
Overfill the pot with water, and the foam will inevitably surge from beneath the lid once the water begins to boil. Instead, choose a large pot and add no more than 2 cups of water for every cup of rice -- although brown rice requires a little extra.
The aim is to cook the rice through absorption, so that the grains draw in the moisture and swell up, rather than cooking at a rolling boil with plenty of space to move around.
Cook Gently
Bring the water to a boil, and then cover the pot immediately and lower the heat to a simmer. Leave it to cook for about 18 to 25 minutes, depending on the variety of rice.
If foam does escape, lift the lid and skim it away, but don't stir the rice, which will turn it mushy. The golden rule for fluffy rice is to leave the lid on and let the steam cook the rice. At the first sign foam is building, lower the heat or remove the pot from the burner, rather than taking off the lid.
Once the rice has absorbed the water, leave it to rest off the heat for a further 5 minutes, during which it will continue to absorb moisture.