The Science of Sourdough Starters

Tim Chin wrote . . . . . . . . .

These are wild times. Many are stuck at home, locked into a relentless repetition of time and place in which weekends mean nothing, and distance means everything. The crisis outside our windows and walls rages on. And it seems that everyone, from professional bakers suddenly out of work to first-time dabblers, is making sourdough bread. My Instagram feed has become an endless flood of blistered, perfectly imperfect boules and batards, and is peppered with snapshots of fledgling baby starters bubbling away.

Why the sudden interest in sourdough, and baking in general? There’s plenty of bread on the store shelves. That’s not the problem. Maybe it’s the relief of a time-intensive, all-consuming endeavor. Or maybe making sourdough plays a more abstract role. “I think [sourdough] bread is a symbol for home, for comfort and community,” says Daniela Galarza, Features Editor at Serious Eats. The concept of taking raw ingredients and microorganisms—ones that aren’t dangerous pathogens like the novel coronavirus—and making something that’s nourishing provides solace. “It gives people a sense of control that they otherwise don’t have in other parts of their lives right now.”

When using hand sanitizer and fanatical handwashing have become ethical and civic responsibilities, cultivating a starter seems to fly in the face of what we’re supposed to be doing right now. And yet here we are, baking up a collective storm and comparing crumb shots on social media. Making sourdough also taps into that primal drive to survive and be self-sufficient in times of duress: While murder hornets and a deadly pandemic threaten your very existence, at least you can bake a nice loaf for yourself. All you need is some flour, water, salt, and your own two hands.

Now—perhaps more so than any other time in your life thus far—you have the time to make a starter. You’re stuck at home. You can afford to tend to something, to give it both your mental and physical attention.

But before making a starter, it helps to understand what it is. There’s an entire microbial universe at work that leads to that crackling crust, that creamy, honeycombed crumb with an impossibly complex flavor, and that absurdly photogenic loaf of your dreams.

Let’s take a look at what’s really going on under a sourdough starter’s bubbling hood.

Wild Beginnings

Years ago, I used to work at a neighborhood restaurant in the East Village. My pastry chef at the time maintained a modest sourdough program, churning out several loaves and baguettes daily. She had affectionately named her starter ‘The Bitch.’ It lived in a crusty, red-lidded, 12-quart Cambro container with a weathered tape name tag in big, defiant upper-case letters. Every day, it was the first item on my prep list: Feed The Bitch. Sometimes I fed it rye flour. Most times I fed it white wheat flour. Other times, I treated it to cider or beer. And, occasionally, I would arrive late on a warm summer morning to find it spilling out onto the tile floor, angrily gurgling away from the speed rack. The Bitch was a fickle—albeit essential—coworker.

A sourdough starter—or levain, if you’re French or fancy—is a complex community of microbes used to leaven breads, imparting a distinct sour flavor and light texture along the way. Like many ferments, starters have been around for thousands of years, with the earliest known leavened bread dating back to 3700 BCE in Lausanne, Switzerland. In fact, it’s only in the last 150 years or so that commercial baker’s yeast has come into fashion, while the slow, plodding, sometimes mercurial process of natural leavening has faded away, only to be found in artisanal bakeries, restaurants, and enthusiasts’ homes. Commercial yeast has its merits: It works fast, it’s convenient, shelf stable, and, until now, it’s been readily available. Since the onset of the pandemic, yeast has all but disappeared from store shelves, as manufacturers race to keep up with demand. Sales of baking yeast skyrocketed 647.3 percent in March 2020 compared to the year prior, according to Nielsen.

But sourdough has always been, and will always be there, as a reliable way to make bread.

Sourdough Starters Versus Commercial Baker’s Yeast

Take a bite out of a slice of sourdough and another from a loaf made from industrial baker’s yeast and you’ll notice a difference right away. Sourdough breads just taste better—they’re more complex, more aromatic, and more adaptable to a wider range of flavors than commercial yeast. On the other hand, breads made with commercial yeast have a distinct calling card: A monotone, sweet, beer-like aroma that often dominates in breads like a brioche or a white pullman loaf. Baking with a sourdough starter can bring other flavors to the fore, such as the caramel, earthy notes of whole wheat or the subtle sweetness of dairy. This improved flavor comes from a sourdough starter’s microbial diversity, a feature that commercial yeast lacks.

Sourdough breads are also arguably more easily digestible for most people, with a greater bioavailability of nutrients, and are well-tolerated by those with certain sensitivities to commercial baker’s yeasts, sugars, or other additives.

That doesn’t mean breads made with conventional yeast are bad. They have their place in the world of baking, too. But sourdoughs are their own beast, and there’s a lot going on that makes them so special.

How Sourdough Starters Work

The abridged version of the process goes something like this: mix equal parts flour and water in a jar and wait. Take some of that pasty sludge out and discard it; stir in more flour and water, and keep waiting. After some period of time repeating this process over and over, you produce a bubbling, doughy-gooey mass that rises and falls with some predictability. Over time, this mixture contains the proper collection of yeast and bacteria that can leaven bread and bestow that distinctive tangy, creamy flavor and light texture that we know and love—it becomes a sourdough starter. In exact terms, we say a starter has fermentative power—the ability to convert sugars into products like ethanol, carbon dioxide, and organic acids.

Simple, right? Not so fast.

Microbial Power

Here’s the long version: A sourdough starter is a culture of microorganisms. Where do those microbes come from? They’re everywhere: In the flour you use, in the air, on your hands, in the jar, maybe even on the spatula or spoon you use to stir. Common belief holds that the majority of microbes come largely from flour and, to a much lesser extent, the surrounding air. But there’s evidence that yeast and bacteria come from less obvious places: Pulling data from bakers across the globe, this study suggests that some of the diversity of microbes and flavor differences between starters comes from the microbes that live on the hands of those bakers (known as the skin microbiome).

Starters rely on one of the fundamental forces of evolution: natural selection. You’re honing a microbial ecosystem and harnessing it for bread making. How do these microbes grow? When flour and water mix, enzymes (amylases) in flour convert long starch molecules into simple sugars, providing the perfect fuel for microbial reproduction.

In the world of sourdough starters, the two most important microbes are yeasts and lactic acid bacteria. Let’s break those down in detail.


Yeasts are a diverse set of single-celled microorganisms that make up approximately 1 percent of the entire fungus kingdom. There are more than 1500 known species of yeasts. The species we know best is Saccharomyces cerevisiae—or common baker’s yeast—which is used in both baking and the production of alcoholic beverages like beer. But there are many more yeasts that are useful in food production.

Yeasts contribute mainly the leavening power of a dough, and somewhat to the flavor and aroma. How does yeast do that? In order to reproduce, most yeasts like S. cerevisiae convert simple carbohydrates (sugars) to carbon dioxide and ethanol. This process is known as alcoholic fermentation. As the yeasts continue to feast on available sugars, they multiply. This reproduction rapidly occurs at warm temperatures (between 86–95°F, or 30–35°C), but also occurs at lower temperatures, although at a slower rate. The production of carbon dioxide creates gas bubbles in dough, which, when trapped in a well-developed gluten matrix, expand the dough. When baked at a high temperature, these bubbles expand further as more and more carbon dioxide is produced until the yeasts die off, resulting in that airy, spongy loaf we call bread.

As you might expect, given the sheer diversity of yeasts, S. cerevisiae isn’t the only species living in a sourdough starter. The reality is much more complex. In studies of starters from around the world, DNA sequencing from varying samples has revealed the presence of a wide array of wild yeasts: Saccharomyces servazzii, a funky-smelling prolific producer of carbon dioxide with profound leavening power (it’s so powerful that it’s even a bane to industrial food production, where it causes packaging to explode); or Saccharomyces unisporus, found more commonly in liquid and warm starters; Pichia anomala, which produces isoamyl acetate, which smells like artificial banana; and no less than seven other species of yeasts, all with varying characteristics and functions. The most commonly occurring yeasts include S. exiguus, S. cerevisiae, and Candida milleri (or humilis).

The differing ratios of these yeast populations alone is enough to explain the degree of variability between sourdough starters. But yeasts are just one side of the microbial coin.

Lactic Acid Bacteria

Lactic acid bacteria (LAB) are rod-shaped or spherical, and primarily produce lactic acid. Much smaller than yeasts, they are found in decomposing plants, dairy products, on the skins of vegetables, fruits, and even on your own fingers. In a typical starter, LAB outnumber yeasts by as much as 100 to 1. Like certain yeasts, LAB digest simple carbohydrates, but instead of the alcohol created by yeast, LAB mostly produce sour lactic acid as a byproduct.

Why are LAB important in sourdough? First, the production of lactic acid (as well as acetic acid) lowers the pH of your starter to around 3.5 (and as high as 5). This lowering of pH results in that characteristic sour flavor of sourdough. Second, a low pH eliminates unwanted pathogens like enterobacteria or Staphylococcus. Simply put, microbial baddies can’t survive in an acidic environment. This feature alone is the driving force behind lacto-fermentation, the age-old technique of preservation that has produced foods like kimchi, sauerkraut, and kosher dill pickles. A low pH also gives sourdough a longer shelf life than other breads by inhibiting mold growth. Finally, LAB release protease enzymes that break down gluten over time, resulting in a softer, lighter texture.

They are generally classified into two groups: homofermentative and heterofermentative strains.

  • Homofermentative (or homolactic) LAB only produce lactic acid. They prefer temperatures between 86 to 95°F (30–35°C), though they grow at lower temperatures as well. They produce flavors characterized by dairy, cream, or yogurt notes. Bacteria in this category include Lactococcus, Enterococcus, Streptococcus, Pediococcus, and L. acidophilus.
  • Heterofermentative LAB produce lactic acid, but also acetic acid, ethanol, and even carbon dioxide (thus providing some leavening power). These bacteria thrive at temperatures between 59 and 72°F (15–22°C), but can grow over a much wider range as well. They impart a sharper, more vinegar-like tang to foods, likely due to the extra production of acetic acid. The most relevant species are L. plantarum and L. fermentum, among others.*

As with yeasts, a single sourdough starter will likely contain several species of LAB over the course of its lifetime. For instance, there’s L. sanfranciscensis, the bacteria for which the San Francisco-style sourdough is named, which produces a distinctly tangy flavor. Early on in development, species like homofermentative Pediococcus, Enterococcus, Streptococcus, and Weisella bacteria have been shown to predominate. But evidence suggests that over time, stable sourdough cultures contain mostly heterofermentative LAB such as L. fermentum and L. plantarum, which outcompete less adaptable homofermentative lactobacilli. (In other words, a stable starter tends to have a more sour aroma, and imparts more sourness to breads, than a young, one-week-old starter due to the additional production of acetic acid from heterofermentative LAB.)

Putting it all Together: A Story of Symbiosis

How are yeasts and lactic acid bacteria able to coexist peacefully in a starter? Like any bustling city, there are limited resources in a sourdough culture. Let’s call those resources simple sugars, of which there are several: glucose, fructose, and maltose, to name a few. Yeasts like C. milleri and S. cerevisiae prefer to feed on glucose and fructose. Meanwhile, LAB such as L. sanfranciscensis thrive off of maltose. A stable starter features a balance of microbes that don’t compete much for each others’ food.

Both yeasts and LAB work to make their surroundings inhospitable to most other microbes. Yeasts give off ethanol, but oddly enough, the LAB can tolerate ethanol quite well. On the flip side, LAB secrete acids, but wild yeasts are also tolerant of the increasingly acidic conditions. To top it all off, yeast cells produce additional amylase enzymes as they reproduce, which convert additional starch to simple sugars to help feed the whole gang. These two microbes survive, thrive, and outcompete others in a stable starter culture—in perfect symbiosis. That’s the kind of elegant, seamless teamwork that would make NBA Hall of Famer Phil Jackson’s legendary triangle offense look like a game of 4th grade pickup basketball.

That was a lot of hard microbiology. Fortunately, you don’t need to retain a lick of it to successfully make your own starter. But like most granular topics in cooking and baking, it helps to understand what’s actually happening.

Source: Serious Eat

A Milder Hair Dye Based on Synthetic Melanin

With the coronavirus pandemic temporarily shuttering hair salons, many clients are appreciating, and missing, the ability of hair dye to cover up grays or touch up roots. However, frequent coloring, whether done at a salon or at home, can damage hair and might pose health risks from potentially cancer-causing dye components. Now, researchers reporting in ACS Central Science have developed a process to dye hair with synthetic melanin under milder conditions than traditional hair dyes.

Melanin is a group of natural pigments that give hair and skin their varied colors. With aging, melanin disappears from hair fibers, leading to color loss and graying. Most permanent hair dyes use ammonia, hydrogen peroxide, small-molecule dyes and other ingredients to penetrate the cuticle of the hair and deposit coloring. Along with being damaging to hair, these harsh substances could cause allergic reactions or other health problems in colorists and their clients. Recently, scientists have explored using synthetic melanin to color human hair, but the process required relatively high concentrations of potentially toxic heavy metals, such as copper and iron, and strong oxidants. Claudia Battistella, Nathan Gianneschi and colleagues at Northwestern University wanted to find a gentler, safer way to get long-lasting, natural-looking hair color with synthetic melanin.

The researchers tested different dyeing conditions for depositing synthetic melanin on hair, finding that they could substitute mild heat and a small amount of ammonium hydroxide for the heavy metals and strong oxidants used in prior methods. They could produce darker hues by increasing the concentration of ammonium hydroxide, or red and gold shades by adding a small amount of hydrogen peroxide. Overall, the conditions were similar to or milder than those used for commercially available hair dyes. And the natural-looking colors deposited on the hair surface, rather than penetrating the cuticle, which is less likely to cause damage. The colored layer persisted for at least 18 washes.

Source: American Chemical Society

Today’s Comic

Your Chicken Is No Longer Pink. That Doesn’t Mean It’s Safe to Eat.

wrote . . . . . . . . .

As we wait out this pandemic, chances are you’re at home, cooking. Perhaps you’ve baked a million loaves of bread and your sourdough starter is overflowing. If Google Trends is any indication of what comes next, after “banana bread” and “pancakes,” people are seeking “chicken recipes.”

Chicken is America’s most popular meat. But undercooked chicken, when contaminated, is also a leading source of food-borne illness. So how do you avoid giving yourself and your isolation-mates food poisoning?

Many people, including Solveig Langsrud, a scientist at the Norwegian Institute of Food, Fisheries and Aquaculture Research, assume chicken follows a simple rule-of-thumb: Pink chicken turned white means “done.” It’s similar to how we cook other meats.

“Consumers can see that if you have a hamburger, and it turns from red to brown, it’s approximately around the temperature where the meat becomes safe,” said Dr. Langsrud.

But was this true? Did it line up with temperature recommendations?

As scientific literature offered no clear answer to her questions, Dr. Langsrud and her colleagues have identified common problems with recommendations and practices for cooking chicken safely at home. In a study published Wednesday in PLOS ONE, they showed that home cooks often follow intuition and color, disregarding temperature recommendations. Intuition and color sometimes aren’t enough to ensure safety. These can be alleviated with a few expert tips.

To study how cooks at home follow safety recommendations, researchers filmed 75 households in five European countries. From a random but nonrepresentative sample, they also conducted an online survey of nearly 4,000 households in the same countries that say they cook chicken.

Worried that chicken would dry out, most home cooks determined doneness by color and texture inside the meat, they found. Few bothered with thermometers, claiming they took too much time, were too complicated to use, didn’t fit in the chicken or weren’t necessary (although easy-to-use thermometers are inexpensive and widely available).

In additional lab experiments, the scientists injected raw chicken breast fillets with a cocktail of campylobacter and salmonella. These bacteria are common contaminants of chicken, and cause millions of sicknesses, thousands of hospitalizations and hundreds of deaths each year in the United States. They cooked the breasts on a commercial grill plate until they reached core temperatures ranging from 122 to 158 Fahrenheit (the World Health Organization’s minimum temperature for safe chicken), and they discovered something surprising.

At 158 degrees, but not lower, bacteria inside the chickens’ cores was reduced to safe levels, and when cut open its flesh appeared dull and fibrous, not glossy like raw chicken. But meat began changing from pink to white far below this threshold, and most color change occurred below 131 degrees Fahrenheit. Sometimes, the chicken’s core would be safely cooked, but unsafe levels of bacteria still lingered on surfaces that hadn’t touched the grill plate.

Many people think chicken is safe before it is, Dr. Langsrud said. Her advice?

You can check the core for fading pinkness, dulling glossiness and more apparent fibers, all signs of degrading proteins and cooking meat. But those alone won’t bring you safety.

You’re really better off buying a thermometer. Ask a salesperson how it works and where to measure temperature, said Bruno Goussault, a scientist and chef specializing in precise-temperature cooking at the Culinary Research and Education Academy in Paris and Washington, D.C. Dr. Goussault was not involved in the study.

Use it to “follow the temperature,” he said, by measuring often. Temperature still increases in the meat’s core after it is removed from a heat source. Depending on thickness, a chicken breast’s core temperature, for example, may increase 41 degrees Fahrenheit in the 10 minutes after it is removed from heat.

The United States Department of Agriculture’s Food Safety and Inspection Service’s guidelines for cooking chicken at home suggest a minimum core temperature of 165 degrees Fahrenheit. But using the same recommended temperature for legs and breasts can result in Thanksgiving turkey effect — dry breast and juicy drumsticks — because white meat cooks at lower temperatures than dark.

Americans still should respect these guidelines, says Dr. Goussault, but that doesn’t mean we must settle for dry chicken. He prefers a sous-vide method that involves vacuum sealing and cooking in a water bath at exact temperatures to consistently arrive at beautiful, juicy and pathogen-free chicken. But you don’t need to be a sous-vide master.

Try buying and cooking breasts and legs separately, Dr. Goussault said. Bring the breast’s core to 165 degrees Fahrenheit, he said, and the leg to between 168.8 and 172.4 Fahrenheit.

And remember: Chicken surfaces need love too. Unless the inside of a chicken was contaminated during processing, the outside is where you’ll find most bacteria. Boiling it, or searing it uniformly, will ensure heat kills all surface bacteria.

If you really want to safely gauge temperature for a whole chicken, insert a pop-up thermometer into the thickest part of the thigh before roasting it, Dr. Goussault suggests. By the time the probe pops, the breasts will have long cooked. They will likely be dry and far from his standards of culinary perfection. But you’ll be sure to, as Dr. Goussault says in French, “dormir sur ses deux oreilles,” or, figuratively, “sleep peacefully.”

Source: The New York Times

Here’s the Science Behind Why Gin and Tonics Taste So Good

Nicholas Mancall-Bitel wrote . . . . . . . . .

It’s not your imagination: gin and tonic water actually taste better together than apart. The duo is greater than the sum of their parts thanks to their chemical makeup; your nose, mouth, and brain are wired to light up when they encounter the cocktail. Now, if only food scientists could figure out exactly why.

“One of the reasons I love talking about food chemistry and the gin and tonic problem in particular is that we don’t know,” Matthew Hartings, a faculty member in the department of chemistry at American University who has put a lot of thought into the mystery of the delicious G&T, shared. “We have some ideas, but a full account of it, we don’t know.”

Let’s start with what we do know. What we taste, and more importantly what we smell, arises from molecules inside the drink. In the case of a G&T, these molecules come from botanicals — primarily juniper — infused into the gin (which is drawn out by ethanol during distillation) and from quinine in the tonic, which gives the mixer its unique bitter taste. These molecules are delivered to our mouths by drinking or to our noses, where most of our flavor receptors actually are, by evaporation. While ice adds a cool crispness to the taste, it also dampens the molecular activity. This is why extra bubbly tonic helps to deliver more flavor — by transporting the chemicals up the liquid and into our mouths.

The next piece of the puzzle is how we taste. Molecules in your drink fit into protein receptors in the nose and mouth, triggering signals that go to your brain and giving you a sense of taste and smell.

But we’re not simply talking about individual molecules in the case of gin and tonic. We’re talking about aggregate molecules, which combine individual chemicals into new molecules that taste completely different. Unlike oil and water, which separate violently, the molecules in gin and tonic water naturally attract one another.

“When we start talking about how molecules are attracted to one another,” Hartings explained, “the general rule of thumb is if two molecules look like one another, and they have the same patterns of carbons and hydrogens and oxygens, and they have the same backbones and substance, they’re going to be attracted to one another.” If you want to get into the scientific weeds about it, similar chemical structures generate electric dipoles that attract one another.

Molecules in gin and tonic water naturally attract and form aggregates, and these aggregates — along with some individual molecules — float up into the receptors within your nose and mouth. From here, things can get a bit more complicated.

Gin molecules can fit into certain proteins while tonic molecules can fit into some of these same proteins. The same goes for the aggregate molecules, which can fit into some receptors that work with each ingredient and also some new proteins. All of these interactions send different signals to the brain, but size and shape aren’t the only things that matter. “How long these molecules are in that flavor receptor and how tightly they bind all affect the signal that gets sent to your brain,” Hartings said. Plus, it’s not as if the molecules are lining up politely to take turns sending different signals. “It’s a battle royale — these molecules are duking it out to see which one will go into this flavor receptor.”

The sheer complexity explains why food scientists still can’t figure out why gin and tonic taste especially good together. “You’re thinking about hundreds or thousands of different molecules in a glass and then the several hundred different kinds of proteins in your nose collecting all those interesting flavors,” Hartings said. “And you have to think about how all those molecules interact with one another, how they interact individually with those proteins, and how those proteins interact on a whole with all those molecules at once. It’s a big messy problem.”

Compound this challenge with the addition of sugar in the tonic water and lime juice in the garnish, not to mention the various combinations of botanicals that gin distillers can use, and you have a recipe for an incredibly difficult scientific quandary. You also have the recipe for a darn simple drink that’s utterly, mysteriously delicious.

Source: Thrillist

New Textile Could Keep You Cool in the Heat, Warm in the Cold

Imagine a single garment that could adapt to changing weather conditions, keeping its wearer cool in the heat of midday but warm when an evening storm blows in. In addition to wearing it outdoors, such clothing could also be worn indoors, drastically reducing the need for air conditioning or heat. Now, researchers reporting in ACS Applied Materials & Interfaces have made a strong, comfortable fabric that heats and cools skin, with no energy input.

“Smart textiles” that can warm or cool the wearer are nothing new, but typically, the same fabric cannot perform both functions. These textiles have other drawbacks, as well — they can be bulky, heavy, fragile and expensive. Many need an external power source. Guangming Tao and colleagues wanted to develop a more practical textile for personal thermal management that could overcome all of these limitations.

The researchers freeze-spun silk and chitosan, a material from the hard outer skeleton of shellfish, into colored fibers with porous microstructures. They filled the pores with polyethylene glycol (PEG), a phase-changing polymer that absorbs and releases thermal energy. Then, they coated the threads with polydimethylsiloxane to keep the liquid PEG from leaking out. The resulting fibers were strong, flexible and water-repellent. To test the fibers, the researchers wove them into a patch of fabric that they put into a polyester glove. When a person wearing the glove placed their hand in a hot chamber (122 F), the solid PEG absorbed heat from the environment, melting into a liquid and cooling the skin under the patch. Then, when the gloved hand moved to a cold (50 F) chamber, the PEG solidified, releasing heat and warming the skin. The process for making the fabric is compatible with the existing textile industry and could be scaled up for mass production, the researchers say.

Source: American Chemical Society