Is slow cooking in an oven safe with long cooking times I've been reading about slow cooking in an oven and I have some questions.

We cook meat until it reaches a certain temperature; we can assume this temperature is deemed safe by food experts! However, I'm confused about what makes the food safe! Is it the heat of the meat, or the heat of the oven, or both?

The reason for this question, is on some site (which I can't find now), they suggested (with Lamb), not to slow roast it under 135°C (with no reason why). However, I have a friend who is a chef, and he will slow roast lamb at 100°C.

Let's assume pork must be cooked to 70°C for it to be considered safe. Is it considered as "safe" to cook it at 100°C for a 'long' time (until the pork is at 70°C), compared to cooking it at 200°C for a shorter time (until pork is at 70°C)? Does the length of cooking time, at the lower heat mean bacteria could actually spread? 

So, to summarise, if I cook meat (and I assume lamb and pork obey the same rules/laws) at a high temperature am I killing more bugs (due to the higher temperature) than if I cook it at a lower temperature?
In general, the issue is how long the food spends at the middle temperatures, warmer than refrigeration, and not as hot as truely safe cooking temperatures, the so called "danger zone" from 40-140 F, 4-60 C.

While an over simplification, pathogens do not grow well outside of the danger zone because it is so cold that they are inactive, or so hot that they are distressed or killed.

Slow cooking, done properly, is safe because the time spent moving through the danger zone to the safety of temperatures above about 140 F / 60 C is minimized, thus minimizing the opportunity for any pathogens to grow and create toxins; then, at temperatures above the threshold (if held for several minutes), most pathogens are killed.   (Do not take this to mean food is "sterilized" as spores can remain, that can become active later.)



Foods are not cooked to a single temperature to be considered safe.  Instead, there is a combination of time and temperature at which they are considered safe, assuming no other problems existed.   This is mere seconds at 165 F / 74 C, but perhaps half an hour (going by memory, so don't take this as gospel) at 135 F / 57 C.

Still, general audience food safety recommendations suggest temperatures of 160 F / 71 C because the 99.9% kill time at that temperature is only a few seconds--it doesn't require careful monitoring or timing, and so is a simple rule to follow.

275 F / 135 C (I see you edited the question) is definitely a temperature range for slow roasting that is safe when done properly.

135 F / 57 C is possible, but on the borderline, although it would require very precise control not usually available in an oven; this is more the realm of sous-vide cooking techniques.  It also requires longer periods held at temperature, and so is not a method for casual cooks.

100 F is not safe.  It is well into the temperatures at which food is being incubated so the pathogens have a cozy environment for growth.