Heat treatment and preservative application have been widely used during postharvest storage of many fresh products, but the effect of their combination on citrus storage has rarely been investigated. In this study, the optimal heat treatment (HT) conditions and HT combined with preservative treatment were investigated for Ponkan fruit (
Ponkan (
Application of preservative is an effective way to reduce the decay rate, which can control the growth and spread of microorganisms. The most commonly used preservative mixture in the Ponkan industry contained 200 mg L−1 iminoctadine tris (albesilate), 100 mg L−1 2,4-dichlorophenoxyacetic acid, and 200 mg L−1 imazalil [
Heat treatment (HT) is one of the most commonly used physical measures to reduce fruit decay, which has been reported to be effective in decreasing fly infestation, inducing resistance to chilling injury and enhancing the biocontrol effectiveness of antagonistic yeasts [
In this study, the effect of HT combined with application of preservative mixture, including iminoctadine tris (albesilate), 2,4-dichlorophenoxyacetic acid, and imazalil, on storability and quality of Ponkan fruit was evaluated by analyzing the decay rate, weight loss, color, superficial microbial population, ethanol content, acetaldehyde content, pectin content, and fiber content during postharvest storage. The future applications of this method are discussed.
Fruits of uniform size and color were picked at economic harvest time from an orchard located in Quzhou, China, in 2012–2014.
Fruits used for heat treatments were picked in 2012. The control fruits were submerged in water at room temperature (RT), and the heat treatment groups were submerged in hot water at 55°C for 20 s, 30 s, and 40 s, respectively. After the treatments, fruits were stored in a ventilated warehouse at 10°C and were sampled at 0, 30, 60, 90, and 120 d storage.
Fruits used for combined HT and preservative treatments were picked in 2013 and 2014. The preservative mix includes iminoctadine tris (albesilate), 2,4-dichlorophenoxyacetic acid, and imazalil. 0%, 25%, 50%, and 100% (w/v) of the preservative dosage used in production, which contained 200 mg L−1 iminoctadine tris (albesilate), 100 mg L−1 2,4-dichlorophenoxyacetic acid, and 200 mg L−1 imazalil, were dissolved in both RT and 55°C water and were used for the HT combined preservative treatments. The CK, HT, 25% C (“C” represents the concentration of preservative used in production), HT + 25% C, 50% C, HT + 50% C, 100% C, and HT + 100% C fruits were submerged in corresponding preservative solutions for 20 s. After the treatments, fruits were stored for 0, 30, 60, 90, 120, and 150 d in a ventilated warehouse at 10°C and the fruits with 150 d storage were transferred to shelf storage for analysis at 7 and 15 d. These experiments were repeated twice over two years.
Each treatment contained 300 fruits, which were divided equally into three groups and were considered as three biological replicates. Fruits with obvious appearance of decay were recorded on each sampling day. Decay rate were calculated according to the following formula: decay rate (%) =
Thirty fruits were randomly selected from each treatment and marked. The weight of each fruit was regularly recorded on each sampling day. The weight loss rate was calculated according to the following formula: weight loss rate
Color measurement was carried out with a Hunter Lab Mini Scan XE Plus colorimeter (Hunter Associates Laboratory, Inc., Reston, VA) at four evenly distributed equatorial sites of each fruit. The CIE 1976
Ethanol and acetaldehyde production was determined with a gas chromatograph instrument (Agilent 6890N, Folsom, CA, USA) with an FID column (HP-INNOWAX, 0.25 mm, 30 m, 0.25
Microbial medium was obtained from peel of Ponkan fruit and cultured on potato dextrose agar (Difco, Detroit, MI). Twenty grams fruit peel was put into a blender and homogenized with 100 mL sterile water. 1 mL mixtures for each sample were taken out and homogenized with 9 mL sterile water. A volume of 100
One gram fruit tissue samples were homogenized with 25 mL 95% ethanol, followed by 30 min boiling water bath. Then the homogenate was centrifuged at 8000 rpm for 30 min to remove the supernatant. Repeat the extraction step for three times. The resulting pellet was incubated with 20 mL distilled water at 50°C water bath for 30 min to dissolve the pectin. The homogenates were then centrifuged at 8000 rpm for 15 min. Transfer the supernatant into a 100 mL volumetric flask and wash the precipitate with distilled water. Move the supernatant into the volumetric flask and dilute with distilled water to volume. This is the soluble pectin. Add 25 mL 0.5 mol L−1 sulfuric acid into the precipitate and incubate it in boiling water for 1 h to dissolve the pectin. The homogenates were centrifuged at 8000 rpm for 15 min; then transfer the supernatant into a 100 mL volumetric flask and dilute it with distilled water to volume. This is the protopectin. Incubate 1 mL of the extracted solution with 6 mL concentrated sulfuric acid for 20 min in boiling water. Add 0.2 mL 1.5 g L−1 carbazole-ethanol into the mixture, and keep it in dark for 30 min. The absorbance of the reaction mixture was measured at 530 nm to calculate the content of galacturonic acid by contrast with the standard curve. The total pectin content includes both the solution pectin and protopectin.
Add 5 mL mixture of acetic acid and nitric acid into 100 mg drying samples, incubated with boiling water bath for 60 min; the homogenate was centrifuged at 5000 rpm for 10 min to remove the supernatant. The precipitate was washed with 8 mL water and 8 mL acetone, respectively, and centrifuged at 5000 rpm for 10 min to remove the supernatant. The precipitate was dried at 30°C for 30 min, and then 9 mL 67% suspension samples were stewing for 1 h. Repeat the step for three times. Dilute the supernatant for 50 times. The reaction mixture consisted of 1 mL supernatant, 1 mL phenol, and 5 mL concentrated sulfuric acid (ice cold), incubated at RT for 30 min. The absorbance of fiber was measured at 490 nm.
Standard errors and figures were drawn using Origin 8.0 (Microcal Software Inc., Northampton, MA, USA). The test of statistical significance was based on the total error criteria with a confidence level of 95.0%, calculated by SPSS Statistics 20.0 Software.
The decay rate of Ponkan fruit increased continuously in all the treatments during storage. No decayed fruit were found in heat treatments until 90 d storage, while the control fruit reached a decay rate of 3.33% at 90 d storage. Compared with the control, the decay rate was reduced by 36%, 25%, and 25% in heat treatments at 55°C for 20 s, 30 s, and 40 s, respectively (Table
Effects of different heat treatments on decay rate of Ponkan fruit during storage.
Treatments | Decay rate (%) | ||||
---|---|---|---|---|---|
0 d | 30 d | 60 d | 90 d | 120 d | |
CK | nd | nd |
|
|
|
55°C 20 s | nd | nd | nd |
|
|
55°C 30 s | nd | nd | nd |
|
|
55°C 40 s | nd | nd | nd |
|
|
“nd” indicates no decayed fruit observed.
The decay rate of Ponkan fruit increased continuously in all the treatments during the whole storage period. Compared with the control, the decay rates in all the treated samples were significantly decreased during 90 to 150 d storage. At 150 d storage, the decay rate in the control reached a level of 30%, while all the treated samples exhibited decay rates lower than 18%, with a ranking order of HT, 25% C, 50% C, HT 50% C, HT + 25% C, HT + 100% C, and 100% C from high to low (Figure
Effects of different combined heat and preservative treatments on the decay rate (a), shelf life (b), weight loss (c), and CCI (d) of Ponkan fruit during storage. “C” represents the concentration of preservative used in production. The error bars represent the standard errors. Letters on columns represent significant differences at the 0.05 level.
From the above results, the combined heat plus 25% preservative treatment significantly decreased the decay rate of Ponkan fruit during storage, showing a similar effect to that with 100% preservative treatment. This may be because the heat treatment was effective in enhancing the efficacy of preservative [
The epiphytic microbial population increased in all the treatments during the whole storage period. No significant difference was investigated between the treated and untreated samples during the early storage period. Compared with the control, the 100% C and HT + 100% C treatments significantly decreased the epiphytic microbial population during the later storage period. At 120 d storage, the microbial population was 363, 290, 162, and 60 CFU L−1 in CK, HT + 25% C, HT + 50% C, and HT + 100% C treatment, respectively, showing a decreasing trend with increasing preservative concentration. At 150 d storage, the microbial population was decreased to 65% and 60% by 100% C and HT + 100% C treatments, respectively, while no significant difference was observed between the other treatments and the control (Figure
Effects of combined heat and preservative treatments on the epiphytic microbial population of Ponkan fruit during storage. “C” represents the concentration of preservative used in production. The error bars represent the standard errors. Three repetitions were used in this analysis.
The content of ethanol in fruit pulp showed an increasing trend during storage in general, and it occurred more rapidly during the later storage period. A sharp increase was observed in HT treatments at 90 and 120 d storage compared with the control and other treatments. During the late storage period, the ethanol content of fruits in the HT + 25% C treatment was significantly lower than with the 25% C treatment alone. At 150 d storage, the ethanol contents in fruits of all the combined treatments were lower than those where preservative was used alone (Figure
Effects of combined heat and preservative treatments on ethanol (a) and acetaldehyde (b) contents in the fruit pulp during storage. “C” represents the concentration of preservative used in production. The error bars represent the standard errors. Different superscripts between columns represent significant differences between samples at the 0.05 level.
Acetaldehyde and ethanol accumulated under anaerobic conditions during fruit development or storage, for example, through coating with films or in modified atmospheres or packages [
The variation of fiber contents was relatively stable in all the treatments during storage. Compared with the control, the fiber content was lower in HT treatments but showed no regular trend during the whole period. No significant difference was observed between samples in 50% C and HT + 50% C treatments or between the 100% C and HT + 100% C treatments. However, the HT + 25% C treatment significantly increased the pectin content between 60 to 150 d storage when compared with the 25% C treatment alone (Figure
Effects of combined heat and preservative treatments on pectin content in the fruit peel during storage. The error bars represent the standard errors. “C” represents the concentration of preservative used in production. Different superscripts between columns represent significant differences between samples at the 0.05 level.
The fiber contents showed irregular variation in all the treatments during storage. No significant difference was observed in fiber contents among different treatments at 30 and 60 d storage. Generally, the combined treatments increased the fiber content compared with the single preservative treatments from 90 to 150 d storage. Significant differences were investigated in all the combined treatments when compared with the corresponding single preservative treatments at 150 d storage, indicating that the HT treatment was effective in increasing the fiber content in fruit peel during long time storage (Figure
Effects of combined heat and preservative treatments on total fiber content in the fruit peel during storage. “C” represents the concentration of preservative used in production. The error bars represent the standard errors. Different superscripts between columns represent significant differences between samples at the 0.05 level.
Pectin and fiber are important constitutes of the main structural support in the cell wall, playing important roles in material transportation and protection against adversity. Previous reports showed that the cell wall participated in perception of external signals by changing the components in response to various environmental stimuli or stresses [
Heat treatment at 55°C for 20 s was effective in extending the storage life of Ponkan fruit. Furthermore, heat combined with 25% preservative treatment significantly decreased the decay rate of Ponkan fruit during storage, showing a similar effect to that observed with 100% preservative treatment. This may be because the heat treatment was effective in enhancing the efficacy of the preservative, permitting a decrease in the amount of chemicals used for decay control in Ponkan fruit. In addition, the increased pectin and fiber contents in fruit treated with HT combined with preservative suggested that the structure and intensity of cell wall were affected so as to prevent fungus infection and enhance fruit resistance during postharvest storage.
The authors declare that they have no conflicts of interest.
Dandan Tang and Qiong Lin contributed equally to this work.
The authors thank Professor Donald Grierson from the University of Nottingham (UK) for his efforts in editing the manuscript language. This research was supported by the Project of the Science and Technology Department of Zhejiang Province (2013C02019-1) and the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2012BAD38B03).