— Never put a cooler in the trunk but transport it in the passenger area of a vehicle where it can benefit from air conditioning.
— When you get to the picnic site, remove the cooler from the car and set it in the shade.
— Pack drinks or non-perishable snacks in a separate cooler so you don't have to open the one containing the food.
— Refrigerate all planned picnic foods so they're cold before you pack them in a cooler.
— Never take home leftovers from a picnic. Discard any prepared food not eaten.
Source: Dave Pavletic, food safety manager, Middlesex London Health Unit.