Why there is a delay before canned fruits float to the top?

Question

This question may appear silly compared to others asked on this site, yet, I can't get my head around it.

After pouring boiling water into a canning jar with berries, berries stay at the bottom of the jar for about 15 seconds. After that, they begin to float to the top. It takes them several minutes before they start to sink to the bottom again. After hours, the old water is replaced by new boiling water, but the berries do not float anymore, always staying at the bottom.

If I were to ignore the delay before they float when the jar is filled with boiling water for the first time, I would immediately answer that the reason why they float and then sink is because the berries contain air inside of them, that decreases their overall density, making them less-dense than water they are in, thus, making them float to the top. Air can be observed to leave the berries and after some time, when enough air has left, the overall density of the berry increases and the berry sinks.

However, this response woudn't explain why there is an initial delay of about 15 seconds before the berries begin to float. If air was the sole reason why they floated, wouldn't they float immediately?

I had a hypothesis as for why that might happen: When the water has been poured into the jar, it is mixed, with temperature around equal everywhere. After some time, the colder water settles at the bottom of the jar. The colder water is known to have higher density. Could it be that the settlement of the colder water with higher density causes berries to float, in combination with the contained air?

This explaination, however, is also wrong, probably. If the berries do not float immediately, it means their density is higher than the average density of the water, since the water is initally mixed with its temperature (and thus, density) averaged at every point. After the water is arranged colder to hotter from bottom to top, the water at the top would have higher density than average. If the berries are more dense than averaged water density, they canot float to the top, where the density of the water is even higher. So, the hypothesis must be false.

I don't have other explanations for this phenomenon. Sorry for this childish question, but maybe you could explain why that happens?

Answer 1

All fruits have hair, and a water resistant wax finish which lets rain drops roll off. It also provides a surface film of air bubbles if the fruit is immersed in water. Most easily observable with raspberries and peaches.

If the air bound to the berry is heated, there is some uplift, just enough to bring the overall density below that of water.

Hot water dissolves the wax film coating the berry and in a span of a few seconds, the bubbles on the berry surface burst.

Like foils, fruits falling into cold water swim. The air skin is an evolutionary benefit for dissemination, becasue it sllows the berry to find a pollinating bird before it can sink into water or rot.

Answer 2

Just a guess. If berries contain air, the air expands as the berry gets hot. This increases the volume of the berry, but not its weight. The berry floats when the weight of the berry is less than the weight of the same volume of water.

Perhaps it takes 15 seconds for the berry to heat enough.

Eventually either the air escapes, decreasing the berry's volume. Or water soaks into the berry, increasing its weight. Either or both of these would sink a berry, and prevent it from floating when more hot water is poured over it.