Abstract
Coffee quality, and therefore its price, is determined by coffee species and varieties, geographic location, the method used to process green coffee beans, and particularly the care taken during coffee production. Determination of coffee quality is often done by the nondestructive and fast near infrared spectroscopy (NIRS), which provides a huge amount of data about the samples. NIRS data require sophisticated, multivariate data analysis methods, such as principal component analysis, or linear discriminant analysis. Since the obtained data are a set of spectra, they can also be analyzed by signal processing methods. In the present study, the applications of two novel methods, detrended fluctuation analysis (DFA) and yield stability index (YSI), is introduced on NIR spectra of different roasting levels of coffee samples. Fourteen green coffee samples from all over the world have been roasted on three different levels and their NIR spectra were analyzed. DFA successfully differentiated the green samples from the roasted ones, however, the joint analysis of all samples was not able to differentiate the roasting levels. On the other hand, DFA successfully differentiated the roasting levels on samples level, which was strengthened by a 100% accurate agglomerative hierarchical clustering. YSI was first used in NIR signal processing and was able to detect that a light roast is the most stable among all roasting levels. Future research should focus on the application of DFA in terms of the analysis of the effects of other transformation methods of the spectra.
Highlights
Coffee is one of the most marketed products worldwide
First derivatives remove additive baseline shift so this is very useful in NIR spectroscopy
NIR spectra tend to have linear baseline increases and these are removed by second derivatives which have negative peaks where the original had a peak and are more readily comprehensible
Summary
The quality of the beverage depends on the physical properties and chemical composition of raw coffee beans, respectively, on the roasting process. The quality of the coffee beverage is manifested differently depending on its geographic origin, and there are notable variations in sensory profile according to country of origin, microregions, and even different planting locations in the same farm [1]. Over 100 species within the genus Coffea are catalogued. Despite this diversity, only two species are of great importance in the world market, C. arabica L. and C. canephora. These two species have distinct characteristics, especially regarding the quality of the beverage. Price, is determined by coffee species and varieties, Processes 2020, 8, 913; doi:10.3390/pr8080913 www.mdpi.com/journal/processes
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