Changes in the total water-soluble polyphenol content and antioxidant effect of coffee as a function of the roasting temperature (en-US)

Friday, October 9, 2015

Authors: Anita Imre, László Somogyi, Anita Soós, Katalin Szántainé Kőhegyi

 

In our work, Brazilian arabica and Ugandan robusta coffee varieties were compared in terms of water-soluble polyphenol content and antioxidant effect, as measured by ferric reducing ability. Because, of secondary processing procedures, roasting has the greatest impact on the physical and chemical properties of coffee, measurements were performed as a function of roasting intensity, and they were also extended to determine the degree of weight loss and volume growth. In our study, roasting time was kept constant, only the temperature of the second stage of roasting was varied. Our results confirmed the fact that, at increasing roasting temperatures, the volume of coffee beans increases to a larger extent, while their weight gradually decreases.

 

There were differences observed between the two varieties regarding the degree of increase and decrease. Results of our chemical analyses were consistent with literature data, according to which the content of phenolic components of robusta coffee is higher than what can be measured in arabica coffee. The total water-soluble phenolic component content was significantly reduced in both varieties as a result of roasting. The antioxidant activity of the coffees was also investigated. During these measurements we found that green beans of the two varieties have nearly identical FRAP values. However, there have been differences between the values of antioxidant capacity and changes in them for arabica and robusta, as a function of roasting intensity. We found for both coffees that the greatest antioxidant effect was experienced in the case of medium roasting, and it had a maximum at 198 °C for robusta, and at 180 °C and 192 °C for arabica. As a result of our measurements it can be stated that medium- roasted coffees have higher antioxidant activities, as opposed to samples roasted at higher temperatures.

 

 

2. Introduction

The composition of coffee is extremely complex. The ratio of components is determined by the type and variety of the coffee, and also by the agricultural procedures used, crop maturity and the storage conditions of green coffee. Concentrations of the ingredients can also be modified by the procedures applied during the industrial processing of green coffee, as well as the method used for preparing the coffee drink. In terms of biological activity – in addition to caffeine – phenolic compounds are considered significant. Phenolic components that occur in coffee seeds are primarily present as esters of hydroxycinnamic acid and quinic acid, collectively called chlorogenic acids. Thanks to their antioxidant properties, many beneficial health effects in humans are known, they have antiviral, blood sugar lowering and hepatoprotective effects [3], [7]. The main components of the phenolic fraction of coffee beans are chlorogenic acids. The group of chlorogenic acids includes several different compound groups and their isomers, which are esters of one molecule of quinic acid and one to three specific trans-hydroxycinnamic acids. Hydroxycinnamic acids are trans-phenyl-3-propenoic acids, with various functional groups on the aromatic ring.

 

The one that occurs in the largest amount in coffee is caffeic acid (3,4-dihydroxycinnamic acid, CA), followed by ferulic acid (3-methoxy-4-hydroxycinnamic acid, FA) and p-coumaric acid (4-hydroxycinnamic acid, pCoA) [5], [8]. During roasting, the chlorogenic acid content of the coffee beans is altered, transformed. When roasting coffee, chlorogenic acids participate in the development of color, flavor and aroma. Due to their thermal instability, they are almost completely decomposed when exposed to intensive roasting. During roasting, chlorogenic acids isomerize and turn into quinolactones through dehydration and formation of an intramolecular bond, and they also hydrolyze and decompose to low molecular weight components [4]. Chlorogenic acids also participate in the formation of compounds with a higher degree of polymerization, such as melanoidins. As a result of strong roasting, each percent of loss in dry weight is accompanied by a chlorogenic acid loss of 8 to 10% [5].

 

Substances with antioxidant properties are easily oxidized when coming into contact with atmospheric oxygen, so they can protect other valuable biomolecules, which play very important roles in the human body, from oxidation and the harmful effects of free radicals. Antioxidants react with free radicals so that they are eliminated by reduction via electron transfer, so they are basically „scavenger” compounds. In addition to phenolic components and products of the Maillard reaction (a non-enzymatic browning reaction) and caramelization, small molecules, such as caffeine, also contribute to the antioxidant effect.

Other compounds are produced during roasting, either as intermediates of the Maillard reaction or as polymerized phenolic components, such as condensed tannins, which are also effective antioxidants [10]. The antioxidant activity of the coffee drink is strongly influenced by the roasting process. While the amount of phenolic antoxidants naturally occurring in coffee (mainly chlorogenic acids) is reduced during roasting, the total antioxidant content does not decrease, it might even increase. This is due to the formation of compounds possessing antioxidant effects, mainly the products of the Maillard reaction. When investigating these compounds it was found that they have antioxidant

properties [2], [9].

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