Sugar syrup with orange juice foams excessively Today I made an orange chiffon cake with buttercream frosting. I used Rose Levi Beranbaum's Classical buttercream recipe, in which 100 g of sugar and 90 g of water are cooked to a softball-stage syrup (114.5 Celsius) and then added to the yolks. 

After making the cake batter, I had 50 g of just-squeezed orange juice left over. So instead of using 90 g water, I used 50 g of orange juice and 40 g of water for the syrup, hoping to have a more fruity flavor in the buttercream. 

Shortly after passing 100 Celsius, the syrup started to foam, throwing very large and stable bubbles. I was very busy beating them back, using both my stirrer and blowing on them to pop them. But the foam still almost managed to climb out of the pot, it did form a heap above the upper wall level (the actual syrup only covered about 1-2 cm at the bottom of the small pot). I was also afraid that I will get an uneven heating, as the syrup forming the cell walls, with all the air inside, was probably not at the same temperature as the syrup puddle at the bottom. 

The moment I took the syrup off the heat and poured into a cold cup, the foam disappeared completely. 

Is that foam really a problem? Could it have ruined the syrup, or do I simply take a deeper pot and live with it? 

Also, what caused the foam? Was it the orange juice, or can it happen without it too? If it happens without, what is the most likely trigger for it (I have made syrup before, and cannot remember such strong foam-building). 

Regardless of whether it affects the end result, I found it difficult to work with. Is there something I can do to prevent it?
I don't know what exactly it is about citrus - whether a particular combination of proteins, essential oils, lots of small particulates, or something else - but in my experience this is common to many citrus fruit.  It seems to be a general property that they capture gas bubbles very effectively, leading to froth.  Citrus froths impressively whenever it's heated, shaken, or carbonated, and it probably didn't help that your syrup would have been fairly viscous.

I suggested in another answer about excessive foaming that reducing the heat can reduce the amount of gas escaping from solution, and thus reduce frothing. (This is also recommended in the marmalade recipe provided by @Debbie in comments.) But I suspect this would slow or interfere with your attempts to reach a soft-ball consistency.

Another thing that helps is clarification - removing small particulates seems to reduce citrus' tendency to foam. There are several methods being used by leading bartenders to clarify juices for carbonation, the easiest of which is filtration.  You could run your juice through a fine coffee filter, which would take a while but be easy to do for small amounts at home. If you want to get really serious about it, you could also try something like agar clarification... but that's almost certainly overkill for a spontaneous decision to use a spare 50 g of juice. Maybe if you were producing this at commercial scales...

At any rate, the foaming probably won't ruin your syrup.  Any effects due to uneven heating are likely to come out in the wash of other factors, such as the added proteins and acids in the juice. Using a taller pot and stirring well are probably the simplest ways to deal with this as a minor annoyance, unless you need a more rigorous solution for working in larger amounts.