AutoBooch — Automate Your Kombucha Brewing System With a Raspberry Pi

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Introduction: AutoBooch — Automate Your Kombucha Brewing System With a Raspberry Pi

Science of Cooking

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Science of Cooking

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This is an entry in the
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Tired of unpredictable brewing times for your kombucha? Worried about the health of your SCOBY pellicle? Look no further! The AutoBooch has your back!

Don't know what a SCOBY is? Have no fear—we cover that here.

A growing number of people drink kombucha, for a variety of reasons. Known by various names, such as 'mushroom tea' or 'Manchurian Mushroom,' the etymology behind the strange word is not definitively known. My favorite origin story comes from the legend of the Korean physician Kombu, whom prescribed it to the Japanese Emperor Inyoko, in the fifth century. Hence the word 'Kombu Cha': the tea of Kombu. I apply the se non è vero, è ben trovato rule and choose this one.

There are several purported health benefits, including probiotics, useful enzymes and malic acid and other beneficial acids, but there is no scientific consensus yet. I personally drink it because I like the taste.

However, the process of brewing kombucha is not the simplest, especially at first. Part of the problem is that the times for the primary and secondary fermentation stage can vary quite a lot, requiring regular testing to see if it has adequately macerated. This project helps make this part of the process more predictable by controlling the temperature using a Raspberry Pi, a relay, and a heating pad.

I'll sneak in a bit of science, and end with some pro tips for the adventurous.

First, you need to make the kombucha!

(I don't care about your hackneyed kombucha recipes. Get me to the automation stuff in Part 3!)

Step 1: Making a SCOBY From a Bottle of Kombucha

The main engine for brewing kombucha is the interesting gloop of life-forms known as a SCOBY— a Symbiotic Culture Of Bacteria and Yeast. Technically, the floating part is known as a pellicle—the SCOBY itself is a complicated interaction, much of which occurs throughout the entire volume of the mixture (thanks to voxamps2290 for the clarification!)

The SCOBY pellicle is a complex, scabrous-looking, pad (or 'zoogleal mat') that floats at the top of the brew. The SCOBY as a whole consumes a sweet tea, and excretes a yummy, not very sweet, fermented beverage. The bacteria are various species of Acetobacter (mainly Acetobacter xylinum) and strains of yeast (one or more strains of Saccharomyces cerevisiae—Latin for 'sugar-eating beer fungus'—and Brettanomyces, Zygosaccharomyces, even sometimes Candida kefyr[1,2]) forms a colony within a bacterial matrix, forming a rubbery film that becomes thick and brown over time (and, if you get bored, you can turn into a fencing jacket). The yeast consume the sugar in the raw brew (usually black tea and sugar), producing a small amount of ethanol. In turn, the bacteria consume most of the ethanol and some elements of the tea (including a reasonable fraction of the caffeine). The ethanol is oxidized, creating acetic acid ('vinegar') and other acids (gluconic, lactic, and perhaps usnic and malic acid), as well as glucose and vitamins may be formed in various concentrations, although how much of each is still debated.

Symbiotic systems in general are very finely tuned. So engineering one of these puppies yourself is going to require a PhD in microbiology, a fully stocked biotech lab and a decade of dead ends (but then you could just battle malaria instead—your call). Failing this, there are three main ways to obtain a SCOBY yourself:

  1. Take a cutting from a friend/frenemy — literally cut a chunk of another healthy SCOBY.
  2. Purchase a SCOBY from a store or online (roughly $10).
  3. Grow one yourself from a bottle of raw, unpasteurized kombucha.

I'll run you through the last step, the most difficult path, padawan: growing one yourself.

You will need:

  1. One bottle of raw, unpasteurized kombucha (I used GT's Organic Raw Kombucha).
  2. Two bags of inexpensive black tea. Do not use tea with any added flavoring agents. Just pure black tea.
  3. A quarter cup of inexpensive white sugar.
  4. Large diameter, non-metallic, bowl with enough capacity for a couple of kombucha bottles. A pot for boiling.

Empty the contents of the kombucha bottle into the bowl.It should not be metallic, as ions can interfere with bacterial growth (looking at you, Chromium).

Boil enough water to fill the rest of the bowl (panel 1 in the figure above). Dissolve the sugar in the water, and add tea bags to make a sweet tea, removing bags when a rich color has developed. Allow to cool so that it is mildly warm to the touch. Pour into the bowl. Cover with a breathable cloth and store in a dark place for five days to a week, depending on the ambient temperature.

At this stage, a thin, yet strong, film should have formed on the top layer of the tea (see the panels 1 - 4 in the figure above). Repeat the above stage once or twice. The SCOBY pellicle should now have formed a fairly strong, thick matrix. Feel free to sample small amounts of the resulting tea. You will notice the flavor profile moving from a sweet tea, becoming drier until it becomes more acidic. You will notice a complex vinegary scent developing.

Eventually, you should end up with a healthy looking SCOBY pellicle. Name it something cool, like SCOBY-DOO, and post it all over social media. After time, noticeable layers form and it thickens greatly. There seems to be an entire cottage community devoted to 'SCOBY porn,' showing off large thick pellicles. Unfortunately, there are also common questions about rival bacterial growths on less healthy pellicles. There should not be black or wildly colorful growths on your pellicle (maybe see a doctor, too). You should aim for a finished pH of roughly 2.5 - 3.5, and try to keep your SCOBY in acidic conditions to protect it from invasive microbes, as it's a jungle out there. As with anything that may involve your health and safety: do independent research if you are concerned!

[1] Mayser, P. et al.: "The yeast spectrum of the ‘tea fungus Kombucha’," Mycoses 38(7-8): 289—295 (1995)

[2] Jarrell, J., Cal, T. & Bennett, J.W.:"The Kombucha Consortia of yeasts and bacteria," Mycologist 14(4):166—170 (2000)

Step 2: Continuous Primary Fermentation

Once you have your SCOBY healthy, with a pellicle to desired size, you can embark upon your brewing career. Goodbye expensive commercial kombucha! Making your own should be inexpensive and fun. However, it can be annoying. To find out how, keep reading.

The first, most important, step is primary fermentation, where the yeast consumes the sugar and the bacteria aid oxidation of the resulting ethanol and consume some complex compounds in the tea. For a one gallon brew (that's 3.8 litres outside the US), you will need:



8 black tea bags (cheap is good).

1 cup white sugar (plain is fine).

1 gallon filtered/unchlorinated water (hard water is actually great for brewing!)

(optional) 1 tbspn nutritional yeast (sometimes called 'brewer's yeast').



Put enough water to cover a large pot a few inches/deci-metres. Add sugar and nutritional yeast if you desire*. Stir, bring to boil, then remove from heat. Add tea bags after a few minutes and steep for another five minutes or so. Remove tea bags and add remaining water. Allow to cool to just above room temperature (I usually aim for around 75°F (24°C)). Now you are ready to add to your primary fermentation vessel and your new SCOBY.

You have probably seen, in other awesome how-to guides, that you need to wash your hands carefully, because when you handle your pellicle before each fermentation—wait, you handle your pellicle? That's for chumps! We're not chumps, are we? We're Instructabelievers. That's why we opt for the continuous primary fermentation method. If you are not a lazy person, then please move on to the next step.

Instead of using a simple jar, making a mess, and raising ire in the kitchen every time you have to refill your primary fermentation vessel, you will need the following materials:



One gallon glass Mason jar, with a 5/8 inch (16 mm) hole in one side, about two inches from the bottom (I used a Circleware Mini Yorkshire Mason Jar Glass Beverage Drink Dispenser, $15).

A stainless steel dispenser spigot (or 'tap'; I used Stainless Works' SSS010 Stainless Steel Beverage Dispenser Replacement Spigot, although it appears currently unavailable. $10 - $15, but food-grade plastic will do).

Muslin cloth to cover the top.

A rubber band.



Attach the spigot such that there are no leaks and you can turn it easily. If it's your first time, gently transfer your pellicle, and some of the fermented brew, to the vessel (to be sure, wash your hands and rinse in white vinegar first). Then gently pour your tea onto the SCOBY and add water if necessary to top it up. Place the Muslin cloth on top, stretching it so there is no danger of sagging into your pellicle floater. Now put the rubber band over the cloth to hold it in place. If it's not your first time, welcome to Easy Street. You are a certifiable continuous brewer. Just pour out the raw 'booch into your secondary fermentation system, close the spigot, and pour in your sweet tea, without bothering your pellicle.

Now, remember that UV is good at killing off nasty germs? Most (but not all) bacteria are killed by UV. Although glass is largely opaque to much of the ultra-violet band (specifically, wavelengths less than about 370 nm), you're still going to want a dark place. With a reasonably constant temperature. This is where the automation part comes in.

*This is a pro-tip secret, and you heard it here first, dear reader. Yeast thrive on a little bit of protein. The best source of protein is... dead yeast. Just make sure you boil it, to make sure there are no upstart, nouveau riche, strains to compete with your SCOBY.

Step 3: Optimize Your Brewing Temperatures Via Automation I — Hardware

Many recipes for brewing kombucha require arcane rules of thumb, some mystic lore and a bit of tasting along the way, to determine if a batch has brewed. This can range from a few days, in summer months, to well over a week when the outside temperature drops. To make this consistent—and maximize throughput—controlling the temperature through the colder months is key. This is a perfect fit for a Raspberry Pi!

To control heating for your kombucha brewing, you will need the following:

  1. A Raspberry Pi (I use a Model 3B ($36), but I have used my ol' original Model B for similar applications).
  2. A heating mat (I use Propagate Pro's 12" diameter Brewing & Fermentation Heat Pad, $25).
  3. A DS18B20 temperature probe (I got mine as part of the OSOYOO 2017 Ultimate Starter Working Learning Kit for Raspberry Pi 3 (whole lotta goodies for $21), but you can easily find one for $8).
  4. A relay switch (I use DZS Elec's 4 Channel 5V Relay Module ($8), but you only need one channel for this project ($6)).
  5. Jumper cables (I soldered a few together to get the length I needed; ~$2).
  6. Aluminum (Aluminium :) ) foil.
  7. Wide clear tape.
  8. An empty Aluminum drink can.
  9. Rubber band, large enough to encircle your primary fermentation vessel.
  10. Some old towels or other thermal insulation.

First, you will need to set up the software on the Raspberry Pi. I assume the Pi already has a Debian-like GNU/Linux operating installed, you have a way of interacting with a shell terminal, and that you have root ('administrator') privileges.

The DS18B20 temperature probe is pretty neat. It makes use of the '1-Wire' bus interface to send digitized temperature information to the Pi, meaning it doesn't have to be calibrated against a reference voltage etc., which would be a real pain. However, it does mean that you need to set the Pi up to 'speak' 1-Wire (w1) language. This means you'll need to:

  • Ask your Pi to add the GPIO pins to your Device Tree. Edit the file (as root) the /boot/config.txt file:
sudo nano /boot/config.txt<br>

This uses the text editing program, nano, to edit the file as root (however, you should embrace the dark side and learn vim). Add the following line (and save the changes (Ctrl-X)):

dtoverlay=w1-gpio  
  • Activate the w1 kernel modules, so it can speak 1-Wire lingo:
sudo modprobe w1-gpio<br>sudo modprobe w1-therm<br>
  • Power off the Pi
  • Connect your thermal probe

This involves selecting a GPIO pin to be the designated read-out. Here, we take GPIO4. The wiring schematic provided here refers to the Raspberry Pi 3B GPIO pin configuration; please make sure you check your model's pin configuration. You will also need 3.3 V (3V3) power and an Earth (GND). It's safer to connect the jumper leads to both the Pi and the temperature probe when the Pi is powered off.

  • Restart the Pi
  • Check your probe identifier prefix

This involves looking at the directories created in the correct device directory:

ls /sys/bus/w1/devices/<br>

This should display 'w1_bus_master1' and some gobbley-gook, which is the serial number (mine says ''28-04169314f7ff"). If the prefix for your device is '28,' then hooray! You don't need to change any code. Otherwise, note the first two digits of this code. We'll edit the script to use this later. The temperature is read out into a file called 'w1_slave' in the serial number directory.

  • Connect the heating pad to the relay

Note that the heating pad is a mains device, and can be unsafe if you do not perform correct procedure. If you do not feel comfortable dealing with mains voltage, then seek assistance, or do not go on.

We'll be introducing the relay to intercept one of the two leads coming from the mains plug. Separate a segment of the lead, such that each of the two wires is still insulated. Now sever one side, and strip about a half inch of insulation from the end of each side of the cut wire. Now wire the lead into one of the relay channels.

The relay requires power and a switch. Connect GPIO17 to the appropriate switch input (e.g. 'In1'), and connect the 5V and GND to the appropriate locations on the relay (the model should have this labelled).

  • Power on the Pi and test the relay switch (without connecting the mains)
  • Make the thermal coupling between the temperature probe and your brewing vessel

You want to make a good thermal 'circuit'. Aluminum foil is a great thermal conductor, but it doesn't have great shear strength (i.e. it tears easily). So reinforce the back side of it with wide clear tape. Making mechanical and thermal contact between this and the sensor will require a bit of ingenuity on your part, but you can handle it.

  • Place the whole shebang somewhere dark and insulated

I use a few towels large enough to cover the primary and secondary vessels. Ideally this would be a cupboard etc., but you will note that my current set-up is open (this is because I use this Pi for a range of projects; don't worry, I make sure to cover it so it's very snug).

Now you're ready to run the software.

Step 4: Optimize Your Brewing Temperatures Via Automation II — Software

Once you're happy the relay and hardware is functioning, then we can run the AutoBooch script in anger! Or perhaps peace and tranquility is your thing. AutoBooch is programmed to not judge.

  • Download or copy the attached Python script (auto_booch.py) onto your Pi

If you're comfortable with git, you could always clone this project from GitHub:

git clone <a href="https://github.com/RaInta/AutoBooch" rel="nofollow">  https://github.com/RaInta/AutoBooch>
  • Enable execution privileges for the script:
chmod +x auto_booch.py
  • Edit the DS18B20 prefix if necessary

If your DS18B20 prefix was not '28' then you will need to change it. Edit line 50 of auto_booch.py so that

ds1820_prefix = '28'

reflects your own prefix.

  • Set up the Pi to run this script regularly as a cron job

Cron is a lightweight scheduler for Linux commands. You can create a 'cron job' by the following:

crontab -e

Create a new line so that the script is run every half an hour:

*/30 * * * * /path_to_your_directory/auto_booch.py

Given the thermal energy required to heat or cool a gallon of water a degree Celsius is about 16 kJ, unless you live in an active volcano, sampling every half hour is more than adequate. The heating pad is designed to heat very gently. I have set the optimum temperature range as between 24°C — 27°C (75°F — 81°F), based on recommended brewing temperatures for Saccharomyces cerevisiae.You can alter this in the script if desired.

You may notice that the script has options for logging and checking the state of your heating pad. These files are stored in the same directory as your script and are called 'brew_log.txt' and 'heating_pad_state.txt'. You can use this to check the performance data (see the graph above). You can see how I did this analysis here.

  • Run the script and cover your primary fermentation vessel

Let fermentation run its natural course for some time. How long comes down to personal preference. You should find the raw kombucha tasting progressively drier and sourer. I let mine go for a little less than a week, because I like it fairly sour. Be aware that some ethanol will be produced in both primary and secondary fermentation, so take care if you should not imbibe even small amounts of alcohol.

Step 5: ​Secondary Fermentation

Once you are satisfied with your raw product, it's time to jazz it up! This is the secondary fermentation stage. This is for flavor and effervescence. Pour your primary ferment into adequately strong glass vessels. Commercial kombucha bottles work great, as do moonshine-style cider growlers. There is still some active yeast in the raw product. If you add some more sugar, fermentation will continue. If you limit exposure to oxygen, this will cause the carbon dioxide produced in the brewing process to dissolve into the beverage, making it fizzy.

This is usually made by adding fruit or vegetable juice. Grated ginger is a popular favorite. Just cover up your secondary vessels with your primary and you're good to go. I usually do this for four days, as I like it fizzy. Be very careful opening your vessel after this. Sieve off the floaters. Refrigerate and enjoy the fruits of your labor.

I will share here two recipes that, as far as I know, are original. It's a thank you for reading this far through a long Instructable!

Brahma Astra

  • 1 tbspn fresh grated ginger root
  • 1 tspn lemon juice
  • 1 tbspn honey
  • 1 dash Cayenne pepper

Amounts are roughly per 300 ml bottle. This is something I converted from a drink the Hare Krishnas taught me for a cold remedy. It works great in kombucha, but you might find the Cayenne a bit spicy! Mix thoroughly.

Tamarind

  • 1 tbspn tamarind paste

Again, per bottle. I had a lot of tamarind not being used. Heat it up to reduce viscosity.

Other favorites are strawberry and blackberry (you can use frozen berries, just boil them for a few minutes). I have used frozen orange juice in a pinch. If a banana is going brown, throw that in! You have to really like banana though, and I suspect this in particular produces more than the usual amount of ethanol.

And, as they say in Hungary: egészségedre!

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    1 Questions

    Awesome instructable! I recently found out a second fermentation stage just by accident(actually year ago, but who cares) and i thought it's cool, but after reading your instructable i see that there is still a lot to learn.

    Could you, please, explain more about continuous primary fermentation. Your SCOBY pellicle is growing isn't it? How big(thick) do you keep it? And what do you do with remains? Do you keep the old one or new part of it?

    I am wondering about dark brown coloration you have on pellicle - how old is it or what is it? I have little lighter brown coloration, but on downside of pellicle. Upwards it's clean and shiny, like everything around that dark spot.

    Second fermentation stage is incredible and i enjoy it so much that i tried to remake it into continuous using jar with lid and spigot. But the problem is that i can't fully limit access to oxygen because i couldn't find adequately strong jar for such capacity(at least 4L). Maybe you know some method or tool like brewers airlock but with manual control of over-pressure level? It would be awesome to have fizzy drink in high volume.

    Have you tested alcohol content in it?

    1

    Hey Vitalij!
    Good questions—I'm still learning myself :)


    > Your SCOBY pellicle is growing isn't it? How big(thick) do you keep it? And what do you do with remains? Do you keep the old one or new part of it?

    Yes, it grows in layers with each new batch. I have not done anything with it, just kept it au naturale. I wanted to keep the respective colonies in a high population in case any rival yeast or bacteria try to invade. It also provides a physical barrier to the air, so the only exposure to the air is via the pellicle, so the only means of oxidation is via the bacteria in the colony matrix. It's something of a monster now, very thick and leathery at the top with lots of tendrils at the bottom. But started as a wee guy, a thin layer in a bowl from a bottle of kombucha. However, it might be time to start sharing it, as I can't let it grow indefinitely!

    > I am wondering about dark brown coloration you have on pellicle - how old is it or what is it? I have little lighter brown coloration, but on downside of pellicle. Upwards it's clean and shiny, like everything around that dark spot.

    It's a complex interaction, but I believe light and dark brown, or beige, is fairly healthy. It has gotten darker with age, but I think part of the loss of translucence is because there are so many layers of material. I have only ever had successful primary fermentation, which is a little more than one per week for almost eight months now. However, I have seen a lot of photos of weird colorations, such as black, white or dark red mold on top, usually with the obligatory "Is my SCOBY OK?". I think the answer there is usually "no," but I don't know how to tell from just the color alone.

    > Maybe you know some method or tool like brewers airlock but with manual control of over-pressure level? It would be awesome to have fizzy drink in high volume.

    If this is what you're after, what about a 'kegerator' or other carbonation system (one brand is 'SodaStream')? Then you'll only be limited by the rate of your primary. Personally, my second ferment is faster than my primary, so it's the rate limiting step anyway. But, to answer your first part, I have no idea of a continuous 2F; I find it hard to picture mentally because you want to limit exposure to oxygen, and retain pressure so the carbon dioxide dissolves adequately.

    > Have you tested alcohol content in it?

    No. But I'd really like to!

    Problem is, it's hard to tell using the specific gravity because the final products (both primary and secondary fermentation stage) are such complex mixtures. I have a refractometer, but the residual sugars and things like acetate throw it off. So that leaves mass spectrometry (which I don't have). Maybe that's another Instructable? ;)

    Cheers,

    Ra

    14 Comments

    Hi Ra,

    Great answer(Devs of Instructables might add reply to an answer option aka answer to an answer).

    > It's something of a monster now, very thick and leathery at the top with lots of tendrils at the bottom.

    Thats a relief. I did the same thing, but when it's so big it have a great appetite - he eats sugar faster than i can add it and eventually it produces a lot of vinegar which destroys the taste of the drink. Now i keep pellicle max 10cm thick. And it needs around 2-4 days to produce light kombucha. As i use it substantionally for 2nd stage it's enough for me.

    > However, I have seen a lot of photos of weird colorations, such as black, white or dark red mold on top, usually with the obligatory "Is my SCOBY OK?". I think the answer there is usually "no," but I don't know how to tell from just the color alone.

    Same here. For me i decided that healthy color is the one it gets when it's young(top layer) or old(bottom layer): everything between #ffccXX - #ff77XX*. Sometimes i get more darker spots on top i suppose when it gets burned by undissolved crystals of sugar.

    * the color i provided is approximate, but like you said light\dark brown.

    > It also provides a physical barrier to the air, so the only exposure to the air is via the pellicle, so the only means of oxidation is via the bacteria in the colony matrix.

    Thanks to you i have some new ideas now. Have you tried to keep the 1st stage jar closed(not fully, so it won't blow up, but enough to maximally limit access to oxygen? It's interesting how it will affect the taste of kombucha at this stage(...and SCOBY itself).

    > If this is what you're after, what about a 'kegerator' or other carbonation system (one brand is 'SodaStream')?

    Thank you for a tip, but i'm searching kind of a natural solution to keep things as simple as possible. I'll try idea with balloon on top of jar. Theoretically it will "breath" accordingly to pressure. Another problem is to get oxygen and keep CO2 inside at the same time.

    > I find it hard to picture mentally because you want to limit exposure to oxygen, and retain pressure so the carbon dioxide dissolves adequately.

    Totally agree to you, but i believe that solution exist we just have to find it).

    > Yeast thrive on a little bit of protein. The best source of protein is... dead yeast.

    Have you tried another sources of protein?

    Best wishes,

    Vitaly

    Hey Vitaly,
    Some great questions.

    > Devs of Instructables might add reply to an answer option aka answer to an answer

    I'm with you there. I initially thought I screwed up my reply because it hadn't indented. Oh well, a work in progress I suppose.

    > he eats sugar faster than i can add it and eventually it produces a lot of vinegar which destroys the taste of the drink. Now i keep pellicle max 10cm thick. And it needs around 2-4 days to produce light kombucha. As i use it substantially for 2nd stage it's enough for me.

    Yep, I have certainly reduced my 1F brewing time substantially (but I do like it pretty vinegary). The good thing about paring your SCOBY down is that you can share it, or make other weird things with it. Or experiment with things like coffee etc. (never tried, but apparently it's a one-way street).

    > Sometimes i get more darker spots on top i suppose when it gets burned by undissolved crystals of sugar

    For me at least, the coloration comes from the tea itself, as I use cheap white sugar and make sure it's dissolved well. It's far from a supersaturated solution too, so I doubt I have crystallization.

    > everything between #ffccXX - #ff77XX

    Ha ha, nice! Spotted the web/full-stack dev ;)

    > Have you tried to keep the 1st stage jar closed(not fully, so it won't blow up, but enough to maximally limit access to oxygen?

    I *think* for the bacterial colonies to thrive, you require aerobic activity. Exactly how much, I'm not sure. That's the cool thing about the zoogleal mat, that it provides a somewhat impervious membrane.

    As for closing it up, I already have problems with my guy's multiple escape attempts! No way I'm sealing my current set-up ;)

    > Thank you for a tip, but i'm searching kind of a natural solution to keep things as simple as possible. I'll try idea with balloon on top of jar.

    This would certainly work—as long as your balloon was "large" (i.e. ~pressure rated) enough. It's actually a neat mechanical fuse, as the balloon would pop before your vessel exploded. However, it would still be messy!

    > Have you tried another sources of protein?

    No I haven't. I'm a cheap-skate, and inherited this idea from brewing other beverages. You can purchase special yeast nutrient packs for wine and beer. These contain vitamin B, I think a little soluble magnesium, iron and protein. But the right protein profile comes from the residue of yeast itself, which nutritional yeast is the same species (although likely a different strain). Some brewers keep their own 'bug' going by topping it up a little. Where I live, the water is quite hard (a lot of dissolved minerals) so is naturally great for brewing.

    Thanks so much for the discussion. Let me know how you go!
    Cheers,
    Ra

    Hey there, great article. Very interesting. But I also realised after reading that I have seen this scooby thing a few times before. If I find a forgotten mug of tea down the back reaches of the workshop- there is this rubbery browny white thing floating. This gets hurredly discarded. Never knew it could be part of something potentially "healthy"

    Aha! It sounds like what you might have there is more of a COB—a Culture of Bacteria—than a SCOBY. An entirely different beast altogether. It would be better left as a scientific experiment than consumed. Although it still forms a zoogleal mat (sorry, I just love that term and try and use it frequently), it's from plain bacterial action on the milk from your tea.

    If you're really lucky, it may be some Lactobacillus bulgaricus, Streptococcus thermophilus, or *chime sounds* the revered Lactobacillus acidophilus, and you could make yoghurt out of it. But I wouldn't count on it, and it's easy enough to make your own yoghurt without resorting to the ol' forgotten workshop mug.

    Better off not risking food poisoning in the workshop, and stick to chucking it!

    Awesome instructable! I've tried my hand at kombucha a few times and always failed dismally. I don't think I've had my secondary fermentation right. How long do you keep it in that process? It looks like you have a brewer's airlock in there, so it's not a pressurised vessel? Is adding sugar optional in the second stage?

    Hi Richie!
    Did you manage to get a SCOBY pellicle formed? This is a sensitive time, you have to have a bit of patience until you get the gelatinous layer forming.
    Re: the secondary fermentation: to be honest, most of the time I don't use the brewer's airlock, as this doesn't give maximum effervescence. I usually use fully sealed glass containers that can handle a fair bit of over-pressure, and fill them almost to the top, to mimimize exposure to oxygen. The one you see in the photo here is banana, which I found tends to brew so violently that I use an airlock for it. Note that I'm pretty sure the alcohol content was a lot higher than other brews I've made. Be warned! ;)
    Let me know if I can clarify anything further.

    Oh, and I forgot to answer on the timing...
    This comes down to your personal preference. If you like it drier and sour-er (which I do), then do the primary fermentation for longer. I do it for six days to a week. Secondary is usually well done in three days. You have to be very careful when opening the vessel (I sieve off the crud that floats to the top before refrigerating)!

    Hi, Ra!

    Thanks for the replies. I did get the pellicle formed. The flavour of my brew wasn't great, and it was flat, not fizzy enough. I think it may have been affected by UV, poor temperature control, and insufficient secondary brew time. Maybe even insufficient primary brew time.

    A thought I had for the temperature control is to use an aquarium heater. They're quite cheap, waterproof, and have a built-in thermostat.

    R

    Ahhh, I see. Well there are plenty of other variables!
    For your primary fermentation, are you using plain black tea? And plain sugar? Is your vessel non-metallic?
    Getting enough effervescence can be tricky. In that stage, you want the fermentation to be anaerobic, so fill your secondary vessels almost completely. Some people recommend 'burping'—periodic release of pressure by opening the caps, but I found that made my product weakly fizzy. Now I just let it go without looking at it. I haven't had an explosion yet (touch wood).

    You definitely could use an aquarium heater. The only thing is that you have to figure out a way to get it to sit right in your fermenter (without interfering with your SCOBY pellicle) and then you have to clean it periodically. Another issue is that it's a centralised source of heat, so you have to be careful to not burn anything with it. The beauty of the brewing/propagating mat is that it's pretty gentle and the heat is distributed across the bottom. Funnily enough, the reason why my Raspberry Pi is located where it is in this Instructable is because one of its other duties is to automate the lights (still aren't happy with my automated fish feeding project :( ).

    *automate the lights—in my aquarium :s