Where to find health-conscious food manufacturer/brand executives to better understand their pain points related to customer reach and interactions?

Try the International Food Service Executives Association (https://www.ifsea.org/). Their membership is incredibly broad. You can also try connecting with them on LinkedIn. The hard part is finding the first couple of executives, if you can tap into their networks, then you will find success.

Concerning email formats, use subject line: "Quick Question"

Then use a brief straight forward message:
"Dear XYZ,

I am ABC and I am trying to create a start up to help food brands. Could I ask you a few questions to learn about needs for food brands?

Sincerely,

ABC"

There are several online survey groups that employ panels of professionals that you can survey. A recent favorite is Centiment. But Qualtrix is also popular.

To start, it sounds like you are seeking a few people to validate your product concept and provide feedback on your direction. If that's the case, you may want to browse LinkedIn and reach out directly to the founders of some brands you know and personally use. Mentioning that personal connection will go a long way to getting a response.

Once you get beyond that initial phase and you want to start scaling, that's another story. Are you targeting DTC, grocery, or foodservice (restaurant) brands? There are trade associations that could get you a good start, depending on which segment you're aiming to target.

In terms of an email format that works, trial and error -- along with careful measurement -- might be your best approach. Try some different formats and see what resonates with your audience. Keep in mind that you're likely reaching out to people who hear a LOT of sales pitches, so keep in mind the abbreviation WIIFM (What's In It For Me, where me=your prospective client) -- and shorter is almost always better. Your goal in sending an email is to schedule a meeting, not tell the entire story of your offering.

I'd be happy to talk in more detail if you'd like to schedule a call.