Chemistry and Cheese

Chemistry and Cheese

Cheesy Science

By Sarah Mullen Gilbert 

We Love Cheese

We love cheese. Love it, love it, love it. And we are eating more of it—in the past 30 years, the average cheese consumed per person in the United States has increased 41%, up to 36 pounds of cheese per person. Mozzarella consumption was up 178% in the same period.

But cheese is kind of weird. It is storable milk. It can last weeks or years longer than milk and there is such a variety of cheese. But when did we start eating cheese? We don’t know the exact origin but at some point, someone must have thought, “You know that stuff at the bottom of the milk? That stuff that dried out and sat for weeks? I’m going to eat that!” And when they survived, proto-cheese was born.

Multiple Choice

In the past 30 years, the average cheese consumed per person in the United States has increased

• 40%

• 41%

• 42%

• 50%

Open Ended

Why do you think Mozzarella has increased by 178%?

Making Cheese

Humans have used practical knowledge of chemistry and biology to preserve food for millennia, long before refrigeration and other modern technology. To make cheese, bacteria digest sugars in milk and produce lactic acid. The additional lactic acid lowers the pH and hinders the growth of harmful organisms. By turning milk into cheese, its shelf life is extended from about three weeks to two decades, or even longer.

Multiple Choice

As pH goes down, what happens to cheese?

• It makes it more acidic

• It hinders the growth of harmful organisms

• It last longer

• all of these

• none of these

All cheese starts as milk. 

The primary sources are milk from cows, goats, and sheep, though other mammals such as water buffalo are also used around the world for milk production.

Milk

Cow’s milk compared to hard cheddar. Cow’s milk is mostly water; cheese making removes much of the water and concentrates the remaining fat, protein, and other components.

Notice the percentage of protein.

Cheese

What is different in these percentages?

Multiple Choice

Proteins percentage in cheese goes up because...

• Bacteria must create more protein

• The loss of water changes the ratios

• Most of the protein is added

• None of these

pH and cHeese

The pH measures the concentration of hydrogen ions (H⁺) in a solution, with most solutions between the range of 0 and 14. The more acidic the solution, the lower the pH, with 7 considered “neutral” and above 7, “basic.” Milk has a pH between 6.6 and 6.7. pH is a logarithmic scale, so one unit differs by a factor of 10. A pH of 6 has ten times the H⁺ concentration than a solution with a pH of 7. This logarithmic scale is why even apparently subtle pH differences can be quite significant.

Multiple Choice

On the pH scale, Milk is ....?

• slightly acidic

• slightly basic

• neutral

Making Cheese

To make cheese, milk is pumped into a large tank and warmed to the right temperature. Two different bacteria are used in this process: mesophilic and thermophilic bacteria. Mesophilic bacteria grow best in moderate temperatures, typically between 20 and 45°C (68 and 113 °F). They are used to make mellow cheeses such as cheddar, gouda, and Colby. Thermophilic bacteria thrive between 45 and 122 °C (113 and 252 °F) and are used to make sharper cheeses such as Gruyère, Parmesan, and romano.

Thermophillic

There = heat and Phillic = loves

The opposite of Phillic is "phobic"

Milk has a pH between 6.6 and 6.7.

That makes it below 7 or slightly acidic

Multiple Choice

What does the term Thermophilic mean?

• Likes heat

• dislikes heat

• is made of plastic

• none of these

Multiple Choice

What do you think "Meso" Means in Mesophillic?

• high

• middle

• low

• all of these

Meso = middle

This cheese and bacteris work at

In the tank, the bacteria ferment the sugar present in milk,

• called lactose (C₁₂H₂₂O₁₁),

• into lactic acid (CH₃CHOHCOOH),

• as follows:

  C₁₂H₂₂O₁₁ + H₂O ⇾ 4 CH₃CHOHCOOH

Lactic acid from Lactose

As more lactic acid is produced, the milk’s pH lowers. “pH is our indicator of activity,” says Jeremy Stephenson, cheesemaker at Spring Brook Farm in Reading, Vt. “When the pH changes, we know the bacteria are alive and well. By measuring the pH, we are measuring the activity of these bacteria and assuring that the fresh curd is on the right path to becoming cheese.”

Multiple Choice

What causes Lactose to become Lactic acid?

• Bacteria

• Polymerization

• Casein

• None of these

Coagulates

After the bacteria replicate and culture the milk at the optimal temperature, the milk coagulates and changes from a liquid into a firm, rubbery material. This change takes an hour or two and is possible because of the casein proteins in milk.

Multiple Choice

What does coagulate mean?

• change to a gas

• change to a solid

• change to a liquid

• none of these

Micelles

After the bacteria replicate and culture the milk at the optimal temperature, the milk coagulates and changes from a liquid into a firm, rubbery material. This change takes an hour or two and is possible because of the casein proteins in milk. Casein molecules aggregate into spheres called micelles. The outer layer is negatively charged, which allows the micelles to remain dispersed in liquid milk. To form cheese, the proteins must coagulate, or stick together

Multiple Choice

What makes a structure negatively charged?

• More protons

• more neutrons

• more electrons

• less electrons

Casein is a Protein

Casein comprises 80% of the protein in milk. The casein molecules are normally wrapped into compact spheres that are packed together with calcium and phosphate ions to form microscopic micelles. Acid causes the casein molecules to partially unfold and link with each other. The interconnected micelles form a mesh-like structure that causes the milk to gel into a semisolid.

Multiple Choice

What links in this process?

• monomers

• atoms

• milk curds

• micelles

Soft Cheese

Soft cheeses, such as cream cheese, coagulate slowly. As bacteria produce lactic acid, the other layer of the casein micelle becomes less and less polar. The micelles begin sticking together at around pH 5.3, with full coagulation after 24 hours, at pH 4.6.

Hard Cheese

Hard cheeses, such as Colby and Swiss, require a faster coagulating phase and a firmer resulting curd, so cheesemakers add a substance called rennet. The chymosin enzyme in rennet cuts the negatively charged ends on the micelles’ surfaces. No longer polar, the micelles are repelled by water and begin sticking together. The micelles form chains, which extend in all directions and interlock into a three-dimensional matrix to trap the milk-fat molecules. The more acidic the milk (the lower the pH), the faster this coagulation occurs and the firmer the curd.