Non-Saccharomyces Yeast: Complexity & Sweetness

This is the second article in my series on non-Saccharomyces yeast. In the first, I challenged the concept that Saccharomyces yeast is ideal for cider. While yeast is a critical element that defines the essence of cider, I asserted the view that we needed to break away from our current beliefs about yeasts. I proposed the concept that Saccharomyces cerevisiae could be a spoilage yeast for cider and introduced the idea that non-Saccharomyces yeast may be the best yeast for making hard cider. This is because non-Saccharomyces yeasts can create complex aromas and natural residual sweetness, two key features that I think most cider makers seek and that I think improve cider quality. These are also characteristics that Saccharomyces cerevisiae yeast can struggle to create in cider(1). Let’s explore how non-Saccharomyces can produce high quality ciders by creating complex aromas and natural sweetness compared to Saccharomyces.

Aroma Complexity

The Source of Aroma Compounds In Hard Cider

Cider aroma is usually concentrated in two key area: esters and higher alcohols (also called fusel alcohols). Therefore, we might think that we should use the yeast that creates the most esters and higher alcohols. We would review various research papers and conclude that Saccharomyces cerevisiae usually creates the most esters so it must be the best yeast to use. But, aromas are not straightforward. First, research shows that aromas are not binary, meaning that aromas don’t work independent of each other(2). The higher number of different compounds present, the more variance you have in the results. Also, aromas do not need to reach a threshold level to have an impact. Aromas mix and can suppress or enhance the final characteristics of a cider. For example, aging cider with oak will impart the compound vanillin, which smells of vanilla. However, how strong that vanilla flavor is will depend on the other compounds in the cider. Research has also shown that the threshold level of a compound increases as other compounds increase. Highly complex ciders and wines require more of any specific compound to be present for it to be noticed. Also, the minutia of compounds below the threshold can enhance or detract from aromas above the threshold limit. This highlights an important factor. It’s not just the amount of an aroma compound created by the yeast during fermentation but also the breadth of aromas. Saccharomyces cerevisiae may create a higher volume of esters but, if there aren’t a multitude of compounds created, the cider can be one dimensional. Complexity requires breadth of compounds versus the magnitude of a compound. I find that most commercial Saccharomyces yeast strains produce a one dimensional cider. One of the key reasons is because they dominate the fermentation. This usually occurs when you inoculate the juice with a Saccharomyces yeast as other yeast are killed or suppressed.

Allowing a wild fermentation to occur usually means multiple types of yeast are initially competing and contributing to the complexity of the aroma(4). As we have discussed, these other yeasts are non-Saccharomyces strains. They are the yeast found on the fruit and in the orchard. My research and own experiments using non-Saccharomyces yeast have shown that they each have unique characteristics. Lachancea thermotolarens creates fruity but sour notes, Pichia kluyveri creates tropical fruit aromas, and Candida zemplinina add smoky and spicy characteristics. These are just a few of the multitude of yeasts that are naturally found on fruit, which highlights one of the challenges of using wild non-Saccharomyces yeast. A wild ferment will have a multitude of yeasts and aromas but, not all are necessarily desirable. For example, some can create rotten or wet aromas or even worse. While a little of these undesirable aromas can actually be positive, there is a tipping point where they detract from a cider. Fusel alcohols are similar. A little fusel alcohol can go a long way. The more you create, the more your cider can start tasting like cleaning solvent, nail polish remover, or simply of alcohol. Controlling the yeast that will dominate the ferment is desirable.

Inoculating the yeast that will ferment is a way to control the aromas of your cider. However, inoculating with Saccharomyces usually creates another problem. Apple juice has a low sugar level when compared to wine. When comparing to beer, the sugars in apples are not very complex. Inoculating with Saccharomyces yeast is like throwing gasoline on a fire. This highlights the another challenge that Saccharomyces strains can cause, which is the rate of fermentation. You might hear people recommending cool fermentation temperatures because it produces a more aromatic cider. Temperature can directly impact the type of aromas produced but, more importantly, it impacts the amount of aromas produced, usually in the form of esters. Surprisingly, research has shown that cooler ferments produce fewer esters. This is counter to the idea that a cooler fermentation would produce a more aromatic cider. However, cooler ferments are slower fermentations and slower fermentations means slower release of CO2, which impact volatile compounds like esters.

Aromas are volatile compounds, which means they are easily dispersed or released. Guess what happens when a fermentation is vigorous and releases a lot of CO2. It also releases a lot of volatile aromatic compounds. A short fast fermentation will result in a cider that is not as aromatic because it will lose more of its aromas than a slow fermentation. Saccharomyces cerevisiae fermenting apple juice can complete fermentation in days. Is there any wonder why many Saccharomyces fermentations are not complex. The other interesting aspect of Saccharomyces yeast is that most don’t like cooler temperatures. They perform well at warmer temperatures but when you have as little sugar as apple juice, I think the gasoline on a fire analogy is appropriate. Guess what the fermentation characteristics of most non-Saccharomyces yeast are. First, most are more tolerant to cold temperatures so you can ferment cool or warm. Second most are just slower fermenters. There pathways and processes don’t appear to be as efficient as Saccharomyces cerevisiae. That means that even if you are like me and have to ferment in a warm climate, 70-74F/21-22C, using non-Saccharomyces yeasts will be less vigorous and take more time. I have found the non-Saccharomyces strains taking 1.5 to 4 times as long to ferment. That may not be as desirable if you are trying to get more product out the door but, it is very desirable if you want to retain as many of those complex volatile aroma compounds as possible.

Natural Sweetness

Glycerol & Non-Saccharomyces Yeast

I have yet to find a cider maker that didn’t desire to have a method for creating natural sweetness in cider. I am not talking sugary sweet with 20-30 points (1.020-1.030) of gravity left. Though, we should remember that even 20 points of sugar can be masked by high levels of acids. I am talking about a method that will leave something with 10 or less points of sugar and maybe even still be able to produce CO2 for bottle conditioning. Sounds impossible I know but, I am starting to find that it isn’t. The reason is because there are naturally occurring compounds that produce sweetness. Some of these, like sorbitol, we know. It’s a compound commonly found at higher levels in pears than apples. This is why I often recommend adding 10-15% pear juice to a blend. It can give it a few extra points of residual sweetness naturally. However, there are other compounds that do the same thing. One of these compounds is glycerol. It’s a viscous compound that improves mouthfeel and sweetness. When we compare Saccharomyces and non-Saccharomyces yeast, which do you think produces more glycerol? If you said non-Saccharomyces, you win the prize. For example, Lachancea thermotolarens has been shown to produce significantly more glycerol than Saccharomyces strains(4). It does this because it’s pathway to produce ethanol is less efficient. Instead of producing ethanol, it diverts to a pathway that produces glycerol. Higher glycerol is produced and lower ethanol. But it may not just be higher levels of polyols like glycerol or sorbitol creating the residual sweetness.

I have found that many non-Saccharomyces strains appear to stop fermenting with residual sugars present. I now have 25 fermentations and growing with non-Saccharomyces strains. I have been able to create several with a specific gravity above 1.004 and even a couple above 1.010. These have all been stable without filtering, pasteurizing, or addition of chemical preservatives (sulfite and sorbate). The ones above 1.010 came from juice with a naturally high starting gravity (above 1.080). I have also had some that initially appeared to have stopped between 1.010 and 1.020 but, after aging for several months, dropped further. Some of these went to 1.000 while others were in the 1.004-1.006 range. All of my fermentations so far have been with raw, untreated, and freshly pressed juice. That means there were natural yeasts present and most likely some Saccharomyces cerevisiae “spoilage” yeast from my environment. However, all the cider took longer, smelled better, and often appeared to stop with some residual sweetness. Some strains, like Pichia kluyveri, always seemed to go dry, 1.000-1.002. However, I have rarely experienced a gravity reading below 1.000 with any non-Saccharomyces strains, which would indicate higher probable levels of compounds like glycerol.

While further research and experiments are needed, this data supports the research papers that I have read on non-Saccharomyces strain(1,4). Non-Saccharomyces strains can be less alcohol tolerant, which is why most research recommends co-inoculation with Saccharomyces. Why? Because 95% of the research papers are catering to the wine industry and with high levels of sugars, non-Saccharomyces strains would not ferment grape juice to completion or even close in many cases. But, apple juice usually has half the amount of sugar that grape juice has so most non-Saccharomyces yeasts are capable of fermenting apple juice to dry or almost dry levels. If we are looking for a few extra points of residual sugar, non-Saccharomyces strains offer the ability to provide this by creating more polyols or by sticking a fermentation. Even if Saccharomyces strains from my processing equipment are present in my juice, I was still able to naturally produce ciders with higher levels of residual sugars. That occurred because the non-Saccharomyces strains exhausted the available nutrients, produced higher levels of glycerol, or acted as killer strains. Either way, it highlights the ability of non-Saccharomyces strains to create ciders with some level of natural residual sweetness even in non-sterile juice.

Hopefully, you are starting to understand my assertion that Saccharomyces yeasts should be considered a spoilage yeast and that non-Saccharomyces yeasts offer a better way to produce complex aromas and residual sweetness. The challenge we face is how to assert some control over the fermentation process that wild ferments don’t offer. We will need to explore the concept of inoculating for control, which will be the next article in this series.

(1) J. Wei be associates, Characterization and screening of non-Saccharomyces yeasts used to produce fragrant cider, LWT – Food Science and Technology, 107, 191–198, 2019

(2) L. culleré and associates, Gas Chromatography−Olfactometry and Chemical Quantitative Study of the Aroma of Six Premium Quality Spanish Aged Red Wines, J. Agric. Food Chem., Vol. 52, No. 6, 2004

(3) W.F. Morrissey and associates, The role of indigenous yeasts in traditional Irish cider fermentations, Journal of Applied Microbiology, 97, 647–655, 2004

(4) S. Benito, The impacts of Lachancea thermotolerans yeast strains on winemaking, Appl Microbiol Biotechnol., Appl Microbiol Biotechnol., Aug, 102, 2018

Understanding how yeast create great cider will help you make better cider. Knowledge and sharing it is why I wrote my book, launched this website, and provide products and recommendations on The Shop page. It is why I started offering non-Saccharomyces yeast strains in the Cider Yeast section of the shop. If you are interested in supporting, check out the shop. As with everything, my goal even with the shop is to help you make better cider.

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