How do I keep food and drink cold on a day out?

A cool bag does not make anything cold. It slows down warming, which means everything you put in it should already be cold and the bag itself should be too.

Safety

What you'll need

• A cool bag or box, ideally with a firm lid
• Solid freezer blocks rather than small gel packs
• A separate bag for drinks

Step by step

1. Chill everything the night before.Food and drink straight from the fridge, and the cool bag itself in a cold place. Packing room-temperature items means the ice packs spend their capacity cooling them rather than keeping them cold.
2. Freeze the bag if it fits.A soft cool bag folded into the freezer overnight starts the day genuinely cold. It makes a noticeable difference for no effort.
3. Use large solid ice blocks.A few big blocks last far longer than many small gel packs, because there is less surface area relative to their mass. This is one of the biggest practical differences.
4. Freeze drinks and use them as the ice.Bottles of water or cartons of juice frozen solid act as ice blocks and become drinks as they thaw. Leave headspace in bottles, as liquid expands when frozen.
5. Put the cold at the top.Cold air falls. Ice packs on top of the food cool downwards through the bag, which works better than sitting them underneath.
6. Fill every gap.Empty space fills with air that warms quickly. Pack tightly, and use a tea towel or crumpled paper to fill what is left.
7. Use two bags if you can.One for drinks and one for food. The drinks bag gets opened constantly, and every opening lets the cold out — keeping them separate protects the food.
8. Keep raw and ready-to-eat separate.Raw meat for a barbecue sealed and at the bottom, well away from anything eaten as it is. Cross-contamination in a cool bag is a real risk.
9. Keep it out of the sun.In shade, under a blanket, or in the footwell of the car rather than the boot. A car boot in summer reaches temperatures that defeat any cool bag.
10. Open it as little as possible.Decide what you need before opening, and take it all out at once. Repeated brief openings warm the contents faster than one longer one.
11. Choose food that travels well.Hard cheese, cured meats, whole fruit, bread, crisps and tinned drinks are far more forgiving than egg mayonnaise, cream cakes and cooked rice.
12. Be strict about the high-risk items.Cooked rice, cooked chicken, dairy, anything with mayonnaise or cream. These have the shortest safe window and cause most of the problems.
13. Take a thermometer for a long day.A small fridge thermometer in the bag tells you whether the contents are still below 8°C. Cheap, and it removes the guesswork.
14. Throw away what has been warm.If the bag has been warm for hours, discard the perishable contents rather than judging by smell. Food that causes illness often looks and smells entirely normal.
15. Wash the bag afterwards.Wipe it out and let it dry fully open. Cool bags left damp and closed develop mould and a smell that transfers to everything.

Tips

• Chill the bag as well as the contents. A cool bag only slows warming — it cannot cool anything down.
• A few large ice blocks beat many small gel packs, because of the surface area to mass ratio.
• Separate bag for drinks. Every opening costs you cold, and the drinks bag gets opened constantly.

Common mistakes

• Packing room-temperature food and expecting the bag to cool it — A cool bag is insulation, not refrigeration. Warm contents use up the ice packs' capacity and the whole bag ends up warmer for the rest of the day.
• Putting the cool bag in the car boot — A boot in summer sun becomes very hot and stays hot, which no domestic cool bag can counteract. The footwell of the air-conditioned cabin is far better.

If it doesn't work

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