fermentation – Science and Food

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Sauerkraut

May 3, 2016/in Science & Food/by Grant Alkin

Photo credit: Anders Adermark (cmbellman/Flickr)

Fizzy, bitter, yeasty, sour, floral, and sometimes just downright offensive—there are a dazzling array of adjectives that can come to mind when you think of fermentation. Fermentation is one of world’s oldest and simplest culinary traditions. Serendipitously discovered in ancient times as a means of preservation, flavor enhancement, and intoxication, it has exploded as an art and scientific field in recent years. Aptly described by Chef David Chang as when “rotten goes right,” fermentation is a process that harnesses the power of benign microbes to produce complex flavors and can transform a seemingly rotten pile of vegetables into a curiously palatable delight (1). It’s fermentation we can thank for transforming a lonely, simple cabbage into a bustling hotspot for microbial activity that we know as sauerkraut.

German for “sour cabbage,” sauerkraut is distinctively tangy, floral, and surprisingly simple to create. It requires no specialized ingredients or starters, demanding just cabbage and salt (1). Sauerkraut is an example of wild fermentation, a process that exploits microbes native to the surface of cabbages. To make a batch, begin with finely shredded cabbage with roughly 3 tbsp salt per 5 lbs of cabbage. Give your fingers a workout by gently massaging the cabbage mix and after a few minutes, you’ll notice that the cabbage appears to be sweating (2). This sweat is the basis for our brine, whose presence is absolutely vital in our cabbage-to-sauerkraut transformation.

This brine sets a stage for successful sauerkraut fermentation, a fermentation made possible by the flavorful collaboration between several different microbes. These microbes, specifically lactic acid bacteria and yeast, thrive in salty, anaerobic environments, much like in the brine (1). Although the brine is simple, comprising just salt and water, it must be carefully controlled so as to provide the lactic acid bacteria and yeasts with a competitive advantage over other undesirable organisms. Too much salt and you ruin the palatability of our sauerkraut, but too little salt and you risk creating an environment favorable to spoilage or pathogenic bacteria (3).

As the microbes feed on sugars in cabbage, they mainly produce lactic acid, which like salt lends taste in addition to antimicrobial effects. As the brine becomes enriched with lactic acid, the pH declines and the sauerkraut begins to develop tart notes. These effects inhibit the growth of our unwanted microbes, which tend to be sensitive to acidity. Secondary products can also include carbon dioxide, alcohol, and acetic acid, all of which can suppress the growth of our unwanted organisms, too (3).

After massaging long enough, you should have enough brine to prepare your cabbage for jarring. Employing a willing and eager fist, pack your cabbage into a jar so that it’s completely submerged beneath the recently-created brine—you may also want to consider weighing it down. Ensure that your cabbage is entirely covered by brine, otherwise you risk inviting the growth of harmful aerobic bacteria into your jar. Anything left exposed to air is susceptible to mold or invasion of other organisms (2).

Depending on your preferences, you can let your sauerkraut ferment for just a few days or up to several weeks if you prefer stronger flavors. Leave it in a closet, on your countertop, or bury it in your backyard—it’s totally up to you!

References cited

McGee, Harold. On food and cooking: the science and lore of the kitchen. New York: Scribner, 2004. Print.
Katz, Sandor E. Wild Fermentation: The Flavor, Nutrition, and Craft of Live-Culture Foods. White River Junction, VT: Chelsea Green Pub, 2003. Print.
Katz, Sandor E., Michael Pollan. The Art of Fermentation: An In-Depth Exploration of Essential Concepts and Processes from Around the World. White River Junction, VT: Chelsea Green Pub, 2012. Print.

About the author: Mai Nguyen is an aspiring food scientist who received her B.S. in biochemistry from the University of Virginia. She hopes to soon escape the bench in pursuit of a more creative and fulfilling career.

Read more by Mai Nguyen

Fermentation Revival & Mind-Altering Microbes

April 21, 2016/in What We're Reading/by Grant Alkin

Sandor Katz and Dr. Elaine Hsiao will be joining us at our next 2016 public lecture, Microbes: From Your Food to Your Brain. Get to know them beforehand, as Sandor Katz talks about his book, The Art of Fermentation, on NPR: Fresh Air and Dr. Hsiao shares her fascination with the microbiome at a TedxCaltech talk.
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Sandor Katz

April 19, 2016/in Profiles/by Grant Alkin

Sandor Katz, a self-proclaimed fermentation revivalist, became hooked on fermentation with his first homemade batch of sauerkraut, earning him the nickname “Sandorkraut”. As an AIDS survivor, he considers fermented foods an important part of his health and well-being. His 2003 book, Wild Fermentation, was lauded by Newsweek as the “fermentation bible”, and his 2012 book, The Art of Fermentation, received a James Beard award and was a finalist at the International Association of Culinary Professionals. In 2014, Katz received the Craig Claiborne Lifetime Achievement Award from the Southern Foodways Alliance.

See Sandor Katz May 11, 2016 at “Microbes: From Your Food to Your Brain”

What hooked you on cooking?: I’ve always loved eating, and my parents both cooked and we were always expected to help in the kitchen. I got especially interested in cooking from scratch, and understanding and experiencing how the raw products of agriculture are transformed into the foods we love to eat.

The coolest example of science in your food?: Our food is all biology and chemistry. Of course, I am most tuned into the biological processes, though at a certain level they break down to chemistry. One question that I have thought a lot about is: Why is fermentation practiced everywhere? I do not know this to be absolute fact, but I have been unable to find any counter-example. And the reason that we now understand is that all of the plants and animal products that make up our food are populated by elaborate microbial communities. Microbial transformation of our food is an inevitability and the question is how do we deal with that fact.

The food you find most fascinating?: I’m endlessly fascinated by kefir, a fermented milk, or rather by the culture that produces it, undulating rubbery masses known as kefir grains. These symbiotic communities of bacteria and yeast (SCOBIES) are incredibly complex, with more than 30 distinct organisms that have been identified.

What scientific concept–food related or otherwise–do you find most fascinating?: Co-evolution. How we have evolved with plants and microorganisms and how they have influenced what we are and how we have influenced what they are, and how each of these organisms has influenced the others. All the foods that’ve been interested in come about as a result of these co-evolutionary relationships.

Your best example of a food that is better because of science?: Certainly fermentation is better understood because of science, and it is possible thanks to science to replicate particular ferments in environments other than those in which they emerged as spontaneous outcomes. However, it is important to note that science has had negative effects on fermentation practices as well as positive ones. The presumption that large undefined communities of organisms are dangerous has led to the replacement of traditional starter cultures with pure culture starters that cannot easily be perpetuated, thus diminishing the ability of home, village, or small-scale producers to perpetuate cultures and thereby breeding dependence on starters purchased from a lab for each batch.

How do you think science will impact your world of food in the next 5 years?: I’m very excited by all the new findings in microbiology, especially our emerging understandings of microbial communities in different environments and their complex interactions. My hope is that our growing understandings of the functional importance of microbial communities will help us move beyond the war on bacteria, and embrace bacteria as our ancestors, allies, and greatest protection.

One kitchen tool you could not live without?: My crocks! Vessels are the most basic of kitchen tools.

Five things most likely to be found in your fridge?: Milk, yogurt, starter cultures, beer, miso. Kraut and kimchi might be in the fridge, or might be on the counter.

Your all-time favorite ingredient? Favorite cookbook?: Brussels sprouts, along with almost any cruciferous vegetable. They are so delicious and versatile! I like to check out cookbooks and explore recipes from diverse sources, but Joy of Cooking is my enduring go-to.

Your standard breakfast?: I love to use my sourdough starter to make savory vegetable sourdough pancakes, incorporating almost any vegetables, leftover grains if I have them, and cheese, topped off with fried eggs and yogurt-hot sauce.

Stinky Tofu

December 8, 2015/in Science & Food/by Grant Alkin

You may be familiar with stinky tofu’s strong, pungent odor that makes you wrinkle your nose in disgust. Although this dish certainly lives up to its name, taking a bite into its crunchy, deep-fried exterior that gives way to warm, firm tofu might just make you a stinky tofu convert. A popular street food in Taiwan, Hong Kong, and parts of China, stinky tofu is a fermented tofu dish that is often deep-fried, drizzled with a salty sauce, and served with a side of pickled vegetables. This dish can also be found simmered in spicy hot pot, grilled on skewers, and mixed into rice porridge. So what is the secret to its stink?

Stinky tofu from Luodong township in Yilan, Taiwan.

It starts with the brine used to ferment the tofu. The conventional process involves adding shelled shrimp and vegetables such as bamboo shoots and Chinese green cabbage to salt water in wide mouth jars. These jars are then exposed to air for several months to allow the brine to undergo natural microbial fermentation that results in its unique stinky smell. Bricks of tofu are then soaked in the liquid for 4-6 hours to develop its flavor [1].

The major bacteria strains that that contribute to the fermentation process include Bacillus sphaericus, Enterococcus gallinarum, Acinetobacter spp., and Corynebacterium spp. [1]. Microbes of the Bacillus genus secrete proteases that hydrolyze, or break down, the tofu proteins into their constituent amino acids (aka the building blocks of proteins) and peptides (molecules made of multiple amino acids) [2]. Two important sulfur-containing amino acids, cysteine and methionine, degrade during the fermentation process and form various sulfides that are responsible for those sulfurous, meaty, and onion odors [3].

Stinky tofu that have been marinated and are ready for deep-frying

If that is not stinky enough, the volatile flavor compound that contributes most to stinky tofu’s smell is indole, which is associated with fecal and animal odors. Less potent flavor compounds include esters, alcohols, aldehydes and ketones, which confer sweet and fruity odors. In fact, a research study analyzing volatile flavor compounds in a sample of fermented stinky tofu identified a total of 39 compounds that contribute to its smell [3]. Considering how molecules become even more volatile when heated (diffusion speeds up with higher temperatures), helps to further explain why you get a burst of stinky odor when stinky tofu is deep-fried [4].

Stinky tofu definitely lives up to its name, but don’t let the smell deter you from trying the dish. After all, if you end up loving it you will never have trouble finding a stinky tofu stand if you just follow your nose!

References cited

Chang, H., Wang, S., Chen, J., & Hsu, L. “Mutagenic Analysis of Fermenting Strains and Fermented Brine for Stinky Tofu.” Journal of Food and Drug Analysis. 9.1 (2001): 45-49. Web.
Stinky tofu. Microbe Wiki.
Liu, Y., Miao, Z., Guan, W., Sun, B. “Analysis of Organic Volatile Flavor Compounds in Fermented Stinky Tofu Using SPME with Different Fiber Coatings.” 17 (2012): 3708-3722. Web.
Hui, Y.H. (2007). Handbook of Food Science, Technology, and Engineering (Vol. 4). Hoboken, NJ: Wiley & Sons, Inc.

About the author: Catherine Hu received her B.S. in Psychobiology at UCLA. When she is not writing about food science, she enjoys exploring the city and can often be found enduring long wait times to try new mouthwatering dishes.

Read more by Catherine Hu

Kombucha Brewing: The Process

November 10, 2015/in DIY Kitchen Science/by Grant Alkin

Photo credit: Mgarten (Wikimedia Commons)

At first glance, making kombucha sounds straightforward. After all, kombucha is fermented tea, which tells all you need to know about making it: take some tea and ferment it. Unfortunately, brewing kombucha is not that simple, as evidenced by the plethora of information and recipes found on the Internet. For those who have ever contemplated or even decided to begin brewing kombucha for the first time, don’t let the wealth of kombucha information intimidate you. Here, we break down the process of kombucha brewing and experimentation, supplying you with the scientific rationale for each step. Understanding the science of each stage may allow for a more successful and experimental brewing without having to rely on a recipe.

1. Making the tea base.

The tea base is nothing more than sweetened tea, so it is easy enough to make. However, the amount of tea and sugar used will affect the flavor of the resulting kombucha. The exact proportion of water to tea to sugar can be modified to suit personal tastes. For the varieties of teas and sugars suitable for making kombucha, check out our previous post on the ingredients that go into making kombucha.

In general, for every 1 cup of boiled water, steep 1 tea bag or 1 ounce of loose leaf tea; this should be left to steep for 3 – 5 minutes, with deviations depending on the type of tea and desired tea strength. Certain teas, such as green and white teas, have subtle flavor profiles that may result in a bland-tasting kombucha. To obtain a more concentrated flavor with delicate teas, use more tea bags, do multiple infusions, or combine the green or white teas with a more robust black tea.

If you use tea bags, adding more of them can help increase the amount of flavor compounds in the brewed tea, creating a more concentrated green or white tea flavor. Avoid steeping the teas for too long; steeping teas longer than the recommended time results in the extraction of more bitter compounds. This over-extraction will create a more bitter tea base. The same caution equally applies to loose leaf teas.

If you use loose leaf teas, multiple infusions will help concentrate the flavor without the risk of over-extraction. A proper method for multiple infusion involves steeping a large amount of tea leaves for 20 – 45 seconds in just enough hot water to cover the leaves. The brewed tea is removed, and another small amount of hot water is added to the leaves and steeped for another short amount of time. This can be repeated 3 – 15 times, depending on the type of tea. This method uses twice the amount of tea leaves with half the amount of hot water [1], essentially concentrating the flavor compounds that diffuse out of the tea leaves. Multiple infusions may not be as effective with tea bags; the tea fannings used for tea bags have small surface areas, and so most, if not all, of the flavor compounds will have quickly diffused into the water in the first steeping.

Sugar can be added to the boiling water before or after steeping the tea, as long as the sugar source completely dissolves. Typically, 1 cup of sugar is added for every 4 cups of boiled water.

2. First fermentation.

Once the tea is finished steeping and the sugar is dissolved, remove the tea bags or strain out the leaves – this is the completed tea base. Tossing the SCOBY (Symbiotic Culture of Bacteria and Yeast) into this freshly-completed tea base willy-nilly will negatively affect the fermentation process, as the microbes within SCOBY thrive best at specific temperatures and pH levels. To ensure a successful fermentation, the tea base has to be adjusted for temperature and pH create a suitable environment for the SCOBY.

Optimum temperature. Recall that SCOBY is alive; wait for the tea base to cool down to at least below 90°F (32°C) before adding SCOBY. A hot tea base would destroy the SCOBY microorganisms, resulting in a complete lack of fermentation. Conversely, do not add SCOBY to a tea base that has been refrigerated to below room temperature, as this would encourage the microbes to go into a dormant state, leading to a very sluggish fermentation process. The optimal temperature to add the SCOBY is between 77°F (25°C) and 90°F (32°C); , as this is the range which SCOBY microorganisms such as Acetobacter and yeast grow best [2,3].
Optimum pH. SCOBY bacteria are acidophiles, meaning that these bacteria thrive in acidic environments. Excluding herbal teas, the teas used for kombucha generally have low pH ranging from 2.9 to 6.3 [4,5]. While this is considered acidic, the pH of the tea base may not be at the optimal range for the Lactobacillus and Acetobacter that inhabit SCOBY, which thrive around pH 5.0 – 6.3 [6,7]. To remedy this, a starter liquid is added to the tea base, which is the liquid that the SCOBY was stored in. Since the starter liquid houses both Lactobacillus and Acetobacter, which produce acid by oxidizing sugar to lactic acid and ethanol to acetic acid, the starter contains a mixture of lactic and acetic acid at a buffered pH that is ideal for the SCOBY. In general, 1 cup of starter liquid is used for every 2 cups of tea base. If there is not enough starter liquid, then plain, store-bought kombucha can be used in lieu of the starter.

SCOBY is added to the tea base in a wide-mouthed container, often a glass jar, to allow for gas exchange and left to ferment for 7 to 10 days at room temperature. During this first fermentation, oxygen has to be abundantly available for Acetobacter, which requires oxygen to grow (it is an obligate anaerobe) [7]. However, leaving the container uncovered puts the kombucha at risk for contamination by fruit flies. Covering the jar with a tightly-woven cloth or paper towel and an elastic band can keep out fruit flies while permitting oxygen availability for the fermenting kombucha. The longer the fermentation period, the more vinegary the flavor and the lower the sugar content.

Kombucha undergoing the first fermentation. Photo credit: Amy Selleck (amyselleck/Flickr)

3. Remove the SCOBY.

To end the first fermenation, simply remove the SCOBY from the kombucha. From here, there are two options: reuse the SCOBY for another batch of kombucha or store it for later brewing.

Reuse: Make another tea base. For the starter liquid, it would be easiest to use the kombucha that the SCOBY was previously removed from.

Store: Store the SCOBY in a tea base/starter liquid mixture. This can be kept at room temperature for up to three weeks, depending on the volume of the storage mixture, before the microbes exhaust the nutrients. For longer storage, place the SCOBY mixture in the refrigerator. SCOBY become dormant in cold temperatures, but this does not mean the microbes cease activity altogether. Rather, in dormancy, cell division halts and the microbes’ metabolism slows significantly [9]. In storage, the SCOBY will continue to ferment its storage mixture, albeit at a slower rate than if left at room temperature. To maintain SCOBY viability, replenish the storage mixture every 4 – 6 weeks by removing 50 – 80% of the liquid and replacing that with new sweetened tea [8]. The main idea is to provide continuing fuel for the microorganisms. It is also possible to simply add ¼ cup sugar per quart of storage mixture every 4 – 6 weeks [8], but keep in mind that the dormant microbes are still carrying out cellular functions which require nutrients and water. The stored SCOBY will reduce the volume of its storage mixture, and so additional tea is required to prevent the storage mixture from drying up.

4. Second fermentation.

Pour the kombucha into bottles and cap them, leaving the bottles out at room temperature. If a flavored kombucha is desired, this is the step to add flavoring ingredients. Although SCOBY was removed at the end of the first fermentation, not all the microorganisms were attached to the cellulose matrix, especially if the microbes were newly-cloned during that previous fermentation period. There will still be kombucha microbes present to perform a second fermentation.

As this second fermentation occurs in a closed system, CO₂ produced from the yeast cannot escape the kombucha as it did during the first fermentation. As a result, the kombucha becomes carbonated during this step. Further, the kombucha microbes will continue to metabolize any remaining sugar to produce lactic acid, acetic acid, ethanol, and CO₂, so the kombucha will become less sweet but tangier.

After 1 to 3 days, depending on how quickly carbonation occurs, store the kombucha in the fridge. This stops fermentation and carbonation because the significantly decreased temperature causes the microbes to go into a dormant state. And voilá! You have your first batch of kombucha!

Photo credit: thedabblist (64636759@N07/Flickr)

While making kombucha is a lengthy process that can take up to two weeks to complete one batch, and perfecting the recipe to your own taste will involve making many batches, there is perhaps nothing more satisfying than a successful and delicious kitchen experiment.

The process described in this post was based off of kombucha recipes from The Kitchn and Food52.

References cited

Thoughts on Re-steeping. Teatrekker’s Blog. 22 Sept, 2013.
Science of Bread: Yeast is Fussy about Temperature. Exploratorium.
McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Simon & Schuster, 1997.
pH Values of Common Drinks. Robert B. Shelton, DDS MAGD.
Singh, S., Jindal, R. Evaluating the buffering capacity of various soft drinks, fruit juices and tea. Journal of Conservative Dentistry, 2013; 13(3): 129-131.
Rault, A. Bouix, M., Béal, C. Fermentation pH Influences the Physiological-State Dynamics of Lactobacillus bulgaricus CFL1 during pH-Controlled Culture. Applied and Environmental Microbiology, July 2009; 75(13): 4374-4381.
Hwang, J. W., Yang, Y. K., Hwang, J. K., Pyun, Y. R., Kim, Y. S. Effects of pH and dissolved oxygen on cellulose production by Acetobacter xylinum BRC5 in agitated culture, 1999; 88(2): 183-188.
Take a Break from Making Kombucha Tea. Cultures for Health.
Lahtinen, S. J., Ouwehand, A. C., Reinikainen, J. P., Korpela, J. M., Sandholm, J., Salminen, S. J. Intrinsic Properties of So-Called Dormant Probiotic Bacteria, Determined by Flow Cytometric Viability Assays. Applied and Environmental Microbiology, July 2006; 72(7): 5132-5134.

About the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.

Read more by Alice Phung

Kombucha Brewing: The Ingredients

November 3, 2015/in DIY Kitchen Science/by Grant Alkin

Kombucha with SCOBY. Photo credit: thedabblist (64636759@N07/Flickr)

Craving some kombucha without the grocery store prices? Why not try brewing your own kombucha? As a fermented tea drink that is brightly effervescent, deliciously tangy, and slightly sweet, having some kombucha on hand could add a little spring to these cold seasons. On top of that, the brewing and fermentation involved in kombucha-making requires a little scientific know-how and quite a bit of trial and error to perfect the flavor to your liking. Think of it as having a science experiment in your kitchen!

At first glance, making kombucha appears fairly simple, as there are only four basic ingredients that go into it: water, tea, sugar, and a “Symbiotic Colony of Bacteria and Yeast,” SCOBY. If a flavored kombucha is desired, specific flavor ingredients can be added too. A cursory investigation into each ingredient, however, may bring up some questions. What type of tea makes the best-tasting kombucha? What is SCOBY and where can you source it? Is it possible to brew a sugar-free kombucha? Here is your scientific guide to making kombucha. We provide some scientific information regarding each component to help make an informed decision in choosing the ingredients that would create the kombucha that best aligns with your preferences.

SCOBY

What is it?

SCOBY is the most important component of kombucha, since it is the only thing standing between ordinary, sweetened tea and kombucha. Other fermented foods which utilize a similar symbiotic culture include kefir, ginger beer, vinegar, and sourdough. SCOBY is a grayish-white or beige, squishy mass floating within the brewed culture, and it is responsible for the distinct vinegar-like flavor, trivial alcohol content, and characteristic carbonation of kombucha. However, to call this leathery, stringy mat a symbiotic colony of microbes is a scientific misnomer. Biologically, a colony implies a coexisting group of individuals within the same species; a microbial colony is a cluster of microorganisms which have descended from a single cell, a common ancestor. SCOBY, on the other hand, is a symbiosis of multiple bacterial and yeast species cohabiting a cellulose matrix [1]. It may be more accurate to describe SCOBY as a biofilm, a colony of several microbial species attached to one another on a surface.

A symbiotic culture of bacteria and yeasts. Photo credit: Robert Anthony Provost (twon/Flickr)

As the name implies, SCOBY is alive. A study on the microbial populations existing in SCOBY reveals that the bacterial genus Gluconacetobacter is the most abundant [1]. Gluconacetobacter is responsible for the biosynthesis of the cellulose matrix that the SCOBY microbial population resides within. In other words, this genus of bacteria enables easy handling by creating the solid, stringy, floating mass that SCOBY is visually famous for. The next most abundant SCOBY bacteria belong to the genera Acetobacter and Lactobacillus [1], both of which give kombucha its acidic, vinegary taste by oxidizing ethanol to acetic acid and sugar to lactic acid, respectively. The yeast population of SCOBY primarily consists of the genus Zygosaccharomyces [1], which is notable for its high sugar, high alcohol, and high acid tolerance [2]. Yeasts in SCOBY generate CO₂ and thus provide carbonation; they also produce alcohol, some of which is metabolized by Acetobacter into acetic acid. It is worth noting that the microbial composition of SCOBY may vary over time [1], possibly due to rapid growth, contamination, and/or random mutations. This compositional change may lead to flavor differences among different batches that have used the same SCOBY.

Where do I get it?

Home-brewing stores and online marketplaces are the more common places to buy SCOBY. For the more ambitious, there is also the option to culture SCOBY at home. Given that it is a collection of living organisms, you need to start with some pre-existing collection of kombucha microbes.

To make SCOBY at home, a modest amount of store-bought or homemade, unflavored and unpasteurized kombucha is required. Kombucha often contains a small amount of SCOBY left behind from the brewing process. To begin, place about 1 cup of kombucha with 7 cups sweet tea in a covered container and store for 1 to 4 weeks. In storage, the SCOBY microbes multiply and aggregate, with Gluconacetobacter synthesizing the cellulose that enables the microorganisms to grow together in that signature rubbery mass. For more detailed instructions, check out The Kitchn’s recipe for home-grown SCOBY.

Teas

Which tea?

Kombucha would not be kombucha without tea, but with so many varieties and forms to choose from, it’s easy to get lost. In general, teas are categorized by how the tea leaves (from the plant, Camellia sinensis) were processed, which affects the flavor, caffeine content, and color of the brewed liquid. Varieties among the basic tea categories arise from the geography of C. sinensis, growing conditions, time of harvest, and production processing, giving rise to notable flavor differences. The type of tea chosen will influence the prominent flavor profile of the finished kombucha. For the adventurous, different teas can be mixed together to create a unique kombucha flavor base.

Left to right: green tea, yellow tea, oolong tea, and black tea. Photo credit: Haneburger (Wikimedia Commons)

Black: The most common choice for brewing kombucha, black teas undergo full enzymatic oxidation during production, which gives the drink a dark brown color [3]. Furthermore, complete oxidation of the tea leaves gives black teas a deep malt, caramel, or toasty flavor. This rich tea flavor enables a quick brew without flavor loss during kombucha fermentation.
Oolong: Literally translating to “black dragon tea”, oolong teas are partially oxidized, ranging from 8-85% oxidation depending on the tea producer. Oolong flavor profiles fall between the robustness of black teas and the delicacy of green teas, with tones ranging from smoky and buttery to floral and fruity, depending on the amount of oxidation the tea leaves were processed.
Green: During production, the oxidation process is stopped early; the tea leaves undergo minimal oxidation, giving green tea a more grassy, floral flavor when compared to other types of teas [3]. Due to their light and subtle flavors, green teas may have to be steeped many times for full flavor, and kombucha with a green tea base may have to be brewed longer.
White: Unlike the other teas, white teas are made using only the buds of the C. sinensis plant. Additionally, some white tea varieties use buds that have been steamed or baked, which inactivates enzymatic oxidation. The minimal or absence of oxidation gives white teas a very delicate and subtle grassy flavor, and so this tea may have to be steeped multiple times and a kombucha with a white tea base may have to be brewed for a long time.
Pu-erh: Pu-erh stands apart from other teas that use sinensis leaves by an additional fermentation step after the leaves are dried. Fermenting the tea leaves gives pu-erh teas a complex, sweet, earthy flavor profile that the other teas do not have [3].
Herbal: Unlike the above four categories, herbal teas rely on steeping plant parts that do not come from sinensis. Herbal teas are strongly advised against for kombucha brewing, as the plants that are used often contain volatile oils that have anti-microbial and/or anti-fungal activity. Some common anti-microbial volatile oils found in herbal teas include lavender oil (from lavender teas), peppermint oil (peppermint teas), and eugenol oil (chai teas) [4], all of which can destroy the bacteria and yeast in SCOBY. A damaged SCOBY will not be able to ferment or carbonate the kombucha batch.

Loose leaf or tea bags?

Tea bags are cheaper and easier to find at the grocery store, but tea bags typically contain fannings or tea dust, which are broken remnants of tea leaves. These remnants were either purposefully crushed for packaging into tea bags or are the leftover fragments after the loose leaf teas are packaged. In contrast, loose leaf teas cost more than their tea bag counterparts and are primarily found in tea specialty stores, but the leaves are much bigger than the fannings found in tea bags. The primary difference between loose leaf and tea bags are the size of the tea leaves, which will affect taste and brew time. Tea leaf sizes do not always correlate to the quality of the tea [5].

Where tea brewing is concerned, fannings have a much greater surface-area-to-volume ratio due to the small particle size, and so will brew much quicker than loose leaf teas. Furthermore, crushed tea leaves may increase the strength of the brewed tea [5]. However, loose leaf teas generally offer more complex, nuanced flavor profiles which tea bags lack. The form of tea to use for brewing kombucha overall depends on personal taste preferences.

Sugar

Which sugar?

At first glance, white sugar seems like the only option, given its ubiquity. For those wishing to experiment a little further, there is no reason to try other sugar sources, since the sugar-metabolizing microbes in SCOBY are not sucrose-specific. There are a couple of notes to consider when choosing the type of sugar:

Brown sugar is sucrose sugar that contains molasses, which may add a molasses flavor to the kombucha.

Raw sugar tend to have bigger crystals, since it is less refined. Bigger sucrose particles may affect its ability to completely dissolve in the kombucha, especially at or below room temperature. If the sugar crystals are not completely dissolved, there may be less sugar in solution available for the bacteria and yeast to metabolize. This could perhaps lead to a more yeasty, rather than fizzy kombucha.

Honey is a mixture of glucose and fructose, with its golden color deriving from non-sugar components such as pollen. Other microorganisms may also be found in honey [6], so using honey for brewing kombucha runs the risk of microbial contamination which may affect SCOBY efficacy.

Sugars extracted from plants or trees other than beets and sugar canes are fair game for brewing kombucha. A few examples include maple syrup, coconut sugar, and palm sugar. Agave nectar, despite health claims, contains a higher fructose content by weight than high fructose corn syrup [7].

Sugar substitutes, such as stevia, xylitol, and glycerol, are sugar alcohols. SCOBY is unable to metabolize sugar alcohols, and so adding artificial sweeteners would not be effective at all in brewing kombucha.

How much sugar?

In kombucha, sugar is used as a food source for the SCOBY, not as a sweetener as in many other recipes. The end product has far less sugar than was originally added to the first fermentation period, as the SCOBY has metabolized most of it to create the vinegary flavor and carbonation. Therefore, adding sugar is necessary for successful fermentation.

Too little sugar, and the SCOBY does not have the necessary fuel to undergo prolonged fermentation, leading to an unsweet, not very acidic, and possibly flat kombucha. Too much sugar may cause the yeast to over-proliferate, outnumbering the other SCOBY microbes. This both decreases the efficacy of the SCOBY and decreases the flavor and carbonation of the resulting kombucha. The exact amount of sugar varies among recipes, and can be experimented with to suit personal preferences.

Flavorings

For a more unique kombucha, flavors are often added near the end of the kombucha brewing process, after the batch has undergone its initial fermentation period. Just like every other component that goes into kombucha, the choices for flavoring are abundant.

Herbs and spices: Since herbs and spices tend to have strong flavors, adding a little bit can go a long way. Keep the amount to a minimum, as some herbs and spices may contain antimicrobial activity, and adding too much may harm the microbes on SCOBY, making the second fermentation period unlikely to occur successfully.

Fruits: Whether fresh fruit or fruit juice is used, be sure to keep an eye on the batch after adding the fruits. Fruits and fruit juices introduce an extra sugar source for the SCOBY during the second fermentation period; the yeast cultures in the SCOBY go into “overdrive” with this added amount of sugar. While this may lead to a fizzier kombucha, the increased carbonation will create a pressure-build up within the container. Opening the container may risk a small kombucha explosion or the container may burst open from the pressure built up.

Kombucha flavored with raspberries. Photo credit: Lukas Chin (Wikimedia Commons)

Extracts and infused waters: Like herbal teas, be sure that the extracts are oil-free as to avoid volatiles that contain anti-microbial activity. A few examples of water-based extracts would be lemon extract (not lemon oil), almond extract, and vanilla. Infused waters include rose water and orange blossom water.

With a little bit of background knowledge, kombucha brewing could become your favorite science project. Explore the possibilities!

References cited

Marsh, A. J., O’Sullivan, O., Hill, C., Ross, R. P., Cotter, P. D. Sequence-based analysis of the bacterial and fungal compositions of multiple kombucha (tea fungus) samples. Food Microbiology, April 2014; 38:171-178.
C. Fugelsang, “Zygosaccharomyces, A Spoilage Yeast Isolated from Grape Juice.”
Types of Tea. TeaSource. 2013.
Thosar, N., Basak, S., Bahadure, R. N., Rajurkar, M. Antimicrobial efficacy of five essential oils against oral pathogens: An in vitro European Journal of Dentistry, Sept 2013; 7:71-77.
Does the size of your tea leaf matter? Octavia Tea. 18 November, 2011.
Olaitan, P. B., Adeleke, O. E., Ola, I. O. Honey: a reservoir for microorganisms and an inhibitory agent for microbes. African Health Sciences, Sept 2007; 7(3):159-165.
Bowden, Jonny. Debunking the Blue Agave Myth. Huffington Post. 17 April, 2010.

About the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.

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The Wonders of Baker’s Yeast

August 11, 2015/in Science & Food/by Grant Alkin

Among life’s simplest joys: smelling freshly baked cinnamon rolls wafting through the kitchen, sliding the tray of artfully coiled pastries from a warm oven, and marveling at their golden crust and fluffy interior. An ideal cinnamon roll features a potent cinnamon-sugar mixture oozing in sticky spirals. It’s often topped with a generous smear of tangy cream cheese icing that’s tempered with notes of orange peel and vanilla, sweet and rich enough to catapult you back to childhood. While the filling and icing are notable qualities, what really makes or break a cinnamon roll is its texture. Cinnamon rolls may simply serve as a vehicle for sugar and icing, but their bready foundation boasts an often-understated value. Imagine greedily lunging for a roll and biting into it, only to discover that it’s a rock-hard spiral of disappointment, instead of an airy and delicate pastry with a tender crumb. The science behind the texture of a perfectly fluffy cinnamon roll lies in the yeast.

Photo credit: Mai Nguyen

When you’re browsing the baking aisle in the grocery store, you may be overwhelmed or confused by the sheer number of different forms of yeast available—you’ll find loose granules in packets and jars, bricks, discs, and fast-rising, instant, or active dry. Despite the multitude of forms the yeasts can come in, they’re all merely purified and processed versions of the same organism. Saccharomyces cerevisiae, or baker’s yeast, is a microorganism used in professional and home kitchens alike primarily as a leavening agent for baked goods (1).

The three most commonly/commercially available forms of yeast are:

Caked yeast: This moist block consists of fresh, living cells that are packed tightly together. This form of yeast shows substantially higher leavening activity than its dried forms. Caked yeast is highly perishable and has a shelf life of only one to two weeks. More commonly, you’ll encounter yeast granules in packets or jars, widely available as active dry or instant.
Active dry: Active dry is a granular form of yeast that has been dried at high temperatures. These granules are comprised of yeast clusters that are encapsulated in a protective coating of yeast debris that formed on the surface of the granules during the drying process. These yeast cells are dormant and need to be rehydrated in warm water before being used. Simply sprinkle the granules in warm water (around 110°F), stir, and wait five to ten minutes. Water will dissolve the protective coating surrounding the granules, releasing the revived yeast cells from within. As the yeast become active, you should see a foamy layer of bubbles forming at the surface, which is carbon dioxide being released.
Instant rapid-rise yeast: Boasting higher viability and increased CO₂ production, instant rapid-rise yeast is dried at more gentle temperatures than active dry, so more yeast cells survive this drying step. Bakers can add instant rapid-rise yeast directly to the flour, eliminating the need for prehydration. Because instant rapid-rise yeast produces carbon dioxide more vigorously than active dry yeast, these two forms of yeast should not be used interchangeably.

Granules of active dry yeast
Photo credit: Mai Nguyen

Instant rapid-rise yeast
Photo credit: Mai Nguyen

How does this tiny organism transform a dense blob of dough into a puffy masterpiece? To harness its leavening power, we rely on the phenomenon of fermentation. In the first steps of bread baking, water, yeast, flour, and salt are combined. Kneading hydrates the flour and after just a few minutes of manipulation, the dough becomes noticeably stretchier and more pliable. Water enables individual protein molecules in the dough, glutenin and gliadin, to link together to form long, elastic chains of a protein called gluten. These individual gluten strands combine to form a mesh-like network which gives bread its structure and chewy texture (2). Meanwhile, the addition of water also activates enzymes in the flour known as amylases which break down the flour’s starches into simple sugars, providing food for the yeasts (3).

The yeasts feed on these simple sugars and convert them into ethanol and carbon dioxide gas (CO₂). This is where the magic begins. As carbon dioxide is released into the dough, it becomes trapped in the gluten matrix. As more and more CO₂ bubbles form, the protein network stretches, inflating the dough. Depending on the recipe, dough can spend between an hour to several days rising and can expand two to four times its original size. This initial rising step is often referred to as bulk fermentation.

Like many other types of yeasted breads, a classic yeast-based recipe for cinnamon rolls calls for two rising steps. After the dough has been kneaded and has undergone bulk fermentation, it’s time to roll out the dough and shape it to prepare it for the second rising step, known as proofing. Many recipes for yeasted breads will instruct you to “punch down” dough after the initial rise. In this step, we turn and fold the dough, fill it with a cinnamon-sugar mixture, shape it into coils, and allow them to rise into bloated versions of their former selves (2). This “punching down” or turning step serves a couple of purposes: it stretches the gluten and expels excess CO₂ buildup trapped in the dough from the bulk fermentation step, which can inhibit any further yeast activity. Handling the dough at this stage also redistributes yeast, moisture, heat, and sugars throughout the dough for optimal lift and flavor.

A noteworthy point: while our goal is to encourage yeast proliferation and to optimize the production of CO₂ and flavor molecules, bakers should be cautious of overfermentation. If yeast fermentation happens too rapidly or continues for too long, gas bubbles can overinflate and burst, causing our dough to collapse (3). The excess of CO₂ can also cause the yeast to leave behind many unwelcome tasting flavor compounds and the bread may end up tasting like alcohol.

In our final phase, our twice-risen dough is placed into the oven. Once inside, the dough experiences one last rise thanks to the high heat. The heat causes CO₂ present in the dough to expand and for about the first ten minutes in the oven, the rising temperatures stimulate a rapid burst of activity in the yeast, causing them to produce even more CO₂. Water and ethanol byproducts in the dough will also expand during heating. This causes the bread to rise dramatically in the oven a phenomenon known as oven spring (3). Eventually, the CO₂ and alcohol are expelled from the bread and the yeast cells succumb to a dry, hot death once temperatures exceed 140°F (2).

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Photo credit: Mai Nguyen

Behind a cinnamon roll—or any kind of yeast bread —lies an intricate chemistry involved in its creation. Without the wonders of yeast and fermentation, bread wouldn’t exist as we know it today.

References Cited

McGee, Harold. On food and cooking: the science and lore of the kitchen. New York: Simon & Schuster, 1997. Print.
Crosby, Guy. The Science of Good Cooking. Brookline, MA: Cook’s Illustrated, 2012. Print.
Bernstein, Max. “The Science of Baking Bread (And How to Do It Right).”Serious Eats. 1 Oct. 2014. Web. 11 Aug. 2015.

Read more by Mai Nguyen

International Variations of Yogurt: A Cultural Exploration of Milk

June 30, 2015/in Science & Food/by Grant Alkin

Photo credit: Robert Koehler

It’s a dessert, it’s a condiment, it’s a breakfast staple. Yogurt can be consumed in a myriad of ways; there also exist several variations of yogurt around the world that differ dramatically in taste and texture.

It all begins with milk—be it from a cow, sheep, buffalo, donkey, or goat. In standard western forms of yogurt, the art of yogurt-making begins by heating milk to 85°C and holding for 30 minutes or at 90°C for 10 minutes. Applying heat denatures the whey protein, lactoglobulin, and plays a pivotal role in determining the yogurt’s final creamy texture. Without this protein denaturation, the milk proteins won’t set together in an organized matrix, but will instead cluster together and form curds.

In the second phase, fermentation, milk is first cooled to a temperature within the range of 30°C-45°C, a range well tolerated by the microbes that play a role in fermentation. Traditional yogurt-making in the western world relies on bacterial cultures containing Lactobacillus bulgaricus and Streptococcus thermophilus, which can be directly added to milk in the form of a packaged starter (similar to yeast) or taken from a previously-made batch of yogurt. The bacteria then convert the milk sugar, lactose, into lactic acid, which firms the yogurt and provides it with its tart and tangy taste.

Fermentation conditions heavily influence the flavor and consistency of the final product. Different types of bacteria thrive in different temperature ranges and can produce several variations in consistencies, ranging from smooth and creamy to thick and jelly-like. At lower temperatures, bacteria produce lactic acid much more slowly and it can take up to 18 hours for the yogurt to set; by contrast, lactic acid bacteria working at higher temperatures can set the milk proteins in just two or three hours. A rapid, high-temperature gelling will result in a firm yogurt, whereas a low-temperature, slow gelling will produce a more delicate, tightly packed protein network. [1]

Throughout the world, we’ll find several different kinds of yogurt or yogurt-like products. Significant textural or taste changes can be made with simple tweaks in the preparation or fermentation process, and we’ll explore a few of these here.

Greek yogurt

Greek yogurt has become ubiquitous in grocery stores over the past decade and is loved for its rich flavor and thick consistency. The secret behind its popularity lies in the straining step. Straining allows the liquid whey component of milk to drain away and also removes some of the lactose, leaving behind a product with reduced sugar and nearly double the protein when compared to its non-strained counterparts. Although not actually of Greek origin (its origins still remain unclear), this strained yogurt is also popular throughout the Middle East and Central Asia [1,2].

Viili

This slimy dairy product topped with mold may sound like a failed kitchen experiment, but is in fact a yogurt-like product held dear to many Scandinavians. Known as viili to the Finnish, långfil to the Swedes, or tättemjölk to those in Norway, this ropy milk product is so thick that it needs to be cut with a knife when served. The main culprit in viili’s ropiness is a mesophilic strain of lactic acid bacteria called Lactococcus lactis subspecies cremoris. During fermentation, this strain of lactic acid bacteria produces long strands of slimy sugars known as exopolysaccharides that create the characteristic texture and flavor profile of viili. On its surface, you’ll find a velvety layer of mold formed from G. candidum, which lends this dairy product fruity and savory notes. The mold also consumes lactic acid, reducing the acidity of viili, giving it a relatively mild acidic flavor. To make viili, you’ll need milk and a viili starter, which contains both Lactococcus cremorisx and G. candidum [3].

Kefir

Although not technically yogurt, kefir is a bubbly, mildly alcoholic, Russian-derived equivalent. What distinguishes kefir from yogurt is that instead of relying solely on lactic acid bacteria for fermentation, it’s made from kefir grains, which are large cauliflower-like complexes composed of lactobacilli bacteria and yeasts. Kefir also sets itself apart from other fermented milk products in that its fermenting microbes exist in these relatively large, popcorn-sized ‘grains,’ instead of being evenly dispersed throughout the milk. While bacteria are busy converting a portion of the lactose into lactic acid, yeasts from the kefir grains also convert lactose into ethanol and carbon dioxide. The result is a tangy, yeasty, and effervescent beverage [4]. Kefir boasts higher probiotic activity than typical yogurts, making it a fermented dairy product of choice among health enthusiasts.

Kefir grains. Photo credit: Chiot’s Run (chiotsrun/Flickr)

Ayran

This national drink of Turkey is made by diluting natural yogurt with ice water and salt. Throughout Turkey, different regional variations of ayran exist, with the most well-known version originating from a town called Susurluk. In Susurluk, local variations of ayran are made from a mixture of cow, buffalo, and sheep’s milk, giving it a distinctive creamy and foamy quality. A notable feature of Ayran is its lower shelf-life when compared to other fermented milk products. Stabilizers are added to ayran to prevent the water and milk mixture from separating, but salt hinders the effects of stabilization [5].

Yakult

Photo credit: Dezzawong/Wikimedia Commons

Now found all over the world and most notably in vending machines throughout Asia, Yakult is a probiotic drink that was developed in Japan. Japanese microbiologist, Minoru Shirota, was searching for a strain of bacteria that would benefit digestive and overall health. His work led him to Lactobacillus casei Shirota, which he cultivated and used to develop Yakult. Yakult is made from adding this unique bacteria strain to skim milk, water, and sugar, and is often enjoyed for its sweet and fruity natural flavors. As seen above, Yakult can also be found in several flavor variations as well.

Cultured milks are a culinary marvel, especially when you consider how many different forms it can take. Head to the dairy aisle and try a new variation of yogurt, or why not attempt making some at home in your own kitchen? With all the different forms out there, you’ll surely find one you enjoy.

References cited:

McGee, Harold. On Food and Cooking. The Science and Lore of the Kitchen. New York: Scribner, 2004. Print.
Lalime, Jennifer. “How to Make Greek-Style Yogurt“. The Feed.
Salminen, Edith. “There Will be Slime“. Nordic Food Lab.
Farnworth, E. R. Kefir—a complex probiotic. Food Science and Technology Bulletin.
“Fame of Foamy Ayran Goes Beyond Borders“. Hurriyet Daily News.

Read more by Mai Nguyen

Lauryn Chun

June 9, 2015/in Profiles/by Grant Alkin

Lauryn Chun runs small-batch food business Mother-in-Law’s Kimchi and is the author of The Kimchi Cookbook. Chun revolutionized kimchi by bringing it out from the margins of traditional side dish-dom to center stage as a main course.

What hooked you on cooking?: My earliest childhood memories in Seoul, Korea – foraging for wild herbs and plants on walks with my grandmother, watching my grandmother and mother cooking in the kitchen gave me a sense of comfort in the kitchen. I loved all the aromas and witnessing how ingredients transformed into a meal and an event. It felt like magic for me as little kid. Creating a dish to feed the entire (extended) family formed a deep appreciation for food as a gift, nourishment and an event.

The coolest example of science in your food?: Watching the initial state of lactic fermentation while making my first kimchi batch, witnessing the continuous state of change as kimchi ferments. The ‘exposion’ and bubbling of kimchi’s anaerobic state and pressure inside lids in jars that causes kimchi to expand with oxygen when opening – kind of like a fake toy snake poping out of a can. Watching the live active lactic bacteria fermentation in action. It is magical and there’s a reverence for nature’s ability to make food safe through lactic bacteria at its simplest state which is essentially salt (brine) and vegetable.

The food you find most fascinating?: I like simple foods and flavors with texture and taste that is balanced. I do think that kimchi is absolutely fascinating the way I think about balance of flavors and textures. By taking process of vegetable’s natural fermented state of acid (making its own vinegar) and flavors, it is akin to ‘cooking’ a dish to achieve a balance of flavors and textures. The latin word ‘fevere’ which is root of word ‘fermentation’ means to boil with foam – a perfect description of how live bacterias are working to break down the natural state without heat. When we are creating a dish in the kitchen, it is the flavors of adding acids and flavors to achieve a balance of taste that is pleasant in our mouth when we taste.

What scientific concept–food related or otherwise–do you find most fascinating?: Chemistry of taste and physiology of what we taste, our connectivity in brain that tells us something tastes delicious, balance of flavors, texture and desirability. I think my truly taking the time to taste foods is the best way to nurture ourselves and future generations eating foods for good health and ethics.

Your best example of a food that is better because of science?: Kimchi fermentation, soy sauce, cheese making, wines.

How do you think science will impact your world of food in the next 5 years?: I think it can go either direction of good, using science to have better understanding of natural unprocessed foods or bad with corporate profit and industrial scaling of production to manipulate nature through bio-engineered foods and seeds.

One kitchen tool you could not live without?: Cusinart Mini hand blender-chopper, takes up no space in the cabinets and so versatile, 15 years and counting…

Five things most likely to be found in your fridge?: Kimchi, kimchi and kimchi…. And variety of cheeses, chile flakes, soy bean paste, mustard…

Your all-time favorite ingredient?: My all time favorite ingredient would be yellow onions as they create such a base of flavor in every type of cooking.

Favorite cookbook?: Favorite cookbook would be Marcella Hazan’s Essential Italian Cooking.

Your standard breakfast?: Usually something savory like a poached egg and toast or healthy museli and definitely coffee.

Sour Beers & Skunky Beers

May 7, 2015/in What We're Reading/by Grant Alkin

“Fermenting yeasts produce more than just ethanol and carbon dioxide. They make flavorful, aromatic molecules: acids and esters. But which ones make which ones?” wonders William Bostwick as he attempts to recreate a sour beer in his kitchen in San Francisco’s Mission District. If you’re more interested in preventing your beer from getting skunky than making your own, we found some chemistry to help you out.
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