The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

Back to Uni. First Lecture

mwilson's picture
mwilson

Back to Uni. First Lecture

Yeast metabolism. And you thought sugar just got converted to CO2 and Alcohol...

Mini Oven's picture
Mini Oven

If only all our crap could be recycled.  No wonder we love the little beasties!  :)

So this is what happens during the life cycle of a yeast cell.  

What happens when we build up a mass of these and cut their lives short as when we bake their cute little cells bottoms?  Everything less or just some of the end products?

mwilson's picture
mwilson

90+ % is just a simple conversion of sugar to CO2 and alcohol but sometimes the yeast need to utilise energy to look after themselves. The diagram shows all possible routes of creating energy.

Baked yeast will explode and release their products and some of these help to create flavour compounds..

mwilson's picture
mwilson

Plating microbes (specifically wine yeast) today in agar.,,

Mini Oven's picture
Mini Oven

in the agar-agar?  

mwilson's picture
mwilson

ok, ok, It's not just agar, It does contain nutrients. I have the recipe on file and will come back to you on that...

There are different "media". In the photo the blue looking agar is designed to create an anaerobic environment while the burnt-orange looking one is just the standard one.

Mini Oven's picture
Mini Oven

correspond to the colors in the metabolism diagram???  Different by-products?

mwilson's picture
mwilson

Good guess but no sorry. The different colours of the agar represent their type (ingredients) for the purpose of growing different microbes (yeast or bacteria) under different conditions.


Yeast growth.


The black marks are from counting the cells which were diluted many times before plating.

PS. I'm not sure of the relevance of the colours in the yeast diagram...

Elsie_iu's picture
Elsie_iu

Last summer I did an estimate of the population of my starter (100% hydration, half rye half red wheat). It was inoculated onto several MRS plates of various concentration after serial dilution. They were then incubated at 38°C for 24 hours.

The viable cell count result was 10^12 microbes per g of starter. It seemed high when compared with that published in journals. There were certainly errors in my experiment. For instance, the bran kept blocking the pipette tips. My counting skills could play a role as well since my vision went blurry after counting 10 plates or so...

 

10^(-7) concentration, immediately after incubation

 

Same plate when stored at room temperature for a few days

P.S. When observed under the microscope, the large yellowish colonies were yeasts and the tiny white spots were mainly bacillus.  

mwilson's picture
mwilson

Well done for counting 10 plates! I fatigued halfway through counting just 1!

In the LAB work I was doing we ended up using about 7 different plate media. It was a Sherlock Holmes affair... In groups we were given one unknown yeast and one unknown bacterium (all wine related of course) that we had to identify by assessing on which media they grew and how they grew.

We also did Gram staining. It was pretty cool to be honest!

Thanks for sharing your results.

Mini Oven's picture
Mini Oven

So cool!  

 

dabrownman's picture
dabrownman

alcohols, other acids and other compounds that yeast produce to give bread its great flavor profile.  Is it my old eyes nit working right or is there no large CO2 production mentioned in the chart?  Ethanol is there but, the other main bi-product seems to be missing?

Thanks for the chart!

Happy baking in 2020 Michael!

mwilson's picture
mwilson

Yes, no CO2. I'm not sure of the reason for its absence but I doubt it is an oversight. It may be because CO2 can't be re-metabolised unlike many of the other compounds.

But in any case I can tell you where CO2 fits into that diagram. On the thick black line from Pyruvate to Ethanol when pyruvate is converted to acetaldehyde that is where CO2 is released. Pyruvate is decarboxylated to a acetaldehyde.

Nice to hear from you and Happy baking!