What was the main food source in California during the Spanish and Mexican periods? I'm expecting it was either corn or wheat, but I don't know which.
Here's an interesting article on the topic, from the Santa Barbara Independent newspaper. It's titled, "What Did the Early Spanish Settlers Eat?" and should answer your questions:
The primary crop was wheat, in addition to significant amounts of corn, beans, barley, and peas. As the mission’s water system developed, more sophisticated irrigation techniques were possible, making for increased agricultural yields.
As Ben Crowell observed above, while the missions produced grain and legumes, the rancho economy was extremely focused on cattle production. Californios were rich in products made from leather, tallow, and beef. Meat was often wasted after slaughter, left to rot on the bone, or boiled up in tallowmaking. They had so much beef they could have made a great profit selling it to the Russians, if they had arranged better salt production. It seems safe to say that Californio households were eating their fill of beef.