## 2 Materials and Methods
### Bionic Olfaction Detection System  
The system employed in the experiment was a portable fish meal quality detection system, based on bionic olfaction. As shown in Figure 1, the system primarily consisted of four parts; (I) a gas acquisition and transmission module; (II) a control processing storage module with strawberry pie as the core; (III) an ARPI600 data acquisition module; and (IV) a sensor array module. The sensor array module was the core component of the device and primarily consisted of 10 MOS (metal oxide semiconductor) gas sensors. The models of each sensor, and the gases that can be measured, are shown in Table 2. The ten gas sensors were selected in order to cover the volatile organic compounds in fish meal. Gas sensors had broad spectrum response characteristics and can respond to a variety of gas components. This characteristic enabled the constructed array to test a variety of samples. The loop measurement voltage of the gas sensor array was (\(5\pm 0.2\)) V, and the heating voltage was (\(5\pm 0.2\)) V. In addition, the electronic nose was equipped with temperature and humidity sensors to measure the environmental temperature and humidity information to compensate the influence of the measuring environment on the gas sensor. The temperature and humidity sensors were integrated components (Guangzhou Aosong Electronics Co., Ltd., Guangzhou, China). The model is DHT11, and the measuring voltage is (\(5\pm 0.01\)) V. The temperature measuring range was 0-50 \({}^{\circ}\)C, the relative humidity measuring range was 20%-90%. The key component of the device was the self-designed sensor sampling chamber, which consisted of an upper shell, a center body, and a lower shell. The shape of the upper shell with an air outlet was a regular prism; the first mounting cavity had the shape of a regular prism, and mounting holes were formed in the center of each side surface. The number of sensors was the same as the number of the side surfaces of the upper shell; each sensor was mounted in each mounting hole. The shape of the central body was a regular prism; the size of the central body matched the size of the first mounting cavity; a plurality of gas channels were uniformly arranged along the surface of the central body, and the number of the gas channels was equal to the number of the side surfaces of the central body. An air inlet was arranged in the center of the lower shell. This ensured the synchronization and uniformity of the detection of sample gas by each sensor. At the same time, it also ensured the full contact between the sample gas and the sensor, and reduced the response and recovery time. The volume of the gas sampling chamber is only 0.059 L, which effectively reduced the volume. The material used in the gas sampling chamber is polytetrafluoroethylene (PTFE) that has the characteristics of thermo-stability, slushing, not easily bonding, and good sealing. It is suitable for manufacturing the gas sampling chamber. First, the detection device was pre-heated with electricity for half an hour to eliminate the any instability in the instrument. In this experiment, the gas flow rate was set at 2.2 L-min\({}^{-1}\). Then, the valve 1 was opened and the valve 2 was closed, the device was cleaned with purified air that was filtered by activated carbon for 77 s. Last, the fish meal sample (80 g) was placed in a 250 mL high borosilicate headspace vials; the valve 2 was opened and the valve 1 was closed, the top air generated by the sample was pumped into the gas sampling chamber of the detection device by a micro air pump and chemically reacts with the gas sensor located in the sampling chamber, which caused the resistance of the sensor to change. The detection time of the device was 39 s and the data sampling interval was 1 s. After each sampling, the device needed to be cleaned and restored to test the next