pressure cooker overflowing while cooking soup

Question

All my life I've cooked soup in a big, old (over 30 years old) pressure cooker. I'd boil all the vegetables in it, open it to blend them all together, then add cabbage and let it boil anew.

Now, in an attempt to boil the vegetables for less time and, therefore, avoid losing so many nutrients, I've started blending the raw vegetables. That way, the whole thing only needs to boil once and the flavours are much stronger. What I do is simply blend the vegetables with some water until it is a thick 'juice'. Then I add some more water, to make it less thick, add the cabbage and put it on the cooker.

Unfortunately, the blended 'juice' starts foaming (I'm not quite sure if that's the accurate word, since I'm not a native speaker, but to me it looks like a thick vegetable foam) and often spills over the little opening for releasing some of the pressure. I've tried boiling it in a normal pot, and the situation was the same: whenever the soup comes to a boil, the foam starts climbing up the pot and, if the fire isn't immediately turned down, it will overflow.

Can someone explain why this happens now when it didn't before? And how can I avoid it overflowing?

Answer 1

Simply put: it's happening because you're now blending the vegetables prior to cooking.

All vegetables are composed of plant cells, which have relatively rigid, fibrous walls composed of various starches; these trap the moisture and nutrients that the plant needs to survive, and provide structure. As you cook (applying heat) those starches will soften and become less rigid. Some of the cells will burst and allow their interiors to leach out, but on the whole the structure slowly deforms and collapses as you cook. Once softened, the structure can be easily pulverized/blended.

If you blend your vegetables prior to cooking, the starches are too tough to completely destroy; they get shredded into small fibers instead. If you then apply heat, the fibers (and various proteins contained in the plant) will still link up with one another, and are still tough enough to provide some structure while also being extremely light. As a result, they'll form bubbles that trap steam as it boils out of the solution, producing the foam that you're seeing.

You could try adding an enzyme like pectinase to break down pectin, one of the compounds responsible for the foam. This will only be partially effective because there are other starches which contribute to the foaming effect, and pectinase works on pectin only.

You could also try reducing the heat and letting your blended vegetables only come to a bare simmer; that will reduce the steam being forced out of solution, and thus reduce the number of bubbles that form. This wouldn't really be an option in the pressure cooker, which relies on higher heat to produce pressure and thus faster cooking.

But really, the easiest way to avoid this would simply be going back to your old method of boiling before blending. The foam is tricky enough to deal with that you won't gain much in terms of convenience by just boiling once. And if you want to preserve the nutrients from the raw vegetables, blending first may do more harm than good because you're exposing them to additional oxygen and heat which break them down. There's not really a lot you can do to protect those nutrients from the high heat of a pressure cooker at any rate. If you want to preserve them as much as possible, you'll want to try a juicer or some other process that doesn't involve the application of (much) heat.