Ben asks via twitter,
- Ben Ostrowsky (@benostrowsky) July 30, 2012
@thefoodgeek Is duration the only difference between blanching and parboiling?
From a pure technique perspective, the answer to your question is mostly, "Yes." Blanching is a shorter process than parboiling. However, when you think about why you would parboil something vs blanching it, then there is a world of difference.
The purpose of parboiling is to prepare something for a later stage of cooking. Par-cookingtechnique in general, where cookingtechnique could be baking or roasting or whatever, means that you are partially cooking, roasting, baking, boiling, etc. the food in question. The rest of the cooking will happen though, whether immediately or after a storage period.
Let's take a beef stew, for example. If you want to have carrots and beef in this stew, then you can't just dump them in the pot all at the same time and expect the carrots to be cooked through before the beef is so completely overcooked as to make any stew-eater filled with sadness. We discussed this somewhat in Slow Cooking Secrets.
On the other hand, if you make several loaves of bread, but you don't want to serve them all at once, then you could par-bake most of the loaves and freeze them, so that you merely have to finish the baking process later to have fresh bread. It won't be as good as if you baked it all at once, but it's better than most of the sliced bread you'll get from the bread aisle.
When you're blanching, though, you aren't usually trying to even out cooking times. I've mentioned before that vegetables are alive when you cook them, which seems kind of gross when I put it like that but it's really better than the alternatives. There are biological processes going on that can cause you trouble if you don't stop them, such as fruits that convert their sugars to starches or over-ripen. Blanching stops those processes, but you want to avoid actually cooking the food.
When I said the answer to your question was, 'mostly, "Yes,"' it's because there's usually an extra step to blanching, which to is quickly remove the heat from the thing you're blanching right after you're done. This process is called "shocking", and it involves dipping the recently blanched item in water (or ice water) to quickly pull the heat away. Water can hold a lot more heat than air, and it can transfer it faster, so letting something sit in the air causes it to keep warm longer than dipping it in water. I'm confident you've experienced this process in many ways over your life. The ice helps a little, but if you're not doing many, many shocks, it's usually not necessary. Doesn't hurt, but isn't necessary.
The other reason for blanching is usually for greens, and it's to make them greener. Greens lose some greenness in day-to-day life due to oxygen attaching itself to the outside of the leaf. Blanching will pull that oxygen away and bring out the brightness. Also, if you add some baking soda to the water, the green pigment will be made more vibrant by the alkaline water. Finally, because some of the processes that cause greens to lose their vibrancy when they are mishandled are stopped by the heat, this step will keep your pestos, and similarly roughed-up leaf preparations, a lovely green hue.