Top cropping is a method of harvesting yeast from beer that is in the 2nd or 3rd day of active fermentation. Ale yeast are also known as top-fermenting yeast. During fermentation the unique shape of the ale yeast's surface allows it to attach to rising CO2 and travel to the beer's surface. Top cropping was the norm years ago. It is how breweries managed to reuse yeast for hundreds of years.
Today bottom collection is common practice. Shallow, wide and open is the optimal fermentor shape for top cropping and this shape also takes up the most space. You can fit a larger volume of beer in a 100,000 sq. ft. building using cylindrical shaped fermentors then flat shallow ones. There are some instances where the tall narrow shape of modern cylindrical fermentors prevented the yeast from forming enough of a head to successfully harvest enough yeast from the top. Top cropping is becoming more popular among craft and homebrewers. The yeast that rise to the surface after the first 12 hours of active fermentation, are the most viable, active and are relatively clean.
The shape of the fermentor dictates the ease of top-cropping. Fermentors with large top openings are much easier to harvest top yeast from than fermentors with small top openings. On open top fermentors, you can collect the yeast with a shovel, spoon, a cup as long as the methods are sanitary. On small top fermentors some sort of device must be used like a vacum, or pump or collecting via a special blow off tube contraption.
Below I will show you my method for top-cropping.
Step 1. Take 4 quart glass canning jars and place in pot along with lids and 1 large stainless steel spoon and tongs. Fill pot with distilled water until water is above top of the jars.
Step 2. Boil for 30 minutes.
Step 3. Place lid on jars leaving them full of water.
Step 4. Allow to cool to below 70f.
From this point on a spray bottle filled with star-san is our best friend. I spray everything. Jars, fermentor lids, spoon everything. I keep a piece of aluminum foil on the shelf by the fermentors to set everything on while working and I spray star-san on that as well. I even like to blast the air before I start opening fermentors.
Step 5. Take the lids off 2 of the jars and discard the water.
Step 6. Open fermentor and verify a nice thick krausen.
Step 7. Scoop out krausen placing it in the 2 empty jars (scrape back the scummy looking stuff, try taking the creamiest yeasties) I took about 10 scoops from the 3522 (ale pail) and about 30 scoops from the US-05 (10 from ale pail and 20 from my 20 gallon fermentor).
Step 8. Allow foam to settle then fill with water from the other 2 jars. Make sure you fill it to the brim. You will have to fill and let foam settle and then top it off. Place lids on jars loosely. Label with yeast name and the date.
Step 9. Refrigerate between 36f and 40f. Once chilled tighten lids.
I use Mr. Malty to estimate cell count. Use the "Repitching from Slurry" tab changing the yeast concentration setting to "Thick".
Also don't forget to practice good sanitation throughout the process.