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-   -   Petri Dishes (http://www.homebrewtalk.com/f163/petri-dishes-221312/)

ChickenHops 01-29-2011 12:51 AM

Petri Dishes
I decided to try propagating yeast earlier this month. I made my first wort based agar mixture. I made up some slants. I had some agar left over so I made up three petri dishes. I've yet to inoculate the slants. They all look great. No sign of anything growing. The petri dishes are another story.

I streaked one plate from the cake off the bottom of a Brown Ale that I racked last week (WLP001). I let them sit for about a week. Images of the results are below.

There was a little moisture in the dish that I streaked, but otherwise things look fine. It looks a little rough perhaps but this was my first attempt at streaking.


The next dish was interesting because I found one colony growing. I hadn't streaked or inoculated this plate. I poured the wort into the plate after the agar and the dish came out of the pressure cooker (as opposed to putting the agar in the dish and then heating). How a yeast cell, and only a yeast cell, got there, I have no idea.


The day after pouring agar I broke the lid of the third petri dish. I put another lid on it, just to see what might happen after it was exposed to open, unfiltered air for about 20 minutes. My lab is in a room far away from any brewing or food handling area. I would consider it quite clean. Just goes to show that little bugs are everywhere. I have no clue what this nasty looking colony is. If anyone can identify it I'd like to know what it might be.


Again, the slants look fine. I'm going to inoculate them this weekend (close to an alcohol lamp). Also, I hope the images show up. This is my first post on HBT.

BlindOwl 01-29-2011 01:04 AM

The third dish you mention is likely black mold, really common around bathroom areas. The second one is definitely mold as well though it looks like it hasn't sporulated like the third yet. They are definitely not yeast though. The streaking on the first looks good with no mold or anything so should be good to go. Welcome to yeast culturing!

ChickenHops 01-29-2011 01:27 AM

Thanks for the reply, BlindOwl. I appreciate knowing what the organisms likely are. It gives me an even greater appreciation of the importance of sanitation in brewing.

mvnegassi 01-30-2011 02:28 AM

That first plate is beautiful.

Strange though that you've got stuff on the second two. I can keep my un-inoculated plates for months with no problem. I even take off the tops and drain out condensations once in awhile. Do you work with a flame nearby to create an updraft?

ChickenHops 01-30-2011 03:19 PM


Originally Posted by mvnegassi (Post 2596962)
That first plate is beautiful.

Strange though that you've got stuff on the second two. I can keep my un-inoculated plates for months with no problem. I even take off the tops and drain out condensations once in awhile. Do you work with a flame nearby to create an updraft?

No bunsen burner or alcohol lamp. I used up a candle on the slants. So no flame or updraft with the dishes. Furnace may also have been running. I didn't close all vents, although I have now.

Here's how the slants look.


Admittedly, I used sloppy techniques on the dishes. I made them as an afterthought with excess agar, after preparing slants. I'm putting together a small lab area in the corner of a den/office and this was my first attempt. I didn't have all the equipment yet.

Propagating yeast is like all other aspects of brewing. You make mistakes. You learn. You adjust and hopefully improve. Little things can make a difference. Although after 10 batches I've never had an infection in beer (that I know of).

El_Exorcisto 01-30-2011 03:41 PM

Keep your dishes upside down to keep condensation from drowning your colonies. The streaks look good on the first.

riromero 01-30-2011 03:58 PM

Nice. My slants are all scattered here and there. Out of curiosity: How many slants of one yeast strain to you make?

ChickenHops 01-30-2011 06:55 PM

These are my first slants and I haven't inoculated any yet. I'll probably make 4 from each new yeast pak/vial that I open.

I'll make 4 from the streaked plate. The streaks were from the trub of an 1.060 OG Tumbler Clone, fermented with yeast washed from a 1.052 OG PA (WLP001). Both fermented down extremely well and both ales are excellent.

I don't have a convenient LHBS where I can get yeast. Further, I'm only brewing once a month -- different beers using different yeasts. So there's a long lag time between using the same yeast. I suppose I could keep using washed yeast kept in jars, but I understand that slants have a much longer shelf life. I don't mind building up a starters from slants. I equipped my "lab" for that very purpose (i.e. properly sized glassware, stirplate, etc.).

I learned both from this forum and then from actual brewing that an adequate starter of healthy yeast is important for making good beer. In my first half dozen batches I didn't pitch nearly enough yeast. The beers still turned out, but pitching more yeast has resulted in even better beer.

BTW, make sure that whatever you put in a pressure cooker can handle the heat. This tube rack was supposed withstand 250 F. Perhaps 251 F was just too much. The slants were OK, but difficult to remove. :)


DrawTap88 01-31-2011 01:17 PM

Good job so far. The only thing that I would recommend is streaking a second plate, without reinocculating your loop, so you get better separation of each colony. That will allow you to make sure that 1) you're only getting the isolated yeast you're trying for, 2) you can see if any of the colonies are also growing with mold or bacteria, and 3) colonies growing that close together and in colonies that small could be petite mutants.

If you haven't already checked out First Steps in Yeast Culture or Yeast, I suggest you do, as they are great refrences to slanting and yeast health.

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