Katie remarked, “Juiciness is just about how much juice is in the food, right?”
Not exactly! We all like a juicy bite of meat (or fruit, or vegan meat, or what have you); but the phenomenon of juiciness is all about what happens during that bite. It’s not enough for food to contain moisture; it has to fill your mouth with a marvelous juicy sensation.
The scientific literature* about juiciness of meat (there’s plenty) says elaborate things like:
Attempts to model juiciness have found it to be highly complex. One of the best models is a three-dimensional one, involving the effect of ‘time’ in the mouth on the ‘degree of structure’ and the ‘degree of lubrication’.
That is to say, it’s complicated, and it’s subjective.
Here’s a science experiment to try: Vacuum-seal two chicken breasts in two separate sous vide bags. Cook one at 145 degrees for an hour (as long as it spends at least 9 minutes at 145, it's bacterially safe); and cook the other one at 170 degrees for an hour. Let them cool, then unseal and taste them.
No liquid will come out of (or go into) the bags during cooking, so the cooked samples will have the same amount of moisture. (You can weigh them before and after cooking to prove that.) Does that make them equally juicy when you eat them? Spoiler: No. The 145-degree sample will be mouthwatering and succulent, but the 170-degree one will have turned into a tough, cottony wad.
Why? Because juiciness is more than just the wetness of food; it’s a sensation. In the chicken cooked at a lower temperature, the protein in the muscle is plump with moisture, and the first bite squeezes it out into your mouth in a gush. The muscle fibers move easily as you bite through them.
In the overcooked sample, the muscle proteins have contracted and coagulated. The muscle fibers are no longer plump with juice; instead, they’re coarse and mealy, their liquid sitting loosely between the strands. Now, with each chew, liquid leaks out of the meat little by little, with none of the appetizing gush that we perceive as juiciness.
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The other major component of juiciness is salivation. If our mouth waters when we take a bite of food, we perceive that as juiciness. That’s why sour candies (which contain no liquid) and sour lemonade (which is all liquid) are both refreshingly juicy. Acidity stimulates salivation.
So does fat. That means that richer meats, like a well-marbled steak, come across as juicier, even though they may not contain all that much moisture. The luscious flood as we bite into the meat comes partly from the steak and partly from our mouths.
The same principles can be applied to any food. A firmer apple tastes juicier than a softer one because it bursts more when you bite into it. If it’s a tart apple, it tastes even more juicy.
Finally, a trick I lifted from Harold McGee years ago and never looked back: If you add a little butter to homemade caramels, the fat gives them a juicier feeling when you chew them.
* “Juiciness - its importance and some contributing factors” by R.L. Winger and C.L. Hagyard, in Quality Attributes in Meat, Poultry and Fish Products, 1994.