Last week we went to Grand Canyon and DesertRoomie had a sudden jones for some Coca Cola. She bought a four-pack of 20-oz. plastic bottles in Cameron and drank one on the way in, then another there (a refrigerator was in the room). The remaining two bottles came home with us and were forgotten in the car here in Phoenix for three days, then moved to the refrigerator here for a couple days. The daily highs while they were the car were 105 to 108 so it likely reached 120 or a bit more in the closed car.
A day or so later I opened one; it was god-awful. It was practically flat, not sweet at all and had a totally weird aftertaste. This was not diet Coke so there weren’t any complex sweeteners to break down oddly, just sugar and HFCS. As for the flatness, I thought a defective cap might let the carbonization escape so I tried the remaining bottle; it was just as flat.
So what happened? The heat, of course, but it seems surprising that just a couple days could cause such major changes. Cool, dry place indeed.
Even bottled water tastes awful if it’s been in a car for a day or two with temperatures over 80 or 90. I assume it’s the taste of the plastic leeching into the water.
As for them being flat, I’d guess that the higher pressure from the heat forced the air/CO2 out of the bottle. Likely through the seal between the bottle and cap (as opposed to directly though the plastic). In fact, they may even be designed to do that to avoid a bigger mess or injury if the bottle explodes.
You could do an experiment. Leave another one in your car on a really hot day, but have it inverted. That way if it is just the heat pushing the CO2 out, the soda will probably go with it and you’ll see the evidence (in the plastic bag you put it in to conduct an otherwise messy experiment).
I think glass bottles would have been even worse. The last time I was in Phoenix it was around 110 degrees (in May!) and I left a six pack of club soda in glass bottles in a box in the back of my truck. They all exploded at some point and sent glass shards and water everywhere, like little Canada Dry hand grenades.
How hot your car got depends on where it was parked. If it spent an hour or more in the sun, 120°F is naïvely optimistic, 150° is much more likely. I had a car window blow out from a closed car sitting in the sun, in an area where the midday temperature rarely exceeds 95°.
I’ve never had glass bottles explode but a couple of cans in a 6-pack did once (I’m a slow learner). Luckily the pack was wrapped in a beach towel – that’s how it was overlooked – and the towel absorbed most of the sticky mess.