原著論文
雪冷熱を利用した貯蔵におけるアスパラガス若茎の品質変化
2014 年 83 巻 4 号 p. 327-334
詳細
2014 年 83 巻 4 号 p. 327-334
Asparagus has a short shelf life. A temperature of 0–2°C with a relative humidity of 95–100% is well known as the ideal storage environment for asparagus spears. The quality of spears stored in a snow vault and snow mount (Snow) was compared with those stored in an electric refrigerator (Refrigerator). Asparagus spears of ‘Grande’ and ‘Gijnlim’ were stored in Snow and Refrigerator for 0, 2, 5, 10, and 20 days; then, 1) physical and external appearance characteristics such as hardness, weight, and surface color, and 2) features of internal quality such as Brix, sugar, ascorbic acid, and rutin content in the spears were investigated. Although temperature and relative humidity fluctuated largely in the ranges of 3–6°C and 65–85% in Refrigerator, those in Snow were almost completely stable at 0–1°C and 100%. In ‘Grande’, the weight of the spears stored in Refrigerator decreased dramatically compared with that of spears stored in Snow. The external appearance of ‘Gijnlim’ spears was preserved until the 10th day, but loose tips were observed on the spears in both Snow and Refrigerator on the 20th day. ‘Grande’ spears stored in Refrigerator turned slightly brown and wilted at the surface of the basal part compared with Snow-stored spears. No loose tips were observed on ‘Grande’ spears. There was also no significant difference in the internal quality of spears between those stored in Snow and Refrigerator, in both varieties. CO2 emissions in snow storage were reduced to half of those in refrigerator storage in LCA analysis and no CO2 emissions were identified during the storage period in Snow. From the perspectives of energy utilization and quality preservation, snow appears to be one of the better alternatives for spear preservation than use of a refrigerator.
Asparagus is currently receiving particular attention because it is one of the healthy vegetables containing functional ingredients like polyphenols, including rutin (Maeda, 2005). It is expected to sell well in the long term.
Asparagus spears are very active metabolically and highly perishable during handling and storage (Lill et al., 1990). Respiration rate and transpiration in asparagus are high compared with those of other vegetables (Katoh et al., 1983; Lill et al., 1990). Cell senescence is induced by harvest and consumes much of the sugar and water in the plant (Irving and Hurst, 1993). However, it takes a few days to deliver harvested asparagus to consumers. Asparagus spears are harvested intensively over a short period in an open field. If harvested spears can be stored for a certain period, it will lead to a stable supply for consumers. Therefore, it is important to suppress the activity of high metabolism in the young spears and to extend their shelf life. Controlled atmosphere conditions (King et al., 1986) and the use of compressed mixed argon and xenon gases (Zhang et al., 2008) were examined for the purpose of extending the shelf life of asparagus spears; however, these systems are expensive.
It has been said that the ideal storage conditions for asparagus spears are a temperature of 0–2°C with 95–100% relative humidity (Paull, 1999). Such conditions can be realized in a snow vault. Snow has been used as a coolant in snow-covered regions. Although utilizing snow for the storage of vegetables is a traditional approach, there are some scientific reports on vegetable storage in snow (Ishihara et al., 2005; Jishi and Araki, 2013; Kobayashi et al., 1993; Muramatsu, 1986; Nakamura et al., 2001). The storage conditions in a snow-storage room or snow vault realized a temperature of exactly 0°C and relative humidity of 100% similarly to an electric refrigerator (Nakamura and Osada, 2001).
Vegetables with storability, such as carrots, radishes, and cabbages, were mainly successfully stored under snow cover or in a snow vault for about 3 months in previous studies. The contents of soluble sugars, organic acids, and free amino acids changed upon snow storage; however, their changes differed according to the vegetables examined (Kobayashi et al., 1993).
Nakamura and Osada (2001) examined the storage of tomatoes in a snow room. In their study, lycopene and β-carotene levels were higher in tomatoes stored in a storage room cooled by snow than in those stored in an electric refrigerator; however, the β-carotene level of carrots stored in a storage room cooled by snow was lower than in those stored in an electric refrigerator. Ishihara et al. (2005) reported that the volatile compounds in carrot changed during storage under snowy ground. From these investigations, changes of the inner quality of vegetables should be checked when vegetables are stored with snow.
As for perishable vegetables, an increase of the brightness of leaves was observed upon the snow storage of spinach (Nakamura and Osada, 2001). However, there have been few reports on snow storage in perishable vegetables including asparagus spears.
Snow cooling also contributes to a reduction of CO2 emissions compared with electrical cooling (Kobiyama, 2003). With the extension of the cropping season and the enlargement of distribution areas, the energy consumption of greenhouses in winter and the energy consumed by pre-cooling vegetables and shipping them to market are increasing. Utilizing renewable energy is now one of the essential tools to reduce greenhouse gas emissions.
In the present experiment, in order to evaluate the effectiveness of snow for the storage of asparagus spears, the external appearance and internal quality of asparagus spears after snow storage were investigated, and compared with the results from an electrical refrigerator and an experiment room. Moreover, CO2 emissions from snow and refrigerator storage conditions were determined. The possibility of the snow mount as a substitute storage facility was evaluated in the storage of asparagus spears for future marketing as a low-carbon approach.
Green asparagus ‘Gijnlim’ was grown in the experimental farm of Hokkaido University, and spears were harvested on May 28 (the first trial) and June 12 (the second trial), 2011. The spears were cut at a length of 25 cm, ranging from 13 g to 33 g in weight. The individual weight of harvested spears was not measured because the change of inner quality was mainly investigated in 2011. Harvested spears were stood straight up in a 2 L measuring cup without wrapping (Fig. 1). Such cups with these spears were set in 3 kinds of storage room as follows: 1) A snow vault (Snow) was made underground next to melon research facilities in Yubari City (latitude 43°, longitude 142°, altitude 480 m). The size of the vault was 450W × 360L × 270H (cm) and it was filled with snow. 2) An electric refrigerator (Refrigerator) (SZE19-151D; SANYO, Osaka, Japan): Its size was 270W × 180L × 207H (cm) and its cooling system was forced air circulation. 3) An experimental room (Exp. Room): The storage in Exp. Room was set as a negative control to confirm the perishability of asparagus spears. Spears were put in a corrugated carton and set in a room with a steady temperature, 26.3 ± 1.7°C (mean ± standard deviation, SD). Five spears were collected for biological and chemical analyses at various time points: 0, 2, 5, 10, 20, and 30 days after the beginning of storage. The investigation of spears stored in Exp. Room was discontinued after 10 days because of severe senescence. The experiment was replicated three times.
Storage of asparagus spears in snow vault and snow mount. Vertical asparagus spears placed in measuring cups (left). Underground snow vault in Yubari City was used for ‘Gijnlim’ in 2011 (middle) and snow mount in Hokkaido Univ., Sapporo City, was used for ‘Grande’ in 2012 (right).
Green spears of ‘Grande’, grown in a greenhouse in Kuriyama Town, Hokkaido, were harvested on April 26, 2012. The spears were cut at a length of 25 cm and these fresh weight was 24.6 ± 1.9 g (mean ± SD). Storage conditions of the harvested spears were applied in the same way as in 2011 except for the snow storage. Snow cooled storage in 2012 was arranged in a different way from our former experiment. 1) A snow mount (Snow) was set up in the experimental farm of Hokkaido University because it takes 2 hours to travel from Yubari City to Hokkaido University, Sapporo, and we wanted to avoid the time lost by traveling from the snow vault to the laboratory when analyzing spear quality in 2012. The snow mount was produced by using snow cover around the office and a crop-storage facility of the experimental farm. The base diameter was about 10 m and the height was about 4 m. A tractor and truck were used for snow mount production, and their fuel (diesel) consumption was recorded for life cycle assessment (LCA). The storage space in the snow mount was about 70W × 45L × 35H (cm). Two liter measuring cups of spears were set into the snow mount. 2) Refrigerator and 3) Exp. Room were applied in the same manner as in the ‘Gijnlim’ examination in 2011. Spears were stored in a snow-cooled space, a refrigerator, and an experimental room, for 0, 2, 5, 10, and 20 days. Before storage, the fresh weight of 5 spears was measured for every plot because weight loss had been observed in the 2011 examination. The investigation of spears in the experimental room was discontinued after 10 days because of severe senescence. The experiment was replicated three times.
Temperature and relative humidity (RH) measurementsData loggers (Ondotori Jr. TR-74Ui; T&D Corporation, Nagano, Japan) were set in Snow, Refrigerator, and Exp. Room to record the hourly temperature and relative humidity. Data collection was carried out throughout the experiment.
Spear quality assessmentsFive spears were randomly collected at each storage period, 0, 2, 5, 10, and 20 days from the beginning of storage in all 3 storage conditions in both 2011 and 2012 examinations. Spear fresh weight after storage was measured for every plot only in 2012. They were cut at a length of 24 cm for physical investigation. Three grams of each sample was cut longitudinally, freeze-dried and powdered.
Spear hardness was measured using a Texture Analyzer (TA-XT2i; Stable Micro Systems, Godalming, U.K.) using both ‘Gijnlim’ and ‘Grande’. Maximum load (N) was recorded when a cylinder (ϕ 2 mm) penetrated a spot 8 cm away from the tip at a speed of 2 mm·s−1 in ‘Gijnlim’ and 12 and 22 cm away from the tip in ‘Grande’.
The surface color values, namely, L*, a*, and b*, of the stored spears were measured 4, 12, and 22 cm away from the tip using a color-difference meter (Color Reader CR-10; Konica Minolta, Inc., Tokyo, Japan) in ‘Grande’. L* represents the lightness from black = 0 to white = 100. Positive a* indicates red-purple and negative a* indicates a bluish-green color. Positive b* indicates yellow and negative b* indicates a blue color (McGuire, 1992; Siomos et al., 2001). External appearance of the spears was also visually observed.
After measuring physical quality, the Brix value was measured in both ‘Gijnlim’ and ‘Grande’. Spears were crushed to make paste, and the extracted juice was dropped on a Brix meter (PAL-1; Atago, Tokyo, Japan).
Asparagus paste was also used for ascorbic acid analysis in ‘Gijnlim’. Ten grams of the paste and 40 mL of meta-phosphoric acid (5%) were mixed and homogenized using a homogenizer (Polytron; Kinematica AG, Lucerne, Switzerland). Fifty milliliters of distilled water was added to the homogenized solution and filtered using filter paper. Ascorbic acid of the filtrate was measured using a reflectometer (RQflex® plus 10; Merck KGaA, Darmstadt, Germany).
Soluble sugar was extracted from 50 mg of freeze-dried powder using 1 mL of 80% ethanol. One milliliter of 20 mM lactose was also added as the standard. The extraction was performed for 30 minutes at 70°C, after which 1 mL of distilled water was added and centrifuged (10000 rpm, 5 min). The supernatant was then concentrated to 500 μL using a centrifugal evaporator. The contents of fructose, glucose, and sucrose were determined by high-performance liquid chromatography (HPLC) analysis using an NH2P50-4E column (ϕ 4.6 mm × 250 mm) (Shodex; Showa Denko, Tokyo, Japan) and an L-5020-type pump (Hitachi, Tokyo, Japan). The mobile phase was 75% acetonitrile. Analysis was performed at a column temperature of 35°C with a flow rate of 0.7 mL·min−1.
Rutin was extracted from 20 mg of freeze-dried powder of the spears by using 1 mL of 80% methanol in the first experiment for ‘Gijnlim’. The extraction was conducted for three hours at Exp. Room. Sample solutions were centrifuged at 10000 rpm for 10 min and the supernatant was used for measurement. Rutin content was determined by HPLC analysis using a Shimadzu LC-9A system (Shimadzu Corp., Kyoto, Japan) equipped with a Waters Sunfire C18 (ϕ 4.6 mm × 250 mm) column (Waters Corp., Milford, MA, USA). The mobile phases consisted of acetonitrile (A) and 0.1% trifluoroacetic acid (B), using a linear gradient system. The gradients were 0–20 min, A:B = 16:84 → A:B = 40:60, 20–30 min, A:B = 40:60 → A:B = 60:40, 30–35 min, A:B = 60:40 → A:B = 16:84, and 35–40 min, A:B = 16:84. Each run was monitored at a wavelength of 354 nm using a photodiode array detector. Analysis was performed by running each sample for 35 min at a column temperature of 40°C with a flow rate of 1.0 mL·min−1. Rutin quantitative analysis was conducted by the external standard method. A standard curve was drawn with solvents of 50 mg·L−1 and 200 mg·L−1. Rutin content in Exp. Room-stored spears was analyzed only on the 0th and 2nd days of the storage period because of severe senescence after the 5th day.
Statistical analysisThe values of weight loss, Brix, sugar, ascorbic acid, and rutin contents represent the means of three replications, and those of spear surface color and hardness represent the means of 15 measurements. Significant differences of the mean values between Snow and Refrigerator were analyzed using t-test, and those among Snow, Refrigerator, and Exp. Room were analyzed using Tukey’s test. Statistical analysis was performed using SPSS statistics software, ver. 16.0.1 (IBM, Armonk, NY, USA).
Life cycle assessmentsThe CO2 emissions of the 2 storage methods, electric refrigerator and snow mount, were compared in 2012 (Table 1). Electric power consumption of the refrigerator was recorded during the spear storage period, 20 days. CO2 emissions from the refrigerator use were calculated using the MiLCA program Ver. 1.1.2.50 (Japan Environmental Management Association for Industry) and IDEA Ver. 1.1.0 (National Institute of Advanced Industrial Science and Technology).
CO2 emission life cycles of electric refrigerator and snow mount.
A tractor and truck were used for making the snow mount. The amount of diesel used for the collection and transportation of snow and for making the snow mount was recorded. The CO2 emissions were calculated by using the CO2 emission coefficient of diesel, 2.62 kg-CO2·L−1, cited from the Ministry of the Environment (Calculation method and the list of emission factors in calculation, report and publication system; Ministry of the Environment website, http://ghg-santeikohyo.env.go.jp/files/calc/itiran.pdf, accessed on Feb. 17, 2014). The processes of manufacturing the tractor and truck were not included in the CO2 emissions.
In Snow, the temperature was almost always constant at 1°C, and the relative humidity was 100% (Fig. 2). This was the same environment as used in previous experiments (Kobayashi et al., 1993; Muramatsu, 1987). Temperature and RH fluctuated in the ranges of 3–6°C and 65–85%, respectively, in Refrigerator because of forced air circulation. The Exp. Room temperature varied from 21.5 to 22.5°C. The ideal storage conditions for asparagus spears were realized by using snow.
Temperature and RH change in Snow, Refrigerator, and Exp. Room. Shown data are from June 13 to June 19, 2011.
Mean ± SD of spear fresh weight before storage were 25.0 ± 1.2 g, 25.4 ± 1.5 g, and 21.7 ± 1.8 g in Snow, Refrigerator, and Exp. Room, respectively, in 2012. There was a significant difference in the change of spear fresh weight among the storage types. The spear weight decreased 0.39 g per spear (1.6%) until the 20th day in Snow, compared with 4.39 g per spear (16.8%) in Refrigerator (Fig. 3). In Exp. Room, the spear weight decreased 4.58 g per spear (22.7%) until the 5th day. Such sudden weight loss proved that asparagus spears are very perishable and their continuously high transpiration after harvest was recognized (Katoh et al., 1983).
Effect of storage condition on weight loss of spears during storage in ‘Grande’, 2012. Vertical bars indicate standard error (SE) (n = 3). Means followed by same letters are not significantly different among treatments at 5%, Tukey’s test. Means followed by *** are significantly different among the treatments at 0.1%, by t-test between Snow and Refrigerator.
The spears were stored with no wrapping in the experiments in 2011 and 2012. In a previous study, spears wrapped with perforated polyethylene bags lost more water than spears wrapped with unperforated polyethylene bags (Lill, 1980). Thus, if spears are stored in a refrigerator, they need to be wrapped to retain their water content. On the other hand, snow makes it possible to store the spears without wrapping because of their contention of moisture.
In Exp. Room, spear hardness increased significantly by about 1.2 N for 2 days in ‘Gijnlim’ (Fig. 4). However, there were no significant changes in Snow and Refrigerator in ‘Gijnlim’, in 2011, and Snow, Refrigerator, and Exp. Room in ‘Grande’, in 2012. From these findings, there was little effect of water loss on the hardness of stored spears as the lignification process continues after harvest (Clore et al., 1976; Lill, 1980).
Effect of storage condition on change in surface color measured 8 cm away from the tip in ‘Gijnlim’ and 12 cm away from the tip in ‘Grande’. Vertical bars indicate SE (n = 15). Means followed by same letters are not significantly different among treatments at 5%, Tukey’s test. ns: not significant.
In terms of the color of the spears in Snow, they had a higher L* value on the 10th and 15th days, a lower a* value from the 10th to 20th days, and similar b* values from the 5th to 15th days at 12 cm away from the tip, compared with Refrigerator spears (Fig. 5). This indicated that the storage in Snow maintained the better external appearance of the spears.
Effect of storage condition on change in surface color measured 12 cm away from the tip in ‘Grande’. Vertical bars indicate SE (n = 15). Means followed by same letters are not significantly different among treatments at 5%, Tukey’s test. Means followed by *, **, and *** are significantly different among the treatments at 5%, 1%, and 0.1%, by t-test between Snow and Refrigerator. ns: not significant.
In ‘Gijnlim’, stored spears lost commercial value after 5 days due to severe wilting and browning in Exp. Room. The spear tops had loosened in Refrigerator by the 20th day (Fig. 6). In ‘Grande’, however, such top-loosening spears were not observed by the 20th day. Serious wilting was seen at the basal part of Refrigerator-stored spears from the 15th to 20th days, whereas it was only slightly observed in Snow-stored spears (Fig. 6). From the perspective of external appearance, the shelf life of asparagus spears is considered to be 15–20 days, and ‘Grande’ had greater storage ability than ‘Gijnlim’. Therefore, the present observations may well support the results of Kitazawa et al. (2011), Motoki et al. (2008), and Uragami et al. (1995), considering that the year, harvest season, and conditions differed between ‘Gijnlim’ and ‘Grande’.
External appearances of stored spears. Loose tips were observed on ‘Gijnlim’ spears (upper left) but not on ‘Grande’ spears (upper right) on the 20th day in both Snow and Refrigerator storage. Wilting was seen at the basal part of Refrigerator-stored spears (lower left) but not on Snow-stored spears (lower right) on the 15th to 20th days in ‘Grande’.
There was no significant difference in the Brix value until the 20th day in Snow and Refrigerator, except for the 15th day in ‘Grande’. The value was around 6% in ‘Gijnlim’ and ‘Grande’ (data not shown). ‘Gijnlim’ showed rapid deterioration in the present examination, as well as the study by Kitazawa et al. (2011); however, the relationship between the rapid deterioration and internal quality was not clarified in ‘Gijnlim’.
A special difference was not recognized in the tendency for decreases of each sugar (data not shown). The total content of soluble sugar, fructose, glucose, and sucrose, was slightly decreased in all experiments (Fig. 7). There was no significant difference between Snow and Refrigerator. In the first trial of ‘Gijnlim’, the content had decreased 45.1 mg compared with the initial sugar content (361.9 mg) in Snow and 34.4 mg in Refrigerator by the 20th day of storage. In the second trial, the content had decreased 26.2 mg compared with the initial sugar content (309.8 mg) in Snow and 19.8 mg in Refrigerator. A similar result was seen in ‘Grande’. The decrements of soluble sugar content differed among trials, but with no significant difference being recognized between Snow and Refrigerator. The content difference between the first and second trials was possibly due to the difference of the harvest season (Bhowmik et al., 2002).
Effect of storage condition on changes in total soluble sugar content in ‘Gijnlim’ (left: the first trial; right: the second trial). The quantity of total soluble sugar is the sum of glucose, sucrose, and fructose. Vertical bars indicate SE (n = 3). Means followed by same letters are not significantly different among treatments at 5%, Tukey’s test. ns: not significant.
Ascorbic acid content decreased with progression of the storage period. Especially in Exp. Room, the content had decreased 21.0 mg (54.3%) by the 5th day (Fig. 8). A significant difference could not be found between Snow and Refrigerator, and ascorbic acid content had decreased 14.4 mg (37.2%) and 13.0 mg (33.6%) by the 20th day, respectively. Hirai et al. (2007) reported that ascorbic acid content decreases rapidly after harvest. In addition, Nakamichi et al. (1982) found that the higher the temperature in the storage room, the more rapidly the ascorbic acid content decreases. The present study showed similar results.
Effect of storage condition on changes in ascorbic acid content of ‘Gijnlim’ in the first trial. Vertical bars indicate SE (n = 3). Means followed by same letters are not significantly different among treatments at 5%, Tukey’s test. ns: not significant.
In Exp. Room, rutin content decreased after 2 days (Fig. 9). In Snow and Refrigerator, the content decreased along with the storage period. At the 20th day, rutin content of spears in Snow and Refrigerator remained at 76% (4.42/5.85 mg) and 62.2% (3.64/5.85 mg), respectively, with no significant difference between them. Rutin is a relatively stable substance, but its content decreases during spear storage. A similar result was reported by Xiong et al. (2005).
Effect of storage condition on changes in rutin content of ‘Gijnlim’ during storage in the first trial. Vertical bars indicate SE (n = 3). ns: not significant.
The calculated value of CO2 emissions for the electric refrigerator used in the present examination was 99.7 kg-CO2 per 20 days using MiLCA and IDEA programs (Table 1). Diesel was used as the fuel for the tractor and truck when the snow mount was established and 48.5 kg-CO2 per 20 days was emitted by calculation using the CO2 emission coefficient of diesel (2.62 kg-CO2·L−1).
In addition, although 81.8 kg-CO2 was used from the electricity for operation of the electric refrigerator, no CO2 emissions were recognized for cooling of the storage space in the snow mount. This shows that utilizing the snow mount will contribute to reducing CO2 emissions. However, cooling ability will be affected by the volume of storage space (room) and the number of stored spears. An exact comparison of storage ability and CO2 emissions should be carried out in future studies.
Reductions of oil consumption and CO2 emissions by snow utilization for air-cooling have been reported (Kobiyama, 2003), and some crops have begun to be stored in storage rooms cooled by snow (Nakamura and Osada, 2001). Snow cooling should also be applied to asparagus spear storage and could contribute to the reduction of greenhouse gas emissions.
On the basis of the obtained results from the total experiments, there was no difference in internal quality, including levels of ascorbic acid and antioxidants such as rutin, between the spears stored in Snow and Refrigerator. In addition, the external appearance was better maintained in Snow than in Refrigerator with forced air circulation. Moreover, snow cooling provided more ideal storage conditions than the refrigerator for asparagus spears. Nakamura and Osada (2001) examined the snow storage of spinach and reported that the L* value obtained using a color-difference meter increased. Spinach is a perishable vegetable with high respiration and transpiration at cool temperatures, similarly to asparagus. There are few reports on the snow storage of perishable vegetables. It was revealed that the quality of asparagus spears stored in Snow was similar to that of those stored in Refrigerator. The maximum storage period for asparagus spears without wrapping is suggested to less than 15 days, from the external appearance.
In the marketing of fresh vegetables, low-carbon systems using natural resources are increasingly important. Fortunately, the use of snow-cooled systems can be a useful technique in the storage of asparagus spears, one of the perishable vegetables, in snow-covered regions.
We wish to thank Mr. S. Ichikawa and Mr. T. Kawai, technicians at Hokkaido University, and Mr. T. Yokota, president of Kensei Sangyo Co., Ltd., for their generous technical assistance in the maintenance of the snow vault and snow mount.
