Study of the effect of microwave roasting method on volatile compounds and some physicochemical attributes and sensory properties of green coffee bean

Coffee is one of the most popular beverages in the world, which contains large amounts of bioactive compounds and micronutrients that have beneficial effects on human health.Raw and unprocessed coffee beans are green and have a weak flavor and must be roasted before consumption to achieve the desired taste and flavor. The composition of coffee varies greatly during roasting and many factors, including the roasting method, have a great impact on the type and amount of compounds in coffee. Microwave as one of the roasting methods has advantages that have expanded its application in food processing. The aim of this study was to compare the effect of using the microwave as one of the roasting methods on the physicochemical including the moisture content, hardness, instrumental color, antioxidant activity, type and amounts of aromatic compounds, and sensory properties of green coffee beans with the common method of using hot air. For this purpose, 200 g of coffee beans of uniform size were used for each one of the roasting treatments.

In the present study, Arabica green coffee (Coffea arabica) beans produced in Brazil were roasted using direct oven hot-air (220 °C) for 15 minutes and various microwave powers (650, 750, and 900 watts) for 1.5 minutes. Moisture content was measured by accurately weighing ground coffee samples after drying them in an oven at 105 °C until constant weight. The hardness of coffee beans was investigated using a wedge probe (A/WEG) connected to a Texture Analyzer TA-XT2i equipped with a load cell of 25 kg. Instrumental color characteristics including lightness (L*), redness (a*), and yellowness (b*) were measured using a Konica Minolta Colorimeter CR-400. The antioxidant activity of coffee beans was assessed by DPPH• radical scavenging assay. The aromatic compounds of coffee samples were extracted and identified by the solid-phase microextraction (SPME) method and gas chromatography-mass spectrometry (GC-MS), respectively. Organoleptic attributes of brewed coffee samples including appearance color, odor intensity, taste, and overall acceptability were evaluated with a group of 10-trained panelists using an 11-point scale. All experiments were carried out using a completely randomized factorial design and the data reported were the mean of a minimum of three replicates.One-way analysis of variance (ANOVA) test was taken using SPSS Software and Duncan’s multiple range test was used to show significant differences of the mean values at P ≤ 0.05.

The results of data analysis showed that the moisture content of coffee beans was significantly reduced by roasting and the lowest moisture content was observed in coffee roasted by microwave at 900 watts. The results also showed that roasting reduced the hardness of coffee and microwave-roasted beans had less hardness and the lowest texture hardness was observed in microwave-roasted coffee beans at 900 watts. Roasting decreased the lightness (L*) and increased the intensity of redness (a*) and yellowness (b*) of coffee beans, and in microwave-treated samples at higher powers, more differences were observed in the instrumental color characteristics of roasted coffee with the oven. The results revealed that the roasting process reduced the antioxidant activity of coffee in different concentrations; however, microwave-roasted coffees had more antioxidant activity than oven-roasted coffees. The results of the analysis of volatile compounds of coffee samples using solid-phase microextraction method by gas chromatography-mass spectrometer (SPME-GC/MS) showed that the content of total acidic compounds in roasted beans was significantly reduced compared to green coffee beans, while the amounts of alcoholic, ketone, aldehyde, furan, pyrazine and phenolic components in roasted coffees raised significantly compared to green coffee. The results showed that the roasting method had a significant effect on the type and amount of volatile compounds in coffee (p < 0.05). During the roasting process, some compounds were completely destroyed and some new compounds appeared instead. In the present study, the highest amount of volatile compounds was observed in coffee roasted by microwave at 650 watts. Furfuryl alcohol, 2-methylpyrazine γ-butyrolactone, and Pyridine were the predominant volatile compounds in roasted coffee. The results of the sensory evaluation showed that roasting had a significant effect on the sensory properties of coffee, so that roasting increased the acceptance of the appearance color of coffee, and coffee roasted by oven and microwave (650 watts) had the highest score. In addition, coffee roasted by microwave with a power of 650 watts had a significantly higher odor and taste score. Roasted coffee with 650 watts of microwave and oven had the highest overall acceptability, while samples prepared with 900 watts of the microwave with green coffee had the lowest overall acceptability.

In conclusion, based on the results of the present study, it could be stated that the use of microwave power at 650 watts due to the ability to create more aromatic components in coffee beans, better sensory acceptance, and more antioxidant activity could be a suitable method for roasting coffee beans.