A pot of boiling water tops out at 212°F (100°C) no matter how long it stays on the burner, and that ceiling is most of the reason pasteurization and sterilization are different processes rather than two words for the same thing. Pasteurization works entirely within that ceiling. Sterilization requires going past it, and getting past it takes pressure.
What each temperature actually kills
Most of the bacteria that make food unsafe or spoil it are working cells — active, growing, and about as heat-tolerant as an egg white. A moderate temperature held for a short time is enough to finish them off, which is exactly what pasteurization is: heat food to somewhere around 145–165°F (63–74°C) for anywhere from fifteen seconds to half an hour, depending on the method, and Salmonella, Listeria, and the everyday spoilage bacteria in milk or juice are done.
Some species can also build a second form of themselves called an endospore, a dormant, armored capsule that bacteria like Clostridium botulinum and Bacillus cereus produce when conditions turn hostile. A spore barely metabolizes. Its coat shrugs off heat that would destroy the same organism's active cells outright, and some spores shrug off a full hour at a rolling boil without much trouble. Pasteurization never touches them. It was never built to.
Sterilization is the process aimed at that gap: heat hot enough, held long enough, to take out the spores too, not just the cells that are already active.
Why sterilizing means using pressure, not more time
Reliably destroying a spore takes something in the neighborhood of 240–250°F (116–121°C), sustained. Water boils at 212°F at sea level and stops there — push more heat into it and you get faster boiling, not a hotter pot, because the extra energy goes into making steam instead of raising the temperature. A sealed, pressurized vessel is what gets around that, since trapping the steam raises the boiling point along with it. That's why real sterilization happens in a pressure canner, a commercial retort, or an autoclave, and not in a stockpot with a lid clamped on.
Why some canning recipes skip the pressure canner
A boiling water bath holds jars at 212°F. That's pasteurizing heat, not sterilizing heat — it kills active spoilage organisms and most pathogens, and it leaves any spores in the jar alive and waiting.
For high-acid foods, that's fine. Tomatoes, pickles, jams, and most fruit sit below pH 4.6, and Clostridium botulinum spores can't germinate or produce toxin in an environment that acidic. Leaving them dormant is enough, so a water bath does the whole job. Some tomato recipes add bottled lemon juice or citric acid for exactly this reason — not every variety is acidic enough on its own to guarantee it.
Low-acid vegetables have no such floor. Green beans, corn, and most meats sit well above that threshold, and a boiling water bath seals them into a jar with surviving spores, no oxygen, and no acid to stop them — precisely the environment C. botulinum needs to wake up and start producing toxin. That's the specific reason USDA guidance requires a pressure canner for low-acid foods: it's the only home method that reaches actual sterilizing temperatures.
Why UHT milk tastes different from the milk in your fridge
Ordinary pasteurized milk is flash-heated to around 161°F (72°C) for about fifteen seconds, which knocks out pathogens and buys a couple of refrigerated weeks before spoilage organisms and any surviving spores catch up. UHT — ultra-high-temperature — milk is a sterilization process instead, flashed to roughly 275°F (135°C) for a couple of seconds, hot enough to take spores out along with everything else. That's the entire reason it can sit sealed on a shelf for months.
The extra heat isn't free. Whey proteins in milk denature at those temperatures and release sulfur compounds, and the milk sugars brown slightly through the Maillard reaction — the same reaction that browns a steak, just barely getting started. That faint cooked or caramelized note people notice in UHT milk and can't quite name is that reaction. Pasteurized milk never gets hot enough to trigger it.