My guess would be no valve atall, the dosers prevent siphons and the return flow would provide enough pressure to keep the food in the line until the dosers turned on. so the only time food would enter the return would be when the dosers are pushing it in.I also did something similar. If you want to take it up a notch, add a few $35 mini magnetic stirrers underneath your bottles. Program them to turn on for 10 minutes before each feeding.
What type of valve did you use to inject into the pass-through pipe? Is it a solenoid?
Sorry for the delayed response...I guess I missed the notification . I am feeding Reef Nutrition food...What are you feeding? More details. How do you keep the line outside of fridge cold, I plan on setting up a nps tank and this might be what I was looking for.