The fourth of my “Your Home Yeast Lab Made Easy” series is now up on youtube. This video covers an advanced topic – streak plating. I was not planing on covering this topic so early in my video series, but I was streaking yeast for another project and decided to take advantage of the situation.
Streak plating is used any time you need to purify a strain of yeast (or other microorganism). This method allows you to pull single, genetically pure strains of a microorganism out of a mixed culture. This mixed culture could be any number of sources – an old strain of yeast whose characteristics are starting to drift, a contaminated batch of yeast, a mixed culture (e.g. those made by Wyeast, or from a bottle of sour beer), or even from a wild ferment you started using yeast from your back yard!
The principal of streak plating is simple – a small amount of the source yeast is spread across an edge of a petri dish. The yeast/bacteria/etc concentration in this streak are at a high density, so you swipe a sterilized loop across the first set of streaks, picking up a small number of the organisms, and then streak over another portion of the plate. You repeat this two more times, each time diluting out the yeast/etc further.