If you make enough hard cider, you will inevitably start to explore yeast. Whether you are using commercial strains or just letting nature run its course, yeast is such a critical component in making great craft cider. It impacts your residual sweetness, aromas, tastes, clarity, and many other aspects of your cider. Working in conjunction with the compounds in your juice, it’s what creates your cider. Wanting to understand more about yeast is important. This desire to understand usually leads to wanting to harvest and manage the yeast you are using. Harvesting and maintaining yeast is even important if you want a natural or wild fermentation. Natural ferments are blends of yeast and bacteria that are unique to your process. This process includes your apples, your equipment, and even your physical location, which all contribute to the yeast and bacteria micro-flora found in your cider. This micro-flora can change each year so if you want to reproduce it, you have to cultivate it. That means you need to expand your cider making skills into yeast harvesting and culturing. Let’s explore some of the basic tools and terms you need to harvest and culture yeast. One of the most important parts of harvesting and culturing yeast is agar.
Agar is a gelatinous compound formed from polysaccharides. It is used to fill plates and slants, which are used to identify, culture, and store yeast and bacteria samples effectively. Without agar, it would be difficult to utilize yeast in the ways we do because we couldn’t isolate and grow colonies easily. Some interesting background on agar is that it is actually a food source in Asia and comes from algae(1). Japan was the largest producer until World War II. The algae used to make agar is not farmed but requires harvesting or collection from beaches. As you might imagine, like all things harvested from the earth, there are sustainability concerns with it. While gelatinous, agar is not gelatin. It has a higher melting temperature and the polysaccharides that form it are not generally digestible by yeast and bacteria. This is how it came to become the substance used for plates and slants instead of animal derived gelatin and other substrates. Being more stable allows for the agar to be mixed with various compounds and used for numerous objectives. White and Zainasheff in their book, Yeast, provide an excellent example of how different agar compounds are created and used for isolating and culturing yeast. The following modified table is an example of some of these agar compounds and how they can be used to cultivate organisms and diagnose when your cider might have organisms you don’t want(2).
| Agar Compound | Description/Use |
|---|---|
| Yeast Peptone Dextrose (YPD or YEPD) | Used for general yeast cultivation consisting of nutrients and sugar |
| Wallerstein Differential (WLN) | Used for aerobic cultures to identify wild yeast, bacteria, and mold – contains cycloheximide that prevents S. cerevisiae growth |
| Lee’s Multi-Differential Agar (LMDA) | Used to isolate bacteria, usually for aerobic growth like acetic acid – acid producer change color |
| Hsu’s Lactobacillus and Pediococcus (HLP) | Used to identify anaerobic bacteria Lactobacillus and Pediococcus |
| Lin’s Cupric Sulfate Media (LCSM) | Used to detect wild non-Saccharomyces yeast – inhibits most S. cerevisiae growth |
| MacConkey Agar | Used to identify enteric bacteria like E. coli and salmonella and lactose fermenters |
You can purchase agar compounds and make your own plates and slants or you can acquire pre-poured versions. If you are just starting out, the pre-made versions are more expensive but quite convenient. They along with sterilized loops make the harvesting process much easier. You can see what plates and tools I use in The Shop. Make sure to explore my other yeast harvesting and cultivation articles for tips and information on how to propagate yeast.
(1) S. Shuzhen, A Brief History Of Agar, The Bug Report, http://www.asianscientist.com/2016/01/columns/history-agar-microbiology-lab/, 2016
(2) C. White and J. Zainasheff, Yeast The Practical Guide to Beer Fermentation, Brewers Association, 2010
Want to learn more about harvesting yeast? Checkout some of the articles below or search yeast on PricklyCider.com and find even more.
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