Back in July, I posted about a problem I had with a batch of cider failing to carbonate. After some more research I think I have zeroed in on what had gone wrong and thought it worth posting the outcome for others who might strike the same problem.
The short answer seems to be…
For those who want to dig bit deeper, the long story is…
When I looked at my past notes, a common factor in sticking fermentations was S04 yeast (my usual “go-to”). Similar batches with Mangrove Jack's M02 didn’t have a problem. It seems that yeasts with a high nutrient demand, may stop fermenting and stick if the nutrients become depleted.
Although I couldn’t find any published data regarding S04 nutrient demand, it is an ale yeast which are generally “high demand”. On another forum, MJ have acknowledged that they add micro-nutrients to the 9g packs of M02 so the problem is avoided with small batches using that yeast. Also, in a recent paper about their “new” yeasts, Fermentis suggest adding nutrients when making “American Dry Hard Cider” which is probably the style that most of us make. They indicate that lack of nutrients readily stops their TF6 yeast above SG 1.000 for residual sweetness. Hmm, nutrient depletion was starting to look like a “smoking gun”.
Incidentally, Scott Labs have YAN demand data on wine yeasts including many used for cider but unfortunately not for S04. The popular 71B and DV10 yeasts are classified as “low demand” so these may be a better choice for some of my ciders.
I had previously dealt with stuck fermentation issues by going through the painful process of decanting the cider into a bottling bucket, adding a starter of yeast and DAP nutrient and re-bottling, but it seems that maybe I didn’t need the extra yeast, only enough nutrients for the residual yeast to finish the job. This might also explain why S04 has a reputation for finishing a bit above 1.000 thus leaving some sweetness (i.e. if the nutrients are exhausted before the SG reaches 1.000 fermentation stops, leaving residual sugar).
Both Jolicoeur and Lea suggest that up to 10ppm of YAN (50ppm of DAP or similar nutrient) is needed for each 0.010 drop of SG and this should be a starting point to get stuck fermentations going again. Jolicoeur also points out that with keeving, the deliberate control of nutrients is used to stop fermentation (and so produce a sweet cider) and that other methods of limiting nutrients (like several rackings to leave yeast and nutrients behind) can be used to get similar results.
It is interesting that wine industry research suggests that 150ppm of YAN is desirable for complete fermentation of white wine and that additions of YAN are best done progressively during fermentation after the initial consumption of the native YAN has taken place.
Although apples are inherently low in YAN, the juice from “normal” apples might have 80 -120 ppm YAN which should be enough for complete fermentation. However, in some apples the nutrient level can be as low as 50ppm or even lower, leading to a sluggish or stuck fermentation.
In my case it is likely that my apples were low in YAN. They were from old unfertilised trees, were late maturing ripe or over-ripe apples and were late picked, all of which I understand can result in low YAN. Unfortunately, because of adverse seasonal conditions over the past year (wet cold spring and summer), my crop was a candidate for this “triple whammy”. Also, it seems that the level of YAN in apples can change from year to year because of growing conditions, so you never really know what you have.
So as suggested in the original post, after adding a “pinch” of DAP to a 100ml sample of a problem batch “to see what happens”, fermentation re-started from where it had stuck and quickly continued down to 1.000. On that basis I decanted the troublesome bottles, added 0.25g of DAP per 5-litre batch (50ppm) and re-bottled. Fermentation kicked off and carbonation started to build up.
After pasteurising to stop fermentation at 1.004, I now have another semi-dry, carbonated “Winter Harvest” cider as planned.
Happy days!
The short answer seems to be…
Fermentation can stick when the nutrients have been exhausted by the yeast, i.e. the natural YAN (Yeast Assimilable Nitrogen) nutrient in the juice has been used up.
For those who want to dig bit deeper, the long story is…
When I looked at my past notes, a common factor in sticking fermentations was S04 yeast (my usual “go-to”). Similar batches with Mangrove Jack's M02 didn’t have a problem. It seems that yeasts with a high nutrient demand, may stop fermenting and stick if the nutrients become depleted.
Although I couldn’t find any published data regarding S04 nutrient demand, it is an ale yeast which are generally “high demand”. On another forum, MJ have acknowledged that they add micro-nutrients to the 9g packs of M02 so the problem is avoided with small batches using that yeast. Also, in a recent paper about their “new” yeasts, Fermentis suggest adding nutrients when making “American Dry Hard Cider” which is probably the style that most of us make. They indicate that lack of nutrients readily stops their TF6 yeast above SG 1.000 for residual sweetness. Hmm, nutrient depletion was starting to look like a “smoking gun”.
Incidentally, Scott Labs have YAN demand data on wine yeasts including many used for cider but unfortunately not for S04. The popular 71B and DV10 yeasts are classified as “low demand” so these may be a better choice for some of my ciders.
I had previously dealt with stuck fermentation issues by going through the painful process of decanting the cider into a bottling bucket, adding a starter of yeast and DAP nutrient and re-bottling, but it seems that maybe I didn’t need the extra yeast, only enough nutrients for the residual yeast to finish the job. This might also explain why S04 has a reputation for finishing a bit above 1.000 thus leaving some sweetness (i.e. if the nutrients are exhausted before the SG reaches 1.000 fermentation stops, leaving residual sugar).
Both Jolicoeur and Lea suggest that up to 10ppm of YAN (50ppm of DAP or similar nutrient) is needed for each 0.010 drop of SG and this should be a starting point to get stuck fermentations going again. Jolicoeur also points out that with keeving, the deliberate control of nutrients is used to stop fermentation (and so produce a sweet cider) and that other methods of limiting nutrients (like several rackings to leave yeast and nutrients behind) can be used to get similar results.
It is interesting that wine industry research suggests that 150ppm of YAN is desirable for complete fermentation of white wine and that additions of YAN are best done progressively during fermentation after the initial consumption of the native YAN has taken place.
Although apples are inherently low in YAN, the juice from “normal” apples might have 80 -120 ppm YAN which should be enough for complete fermentation. However, in some apples the nutrient level can be as low as 50ppm or even lower, leading to a sluggish or stuck fermentation.
In my case it is likely that my apples were low in YAN. They were from old unfertilised trees, were late maturing ripe or over-ripe apples and were late picked, all of which I understand can result in low YAN. Unfortunately, because of adverse seasonal conditions over the past year (wet cold spring and summer), my crop was a candidate for this “triple whammy”. Also, it seems that the level of YAN in apples can change from year to year because of growing conditions, so you never really know what you have.
So as suggested in the original post, after adding a “pinch” of DAP to a 100ml sample of a problem batch “to see what happens”, fermentation re-started from where it had stuck and quickly continued down to 1.000. On that basis I decanted the troublesome bottles, added 0.25g of DAP per 5-litre batch (50ppm) and re-bottled. Fermentation kicked off and carbonation started to build up.
After pasteurising to stop fermentation at 1.004, I now have another semi-dry, carbonated “Winter Harvest” cider as planned.
Happy days!
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