Yeast Growth Experiment

I didn’t see a forum category for posting and updating experiments, maybe that should be another category separate from this one?

After a basic test run ~2 weeks ago, I decided to try posting my second experiment run here. I am still developing a process for setting up and running an experiment, so I would welcome any suggestions.

Goal: Evolve a colony of yeast in different conditions to see what happens, develop an experimental process for future experiments, and to identify opportunities for improvement.
Organism Strain: Fleischmann’s RapidRise Instant Yeast strain
Nutrient Media: LonoLife Bone Broth 15g + ~10 Oz water (tap)
Alternate Media: ~6.1 molar saltwater (~87g NaCl + ~270mL tap water)

Experimental Method
I plan on running this experiment for ~1-2 weeks. When I refer to an “experiment-day,” I am referring to a 24 hour period starting from whenever the experiment started. If I start an experiment at 8PM Monday, 1 full experiment-day would from 8PM Monday to 8PM Tuesday.
Nutrient Solution: Nutrients are added regularly at a rate of 0.5 ml every 20 minutes, which is 1.5 ml every hour.
Salt Solution: Saltwater is on a half-on, half-off schedule, where the first 12 hours of any experiment-day have no saltwater added, and the second 12 hours of any experiment-day have some amount of saltwater added regularly. The dose of saltwater added, and the interval between doses, is calculated based upon a desired molarity and the amount of nutrients added every hour (i.e., a constant 1.5ml/hr, for now…). The desired molarity starts at 0.2M and increases by 0.2M every experiment-day.

Example: At hour 13, I want to add saltwater. Because it is experiment-day 1, the desired molarity is 0.2M. To achieve this, the script will add approximately 0.052 ml of 6.0M saltwater every hour, resulting in a combined volume of 1.55 mL of nutrient solution and 6.0M saltwater being added every hour. At hour ~248, on experimental-day 10.5, the target molarity is now 1.4M. Since nutrient solution is still being added at a rate of 1.5 ml/hr, the script will add 0.45 ml of 6.0M saltwater every hour. The program will calculate the amount of saltwater to be added every hour. There is a little bit of logic to calculate the saltwater dose volume and dosing intervals to avoid impracticalities, but the important part takeaway is the rate of saltwater being added per hour.

Temperature Schedule: The temperature changes every 8 hours following this schedule: [21C → 28C → 34C → 37C → 34C → 28C → repeat from beginning]

I also manually started the stirrer at 600 rpm and the od600/growth/etc automations at the beginning of the experiment.

Experimental Setup
I used 3 glass jars (250ml) to hold the media solution, saltwater solution, and waste. First, I sterilized all glass jars, as well as the chamber/cap/stir bar (unscrewed, but cap resting on top), by boiling water in a large cooking pot with a glass lid on top. Once I was satisfied, I turned off the heat and kept the glass lid on, and removed the glassware as needed.

For the saltwater solution, I weighed out 87.44g of salt and 252g of tap water, and mixed them. Initially, I was targeting a 6.0M solution (just below the saturation point of ~6.1M at 20C). My roommates helped a bit, and while trying to get the salt to dissolve, I’m pretty sure water and/or salt was added/evaporated.
In the future, I think a better approach would be:

  1. Decide on using supersaturated saltwater (6.1M) instead of almost supersaturated saltwater (6.0M)
  2. Calculate the desired volume of saltwater, and then weigh out that much water.
  3. Calculate and weigh out enough salt to supersaturate the amount of water. Add in a little extra to ensure saturation.
  4. Pour the water into a coffee mug, and heat the water to a boil in the microwave (watch the microwave the entire time).
  5. Mix the salt into the heated water until fully dissolved. No solid salt should remain.
  6. You should have an excess of saltwater by volume, so weigh out the desired volume of saltwater. (keep in mind, saturated saltwater has a density of 1.2g/ml)
  7. Pour the saturated saltwater into the glass jar. Done.

Nutrient Solution: The beef broth recommended mixing each packet (15g) with 10 oz of hot water. I sterilized the water by boiling ~270g water (can use microwave) and mixing in the beef broth while still hot. With the beef broth still near-boiling, I added it to the nutrient jar, hoping that by adding the broth to the jar while still near-boiling, it would be self-sterilizing until I capped the jar.

A picture of my setup.

I mixed the yeast powder in room-temperature water I had previously boiled and then used a small syringe (sterilized with 70%ISO) to transfer ~ 3ml of yeast-water to the pio-chamber. I then filled the pio-chamber to running volume with nutrient media. Then, I started the experiment.

Ideas for improvement

  • Find a better way to prepare and store nutrient/salt solutions. It is difficult and a hassle to sterilize it and my current setup doesn’t allow me to easily sterilize or store it. I plan on using IV bags or plant watering bags to store my solutions. I could prepare the solutions ahead of time, sterilize them by placing them in water heated to a boil, store them in the freezer, and then unthaw when I run the next experiment.
  • I’ll add in more later as I think of them

Test Code

from pioreactor.background_jobs.dosing_control import DosingController
from pioreactor.background_jobs.temperature_control import TemperatureController
from pioreactor.utils.timing import RepeatedTimer
from pioreactor.whoami import get_unit_name

class Schedule():
    def __init__(self, salt_value_array, dose_value_array, temp_value_array, **kwargs):
        self.salt_value_array = salt_value_array
        self.dose_value_array = dose_value_array
        self.temp_value_array = temp_value_array
        self.dose_state_array = {
                'run_flag': True, 'run_interval': 20, 'run_counter': 0,
                'update_flag': True, 'update_interval': 60, 'update_counter': 0,
                'run_target': 0.5, 'previous_run_amount': 0, 'name': 'nutrient_media',
                'interval_counter': 0}
        self.salt_state_array = {
                'run_flag': True, 'run_interval': 20, 'run_counter': 0,
                'update_flag': True, 'update_interval': 720, 'update_counter': 0,
                'run_target': 0.1, 'previous_run_amount': 0, 'name': 'salt_media',
                'interval_counter': 0} # 720 update interval -> 12 hours
        self.temp_state_array = {
            'run_flag': True, 'run_interval': 20, 'run_counter': 0,
            'update_flag': True, 'update_interval': 480, 'update_counter': 0,
            'run_target': 28, 'previous_run_amount': 0, 'name': 'temp_control',
            'interval_counter': 0} # 480 update interval -> 8 hours

    def check_for_updates(self, state_array):
        if counter - state_array['run_counter'] >= state_array['run_interval']:
            state_array['run_flag'] = True
            state_array['run_counter'] = counter
        if counter - state_array['update_counter'] >= state_array['update_interval']:
            state_array['update_flag'] = True
            state_array['update_counter'] = counter
        return state_array

    def update_target_values(self, state_array):
        if state_array['update_flag'] == True:
            if state_array['name'] == 'salt_media':
                target_molarity = self.salt_value_array[state_array['interval_counter']]
                if target_molarity == 0:
                    state_array['run_target'] = 0
                elif target_molarity > 0:
                    values = self.calc_salt_dosing(target_molarity)
                    state_array['run_target'] = values[0]
                    state_array['run_interval'] = values[1]
                state_array['interval_counter'] += 1
                state_array['interval_counter'] %= (len(self.salt_value_array)+1) # loop around to begining to prevent out-of-bounds

            elif state_array['name'] == 'nutrient_media':
                state_array['run_target'] = state_array['run_target'] # Not currently in use.
            elif state_array['name'] == 'temp_control':
                state_array['run_target'] = self.temp_value_array[state_array['interval_counter']]
                state_array['interval_counter'] += 1
                state_array['interval_counter'] %= (len(self.temp_value_array)+1)

            state_array['update_flag'] = False
        return state_array

    def get_run_values(self):
        media_ml = 0
        alt_media_ml = 0
        waste_ml = 0
        temp_target = 0

        if self.dose_state_array['run_flag'] == True:
            media_ml = self.dose_state_array['run_target']
            self.dose_state_array['run_flag'] = False
        if self.salt_state_array['run_flag'] == True:
            alt_media_ml = self.salt_state_array['run_target']
            self.salt_state_array['run_flag'] = False
        if self.temp_state_array['run_flag'] == True:
            temp_target = self.temp_state_array['run_target']
        temp_target = self.temp_state_array['run_target']
        waste_ml = media_ml + alt_media_ml
        run_value_settings = {'media_ml': media_ml, 'alt_media_ml': alt_media_ml, 'waste_ml': waste_ml, 'temp_target': temp_target}
        return run_value_settings
    def calc_salt_dosing(self, diluted_molarity):
        # takes a target molarity and returns saltwater dose volume and inter [minutes/dose]
        # Settings assume 6.0 molarity NaCl-water solution
        solution_molarity = 6.0 # NaCl saturation point ~= 6.1 molarity in 20C H2O
        salt_dose_volume = 0.1 # Starting dose volume

        fraction = diluted_molarity/(solution_molarity - diluted_molarity) # V_salt = V_Nut * M_dil / (M_NaCl - M_dil) = V_Nut * fraction
        nutrient_volume_per_hour = 60*self.dose_state_array['run_target']/self.dose_state_array['run_interval']

        salt_volume_per_hour = nutrient_volume_per_hour * fraction
        salt_dose_interval = round(60*salt_dose_volume/salt_volume_per_hour)
        while salt_dose_interval < 20:
            salt_dose_volume += 0.1
            salt_dose_interval = round(60*salt_dose_volume/salt_volume_per_hour)
        values = [salt_dose_volume, salt_dose_interval]
        return values

    def run_execute(self):
        self.dose_state_array = self.check_for_updates(self.dose_state_array)
        self.dose_state_array = self.update_target_values(self.dose_state_array)
        self.salt_state_array = self.check_for_updates(self.salt_state_array)
        self.salt_state_array = self.update_target_values(self.salt_state_array)
        self.temp_state_array = self.check_for_updates(self.temp_state_array)
        self.temp_state_array = self.update_target_values(self.temp_state_array)
        run_value_settings = self.get_run_values()
        return run_value_settings

# Lists of setpoints for dosing, salting, and temperaturing
salt_value_array=[0, 0.2, 0, 0.4, 0, 0.6, 0, 0.8, 0, 1, 0, 1.2, 0, 1.4, 0, 1.6, 0, 1.8, 0, 2, 0, 2.2, 0, 2.4, 0, 2.6, 0, 2.8, 0, 3, 0, 3.2, 0, 3.4, 0, 3.6]
temp_value_array=[21, 28, 34, 37, 34, 28]

sc = Schedule(salt_value_array, dose_value_array, temp_value_array)

# "silent" automation is basically: do nothing. We'll manually invoke dosing later.
dc = DosingController(
tc = TemperatureController(

        1: {'media': 0.5, 'alt_media': 0.0, 'temperature': 35},
        2: {'media': 0.5, 'alt_media': 0.0, 'temperature': 35}

counter = 0

previous_temp = 0

def update():
    global counter
    global previous_temp
    run_value_settings = sc.run_execute()
    temperature = run_value_settings['temp_target']
    media_per_dose = run_value_settings['media_ml']
    alt_media_per_dose = run_value_settings['alt_media_ml']
    waste_ml = run_value_settings['waste_ml']

    if temperature is not previous_temp:
        previous_temp = temperature
    dc.automation_job.execute_io_action(alt_media_ml=alt_media_per_dose, media_ml=media_per_dose, waste_ml = waste_ml)
    counter += 1
# this RepeatedTimer will run `update` every 60 seconds

scheduler = RepeatedTimer(60, update).start()

# block here
1 Like

Very cool @realPeteDavidson! Thanks for sharing!

A couple of ideas of the top of my head:

  • The dilution rate of 1.5ml / hour suggests you will need your culture to be growing at a rate faster than 1.5ml / (your steady state volume¹ in mL). Ex: if your steady-state volume is 14ml, then 1.5/14 = 0.107, so if your yeast are not growing at a rate higher than 0.107h⁻¹, the culture can be diluted out.

  • I have used both an old pressure cooker (like from the 70s), and later a newer 6 qrt pressure cooker to sterlize media. Ex: I’ll mix media powder and water into a mason jar, lightly screw on a plastic cap, and use that as media stock. I’ve even drilled holes into those plastic lids to put tubes through.

  • The UI will show how much media/alt-media has passed through. This is useful to determine how much media is remaining, and the rate of consumption of media.

  • Hidden feature: one additional Overview graph you may be interested in is turned off by default. In the config, look for section [ui.overview.charts] and key fraction_of_volume_that_is_alternative_media. When flipped to a 1, this produces a chart in the Overview that shows the fraction of alt-media in the vial. This can be used to gut-check what’s going on in the vial.

  • I noticed you had to extend the length of one of the pump’s wires. Can you tell me more about that?

  • Did you design those dovetail bottle holders? :star_struck:

¹ This volume is determined by the position of your waste tube in the liquid. Docs here on that

Thanks for the suggestions.

  • Ex: if your steady-state volume is 14ml, then 1.5/14 = 0.107, so if your yeast are not growing at a rate higher than 0.107h⁻¹, the culture can be diluted out.

Do you have some sort of source for this? I am pretty sure I’ve some links to different research articles somewhere on your website/forums, but I can’t remember where or what it was specifically for. If you have a link to an informative article on steady-state volume/dilution/growth rate, I’d be interested in skimming through it.

  • Ex: I’ll mix media powder and water into a mason jar, lightly screw on a plastic cap, and use that as media stock. I’ve even drilled holes into those plastic lids to put tubes through.

The caps I’m currently using are printed in PETG, which has a glass transition temp of 85C, so they aren’t ideal for autoclaving. I like your suggestion and might try it out. I think I might also try printing them in ASA though, which has a glass transition temperature of 112C, and might be suitable for a low-temp autoclave.

  • I noticed you had to extend the length of one of the pump’s wires. Can you tell me more about that?

In addition to the two motors I purchased from your site, I also bought 2 ~$10 pumps off Amazon to see how they compared. They arrived without any leads, so I had to solder red/black wires to the leads, and then I connected them to the Pi with some jumper wires. It’s not ideal, but eventually I plan on ordering the proper connectors/crimper.

  • Did you design those dovetail bottle holders? :star_struck:

Yeah! :smiley: I really liked the ability to have them all connected together and not migrating in every different direction. I measured the dimensions of your dovetails and made sure my design was compatible. They hold 71mm diameter glass bottles and iirc they’re 30mm height and are level with your holders.

Actually, I just now realized this isn’t visible from the photo, but I’m using a Raspberry Pi 3 Model B (gifted to me ~6 years ago by a friend, that I had lying around and unused). The dovetail platform didn’t fit my Pi, so I made my own and added the dovetails to it as well, and it is currently connected to the pump holders in the picture.

I just put up some information on the connectors we use: Hardware connection assembly | Pioreactor Docs

The dovetail platform didn’t fit my Pi, so I made my own and added the dovetails to it as well, and it is currently connected to the pump holders in the picture.

Do you mind sharing the design here? Others will likely be interested

w.r.t. dilution rates in chemostats, an easy read is on wikipedia. We’ve written about chemostats et al. on our blog before.

Awesome, the information on the connectors will be useful.

Yeah, I don’t mind sharing the designs. Should I upload them to I should be able to upload them sometime next week.

Experiment Update

I tried letting the experiment run over the weekend while I was out of town. Unfortunately, there was a power outage and the experiment stopped running and was sitting idle for ~3-5 days while I was away. I will probably try and restart the experiment in the next few days and might try and find improved ways of preparing everything.

I uploaded several of the models I designed and have been using for my setup.

  1. Raspberry Pi 3B+ Holder
  2. Bottle Holders with Dovetails
  3. GL45 Compatible Caps with tubing holes

Let me know if this is what you had in mind. I stopped actively working on the designs once they were ‘good enough’ for my purposes, but I would be willing to continue improving the models if anyone has suggestions for improvement.

Also, let me know if there are any changes you would me to make to the pages on Printables. Let me know if you would like any changes made to descriptions or to the tags (I can add a specific pioreactor tag to all my pioreactor-related uploads to make it easier to find).

1 Like

What you have there is great, thanks for sharing! I may print some of these myself.

Unfortunately, there was a power outage and the experiment stopped running and was sitting idle for ~3-5 days while I was away

Ah, that’s a shame. I’ve had that happen, too. While one could program to restart jobs on reboot, it’s too often an unsafe thing to do, so I’ve never added any functionality for that.

I just restarted the experiment and ran it using python >/dev/null 2>&1 & disown Looks like everything has started out running ok. I’ll let it run for ~4-5 days and hopefully will have some results to share.

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I got around to updating the Raspberry Pi 3 B+ Holder. Check it out when you have a chance.

1 Like