I like the cold. Anything over twenty degrees is a bit too hot for me, and the extra heat from being in full sunlight drives me to skulk from one patch of shade to the next like I'm on a stealth mission 24/7. Other people relish the heat, actively seeking it out.
Everyone has their optimum temperature, and bacteria are no different! From Psychrophiles growing in the fridge, to Thermophiles living in volcanic vents under the sea, different bacteria tolerate different temperature ranges. So what does that mean for us?
When I get too cold I pop a jumper on, too hot and I open the window. Bacteria don't have the luxury of clothes or windows though, so they suffer temperature changes a bit more dramatically.
Heat speeds up biochemical reactions by providing energy, so the warmer things are the faster things grow, as a general rule of thumb. But too much energy is a bad thing, just like giving an already hyperactive child a can of Red Bull. Over a certain point, there's too much heat for the bacteria's proteins to handle and they start to denature, making them stop working, leading to the death of the cell. That's why cooking food makes it safer; the heat kills any bacteria lingering there so they can't cause disease.
Cold, on the other hand, is less final. Reduced heat means reduced energy, but this just slows the bacteria down rather than killing them outright. In the lab I keep my bacteria at minus eighty degrees, which would kill me but is relatively fine for the bacteria; they're still alive, they just aren't growing or doing much metabolically. This is why fridges and freezers keep food from going off for longer; the bacteria that make the food go off are slowed down so it takes them longer to grow and degrade the food. They don't kill bacteria though, so we still need to cook frozen food before eating it (that means you, people who eat frozen chips... you oddballs).
This is all fine; keep cell numbers low in the fridge then cook them so any that are there die. Easy! But there is a problem; heating food up from sub-optimal temperatures for the bacteria to... super-optimal temperatures? Too hot, anyway. Heating from one to the other brings the bacteria nicely through their happy temperature where they grow at their fastest. For pathogenic bacteria, this is the 'danger zone' where they grow, produce toxins, and generally make mischief. To minimise risk of food poisoning, we need to make the time spent in the danger zone as short as possible so they don't grow too much before dying from the heat. In other words, we need to heat the food as quickly as we can, all the way through so the heat reaches all the bacteria in the deepest parts of the food. One easy way to do this is pre-heating the oven! I always thought everyone did this, but apparently not everybody does. Not pre-heating the oven makes the food gently warm up, maximising time in the danger zone and thus maximising food poisoning risk.
Another way to minimise time in the danger zone is to only go there once; reheating food, and refreezing food, gives pathogens extra time at their optimal temperature, which is bad for us. This is especially a problem for rice; Bacillus cereus food poisoning is common with reheated rice, as they get lots of time at their optimal temperature to grow and produce their toxins. Cooking may then kill the bacteria, but the toxin is heat stable so you'll be eating a lot of it. About two hours later you'll be saying hello to your rice based dinner again!
These are all the bad, food based things that this optimal temperature thing has an impact on, but there are many really good applications for it, especially for me in the lab (and other scientists too, I'm not special). I'll do a part two of this post soon covering those, stay tuned!