During the roasting process, coffee bean components undergo structural changes leading to the formation of melanoidins, which are defined as high molecular weight nitrogenous and brown-colored compounds. As coffee brew is one of the main sources of melanoidins in the human diet, their health implications are of great interest. In fact, several biological activities, such as antioxidant, antimicrobial, anticariogenic, anti-inflammatory, antihypertensive, and antiglycative activities, have been attributed to coffee melanoidins. To understand the potential of coffee melanoidin health benefits, it is essential to know their chemical structures. The studies undertaken to date dealing with the structural characterization of coffee melanoidins have shown that polysaccharides, proteins, and chlorogenic acids are involved in coffee melanoidin formation. However, exact structures of coffee melanoidins and mechanisms involved in their formation are far to be elucidated. This paper systematizes the available information and provides a critical overview of the knowledge obtained so far about the structure of coffee melanoidins, mechanisms of their formation, and their potential health implications.

The polysaccharides present in coffee infusions are known to contribute to the organoleptic characteristics of the drink, such as the creamy sensation perceived in the mouth known as "body", the release of aroma substances, and the stability of espresso coffee foam. To increase the knowledge about the origin, composition, and structure of the polysaccharide fraction, the high molecular weight material (HMWM) was extracted with hot water from two green and roasted ground arabica coffees: Costa Rica (wet processed) and Brazil (dry processed). The polysaccharides present in the green coffees HMWM were arabinogalactans (62%), galactomannans (24%), and glucans, and those found in roasted coffees were galactomannans (69%) and arabinogalactans (28%). The polysaccharides of the HMWM of the roasted coffees were less branched than those of the green coffees. The major green coffee proteins had molecular weights of 58 and 38 kDa, and the 58 kDa protein had two subunits, of 38 and 20 kDa, possibly linked by disulfide bonds. The protein fraction obtained from roasted coffees had only a defined band with < or =14 kDa and a diffuse band with >200 kDa. The majority of the galactomannans were precipitated with solutions of 50% ethanol, and the size-exclusion chromatography of the roasted fractions showed coelution of polysaccharides, proteins, phenolics, and brown compounds. The use of strong hydrogen and hydrophobic dissociation conditions allowed us to conclude that the phenolics and brown compounds were linked by covalent bonds to the polymeric material.

A method involving fractionation in ethanol aqueous solutions, anion exchange chromatography, and immobilized copper chelating chromatography was developed to obtain high molecular weight anionic melanoidin populations from coffee infusions. Six anionic fractions with different physicochemical properties (ethanol solubility and chelating ability) and chemical composition regarding carbohydrate as well as protein nature and content were isolated. Fractions with similar chemical composition were obtained for light-, medium-, and dark-roasted coffee infusions. These melanoidin fractions accounted for 30-33% of the cold-water soluble high molecular weight material, independently of the degree of roast in coffee. The nature and abundance of the different polysaccharides in each fraction were dependent on their ethanol solubility. The 50% ethanol insoluble melanoidin populations contained mostly galactomannan-like carbohydrates, and the fractions obtained with 75% ethanol contained mostly arabinogalactan-like carbohydrates. The melanoidin populations with chelating properties presented significantly lower carbohydrate content and, from these, the 75% ethanol soluble fractions were almost devoid of carbohydrate material. The results obtained suggest that the chelating ability of these coffee melanoidins is modulated by their carbohydrates.

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