What is the correct procedure for cooling and reheating leftovers?
From a food safety perspective, the most important thing is that you get your leftovers cooled as quickly as possible, so that they don't spend too long in the temperature "danger zone". This can be tricky, because you essentially have to move from cooking temperatures to refrigerated temperatures within two hours (generally less because the time is cumulative, and includes time that the ingredients spent in the danger zone before you cooked them).

Here's what I do at home:


Separate into smaller portions. For me, this usually means individual servings, but you can package 2-3 servings together; the point is to avoid having a single large mass of food. I do this for a couple of reasons. First, it makes reheating convenient; rather than heating up more portions than you're going to be eating, or scooping out a single portion for separate reheating (dirtying some extra dishware in the process) you can simply grab as many portions as desired and go. Second, by separating the portions you are increasing their surface area to volume ratio, and therefore how quickly they will cool (another reason I like individual servings is that it maximizes this effect). This is a basic trick from my restaurant days; experienced professional cooks will know that to cool a large batch of soup safely, you need to either get the whole thing into an ice bath, or break it down into smaller portions.(A thrifty tip here: you can use dedicated plastic or Pyrex food storage containers, but commercial containers are cheap and effective as well. There are all kinds of "deli" and "disposable" containers intended for food service that are microwave-safe and which can be safely cleaned in the dishwasher. You'll often have to buy them in bulk, but per piece they're very inexpensive, they tend to stack very well, and if you're lucky you can find a decent local takeaway or delivery place that uses them instead of styrofoam. Save whatever you get, wash them, and reuse. It's possible to ruin these with excessive heat, but they're so cheap that it's not too much of a loss.)
Allow to cool on the counter, uncovered, for 10-20 minutes. The goal here is simply to let steam escape; water carries a lot of thermal energy with it (which is why you can easily burn yourself over a pot of boiling water) and letting it get away will cool your food rapidly. The portions don't need to cool completely at this point; if they're warm but no longer actively steaming, that's fine.
Put the portions in the refrigerator. As @Jefromi notes in comments, there's no real risk from putting hot/warm food into the fridge, and a lower external temperature will help your portions cool quickly and safely. Where possible, it's ideal to leave these uncovered and spread out into a single layer so that they will cool quickly. Quite honestly, this isn't always practical, so if you need to cover and stack the portions due to space constraints, cool them a little longer on the counter before putting them into the fridge.
Freeze the portions if you'll be storing them for more than a couple of days. Cooked dishes will generally keep safely in the fridge for 3-4 days; if you need to keep them beyond that, the freezer is your friend. This doesn't work well for everything, but hearty foods like pastas will often freeze reasonably well.
Reheat thoroughly in the microwave. For all that serious cooks might discount the microwave, it's a very effective and efficient reheating device. (Especially if you follow my advice about using microwave-safe plastic containers; if you use something like Pyrex that's oven-safe, you could certainly use that too.) Make sure you heat everything so that it's evenly hot; this may mean stirring a couple of times.


Overall, it's most definitely possible to safely store leftovers, so long as you cool them quickly and reheat them thoroughly. Not only that, but I heartily endorse the practice. Leftovers can be practical, tasty, and a serious time saver. There's no reason to be paranoid about them if you embrace a few simple rules.