Sciencemadness Discussion Board

ideal pH conditions for yeast fermantation

lordcookies24 - 2-1-2019 at 19:08

For school, I am doing a lab report on how different levels of pH affect the rate of alcohol fermentation. After a bit of research, it clearly says that yeast prefers an acid environment.
But now I have two majors questions which are what levels of pH do I need for optimal fermentation and what kind of acid do I want to introduce. My teacher hinted that hydrochloric acid may be toxic to the yeast and seemed to agree with my idea of using acetic acid(vinegar.)
I obviously did some research before coming here but I found no sources for ideal pH conditions or what acid might be toxic from them.


p.s. I am new to this forum so tell me if I posted in the wrong special topic or anything like that.

[Edited on 3-1-2019 by lordcookies24]

happyfooddance - 2-1-2019 at 19:28

https://homedistiller.org/wash/ferment/nutrients

This page has a good primer on yeast acidity.

There are some books in the forum library as well, one is titled Industrial Fermentations

[Edited on 1-3-2019 by happyfooddance]

j_sum1 - 2-1-2019 at 19:47

Welcome to sm, lordcookies. You will enjoy it here. Do take the time to read faq and threads and get a feel for the culture of the forum. It is different from many.other boards.

In general, threads without a reference go in Beginnings (even if the content is quite advanced.) Mods are happy to move if needed. I think this one is ok here though. Please don't double post.

Goid question. A lot of members have significant experience fermenting. There are numerous helpful threads. :)

Sulaiman - 3-1-2019 at 03:02

IF you intend to adjust the pH then first you need to determine the pH before calculations start,
the initial pH will depend upon what it is that you are fermenting,
e.g. pure sugar solution will have a higher pH than fruits.

Yeast fermentation lowers pH, the pH very rapidly drops to just over 4 and slowly goes to pH=4,
a pH lower than 4 is not good for the yeast fermentation,
by some mechanism that I do not know, the yeast fermentation maintains pH at just over 4.

So, I expect that maintaining a pH above 4.2 will require daily monitoring and adjustment.

To lower pH I would not use HCl due to the chlorine ions that may be toxic to yeast, similarly sulphuric acid may not be good as SO2 is toxic to yeast also.
I guess that dilute nitric or phosphoric acid would be ok
but citric or acetic would be more life enhancing for the yeast.

If you are investigating the effect of pH on fermentation rate then
you need to control the temperature as yeast dies if too hot (c35oC), and goes to sleep if too cold.

As above, the nutrients available to the yeast also dramatically effect fermentation rate.

Higher concentrations of alchohol will also kill yeast, so the starting concentration of sugar(s) is important too.

Available oxygen will also influence whether the yeast reproduces or ferments.

So, although your task sounds simple ... it most definitely is not.
There are so many variables to manage.

I suggest that you read up on yeast then ask your teacher for guidance before you start.
If this is just a learning exercise then ok, you will learn a lot,
if this work will form part of your academic assessment then I suggest that you choose a different experiment because to get meaningfull data from this experiment will require a LOT of work, diligence, equipment, time and space.
..................................................................................................
Whilst typing the above I had an idea that might be practical :

Start a large fermentation
. enough sugar(s) to achieve say 10%ABV (about 175 g/l sugars)
. sufficient nutrients (I buy ready-mixed plus I add a little fruit such as rasins, apple, potato,... it seems to help)
. add yeast and a bubble-trap.

After a couple of days the fermentation will be fairly constant, as evidenced by bubble-rate.

Set up as many test tubes as required, each with gass bubbler,
put calculated quantities of acid and base in test tubes (or small flasks etc.)
Add the same quantity of the fermenting liquor to each tube,
mix the tube contents, seal the tube and measure the rate of gas production.
You could measure ml of CO2 per hour, or just 'bubbles per minute'
The rate of CO2 production is proportional to the fermentation rate.

This method would require only an hour or so to prepare the bulk fermentation,
then a few days later LOTS of measurements.

HOW will you measure pH ?

Good Luck.

j_sum1 - 3-1-2019 at 03:13

I had some students do a similar experiment.
They set up a simple fermentation and altered the kind of sugar used. It was a simple task to make concentration, temperature, volume, container shape etc uniform across all the runs. From memory they used evolved gas volume to determine how much fermentation had occurred and ran each fermentation for just 3-4 days.

It was a good experiment. Results were reasonably predictable: fructose and sucrose worked best. Glucose was reasonable. Lactose did not ferment well at all.

happyfooddance - 3-1-2019 at 07:24

Quote: Originally posted by j_sum1  
I had some students do a similar experiment.
They set up a simple fermentation and altered the kind of sugar used. It was a simple task to make concentration, temperature, volume, container shape etc uniform across all the runs. From memory they used evolved gas volume to determine how much fermentation had occurred and ran each fermentation for just 3-4 days.

It was a good experiment. Results were reasonably predictable: fructose and sucrose worked best. Glucose was reasonable. Lactose did not ferment well at all.


You have to train the yeast for the specific sugar.

For example, most "active dry" yeast is trained to consume sucrose (glucose + fructose).

It can be trained in a few weeks to consume primarily lactose or another sugar, in which case it gets used to the sugar, and will preferentially consume that sugar.

I use invert sugar for fermentations, the last leg of my ferment goes faster, but the initial pitch (of dry yeast) lags compared to when I use sucrose.

[Edited on 1-3-2019 by happyfooddance]