The secret to getting a glossy, papery skin on your brownies is to get the sugar very well
dissolved and/or dispersed through your batter. If you can see or feel lots of really big sandy
grains of sugar in your batter, you're probably not going to get that skin. If
you really dissolve your sugar and you're still not getting that nice glossy skin,
then I would guess that maybe you don't have enough sugar, or maybe you have too much water,
or you don't have the right kind of sugar. It has to be sucrose or perhaps some similar sugars, but
most of the common syrups in the kitchen will not do it. Honey brownies, yum. Contrary to what
a lot of very smart people believe, brownie skin does not result from using a specific kind of fat
and it does not result from whipping air into the batter.
It is chiefly about the sugar and secondarily the water. We're going to prove it with experiments.
And the experiments will also explain why other people are wrong about this. Because, in a way,
they're not wrong. Whipping the eggs and the sugar a lot can help you get brownie skin.
Using lots of solid chocolate in your batter can help you get brownie skin. As is so often the case
in cooking, people's methods work — it's simply their reasoning or their explanation of the method
that is false. And they're not wrong because they're stupid. They're wrong because they have
important things to do in their lives. They can't blow a whole week doing 50 test-bakes to isolate
the variables and figure out which one is causal. Only a lunatic like me does that. So let's go.
To start, we're going to need a baseline brownie recipe. Mark Bittman here has the basic,
no-frills and somewhat old fashioned version of nearly every dish in the Western culinary canon
here. We'll bake this. Four ounces of unsweetened chocolate and a stick of butter, melt in a pan or
in the microwave, in goes a cup of sugar and two eggs. Let's see, what else? A splash of vanilla
and a pinch of salt, and finally, a half a cup of flour. Mix it smooth, bake it and it looks
like this. No skin. Boo! Now, let's try that same exact Bittman recipe again, but this time
we'll swap out the granulated sugar for this: powdered sugar, or icing sugar, as the Brits
would say. It's simply an extremely fine grain of sugar, and it's fluffier, more aerated, so you've
got to use one-and-three-quarter cups to get the same weight as a single cup of normal sugar.
Mix that up, and note how much more viscous the batter is. Bake that and ... beautiful
papery skin. Look at the difference. But I have questions. Powdered sugar isn't just
sugar. There's also a small amount of cornstarch in there. It serves as an anti-caking agent.
Could the cornstarch be helping the crust, maybe absorbing water, helping the skin dry? Well,
let's see if we can make our own powdered sugar without cornstarch in it. I blitzed
this for literally a half hour and it's nowhere near as fine as powdered sugar, but it is finer.
So let's make the Bittman recipe with it. Same quantity of sugar by weight,
and that's not the best skin I've ever gotten on brownies, but that batch does have some skin. Now,
let's use the food processor sugar again, but this time add seven grams of cornstarch. If you look at
the nutritional label, you can see that it has 29 grams of sugar but 30 grams of carbohydrates — so
we know that that remaining gram can only be cornstarch. It's 1/30 cornstarch.
Anyway, identical to the previous batch with no cornstarch. Cornstarch is not why
powdered sugar gives us great skin. I used to think it was, but the real reason is any
super-fine grain of sugar is going to dissolve really easily into the batter. And how do we
know that's the real reason? Well, let's talk to Dr. Pia Sörensen at Harvard, co-author of the new
book, "Science and Cooking: Physics Meets Food, From Homemade to Haute Cuisine."
"The magic power of sugar is that you can dissolve a lot of sugar in water.
You can actually dissolve twice the amount, by weight, of sugar in any amount of water."
So doc suggests we try making the Bittman recipe as written, with normal granulated sugar,
and I'm just barely bringing this together. I'm certainly not whipping in air. The only
thing we'll do differently this time is just let it sit for a long time. In fact,
I just mixed it up and then I went to bed. "When you mix all the things together, the sugar
is going to dissolve in the water in the batter. And even if you're not actually adding water,
you're sort of indirectly adding water because there's water in the egg,
there's even some amount of water in the flour, right? There's like 15% water in the flour."
And that little bit of water in these ingredients is enough to dissolve the sugar.
Now you might normally accelerate the dissolution process with a little bit of heat, or maybe with
agitation — stirring it really aggressively. But we want to isolate the variables here. Heat could
cause other chemical changes in the batter, and whipping it could get a lot of air into
the batter. So that's why we're just mixing it up and just letting it sit still for a long time. The
next morning, you can see how much more viscous it is. Dissolved sugar makes liquids thicker.
Bake it, and it's a little lumpen, but that is shiny brownie skin. Again, here's that
same recipe mixed up and immediately baked. Here it is mixed up, allowed to sit overnight,
and then baked. When the sugar is well-dissolved, you get skin. It's almost that simple. But why?
Why does dissolving the sugar get us the skin? "So what I think happens is that, here's this
batter sitting in a pan. It has a little viscosity, then you heat it and any
air that is trapped in the batter is going to try to escape, but already at the very surface,
you have already dehydrated that a little bit. So it's already forming a little bit of a shell
that is trapping the air that is coming up from inside the guts of the brownie. And then there's
this delicate balance where you want this very top layer to be dehydrated enough and viscous
enough so that it both can lift a little bit, but also not break and just go poof right away."
Indeed, the latter scenario is exactly what we see happening in our batches that don't
have skin. They are pockmarked with little popped bubbles. Here's a batch that, for reasons we'll
soon discuss, has some areas with skin and some areas without skin. Look under the microscope. The
skinless areas are just full of little holes where the surface layer could not hold back the gas.
The gas pushed through and left a little hole. If we look at the areas that do have skin,
we can see the surface here has dried in the oven to a hard shell. Gases pushed up under it,
stretching it taut and smooth like a balloon, but the balloon never popped. It stayed solid.
It stayed smooth, and smooth things reflect light much more directly. That's what gloss is.
And what goes better with a glossy brownie? Let's thank the sponsor of this video, Trade Coffee.
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Shipping is also free. Thank you, Trade. Now, let me show you a batch that I made with
powdered sugar so that the sugar would dissolve really thoroughly, and
I increased the sugar quantity by about a third. The surface area dried so hard and candy-like
that the skin couldn't even balloon very much with the gas. It just sat there — a solid, impenetrable
wall. In contrast, here's a batch that I made with the Bittman recipe with powdered sugar,
which normally gives you great skin. But instead of increasing the sugar content,
I increased the water content — just a quarter cup of water there and look, even though that sugar is
very well-dissolved, no skin. The batter was too loose and wet. The surface didn't dry in time
to hold the gasses back. We get a field of popped bubbles instead. So if you've got a brownie recipe
that you really like, but it doesn't give you that nice skin, the first thing that I would try is
just swapping out the sugar for powdered sugar. If that doesn't work, try increasing the sugar a
bit and/or decreasing the water a little bit. And if your recipe doesn't call directly for water,
try removing one of the egg whites. You need the emulsifiers in the yolk, but the protein in the
white is less important and it's mostly water so you can just cut it out. You could also get rid
of the water that's in the butter. Butter is 15 or 20% water, and you can get it out by switching
it with clarified butter, which has no water in it, or you could brown the butter. Browned butter
tastes great in brownies and in the course of browning it you're going to evaporate most of
its water. Or you could swap the butter for oil — no water in oil, right? This batch I made with
powdered sugar and oil instead of butter — inferior flavor, but even better skin.
This experiment also suggests that the type of fat is irrelevant to skin. It's merely a matter of the
water that the butter brings with it. So why do so many people say that cocoa butter is the key
to brownie skin? Melting lots of solid chocolate into your batter, that gives you the skin — lots
of people claim that. I suppose one reason could be that cocoa butter is a form of fat that brings
little, if any, water with it. So if you swap out the butter in your recipe for cocoa butter,
you're going to have less water in the batter and as a result, that surface of the batter will be
able to dry faster in the oven. It could also be that that extra step of heating and melting
the chocolate helps to dissolve your sugar. Heat accelerates dissolution, and if you put the sugar
in the pan with your chocolate and your butter and such as you're melting all of it, that's
going to help dissolve the sugar. Plus if you're melting sweetened chocolate, like chocolate chips,
then it could be the sugar that's already in the chocolate helping you out.
"And that sugar is already dissolved. So you're not running into this problem
with the sugar not dissolving. And that is then, again, causing that crust."
Likewise, this is probably why people think whipping air into the batter will give you
skin. It does, but not because you've aerated the batter. It's because this extra step of
beating the sugar and the eggs until foamy also agitates the sugar in the presence of water,
helping it to dissolve more smoothly. Though, the crust you get from that technique is not
as shiny — I would guess that's because the very small bubbles that you beat into the batter make
the surface less smooth. But it's still good and dry, so it's crusty — because the sugar
was well-dissolved into a minimal amount of water. All of this raises the question,
why not just make brownies with a form of sugar that's already dissolved, AKA a syrup? Well,
here's the Bittman recipe again with no sugar, but an equivalent amount of corn syrup by weight.
No skin at all on those. Here's the same experiment but with
honey instead of sugar. No skin at all. Why not? Well, maybe we have too much water in the batter.
Honey and corn syrup are both about 20% water, and we know from our other experiments that having too
much water in the batter means that the surface layer won't be able to dry in time to form a
crust. So let's try boiling the water out of the honey. Mix up the batter, bake it, and yikes.
Well, maybe something different is happening on the molecular level here. Honey and corn syrup
are mostly glucose and fructose and other sugars that are not sucrose. Cane sugar is sucrose. So
let's try making a pure sucrose syrup. You need a little bit of water to get it dissolving,
but then I boiled it to about 240℉, which according to candy-making references should
mean that most of the water is gone. Mix up the batter, bake the brownies and no skin. Dry batter,
fully dissolved sucrose, no skin. Why not? "Sucrose — it's essentially
two rings linked together. It's one glucose ring and one fructose ring, and they're held together
by one single bond and that's it." And that bond is easily broken with
acid or with heat. A little heat in a not particularly acidic solution won't do much,
at least if you don't leave it in there for very long, but the kind of heat necessary
to create a low-moisture sugar syrup? "What that does when you then heat it is it
pretty quickly breaks that bond, and then you get what's called invert sugar, which is really just
a solution of fructose and glucose. That's it." And as we've previously established, those sugars
don't make brownie skin. One more experiment. Here's the Bittman recipe with powdered sugar,
which would normally give us a great skin, but I'm going to drop in just a little glug
of honey — just a little spoonful. And that is this batch — the brownies with some skin
and some not-skin. Even a small amount of sugar that isn't sucrose disrupts that skin formation.
Why? Well, this is just an educated guess now, but Dr. Sörensen suspects that what's happening is
when the sucrose dries on the surface of the brownie, it recrystallizes a little bit,
and that's at least part of why the resulting skin is so smooth and shiny and crispy.
"You could imagine that there are all these molecules floating around a solution,
and in order for a crystal to form, it's a little bit like almost building a
brick wall, where all of the bricks fit together perfectly. Now, imagine if, instead of a solution
with molecules where they're all the same size, imagine instead you have some of those,
but then you also have maybe even just a few of some other molecules that are not quite like a
half-brick, but they're sort of just a slightly different, maybe they are a little round and now
you want to build that brick wall and have it stick together. You can imagine that it's
not going to fit perfectly. There's going to be little crevices and a little air here and there,
and it's not going to stick together as nicely. So you're not going to be forming those little
crystals again, because those smaller, different ones are interrupting the crystal structure.
So I think that is what the honey does. That's what corn syrup does."
I buy it. There's only one hitch though. In my previous recipe video, linked in the description,
I showed you the original brownie recipe that I came up with in the course of doing all these
experiments. It's got great skin and I used powdered sugar in it, but I also used sweetened
condensed milk. Why doesn't that disrupt the crystal structure? Well, condensed or evaporated
milk is conventionally made in a vacuum pan. You boil it in a low-atmospheric-pressure environment
and that lowers the boiling point. So you can drive off the moisture at a much lower temperature
that isn't going to really affect the flavor of the milk so much. The fact that my recipe
got me skin suggests that the manufacturer of this specific brand, at least, uses real cane
sugar or some other kind of sucrose and that they dissolve it into the milk either in the vacuum pan
or later at some relatively low temperature — in either case, a temperature low enough to not
break the intramolecular bond of the sucrose. "And of course the sugar in milk is lactose, which
is also a two-ringed. It's not a one ring. They're different two-ringed structures than sucrose,
but still, it may be less disruptive than having all these one-ringed structures in there."
However, it occurs to me that some cheaper manufacturers of sweetened condensed milk
might boil their product at a higher temperature to save time, or they might use corn syrup instead
of the more expensive cane sugar. In either case, that would disrupt your skin. So if you're using
my recipe and not getting skin, maybe try a different brand of sweetened condensed milk.
Lastly, there's the issue of the molasses in my recipe. I wanted that nice flavor you get from
brown sugar, but I wanted the crust you get from powdered sugar. So I used powdered sugar and I
called for just a couple of teaspoons of molasses, because after all, brown sugar is just sugar with
molasses in it. Molasses itself is a byproduct of sugar-making, and as a result, it has a lot
of sucrose in it. But depending on what brand you buy and how it was processed, it might have lots
of other sugars in it as well. So if you're doing my recipe and not getting good skin, this may be a
reason why, and/or maybe you're using too much. Here's two batches of my recipe. This one has
about half as much molasses as this one, and this one is shinier. Maybe that's the reason why.
And of course, if you just want brownies with great skin, but you don't have to worry about
any of these variables, just buy boxed brownie mix. The mixes usually give you great skin
because their water-to-sugar ratio is carefully formulated, and because the sugar is very fine.
Look how powdery that mix is — tiny sugar crystals that'll dissolve even better than
coffee into water. Don't forget to save 50% on your first bag from Trade with my link and code
in the description, and happy skin-hunting — because that doesn't sound terrifying.