Green coffee beans
Green coffee beans of Kenya (SL28 variety, provided by SAZA Coffee). The green coffee beans were pre-processed by wet processing and dried to the water content of around 9%.
Determination of water content of green coffee beans at different soaking times
Soaking beans can alter their microstructure (Princewill & Ezinne, 2014; Baquero et al., 2003). When the beans are immersed in water, the water penetrates the cell walls and membranes due to the osmotic pressure, causing them to gradually swell. In addition, the spaces between cells in the beans also expand due to the infiltration of water, resulting in a looser internal structure of the beans.
50 g of green coffee beans with an initial moisture content of 9% were weighed and placed in 250 mL sterile blue-cap bottles, followed by adding 100 mL of distilled water. The beans were then soaked at room temperature for 0, 10, 20, 30, 60, 120, 180, 240, and 300 min, respectively. After soaking, the remain water was wiped off and the moisture content of the beans was measured by moisture analyzer (MX-50, A&D, Japan).
Drying (DR) and freeze-drying (FD)
The first batch of the soaked green coffee beans were placed in the dryer (WFO-500W, TOKYO RIKAKIKAI CO.,LTD, China) and dried at 70°C. For the second batch of soaked green coffee beans, the samples were pre-frozen in the refrigerator at -20°C for 12 hours, then placed in the freeze-dryer (DRC-3L, TOKYO RIKAKIKAI CO.,LTD, Japan) and dried at a vacuum pressure of 40 Pa for 24 hours. The drying chamber temperature was − 40°C, while the heating plate and cold trap were at 40°C and − 50°C, respectively (Cheng et al., 2019).
Freeze-dryer is a device that removes moisture from material by sublimation under low pressure after freezing it. Its basic principle is to remove water from frozen materials using the physical processes of freezing and sublimation (Shukla, 2011). Specifically, in the frozen state, water is frozen into ice crystals. Under low pressure, using the sublimation technique, the ice crystals are directly transformed from solid state to gaseous state, thereby removing the water from the material and achieving drying. Due to the working principle of freeze-drying, high-temperature evaporation-induced thermal damage to coffee beans can be avoided during the processing, while the quality of coffee beans can be maximally preserved.
Short-time heating puffing (HP) and microwave-puffing (MWP)
The experimental methods mentioned above mainly involved two approaches for altering the microstructure of green coffee beans by removing moisture through evaporation and sublimation. Ibtisam M. Kamal, V. Sobolik successfully changed the microstructure of green coffee beans before roasting using two Instantaneous Controlled Pressure Drop (DIC) process (steam and microwaves) (2008). Considering that the principle of puffing process is similar to DIC and has lower cost, the methods used in this section is based on the principle of material expansion caused by moisture evaporation and instantaneous pressure drop, using a puffing machine to change the microstructure of green coffee beans.
The puffing machine introduced in this part is a device used to process various grains and seeds by subjecting them to high temperature and pressure, resulting in an expansion of their cellular structure and a change in their texture. On the other hand, high temperature can cause not only physical alterations but also chemical changes, such as protein denaturation and the Maillard reactions (An et al., 2011; Kim et al., 2018), which we want to avoid when changing the microstructure of green coffee beans before roasting. Therefore, it is necessary to change the heating method or degree during the puffing process, so that it can be suitable for the treatment of thermosensitive materials while changing the microstructure of the material.
This part demonstrates the puffing process based on two different sources of product low-temperature heating. Short-time heating puffing: The third batch of beans were placed into the chamber of puffing machine (SL type, Tachibanakikou, Japan) and sealed, then the chamber was heated by fire at the bottom for 1, 2, 3 min while rotating uniformly. At the same time, the air compressor connected to the chamber was turned on to ensure a pressure of 0.8 MPa in the chamber(Hu et al., 2016; Kim et al., 2020). After 10 minutes, the lid of the chamber was opened. Microwave puffing: The fourth batch of beans were placed into the microwave oven (ER-VS23, TOSHIBA, Japan) and heated at 75°C for 60 s. Then, the heated beans were quickly transferred into the puffing machine chamber connected to the air compressor and the pressure was raised to 0.8 MPa. After 10 minutes, the lid of the chamber was opened instantaneously.
Color
The color of GCB and modified green coffee beans was measured using the colorimeter (CR-200, Reston, MINOLTA, Japan). The colorimeter was calibrated using a white calibration tile and the colors were described using L (luminosity), A (redness/greenness), and B (yellowness/blueness) values outputted by the colorimeter. Finally, the color differences were calculated using Formula 1 (de Oliveira et al., 2016; Berrios et al., 2004).
$$\varDelta {E}^{*}=\sqrt{{(\varDelta {L}^{*})}^{2}+{(\varDelta {a}^{*})}^{2}+{(\varDelta {b}^{*})}^{2}}$$
1
Where ΔE* represents the color change of green coffee beans and modified green coffee beans.
Porosity
The porosity of beans refers to the porosity inside the beans, which is the ratio (varies from 0-100 in percent) of the volume of void spaces or air pockets within the bean to the total volume of the bean. The true density (ρg, g/cm3) and bulk density (ρb, g/cm3) of coffee beans were measured using pycnometer test method and the method stated by Naraganti Snehitha, Ajibola B. Oyedeji and Baryeh, and the porosity of the sample was calculated using Formula 2 ( 2021, 2001).
$$Porosity=\frac{{\rho }_{g}-{\rho }_{b}}{{\rho }_{b}}\times 100$$
2
Scanning Electron Microscopy
The SEM employs an electron beam to scan the sample surface and obtain imaging results, while the electrons in the beam can collide with the molecules in the atmosphere, resulting in the loss of energy and direction, which affects the accuracy and resolution of the imaging results. Therefore, it is necessary to emit electrons under vacuum conditions when imaging with SEM. However, water in the sample can evaporate in the high vacuum environment, producing bubbles and interference. Therefore, when observing the microstructure of the sample using SEM, it is usually required that the sample must be dry (Goldstein et al., 2017). In addition, to avoid errors caused by differences in water content, the final water content of dried beans, freeze-dried beans, and puffed beans (dried at 70°C in an oven after puffing) was all 7.24%.
The surface of the green coffee beans and treated beans was visualized using SEM (JIB-4000, JEOL, Japan) working under vacuum and with the accelerating voltage of 15 kV at ×250 magnification (Aravindakshan et al., 2021).
Statistical Analysis
Experimental data were obtained in triplicates. Data were analyzed using analysis of variance (ANOVA) and Student’s t-test was used for means comparison at the 95% significance level (p < 0.05).