Someone mentioned searing food to "seal in the flavor". My response was that searing was a way to add a delicious crust, and nothing more or less. However, I recently noticed a frozen food mentioning their ground beef product is "flash frozen to seal in flavor".
These are really just buzz words, right? Even if you're talking about juices as opposed to "flavor", you can't seal them in, can you? When the product rests the juice will be redistributed, won't it?
Searing on a grill to "seal in juices" has largely been disproven. Meat loses juice at roughly the same speed regardless of searing the meat first. Searing does produce the Maillard reaction and caramelization which enhances flavor; however, searing first doesn't produce better results. A test performed by Alton Brown in 2008 demonstrated that searing at the end of the cooking process loses less water than searing at the beginning. For more information see this link.
For the Alton Brown results see this link (video).
There is a grain of truth in the claim that flash-freezing beef "seals in flavour". If meat (or anything else) is frozen slowly, large ice crystals form. These puncture the cells, resulting in a mushy texture when the food is thawed. But, because a lot of the cells have burst, all their contents can drain out, too, so you're going to lose flavour. However, it's not really "sealing in" the flavour; rather, it's avoiding doing something bad that would let the flavour out.
However, because of this, essentially all frozen food is flash-frozen. Drawing attention to the flash-freezing is a little misleading, since it suggests it's unusual when, in fact, it's completely normal. It's a bit like explicitly pointing out that the cows were given air to breathe: that's not animal welfare, it's just how cows work.
Is "sealing in the flavor" an actual thing?
No, it isn't.
As Harold McGee explains in his excellent reference work On Food and Cooking (emphasis mine):
The best-known explanation of a cooking method is probably this catchy phrase: "Sear the meat to seal in the juices." The eminent German chemist Justus von Liebig came up with this idea around 1850. It was disproved a few decades later. Yet this myth lives on, even among professional cooks.
Before Liebig, most cooks in Europe cooked roasts through at some distance from the fire, or protected by a layer of greased paper, and then browned them quickly at the end. Juice retention was not a concern. But Liebig thought that the water-soluble components of meat were nutritionally important, so it was worth minimizing their loss. In his book Researches on the Chemistry of Food, he said that this could be done by heating the meat quickly enough that the juices are immediately sealed inside. [...]
Liebig’s ideas caught on very quickly among cooks and cookbook writers, including the eminent French chef Auguste Escoffier. But simple experiments in the 1930s showed that Liebig was wrong. The crust that forms around the surface of the meat is not waterproof, as any cook has experienced: the continuing sizzle of meat in the pan or oven or on the grill is the sound of moisture continually escaping and vaporizing. In fact, moisture loss is proportional to meat temperature, so the high heat of searing actually dries out the meat surface more than moderate heat does. But searing does flavor the meat surface with products of the browning reactions, and flavor gets our juices flowing. Liebig and his followers were wrong about meat juices, but they were right that searing makes delicious meat.
Source: Harold McGee, On Food and Cooking: The Science And Lore Of The Kitchen (Revised Second Edition, 2004), Chapter 3: Meat, The Searing Question, p 161
It's interesting to note that Liebig was the same man who pioneered mass-produced meat extracts.
There is one sense in which searing meat really does seal it.
For a long time it was genuinely believed that searing meat in some way "sealed" it. As other answers have already shown, this is nonsense.
Indeed it's quite easy to prove:
If you put a sealed object that contains a lot of water, fat or air in it into a hot oven, then it's going to get larger as the water (and/or fat and/or air) expands and perhaps burst. We prick sausages precisely so that they aren't sealed.
By the same token, if there's more liquid in the pan after cooking than could come just from the surface then clearly that liquid wasn't "sealed in", was it?
However, if you refer to searing meat before roasting it as "sealing the meat" you aren't being incorrect. And if someone says they consider it incorrect we could ask them if that means they are studying astrology; consider literally means "examine the stars" but we use it to mean "think about" whether or not we believe in astrology and certainly not restricted to cases where we actually draw up a horoscope. By the same token, "sealing the meat" means "searing the meat so the maillard reaction improves the flavour and gives a more pleasant colour" even as used by people who know its etymological origin in a disproven belief that it was actually sealing something "in".
I personally know nothing about cooking but my dear departed mother did. She used a tip that had been passed on by her mother. Before cooking, she would treat meat by pouring on boiling water and then drying. I remember once asking why and she said that she did it to seal the meat. I didn't probe further but I always assumed that it somehow kept the flavour in.
Whether it really had any effect I have no idea but often these traditional methods are rooted in truth.
Well I certainly know the flavor can get out.
I have been experimenting with fried chicken - butter milk and flour - and want to make a kfc style hot and spicy. Easier said than done. I have tried adding bottles of hot chilli sauce, chopping up piles of chilli and adding tablespoons of cayenne pepper. In every case the "Hot" gets out. I think the process of deep frying is either destroying the "Hot" or it is leaching into the oil.
KFC use a pressure fryer and I am using a wok filled with oil. I presume speed of cooking may be critical.
I know when making beer that time at temperature is critical - different chemical reactions occur at different temperatures creating different flavors.
So perhaps you have a situation where the process of applying heat over time destroys the little flavorons and fast cooking and/or bringing the temperature down quickly after cooking avoids it stewing and damaging flavor.
With beer making you certainly want to bring the wort temperature down fast to stop certain chemical reactions that occur at certain temperatures that create cloudiness.
True story: I love beef jerky. The other day when I had a whole topside to play with, I made a batch of biltong on top of the three batches of jerky I had made. Just for fun (and to see what would happen) I seared two of the fillets and treated the other three with vinegar as usual for biltong. I then put all 5 into my dehydrater at 35C and waited for three days. Much to my surprise, while the three vinegared fillets dried out and went firm (as usual), the two I had seared remained as soft and squishy as when I pulled them off the hot-plate.
So, while it seems I didn't discover a tasty new "Charred Steak Biltong" flavour as I hoped, my little experiment may cast some doubt on the allegedly scientific "debunkings" of the "myth".
