Food is a way to increase your population per planet by letting you build more cities (4 food required per city). With crusade, population is only used for raw production or colonizing.
The "arable land" tiles can be farmed to add to the food supply, and all other food tech enhances the food production from these tiles or city population. I have found that the larger the planet, the more likely it is to have "arable land" tiles. Additionally, you can research tech to give all of your planets a base food production of +1 which adds 1 unit of food for every planet in your empire.
You can also research and upgrade your planetary city capital on each planet to increase base population by +1, and this can be done more than once. Note this doesn't require any more food production.
Cities are more useful now and placement is critical, as getting a +1 population bonus to a city increases the available planetary population by a full 25% for each +1, so you don't need many cities to hit the limit of max population equal to the planet's class rating. If you do build a city, always put a hospital next to it for a 50% bump to your planetary population potential.
If you just want increased production on a planet, and don't want to work on food, mine the local asteroids to get +1 raw production from each one within 10 hexes. Each asteroid will be as effective as a unit of population without any "wasted' tiles for food production.
