Why Does Cheese Stink? | The Cheese Professor

Limburger cheese photo credit Depositphotos

Ah, cheese aroma! The butt of jokes and myths, right? But if you spend much time with cheese, you know that aroma is a crucial part of cheese and inextricably linked to cheese flavor. But what is it exactly that makes cheese stink?

The fundamental process of turning milk to cheese involves some pretty remarkable chemical and physical reactions that can be explained scientifically. At its most basic, the cheeesemaking process involves acidification and coagulation of milk, and separation of whey to leave behind the solid curds that become cheese. The subsequent breakdown of proteins (specifically casein) and fats produces the flavors and aromas that make cheese so delectable.

How Volatile Aromatic Compounds Are Made

Bacteria interact with milk
Casein is modified by rennet enzymes
Lactose is metabolized by bacteria
Triglyceride fats are converted into free fatty acids

Brandon Prochaska photo credit Wisconsin Center for Dairy Research

“All the major components of milk—lactose, fat, and protein—and the additives that are used to create the cheese are involved in creating cheese aromas,” says Brandon Prochaska, a sensory coordinator with the Wisconsin Center for Dairy Research. “Milk itself has aromas—although they are generally mild. As milk becomes cheese, the aromas become more complex, more diverse and more intense.” It’s a complicated chain reaction, Prochaska explains. But here is the nickel version: During the cheesemaking process, a community of bacteria (sometimes native to the milk, and sometimes added by the maker) go to work on those milk components. A specific part of the casein is modified by rennet enzymes so that its molecules stick together and form curds. Simultaneously, lactose, the carbohydrate in milk, is metabolized by bacteria, and triglyceride fats are converted to free fatty acids by other enzymes. These are the primary sources of the volatile aromatic compounds that make up cheese’s aroma. These compounds are volatile in that they shed molecules of themselves into the air, where they float around like helium balloons until they reach our aroma receptors. A very complex group of those receptors are located where the roof of our mouth meets our nasal passages. While we smell with our noses, we also smell with our mouths, sort of.

When you take a deep breath of the wafting aromas of a pancake and bacon breakfast, for instance, you smell through your nose, but when you chew that food, more aromatics sneak into your nasal cavity and attach to those super-sensitive receptors in a process known as retronasal olfaction.

In her new book, Flavorama, A Guide to Unlocking the Art and Science of Flavor, flavor scientist Arielle Johnson describes this and other tasting processes in clear terms. “Your nose and tongue function as detectors for these molecules, catching them and sending signals to your brain. Only then does your brain cook up your perception of taste, smell, and flavor from those signals.” The taste buds on your tongue do the basic, yet important work of signaling things like sweet, bitter and salty in one-note expressions, Johnson explains. But your aroma receptors provide the chords and harmonies in detail. In your brain it’s all integrated with memories of previous smelly experiences.

Aromatic Compounds

The aroma compounds at work are numerous and complex, and the combinations of those compounds evolve from the time that the cheese is first formed and throughout the ripening and aging periods.

As Prochaska mentions, some of that is pre-determined by the milk—the animal type and breed and animal diet, to start with, and then by any heat treatment applied to the milk.

Cheesemaking ingredients—including the addition of starter cultures, adjunct cultures, and enzymes, have a role. Cheesemaking processes (including curd heating and washing) all have some impact on the end-result aroma. Ripening temperature and humidity, drying environment, salting procedures, and any washing or other rind treatment can turn a mild cheese into an aromatic show-off. While there are hundreds of aromatic compounds at play in the breath of cheese styles we love, and a nearly infinite number of combinations (several can be detected in any one sample), there are six groups or types of aromatic compounds that are certainly involved.

Esters: These compounds are associated with pleasant aromas, often characterized by sweet, fruity, and floral notes. Parmigiano Reggiano owes its pineapple aroma note to methyl butyrate
Ketones: Including methyl ketones (influential to aroma of blue cheeses), these are formed through the breakdown of triglycerides and the oxidation of saturated fatty acids. 2 heptanone offers fruity aromas in blues.
Lactones: These compounds are associated with pronounced creamy milky, buttery notes.
Alcohols: Many metabolic pathways are involved in the biosynthesis of alcohols encountered in cheese, including lactose metabolism, methyl ketone reduction, amino acid metabolism, and degradation of linoleic and linolenic acids.
Sulfur Compounds: These compounds, such as methional and dimethyl trisulfide, contribute to the characteristic aromas of certain cheeses, like cheddar and blue cheese.

Free fatty acids: FFAs produce butyric acid, which can result in rancid aromas resembling baby vomit that are detected in feta, pecorino Romano and provolone.

The results are familiar to fans of all kinds of cheese.

Aromas of Different Cheese Types

Bloomy rind cheeses should offer earthy aromas of mushroom, soil, and dry leaves—cabbage notes too for a real Camembert. Blue cheeses smell fruity, with anise, licorice notes, and sometimes, the baby vomit.

Cheddar, especially those of the Vermont style, can be quite sulfuric, while almond, cooked milk and sweet cream, notes can be had in Wisconsin Cheddars, with a restrained sulfur aroma. A good deal of pineapple comes out in those Cheddars made with mixed cultures, and a good clothbound cheddar will be earthy with spicy notes.

Soft washed rind cheeses, also called smear ripened, are the stars of the stinky cheese world. These include Taleggio from Italy, French Epoisses, and Red Hawk from California. You may have heard how continually washing the rinds of these beauties with salt brine, beer, or the many other liquids invites brevibacterium linens, a bacteria that results in strong aromas. And if you enjoy the smeared cheeses you know they are mild and buttery once you get to the paste. The aromas on the rind however have been compared to everything from stinky feet and decaying flesh to the warm brackish water of an estuary.

Hard Alpine cheeses like Gruyere usually have a bold aroma (meaty, sweaty) on the rind, but in the paste, they counter with sweet hazelnut, and even chocolate cake. Appenzeller, not technically an Alpine, is a lot like Gruyere in several ways, with more pronounced aromas. Emmentaler is quite different from other Alpines. Thanks to a nearly-overwhelming presence of propanoic acid aroma compounds, you might smell vinegar in Emmentaler.

To learn more about the science of cheese aroma, visit the Cheese Science Toolkit checkout Arielle Johnson’s work, and the 2014 book The Science of Cheese by Michael H. Tunick.