Non-Destructive, Opto-Electronic Determination of the Freshness and Shrivel of Bell Pepper Fruits

Abstract

(1) The objective of the present study was to identify suitable parameters to determine the (degree of) freshness of Bell pepper fruit of three colors (yellow, red, and green) over a two-week period including the occurrence of shrivel using non-destructive real-time measurements (2) Materials and methods: Surface glossiness was measured non-destructively with a luster sensor type CZ-H72 (Keyence Co., Osaka, Japan), a colorimeter, a spectrometer and a profilometer type VR-5200 (Keyence) to obtain RGB images. (3) Results: During storage and shelf life, bell pepper fruit of initially 230-245 g lost 2.9-4.8 g FW per day at 17 °C and 55% rh. Shriveling started at 6-8% weight loss after 4-5 days and became more pronounced. Glossiness decreased from 450-500 a.u. with fresh fruit without shrivel, 280-310 a.u. with moderately shriveled fruit to 80-90 a.u. with severely shriveled fruit irrespective of color against a background of <40 a.u. within the same color, e.g., light red and dark red. Non-invasive color measurements showed no decline in Lab values (chlorophyll content), irrespective of fruit color and degree of shrivel. RGB images, converted into false color images, showed a concomitant increase in surface roughness (Sa) from Sa = ca. 2 µm for fresh and glossy, Sa = ca. 7 µm for moderately shriveled to Sa = ca. 24 µm for severely shriveled rough surfaces of stored pepper fruit, equivalent to a 12-fold increase in surface roughness. The light reflectance peak at 630-633 nm was universal, irrespective of fruit color and freshness. Hence, a freshness index based on (a) luster values ≥ 450 a.u., (b) Sa ≤ 2 µm and (c) the difference in relative reflectance in % between 630 nm and 500 nm is suggested. The latter values declined from ca. 40% for fresh red Bell pepper, ca. 32% after 6 days when shriveling had started, to ca. 21% after 12 days, but varied with fruit color. (4) Conclusion: overall, it can be concluded that color measurements were unsuitable to determine the freshness of Bell pepper fruit, whereas profilometer, luster sensor, and light reflectance spectra were suitable candidates as a novel opto-electronic approach for defining and parametrizing fruit freshness.

Keywords: Solanaceae; agriculture 4.0; bell pepper (Capsicum annuum L.); color; digitalization; false color image; freshness; fruit quality; gloss; light reflection; roughness; shelf-life; storage.

Figure 1

Our designation of shriveling of the bell pepper fruit deployed for the experiment defined as degree of shriveling (d.o.s.).

Figure 2

(a) Gloss measurements using our designed machined holder (left) and (b) marked spots/areas for repeated non-invasive measurements at the same positions (right).

Figure 3

Light reflectance measurements (0° from the normal) with a portable spectrometer.

Figure 4

(a) Weight loss of bell pepper fruit during 12 days storage (top) and (b) daily weight loss (bottom) (averages of yellow, red and green fruits ± SE) and the degree of shriveling. (d.o.s. 1 begin of shriveling, d.o.s 2 slight shriveling, d.o.s 3 moderate shriveling, d.o.s 4 severe shriveling/ discarded for human fresh fruit consumption) (n = 60 pepper fruit).

Figure 5

Weight loss of bell pepper fruit stored for 14 days averaged over all three fruit colors and considering the degree of shrinkage (d.o.s.) (n = 60 pepper fruit).

Figure 6

(a) Loss of the gloss of bell peppers classified by color (top) (n = 20 per color) and (b) averaged gloss values per day, stored over 14 days (bottom) (n = 60 pepper fruit per day; ± SE).

Figure 7

(a) Lack of measurable color change in harvested bell pepper fruit, expressed as a* color values (red or green) of red and green peppers and (b) hue color angle of red, green, and yellow peppers during 14 days of storage (n = 2 measurements per fruit per day).

Figure 8

RGB (left column) and false color images (right column) of fresh, glossy (top), begin of shrivel after day 6 (d.o.s.2) (middle) and severely shriveled (bottom) pepper fruit indicating the degree of roughness between red (peaks) and blue (troughs) in these false color images; magnification × 40.

Figure 9

Percentage spectral light reflection of (a) yellow, (b) red, and (c) green bell pepper fruits in three stages of freshness (shriveling and storage day 0 = fresh fruit, day 6 = d.o.s 2, day 12 = d.o.s 4) (barium sulfate = 100% light reflectance) (n = 20 fruit per color, i.e., per curve; n = 60 overall).

Figure 10

Visualized example for a possible freshness index, integrating the current findings (Figure 6, Figure 9, and Table 2) and two or three parameters depending on available instrumentation and situation (transport, storage, retail); spectral values (Table 2)/thresholds depend on fruit color.