Is Rosehip Syrup boiling in a bottle? My wife made Rose-hip syrup and the final part was boiling it in a saucepan. While still hot she placed it  in a glass bottle. To keep it sterile she plugged it with a wine-saver top and sucked out some of the air - so the (inch or so) space in the bottle is at lower than atmospheric pressure. Large bubbles are spontaneously appearing in the bottle and quickly rising to the top. This has been happening for half an hour. The bottle is still quite warm . It looks like it is on a slow boil: is it?
 A: If you fill a bottle with hot water, the water in contact with the sides cools quickly.  This sets up convection currents, which help to cool the whole bottle, and ensure its temperature is rather uniform.
The syrup is viscous, so this doesn't happen.  The syrup in the centre of the bottle remains hot - maybe 80 C after half an hour.  The coldest part of the syrup is probably where the top surface meets the glass - which might be, say 50 C.  At 80 C, the saturated vapour pressure of pure water is 0.47 bar, while at 50 C it is only 0.12 bar.  The numbers will be slightly different for syrup, of course.
Provided the air left in the bottle has a pressure less than the difference (0.35 mbar), the pressure in the bottle will be below 0.47 bar, causing the hot syrup in the middle to boil.  The bubble of steam will recondense at the top.  Unlike natural convection, which is driven by the tiny density variations in the syrup, the movement of the steam bubble is driven by the very large density difference between steam and syrup, so it now takes over as the most effective method of heat transfer.
This only works because the steam condenses when it encounters cooler syrup at the top of the bottle.  Otherwise the space above the syrup would immediately fill up with steam and boiling would stop.
A: Yes, it's boiling.  The clue is that you said some of the air above the liquid was sucked out, so the pressure is significantly lower than normal atmospheric.
Boiling happens when the gas pressure at the surface of a liquid is lower than the liquid's vapor pressure.  When boiling something in a open container, we heat it to make it boil.  Water boils at 100°C because that is when its vapor pressure reaches one atmosphere.  We sometimes forget that pressure is the other variable that decides when a liquid boils.  You can just as well boil water by lowering the pressure above it as raising its temperature.  There are things called steam tables that give the vapor pressure of water at various temparatures.
This is the reason that water boils at a lower temperature than 100°C at high elevations.  The air pressure is lower, so it takes a lower vapor pressure for water to boil, which requires a lower temperature to reach.  This is also the same reason that the temperature in a pressure cooker is higher than normal "boiling" temperature.  Pressure is deliberately regulated to a higher than normal level in the pressure cooker, which requires higher temperature to keep the water at the boiling point.
