Ben Crane advised his students to take two light sniffs and then one bigger one, as they considered a recent assignment.
"It smells warm," said one student. "Strawberry!" a classmate exclaimed.
The materials for that day's class at the John Hopkins University were supposed to help train noses to probe for scents of oak, cherry, earth and leather, and even paint thinner or a baby's diaper.
"It doesn't smell like a baby diaper," one student said after taking a whiff of white wine, "but it's not good."
The two week winter class is devoted to the science of wine and anything else that is tasty because it has been fermented — the chemical process that converts sugars to alcohol or organic acids.
Crane's class is one of the most popular among Hopkins undergraduates during the brief January term called intersession. Though U.S. Census data shows cooking has long been on the decline across the country, the students are among a group of millennials that are showing an interest not just in making food but getting inspiration from a broader "foodie" culture or a do-it-yourself attitude in the kitchen.
The class trains students to be home brewers or vintners or to make their own yogurt. It also teaches them the science behind why some cheese is yellow and others are oozy, when to know whether to send back a bottle of wine, and why honey can be kept on the shelf but milk spoils quickly.
For some students, it's an outlet to apply lessons of more science-heavy classes in a hands-on way. Sophomore Stephen Brao is studying chemical engineering at Hopkins, but after he dabbled with some home beer-brewing with his father, the class is helping him understand the hobby like a scientist — and giving him some new ideas for career options.
For others who are more focused on the humanities, it's a chance to learn just enough science.
"I'm in the kitchen a lot," said Nicola Willoughby, a senior majoring in writing and film. "I thought it would be cool to learn more about it. If you can understand what's going on when you're cooking, then you can make changes."
This month was the second time the class has been offered. Crane, a doctoral student in Hopkins' Chemistry-Biology Interface program, had taken a similar class while studying biochemistry as an undergrad at the University of Georgia.
More recently, he has tried making his own beer, wine and yogurt, and pitched the class as a way of "demystifying" fermentation.
"I think a lot of people don't realize it's really pretty basic science and microbiology and chemistry going on," he said.
Fermentation is what makes beer and wine boozy, as yeast turns the sugars in grapes or grains into alcohol. But it's also central to dairy products such as yogurt, buttermilk and kefir, and understanding it properly is vital in ensuring food safety.
At a lecture devoted to the science behind dairy products, Crane explained how milk can be transformed into so many other types of food. As a key energy source for infant mammals, it's high in sugar, fat and water content — properties that also make it a hospitable breeding ground for bacteria.
Raw milk starts to ferment within a few hours left at room temperature, while honey is safe on the shelf because the water it contains is not as accessible to organisms on a microscopic level, he explained. It's too dry an environment for bacteria to grow.
A process that involves adding citric acid and leaving milk at room temperature for 12 or 16 hours produces both buttermilk and sour cream, depending on whether the cream is separated out.
Heating milk to about 185 degrees and then leaving it around 110 degrees for several hours is all it takes to start turning it into yogurt. The lactic acid produced during that process gives yogurt its sour taste, while a chemical compound known as acetaldehyde is responsible for the signature tart flavor.
Yogurt's acidity is not just for its flavor, though — it also prevents pathogenic bacteria from growing, instead leaving only the gut-healthy probiotics for which the product is touted.
There is even a scientific term for what happens when the liquid whey separates out of yogurt — syneresis — but Crane explained that there's nothing wrong with it. Some types of yogurt are better than others at maintaining stability as gels — particularly nonfat varieties, many of which have starchy flavors because stabilizers have been added to keep the yogurt homogeneous.
In classroom experiments, the students learned how to at least start the process of fermenting grapes into wine, and how to (and not to) make sauerkraut. One proper batch was made by sealing salted cabbage in a jar, leaving it submerged in the brining liquid that is naturally produced, while another showed that mold and malodorous bacteria can appear if the cabbage is left exposed to the air.
Lessons about wine explained how different compounds create different "notes" of scent. A chemical reaction with natural cork can spoil as much as 10 percent of wines, Crane explained, and that is why, traditionally, restaurants give diners a taste from a bottle before they commit to it.
In final projects, students explored everything from the Australian spread known as Vegemite to unique varieties of beer and cheese to sourdough bread.
The "starters" — fermented mixes of water and flour — that give sourdough bread its trademark flavor are like pets, explained Willoughby and Laura Nugent, a sophomore cognitive science major.
"There are sourdough hotels," Nugent said, where someone will mind and feed your starter to keep it alive. The starter used by the famous Boudin Bakery in California has been alive for more than 150 years and contains a strain of bacteria named Lactobacillus sanfranciscensis in honor of its hometown.
Brao and classmate Turner Schwartz explained that there isn't anything unusual about the brewing process behind Trappist beers — they can vary from 4 percent to 12 percent alcohol content by volume and come in many varieties.
What makes them so sought after is instead their rarity. The Roman Catholic monks who brew them abide by a strict prohibition on profiting from the endeavor, only making enough in sales to keep their monasteries running.
Though much of the class is devoted to alcohol, it's not an excuse for students to get buzzed — at least not in the classroom. Included in a slide from Crane's presentation one Friday was this disclaimer: "A random subsample of wines are heavily salted and very, very not tasty."