According to literature that I have, stronger beer bottles can withstand about 3 atmospheres of pressure (or 44 psi). But beer bottles aren't all made the same. The mass of a refillable bottle can be as much as 40% higher than that of their lighter weight cousins. Higher glass mass probably equals higher strength. Champagne bottles are roughly twice as strong and capable of handling about 6 atmospheres (or 88 psi).
It suggests that bottling an unstabilized cider at an SG of 1.005 or higher in a beer bottle is asking for trouble. Unstabilized cider can be bottled in a champagne bottle at up to SG = 1.010. Both of these cases assume that the cider will continue to ferment to dryness with no pasteurization.
My interpretation of this is that if a 5 point drop puts a beer bottle at a potential for explosion and greater than a 10 point drop puts a champ bottle in that range and heating the bottle (pasteurization) will further increase internal bottle pressure, then (to be safe) the permitted SG drop has to be somewhat less than those levels for bottle conditioning - if pasteurization will follow.
For bottle pasteurization, the recommendation is to avoid beer bottles and to go with champagne bottles - American type that can be capped with standard beer bottle caps. (Though it should be noted that if a champagne bottle is over carbonated, the explosion will have considerably more force versus a beer bottle - should it occur.)
Since I wanted a cider that ended at 1.018, I bottled (in champagne bottles) at 1.023. This allowed a 5 point drop and presumably about 3 atm of carbonation. This also would allow about 3 atm of pressure leeway for pasteurization. (I bottled some in a 10 oz screw cap bottle so that I was able to monitor SG without opening the champagne bottles.)
Another thing to consider is head space. Gas is far more compressible than liquid. Liquid will expand upon heating. If you leave little or no head space, the force on the inside walls of the bottle will be higher and your chance for explosion will be higher. I leave most of the neck of the champagne bottle empty. I know that excess air isnt a good thing for cider. Though, when I bottle, fermentation carbonation generally bubbles out and I try cap quickly before the head space mixes too much with air. CO2 is heavier than air, so minimizing bottle movement after dispensing into the bottle should minimize the ability of air to get into the head space (but capping quickly is still a good idea).
The pasteurization process I use is to put room temperature cider-containing bottles into room temperature water in a canning pot. The bottles are separated from the floor of the pot by a canning rack. One bottle is uncapped with water and a thermometer in it. I raise the temperature slowly until the test bottle is at 150 F and then shut off the heat (at this point the liquid has pushed up 0.5 to 0.75 inch up the neck from where it started). The bottles are left in the pot for 15 to 20 minutes and then removed. When removed, the bottles are placed onto a towel that is spread out flat onto the counter top. I dont touch them for at least a half hour. Literature suggests that most cultured yeast will be killed by this process. I have had no referementation or broken bottles using this process.
Hot bottles should not be placed directly onto cold surfaces. Pyrex glass is designed to withstand significant thermal shock. Beer and champagne bottles (to my knowledge) are not. Thermal shock, by itself, and especially if the bottles have defects, can cause a bottle to break and/or explode (if there is internal pressure). This also applies to the reverse: i.e. putting a room temp bottle into hot water. The greater the temperature difference between the bottle and the water bath, the greater the probability the rapid thermal expansion of the glass will cause the bottle to break. Even a defect free bottle, if pulled out of a freezer, will almost certainly break, if placed directly into very hot or boiling water.
Regardless of heat pasteurization method used, it is probably wise to keep some safety related things in mind (at least this is what I try to keep in mind):
1. Keep track of carbonation level. If SG drop or perception of pressure is too great, abort.
2. The process increases pressure inside an already pressurized bottle.
3. Thermal shock and/or physical shock can cause pressurized bottles to explode.
4. Skin and eyes dont react well to high velocity glass shards.
a. Wear a full face shield or at least safety goggles
b. Wear a thick heavy jacket
c. Wear heavy gloves or thick hot mitts
5. If the bottle explodes, it will be when youve become complacent and have taken no safety precautions, are wearing a T-shirt and shorts, have just removed the bottle from the pot with your bare hands and are admiring its clarity.
) and I want to backsweeten and pasteurize...
