Short answer: Get the heat out before it goes on the truck, then keep it out. Pre-cool the box, load fast, leave air gaps, and open the doors as little as you can.
Steps
Start cold. Produce should arrive at the truck already cool. You cannot chill warm produce on the road. Once cool, keep it cool.
Pre-cool the box to the target temperature before loading.
Load fast. If the box is pre-cooled and loading is quick, turn the cooling unit off while doors are open so the fan does not pull in warm air. Turn it back on when doors close.
Leave air gaps. 1 to 2 inches from the walls, space above the load, and nothing blocking the cooling unit outlet. Pallets or racks let air move underneath.
Back up the cooler. Keep insulated chests and food-grade ice packs for the most sensitive items. Do not let ice touch crops that bruise from cold.
Limit door openings at the stop. Restock the display from the truck in small, planned trips.
Clean the backups. Wash and sanitize ice packs and chests after every use.
If the cooling fails
Move cold items into ice-filled chests right away.
Check temperatures with a probe thermometer, not the dashboard display.
Food held above 41°F for more than 4 hours gets thrown out. Ask your local health department for its rule.
Sources: Rutgers On-Farm Food Safety fact sheets · Hale Trailer, Best practices for reefer loading · Massachusetts DPH, SOPs for mobile food establishments
