Original papers
Application of radio frequency heating in water for extending the shelf-life of fresh-cut Japanese loquat fruit (Eriobotrya japonica)
2021 Volume 27 Issue 6 Pages 847-857
Details
2021 Volume 27 Issue 6 Pages 847-857
Heating is conventionally used for preserving and extending the shelf-life of food products. However, the original taste, flavor, and texture of products can be changed with extended heating. In this study, we applied radio-frequency heating (RF), a technique that produces uniform heating and reduces the heating time, to extend the shelf-life of fresh-cut loquat fruit, also called biwa in Japan. Results demonstrated that the application of RF at 70, 80, and 90 °C maintained the color and textural quality of fruit samples as well as reduced the enzyme activity of polyphenol oxidase, which causes oxidative discoloration, during 21-day refrigerated storage. RF also reduced the heating time by up to five-fold as compared to conventional heating, leading to improved physical, chemical, and aroma attributes of the fresh-cut loquat fruit samples. In addition, RF also reduced the growth of Escherichia coli in the fresh-cut fruit, indicating that RF is a promising technology for extending the shelf-life of fresh-cut fruits and their products without compromising quality and safety.
Recent growth in the fresh-cut produce industry has been mainly attributed to increasing consumer interest in healthy, freshly prepared, convenient, and ready-to-cook fruits and vegetables (Qadri et al., 2015). However, challenges faced by this industry include reduced shelf-life, high rates of enzymatic browning, textural loss, microstructural degradation, and generation of compounds that can impart undesirable aromas and flavors (Lara et al., 2019; Watada et al., 1996).
Loquat (Eriobotrya japonica) is a widely grown fruit in the subtropical regions of China, Japan, India, Israel, and the Mediterranean (Ding et al., 2002). It is a small evergreen tree that bears 3 to 7 cm-long, slightly pear-shaped or oval fruit. The loquat fruit has a characteristic mild and slightly acidic apple flavor, which is much sought after by consumers (Pino and Marbot, 2002). In addition, the fruit is rich in sugars, organic acids, carotenoids, flavonoids, phenolic acids, and vitamins, and the fruit extract is known to have anti-inflammatory, anti-diabetic, anti-cancer, antioxidant, anti-obesity, anti-aging, anti-allergic and antinociceptive effects, and is also thought to have antithrombotic potential. In addition, it has been reported to improve liver, lung, renal, and neuronal cell function (Liu et al., 2016). Unripe loquat fruit contains high amounts of malic acid, and small amounts of citric, succinic, fumaric, tartaric, and ascorbic acids. As the fruit ripens, these acids are converted into sugars such as glucose, fructose, and sorbitol (Hasegawa et al., 2010).
One of the most popular products is fresh-cut loquat fruit, which is mainly used in baking and preserving as well as in the production of sweets and jellies. However, such applications face the challenge of rapid enzymatic browning after peeling and crushing. Studies have revealed that loquat fruit contains a significantly high amount of polyphenols such as chlorogenic, neochlorogenic, hydroxybenzoic, and 5-feruloylquinic acids, which promote the browning process (Ding et al., 2002). To address these undesirable changes and to extend the shelf-life of fresh-cut loquat fruit, several preservation techniques have been devised. For example, Ding et al. (2002) used various concentrations of sulfhydryl compounds to slow enzymatic browning in peeled loquat fruit. Zheng et al. (2010) reported that 1-methylcyclopropene decreased internal browning of the fruit and inhibited polyphenol oxidase (PPO) activity. Kahramanoğlu (2020) attempted to coat cut loquat fruit with edible Opuntia ficus-indica extracts, Nigella sativa oil, propolis extract, cinnamon oil, and extracts of Chrysanthemum coronarium to reduce undesirable storage effects such as weight loss, browning, and decay. Cai et al. (2006) investigated the effect of low temperature in reducing cold injury and extending the shelf-life. A limited number of studies have also focused on the application of heat in improving the shelf-life of fresh-cut fruits especially fresh-cut loquat fruit.
Heating is an effective method to extend the shelf-life of fresh-cut products, by inactivating enzymes and controlling microbes. For example, Lopes et al. (2014) used thermal treatments between 50 and 70 °C to inactivate the enzymes PPO and peroxidase (POD), and to inhibit the growth of yeast and molds in the whole fruit and puree of peach, while Vanden Abeele et al. (2019) investigated the effects of mild heat on PPO and POD activities in fresh-cut lettuce. Koukounaras et al. (2008) found that, when subjected to short-duration heat treatments, fresh-cut peaches retained their quality and firmness, and showed less browning during storage. However, conventional heating (CH) processes usually lead to undesirable product changes, such as reductions in flavor and taste as well as the loss of essential nutrients. New technologies such as ultrasound, irradiation, ohmic heating, high hydrostatic pressure, and pulsed electric field are being used to maintain the sensory and nutritional qualities of food products. Uemura et al. (2017) applied direct resistance heating as a potential substitute for CH. This included ohmic heating with frequencies ranging from 0–1 MHz, radio-frequency (RF) heating between 1–300 MHz, and microwave heating between 300–3 000 MHz.
RF is a form of electrical heating and is one of the emerging alternatives to CH processes for food packed in vacuum-pouches. This method, which is based on dielectric heating, can shorten the heating duration and achieve uniform heating. Kanafusa et al. (2018) showed that RF of vacuum-packed saury resulted in fish that were enriched with low-molecular collagen peptides, with only a minimum heating time. As there are limited studies on the processing of fresh-cut loquat fruit with RF, our study aimed to investigate the immediate effects of RF on the overall quality of fresh-cut loquat fruit and during storage at 5 °C for 21 days.
Chemicals Chlorogenic acid (95% purity) and desoxycholate-citrate agar (DCA) used in this study were purchased from Fujifilm Wako Pure Chemical Corporation (Osaka, Japan).
Fruit material Fresh loquat (Mogi, Eriobotrya japonica) fruit samples, cultivated in Nagasaki, were procured from a local supermarket in Tsukuba, Japan during the harvesting season between May and August 2020. Samples were selected based on their freshness and maturity. Fruit with very soft texture, blemishes, and areas of rot were discarded. The samples were washed with running tap water, peeled, and manually sliced into half pieces (thickness = 7 mm, diameter = 30 mm). Four pieces, weighing approximately 120 g, of fresh-cut loquat fruit were placed in heat-resistant plastic bags (length = 260 mm, width = 150 mm, thickness = 0.20 mm; BR, Meiwa Pax Co., Ltd., Osaka, Japan), to each of which 50 mL of gum syrup sugar solution (12% Brix) was added and vacuum packed for conventional and RF. The sugar solution was used to simulate the same conditions as for commercially available vacuum packed loquat products, and the 12% Brix value was chosen to maintain a similar sugar solution concentration to that of total soluble solids of loquat fruit.
Radio-frequency heating process In each of the RF experiments, three vacuum bags of fresh-cut loquat fruit samples were placed in the RF chamber (height = 150 mm, width = 150 mm) between parallel electrodes (length = 140 mm), as shown in Fig. 1. A distance of 30 mm was maintained between the electrodes. The RF machine used a frequency of 27 MHz and 4 kW produced from a high-frequency source (Seren Industrial Power Systems, Inc., Vineland, NJ, USA). Meanwhile, the vacuum bags were perforated with an assembly jig where the optical fiber thermometer (Neoptix, Inc., QC, Canada) was inserted. The vacuum bags (length = 260 mm, width = 150 mm, thickness = 0.20 mm; BR, Meiwa Pax Co., Ltd.), were then sealed tightly to prevent the entry of gases. The surrounding water was circulated with an external pump and heated using a temperature-controlled bath and a heat exchanger (TB-HE-Ti-02B; Tokyo Braze Co., Ltd.) at 70, 80, or 90 °C. The application of RF power was terminated upon reaching the desired sample temperature of 70, 80, or 90 °C, and the temperature was held constant for 1 min in the surrounding water at 70, 80, or 90 °C, respectively, before rapidly cooling to 40 °C in an ice water bath. The use of the 1-min holding time was based on our preliminary experiments, in which this time duration was sufficient to inactivate PPO enzyme level.
Schematic diagram of the RF process.
Conventional heating (CH) Vacuum packed fresh-cut samples of loquat fruit were conventionally heated in a temperature-controlled bath at 70 °C for 30 min; the core temperature was measured using a thermocouple attached inside the loquat fruit sample and the results were recorded in a datalogger GL220 (Graphtec Corporation, Yokohama, Japan) every second. The duration of 30 min was specifically chosen because of the reduction in PPO activity at this time.
Assessment of physicochemical changes during storage Immediately after the experiment, changes in the color of untreated, RF heated, and CH samples of fresh-cut loquat fruit were monitored using a spectrophotometer (CM-5; Konica Minolta, Tokyo, Japan), and were expressed using the CIE L*a*b* values read from the spectrophotometer. For the results, the L* value was only used to evaluate the color change in the untreated and heated samples because this value describes the change in the lightness of samples. The L* value was also chosen because of its suitability for understanding color changes due to browning in the samples.
Textural changes were evaluated using the Texture Profile Unit (TPU 2DL; Yamaden, Tokyo, Japan). A 6 mm φ probe was longitudinally inserted into the fresh-cut loquat fruit slices at a deformation rate of 2.5 mm/s. The hardness value of each sample, which is defined as the peak force at the first compression cycle, was measured.
PPO activity PPO activity was measured using the standard procedure reported by Son et al. (2015) with some modifications. Five grams of the fresh-cut loquat fruit sample per treatment was homogenized using a handheld homogenizer (Fisherbrand; Thermo Fisher, Tokyo, Japan) in 30 mL of sodium phosphate buffer (0.1 M, pH 6.0), and then centrifuged in a high-speed refrigerated centrifuge (Avanti Centrifuge HP- 25; Beckman) at 12 000 rpm for 30 min at 4 °C; the supernatant was used for the analysis. PPO activity was measured by the increase in absorbance at 420 nm for chlorogenic acid at 25 °C using a spectrophotometer (UV-Vis, V-730; JASCO Inc.). A 0.5-mL aliquot of the supernatant was mixed with 2 mL of sodium phosphate buffer and 100 µL of 0.02 M chlorogenic acid solution. The same mixture in which the supernatant was replaced by distilled water was used as the reference blank. Enzyme activity (units/mL) enzyme) was determined on the linear section of the activity curve. The change in the absorbance value by 0.001 per min was defined as 1 unit of PPO enzyme activity.
Microbial analysis The microbial analysis used in this study was adapted from Lara et al. (2019) with some modifications. Preceding our microbial experiments, 1 mL of Escherichia coli bacterial broth was added to the vacuum bags with the loquat fruit samples. After the RF and CH experiments, 5 g of the fresh-cut loquat fruit samples was aseptically collected from the vacuum bags and transferred to sterile stomacher bags. To each sample, 10 mL of sterile Milli- Q water was added and homogenized in a stomacher for 30 s. One milliliter of each sample was collected for serial dilutions. One milliliter per dilution (100 to 107) was plated on petri dishes, onto which 15 mL of DCA agar was poured before incubating for 24 h at 37 °C. E. coli colonies were manually counted and expressed as CFU/mL (colony forming units per mL of sample). These tests were conducted in at least duplicate on a clean bench at room temperature.
SPME-GC/MS analysis The effect of heating on the volatile compounds of fresh-cut loquat fruit samples was studied according to the method of Tomita et al. (2018), using a GCMS-QP2010 Ultra (Shimadzu, Kyoto, Japan) equipped with an AOC-5 000 autosampler (Shimadzu). For SPME, a divinylbenzene/carboxen/polydimethylsiloxane fiber 2 cm in length and 50/30 µm in diameter (Sigma-Aldrich, St. Louis, MO, USA) was used. Prior to GC/MS measurements, 5 g of loquat fruit samples was transferred to a 20-mL screwcap vial and stored at 4 °C. During the analysis, the samples were initially heated at 50 °C for 10 min with constant agitation at 250 rpm. The headspace volatile compounds were released by exposing the SPME fiber for 20 min and then further desorbed for 5 min in an injection port operating at 250 °C in splitless mode. The Rtx-WAX capillary column (60 mm x 0.25 mm I.D. x 0.25 µm film thickness; Restek, Bellefonte, PA, USA) was used to separate the compounds, with helium as the carrier gas at a flow rate of 2 mL/min. The column temperature was isothermally held at 40 °C for 5 min, and further elevated by 5 °C/min to 180 °C, followed by 10 °C/min to 200 °C, and held for 5 min. The electron ionization mode was used for the MS analysis. The comparison of mass spectra and retention indices with those recorded in the National Institute of Standards and Technology (Gaithersburg, MD, USA) was conducted for peak annotation.
Storage tests To investigate the effect of RF and CH on fresh-cut loquat fruit during storage, the untreated, RF heated and conventionally heated (70 °C) packed samples were stored at 5 °C for 21 d to mimic market conditions. Among the RF heated samples, only the samples heated at 70 °C were chosen for storage tests, as they exhibited superior qualities in terms of color, texture, PPO enzyme activity, and microbial load.
Statistical analysis The experiments were conducted in at least triplicate, and data were reported as average ± standard deviation. SPSS Statistics (IBM Statistics 22, Armonk, NY, USA) was used for one-way analysis of variance (ANOVA) to characterize the texture, color, weight loss, enzyme activity, and microbial analysis data. Duncan's multiple range test was applied as the post-hoc test at a 95% confidence level. Significant differences among treatments (p < 0.05) are indicated by different letters.
Comparison of time-temperature profiles Various treatment conditions and the corresponding maximum temperatures in the center of the RF heated loquat fruit are shown in Fig. 2. Our results showed that the temperature at the center of fresh-cut loquat fruit reached 70 °C after 11.37 min of CH at 70 °C, unlike RF, which required only 2.5, 2.98, and 3.16 min at 70, 80, and 90 °C, respectively, to attain the core temperature. Based on our results, the application of RF can reduce the heating time required to achieve the desired temperature of food by up to five-fold as compared to CH. We speculate that this reduction in heating time could have significantly improved the physical, chemical, aroma attributes of our fresh-cut loquat fruit samples. Kanafusa et al. (2018) showed that with RF, the core temperature of fish reached 120 °C three times faster than with CH. As longer heating causes several undesirable changes, including loss of nutritional quality, flavor and aroma as well as tissue softening, in processed foods, a minimum duration of heating is preferred. Birla et al. (2005) reported uniform heat distribution in oranges with RF. However, the small differences observed in the temperature profiles of RF and CH were attributed to differences in the dielectric properties, and shape and size of fruits, as well as differences in the surrounding medium (Birla et al., 2008).
Temperature profile of RF and CH processes.
Changes in the color of RF heated loquat fruit samples The color of fruits and vegetables is due to their natural pigment contents, which go through several changes during maturation and ripening (Barrett et al., 2010). The major pigments responsible for color are chlorophylls for green, carotenoids for yellow, orange, and red, anthocyanins for red and blue, flavonoids for yellow, and betalains for red. In addition, enzymatic and non-enzymatic browning also contribute to the formation of gray, black, and brown pigments.
Color is one of the most essential attributes consumers take into consideration while purchasing fresh-cut products (Khan et al., 2014). Thus, color changes of the fruit after CH and RF were investigated, as shown in Fig. 3. We found a significant decrease in lightness value (L*) in both RF heated and CH samples. We propose that the samples became darker, probably as a result of heating the sugar solution in the vacuum bags. There were no significant differences (p < 0.05) in lightness value among the samples heated by RF at 70, 80, or 90 °C, and those subjected to CH at 70 °C for 30 min. Based on our results, it could be suggested that using RF as low as 70 °C could possibly extend the color quality of fresh-cut loquat fruit samples during storage.
Color changes of fresh-cut loquat fruit after heating. Different letters indicate significant differences between different treatments (Duncan's test; p < 0.05). RF: radio frequency heating; CH: conventional heating
Textural changes of RF and conventionally heated loquat fruit Softening of freshly cut fruits and vegetables can be attributed to several physiological processes, such as the conversion of protopectins into water-soluble pectins due to hydrolysis, decrease in cellulose crystallinity, movement of ions out of the cell wall, or cell wall thinning (Rux et al., 2019). The effects of heating on the texture of fresh-cut loquat fruit were indicated by changes in the hardness value (Fig. 4), which decreased significantly (p < 0.05) in both RF and conventionally heated samples. As compared to the untreated fruit samples, both RF at 70, 80, and 90 °C, and CH at 70 °C for 30 min decreased the hardness value by 7.5, 18.13, 19.84, and 27.34% respectively. It should be noted that CH at 70 °C resulted in the highest decrease in the hardness values as compared to RF heated samples. Among the RF heated samples, heating at 70 °C resulted in the lowest reduction of hardness value of the samples. Therefore, based on these results, we can assume that increasing the heating temperature could possibly lead to increased reduction of the hardness value of fresh-cut loquat fruit samples.
Textural changes of fresh-cut loquat fruit after heating. Different letters indicate significant differences between different treatments (Duncan's test; p < 0.05).
Sila et al. (2008) reported that thermal processing methods, such as cooking under high pressure, led to the solubilization of total pectic polysaccharides, causing significant fruit softening. In addition, a study by Taherian and Ramaswamy (2009) revealed that cooking lowered the hardness values of root vegetables.
In the present study, the CH samples at 70 °C for 30 min showed the lowest hardness values. We speculate that both the duration and the temperature of heating are important parameters that affect the quality of fresh-cut loquat fruit. For instance, CH at 70 °C for 30 min produced lower hardness values than those of the entire temperature range with RF (70, 80, and 90 °C). These results could be attributed to the longer processing time in the CH method, which may have broken down the pectic materials responsible for the textural quality of fruits and vegetables.
Changes in PPO enzyme activity in RF and conventionally heated fresh-cut loquat fruit The oxidation of phenolic substrates by PPO causes browning of fresh-cut fruits and vegetables (Nguyen et al., 2003). Ding et al. (2002) reported that loquat fruit contain high amounts of polyphenols. The main phenolic compounds in ripe fruits include chlorogenic, neo-chlorogenic, hydrobenzoic, and 5-feruloylquinic acids. Hence, it is necessary to measure the enzyme activity and correlate it with the browning of fresh-cut loquat fruit. In this study, we evaluated the effects of RF to 70, 80, and 90 °C and CH to 70 °C for 30 min on the enzyme activity of fresh-cut loquat fruit, as presented in Fig. 5. Our results revealed that RF at 70, 80, and 90 °C significantly decreased PPO activity by 90.04, 98.74, and 98.21%, respectively. Based on our results, the use of RF as low as 70 °C could inactivate PPO enzymatic activity to significantly reduce levels even at shortened heating times. Among the RF heated samples, we found that there were no significant differences between temperatures of 70, 80, and 90 °C in terms of reducing the PPO enzymatic activity; however, increasing the heating temperature also led to increasing inactivation of the enzyme. Lopes et al. (2014) reported that the loss of PPO activity in peaches occurred after incubation at 70 °C for about 9 min. In addition, PPO showed structural changes with the increase in high thermal processing temperature, finally leading to its inactivation. In contrast, the results for CH at 70 °C for 30 min showed increased PPO activity (by 35.86%) in fresh-cut loquat fruit samples. We speculate that such increases during CH can be due to partial conformational changes in the latent enzyme at lower temperatures (Murtaza et al., 2018). Based on our experiments, longer heating times were required before the core temperature of loquat fruit samples reached 70 °C during CH, which lessened the holding time at this temperature and resulted in partial inactivation of PPO. This partial inactivation could have led to the increase in PPO enzyme activity. We also suppose that protein dissociation could have occurred during this partial inactivation, possibly leading to the release of other enzymes and resulting in other protein and enzyme interactions. In addition, the previously mentioned study also reported that heating at 50 °C continuously for 60 min led to a similar increase in PPO activity.
PPO enzymatic activity of fresh-cut loquat fruit after heating. Different letters indicate significant differences between different treatments (Duncan's test; p < 0.05).
Effects of RF and conventional heating on Escherichia coli levels The microbial content of food products is one of the most essential parameters that must be controlled in order to effectively preserve fresh-cut produce (Bico et al., 2009). Increasing incidences of food-borne infections through fresh-cut fruits and vegetables are being reported; therefore, several heating strategies have been evaluated to reduce microbial growth in these food products. In this section of the study, we investigated the application of RF and CH on E. coli growth. Effects of RF at 70, 80, and 90 °C and CH at 70 °C for 30 min on growth reduction of E. coli in fresh-cut loquat fruit are shown in Fig. 6. Here, the growth of E. coli in the liquid contained in the vacuum bags was evaluated. Our results show that both RF at 70, 80, and 90 °C and CH at 70 °C for 30 min completely eliminated the initial E. coli population of 107 CFU/mL to 0 CFU/mL in the fruit pieces (Fig. 6). In our study, we highlight the beneficial effects of short-term RF application at 70, 80, and 90 °C on E. coli control. We speculate that because of the uniform heating within the RF heated samples, which was achieved within only 3 to 4 min, fast and complete inhibition of E. coli growth could be attained. Among the RF heated samples, there were no significant differences between RF at 70, 80, and 90 °C in terms of reducing E. coli growth. Meanwhile, Zhao et al. (2017) similarly found that RF for 5 min at 27.12 MHz dramatically decreased the population of microorganisms from 7.2 to 3.0 log CFU/g.
E.coli counts on fresh-cut loquat fruit before and after heating. Different letters indicate significant differences between different treatments (Duncan's test; p < 0.05).
Characterization of volatiles in fresh-cut loquat fruit after RF The characteristic flavors of tropical and subtropical fruits are vital attributes that are responsible for their great market demand (Pino and Marbot, 2002). The most abundant volatile compounds of Japanese loquat fruit include 2-methylbutanoic acid, (E)-2-hexenol, octanoic and nonanoic acids, (E)-2-hexenol, (Z)-3-hexenol and hexanol or (E)-2-hexenal, hexanol, 2-methylbutanoic acid, and (Z)-3-hexenol. This study reports for the first time the effect of heating on the volatile compounds of fresh-cut loquat fruit.
Table 1 presents the various compounds detected in an unheated loquat fruit sample. We found that the major contributor to the unique aroma and flavor of loquat fruit is (E)-2-hexenal. Additionally, we detected high levels of (E)-2-hexenal in the unheated loquat fruit. Sun et al. (2020) reported that aldehydes including (E)-2-hexenal were dominant volatiles in Chinese cultivars of loquat fruit. In addition, we detected a total of 19 aldehyde compounds, 5 ketones, one alcohol, and one acid in the untreated fresh-cut loquat fruit (Fig. 7). RF of these samples at 70 °C preserved the major volatile compounds, such as (E)-2-hexenal and hexanal, but decreased their absolute concentrations (Fig. 8). We observed that the original concentration of (E)-2-hexenal in the unheated loquat fruit samples was reduced in the RF heated samples. On the other hand, CH at 70 °C for 30 min led to higher reductions in (E)-2-hexenal concentrations compared to the RF heated samples, and increased the aldehyde dodecamethyl pentasiloxane and acetic acid contents (Fig. 9). Based on these results, it is therefore suggested that the use of a minimum heating duration during the heating of fresh-cut loquat fruit samples could decrease the undesirable effects of CH on aroma and volatile compounds of fresh-cut loquat fruit samples. In a separate study, Varga-Visi et al. (2019) reported that the heating of garlic led to the formation of sulfur compounds. This suggests that heating can generate several new flavors and aromas, some of which can be beneficial, while the others detrimental to fresh-cut fruits and vegetables. Moreover, our GC/MS analysis emphasized the significance of RF with shorter heating times in comparison with CH in preserving the aroma and volatile compounds of fresh-cut loquat fruit samples.
| Peak number | Retention time (min) | Names |
|---|---|---|
| 1 | 4.38 | 2-methyl propanal |
| 2 | 5.09 | Hexamethyl cyclotrisiloxane |
| 3 | 5.97 | 2-methyl butanal |
| 4 | 6.07 | 3-methyl butanal |
| 5 | 6.50 | Ethanol |
| 6 | 7.38 | 2-pentanone |
| 7 | 7.97 | Acetonitrile |
| 8 | 8.13 | Octamethyl- cyclotetrasiloxane, |
| 9 | 8.94 | Toluene |
| 10 | 10.02 | Hexanal |
| 11 | 12.11 | N-β-4-Benzeneethanamine |
| 12 | 12.60 | 2-heptanone |
| 13 | 13.53 | (E)-2-Hexenal |
| 14 | 14.18 | 2,6,11-trimethyl- dodecane |
| 15 | 14.79 | Dodecamethyl- pentasiloxane |
| 16 | 15.23 | 2-Octanone |
| 17 | 16.19 | Dodecamethyl- cyclohexasiloxane, |
| 18 | 19.86 | Tetradecamethyl- cycloheptasiloxane |
| 19 | 20.77 | Benzaldehyde |
| 20 | 21.01 | (E)-2-Nonenal |
| 21 | 22.01 | (E,Z)-2,6-Nonadienal |
| 22 | 22.19 | Hexadecamethyl- heptasiloxane |
| 23 | 22.90 | 2,6,6-trimethyl-1-cyclohexene-1-carboxaldehyde |
| 24 | 23.19 | Hexadecamethyl - octasiloxane |
| 25 | 23.60 | 2-methyl-butanoic acid |
| 26 | 28.86 | trans-β-Ionone |
| 27 | 34.69 | 2,4-bis phenol |
GC/MS profile of fresh-cut loquat fruit volatile extracts. Specific numbers correspond with the listed compounds in Table 1.
GC/MS profile of fresh-cut loquat fruit volatile extracts after RF at 70 °C.
GC/MS profile of fresh-cut loquat fruit volatile extracts after CH at 70 °C for 30 min.
Color changes in untreated, RF, and conventionally heated loquat fruit samples upon storage at 5 °C Changes in the color of untreated, RF heated (70 °C), and conventionally heated (70 °C) loquat fruit samples during storage at 5 °C, as indicated by lightness (L*) value, are shown in Fig. 10. Our results revealed that RF and CH lowered the L* value, as the fresh-cut loquat fruit turned slightly darker after heating. However, darkening of the treated samples, as compared to the bright yellow color of the untreated, fresh-cut loquat fruit, may be due to the heating effects on the surrounding sugar solution in the vacuum bags. During storage, oxidation occurs in fresh-cut loquat fruit samples and leads to undesired color changes. After 21-day storage at 5 °C, the pieces of loquat fruit subjected to RF to 70 °C and CH at 70 °C for 30 min maintained their initial L* values, while those of the untreated samples were significantly decreased. This suggests that RF and CH are both effective techniques for maintaining the color of fresh-cut loquat fruit samples under refrigeration (5 °C). However, in terms of efficiency and practicality, the RF process possesses an advantage over the CH process due to the reduced heating time in RF application, which led to similar results with samples treated with CH in terms of color quality of the fresh-cut loquat fruit samples.
Color changes of loquat fruit during storage for 21 d at 5 °C. Different letters indicate significant differences between different treatments (Duncan's test; p < 0.05).
Textural changes of untreated and RF heated loquat fruit samples upon storage at 5 °C Textural changes in the untreated, RF heated and CH loquat fruit samples (both at 70 °C) during 21-day storage at 5 °C are shown in Fig. 11. We found that RF at 70 °C did not significantly reduce (p < 0.05) the hardness of fresh-cut loquat fruit samples, unlike that of CH samples at 70 °C for 30 min, which showed a significantly lower hardness value. This is likely to be associated with the longer duration of CH, which likely damages the cell structure of fruit tissues. Aside from this, our results also feature the effect of reduced heating times achieved through RF, which significantly maintained the textural qualities of fresh-cut loquat fruit samples. After 21-day storage at 5 °C, the untreated fresh-cut loquat fruit samples exhibited 80% reduction in hardness value. Such softening could be related to the gradual dissolution of pectin during storage, which could pose a problem for the textural quality of fresh-cut loquat fruit. This was unlike the RF and conventionally heated samples, which were recorded to have slightly increased hardness values after 21-day storage. The maintained textural qualities, in terms of hardness, of the heated samples could be attributed to inactivation of specific enzymes related to tissue softening of fresh-cut loquat fruit. Vicente et al. (2002) also reported similar results, in which heat treated strawberry fruit exhibited increased hardness during 14-day storage, likely due to the delaying effects of RF and CH on softening. Sivakumar and Fallik (2013) found that heat treated apples had higher soluble pectin, suggesting the inhibition of uronic acid degradation. Loss of the side chains of neutral sugars during heating may also lead to close packing of pectin strands and prevent enzymatic breakdown during storage, thereby maintaining the textural qualities of heat treated apple samples.
Textural changes loquat fruit during storage for 21 d at 5 °C. Different letters indicate significant differences between different treatments (Duncan's test; p < 0.05).
The results of the present study demonstrated that the application of RF at 70, 80, and 90 °C improved the color and textural qualities of fresh-cut loquat fruit, as well as reduced the PPO activity, which causes oxidative damage during storage. We also observed that RF heated samples showed superior qualities over conventionally treated samples in terms of maintaining the color, textural, and enzymatic qualities of fresh-cut loquat fruit. In addition, RF reduced the growth of E. coli in fresh-cut loquat products, making it a promising technology for extending the shelf-life of freshly cut produce without negative impacts on quality and safety. Our overall results suggest that RF is a possible alternative method to CH in maintaining the quality of fresh-cut fruit and vegetable products.
Acknowledgements This research was supported by grants from the Project of the Bio-oriented Technology Research Advancement Institution, NARO (special scheme project on advanced research and development for next-generation technology).
Conflict of interest There are no conflicts of interest to declare.
