Food trade routes?

[Gregorius]

I've looked everywhere I can find, but I haven't found a reference to how food trade routes are established. It seems to be totally random. I have cities that have an imbalance between production and consumption of 1 unit. This causes regular food shortages. Occasionally, I can set up a food trade route to/from another city which removes the problem. But sending a food caravan often just increases the population without a trade route being established.

So, how to I ensure the creation of a food trading route?