A portable, low-cost and sensor-based detector on sweetness and firmness grades of kiwifruit

During storage, the sweetness of kiwifruit increases and its firmness decreases quickly. Since the sweetness is linked to consumers' taste preference, and the firmness is regarded as the primary indicator of kiwifruit eating ripeness, detecting the sweetness and firmness are welcome by kiwifruit sellers, consumers and processors. To make the detected results intuitive and acceptable to more people, this study aimed to present a portable, lowcost, sensor-based grade detector on sweetness and firmness of kiwifruit. The detector consisted of a control/processing unit, a LED panel and driver unit, a light signal detection and amplifier unit, an input/output unit and a battery. Micro-controller of STM32 was used to do data collection and processing. The LED panel with nine LEDs at peak wavelengths of 800, 810, 850, 880, 900, 940, 970, 1000 and 1100 nm were used as light source. The reflected light from the kiwifruit was sensed by a Si PIN photodiode. The software of the detector was programmed by using C language. Kiwifruit varieties of 'Xuxiang' and 'Huayou' were used as samples, and their sweetness and firmness were divided into four grades. The model of support vector machine was established to determine kiwifruit grades. The validation experiment showed that the identification accuracies of the grade detector were 82.0% for sweetness and 74.0% for firmness of 'Xuxiang' variety, and 85.5% and 91.5% for sweetness and firmness of 'Huayou' variety, respectively. The detection time for one sample was less than 3 s.