When you see a coffee roaster spending upwards of $100,000 for a laser diffraction particle size analyzer, it becomes clear that science plays a pretty important role in the processing of the worlds second largest commodity.
Although science is also involved in the growing and preliminary processing of coffee, this article starts with parchment coffee, a wet processed coffee shipped with the dry parchment skin still adhering to the bean.
This is done to help preserve the integrity of the bean during transit to the roaster, waiting until the last possible minute to uncover it's aromatic splendor.
MOISTURE
Parchment beans have a desirable target moisture range relative to certain quality parameters, generally i the 11% range, but perhaps just as importantly the moisture level has a direct economic impact. The amount of water in a commodity that is sold by the pound, can turn in to a substantial cost or savings, depending on which side of the fence you're sitting on.
Testing for specific types of moisture gives valuable information on shelf life as well as how susceptable a crop is to
microbial growth, such as mycotoxins.
Relative Humidity Sensors provide what is called a Water Activity Reading.Simply put, water activity is the measure of free or available water in a sample, which is the water that therefore becomes available for microbial growth, and reduces shelf life.
There are also the indirect but necessary costs, involved in keeping moisture at ideal levels using dryers and other equipment.
So needless to say, determining moisture levels in parchment coffee, would be a good place to start the science trail.
After separating the parchment from the bean, the coffee has to be dried again, the target being the optimum moisture level for roasting. This will be a subjective value, normally in the 12% range, having to do with the specific profile of the roast. Other variables will
the size and density of the bean.
After the green coffee beans are dried to the appropriate moisture level for roasting, they are carefully roasted until most of the residual moisture has been cooked off, often down to 2% to 3% , down from 12%. The longer the green beans are roasted the darker they get. A color chart or color analyzer is then used to help determine the end point of the roast profile.
It should be noted that other changes in the bean occur during roasting, like pore size, which has direct impact on the ability of oils to escape to the surface of the bean, much the same way free moisture does. Roasting for shorter periods at high temperatures, has been noted to produce greater pore generation than roasting at lower temperatures for longer periods of time. All of these changes result in different variations and shades of colors, which can be measured by one of many different methods to read color, or at least attach a unique numerical value to each variance. The most recent development in color analysis is the Color Track Analyzer
Roasted coffee often ends up at around 2-3% moisture. Some roasters retail the coffee at this ending moisture percentage level with the feeling that it will have a longer shelf life, and be fresher.
Others wet quench not only to help stop the roasting process, but in some cases to add moisture back to the bean, where once again it is sold by the pound.
GRIND SIZE
It has been suggested that a 5% moisture level is ideal for grinding.
(Independant test by QAQC Lab showed
no noteable difference between samples at 2.5% moisture and 4.5% moisture)
In order to gauge whether or not the grinder and coffee are producing the correct size distribution of grinds for the optimal cup of coffee, one needs to conduct a particle size distribution test.
The size of the coffee grind, the temperature of the water, and the length of time the water comes in to contact with the grind. are the key variables now involved in producing a superb, finished cup of coffee
As a rule, the smaller the grind, the less time it needs to be in contact with hot water to produce a fluid with the desirable amount of extractable solids.
Expensive inline systems like laser diffraction particle size analyzers are used by coffee companies that grind coffee in a central location, and then package, but particle size analysis using sieves and sieve shakers, is still the most common method, especially where coffee is ground closer to the brewing time for freshness.
Below is a chart with the most common aperture sizes and sample parameters relative to grind distribution.
In theory, if you could grind a roasted coffee sample so that all of its' particles were the same size, you would then be able to have greater control over the amount of extractable solids, and produce a more consistent cup of coffee. It is practically impossible for a grinder to be so exact, so there ends up being a sort of bell curve as to how the grinds end up being sized, although some roasters use sieving to physically separate out the fines prior to brewing.
It has also been noted that perhaps the most important grind size consideration, is to ensure the fines (Amount passing #30 sieve) , do not exceed 20%
There are also different schools of thought as to what grind distributions might produce the best cup of coffee. Some say that by taking the grinds out of one sieve in particular, you end up with less variance, therefore greater control over the finished product. Others say, that mixing the different grind sizes results in a better tasting and more complex cup of coffee.
The end results are largely subjective, but having the right testing equipment gives you a foundation towards repeatability. The more variables you define and control, the more consistent the end product, and consistency is the most necessary ingredient in effective branding.
Lastly, to ensure that the optimal extractable solids level has been attained, a solids test is performed using a simple TDS Meter, or by loss on drying, using the dry weight capabilities of an infra-red or halogen bulb moisture balance, or indirectly by using a refractometer.
As it turns out, there may be a reason why you can't get your coffee machine to replicate the coffee of your favorite retailer.
CONNECTING THE DOTS WITH COFFEE SCIENCE
Particle Size and Moisture Analysis in Coffee (cont)
Continued below
As with most laboratory testing instruments, operator time, training, and errors, now become more of an issue.
Most moisture analysis is at least initially based on a simple laboratory oven, whereby the sample is heated at a low temperature over a few hours, until the free moisture has evaporated. The ending weight is subtracted from the beginning weight, the difference being the amount of moisture in the sample.
Because of the prohibitive amount of time involved in using an oven for moisture analysis, other methods have been developed, the simplest being a moisture balance, which uses heat lamps and a sophisticated balance to automatically determine the endpoint, which is defined by the decline in the rate of drying, as the critical moisture point is neared, where the line between bound and unbound moisture exists, generally in less than 5 minutes.
A GOOD MOISTURE BALANCE IS AN INVALUABLE AID IN ANY QUALITY CONTROL LAB
Still, care has to be taken in ensuring that the sample is not burned and volatiles are not inadvertantly driven off.Moisture balances are also relatively expensive, ranging from $2,000 to $5,000.
Resistance or capacitance based meters, as well as other indirect methods like Karl Fischer, utilize the electrical properties of water to provide a quantitative analysis, although it should be noted that these methods provide information on total moisture, compared to residual or free moisture.
RESISTANCE BASED MOISTURE ANLYZER
Capacitance based moisture analyzers like the COFFEE PRO Moisture-Mac, offer great speed (10 Seconds), accuracy, reliability, and handle
larger sample sizes.
The coffee beans are dried in preparation for roasting either on a patio, or in a dryer. An initial sample will be dried, sometimes with the help of a sample fluid bed dryer, reducing the drying time from days to minutes.
This density cylinder measures out exactly 1/3 liter, which provides a fixed volume for use in calculating bean density with the SInar AP 6060 Moisture Analyzer.
Generally, larger coffee beans are thought to be the best. The beans are sized in 64th inch increments, starting with 8/64th" up to 20/64tth", using perforated plate sieves, and ideally a sieve shaker.
PERFORATED PLATE BEAN AND GRAIN SIZE GRADING SIEVES
to determine the presence of mycotocins in commodities like coffee.
COFFEE BEAN GRADING
Besides bean color, moisture, and water activity, coffee beans are categorized by their size, as well
as well as their density. Density is simply a measure of the weight divided by the volume. It is an indicator of overall bean quality, as well as having important implications in shipping by container.
NEW LASER TECHNOLOGY PROVIDES NEVER BEFORE SEEN ACCURACY AND REPEATIBILITY IN COLOR AND SHADE DETERMINATION
THIS COMBINATION PH,TEMPERATURE,TDS METER, IS A USEFULT TOOL
FOR COFFEE LABS
A BRIX METER GIVES INDIRECT MEASUREMENT OF DISSOLVED SOLIDS, BUT REQUIRES EXTENSIVE CALIBRATION WORK
