In the coffee industry, there is a need for in-line analysis for physicochemical and functional properties. The principal quality parameters include moisture, blend ratio, roasting degree, and caffeine. Coffee is first harvested as ripe berries and can have a moisture content over 60% before going through multiple drying processes that result in green coffee beans.
Moisture content in green coffee beans is strictly regulated, and the safe range is from 8% – 12.5%. In most countries, beans above 12.5% are not allowed to be traded due to microbial growth, mycotoxin formation, decreased sensory quality, and unstable production conditions, among other unwanted consequences. Beans below 8% are shrunken and often have a poor appearance. Storage stability and keeping moisture consistent in different batches are essential because the moisture has a strong effect on roasting quality. Coffee blending almost always occurs in raw coffee before roasting. This is a crucial part of the process to make coffee with a given flavor and aroma, and it must be continually reproduced, which is often a difficult task due to variations in harvest quality. Blends are often comprised of at least four different varieties to achieve particular flavors and aromas because blending is the only means to account for natural fluctuations in quality.
Once the desired blend ratio is achieved, the roasting process begins. Flavor is locked within green coffee beans and heating starts a series of chemical reactions. Although it can vary due to moisture and other factors, roasting typically begins when the temperature inside the bean reaches around 200°C. As moisture evaporates, aromatic oils are released. Caramelization occurs as starches break down, changing them to simple sugars that brown and alter the color of the bean. Sucrose rapidly disappears during roasting and may disappear entirely in darker roasts. Thus, the color of the beans is an important marker in the roasting process and is an indicator of volatile compound patterns that determine aroma and flavor. Caffeine is an important parameter in coffee as the stimulating effect is the biggest factor in consumer appeal of coffee. If coffee is decaffeinated, it is done by either soaking in hot water or steaming and using a solvent before roasting. After roasting is complete, the roasted coffee beans are moistened, and cold air is blown through them. Some intact beans are sold, but most often, the coffee is ground through rolling mills. Rolling mills consist of several groups of cylinders moving in opposite directions, all adjusted accordingly to reach the final desired level of grinding. Ground coffee is packaged, and higher-quality coffee is usually vacuum packed to diminish the effects of oxidation, which can break down aromas in the coffee and affect flavor.
NIR spectroscopy has emerged as a tool for rapid, non-invasive, and cost-effective analysis of parameters of interest in coffee that could potentially replace traditional reference methods. Moisture is one of the best-measured constituents using near-infrared light because of its strong absorption, and NIR spectroscopy has been demonstrated as an effective tool for determining moisture in green coffee beans. The roasting color of beans and varietal composition of blends are critical parameters in the development of sensory properties of coffee. Color analysis can verify the performance of the roasting and thus the desired characteristics of the final product, while the varietal composition is important for quality as well, especially when comparing the higher quality and more expensive Arabica species to the lower quality Robusta species. NIR spectroscopy has proven to be a feasible method for measuring these parameters in roasted ground coffee. Studies have also been conducted for measuring caffeine and other major alkaloids in coffee. Adulteration is a major problem in many food and beverage products, and coffee is subject to many forms of adulteration. One form of coffee adulteration is mixing a low-quality blend with a high-quality blend, but often coffee can be adulterated with adulterants that are an entirely different constituent. These can include corn, soybean, and wheat. NIR spectroscopy has been examined as a potential method of identifying adulterants in coffee. The potential has been demonstrated for using NIR spectroscopy as an analytical tool for analyzing coffee and replacing traditional methods. Advancements in application development and online analysis continue to move forward to realize the potential of NIR spectroscopy as a method for real-time, online implementation as a process control tool.
Quality Analysis, Classification, and Authentication of Liquid Foods by Near-Infrared Spectroscopy: A Review of Recent Research Developments – Wang, Sun, Pu, and Cheng, Critical Reviews in Science and Nutrition, 2017, Vol. 57, No. 7, 1524-1538
Application of Infrared Spectral Techniques on Quality and Compositional Attributes of Coffee: An Overview – Barbin, Felicio, Sun, et al., Food Research International 61 (2014) 23-32 https://www.sciencedirect.com/science/article/pii/S096399691400009X