[SAM] So in terms of pH I'm going to guess that A was the most basic, and then B is
the most acidic. [CREW] You are almost correct.
[SAM] Today we're making some roasted potatoes.
We at reactions were inspired by J Kenji Lopez-Alt's recipe but, we kind of wanted
to put our own spin on it. So today we're gonna play around with pH, and see how it
affects potato consistency, and then decide which ones we like most. Delicious
roasted potatoes are all about the perfect texture, and today we're out to
find it using chemistry. To get set up I'm filling each of these pots with four
cups, or a liter, of water. Lots of recipes call for boiling the potatoes until
they're soft but not cooked through before throwing them in the oven.
Potatoes are mostly made of water, followed by starch, which is made up of
the polymers amylose and amylopectin. Amylose and amylopectin are found in
packages called starch granules that you'll find throughout a potato.
These starches actually serve as stored fuel for the potato plant, the leafy part
that grows above ground. The texture of a cooked potato is at least partially
related to how phosphorylated it's amylopectins are. When a potato is boiled
the starch granules swell with water, and the amylopectin will break free.
Phosphorylated amylopectins will stick to each other and form a gel as they cool.
This makes your potatoes kind of gummy, and that translates to them holding
their shape. Red potatoes are a great example of this. They have very well
phosphorylated amylopectin, which makes them perfect for a soup or stew.
On the other end of the spectrum are russet potatoes. They don't have well
phosphorylated amylopectin, so when you boil them they'll break
apart more easily, and you'll end up with a fluffier texture, which is perfect if
you're trying to make mashed potatoes. We decided to go with Yukon Golds because
they're nice in between. They'll keep their shape better than a russet potato,
but they'll fall apart more easily than a red potato. With our potatoes picked
out it was time to experiment with texture. Our goal was soft on the inside,
but not so soft that the roasted potatoes would fall apart, and then
crispy on the outside. To try to make this happen we played with pH. So how
does pH affect potato texture? Well we were surprised
to learn that it's incredibly complicated. We talked with a bunch of
experts across the globe and, as it turns out, there are a ton of things going on.
So we're going to get into a few that our experts thought were important. First
let's talk about pectin. This is different from the amylopectin I
mentioned earlier. Pectin in the space between plant cell walls acts like a
glue that keeps the cells stuck together. Pectin is a polymer of repeating
galacturonic acid molecules that are linked together by glycosidic bonds. At a lower
pH, like if we add vinegar to our water, those polymer chains hold together well.
At a higher pH, like if we add baking soda to our water, those glycosidic
bonds linking pectin together start to break down, which leaves you with a
mushier potato. On top of that, a higher pH will also make pectin chains repel each
other, creating a looser structure, giving you an even mushier potato. Another thing
impacting texture are those phosphorylated amylopectins I mentioned
earlier. When phosphoryl groups are added, the whole molecule becomes negatively
charged. When you add a base like baking soda, even more negative ions are thrown
into the mix, making the phosphorylated amylopectin chains repel each other, and
your potato falls apart. In all of these cases acidic water helps hold potatoes
together, and basic water makes them fall apart. So we're going to try five
different pHs to see which one gives us the best consistency for roasted
potatoes. I'm gonna go most acidic, less acidic,
neutral, basic, most basic. I'm going to add three tablespoons of vinegar to
these four cups of water. All right so testing pH. So we'll use pH
strips, but these are pH strips that have never been in a lab. I'd say this is
somewhere between a five and a six, but I definitely want it to be a bit more
acidic, so I'm gonna add a lot more vinegar.
I'm actually going to add entire cup of extra vinegar. Let's test this pH.
Oh that's definitely more orange already. Woo! That's brighter. That's gotta be
somewhere around a four. So that was the most acidic one, here's where the pH came
out for the rest of them. Alright so I'm gonna add five pieces of potato to each
of these pots. Normally you would boil the water first, and then add potatoes—
I guess I cut more potatoes than I thought I did, let's go six—but because we take our
safety seriously, I wanted to make sure I didn't splash boiling water on myself.
Seven, let's do seven. Oh my gosh, perfect! We can do eight. So now I need someone to
come label these for me, because I don't want to know which pot is which. We're
trying to be as scientifically accurate as possible,
meaning I'll be blinded to which potatoes were boiled at what pH. It's
very secretive serious business.
Okay.
I think this one—oh, you're close!
Once you can push a fork through the boiled potato chunks without too much
resistance, they're ready to go. E is done.
I'm gonna add two tablespoons of olive oil to each of these. Here we're coating
the potatoes with oil, and spreading around the starchy paste they get from
boiling. This is important for the Maillard reaction, which we'll get back to in
a second. If this were a recipe and not just an experiment to play with texture,
you'd want to add your favorite spices and seasonings, like garlic, rosemary, salt,
and pepper. We're just here for the science, so we'll leave that part up to
you. Roast the potatoes at 450 degrees Fahrenheit, or 230 degrees Celsius. Leave
them alone for about 20 minutes, then use a spatula to pry them up off the pan and
give them a flip. Cook for another 20 minutes or so, pulling them out when
they're a crispy, golden brown. That color is thanks to the Maillard reaction, a very
complicated, multi-step chemical reaction that breaks down the amino acids and
sugars in the oil and starchy paste mix that we coated our potatoes with. This
reaction will speed up at a higher pH, and slow down at a lower pH, so our
potatoes boiled in baking soda water should come out more brown.Our potatoes
are ready, so I'm going to go through each treatment in order and see how they
turned out. So I'm gonna grab one of these, oh wow.
Maybe I'll do it like this so as not just squash it. So this here is A. That's
nice. Let's see how it tastes.
mmmmmm!
Crispy, but the inside is very, very soft. so that was A, I'm gonna try E now. Seems
pretty perfect looking actually, I must say. I think that E is still a little
less mushy than A, so A is a bit more mushy. They're pretty similar but I would
prefer E. So now we're gonna go with D.
This is way harder. The other ones kind of fell apart,
this did not. I feel like the outside is still, it's a
little bit chewy. It's a little less crispy, but the inside is kind of perfect,
I have to say. Let's try B. This is definitely harder to cut through than D,
and definitely a lot harder than E or A.
It's a little weird. It's pretty hard on the outside and even though once you get to
the center it's soft, it's still a little bit too hard I think, a little crunchy
overall. Okay. Final one, C.
Alright that was way easier to cut through than B, and a little bit easier
to cut through than D. I like that. So if I was gonna assign an award, I would give
E the gold star, C would get the silver, and I think mmm
D or A would get the bronze, I can't decide. So in terms of pH, I'm going to
guess that A was the most basic, E was slightly less basic than that, C was
neutral, followed by D, D is a bit acidic, and then B is the most acidic.
[CREW] You are almost correct. [SAM] What was I off on?
[CREW] So, E was the most basic. [SAM] Oh okay. Huh. I really like
that was my favorite. Awesome. Throw some baking soda in your water
when you make potatoes from now on. The take-home message is if you want what we
think is the perfect roasted potato texture, lay on the baking soda when you
boil. We were all actually shocked at how much baking soda we could use and still
end up with delicious potatoes. So go forth and make some amazing potatoes, and
let us know how they came out in the comments. We'll see you next week. If
you're in the mood for more food science, be sure to check out Serving Up Science,
the newest show in the PBS Digital Studios family. Serving Up Science is
hosted by history buff, science writer, and foodie Sheril Kirshenbaum, who will
give you science based answers to all of your biggest food questions.
What's better, wild or farmed salmon? Should you let your meat rest?
What makes that blue cheese so stinky and delicious? Follow me over to the PBS
Food channel to watch Serving Up Science, and tell them we sent you.