Original papers
Effects of Aqua-gas Drying Conditions on Functional Components and Antioxidant Activities in Egoma (Perilla frutescens (L.) Bitt. var. frutescens) Leaves
2019 年 25 巻 1 号 p. 39-48
詳細
2019 年 25 巻 1 号 p. 39-48
Aqua-gas (AQG; superheated steam containing micro-droplets of hot water) was developed as a pretreatment technology before primary processing, such as sterilization, without sacrificing the material yield and maintaining qualities such as functionality. Its application to drying treatment, however, had yet to be studied. In this study, we examined its applicability to drying treatment of egoma (Perilla frutescens (L.) Bitt. var. frutescens) leaves. Results revealed that when egoma leaves were treated with AQG, they underwent a drying process that progressed with time, as was the case for treatment with superheated steam (SHS). AQG treatment produced dried leaves with a final wet weight-based water content of <10 %. We then investigated the effects of different drying methods on functional components in egoma leaves, and found that such components (e.g., α-linolenic and rosmarinic acids) were preserved during treatment with AQG, or AQG followed by SHS, at levels comparable to those by freeze drying (FD). In contrast, rosmarinic acid content was reduced after drying with SHS alone or hot air drying (HAD). In addition, egoma leaves were successfully sterilized by treatment with AQG, SHS, or AQG followed by SHS, or by steaming followed by HAD. Common bacteria remained viable after HAD treatment. Moreover, egoma leaves dried with AQG, AQG followed by SHS, or steaming followed by HAD had significantly higher DPPH radical-scavenging activity than the FD-treated product. In contrast, such activity was reduced after HAD. In addition, radical-scavenging activity correlated well with total soluble polyphenol content and polyphenol oxidase activity. These results suggest that AQG treatment shows promise as an efficient approach to completing the multiple steps generally required for drying leafy vegetables (blanching, drying, and sterilization) in one step, while preserving functional components.
The effects of functional components in vegetables and other agricultural crops have been well investigated, and both consumers' interest in domestic agricultural products and the importance of vegetables in health-promoting diets have been increasing. Meanwhile, many agricultural crops with high functionality are under-utilized and discarded. Egoma (Perilla frutescens (L.) Bitt. var. frutescens) is one such example. Its seeds are cultivated mainly for oil, while the leaves are rarely used (some are used in kimchi [Korean pickle] production). Egoma leaves contain α-linolenic acid, one of the n-3 essential fatty acids, and rosmarinic acid, a kind of polyphenol. The former has been reported to have an anti-obesity effect (Chicco et al., 2009), improve insulin resistance (Chicco et al., 2009; Muramatsu et al., 2010), and be partly converted in the body to EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) (Ezaki et al., 1999), both of which are reported to maintain/ improve brain function (Okuyama, 1995). Meanwhile, rosmarinic acid has been reported to have high antioxidant (Meng et al., 2008) and anti-allergic effects (Takano et al., 2004). In this study, we focused on the contents of α-linolenic and rosmarinic acids as specific components in egoma leaves for the following reasons: egoma oil contains more α-linolenic acid than any other agricultural oil crop, and constitutes approximately 60 % of the fatty acid components. Meanwhile, the rosmarinic acid content of egoma leaves (approximately 37.0 mg/g DW, Suzuki and Imaizumi, 2013) is reportedly higher than that of rosemary (Rosmarinus officinalis L.) (21.5 mg/g DW, Kido, 2004), where it was first identified and is present in high levels. However, both components are easily oxidized.
Agricultural crops are generally produced at specific times in a concentrated manner, and how best to preserve them has been an issue. Widely used methods for improving storage stability include drying treatment. Hot air drying is a common method, and has been used for tomato (Orikasa et al., 2006), cut apple (Morifusa et al., 2012), kiwifruit (Yoshida et al., 2014), and spinach (Hirota et al., 2000), among others. This method is not effective, however, when functional components are susceptible to oxidation (Luning et al., 1995). The freeze-drying (FD) method is conducted under low oxygen and temperature conditions (Miyasaka, 1987; Yamaguchi et al., 2012); however, this method takes a long time to complete, and thus has a high running cost. In addition, superheated steam (SHS) drying, in which saturated steam is heated to above 100 °C with an external heater and is also conducted under a low oxygen state, has been tested for various materials. For example, SHS-dried potatoes retain high color and vitamin C (Shibata and Mujumdar, 1994), while burdocks, carrots, and lotus root dried with this treatment not only retain their color, but are also functionally enhanced, e.g., increased sweetness (Miyatake, 2008). In contrast, Ichige et al. (2009) reported that, in carrots, the same drying treatment reduced carotene levels compared with air drying, presumably due to the high treatment temperature, confirming that the method is difficult to control.
Therefore, as an improved variant of this method, we attempted to apply an Aqua-gas® (AQG; superheated steam containing micro-droplets of hot water) drying method. Since AQG is characterized by fine water droplets contained under a superheated steam atmosphere, heat-transfer coefficients to materials are extremely high. For this reason, AQG was originally developed as a pretreatment technology before primary processing, such as sterilization, without sacrificing material yield and freshness (Sotome et al., 2005a). Since AQG contains fine water droplets, it is generally considered less suitable as a drying medium than superheated steam, and to our knowledge, there are no reports of the use of AQG to dry vegetables. However, the quality of dried vegetables is greatly affected by not only the drying conditions, but also the blanching conditions as the pretreatment step in the drying process. As mentioned above, the properties of AQG are suitable for blanching vegetables, and can contribute to the effect as a drying medium depending on the conditions, such as the content of fine water droplets. Thus, it is expected that dry products of enhanced quality could be obtained by using AQG as a heating medium for a series of treatments, from blanching to drying.
In this study, we prepared dried egoma leaves by drying with AQG, SHS, SHS combined with AQG blanching, and hot air (HA) in order to develop a rapid and efficient method of drying egoma leaves while maintaining the levels of functional components. We evaluated the levels of α-linolenic and rosmarinic acids, etc., in the dried product to assess the effects of different treatments on the levels of functional components.
1) Materials tested Egoma leaves (of a Korean strain), cultivated and collected in a field of the Shimane Agricultural Technology Center on July 23, 2012, were used after washing and dehydration.
2) Experimental devices An AQG heating device (AQUA COOKER, Prototype No. 1; Taiyo Seisakusho Co., Ltd., Hokkaido, Japan) was used for the treatment and drying. This device generates steam and hot water by allowing water to boil under pressure and then expand through a nozzle into a heating chamber, generating an atmosphere of superheated steam mixed with fine water droplets that can be used to heat a sample. The heating chamber has an internal volume of 0.178 m3. While the flow rates of steam and water droplets were variable, the temperature at which the mixture of superheated steam and fine water droplets could be maintained stably was 110–120 °C.
For SHS drying, a SHS oven (SDAHW; Chugoku Maintenance Co., Ltd.) was used. The heating chamber of this device has an internal volume of 0.058 m3. The device supplies saturated steam from a steam boiler to a heating chamber after secondary heating, and uses it to heat a sample. Heating is possible at approximately 120–250 °C.
For steam treatment as a pretreatment in HA drying, saturated steam generated by a once-through boiler (SAJ-20; Maeda Technical Works Co., Ltd.) was supplied to a steamer using a pressure-reducing valve. For HA drying, a vacuum equilibrium-heating dryer (BCD-2000U; Yahiro Industry Co., Ltd.) was used.
3) Drying procedures Drying conditions are listed in Table 1. The drying treatment methods used were: (a) Aqua-gas (AQG) drying; (b) superheated steam (SHS) drying; (c) AQG followed by SHS drying (AQG->SHS); (d) HA drying for experimental plots, and freeze drying (FD) for a control plot.
| Freeze dried product (FD, control) |
|---|
| (a) Aqua-gas (AQG) dried products |
| ①AQG drying (120 °C, 57.6 g/min, 15 min) |
| ②AQG drying (120 °C, 64.8 g/min, 20 min) |
| ③AQG drying (120 °C, 72.0 g/min, 25 min) |
| (b) Superheated steam (SHS) dried products |
| ④SHS drying (120 °C, 15 min) |
| ⑤SHS drying (150 °C, 7 min) |
| ⑥SHS drying (180 °C, 4 min) |
| (c) AQG treatment followed by SHS dried products |
| ⑦AQG (120 °C, 108.0 g/min, 2 min) → FD |
| ⑧AQG (120 °C, 108.0 g/min, 2 min) → SHS drying (120 °C, 15 min) |
| ⑨AQG (120 °C, 108.0 g/min, 2 min) → SHS drying (150 °C, 7 min) |
| ⑩AQG (120 °C, 108.0 g/min, 2 min) → SHS drying (180 °C, 4 min) |
| (d) Hot air dried (HAD) products |
| ⑪HAD (55 °C, 5 h) |
| ⑫Steaming (2 min) treatment → FD |
| ⑬Steaming (2 min) treatment → HAD (55 °C, 5 h) |
57.6, 64.8, 72.0 and 108.0 g/min indicate the amounts of micro-droplets of hot water
In AQG drying, the gas-phase temperature of the heating medium was set to 120 °C, the total amounts of superheated steam and fine water droplets supplied were 57.6, 64.8, and 72.0 g/min, and the corresponding drying times were 15, 20, and 25 min, respectively (Table 1 ①∼③).
In SHS drying, the steam temperatures were 120 °C, 150 °C, and 180 °C, and the corresponding drying times were 15, 7, and 4 min, respectively (Table 1 ④∼⑥).
In SHS drying after AQG treatment, the sample was first blanched in an AQG heating apparatus at 120 °C for 2 min with a steam–water droplet flow rate of 108.0 g/min. The blanched sample was quickly transferred to a SHS oven, and dried in 120 °C, 150 °C, and 180 °C superheated steam for 15, 7, and 4 min, respectively (Table 1 ⑧∼⑩). In this treatment, a reference sample subjected to FD following AQG treatment was also prepared (Table 1 ⑦).
In HA drying, the drying temperature was set to 55 °C, and the drying duration was 5 h. Samples dried without blanching treatment (Table 1 ⑪) and samples dried after steam-blanching (Table 1 ⑬) were prepared. In this treatment, a reference sample subjected to FD following steam-blanching treatment was also prepared (Table 1 ⑫). In the steam-blanching treatment, the pressure of saturated steam supplied from the boiler was adjusted to 98 kPa with a pressure-reducing valve, and the temperature was set to approximately 100 °C. A 30-g sample was placed on a cloth, laid in a basket steamer, and processed in the steamer.
All experiments were conducted in triplicate, and the data were subjected to the following analyses. In addition, to confirm changes over time in the drying process with AQG and SHS, a temperature sensor was inserted into the stem of an egoma leaf, and changes in sample temperature and moisture content were measured throughout the drying treatments, which were carried out under conditions identical to drying with AQG (Table 1 ①) and SHS (Table 1 ④).
Thus, we applied the drying conditions described above until the moisture content reached 10 % or less. AQG conditions at which the mixture of superheated steam and fine water droplets could be maintained stably were determined to be approximately 120 °C, 57.6–108.0 g/min. As shown in the results below (Fig. 1), drying was complete at 120 °C, 57.6 g/min for 15 min; based on these conditions, drying conditions were therefore set to greater total amounts of superheated steam and fine water droplets, and for an extended drying time. In the SHS drying conditions, drying was complete at 120 °C for 15 min (see Fig. 1); based on these conditions, drying conditions were set to a higher temperature commonly used for drying, and for a shorter time. In AQG followed by SHS drying conditions, the sample was first blanched in an AQG heating apparatus at 120 °C for 2 min with a steam–water droplet flow rate of 108.0 g/min, and then subjected to SHS drying under the same conditions as SHS drying alone. As the HA drying condition, the drying temperature was set to 55 °C for a period of 5 h, a commonly used duration. Steam-blanching for 2 min before HA drying was set as the additional treatment.
Drying profiles of egoma leaves. AQG, Aqua-gas; SHS, Superheated steam. 57.6 g/min indicates the amount of micro-droplets of hot water.
4) Measurement of moisture content The moisture content of a dried sample was measured as follows. The sample was finely pulverized with a Vibrating Sample Mill TI-100 (CMT Co., Ltd.). After the sample was dried to a constant weight in an isothermal drier (DX 300; Yamato Scientific Co., Ltd.) at 105 °C, the weight of the dried sample was subtracted from that of the original sample, and then the difference was divided by the weight of the original sample to give the wet weight-based water content. Thus, the amount of dry matter in each dried sample was determined, and all the following measurement items were calculated per dry matter.
5) Measurement of α-linolenic acid content α-Linolenic acid in samples was quantified according to the method described in Hashimoto et al. (1999) using the standard material from Cayman Chemical (USA) as follows. First, 20 mL of chloroform: methanol = 2: 1 (v/v) was added to 0.5 g of a sample. After stirring and extraction with a Polytron homogenizer for 1 min, the mixture was centrifuged at 2,000 rpm (760×g) for 10 min at 25 °C. Then, 100 µL of the supernatant was weighed out into a stoppered test tube, and 100 µL of 100 µg/mL tricosanoic acid as an internal standard and 2 mL of methanol were added. The resulting mixture was cooled on ice, 200 µL of acetyl chloride was added while stirring, and then the mixture was heated at 100 °C for 1 h. Subsequently, 400 µL of octane and 5 mL of an aqueous solution containing 0.5 mol of sodium hydroxide in a 10 % (w/ v) aqueous solution of sodium chloride were added, and the resulting mixture was vigorously stirred for 10 min. After centrifugation at 2,500 rpm (1,190×g) for 10 min, the upper layer (octane layer) was dispensed into vials and subjected to the following gas chromatography (GC) analysis.
Equipment: Shimadzu GC-14B, column: DB-WAX (0.25 mm × 30 m; J & W Scientific); column temperature: held at 100 °C for 1 min, increased at 20 °C/min to 180 °C, at 2 °C/ min to 240 °C, then at 4 °C/min to 260 °C, and held at 260 °C for 5 min. The detector was FID, the detector temperature was 260 °C, the temperature in the sample vaporization chamber was 260 °C, and the sample injection volume was 1.0 µL.
Methanol solutions containing 10, 20, 50, 100, or 200 µg/ mL of a standard product of α-linolenic acid were prepared, subjected to methyl esterification in the same manner as the samples, and then subjected to GC analysis. Based on the obtained values, a calibration curve was prepared by the least squares method and the contents in samples were quantified.
6) Measurement of rosmarinic acid content Rosmarinic acid in samples was quantified using a standard material from Wako Pure Chemical Co. (Osaka) as follows. Fifty milliliters of 80 % (v/v) methanol was added to 0.4 g of a sample. After reflux extraction at 80 °C for 1 h, the volume was adjusted to 100 mL with 80 % methanol. After filtration through a 0.45-µm filter, the filtrate was subjected to high-performance liquid chromatography (HPLC) analysis as follows.
Equipment: Shimadzu LC-10AB, column: Shim-pac XR-ODS; eluent: solution A: 20 % aqueous solution of acetonitrile; solution B: 80 % aqueous solution of acetonitrile (both containing 0.2 % formic acid); the solution was held at 100 % of A for 2 min, then the eluent was changed from 100 % of A to 100 % of B in 10 min, held at 100 % of B for 3 min, and then held at 100 % of A for 3 min to complete the gradient elution. Column temperature: 40 °C; flow rate: 1 mL/min; detection: UV 280 nm.
Eighty percent methanol solutions containing 100 and 250 µg/mL of a standard product of rosmarinic acid were prepared and subjected to HPLC analysis. Based on the obtained values, a calibration curve was prepared by the least squares method and the content in each sample was determined.
7) Measurement of total soluble polyphenol content Total soluble polyphenol levels were measured by the Folin-Ciocalteu method (Singleton and Rossi, 1965). Ten milliliters of 70 % (v/v) ethanol was added to 1 g of a sample, and the mixture was stirred and extracted overnight. To 80 µL of serial dilutions of the extract, 80 µL each of phenol reagent diluted 5 times with water and a 10 % (w/v) aqueous solution of sodium carbonate was added. The resultant mixtures were allowed to stand for 60 min, and then absorbance values at 650 nm were determined with a microplate reader (Wallac 1420 ARVOsx Multilabel Counter; Perkin Elmer, Japan). The measured values were expressed as gallic acid equivalents per 1 g of dried egoma leaf powders.
8) Evaluation of antioxidant activity To measure DPPH radical scavenging activity, various reagents including DPPH (1,1-diphenyl-2-picrylhydrazyl) and Trolox were used, purchased from Wako Pure Chemical Co. (Osaka). An 800 µM DPPH solution in ethanol was prepared as needed during analysis. The activity was determined by adding 50 µL of 30 % ethanol, 50 µL of 200 mM MES buffer, and 50 µL of 800 µM DPPH solution to 50 µL of the egoma leaf extract used for measurement of the total soluble polyphenol level; the resulting mixture was stirred for 20 min, and then the absorbance at 520 nm was measured with a microplate reader. Values were expressed as Trolox equivalents per 1 g of dried egoma leaf powders.
9) Measurement of polyphenol oxidase activity Polyphenol oxidase activity was measured according to the methods detailed in Wakayama and Sekine (2003) and Tsuzuki et al. (2006). First, a crude enzyme solution was obtained from dried egoma leaf powders as follows. Ten milliliters of phosphate buffer (pH 7.0) containing 5 % ascorbic acid and 1 % Triton X-100 was added to 0.2 g of dried egoma leaf powder, the mixture was stirred and extracted by shaking at 0–5 °C for 1 h and then centrifuged at 10,000 × g at 4 °C for 10 min. The supernatant was collected and used as a crude enzyme solution. To 15 µL of this crude enzyme solution, 210 µL of 10 mM catechol/0.1 M phosphate buffer (pH 7.0) was added as a substrate. The reaction proceeded at 25 °C for 10 min, then 37.5 µL of 10 % (v/v) sulfuric acid was added to terminate the reaction, and absorbance at 420 nm was measured with a microplate reader as described above. A blank was prepared by adding the reaction terminator before the substrate was added, and was otherwise treated identically to samples. The activity of increasing the absorbance by 1 in 1 min was defined as 1 unit, and activity levels were expressed as relative values to the activity of an FD-treated sample (=1).
10) Counting of viable and coliform bacteria Viable and coliform bacteria were counted using standard methods. Briefly, 99 g of sterilized physiological saline was added to 1 g of a dried sample, and the mixture was stirred for 1 min using a Stomacher to yield a sample stock solution (102 diluted solution). For viable bacteria counts, 1 mL of each serial dilution of this sample stock solution was seeded on a plate, followed by the addition of 15 mL of sterilized standard agar medium. After incubation at 30 °C for 3 days, the colonies formed were counted. For coliform counts, 2-fold concentrated BGLB medium was prepared, 10 mL of which was added to a test tube. A Durham tube was then inserted into the test tube, and the whole unit was sterilized. The same amount of sample stock solution was added. After incubation at 35 °C for 2 days, the sample was judged to be coliform-positive or -negative based on the presence or absence of gas formation in the Durham tube.
11) Statistical processing Component levels, and antioxidant and polyphenol oxidase activities of samples were determined and statistical significance tests were performed using the mean values of duplicate measurements of three replicate experiments (n=3). Bactericidal effects were also determined in duplicate, and the data were expressed as mean values of viable cell counts, and the results in coliform bacteria were assessed in three replicate experiments (n=3).
Statistical analyses were performed using the Tukey-Kramer method with R ver. 3.5.0 for Windows (R Core Team, R Foundation for Statistical Computing, Vienna, Austria).
Drying profile of egoma leaves The temperature and moisture content data during AQG and SHS treatments of raw egoma leaves are shown in Fig. 1. The AQG treatment was confirmed to produce a faster temperature increase than the SHS treatment, as reported previously (Sotome et al., 2005a). This is presumably because the fine water droplets in AQG had an impact on the condensed water layer on the sample surface or the temperature boundary layer of the heat medium. The sample temperature exceeded 100 °C after about 300 s even with AQG treatment, suggesting that the drying process was ongoing. The moisture content reached constant values at around 900 s in both AQG and SHS treatments; the wet weight-based moisture content was about 9.0 % and 5.4 % after treatment with AQG and SHS, respectively. This result was a good reflection of the findings of Sotome et al. (2005b). Regarding the likely mechanism by which drying occurs with AQG treatment, the amount of water evaporating from the sample exceeds the amount of fine water droplets in AQG attaching to the sample. The net evaporative water loss water is attributed to an imbalance in radiant heat-transfer from the device wall surface and convection heat-transfer from the vapor phase of the heating medium to the sample, because the gas-phase temperature of the heating medium is as high as 120 °C.
2) Effects of different drying treatments on functional component levels in egoma leaves Table 2 shows measured levels of the main functional components in egoma leaves subjected to different drying methods including AQG. The detailed conditions of the AQG treatment were as described in Table 1. The hot air drying (HAD) treatment of raw egoma leaves reduced the yield of α-linolenic acid to 90 % of that in the control plot (⑪); however, the difference was not statistically significant. Dried egoma leaves in which original levels of α-linolenic acid were nearly completely retained could be prepared by drying with AQG treatment (①∼③), SHS treatment (④∼⑥), and AQG treatment followed by SHS treatment (combined use, ⑦∼⑩). In addition, products from AQG or SHS treatment under various conditions did not show any particular trends in the yield of α-linolenic acid according to treatment duration or temperature. Similarly, HAD treatment of raw egoma leaves without steaming pretreatment (⑪) drastically reduced rosmarinic acid levels to 10 % compared to the control plot, and drying by SHS treatment alone (④∼⑥) drastically reduced the levels to 40–60 % of those in the control plot. In contrast, dried products containing high rosmarinic acid levels exhibited no appreciable loss when the raw leaves were successfully prepared by drying with AQG treatment or AQG treatment followed by SHS treatment. This was because oxidation progressed in the HAD treatment, while in the SHS drying treatment, more decomposition progressed through, for example, the action of polyphenol oxidase, as described below, due to a slower initial temperature increase compared to the AQG treatment.
| Test No. | α-linolenic acid contents (mg/g DW) |
rosmarinic acid contents (mg/g DW) |
|---|---|---|
| FD (control) | 28.2 ± 1.2bc | 33.9 ± 0.2c |
| (100) | (100) | |
| ① | 31.5 ± 1.2ab | 34.5 ± 0.6c |
| (112) | (102) | |
| ② | 28.3 ± 0.3bc | 43.6 ± 0.4ab |
| (101) | (129) | |
| ③ | 30.5 ± 0.3ab | 41.4 ± 0.6ab |
| (108) | (122) | |
| ④ | 30.8 ± 0.9ab | 15.1 ± 1.5e |
| (109) | (45) | |
| ⑤ | 30.0 ± 0.6ac | 20.6 ± 0.6d |
| (106) | (61) | |
| ⑥ | 32.1 ± 1.2ab | 22.0 ± 0.2d |
| (114) | (65) | |
| ⑦ | 28.5 ± 0.3ac | 39.0 ± 1.3b |
| (101) | (115) | |
| ⑧ | 28.5 ± 0.3ac | 41.2 ± 0.4ab |
| (101) | (122) | |
| ⑨ | 33.3 ± 1.8a | 45.5 ± 1.8a |
| (118) | (134) | |
| ⑩ | 30.6 ± 0.9ab | 44.0 ± 0.3a |
| (109) | (130) | |
| ⑪ | 25.4 ± 0.7c | 5.4 ± 0.3f |
| (90) | (16) | |
| ⑫ | 25.3 ± 0.7c | 43.0 ± 0.7ab |
| (90) | (127) | |
| ⑬ | 28.1 ± 1.2bc | 41.7 ± 1.3ab |
| (100) | (123) |
Drying methods and conditions tested (Test No.) are indicated in Table 1.
Values in parentheses are relative compared with the content of each functional component in the control (FD) as 100.
All data are expressed as mean±SE (n=3).
The different superscripts represent the significant differences p<0.05.
These results revealed that drying with AQG treatment was an effective method for preparing dried egoma leaves, evidenced by the retention of high levels of α-linolenic and rosmarinic acids resulting from their reduced oxidation. In addition, drying with the combination of AQG and SHS was nearly equivalent to AQG treatment in terms of quality preservation; however, it was expected to require less processing time for drying in actual operations.
3) Bactericidal effects of different drying treatments on egoma leaves Table 3 presents the bactericidal effects observed in egoma leaves dried under the conditions listed in Table 1. Assuming that the viable bacterial count of the dried control product (lyophilized product) is the same as that of the raw leaves, the latter had a viable bacterial count in the order of 104, and was also positive for coliform bacteria. In contrast, these microorganisms were not detected after drying with AQG, SHS, AQG followed by SHS, or steam followed by HAD. However, viable bacteria were detectable after HAD without steaming pretreatment. This was likely because in AQG treatments, the heat-transfer coefficients to materials were extremely high, almost 120 °C, and in the SHS treatments, the temperature was extremely high at greater than 120 °C. In the steam-blanching treatment, the heat-transfer coefficients to materials were again as high as in the AQG treatment, and the temperature was almost 100 °C. These results indicate that for effective microbial inactivation, treatment with a rapid temperature increase to at least 100 °C was necessary.
| Test No. | bacteria numbers (cfu/g) |
coliforms |
|---|---|---|
| FD (control) | 2.5×104 | positive |
| ① | <300 | negative |
| ② | <300 | negative |
| ③ | <300 | negative |
| ④ | <300 | negative |
| ⑤ | <300 | negative |
| ⑥ | <300 | negative |
| ⑦ | <300 | negative |
| ⑧ | <300 | negative |
| ⑨ | <300 | negative |
| ⑩ | <300 | negative |
| ⑪ | 4.4×104 | negative |
| ⑫ | <300 | negative |
| ⑬ | <300 | negative |
Drying methods and conditions tested (Test No.) are indicated in Table 1.
All data are expressed as means, or the results were confirmed (n=3).
These results demonstrated that drying treatment with AQG alone or AQG combined with subsequent SHS could effectively kill microorganisms on the surface of egoma leaves by rapid heating of the sample surface in the AQG treatment, as described above.
4) Effects of different drying treatments on the antioxidant activity of egoma leaves Next, we examined the effects of drying raw egoma leaves under the conditions in Table 1 on total soluble polyphenol content and DPPH radical scavenging activity. The results are shown in Table 4.
| Test No. | total polyphenol contents (µmol gallic acid eq./g DW) |
DPPH radical scavenging activities (µmol Trolox eq./g DW) |
|---|---|---|
| FD (control) | 112.6 ± 4.5gh | 120.7 ± 2.9f |
| (100) | (100) | |
| ① | 183.1 ± 3.0e | 211.4 ± 0.7d |
| (163) | (175) | |
| ② | 212.5 ± 1.9ac | 236.4 ± 1.9b |
| (189) | (196) | |
| ③ | 205.1 ± 3.8bcd | 231.6 ± 3.8b |
| (182) | (192) | |
| ④ | 109.3 ± 2.5ab | 116.0 ± 1.0f |
| (97) | (96) | |
| ⑤ | 130.3 ± 3.1a | 154.1 ± 1.5e |
| (116) | (128) | |
| ⑥ | 140.6 ± 2.5de | 159.0 ± 2.1e |
| (125) | (132) | |
| ⑦ | 193.4 ± 9.7h | 209.4 ± 1.4d |
| (172) | (173) | |
| ⑧ | 222.0 ± 4.5fg | 220.0 ± 2.8c |
| (197) | (182) | |
| ⑨ | 228.0 ± 1.7f | 254.8 ± 0.8a |
| (202) | (211) | |
| ⑩ | 190.1 ± 4.6ce | 253.6 ± 2.3a |
| (169) | (210) | |
| ⑪ | 50.1 ± 1.6i | 66.9 ± 1.9g |
| (44) | (55) | |
| ⑫ | 212.6 ± 2.5ac | 229.0 ± 2.6c |
| (189) | (190) | |
| ⑬ | 198.6 ± 3.0ce | 203.3 ± 1.1c |
| (176) | (168) |
Drying methods and conditions tested (Test No.) are indicated in Table 1.
Values in parentheses are relative compared with the content and activity of each parameter in the control (FD) as 100.
All data are expressed as mean±SE (n=3).
The different superscripts represent the significant differences at p<0.05.
When raw egoma leaves were directly subjected to HAD treatment, the residual amount of total soluble polyphenol decreased to a level in the order of 40 % of the control (lyophilized) product. In contrast, the amount tended to increase significantly after drying with AQG alone. Meanwhile, it also tended to increase after drying with AQG followed by SHS, and this tendency was significantly higher than that by treatment with AQG alone. The drying treatment with SHS alone reduced the total soluble polyphenol content compared with treatment with AQG or AQG followed by SHS.
Total soluble polyphenol content after drying treatment with AQG alone was maintained at a level comparable to that after conventional drying treatment with steam followed by HAD. This was likely attributable to the blanching effect produced by the AQG treatment. The total soluble polyphenol content and DPPH radical scavenging activity of dried egoma leaves were confirmed to correlate well (r=0.971). These findings demonstrate that AQG drying is an effective means of preserving the original total soluble polyphenol content and DPPH radical scavenging activity of raw leaves. A likely cause of the increased total soluble polyphenol content and DPPH radical scavenging activity after drying with AQG is that internal polyphenols and other antioxidants are released following disruption of the cell wall, as reported by Ogawa et al. (2016).
5) Effects of different drying treatments on the polyphenol oxidase activity of egoma leaves Next, we examined the effects of drying treatments under the conditions in Table 1 on polyphenol oxidase activity of egoma leaves. The results are shown in Table 5.
| Test No. | polyphenol oxidase activities (relative activities) |
|---|---|
| FD (control) | 1.000a |
| ① | 0.027cd |
| ② | 0.023cd |
| ③ | 0.012cd |
| ④ | 0.039cd |
| ⑤ | 0.057c |
| ⑥ | 0.044cd |
| ⑦ | 0.000d |
| ⑧ | 0.000d |
| ⑨ | 0.000d |
| ⑩ | 0.005cd |
| ⑪ | 0.124b |
| ⑫ | 0.137b |
| ⑬ | 0.116b |
Drying methods and conditions tested (Test No.) are indicated in Table 1.
Values are relative compared with the activity in the control (FD) as 1.000.
All data are expressed as mean (n=3).
The different superscripts represent the significant differences at p<0.05.
The enzyme activity was effectively and significantly inactivated, and was almost completely undetectable after the AQG and AQG followed by SHS treatments compared with the FD treatment as the control. In contrast, the enzyme was not completely inactivated when the leaves were dried by treatment with SHS, HAD, or even with steam followed by FD or HAD. This was because the temperature increase was slower in the SHS treatment than in the AQG treatment. The results indicated that polyphenol oxidase inactivation requires that the heat-transfer coefficients to materials be extremely high, and the gas-phase temperature of the heating medium was as high as 120 °C in the AQG treatment. This reflects the results of the total soluble polyphenol content in 4) above.
In this study, it was found that drying with AQG treatment alone or AQG followed by SHS drying was an effective method for preparing dried egoma leaves prior to sterilization, as high levels of functional components and antioxidant activities were retained.
From these results, we are now considering a new continuous on-line drying system, such as AQG blanching in the first chamber followed by SHS drying in the second chamber, capable of the rapid drying of egoma leaves while retaining high functional components.
In this study, we investigated whether AQG, first developed as a pretreatment technology to maintain functionality prior to sterilization without a decrease in material yield, was applicable to drying treatment of egoma leaves. The results demonstrated that AQG dried leaves with a final wet weight-based moisture content of <10 % were successfully obtained, as observed for SHS treatment.
Our examination of the effects of different drying methods on the functional components of egoma leaves revealed that treatment with AQG or AQG followed by SHS produced dried products in which functional components (α-linolenic and rosmarinic acids) were preserved at levels comparable to those in FD products. These results demonstrate that AQG treatment is an effective drying method for preparing dried egoma leaves.
In addition, egoma leaves were sterilized by drying treatment with AQG, SHS, or AQG followed by SHS, or steaming followed by HAD.
Moreover, egoma leaves dried by treatment with AQG or AQG followed by SHS contained high total soluble polyphenol levels comparable to leaves dried by conventional steaming and subsequent HAD treatment. This was attributed to the blanching effect of AQG treatment. In dried egoma leaves, total soluble polyphenol level showed a good positive correlation with DPPH radical scavenging activity level.
In summary, these findings indicate that the AQG treatment is promising as a highly effective drying method capable of completing the treatments (blanching, drying, sterilization) necessary for drying leafy vegetables in a single step while preserving functional components.
