ChemMatters | DECEMBER 2017/JANUARY 2018 9
coagulation for mozzarella and Brie cheeses
are at pH 6.45.
Once the milk has coagulated, the resulting
curd is cut into smaller cubes, changing from
a rubbery solid into cubes floating in liquid
whey. Whey does not precipitate as the pH
drops because of the proteins present. Whey
proteins do not contain phosphorus, so they
remain dissolved in the watery part of milk.
As a result, liquid whey is expelled, removing
water and further concentrating the remaining
milk components. Whey is the liquid by-product of cheese making, contains soluble proteins, and is sometimes used as a substitute
for skim milk or as a fertilizer.
As curds expel more whey, the result is
a drier final cheese. For some varieties of
cheese, such as Swiss cheese, curds are
heated to further remove moisture.
Cheese curds are transferred into molds
to drain completely and to give the cheese
its final shape. Depending on the type of
cheese, the curds are treated differently,
giving it the shape and consistency of the
cheese you buy at the store or at a farmer’s
market. Soft, spreadable curds with low pH
are ladled into bags to hang overnight and
drain. Curds for harder cheeses
are shaped into wheels or
large blocks and are pressed
or weighed down. The added
pressure drains additional whey
and forces the curds together
into the final cheese shape.
While all milk starts at about
the same pH, and most cheeses
finish at a similar pH, the rate of acidification,
or how fast the pH drops, is vital for creating
different types of cheese. This is a natural pro-
cess that results from the fermentation. For
example, when pressing Gouda cheese curds
into molds, they should be at a pH of about
6. 5; in contrast, mozzarella curds should be at
pH 5.25. But when finished, the pH of Gouda
is more acidic than that of mozzarella. The
different taste and texture are accomplished
through different rates of acidification, along
with adding different culture organisms.
To get the right pH at the right time, cheesemakers rely on measurements. “It’s a mix of
art and science,” says Krista Dittman, owner
and cheesemaker at Branched Oak Farm in
Raymond, Neb. “You need to understand the
science and have measurement tools, but you
also need to develop an intuition about when
and what to do.” Dittman uses paper test
strips to gauge the pH of cheeses at critical
points in their development. This information
guides the timing of steps in the process.
“We take lots of pH measurements while
making mozzarella, but we only write down
three: 1) start, 2) when rennet is added, and
3) the end,” Dittman says. “For a typical soft,
spreadable cheese, such as quark, I spot-check
the whey now and again.”
Some cheeses go right to store shelves, such
as cream cheese and cottage cheese. Softer
cheeses, such as Brie and Camembert, are aged
for about two months. Harder cheeses can age
for decades. Cheeses age in a temperature- and
humidity-controlled environment for varying
lengths of time depending on the type. As
cheese ripens, bacteria break down the proteins,
which alters the flavor and texture. The proteins
first break into medium-sized pieces (peptides)
and then into smaller pieces (amino acids).
In turn, these can be broken down into highly
flavored molecules called amines. At each stage,
more complex flavors are produced.
Each of these cheeses starts with milk, but
different bacterial cultures and different rates of
acidification make each variety unique. Using
lactic acid production to preserve food is a
millennia-old process that still results in delicious food today.
We’d tell more cheesy
chemistry jokes but all the
Gouda ones argon.
Rennet is an enzyme and is added to speed up
the curdling process to make hard cheeses.
Aiken, K. The Top 10 Most Popular Pizza Toppings
(Infographic). The Huffington Post, Nov 12, 2013:
[accessed Sept 2017].
Lactose Intolerance: Causes. Mayo Clinic, Feb 9,
con-20027906 [accessed Sept 2017].
Sarah Mullen Gilbert is a science writer who lives
in Iowa Falls, Iowa. This is her first article in
concentration (mol/L) concentration (mol/L)
1 × 10-14 0.00000000000001 0 1 1 × 100
1 × 10-13 0.0000000000001 1 0.1 1 × 10-1
1 × 10-12 0.000000000001 2 0.01 1 × 10-2
1 × 10-11 0.00000000001 3 0.001 1 × 10-3
1 × 10-10 0.0000000001 4 0.0001 1 × 10-4
1 × 10-9 0.000000001 5 0.00001 1 × 10-5
1 × 10-8 0.00000001 6 0.000001 1 × 10-6
1 × 10-7 0.0000001 7 0.0000001 1 × 10-7
1 × 10-6 0.000001 8 0.00000001 1 × 10-8
1 × 10-5 0.00001 9 0.000000001 1 × 10-9
1 × 10-4 0.0001 10 0.0000000001 1 × 10-10
1 × 10-3 0.001 11 0.00000000001 1 × 10-11
1 × 10-2 0.01 12 0.000000000001 1 × 10-12
1 × 10-1 0.1 13 0.0000000000001 1 × 10-13
1 × 100 1 14 0.00000000000001 1 × 10-14
The relationship between H+, OH-, and pH
A kid threw a hunk of cheddar at me.
“Not very mature,” I said.