How does temperature fluctuation affect my brew during fermentation?

[DanInSydney]

After consulting Dr Google i understand:
1. If too hot the yeast will freak out and I’ll risk a banana/solvent flavours
2. If too cold the yeast won’t ferment the sugars

Is that correct?

What about during fermentation? Can’t seem to find anything explaining what could happen if temperature fluctuates during the fermentation period...

 

[Gnomebrewer]

Kind of correct.
1. Yeast actually like it hot. They don't freak out (until it's really hot, like over 40C) - they actually become aggressively active and ferment crazy fast. It's the crazy fast ferment that can make esters/phenols/solvents, which are generally unwanted. It's very yeast dependent though - for example, kveik (Norwegian yeast) strains are known for fermenting quite clean into the high 30's. Some saison yeasts produce desirable flavour compounds in the 30's that they don't at lower temperatures.
2. That can happen. Or they can ferment most sugars but leave behind some of the harder to digest sugars, so attenuation isn't as high as intended. Or they might finish the sugars, but not finish cleaning up by-products (eg. acetaldehyde/green apple) that might have been produced during the ferment.

When temperatures fluctuate, you tend to get more fermentation by-products and (IME) a slower ferment. Brewers who don't control ferment temperatures seem to find their beers benefit from long ferments (three or more weeks) to give yeast time to clean up by-products. You should really try to avoid temperature swings of more than a few degrees C. Slowly increasing the temperature during the ferment can be a good thing though.

 

[Buick Beer Gardens]

Yeast will perform differently at different temps. When hot they are very active. They can also be very active at 64F. Your post leads me to believe you are brewing a hefeweizen? At 64F(+-) the taste will be of clove. Fermenting at higher degree will produce banana. My first hefeweizen fermented at 64F. It has a clove flavor but a slight banana finish. Whatever the case I asked my daughter and her boyfriend to try it. They said is was one of the best hefeweizen they have had.