The Food Lab: Better Home Cooking Through Science

Milk does, on the other hand, add moisture and fat and is
worth including for that fact alone. Heavy cream works
better. Better still is buttermilk, which has a unique tang that
adds depth and complexity to the finished dish.
Bread crumbs may, at first glance, seem like an
unnecessary extender—something added just to stretch your
meat a little bit further—but they are perhaps the most
important ingredient of all when it comes to improving the
texture of a meat loaf. Aside from absorbing and retaining
some moisture as the meat loaf cooks, they physically
impede the meat proteins from rubbing up too closely to one
another, minimizing the amount of cross-linkage and thus
dramatically increasing tenderness. In many ways, the
physical structure of a meat loaf is much like the structure of
an emulsified sauce stabilized with starch. In the latter case,
starch acts like a bouncer, keeping fats from coalescing,
while in the former, bread crumbs do the job, keeping meat
proteins apart. I found that using crumbs from fresh bread
slices ground in the food processor provided better moisture
and binding capabilities than dried bread crumbs.
Finally, mushrooms, while not necessarily a standard
meat loaf ingredient, are an invaluable addition. Why do I
include them under binders and extenders rather than lump
them in with the aromatics? Because they act much more
like bread crumbs than they do like, say, onions.
Mushrooms are extremely porous and are full of flavorful
liquid. At the same time, they are soft and spongy. Just like
bread crumbs, they prevent the meat proteins from
interlocking, increasing tenderness while simultaneously
adding flavor as they slowly release their liquid. In fact,