Coffee Bean Grading has to do with a number of characteristics, one of the most important being size. Not only are larger beans generally more desirable, but bean size , density, and moisture, are all factors that must be taken in to consideration when establishing roast profiles
COFFEE BEAN SIZE GRADING SIEVES
Bean Sizing Sieves are used to separate the coffee beans by 64th of an inch increments, starting with 8/64ths, all the way up to 20/64ths of an inch, or more. Different countries use somewhat different size classifications, and there are different methods available for green coffee grading. The SCAA METHOD and the BRAZILLIAN / NEW YORK Method are both in practice today. (SEE FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS) for more information.
Although the millimeter equivelants on this popular chart, aren't exact, it fortunately doesn't have any real impact on the size analysis. The New York Grading Chart
has exact conversions, but both result in the
same distribution because the tolerences are too small to impact 64th inch seperations.
"Some say there is an art to great coffee. I don't care how artistic you are; there are too many factors in play. You need the technology."
Forbes Magazine, Bob Stiller Green Mountain Coffee
Sieve Shakers are used to agitate the nest of sieves so that the grinds or beans fall through for sizing purposes.This miniature 100 mm DIAMETER Sieve Shaker comes in a handy carrying case, for transport to field locations, where grind or bean analysis can be done on site.
Grind analysis is a key component to producing a.good cup of coffee. Coffee companies spend tens of thousands of dollars on laser systems or less expensive sieving equipment to help analyze particle size and distribution, but why?
Grind distribution affects the extraction rate. Although there is no "correct distribution" this chart shows popular grind distributions for different grinds.
When performing a grind analysis, it is important to use USA Standard, or ISO / ASTM Mesh Designations, especially when dealing internationally. Some companies as well as countries have their own mesh designations, but ASTM (American Society for Testing Materials) introduced standardized designations in the early 1900's to help eliminate confusion.
#12 1.70mm
#16 1.18mm
#20 850 microns
#30 600 microns
#40 425 microns
#50 300 microns
Even size distribution assures consistent extraction rate. Target extractions of dissolved solids should be 18% to 22%
The smaller the coffee particle, the less contact time required to achieve optimal extraction. Water contact times vary between 1 to 4 minutes.Under extraction makes for a weak coffee. Over extraction releases undesireable chemical compunds that result in negative taste quailites like bitterness.
Regular grind size analysis provides important information on grinder settings and grinder maintenance requirements.
TEST 1- In order to verify that a coffee grinder is operating to specification, take a representative sample
of ground coffee, from a freshly calibrated grinder with new burrs, and run the sample through the sieve stack. Because you are just establishing a benchmark, it isn't really necessary to reach the endpoint here, but whatever time frame you use, it must be long enough to show a difference if the grinder is experiencing problems, and short enough to let you know in time to do something about it. We can't really recommend a time frame, because it depends on the performance characteristics of the sieve shaker you have. Use this
sieve shaker amplitude gauge, to catalogue / verify your sieve shakers performance.
TEST 2- If you took one roasted coffee bean and split it in to two pieces, and soaked it in hot water
for 2 minutes, and then took another bean, and ground it in to 1,000 pieces, and performed the same test, you would find a dramatic difference in the end result. That is because the smaller the grind size, the more exposed surface area, the less contact time is required to achieve the desired extraction.
So for each different type of grind, whether it's Espresso, or Automatic Drip, there are optimal
particle size distribution parameters, and water contact times to achieve desired extraction rates.
Luckily someone took the time to painstakingly document the effects of this phenomenon, and provide at least a good starting point. The below chart has been revised with ASTM Sieve Designations.
NOTE: DUE TO THE MORE ANALYTICAL NATURE OF THIS TEST, IT IS IMPORTANT TO REACH THE SIEVING ENDPOINT, WHICH IS DEFINED AS THE POINT WHERE THE AMOUNT OF SAMPLE RETAINED ON ANY ONE SIEVE CHANGES LESS THAN 1% (.1% FOR AUDITS) WITH ONE ADDITIONAL MINUTE OF SIEVING.
