What is the biggest or at least some causes for yeast to produce off-flavors in beer/wine?!

Question: What is the biggest or at least some causes for yeast to produce off-flavors in beer/wine!?
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O2 (oxygen) saturation of the wort!.
Too high or too low a fermentation temp!.
Old (non-viable) yeast!.
Cell pitching rates!.

It can be several things ranging from nutrition to environment!. Yeast are living things and so are subject to certain biologically required parameters in order to function!.

Too high or too low a fermentation temp!.
When it comes to temperature the yeast, as we do, work to keep warm but in an entirely different way!. While we lose energy shivering trying to create heat to warm ourselves, yeast will change its physiological pathways that produce more energy for itself in a "short-cut" manner!. The result is the yeast releases/leaves intermediate compounds in greater concentrations in the beer!. One such compound is diacetyl, a buttery smelling/tasting compound!.

"When the temperature is increased the yeast produces sweet ester compounds we term as fruity!. These compounds are caused by decreased pitch rate of yeast, increased fermentation temperature, and increased original gravity!."

O2 (oxygen) saturation of the wort!.
Oxygen is an important mineral to yeast for much the same reasons as nutrients are!. You want a wort with an oxygen concentration range between 10-15ppm!. If you do not use a saturation, stone shaking your carboy (hold on tight) very vigorously for 45s (seconds) will give you 7-10ppm!. Because it is more difficult to saturate heavy (er) gravity beers with O2 this time needs to be increased!.

Cell pitching rates!.
A good rule to follow is a million cells per milliliter per degree plato!. This means that you need 20 million cells per ml for a 20 degree plato (1!.080 specific gravity) beer!. This is 2-3 of the average yeast smack-packs!. For higher S!.G!. brews above 1!.060 increase cell count pitching rates!. So for batches above this pitch another 16-22 million cells/ml!.

Old (non-viable) yeast!.
This problem causes increased lag phases and poor yeast activity!. I use my yeast through several batches!. But I do have access to a lab to check its viability!. Although, from my experience I am confident in telling you to do the same!. For batches below 1!.058 you should be good to use the yeast for 3 batches until it needs to be either acid washed or cultured!. When yeast are not given the required environmental parameters they self-destruct in a process called autolysis!. The result of autolysis is unpleasant flavors and bitterness!.

In other considerations yeast produce off-flavors when it’s nutrient are not available!. These include copper, zinc, and selenium all found in barley!. Yeast is also dependent on barley for amino acids for its cell membrane!.

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if you brew at too high a temperature you will tend to get fruity or citric off flavors in your beer, if you leave it in the fermenter too long it will draw bitter flavors off the decomposing yeast on the bottom!. If you use the wrong type of yeast for the kind of beer or wine your brewing then you will get a different type of flavor then you may be wanting!. basically use the right yeast, pitch the yeast ahead of time, ferment at the right temp, and siphon it out when its done bubblling and you should be good to go!.Www@FoodAQ@Com

Pertains more to wine:

To high sulphite levels leading to "rotten egg" odors!. Is cleared up with copper compounds!.Www@FoodAQ@Com

Too much or too little oxygen in the wort!.
Too high or too low a fermentation temp!.
Yeast is too old!.Www@FoodAQ@Com

To high a fermentation temperature!.Www@FoodAQ@Com

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