Yeast Harvesting: Plates & Slants

Yeast Harvesting: Plates & Slants
Yeast Harvesting: Plates & Slants

Yeast and apples are the core ingredients of any cider, even natural/wild fermentations. Understanding your yeast, just like understanding your apples, is key to consistently making great craft hard cider. The yeast will impact your residual sweetness, aromas, tastes, clarity, and many other aspects of your cider. Working in conjunction with the apples you use for your juice, yeast creates your cider. It’s important to understand the yeast and the other organisms (i.e. micro-flora) involved in the creation of your cider. Harvesting and culturing yeast is how you effectively manage your yeast, even if you are using natural fermentations. In a previous Mālus Trivium, we reviewed agar. Without agar, it would be very challenging to cultivate yeast and identify bacteria and other organisms. Agar is the substrate that includes various compounds that encourage, identify, or prohibit growth of various organisms. But, that agar has to be put into a container, usually what is called a plate, also known as a Petri dish, or a slant, which is a type of test tube. Let’s explore plates and slants and how you can use them in your harvesting and culturing activities.

Plates:

Plates are usually a thin plastic dish that is filled with different agar types. It will have a lid but is generally not considered airtight, even when sealed with paraffin tape. The lid is to prevent dust and other contaminants dropping on the agar surface. You may hear plates called Petri dishes or Petri plates. Agar and Petri dishes share a history. Julius Petri worked in Robert Koch’s laboratory and was part of the team that “discovered” how agar could be used as a substrate for the isolation of micro-organisms(1). Plates are the tool of choice when trying to isolate an organism, inspect for contamination, or assess the types of organism in the sample.

Samples can be taken in various ways. A plate can be set out letting airborne organisms float and drop on the agar in the plate. You would do this in your cidery if you suspect airborne contaminants. Samples can also be poured or spread on the agar substrate. After collection, the lid is applied and the plate is usually turned upside down and allowed to incubate. I always store and open my plates upside down to help prevent airborne contamination. Based on the agar used, the incubation period will show whether your sample is contaminated or has grown enough that you can select samples for cultivation and/or isolation. Because plates are not airtight, they don’t offer the best choice for long-term storage. You can improve this by sealing them with paraffin tape, which can extend their life for over a year. Storing them in a cold environment will also help. I use my garage refrigerator and a good paraffin tape wrap to help prolong the life of my plates.

Slants:

The other main container utilizing agar is a slant. Slants are really test tubes and they come in various sizes. You can have very small plastic slants like the one in my above graphic or larger glass or plastic test tube similar to how some White Labs yeast strains are shipped. I recently purchased some large pre-filled glass slants. The first question you might be asking is why are these called slants. They are called slants because when the agar is poured into the tube, it is leaned so that the agar is slanted in the tube. This creates more surface area of the agar compound, which aids the growth of the organisms placed in the slant. The next question is probably why wouldn’t you just use plates. If there is so little agar surface area that you want to slant it to create more, why not just use a plate. Remember plates aren’t airtight. Slants usually are. They normally have a screw cap that seals. Personally, I still use paraffin tape around the cap to help ensure it is sealed. This sealing allows slants to be used in different ways versus plates.

First, it helps prolong the life of the sample. While plates are the best tool for assessing and isolating specific organisms, slants are the best for storing and maintaining those samples. If you discover a yeast or even a combination of organisms that make a great cider, you should create a slant of it so each season, you can propagate a fresh batch for use in your cider making. Slants will easily last a year and often more. Just remember to create a fresh culture and new slant each season as well. Doing this will allow you to never run out of your favorite yeast or micro-flora. Remember that all yeast started as wild organisms and if you want to reproduce your own unique craft cider, harvesting and culturing that natural micro-flora is a best way to do it. The other benefit of being able to seal slants is that you can sterilize the slant using an autoclave or other means. Sterilizing allows you to ensure there are not any contaminants that might spoil your sample. You can’t do that easily with a plate.

Does the thought of harvesting and cultivating yeast intimidate you? Don’t let it. You don’t need a clean room and a lot of expensive equipment to work with yeast, especially for home cider makers. You can buy your own agar and pour your own plates and slants but you can also buy pre-made plates and slants. There is a lot you can do with some pre-made plates and slants along with some sterile loops, paraffin tape, and food grade sanitizer. All of these are available from the internet, Amazon and wine/beer supply houses, or your local home brew shop. Exploring yeast and how they impact your cider is an relatively easy next step in your cider making journey. Checkout some of the articles below or search yeast on PricklyCider.com and find even more.


(1) S. Shuzhen, A Brief History Of Agar, The Bug Report, http://www.asianscientist.com/2016/01/columns/history-agar-microbiology-lab/, 2016


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Want more details about making and enjoying cider, check out these posts.

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