Musing: I believe yeast is the most important factor deciding whether a keeve will be successful or not. Let me explain.
Keeving is a process that uses nutrient deprivation to create a naturally sweet hard cider. The main characteristic of a keeve is the formation of a thick cap. It is usually gelatinous but sometimes it can be thick and crusty looking. It is generally brown, which is why it’s often called a brown cap. The brown comes from the oxidation of various compounds, especially phenolics. A big part of the cap, and why it’s usually gelatinous, is because of the pectin it contains. This cap also has yeast and many other compounds including nutrients in it. When you siphon off the clear cider that forms between the thick cap and the sediment at the bottom, you have a liquid that is low in yeast and low in nutrients. These two characteristics mean your fermentation will slow and even stop before all residual sugars are processed. The low yeast count means the biomass will try to expand again but the low nutrients prevents its growth, causing the fermentation to stall. This creates a cider with residual sweetness. The key is the cap and its ability to trap large amounts of the yeasts and nutrients.
It sounds like the perfect solution for creating ciders with some residual sugars. The challenge is that the process is not guaranteed. You can add pectin methyl esterase (PME) and calcium chloride to your juice to help encourage the keeving process, but that doesn’t guarantee a keeve will occur. Even if it does, it usually still requires multiple racking. Also, you rarely find a keeve in cider inoculated with the commercial Saccharomyces yeast strains. I believe the most important element of creating a successful keeve, is the yeast.
Most successful keeves involve wild ferments. I have seen what I thought looked a lot like a keeving cider with commercial Saccharomyces strains but it never lasts. The fermentation becomes too aggressive and the thick brown cap is broken up. Slower fermenting yeast strains, generally non-Saccharomyces yeasts, are less likely to breakup a cap and the PME and calcium form a tougher gelatinous cap. The issue is that even with PME, calcium, and wild fermentation, a cider may still not keeve or if it does, the cap may not hold. My theory is that it is because of the yeast.
Wild ferments can have any number of yeast strains. There is usually a wide range of non-Saccharomyces strains from the fruit but the equipment you use and your press cloth will add strains that are usually Saccharomyces yeasts. With all yeast being equal, no inoculation, the natural progression is that non-Saccharomyces strain dominate initially before Saccharomyces strains become dominate and finish the fermentation. With a keeve, the non-Saccharomyces strains are what are initially fermenting when the brown cap forms. They are slower and less vigorous. A successful keeve may never reach a state where the Saccharomyces yeast begin to dominate. The loss of nutrient and yeast from racking will usually stall the process. I believe this starts to highlight importance of your yeast.
Note that not all non-Saccharomyces yeast are the same. Pichia kluyveri seems to flocculate well and ferment from the bottom where Candida zemplinina tends to create a surface foam and not flocculate. Do keeves where Pichia kluyveri dominate create a successful keeve and those where Candida zemplininina dominate don’t. Maybe both will enable a successful keeve but Zygosaccharomyces rouxii wouldn’t. While you may believe you are conducting a wild ferment, you may actually be inoculating your juice with Saccharomyces strains because your press cloth is only rinsed each year. Keeving can occur naturally without the addition of PME and calcium. Adding these compounds seems to help but they don’t seem to be the common link. The link that I see is yeast.
I have recently had success creating a cider with residual sweetness following some of the keeving principles. I inoculated a juice clarified with pectic enzymes and cold crashing with Pichia kluyveri. The juice was relatively clear and I added peels to the fermenter. No cap ever formed ane the juice never became turbid. It remained relatively clear because the Pichia yeast flocculates and while it does move around, it stays bunched together. After the specific gravity dropped from 1.055 to 1.017, I racked the cider into a keg and continued to monitor the gravity with my Tilt. I then let it drop to 1.011 before racking again, force carbonating, and bottling. The result is a very clear cider that seems to be stuck around 1.010. Time will tell of the ultimate success, but it highlights the importance of inoculation of yeast, even in a non-sterile must. Keeving isn’t a matter of whether the cider gods are blessing you, it’s about whether you are inoculating with the right yeast. I think hidden within many wild ferments are non-Saccharomyces yeasts that make keeving or a similar processes capable of creating cider with some residual sweetness. It’s another reason you really should be inoculating your juice with yeast. It gives you the ability to control it and help you create the best cider possible.
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I am told that in France keeving can be effected without ambient fermentation by slowly bubbling inert gas through the must.
So the polymers and coagulates in the juice seem sufficient….but your idea of wild yeast acting as a binder is very interesting.
It might be informative to calculate the biomass of yeast early in fermentation to try and see if that makes mechanical sense.
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