Markets try to replenish the existing food type before supplying a second one (unless the houses have reached the point of demanding a second food type. Thus, they will in fact go out of their way to find food type 1 rather than pick up food type 2. Because food type 1 is usually wheat, it does look as if wheat has a higher priority, but in fact whatever food is the houses' primary type will get this favored treatment.
If the houses on her route run entirely out of food, there's no longer a favored food type to replenish, and the market buyer will grab whatever food is closest. That food type now becomes the favored food type, and she tries to replenish that before adding a second type. This is why market buyers might ignore a nearby granary overflowing with meat, and keep trekking miles for wheat. As long as they can keep a trickle of wheat in the pipeline, wheat remains the houses' primary food type and meat is ignored. Only when the houses run entirely out of food does that preference go away.
:Does this mean that the first food type available to new housing becomes the prefered type?
Yes, although the notion of a "preferred type" might be a bit misleading. Markets strive to replace the existing food type, whatever it is, without inherently preferring one type over another. If you create new housing in a city whose citizens already eat primarily wheat, then your existing markets, which are already biased toward wheat, are highly likely to provide the new homes wheat first, thereby making it seem to be a "preferred" food.
If you were to build new housing in isolation from the rest of your city, and build a granary nearby that's stocked with meat, and then build a market to serve only those new houses, the new market will get meat from the new granary and the new houses will receive meat as their "primary" food. That new market will subsequently try to replace their meat.
