The lamp is used to create an updraft. The updraft helps to prevent dust from settling on the media while the lid is up for pouring. This technique does not apply to autoclaved plates because the dish is covered when the media is being sterilized. However, anyone who has worked with plates that were autoclaved in a pressure cooker instead of using the standard laboratory technique knows that the amount of condensation that forms inside of the dish is a major problem.
Here a couple of videos of how plates are poured in a laboratory without the aid of a laminar flow hood:
Two videos that demonstrate the entire process, including media preparation
Did you notice the use of a magnetic stirrer? That’s what magnetic stirrers were designed to do in a laboratory. Stir plates are for mixing, not culturing. Like storing rinsed cropped yeast under water, the almost universal use of stir plates in home brewing for culture propagation is yet another laboratory technique/device that has been introduced via the home brewing equivalent of the pass information around a circle game.
LB stands for Luria broth. It is a specific type of medium. I currently use MYGP, which stands for Malt extract, Yeast Extract, Glucose (dextrose), and Peptone (I use soytone). However, I used agar solidified wort for many years. One does not need to use a ton of malt extract to make malt agar plates. Two percent weight-by-volume (w/v) is enough malt extract. A 2% w/v malt extract solution contains 2 grams of malt extract per 100ml of solution. Media is solidified via the addition 1.5% w/v powdered agar (Green BioResearch on eBay sells 100 grams of lab grade agar powder for a reasonable price). In layman’s terms, 1.5% w/v is 1.5 grams of powdered agar per 100ml of solution.
Remember, the updraft zone around an alcohol lamp is smaller than that of a Bunsen burner; therefore, the plates need to be closer to the heat source while pouring. I would not pour more than 5 to 6 plates at a time unless there are many cultures to plate.
The video linked below demonstrates several basic aseptic transfer skills, including the 4-quadrant streak. The 4-quadrant streak is one of the most fundamental skills in microbiology. What it does dilute the culture to the point where single cells are separated on the plate. In the case of yeast, a round, dome-shaped, opaque, white colored, non-fuzzy colony can be assumed to be the offspring of a single yeast cell; therefore, it is a pure culture. Please note that the loop is flamed and the plate is turned 90 degrees between streaks.
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Finally, here is a video made by a home brewer in who is also a Ph.D. microbiologist. I exchanged a few e-mail messages with him a year or so ago because he has access to -80C refrigeration. He uses the old pipette a drop of liquid culture onto a piece of blotter paper that was autoclaved in aluminum foil technique to make cultures that can be mailed to other brewers in the Canadian equivalent of a first class envelope. The blotter paper is dropped into small amount autoclaved 5% w/v wort to revive the culture and then plated for singles. Once again, I cannot overemphasize that plating is a fundamental skill when working with yeast.
