2018 Volume 24 Issue 3 Pages 377-384
The efficacy of 1-methylcyclopropene (1-MCP) on the quality of Japanese apricot was investigated at 25°C. X-ray computed tomography (CT) was employed to measure pixel-based average CT values in terms of gray scale (GS) value. The ethylene production rate and weight loss were minimal, whereas firmness, apparent density and soluble solids content were higher in 1-MCP-treated fruits. The histogram profile was almost the same during the storage period in 1-MCP-treated fruit. Differences in the internal structure between 1-MCP and control fruits could be identified by CT images. The standard deviation (GS) and low-density pixel volume were negatively correlated, whereas average CT value (GS) and peak height were positively correlated with the bio-yield stress and apparent density. X-ray CT images could thus be used for quality evaluation. Further, 1-MCP significantly delayed the ripening of Japanese apricot, thereby maintaining the internal structure at harvesting for seven d.
The Japanese apricot (Prunus shirakaga) has a high fruit value in terms of volatile phenolic content, the effective antioxidant capacity of which, at 150–320 µmol/g trolox equivalent, depends upon the stage of maturity (Mitanni, et al., 2013). Since the fruit firmness sharply falls with decomposition of soluble solids at room temperature, a controlled atmosphere is needed for storage (0°C, 90–95% RH, 2–3% O2+ 2–3% CO2) (i). The high ethylene formation property of Japanese apricot makes it susceptible to high temperature storage and also chilling injury (Paula et al., 2008; Touzeau, et al., 2002). It is therefore necessary to develop a solution for room temperature storage of Japanese apricot.
Ethylene is a naturally occurring plant growth substance that has numerous effects on the growth, development and storage life of many fruits and vegetables (Saltveit, 1999). Climacteric fruits clearly show the highest peaks in respiration and ethylene production. The ripening of fruits is a common phenomenon, occurring either with or without ethylene. Endogenous production or exogenous application of ethylene is a common ripening strategy in the fruit industry. Ethylene can contribute to postharvest ripening as well as senescence and postharvest disorders (Palou et al., 2003). The ethylene reaction is irreversible, meaning that it is not possible to stop the ripening process. Thus, ethylene removal is of great significance in the fruit, vegetable and flower industries.
1-Methylcyclopropene (1-MCP) has been proven to be efficacious in fruits like banana, mango, plum, peach, strawberry, papaya, orange, watermelon, persimmon, etc. and vegetables including lettuce, bitter gourd, tomato, carrot, pea seedlings, mung bean, etc. for inhibiting ethylene production and extending shelf-life in modified and controlled-atmosphere storage (Blankenship and Dole, 2003). 1-MCP efficacy depends on temperature, concentration of 1-MCP used, maturity of the fruits and obviously the fruit type: climacteric or non-climacteric. The ethylene receptor binding mechanism of 1-MCP is well documented (Sisler and Serek, 1997). Most research has shown that 1-MCP delays postharvest ripening in terms of lower ethylene production and respiration rate, leading to minimal changes in firmness and external color (Luo 2007; Luo et al., 2009; Liu et al., 2013). However, there are limited resource materials for internal structural changes after the use of 1-MCP. The internal structure changes after harvesting and with successive days of storage. This change may contribute to alterations in firmness, total soluble solids (TSS), titratable acidity and moisture content.
Non-destructive techniques for external and internal quality evaluation of fresh and processed fruit and vegetables are now a major area of interest in the food industry. X-rays are used to characterize agricultural commodities with good accuracy after validation (Donis-gonza et al., 2014). X-ray computed tomography (CT) is of growing interest for internal quality evaluation of fruit and vegetables when quality parameters are not visible from the outer surface. Two- or three-dimensional images can be acquired from a projected slice of sample by an X-ray CT device, and the image data are analysed by different algorithms depending on the area of interest (Neethirajan et al., 2007). Pixel intensity-based thresholding is the most commonly used algorithm in X-ray image processing for the separation of targeted areas. The histogram produced from an acquired image based on pixel intensity and the histogram properties, such as standard deviation, peak, skewness, kurtosis etc., have been successfully used to differentiate infested wheat kernels of sprouted and non-sprouted wheat (Narvankar et al., 2009; Neethirajan et al., 2007), green matured and ripened mangos and storage of peaches (Barcelon et al., 1999a; 1999b). We assessed the histogram properties after 1-MCP treatment non-destructively. Bercelon et al. (1999) showed the relationship with average CT absorption to moisture, TSS, apparent density and acidity for fresh mango and peach. There is no data for the relationship between average CT absorption and firmness of fruit.
The objectives of the study were to establish the efficacy of 1-MCP in extending shelf-life in terms of internal structure and to reveal changes in image properties during the storage period associated with the physicochemical properties of Japanese apricot.
Materials Fully matured green, defect free and same-sized (25±2 g) Japanese apricot (Prunus shirakaga) was harvested from the Fukuoka Agriculture and Forest Research Institute located in Kurume, Japan. The fruit was divided into two lots for control and 1-MCP treatment. The control lot contained 30 fruits and the 1-MCP lot contained 25 fruits. Five fruits each were packed in 0.02 mm polypropylene bags and stored at 25°C and 55% RH. One bag from each lot was used for capturing X-ray CT images throughout the storage period.
1-MCP treatment Matured green Japanese apricot fruit was treated with 1-MCP (Rohm and Haas China Inc., Beijing, China) within 4 h after harvest at a dose of 1 ppm (according to the manual) for 24 h at 25°C. A 54 cm × 64 cm × 65 cm closed glass box designed with continuous aeration facilities was used for the 1-MCP treatment. A beaker containing 1-MCP in 200 mL of distilled water was placed in the box and continuously agitated by a magnetic stirrer (MGP-101, SIBATA, Saitama, Japan). After 24 h of 1-MCP treatment, every fifth fruit was packed and sealed in a polypropylene bag (size: 25 cm × 30 cm, film thickness: 0.02 mm, Sanipak Company of Japan Ltd., Tokyo, Japan) and stored at 25°C and 55%RH.
Determination of % weight loss Weight loss of fruits during storage was calculated according to the method described by Paniagua et al. (2013). Each measurement was carried out using an electrical balance (EK-1200i, SATOSHOUJI INC., Tokyo, Japan) in terms of the initial weight with three replications.
Measurement of apparent density and total soluble solids Sample volume VS (m3) was measured by the water displacement method, based on Archimedes' principle, using distilled water at 30°C. Then, the apparent density πap (kg/m3) of each fruit was calculated by the following equation, where ms represents sample weight (kg):
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TSS was measured according to the method of Majidi et al. (2011) using a digital refractomer (MASTER-T, Atago Co. Ltd, Tokyo, Japan).
Determination of firmness A penetrometer (Creep Meter RE-3305, YAMADEN Co. Ltd., Tokyo, Japan) equipped with a cylindrical plunger (diameter: 3 mm) was used for firmness testing. The sample was placed on the stage of the penetrometer and penetrated at a speed of 1 mm/s. In accordance with the method proposed by Mallikarjuman (2011), bioyield stress F (N/m2) was obtained.
Measurement of ethylene concentration in packaging during storage The ethylene concentration inside the bag of Japanese apricot was measured in ppm using a gas detector tube (GV-100, tube No. 172 L, CH2:CH2, measuring range 0.2–100 ppm, GASTEC Co., Kanagawa, Japan). The gas was diluted in order to measure higher concentrations of ethylene gas. Measurement was performed in triplicate.
X-ray CT image acquisition and image processing X-ray CT images from five fruits each of the control and treated samples were obtained using an X-ray CT system (Latheta LCT-100, Hitachi Aloka Medical, Ltd, Tokyo, Japan) operated at 50 kV and 1 mA current. The fruit was placed in a holding tube and scanned at 2 mm pitch width. About 20 seconds is required to scan individual slices 2 mm in thickness at the equatorial part of Japanese apricot. The scanned images were reconstructed at −900 to 350 HU by Nashita software to create 8 bit bitmap images (480 × 480 pixels), and the total volume was separated into high-density (−900 ∼ −200) and low-density (−200 ∼ +350) regions to measure the progress of low-density regions that contained voids during the storage period. The seed and external pixels were manually removed using the paintbrush tool in ImageJ (NIH, Bethesda, USA) to measure the average CT value for the flesh part only. The average CT value, standard deviation and peak height were calculated using ImageJ at a 0–255 GS threshold value. An X-ray absorption histogram comparing the pixel ratio and GS value was produced, incorporating all acquired images during the storage period, and was used to compare among the control and 1-MCP-treated fruit. All X-ray computed image data were calibrated against distilled water as the standard material.
Statistical analysis Pair wise one-way ANOVA for means based on Tukey's method was performed on the data for TSS, % weight loss and firmness to identify differences among the control and treated fruits during storage. All statistical analyses were performed with R ver. 3.1.2 (R Core Team, R Foundation for Statistical Computing, Vienna, Austria).
Effect of 1-MCP treatment on ethylene concentration inside packaging during storage The ethylene concentration inside the polypropylene bag suddenly increased in control fruit and peaked within 5 d (Fig. 1A). For 1-MCP-treated fruit, the ethylene concentration inside the package was nearly constant for the first 4 d of storage and then increased gradually. More interestingly, the ethylene concentration of 1-MCP-treated fruit at 5 d was the same as that at 2 d for control fruit. The control fruit showed a perfect climacteric pattern in the ethylene formation curve. The climacteric pattern of Japanese apricot is well established (Luo et al., 2009; Paula et al., 2008), and a similar result was found by Erkan and Eskl (2012).
Physicochemical quality parameters observed during storage: (A) Ethylene concentration inside package, (B) Apparent density, (C) % Weight loss, (D) Bioyield stress and (E) % TSS
Quality parameters observed during storage Figure 1 shows the changes in quality parameters, such as apparent density (B), % weight loss (C), bio-yield stress (D) and TSS (E), during the storage period. 1-MCP-treated fruit showed a lower amount of weight loss and tended to have higher apparent density values. The % weight loss of 1-MCP fruit was much lower than that of control fruit. A continuous decrease in apparent density was observed in control fruit. Fruit ripening is a complex process involving continuous change in internal constituents. The climacteric nature of Japanese apricot involves a high respiration rate, accompanied by higher conversion of complex polysaccharides to simple ones, and simple sugars hydrolyzed to moisture (Othman, 2009; Othman and Mbogo, 2009). The fruit firmness was measured in terms of bio-yield stress. The bio-yield stress sharply decreased in control fruit, whereas this was delayed by two days in 1-MCP-treated fruit. Control and 1-MCP-treated fruits followed the Weibull model, as shown in equation (2). The bio-yield stress at a certain time t corresponding to the storage day was estimated by:
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where F0 is the initial bio-yield stress. The values of α and β were calculated as 3.458 and 0.97 for control fruit and 6.908 and 3.042 for 1-MCP treated fruit, respectively. The fitness curve is shown in Fig. 1 D. The control fruit curve showed a better fit than the 1-MCP-treated fruit, as 1-MCP-treated fruit demonstrated less change in bio-yield stress. At the end of the 7-d storage period, there was a significant difference between the control and 1-MCP-treated fruit. After 7 d, the 1-MCP-treated fruit had an acceptable firmness value of 2×106 N/m2, which was close to the firmness of the control fruit at the second day of storage. TSS content of the control fruit showed an increasing pattern from the first day of storage, whereas the treated fruit started gaining soluble solids after the fourth day of storage. The 1-MCP-treated fruit initiated ripening on the fourth d, followed by increasing TSS content on the seventh d of storage. At the end of the storage period, the TSS content in 1-MCP-treated fruit was significantly higher than that in control fruit. In this study, at the end of the 7-d storage period there were significant differences in firmness, apparent density and TSS content among the control and 1-MCP-treated fruit.
Ethylene has an obvious effect on fruit ripening and senescence. Fruits undergo various types of metabolic reaction during ripening that change their color, flavor and taste to make them acceptable to consumers. 1-MCP has an obvious effect on lowering ethylene production in climacteric fruit and vegetables to extend their green life (Blankenship and Dole, 2003). 1-MCP binds to ethylene receptors to inhibit ethylene production during the ripening stage (Sisler et al., 1996). In this study, the ethylene concentration inside the polypropylene bag of control fruit was much higher than that of 1-MCP-treated fruit. After ripening, ethylene contributes to accelerate metabolic respiration; thus, concentrations tend to decrease in control fruit after ripening due to metabolic activity. 1-MCP could satisfactorily decrease the ethylene production and respiration rate in peaches, pears, oranges, strawberries and bananas (Palou et al., 2003; Porat et al., 1999; Tian et al., 2000). 1-MCP suppresses gene expression associated with the ethylene-signaling pathway in banana fruits to extend the green life at high temperatures (Yan et al., 2011) and also depresses the peel yellowing of Chinese pears (Cheng et al., 2012). After harvesting, the fruit cells remain alive and the acid content decreases with the increment of sugar content, making the fruit sweet. Continued respiration increases the conversion of solids to moisture and volatile gas; thus, during storage, there is a decrease in solids after peak ethylene formation, followed by senescence. The cell wall permeability increases in fruits stored at room temperature, causing moisture loss and creating voids.
X-ray CT image properties X-ray CT images depicting the internal condition of control and 1-MCP-treated fruits during storage are shown in Fig. 2, obtained from images processed by Nashita software to separate low- (−900 ∼ −200 HU) and high-density (−200 ∼ +350) regions. The blue to green color indicates the low-density region. The blue and red colors indicate the void space and water-rich regions, respectively. It is clearly seen that the 1-MCP-treated fruit has no void spaces and fewer low-density regions in the internal structure within seven days, while the control fruit shows higher amounts of gas-filled void spaces. The void spaces begin at the calyx and progress towards the area surrounding the seed. The images demonstrate that the area of maximum moisture is concentrated around the stone region. Ethylene production is first initiated over the calyx of persimmon fruits in response to water stress, and this ethylene contributes to autocatalytic ethylene formation in the pulp of persimmon fruit. The calyx is the main region for initial ethylene production in persimmon fruit, and 1-MCP satisfactorily suppresses this ethylene production through the calyx (Nakano et al., 2002).
Representative internal CT image during the storage period (Top: Control; Bottom: 1-MCP)
Histograms were generated from the first d of harvest and after seven d of storage, incorporating all sliced images obtained from the X-ray CT scanner. The histogram profile was made for the flesh part using ImageJ. Figure 3 clearly shows that the histogram peak decreased in all fruits, but shifted to the left only in the control fruit during storage. However, the degree of lowering of the histogram peak was much smaller in 1-MCP-treated fruit. 1-MCP-treated fruit had a similar histogram profile during the 7-d storage period, while the control fruit showed larger low-density regions including fewer void spaces, which may have caused the lower histogram peak, and the profile shifted to a lower GS value. Here, ΔGS = 0.93, because the GS value was calibrated using distilled water as the standard material. The peak of the histogram depends on the increment of GS value. The differences in the CT histogram profile between 0 and 7 d for 1-MCP treatment and control samples are shown in the attached graph (Supplemental Figure S1). For 1-MCP-treated fruit, changes in the profile occurred only in the peak region of the histogram, whereas profile changes in the control fruit occurred in the peak and lower GS regions.
Histogram profiles for flesh part of Japanese apricot at 0 and 7th (*) day of storage
The X-ray CT image properties of average CT value, standard deviation, peak height and low-density pixel volume were measured. The storage trends of the image properties are shown in the attached graph (Supplemental Figure S2). Figure S2 shows that the image properties for 1-MCP-treated fruit changed minimally as compared to those of the control fruit. The peak height decreased most in the control fruit, followed by the 1-MCP-treated fruit. The standard deviation (GSSD) increased over time in the control and 1-MCP-treated fruit. During storage, the average CT value (GS) decreased significantly in the control fruit, but remained almost constant in the 1-MCP-treated fruit. For 1-MCP-treated fruit, the CT image obtained at the seventh storage d showed nearly the same appearance as that on the day of harvest. Pixel-based approaches are widely used for identifying internal defects and infestation in grain kernels (Narvankar et al., 2009; Neethirajan et al., 2016). X-ray CT was used to detect the internal decay of chestnuts, internal defects of pickled cucumbers, pit presence and infestation of tart cherries (Donis-gonza et al., 2014), and undesirable fibrous tissue in carrot (Donis-gonzález et al., 2015). From the X-ray CT properties, it is clear that 1-MCP promoted retention of the original structure of Japanese apricot for seven d at room temperature.
The correlations of image properties with apparent density and bio-yield stress are shown in Fig. 4. There was a linear relationship between average CT value, standard deviation, low-density pixel volume and peak height of histograms to apparent density and bio-yield stress of Japanese apricot. The standard deviation and low-density pixel volume were negatively correlated with apparent density and bio-yield stress. However, apparent density and bio-yield stress were positively correlated with average CT value and peak height. X-ray absorption depends on internal components as well as the structure of the material. The standard deviation of histogram and low-density region of fruit increased due to the inconsistency in the internal structural network of solid, water and void spaces. In contrast, X-ray CT absorption and peak of histogram increase when the solid content increases and the porosity decreases. X-ray CT images can illustrate the internal moisture, solid and void network structures of fruits. The average CT attenuation coefficient value is an indication of the moisture-to-solid ratio of fruits and has strong relationships with moisture, TSS, pH and acidity of peaches and mangos (Barcelon et al., 1999a; Barcelon et al., 1999b). Thus, X-ray CT images may be used as a good indicator of internal quality during the storage of Japanese apricot fruit.
Relationship among average CT value, standard deviation, low density pixel volume and average peak height to apparent density and bio-yield stress
We concluded that 1-MCP could suppress the ethylene production rate and extend the period of optimum acceptable quality of Japanese apricot during storage at room temperature. 1-MCP could be a very effective option for maintaining fruit firmness. X-ray CT images can be used to identify voids and internal defects within fruits during sorting and grading lines in the fruit industry. Further, the internal quality of fruits could be compared using histogram profiles. Histogram properties have the potential to be used as indicators of postharvest quality parameters during fruit ripening. The image properties showed linear relationships with some internal qualities. X-ray CT imaging may be considered to be a valuable tool for the non-destructive internal quality evaluation of Japanese apricot.
Acknowledgements This work was partially supported by JSPS KAKENHI Grant Number JP26292135 and 17H03899. 1-MCP was kindly provided by Rohm and Haas China Inc.
