Measuring the coffee roasting process

Historically, the art of roasting has been just that – an artistic ability dependant upon the expertise and experience of the roaster. Modern technological advancements have added some science to the art. on-line measuring tools allow roasters to better follow the time/ temperature curve, and replicate those curves to the exact degree. Despite all these technological advancements, however, the tools used to follow the roasting process haven’t gone much beyond what the senses can measure – colour, temperature, and time remain the main factors used in monitoring and reproducing roast profiles. Ongoing work at the university of Zurich, in conjunction with Buhler, is looking to revolutionise the process by opening up a whole new set of parameters. a group of researchers has been examining the potential of measuring the volatile organic compounds (vocs) in the off-gas of a drum roaster. They have successfully measured these compounds through the use of proton transfer reaction time-of-flight mass spectrometry (pts). Mass spectrometry is an analytical technique that ionizes chemical compounds to generate charged molecules. By measuring the mass-to-charge ratios, information about those molecules – in this case about the vocs – can be obtained and filtered to determine any flavour-relevant information. this work is proving that the roasting process can be observed and measured beyond what the senses can measure, using advanced technology to observe the transformation of the bean at the molecular level. Dr. Chahan Yeretzian is Head of Analytical and Physical Chemistry Zurich University of Applied Sciences (ZHAW). He worked with Buhler’s Head of Coffee Dr. Stefan Schenker on a study proving the ability to use PTS to measure off-gasses as an on-line monitoring tool in a study publish in 2011. With ongoing work in the field of roasting consistency, Yeretzian says the study continues to be highly applicable in its potential to cater for the modern demands of roasting operations. “Consistency is a major factor in the quality of roasting operations. And it’s particularly important in capsule manufacturing,” Yeretzian tells GCR. “Capsule companies that offer a range of products, some more fruity, some with darker roast, need to ensure they have the highest consistency in roasting operations to deliver what is being marketed.” Yeretzian says that in the roasting process, the majority of knowledge still lies with the roast masters who have experience in “sensing” the roast. From a research perspective, he says there is limited scientific work to support improving this space. “We’re trying to improve consistency, but we have nothing to compare our work against,” he says. “Three critical attributes of quality coffee are consistency, freshness and flavour profile. But defining the flavour profile is not that easy to do, especially from a scientific perspective.” As a closed system, capsules offer the potential for a consumer to experience a coffee exactly as the roaster intends. Today’s quality coffee is free of defects, and the systems ensure the coffee is extracted to exact parametres. A vital part of the production process is the ability of the roaster to deliver a consistent roast profile. And while many off-line tools exist – tools that can test the roast profile once the coffee has been roasted – the use of on-line tools is much more helpful in the ability to better control the roasting process. The measurements of those tools can be fed back into the roaster, to adjust the parametres as a dynamic system. Enter the potential of using PTS to measure the VOCs in the off-gas. From a technical perspective, PTS can almost instantly deliver an impressive amount of information on the roasting process in real- time. “We believe that the volatile compounds have a big advantage in that they hold a huge amount of information, and not just on visual attributes like colour and so on,” says Yeretzian. “There are hundreds of compounds holding a huge amount of information that can be used to create a flavour profile. By looking at the off-gas, you’re taking a highly sophisticated perspective. It’s a technique that’s very sensitive, and also very fast.” One obstacle the team had to overcome is that the off-gas actually contains too much information. Although the volatile compounds make up less than 1 per cent of the gas, the high resolution of the PTS generates an overload of information. Buhler’s Schenker explains that identifying principle components relative to flavour was an mportant first step in making use of the PTS information. In the 2011 study, they performed 42 roasting experiments at either low, medium, or high inlet temperatures to three varying roast degrees. A principal component analysis then helped identify 23 mass spectral profiles of the activities of the VOCs and their roast degree at the end point of roasting. The result was a detailed pictured of the evolution of the roasting process, allowing the team to establish a predictive model and use the data in on-line monitoring. “Currently with on-line measurements, our tools are essentially temperature and colour,” says Schenker. “These are all just indirect indications of the chemistry that is going on inside the bean. With mass spectrometry we can identify the aroma compounds and get a very accurate idea of what is going on in the roasting process.” Identifying the key aroma compounds, however, is easier said than done when it comes to coffee. With more than 1000 VOCs, Schenker says they relied on previous work in identifying which compounds most affect the taste of coffee. Down the road, this process could be used for more than just observing the aroma development of coffee. Most recently, Buhler’s Schenker has been working with Yeretzian on figuring out exactly how the anti-oxidant characteristics of coffee appear in the roasting process, and how roast profiles could be adjusted to maximise these qualities. This is just one of many studies Buhler has been working on in the space of non-traditional roast profiles. The commercial application of measuring off-gas in PTS, however, is still a little ways away. While the potential is clearly present, Yeretzian points to the cost of a PTS tool – upwards of $400,000 – as the main commercial barrier to the practical application of this research. One promising factor is that because the current PTS tool produces too much information, a more basic and less sensitive instrument, with a lower resolution, could also be used. With the direction of technology producing these tools more cheaply, roasters could one day benefit from all the information that’s currently being filtered out as a puff of smoke.  

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