Does yeast eat sugar?

A reply to my last post talked of yeast eating sugar and inferred the more sugar there is, the more active the yeast is. I thought yeast cells need nutrients, the right temperature range, the right degree of oxygenation - and then they grow and divide. As they divide, enzymes are formed or released, and these enzymes break down fermentable sugars. Is this somewhere near the truth. A reply from a practicing professional brewer would be good also.

They use the sugar as food and all the other factors are what make up the ideal environment for them. The by products of their life cycle are alcohol and Co2. You could try PMing revvy on this

BOBTHEukBREWER said:

A reply to my last post talked of yeast eating sugar and inferred the more sugar there is, the more active the yeast is. I thought yeast cells need nutrients, the right temperature range, the right degree of oxygenation - and then they grow and divide. As they divide, enzymes are formed or released, and these enzymes break down fermentable sugars. Is this somewhere near the truth. A reply from a practicing professional brewer would be good also.

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I think you might be better suited with a biology text book, or the book Yeast.

You have several questions in there.
Yes yeast "eat" sugar, but there are different kinds of sugars. Enzymes do certain things, fungi do certain things aerobic or anaerobic, bacteria do other things. My guess is you are curious at a level which would be best served with a technical text.

Nearly all organisms that we commonly recognize, including yeast, metabolize some form of sugar for energy (plants do too- it's just that they also CREATE sugar with photosynthesis)

Nutrients refer to other compounds that the yeast need to build their own bodies with. Your question suggest that you might also believe that human beings could survive off of nothing but vitamin supplements- these are akin to the "nutrients" that are discussed with regards to yeast.

Grains are mashed to convert starch chains into their constituent sugars, and yeast may use further enzymatic action to break the complex sugars into even simpler ones (just as we break down sucrose into glucose) to make them easier to eat. They don't release enzymes for our benefit, we benefit from their alcohol waste.

More Biology: Yeast are a type of fungus. Fungi eat by extra-cellular digestion--they release enzymes, these break down the sugars into what the yeast need, then they absorb those compounds. I'm not sure if they need to do this to simple sugars, but they do for polysaccharides. I think. I know they use extra-cellular digestion.

"Eating" is not really what the yeast do, but in simple terms that description works.

In general, the simple answer is that the yeast gain energy from the conversion of the sugar into carbon dioxide and alcohol. In simplest terms, yeast eat sugars and poop and pee Co2 and ethanol. Of course, the metabolic behavior of yeast are more complex than that, but that breaks it down into the simplest way to think about it. The sugar is converted to co2 and ethanol by the yeast.

Yooper said:

"Eating" is not really what the yeast do, but in simple terms that description works.

In general, the simple answer is that the yeast gain energy from the conversion of the sugar into carbon dioxide and alcohol. In simplest terms, yeast eat sugars and poop and pee Co2 and ethanol. Of course, the metabolic behavior of yeast are more complex than that, but that breaks it down into the simplest way to think about it. The sugar is converted to co2 and ethanol by the yeast.

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Eating, pooping and pee-ing all used in one thread, just made my Monday!

Happy Craft Beer Week!

thanks, everybody - but you all dodged the issue of effect of amount of sugar on yeast activity, let us consider 4 beers with OG's of 1025, 1035, 1045, 1055 all 3 gal batches all with a sachet of safale04 sprinkled on top at 25 deg C - I think they would all ferment at same rate but as OG goes up take longer to finish.

BOBTHEukBREWER said:

thanks, everybody - but you all dodged the issue of effect of amount of sugar on yeast activity, let us consider 4 beers with OG's of 1025, 1035, 1045, 1055 all 3 gal batches all with a sachet of safale04 sprinkled on top at 25 deg C - I think they would all ferment at same rate but as OG goes up take longer to finish.

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No, they wouldn't necessarily take longer to finish at all. The yeast will reproduce first, and then start fermentation (from aerobic to anaerobic activity).

A full sachet of S04 is overpitching for all of those examples, So they probably wouldn't reproduce much, if at all, before starting fermentation. Since there are plenty of yeast cells, they would be finishing at about the same time.

If the proper amount of yeast is pitched, it's not like a 1.075 beer will take twice as long as a 1.037 beer. They take about the same amount of time, actually.

BOBTHEukBREWER said:

thanks, everybody - but you all dodged the issue of effect of amount of sugar on yeast activity, let us consider 4 beers with OG's of 1025, 1035, 1045, 1055 all 3 gal batches all with a sachet of safale04 sprinkled on top at 25 deg C - I think they would all ferment at same rate but as OG goes up take longer to finish.

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Just because you walk into a supermarket piled to the rafters with food doesn't mean you will eat it all... Or even ANY of it. As with you, how quickly or violently the yeast metabolize what food they eat is largely dependent on the strain involved and the conditions of it's health, how many cells there are and what type of food is available.

Additionally, as beer ferments, the levels of oxygen are depleted and the alcohol goes up. More alcohol will definitely negatively effect the rate of fermentation.

There is no DIRECT correlation to speed of fermentation and amount of food available

I bet he's going to throw out another trick question that you have supposedly "dodged".

BOBTHEukBREWER said:

thanks, everybody - but you all dodged the issue of effect of amount of sugar on yeast activity, let us consider 4 beers with OG's of 1025, 1035, 1045, 1055 all 3 gal batches all with a sachet of safale04 sprinkled on top at 25 deg C - I think they would all ferment at same rate but as OG goes up take longer to finish.

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Is this scenario a personal experience of yours? What are you drivin' at here Bob?

I don't think the yeast activity is dependent on OG, but it is VERY dependent on the number of yeast cells in the carboy and temperature.

Since the cell population doubles every 2-3 hours, the activity will increase exponentially over time, and the more sugars you start with the longer fermentation will take, the more yeast you will have (due to this geometric progression), and the greater activity.

The doubling time is very dependent on temperature. Increasing the temp of the wort will result in lower doubling time, so the population increases more rapidly and so does that activity.

I'd guess that the greater osmotic pressure on the cell in a high OG beer would have a negative effect on fermentation rate, but that would be guessing.

BOBTHEukBREWER said:

A reply to my last post talked of yeast eating sugar and inferred the more sugar there is, the more active the yeast is. I thought yeast cells need nutrients, the right temperature range, the right degree of oxygenation - and then they grow and divide. As they divide, enzymes are formed or released, and these enzymes break down fermentable sugars. Is this somewhere near the truth. A reply from a practicing professional brewer would be good also.

Click to expand...

To add/clarify a couple of points. Yeast are continually making enzymes. Cell division does not have big impact on the formation and release of enzymes involved in fermentation (in a simplistic sense). More yeast does mean more enzymes though. Yeast need oxygen to divide and grow, but not to ferment. The lack of additional nutrients, beyond sugars (that would be vitamins and minerals) will have a large impact on the health of the yeast. Unless you are brewing with distilled water, this is generally not an issue. Like the earlier plant analogy, there is an optimum though and you can buy supplements to insure your have optimally happy yeast.

Brewing yeast are not terribly good at breaking down polysaccharides (as opposed to most other fungi). That is why we have to mash the grains first. They do take up small sugars and metabolize them inside the cell. They also make proteases that they excrete to break down proteins/peptides into small pieces that they can take up to use as nitrogen sources. I read an article a while back warning that too much extracellular protease (too much yeast?) can lead to the loss of foam positive proteins. I'll have to dig that up and reread it.

Yeast eat lots of things and are chronic over eaters. They are part of the growing obesity epidemic. Often deprived of foods they have no self control and will gnaw their own face off.