There are many factors that go into an advertising strategy for a mobile application. I can help you evaluate and determine your best options if you have time today we can conduct a quick phone call. I have a lot of experience in advertising and the options are numerous.
1. Who will be selling the ad space? In house staff, Supply Side Partner etc.
2. How much do you know about your users that can be passed in an ad request?
3. What are your uniques? How many users does you app have currently?
4. What is the open rate?
5. What is your expected unique growth?
6. How many views do you expect each unique to generate on a daily basis?
This is not a comprehensive list of questions just a taste of the sort of areas that will need to be understood in order to build a media kit. You are selling an audience and the more you understand the worth of your audience the better you will be at demanding value.
Finally, you mention that you only want to sell food related ads. I understand that your content is food related and this may be a great fit, however, you will want to maximize your ad yield and limiting yourself to one advertising category may leave money on the table. So, I would like to understand your app and the user parameters before I comment on this strategy.