At the beginning of each game I start with different amount of food. I understand that it is related to how I end the previous game. What is the rule?
This is in the rules here
On page 9
To play a Solo game, start with 0 Food.
and later on the same page
Because you have more permanent Occupations after each game, the goal score that you must reach goes up in each game: In the first game, your goal is 50 points, then 55, 59, 62, 64, 65, 66 and 67 points. After the eighth game, the Solo game series is over. (You can of course play on with all permanent Occupation cards, in which case the goal score increases by 1 point per game.) At the start of each game in the series, you receive 1 food for every 2 points by which you exceeded the goal score for the previous game (rounded down).