Nondestructive Detection of Decay in Vegetable Soybeans Stored at Different Temperatures Using Chlorophyll Fluorescence Imaging

Vegetable soybeans (Glycine max (L.) Merr.) (“Edamame”) have emerged as one of the most commercially important foods, and they are very popular because of their delicious taste, rich nutritional value, short crop cycle, and export value. Vegetable soybeans are a good source of protein, monounsaturated fatty acids (with no cholesterol), anti-carcinogenic isoflavones, dietary fiber, vitamin C (Lascorbic acid), vitamin E (tocopherols), and phytoestrogens (Johnson et al., 1999; Miles et al., 2000). From this, vegetable soybeans have high commercial value as an agricultural product that is healthy for consumers. However, in East Asia, which is the main region of vegetable soybean production, the harvest period is mainly during summer season. Because of the climatic heat and their active respiration, vegetable soybeans are very vulnerable to perishing after harvest (Sugimoto et al., 2010). Therefore, with the rapid worldwide increase in demand for high-quality vegetable soybeans, there is a need for research on how to evaluate the quality and predict the shelf-life of vegetable soybeans. Chemical parameters, such as chlorophyll concentration, vitamin C, and enzymes (Wang et al., 2017) have been used to evaluate the quality of fruits and vegetables, but the process of obtaining these indicators is destructive. Hence, it is important to develop a non-invasive and chemical-free method for estimating the quality of fruits and vegetables. Chlorophyll fluorescence technology, which is related strongly to the photosynthesis reactions, has the advantages of causing no damage and being both objective and compact. It has been widely applied to studies of plant photosynthesis (Baker, 2008), plant adversity and stress (Omasa, 1990; Calatayud et al., 2006), and plant pathology (Meyer et al., 2001). Consumers’ primary demand for vegetable soybeans is good pod appearance, the surface color in particular, and the pods color should be bright green with no sign of yellowing (Konovsky et al., 1994). The loss of green color is indicative of declining freshness and chlorophyll degradation. The decrease in chlorophyll content is due to the decomposition of chloroplast. This will lead to the reduction of photosynthetic activity. In consideration of the chlorophyll fluorescence originates from light-excited chlorophyll a molecule associated with photosystem II (PSII) (DeEll et al., 1999; Maxwell and Johnson, 2000; Henriques, 2009), the ripening and senescence of fruits and vegetables may affect the chlorophyll fluorescence yield. Upon that, in recent decades, chlorophyll fluorescence technology has also been used to investigate the ripening and senescence of horticultural products (DeEll et al., 1999; DeEll and Toivonen, 2003; Gorbe and Calatayud, 2012), including apple (DeEll, 1999), banana (Smillie et al.,