Goal: Horse stays off treat in strangers hand for 30 seconds, 2 cues.
How to teach this:
Train for added duration first.
Then when you have at least 30 seconds ( I usually do more so the horse has more than the skill that is being asked.) ask a friend that knows your horse to come help you train. This time, demo for your friend what you want her to do. Then have her hold the food in her closed fist and start from the beginning. You control the clicker. Work your way to an open hand first, then to 30 seconds. Ideally, this would occur over several short training sessions.
Next, ask a different person the horse knows but may not be as familair with. Retrain from the start with the friend holding the treat and you holding the clicker as before. Start with low level treats and work your way up as before. Training will likely go faster than the first two times. Your goal is to have 10 different people help you train 'Food Zen' with your horse for 30 seconds each before you test.
If your horse is anxious with strangers at all, take it slow and you will want to practice on more epople until he is confident. Choose people that will follow your instructions closely (or have even trained this beofre on their own horse) so the horse will have a positive experience with each new person and build a positive association.