Characterization of time-series fluorescence properties of bean sprouts during storage using excitation emission matrix and fluorescence imaging

Soybean (Glycine max (L.) Merr.) and mung bean (Vigna radiata L.) are popular varieties of bean sprouts, which are used in various Asian cuisines. The freshness of bean sprouts reduces at fast rate during storage. Hence, determining index of monitoring the state of sprouts is required to prevent unnecessary food loss, which is also linked to economical loss. The aim of this study is to reveal fluorescence characteristic of bean sprouts and investigate its potentiality for tracking the freshness state of bean sprouts. The fluorescence spectroscopy and imaging were used. For fresh bean sprouts, the Excitation-Emission Matrix (EEM) showed two fluorescence regions: Excitation (Ex) 270-300 nm with Emission (Em) 300-400 nm and Ex 300-350 nm with Em 400-450 nm, which are suspected to be amino acids and vitamins, respectively. When browning process started, the new fluorescence region occurred at Ex 400-450 nm with Em 450-550 nm. Consistent with EEM, under 365 nm, bean sprouts initially had blue fluorescence emission, and later changed to green when they start spoiling. However, due to higher emission, 420 nm-excitation wavelength was preferable for detecting browning part. With these basic fluorescence information, further application on inspecting both physical change and chemical change of bean sprouts can be easily established.

Macroscopic and microscopic characterization of fluorescence properties of multiple sweet pepper cultivars (Capsicum annuum L.) using excitation-emission matrix and UV induced fluorescence imaging

Sweet peppers are a popular vegetable with various surface colors, such as green, purple, red, or yellow. To characterize the unique fluorescence properties associated with a broad range of sweet peppers of various colors (14 varieties), a fluorescence spectrofluorometer and imaging were used. The results showed that all cultivars in the experiment had blue fluorescence emissions when excited with light in the UV-A region, while chlorophyll fluorescence could be observed in green peppers. The emitted blue fluorescence originated from the epidermis (cuticle layer). The color distribution of these sweet peppers in the a* and b* color space were compared to the image obtained under white LED light. Yellow and red pepper cultivars have thicker, multiple cuticular wax layers and more distinct maturity stages than other sweet pepper varieties observed. With the establishment of this basic fluorescence database, further applications of fluorescence-based techniques and the unification of evaluation methods for pepper quality will be more easily established.

Response of sweet pepper autofluorescence against solar radiation

Shades are adjusted in sweet pepper cultivation, based on solar exposure levels. Pyranometers and photosensitive films have recently been introduced to smart agriculture. However, there are no means of observing biological responses to solar exposure. In this study, we hypothesized that solar exposure levels affect the visible autofluorescence of sweet pepper under 365 nm illumination. To test this hypothesis, we cultivated sweet pepper plants under two exposure conditions, low (half of the normal) and high (the normal). Fluorescence photography (365 nm illumination) revealed that dark-fluorescent peppers only arise when cultivated under high-exposure conditions (0.7-fold decline at emission of 390 nm for high-exposure conditions). Microscopic and spectroscopic observations showed that blue autofluorescence was accompanied by an accumulation of UVB pigments (1.2-factor increase in the absorbance at 300 nm) and epidermal development (1.3-fold thicker cell wall). This study suggests that the autofluorescence of sweet pepper can possibly be used to understand the response of crop to solar radiation at a fruit level in horticulture.