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Chilly bags are the cool way to shop

If you are likely to be delayed getting home with your groceries, keep meat cool in your car with chilly bags and icepacks. A study has shown that in sunny weather, temperatures in your car can soar, putting your food at risk.

We commissioned Christchurch-based Environmental Science & Research (ESR) scientists during 2004–2006 to study domestic food practices including the temperatures of meat and poultry during transport after purchase by consumers.

Chilled products that enter the temperature danger zone (between 4ºC and 60ºC) need to be refrigerated or cooked as soon as possible (within two hours).

Research into meat temperatures during transport

ESR scientists were interested in the difference between meats stored in a chilly bag with an icepack, compared with meat packs left only in their supermarket bags. They recorded the effects of time and temperature on the following packs of meat bought from a supermarket:

• beef mince

• rump steak

• sausages

• chicken drum sticks.

Researchers did experiments over three days in January and June, using the same sunny Christchurch spot each time. They used a car with non-tinted, tightly shut windows and no sunroof. The car had a separate boot compartment. In each case, the experiments ran for several daylight hours, beginning late morning and ending around 5pm.

These worst-case time/temperature scenarios simulated the changes meat might go through while being transported from the supermarket refrigerator to the domestic fridge.

On clear January days, the air temperature inside the car hovered around 40ºC. The air temperature in the boot of the car was roughly 10ºC cooler.

Research results and lessons learned from the study

After just 90 minutes, the surface temperature on a piece of un-insulated steak on the car’s back seat rose to 35ºC – close to optimal growth temperature for many bacteria. In the boot, where the air temperature was cooler, the un-insulated steak rose to 21ºC in the same timeframe.

For meat transported using chilly bags and icepacks both in the back seat and boot, the temperature rose only slightly over the same time.

The types of chilled raw meat used in these experiments would be cooked before being eaten. Cooking would destroy most, if not all, the bacteria present although spores and heat-stable toxins would survive. Of concern is the ready-to-eat meat, such as ham and deli meats. They would be subject to the same temperature increases but would not undergo further cooking to destroy bacteria.

Reassuringly, in a random survey 97% of Kiwis interviewed by ESR said their journey home from the supermarket typically took less than 90 minutes. In terms of delays – such as visiting friends, returning to work or doing more shopping – just under half said they were occasionally delayed while one in five said they experienced delays about once a year.

The greatest risk from bacterial growth during transport home is mainly cases where long transport times are involved. In such situations, a chilly bag and frozen icepack would help keep your food purchases safe. As some delays are unpredictable, always keeping at least a chilly bag in your car is a good idea.

In general, be aware and think ahead on hot sunny days. Shop for meat and poultry just before you head home. If you have to leave chilled foods in your car, take an icepack and chilly bag with you. If you do not have either of those to hand, store your shopping in the boot of your car.

The ESR Domestic Food Practices in New Zealand project report, including research into temperatures of meat and poultry during transport following purchase, is available to download.

Domestic food practices in New Zealand 2005-06, ESR report [PDF: 1139 kb, 82 pages]

Page last updated 17 January 2008