The bubbles in store-bought sodas are made by adding carbon dioxide gas to a flavored based under high pressure. The gas dissolves into the liquid – you can get a similar effect at home using one of these seltzer makers. However, I like to use a trick that has been around for thousands of years: yeast. Yeast makes bubbles in beer, and yeast can also make bubbles in your homemade soda.

How does this work? Yeast, like humans, obtain energy from breaking down sugar molecules. This requires oxygen, and is known as aerobic respiration. If there is no oxygen around, humans will die. Yeast however, have a metabolic trick that allows them to survive: in the absence of oxygen, yeast can derive energy from sugar in a process known as anaerobic respiration, or fermentation. The two byproducts of yeast fermentation are ethanol (alcohol) and carbon dioxide (fizzy bubbles). When making soda at home, we will let our mixture ferment for ~24 hours. This is enough time for the yeast to produce a lot of bubbles, but only a tiiiiiny trace of alcohol:

Materials you will need:

2-liter plastic soda bottle
Water
Sugar (or honey, turbinado sugar, agave nectar………)
Flavorings (herbs, fruit, syrups)
Yeast – preferably champagne yeast from a store that carries brewing supplies. Baker’s yeast will do in a pinch.

Steps:

1. Clean your equipment. For making soda, you don’t need to sterilize everything with bleach (like you would for homebrewing beer or wine), but make sure everything is clean.

2. In a pot, add ~1.75 liters of water, and all of the flavorings you plan to use. Bring the mixture up to a simmer, and let it bubble for 30 seconds. Turn off the heat and let the mixture cool.

3. In a separate bowl, dissolve 1.4 teaspoon of yeast into a small amount of warm water. When the yeast is dissolved, add it to the cooled soda base.

4. Pour the completed soda base into the 2-Liter bottle, and close the cap tightly. Let the bottle sit for 24 to 48 hours. The amount of time is variable, depending on the temperature of your kitchen, etc. At first, the carbon dioxide we be produced as a gas, and will increase the pressure in the bottle. Once enough pressure has been built up, newly produced carbon dioxide will begin to dissolve into the soda base.

You can monitor the fermentation by squeezing the bottle – it should start to feel firm, just like a commercial soda bottle would. Enjoy!

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Note 1: When you open the soda bottle, there will be a lot of initial fizz. However, if you pour a glass out, and the soda is pretty flat, don’t worry. All this means is that the yeast needs more time. Put the cap back on, and let it go another day. Play around and have fun – since yeast are alive, some days they will produce more bubbles than other days.

Note 2: If you use fruit puree or other “chunky” ingredients, you can filter the cooled soda base through a cheesecloth if you wish. The resulting soda will be smoother. Just like buying orange juice, you can choose pulp or no pulp.

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Recently I made a Blueberry-Ginger-Lemon Sparkler. In a pot I combined:

1.75 L water, 1 cup frozen blueberries (pureed in blender), 1 inch chunk of ginger (grated), 1/4 cup sugar, and 1/2 lemon (whole). I brought the mixture to a simmer – behold the cauldron…..

Then, I added 1/4 tsp champagne yeast which I dissolved first in a little warm water. I added the yeast to the *cooled* soda base (minus the lemon half) and pour into a clean 2 L soda bottle. I went to sleep, and the yeast went to work. The next day, I had:

More fermentation resources:

NOVA’s blog has a post on 45-million year old beer

Matt’s blog Brew and Bake had wonderfully detailed instructions for making your own beer!

Craftzine offers a cool video for making soda using a similar technique.

1. Everything has heat
Let’s talk about heat – solids, liquids, and gases are made up of molecules. Molecules vibrate and interact with each other, and this is known as thermal energy. Thermal energy also manifests as heat – a glass of hot water has more thermal energy than a glass of cool water.

2. Heat passes between any two substances 
Heat (or thermal energy) is transmitted between substances, where thermal energy from the warmer object always passes to the cooler object. If you touch a hot iron, thermal energy from the iron passes into your hand, and I have a scar to prove it. Conversely, if you hold a cold soda, thermal energy passes from your hand to the soda. The process of heat (thermal energy) moving from one source to another is called conduction (details, with math here).

3. But, heat does not pass evenly between substances
However, thermal energy does not pass through all substances evenly. Metals, like copper and aluminum conduct thermal energy extremely well. The most important point here is that thermal energy passes through water 30 times faster than through air (geek details here)*! Think about it – the “insulation” in many travel coffee mugs is a trapped layer of air. Heat does not pass through air very well, so this slows down the transmission of heat from your coffee to the outside, keeping it warm and delicious for a long time.
*of course, nothing is this simple. I’m ignoring some other aspects of heat transmission, particularly convection.

4. If you want cold drinks now, do not put them in the refrigerator
So, if you need to chill some beer or wine, by far the fastest way to do this is by submerging your drinks in ice water. This will only take 5-15 minutes!

Heat conduction is neat! Tell me more….
-You can think about this problem in reverse. If you need to thaw something that is frozen, like a piece of chicken, don’t leave it on the counter or in the fridge. Instead, place it in a bowl of warm water – you’ll be amazed at how fast it will thaw.
-When you’re going out in the cold, dress in layers. There will be air pockets between each clothing layer. As you now know, air is a poor conductor of thermal energy, so the layers of clothing will help you retain your body heat better then a single, thicker sweater.
-Otters have thick fur coats that keep air bubbles next to the otters’ skin, which keep the otters warm while they’re diving. Smart move, otters!