The effects of temperature on grinding

One of the most elusive factors in any aspect of coffee making is consistency. In fact, it is arguable that at the heart of just about every new technology and method for making coffee better is the quest for consistent results. In order to achieve consistency in coffee making, however, one must first have a complete understanding of all of the variables at play in a particular process – such as grinding – and how changes in those variables affect the result. It was this desire to achieve some clarity about a stage in the coffee-making process that is largely shrouded in uncertainty – and not a little folklore – that a team of 11 coffee professionals and career scientists set out to examine the effect of bean origin and temperature on grinding coffee. Leading the team was Australian scientist, Dr Christopher Hendon, a coffee enthusiast who works in the Department of Chemistry at Massachusetts Institute of Technology in the US. Also on the team was one of Australia’s best-known baristas – 2012 World Brewers Cup Champion, partner at Australian specialty coffee roaster, Sensory Lab, and creator of the Barista Hustle website – Matt Perger. Hendon and his team carried out a series of experiments to determine how bean origin, processing method, roast level and temperature can affect the results of grinding. According to Hendon, the idea for the experiments came about as a result of an argument about the topic in that most democratic of scientific forums – Twitter. “Both sides had very little data to support their postulations,” Hendon tells Global Coffee Report, leading the coffee-obsessed scientist to embark on his quest to find some definitive answers. As a result, a team of volunteers from both the science and coffee worlds were assembled to get to the bottom of the matter. Perger’s role came in the form of lending a coffee professional’s know-how to the design of the experiment. However, Perger’s cynicism about much of the received wisdom that influences the thinking of many in the coffee industry was key to his enthusiasm for the subject. “What was most important to me was making sure that we removed any coffee dogma from the thinking that went into this so that we could get the best, most scientifically accurate results,” Perger says. “It is easy for people to go into these things with something of a ‘coffee mindset’ that comes with a lot of preconceived beliefs that are flawed from a scientific perspective, so we needed to make sure that was not the case here.” The team sought their answers through two main tests. First, they ground coffee of different origins, roasts, and processes at the same setting and measured the resulting grind samples. Then they changed the temperature of one coffee to four distinct levels, ground a sample while it was still at each temperature, and measured the resulting grind samples. In order to judge the results of the experiments, the team examined the particle size distribution resulting from each grind. “We assumed that the most important metric for measuring ground coffee is the particle size distribution,” Perger says. “That is, a measurement of the diameter of every single coffee grind in a sample. According to the study itself, which was published on Nature magazine’s subsidiary, Scientific Reports, “[particle size] is of critical importance in coffee brewing because variable accessible surface area causes the small particles to extract more rapidly relative to larger ones. As a result, brewing coffee is challenging with variable particle size, especially in espresso-style pressurised brews, where packing effects become important.” Perger says that method of measurement was chosen over flavour because, while flavour is important, it’s much more difficult to objectively quantify.  Also, the study was probing a material property, rather than its flavoursome characteristics. In order to measure the particle distribution, the team used a laser diffraction particle size analyser (LDPSA). Hendon says that the team set up the LDPSA inside a the Colonna and Smalls café in the UK during normal operating hours to get results that most accurately reflected those that would come from a normal coffee-making environment.  This was something of a departure for Hendon, who is not usually so closely involved in physical experiments. “I am usually a theoretical chemist,” Hendon tells GCR, “[so] it was peculiar for me to even touch the equipment myself, given how detached I typically am from experimental sciences.” Ultimately, while the team found that the differences in origin, processing and roast of the coffee had very little influence on the particle size distribution, the influence of differences in temperature was a whole other story. “Manufacturers place emphasis on temperature stability because it is at least experimentally known that temperature fluctuations cause grind size fluctuations,” Hendon says. “We shed light two-fold on the problem: not only is cooler coffee grinding in a mathematically more elegant manner, but the cooling of the burrs should yield a similar result to cooling the beans.  “More importantly, I think its worthwhile noting that cooling is an absorption process whilst heating is ambiently radient,” Hendon adds. “Hence, heating is intrinsically poorly conceived given that the more you grind the hotter you go, making it predictable but dependent on the volume of grinding and time between shots.  Cooling gives much better capability for temperature control.” While the best results were achieved with samples of frozen coffee, the experiments found that the results started to become markedly less consistent at around 20 degrees Celsius. And it is this finding, says Perger that is likely to be the key takeaway for café owners and baristas. “They don’t necessarily need to be held in a temperature-controlled environment – though that would help – what is most important is keeping them below that critical temperature [20 degrees], which should not be too hard in most cafés, so long as it’s not an extremely hot day,” he tells GCR. While coffee perfectionists such as Perger and other baristas working in the specialty field will get the best results from their coffee by keeping weighed doses of beans stored in the freezer – assuming the beans are packed in a H2O-free environment – this research shows that in order to improve consistency, everyone should be thinking about how to keep the temperature of their beans down. The easiest way to achieve this, Perger says, is to stop holding beans in the chute of the grinder. “Any grinder that holds coffee in a chute before it’s ground is flawed because they are bound to heat up in there,” he says. “The future is in grinders that hold the beans in a separate climate-controlled chamber and drop them into the grinding chute per dose.” For a dedicated coffee nerd such as Perger, the study also yielded some interesting findings about the role that fines play in achieving the perfect extraction, which can be read about in more detail on the Barista Hustle website. As for Hendon, this was not the first, and will certainly not be the last time that he focuses his scientific mind on the world of coffee. “My next papers will be on the effect of adding sugar to coffee, and separately the benefits of low pressure ‘espresso’,” he tells GCR. Stay tuned. GCR

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