A picnic is the hardest test an insulated bag ever faces. The food leaves the fridge in the morning, travels in a warm car, then sits on grass or sand in full sun for several hours while everybody keeps opening it for another sausage roll. Almost every step of that sequence works against you, and the usual response is to hope for the best.

Sun on the bag is the biggest single factor

Insulation slows heat entering through the fabric, but direct sunlight on a dark exterior heats that fabric to well above air temperature. A bag in the shade of a tree, a windbreak or even a folded towel stays dramatically cooler than the identical bag two metres away in the sun. Shade first, everything else second.

Pack twice the cold you think you need

Household advice about perishable food and warm conditions gets stricter as the temperature rises, and a beach in August is not a mild office. Two or three properly frozen packs in a large tote is a sensible minimum, arranged around and above the food. A well-built cooler tote from a range such as https://mexsus.com/ gives you the volume to do that without squashing the salad.

Two bags beat one

The reason picnic food warms so fast is constant opening. Split the load: one bag for drinks and snacks that people raid all afternoon, one bag for the perishable food that stays shut until you eat. Every time the second bag stays closed you gain more than any upgrade in insulation would give you.

Sand, grass and the wet base

Dry sand in the sun gets far hotter than the air above it, so a bag placed directly on it is being heated from below. Put it on a towel, a bag, or in a hollow of shaded ground. Damp grass is the opposite problem: the base of the bag soaks up water and the lining stays wet in the boot on the way home.

• Shade the bag before you unpack anything else
• Separate the constantly-opened drinks bag from the food bag
• Never set a cooler directly on hot dry sand
• Take a bin bag; leftovers should come home, not sit out
• Dry the base and lining thoroughly the same evening

What not to take to the beach

Some foods simply do not belong on a hot sand outing. Anything with a cream filling, rice salads, cooked shellfish, home-made mayonnaise dressings and soft cheese all lose their margin very quickly. Cured meats, hard cheese, firm fruit, breads and dry baked goods handle the day far better.

Leftovers on the way home

The temptation after four hours in the sun is to put the remaining chicken back in the bag and finish it for supper. If the packs have thawed and the bag is no longer cold, that food has been out of control for the whole afternoon. It is not worth the gamble; sandwiches and salads that have been warm all day belong in the bin.

The clean-up nobody does

Beach bags come home with sand in the seams and a puddle of melted ice in the base. Left zipped in a cupboard, that becomes a smell within a week and a mould patch within a month. Rinse the interior, wipe with soapy water, and stand the bag open until it is bone dry.