The most obvious and prominent change in coffee seeds observed during the coffee production process is in the colour of seeds or ground coffee. The colour of the coffee seeds directly impacts the flavour and quality of the end product. With each production step, coffee seeds undergo various chemical and physical changes that result in the variation of the consistency and the colour of the seeds. When the coffee seeds reach the peak of their roasting process, it is important to gauge the exact moment the roast reaches its full-colour potential. Doing this ensures that the colour consistency of coffee seeds remains the same through the entire batch to achieve the perfect coffee blend. Therefore, it is in the best interest of the coffee manufacturers to understand the importance of effective colour measurement and to use productive tools to narrow down the margin of error.
Coffee production process begins with the selection of raw coffee seeds to ensure colour consistency for the next steps. This step is paramount for detecting any contamination or imperfections in the raw seeds to streamline the selection process of raw seeds and also to reduce the quantity of wasted products and the time involved in further steps.
Next comes the roasting, in which green coffee seeds are transformed into brown seeds at a temperature of about 287°C. The roasting process transforms the density and the flavour of the seeds by changing their colour from brown, medium-brown to dark-brown through heat. There are various stages involved in the roasting process, due to which the colour of coffee seeds varies with every stage. As the temperature continues to rise, the colour of coffee seeds changes to a darker shade of brown. The relationship between the seed colour and roast progression, and consistency of seed colour determine the quality and flavour of the coffee.
The idea of using colour sensors for inline measurement during the roasting process is to measure the exact points of colour change, which is prompted by change in temperature. Ideally, the roasting process should stop once the roast reaches its ideal colour. This proposed system allows the operators to predict the internal temperature of coffee seeds based on the colour. This can potentially prevent over roasting of coffee seeds and maintain the quality of the finished products.
The challenge in using colour sensors to determine the ideal roasting time during the coffee roasting process is to accurately identify the shade of colour that signify perfectly roasted seeds. The optical properties of the light source, the coffee seeds, and the observer altogether affect the colourfulness, saturation and hue of the coffee seeds. These factors may contribute to inaccuracies during colour measurement, which impacts the quality of the final products.
Assessment of colour is based on its hue, brightness, colourfulness and saturation. Based on these four properties, each perceivable colour can be assigned to their location in the colour space. The larger the difference between the colour in the colour space, the clearer we can perceive the difference with our eyes.
A colour sensor obtains these data by focusing object with a white light spot generated from, generally, a modulated high-power white light LED (light-emitting diode). The reflected light from the object is directed to a spectrometer that separates the spectral components of the received light. The colour detector separates the spectrum into long-, medium-, or short-wave light component (X = long, Y = medium, Z=short) and finally transform them into the L*a*b colour values. The colour distance between each colour in the colour space is defined as ΔE in the system. This value denotes the accuracy of the colour measurement, which can be varied depends on the requirements in different industry.
The technology advancement has enabled the development of colour sensors that are capable of recognising of individual colours from their coordinates in the colour space, apart from simply identifying their reference colours by comparison. Furthermore, the modern colour sensors are compact and can measure at high speed which is suitable to be used for measuring in-line process. This is ideal for the coffee roasting as the colour changes can be detected as they undergo the roasting process.
Prior to the testing, the colour of the coffee seeds should be measured at normal daylight by recording the colour measurement of a coffee sample from different angles. These are inputted into the system which serve as the reference value for the measurement. As the coffee seeds undergo roasting in the process, the sensor detects whether it falls within the ΔE value set by the reference value. Any value that falls outside this range will signify under-roasting or over-roasting.
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