Isoamyl acetate issue - banana

[Morgan]

We source fruit from several orchards, and one in particular often seems to yield a cider with a distinct banana ester that I don’t particularly care for.  It’s shown up in at least three separate batches with different yeasts in use and different tanks.  None of our other batches from other orchards show this issue, so it seems linked to their fruit.

Could this be a nutrient issue of some sort?  We give everyone a healthy dose of Fermaid-o but haven’t specifically tested for YAN yet.  Perhaps some other nutrient is deficient?

The fruit from this orchard also tends to have lower acidity and higher sugars overall compared to others as it’s in a hotter summer microclimate.  Typically the blend is heavy on wickson and pink lady, with a bit of cinnamon spice, ashmead, and Roxbury russet.

Any ideas?

Best,

Morgan

[Matthew Moser Miller]

Hi Morgan,

What are the fermentation conditions (temp., container type, volume, your location, etc)? I wasn't able to access the full papers, but when I searched for "isoamyl acetate fermentation," a number of the abstracts suggested heightened isoamyl acetate production in higher temp ferments and in semiaerobic (as opposed to anaerobic or aerobic) conditions. I'm surprised that your juice would be lower acid, with Wickson and Roxbury Russet as notable components; where are these being grown? Also, are you getting the fruit and pressing it yourself (fresh or from cold storage), or is it coming to you as juice? And are all these fermentations happening at the same time of year, or at different points? If they're being stored for a while before pressing, you'd obviously have a decline in acidity as the fruit ages.

Another thought that's just come to me: if it's being pressed for you, I wonder if the inoculation from their press equipment may be able to outcompete your pitched yeasts (or do so later in the fermentation, where the papers suggest heightened isoamyl acetate production under certain conditions). You also didn't mention the degree to which you're knocking back the native populations before inoculating?

If all processing is using the same equipment, at the same time of year, and only this one orchard's fruit has these characteristics, then it seems logical that it's something about the fruit itself. I've never had an isoamyl acetate issue, but it may be worth trying to ferment it at lower temperatures and anaerobic (if you have access to SS) or well-oxygenated (if you don't) conditions. I seem to remember something about oxygen issues in large-volume ferments, but I've never worked on that large a scale.

Hope this helps,

Matt Moser Miller

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[AW]

Wow, tons of ester questions lately.  I've spent the past month or so reading the scientific literature on this subject and there are few easy answers.  Attempted optimization of many parameters seem to follow an "uncanny valley" pattern where better results can be obtained by BOTH decreasing and increasing nutrients, oxygen, pitch rate, temperature...etc.  Its very difficult to piece together the full landscape of possible outcomes.  

This paper (https://www.sciencedirect.com/science/article/pii/S0023643824004444) suggests that isoamyl acetate correlates with more reducing conditions.  However that is within the range tested...and as Matt mentions its possible that other investigators have found that more oxidizing conditions are better in similar or different conditions.  

Would you characterize this fermentation as "stressed" or "healthy"?  In either case it could be due (as Andrew says) to an abundance of precursors in the juice...either isoamyl alcohol in some form or the amino acid leucine.  If its a "stressed" fermentation there might be some strategies that could mitigate this ester.  It could undergo trans-esterification as the cider finishes fermentation (do nothing, maybe age on lees or rouse the lees).  If the yeast is converting leucine to isoamyl acetate you might add DAP to distract the yeast from the more complex nitrogen source.  Temperature is a double edged sword...colder will tend to produce less isoamyl acetate, but it will be more easily retained.  Blending down with a less aromatic cider might be the best bet....this ester is usually considered desirable unless its at very high concentrations.  

[LL]

Hi Morgan,

To my understanding, acetate esters can be produced in abundance by the non-Saccharomyces yeasts that are active early in the fermentation. They are typically present in higher numbers than Saccharomyces on the fruit.

It could be that the fruit from that orchard for some reason has a slightly different micro -flora, with more of the ester producing yeasts present. Or it could be that the lower acidity in that fruit favours their development.

Another thing to consider if is that fruit has been waiting for longer after harvest before pressing. Letting the apples rest could allow for native yeasts to start multiplying. It could also explain slightly higher sugar content, since the fruit will dry out a little while resting. Acidity also tends to diminish when the apples rest. But if these apples haven't waited for longer than the others, that is of course not the reason behind the esters.

These early, non-Saccharomyces yeast are in general more sensitive to sulfites than Saccharomyces. So if these yeasts the are the cause, sulfites might help you.

Best regards,

Love