Sweet potato production methods and their uses
The method of high-temperature treatment and gradual temperature reduction before low-temperature storage addresses the issue of chilling damage and sugar content loss in sweet potatoes, ensuring stable distribution with increased sugar content.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- TOKYO UNIVERSITY OF AGRICULTURE
- Filing Date
- 2022-07-06
- Publication Date
- 2026-05-15
AI Technical Summary
Conventional methods for storing sweet potatoes at low temperatures increase sugar content but cause damage when returned to room temperature, and high-temperature treatment decreases sugar content.
A method involving high-temperature treatment at 38°C to 43°C for 1.0 to 2.5 days, followed by gradual temperature reduction over 4.0 to 10 days to 3.0°C to 7.0°C for 20 to 40 days, and then raising to 15°C or higher, to acclimate sweet potatoes before storage, thereby suppressing chilling damage and maintaining sugar content.
The method effectively prevents chilling damage and maintains increased sugar content in sweet potatoes, even when returned to room temperature, enhancing commercial value and stability during distribution.
Smart Images
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Abstract
Description
Technical Field
[0001] The present disclosure relates to a method for producing sweet potatoes.
Background Art
[0002] Sweet potatoes are fresh fruits and vegetables that are easily damaged by low temperatures, and it has been conventionally known that the suitable storage temperature is about 13°C. For example, Non-Patent Document 1 describes that storing sweet potatoes at a lower temperature can promote starch saccharification while spoilage occurs due to low-temperature damage. Non-Patent Document 2 describes that heat-treating sweet potatoes can suppress low-temperature damage during subsequent low-temperature storage.
Prior Art Documents
Non-Patent Documents
[0003]
Non-Patent Document 1
Non-Patent Document 2
Summary of the Invention
Problems to be Solved by the Invention
[0004] The inventors of this invention have found that, while the technology described in Non-Patent Document 2 can suppress cold damage to sweet potatoes during low-temperature storage, it has the problem that the damage increases when the sweet potatoes are returned to room temperature from a low-temperature environment. Furthermore, the inventors have found that when high-temperature treatment is performed using the technology described in Non-Patent Document 2, the sugar content decreases when the sweet potatoes are returned to room temperature from a low-temperature environment. Thus, with conventional technologies, even if the sugar content of sweet potatoes can be increased by low-temperature storage, it has been difficult to distribute them to the market while suppressing the occurrence of damage and the decrease in sugar content. Therefore, there has been a need for a technology that can suppress the occurrence of cold damage to sweet potatoes while suppressing the decrease in sugar content of sweet potatoes whose sugar content has been increased by low-temperature storage. [Means for solving the problem]
[0005] The present invention can be realized in the following forms.
[0006] (1) According to one embodiment of the present invention, a method for producing sweet potatoes is provided. This production method includes: a first step of storing harvested sweet potato tubers at a temperature of 38°C to 43°C for 1.0 day to 2.5 days; a second step of storing the tubers after the first step while lowering the temperature; a third step of storing the tubers after the second step at a second temperature of 3.0°C to 7.0°C for 20 days to 40 days; and a fourth step of raising the temperature of the tubers after the third step to 15°C or higher. The second step includes an acclimatization step of gradually lowering the temperature from a first temperature of 8.0°C to 12°C to the second temperature, and the acclimatization step is a step of lowering the temperature by a range of 0.5°C to 1.5°C per day over a period of 4.0 days to 10 days. This production method suppresses the occurrence of chilling damage while also preventing a decrease in the sugar content of sweet potatoes whose sugar content has been increased through low-temperature storage.
[0007] (2) In the sweet potato production method described in (1) above, the second step may include a step of lowering the temperature of the tuber to 25°C or below before the acclimatization step. This form of production method makes it possible to suppress fluctuations in the temperature of the tuber in the initial stages of the acclimatization step.
[0008] (3) In the sweet potato production method described in (1) or (2) above, the first temperature may be 9.0°C or higher and 11°C or lower. This form of production method can further suppress the occurrence of low-temperature damage.
[0009] (4) In the sweet potato production method described in any one of the above items (1) to (3), the second temperature may be 4.0°C or higher and 6.0°C or lower. This form of production method can further increase the sugar content of the sweet potatoes.
[0010] (5) In the sweet potato production method described in any one of the above items (1) to (4), the period of the acclimatization step may be 4.5 days or more and 7.0 days or less. This form of production method can further suppress the occurrence of low-temperature damage.
[0011] (6) In the sweet potato production method described in any one of the above items (1) to (5), the acclimatization step may be a step of lowering the temperature by 0.8°C or more and 1.2°C or less per day. This form of production method can further suppress the occurrence of low-temperature damage.
[0012] (7) In the sweet potato production method described in any one of the above items (1) to (6), the temperature of the first step may be 39°C or higher and 41°C or lower. This form of production method can further suppress the occurrence of low-temperature damage.
[0013] (8) In the sweet potato production method described in any one of the above items (1) to (7), the duration of the first step may be 1.5 days or more and 2.5 days or less. This form of production method can further suppress the occurrence of low-temperature damage.
[0014] Furthermore, the present invention can be realized in various forms. For example, it can be realized in the form of a method for improving the sugar content of sweet potatoes, a method for maintaining the sugar content of sweet potatoes, a method for suppressing low-temperature damage to sweet potatoes, a method for storing sweet potatoes, a method for processing sweet potatoes, a sweet potato storage system, a sweet potato processing system, a method for producing processed sweet potatoes, and so on. [Brief explanation of the drawing]
[0015] [Figure 1] A process diagram showing the steps involved in sweet potato production. [Figure 2] An explanatory diagram showing the Brix measurement results in Experiment 1. [Figure 3] An explanatory diagram showing the Brix measurement results in Experiment 2. [Figure 4] An explanatory diagram showing the Brix measurement results of raw potatoes in Experiment 3. [Figure 5] An explanatory diagram showing the Brix measurement results for heated sweet potatoes in Experiment 3. [Modes for carrying out the invention]
[0016] A. Embodiment Figure 1 is a process diagram showing the procedure for a sweet potato production method as one embodiment of the present invention. The sweet potato production method in this embodiment (hereinafter also simply referred to as the "production method") includes a first step of storing the harvested sweet potato tubers at a temperature of 38°C to 43°C for 1.0 day to 2.5 days; a second step of storing the tubers after the first step while gradually lowering the temperature; a third step of storing the tubers after the second step at a second temperature of 3.0°C to 7.0°C for 20 days to 40 days; and a fourth step of raising the temperature of the tubers after the third step to 15°C or higher. The second step includes an acclimatization step in which the temperature is gradually lowered from a first temperature of 8.0°C to 12°C to the second temperature. The acclimatization step is a process of lowering the temperature by a range of 0.5°C to 1.5°C per day over a period of 4.0 days to 10 days.
[0017] This production method can be implemented using, for example, equipment capable of controlling the temperature of a space. More specifically, it can be implemented using, for example, non-mobile equipment such as warehouses or incubators, or mobile equipment such as vehicles or ships. Furthermore, a portion of the process in this production method may be carried out without using special equipment, by holding sweet potato tubers in a space maintained within a certain temperature range.
[0018] The sweet potato variety used in the production method of this embodiment is not particularly limited, but examples include Kokei No. 14 (Naruto Kintoki). This production method makes it possible to suppress the decrease in sugar content of sweet potatoes whose sugar content has been increased by low-temperature storage during distribution, etc. For this reason, it can be used particularly suitably for varieties that do not have a relatively high sugar content, such as Naruto Kintoki.
[0019] In the first step, the harvested sweet potato tuberous roots are stored at a temperature of 38°C or higher and 43°C or lower for 1.0 day or more and 2.5 days or less (step P10). In the following description, the implementation of the first step is also referred to as "high-temperature treatment". From the perspective of further suppressing the occurrence of low-temperature injury, the temperature in the first step is preferably 39°C or higher and 41°C or lower. From the perspective of further suppressing the occurrence of low-temperature injury, the period of the first step is preferably 1.5 days or more and 2.5 days or less.
[0020] In the second step, the tuberous roots after the first step are stored while reducing the temperature (step P20). The second step includes an acclimation step (step P24). The acclimation step gradually reduces the temperature from the first temperature to the second temperature by reducing the temperature in the range of 0.5°C or higher and 1.5°C or lower per day over a period of 4.0 days or more and 10 days or less. Here, the first temperature is 8.0°C or higher and 12°C or lower, and the second temperature is 3.0°C or higher and 7.0°C or lower. A detailed description of the acclimation step will be given later. In the following description, the implementation of the acclimation step is also referred to as "stepwise conditioning treatment".
[0021] The second step may include a step (step P22) of reducing the temperature of the sweet potato tuberous roots to 25°C or lower before the acclimation step. By including the step of reducing the temperature of the sweet potato tuberous roots to 25°C or lower, it is possible to suppress the variation in the temperature of the tuberous roots at the initial stage of the acclimation step. Note that the step of reducing the temperature of the sweet potato tuberous roots to 25°C or lower may be omitted.
[0022] From the perspective of further suppressing the occurrence of low-temperature injury, the period of the acclimation step is preferably 4.5 days or more and 7.0 days or less. In the acclimation step, from the perspective of further suppressing the occurrence of low-temperature injury, it is preferable to gradually reduce the temperature from the first temperature to the second temperature in the range of 0.8°C or higher and 1.2°C or lower per day. Also, from the perspective of further suppressing the occurrence of low-temperature injury, the first temperature is preferably 9.0°C or higher and 11°C or lower. Also, from the perspective of further increasing the sugar content of sweet potatoes, the second temperature is preferably 4.0°C or higher and 6.0°C or lower.
[0023] In the third step, the tubers from the second step are stored at the second temperature for 20 to 40 days (step P30). In the following explanation, the third step will also be referred to as "low-temperature storage." It is believed that the sugar content of the sweet potatoes can be increased by performing the third step. From the viewpoint of increasing the sugar content of the sweet potatoes while suppressing an excessively long time until shipment, the third step is preferably performed for 25 to 35 days.
[0024] In the fourth step, the temperature of the tuber after the third step is raised to 15°C or higher (step P40). The temperature in the fourth step is assumed to be the temperature during distribution after the sweet potatoes are shipped, and may be any temperature, such as room temperature. The duration of the fourth step is not particularly limited, but may be any period of one day or more. The fourth step may be implemented during the distribution process after the sweet potatoes are shipped.
[0025] The inventors of this invention discovered that by storing sweet potatoes under the above conditions, it is possible to maintain their sugar content while suppressing the occurrence of chilling damage even when the sweet potatoes are returned to room temperature from a low-temperature environment, leading to the present invention. By performing the first step (high-temperature treatment) and the acclimatization step (stepwise conditioning treatment) in this order before the third step (low-temperature storage), it is possible to maintain the sugar content while suppressing the occurrence of chilling damage even after the fourth step. Examples of chilling damage to sweet potatoes include rot, browning, softening, and pithiness.
[0026] The mechanism by which the above effects are achieved is not clear. However, a possible mechanism is that the first step (high-temperature treatment) promotes the formation of a cork layer in the sweet potato tuber, thereby suppressing decay from the wound. Generally, it is known that the mycelium of blue mold, the main pathogen of sweet potatoes, dies at 40-45°C, and that Rhizopus fungi, which cause soft rot, become unable to grow at temperatures above 35°C. From this, it is thought that high-temperature treatment at around 40°C has a greater bacteriostatic effect compared to the general curing temperature of 30-35°C. By combining high-temperature treatment with stepwise conditioning, it is presumed that the formation of the cork layer is further promoted, and the effect of suppressing decay and weight loss is enhanced compared to a method of storing at low temperatures immediately after high-temperature treatment, i.e., a method in which the second step is omitted.
[0027] Generally, sweet potatoes are known to have a higher optimal storage temperature compared to fruits and vegetables such as tomatoes and eggplants. In other words, sweet potatoes are more sensitive to low temperatures than fruits and vegetables such as tomatoes and eggplants. Fruits have a low pH and less contact between the soil and the edible part during cultivation, whereas sweet potatoes are grown in the soil, resulting in a large number of attached microorganisms, and they are also prone to damage during harvesting. For these reasons, it is not appropriate to directly apply storage methods used for other fruits and vegetables to sweet potatoes.
[0028] According to the sweet potato production method of this embodiment described above, sugar content can be maintained even after the fourth step while suppressing the occurrence of low-temperature damage. As a result, the sugar content of sweet potatoes can be increased while suppressing a decline in quality, thereby increasing the commercial value of sweet potatoes. Furthermore, since the sugar content of sweet potatoes can be increased in a relatively short period of time, and the decrease in the increased sugar content can be suppressed, the lead time until shipment can be shortened. Therefore, according to this production method, sweet potatoes with increased sugar content can be stably distributed to the market. Sweet potatoes produced by this production method can also be called processed sweet potatoes.
[0029] B. Examples The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.
[0030] (1) Experimental materials Medium-sized "Naruto Kintoki" sweet potato tubers from Tokushima Prefecture were used. After the harvested sweet potato tubers were transported to Tokyo University of Agriculture, experiments 1-3 described below were conducted. Experiment 1 was conducted with sand attached to the tubers, while experiments 2 and 3 were conducted with the tubers washed, as is the case in actual distribution. The sweet potato tubers were placed in cardboard boxes and stored under the temperature conditions described later. To prevent the tubers from drying out during storage and shelf life testing, the cardboard boxes were covered with polyethylene bags with a thickness of 0.024 mm and a capacity of 45 L. These bags had approximately 100 punched holes with a diameter of 6 mm.
[0031] (2) Experimental method <Experiment 1> Cardboard boxes containing sweet potato tubers were stored under temperature conditions of 13°C, 10°C, and 5°C, respectively. Brix values were measured after 15 and 30 days of storage. Three raw sweet potatoes were used for Brix measurement in each group. The sweet potato tubers were divided into three sections: upper, middle, and lower, with the middle section used as the sample. 5g of the sample, grated using a fine-mesh plastic grater, was mixed with 10ml of pure water (twice the amount), ground in a mortar, filtered through gauze, and measured using a digital refractometer (PAL-1, manufactured by Atago Co., Ltd.). In addition, five tubers from each group were examined for damage after 30 days of storage. The percentage of tubers that did not show damage was calculated as the healthy tuber rate.
[0032] <Experiment 2> Cardboard boxes containing sweet potato tubers were stored for 30 days under temperature conditions of 13°C and 5°C, respectively. A high-temperature treatment (HS) group was also established, where the tubers underwent high-temperature treatment before being stored at 5°C. In the HS group, the tubers were treated for two days in an incubator set at 40°C, then allowed to cool to room temperature before being stored at 5°C. After storage, a shelf life test was conducted on 10 of the 15 sweet potato tubers in each test group. The shelf life test was conducted for 20 days under temperature conditions of 20°C.
[0033] Brix values were measured before the experiment began (Before storage), after 30 days of storage (After storage), and after the shelf-life test (After shelf-life). Brix was measured using the same method as in Experiment 1. In addition, the presence or absence of decay, softening, browning, and pithiness was investigated as a way to check for the occurrence of problems. Five tubers from each group were examined after 30 days of storage, and ten tubers were examined after the shelf-life test. Even if multiple problems occurred in a single tuber, the presence or absence of each problem was evaluated separately. The percentage of tubers that did not develop problems was calculated as the healthy tuber rate.
[0034] <Experiment 3> Table 1 below shows the treatment conditions for each test group in Experiment 3. Cardboard boxes containing sweet potato tubers were stored for 30 days at storage temperatures of 13°C and 5°C, respectively. In addition, a high-temperature treatment (HS) group was established, in which a high-temperature treatment was performed as a pretreatment before storage at 5°C. The HS group was carried out under the same conditions as in Experiment 2. Furthermore, a test group was established in which a conditioning treatment (CD) was performed as a pretreatment before storage at 5°C. More specifically, before storage at 5°C, there were two groups: CD1, in which the tubers were stored at 10°C for 5 days, and CD2, in which the temperature was gradually decreased from 10°C to 6°C by 1°C per day. In addition, an HS+CD1 group was established in which tubers that had been cooled after the high-temperature treatment similar to that in Experiment 2 were subjected to the same conditioning treatment as in CD1. In addition, an HS+CD2 group was established in which tubers that had been cooled to room temperature after the high-temperature treatment similar to that in Experiment 2 were subjected to the same conditioning treatment as in CD2. After pretreatment, the tubers were stored at 5°C for 30 days. Following storage, a shelf life test was conducted on 15 of the 25 sweet potato tubers in each test plot. The shelf life test was carried out for 20 days under a temperature of 20°C.
[0035] [Table 1]
[0036] The weight of sweet potato tubers was measured before the experiment began (Before storage), after 30 days of storage (After storage), and after shelf-life testing (After shelf-life testing), and the weight loss rate was calculated. The weight loss rate was calculated by dividing the difference in weight between before the experiment and immediately after storage or after shelf-life testing by the weight before the experiment and expressing the result as a percentage. In addition, Brix values were measured before the experiment began, after 30 days of storage, and after shelf-life testing. Brix measurements were performed using the same method as in Experiment 1, and both raw and cooked sweet potatoes were used as the measurement subjects. More specifically, three sweet potato tubers in each group were divided into upper, middle, and lower sections. The middle section was used, and the middle section was divided vertically into two parts. One part was used as a raw sample, and the other as a cooked sample. Heating was performed using the following method. Sweet potatoes were wrapped in pure, damp kitchen paper, then wrapped in food wrap, heated in a 300W microwave for 3 seconds per gram, and then shredded. The same method as in Experiment 2 was used to investigate the occurrence of damage. Damage was investigated by examining 10 sweet potatoes from each group after 30 days of storage, and then 15 sweet potatoes after the shelf life test.
[0037] (3) Experimental results <Experiment 1> Figure 2 is an explanatory diagram showing the Brix measurement results in Experiment 1. In Figure 2, the different lowercase letters "a" and "b" indicate that a significant difference was observed at the 5% level in the Tukey test for differences in storage temperature, while the different uppercase letters "A" and "B" indicate that a significant difference was observed at the 5% level in the Tukey test for differences in storage period. The bars in Figure 2 represent the standard error.
[0038] After 15 days of storage, Brix values were similar in all test groups (13°C, 10°C, and 5°C), with no significant differences observed. However, after 30 days of storage, Brix values were significantly higher in the 5°C storage group compared to the 13°C storage group. Nevertheless, after 30 days of storage at 5°C, rot and other damage were observed in 60% of the tubers (figures and tables omitted).
[0039] <Experiment 2> Figure 3 is an explanatory diagram showing the Brix measurement results in Experiment 2. In Figure 3, the different lowercase letters "a" and "b" indicate that a significant difference was observed at the 5% level in the Tukey test between the data of each test group after cold storage. In Figure 3, the different uppercase letters "A" and "B" indicate that for each test group, a significant difference was observed at the 5% level in the Tukey test between the data after cold storage and the data after the shelf life test. The bars in Figure 3 represent the standard error. In Figure 3, "*" indicates that for each test group, a significant difference was observed at the 5% level in the t-test between the data after cold storage and the data after the shelf life test. In Figure 3, "ns" indicates that for each test group, no significant difference was observed at the 5% level in the t-test between the data after cold storage and the data after the shelf life test.
[0040] The average Brix value before storage was 8.9%, while in the test group stored at 13°C, the average Brix value was 8.0% or less both after 30 days of storage and after the shelf life test. In the test group stored at 5°C, the average Brix value after 30 days of storage was approximately 12%, regardless of whether high-temperature treatment was performed, which was higher than in the test group stored at 13°C. In the test group that did not undergo high-temperature treatment, the Brix value after the shelf life test decreased significantly. In contrast, in the test group that underwent high-temperature treatment, the Brix value after the shelf life test was maintained at the same level as before the shelf life test, i.e., after 30 days of storage.
[0041] The results of the investigation into the occurrence of defects in Experiment 2 are shown in Table 2 below. In Table 2, for each test group, the percentage of healthy roots (%) and the incidence rate of each type of defect (%) are shown after 30 days of storage (After storage) and after shelf-life testing. The types of defects are shown as decay, softening, browning, and cracking. In Table 2, the different signs indicated by "a" and "b" indicate that a significant difference was observed at the 5% level in the chi-square test between the test groups.
[0042] [Table 2]
[0043] In the test plots stored at 13°C, all tubers had developed rot after 30 days of storage, with a healthy tuber rate of 0%. Among the test plots stored at 5°C, the healthy tuber rate after 30 days was 20% in the plot without high-temperature treatment, but in the plot with high-temperature treatment (HS plot), the occurrence of rot was suppressed, and the healthy tuber rate after 30 days was 60%. Even in the high-temperature treatment plot (HS plot), the healthy tuber rate after the shelf life test decreased to 10%. In the tubers after the shelf life test, softening and browning occurred, which were not observed in the tubers before the shelf life test, and the degree of pithiness also increased.
[0044] The results in Figure 3 and Table 2 revealed the following: Storage at 5°C for 30 days resulted in an increase in Brix values compared to before storage (5°C group, HS group), indicating that low-temperature storage can accelerate saccharification. However, low-temperature storage caused damage to many tubers (5°C group). High-temperature treatment before low-temperature storage suppressed decay both after 30 days of low-temperature storage and after the shelf-life test, but sometimes promoted softening, browning, and pithiness (HS group). Thus, high-temperature treatment alone before low-temperature storage did not provide a stable damage reduction effect.
[0045] <Experiment 3> The results of the weight loss rate in Experiment 3 are shown in Table 3 below. In Table 3, the weight loss rate (%) after 30 days of storage and after shelf-life testing are shown for each test group. The standard error is also shown for the weight loss rate values. In Table 3, the different signs indicated by "a" to "c" indicate that a statistically significant difference was observed at the 5% level in the Tukey test between the test groups.
[0046] [Table 3]
[0047] In the test groups that did not undergo high-temperature treatment or conditioning, the weight loss rate was lower when stored at 5°C than when stored at 13°C. In the test groups that underwent either high-temperature treatment or conditioning treatment alone (HS, CD1, and CD2 groups), the weight loss rate was similar to that of the test group that was stored at 5°C without pretreatment (5°C group). In contrast, the average weight loss rate in the test group that combined high-temperature treatment with conditioning (storage at 10°C for 5 days) (HS+CD1 group) was 3.80%, which was a relatively high value. Furthermore, in all test groups, the weight loss rate tended to increase after the shelf life test compared to after 30 days of storage. The weight loss rate after the shelf life test in the test groups that combined high-temperature treatment and conditioning (HS+CD1 and HS+CD2 groups) was similar to that of the test group that was stored at 5°C without pretreatment. In contrast, the average weight loss rate after the shelf life test in the test groups (CD1 and CD2) that underwent conditioning treatment without high-temperature treatment was relatively high, exceeding 9%.
[0048] Figure 4 is an explanatory diagram showing the Brix measurement results for raw potatoes in Experiment 3. Figure 5 is an explanatory diagram showing the Brix measurement results for heated potatoes in Experiment 3. In Figures 4 and 5, the different lowercase letters "a" to "c" indicate that a significant difference was observed at the 5% level in the Tukey test between the data of each test group after low-temperature storage. In Figures 4 and 5, the different uppercase letters "A" to "E" indicate that for each test group, a significant difference was observed at the 5% level in the Tukey test between the data after low-temperature storage and after the shelf life test. The bars in Figures 4 and 5 represent the standard error. In Figures 4 and 5, "**" and "*" indicate that for each test group, a significant difference was observed at the 5% and 1% levels in the t-test between the data after low-temperature storage and after the shelf life test. "ns" indicates that no significant difference was observed in the t-test between the data after low-temperature storage and after the shelf life test for each test group.
[0049] As shown in Figure 4, the Brix value of raw potatoes after 30 days of storage was above 9.3% on average in all test groups stored at 5°C, which was significantly higher than the test group stored at 13°C (13°C group). After the shelf life test, the Brix value of raw potatoes tended to decrease in more than half of the test groups compared to after 30 days of storage. On the other hand, the Brix values of raw potatoes in the test group stored at 13°C (13°C group), the test group that underwent high-temperature treatment alone (HS group), and the test group that combined high-temperature treatment with a conditioning treatment in which the temperature was gradually lowered from 10°C to 6°C by 1°C per day (HS+CD2 group) remained at a similar level to before the shelf life test, i.e., after 30 days of storage, even after the shelf life test. The Brix value after the shelf life test was highest in the HS+CD2 group, increasing by approximately 2.7 times compared to the value before storage.
[0050] As shown in Figure 5, the Brix value of the heated sweet potatoes was 8.7% in the 13°C group after 30 days of storage, showing almost no change from the 8.4% before storage. The Brix value after 30 days of storage was higher than that of the 13°C group in all test groups stored at 5°C. After the shelf life test, the Brix value tended to decrease in more than half of the test groups compared to after 30 days of storage. On the other hand, the Brix values of heated sweet potatoes in the test group stored at 5°C without pretreatment (5°C group), the test group treated with high temperature treatment alone (HS group), and the test group treated with a combination of high temperature treatment and conditioning treatment in which the temperature was gradually lowered from 10°C to 6°C by 1°C per day (HS+CD2 group) remained at a similar level to before the shelf life test, i.e., after 30 days of storage, even after the shelf life test.
[0051] The results of the investigation into the occurrence of damage in Experiment 3 are shown in Table 4 below. In Table 4, the percentage of healthy tubers and the incidence rate of each type of damage are shown for each test group, both after 30 days of storage and after the shelf life test. In Table 4, the different signs indicated by "a" to "d" indicate that a statistically significant difference was observed at the 5% level in the chi-square test between the test groups.
[0052] [Table 4]
[0053] The percentage of healthy tubers after 30 days of storage was lowest in the 13°C group, at 10%. The 13°C group also had the highest incidence of browning. The percentage of healthy tubers after 30 days of storage in the 5°C group was 60%. In the groups treated with high temperature treatment and / or conditioning treatment alone or in combination, the percentage of healthy tubers after 30 days of storage was over 70%. After the shelf life test, the incidence of rot, softening, and browning increased in most groups compared to 30 days after storage. In the group stored at 5°C without pretreatment (5°C group), the percentage of healthy tubers after the shelf life test was 27%, with rot and browning being the main problems. In the HS group, the percentage of healthy tubers after the shelf life test was 67%, about twice as high as the healthy tuber percentage in the 5°C group. In contrast, in the CD1 and CD2 groups, rot, softening, and browning occurred frequently, and the percentage of healthy tubers was 7% or less. Thus, while conditioning treatment before low-temperature storage suppressed the occurrence of damage after 30 days of low-temperature storage, it was found that the occurrence of rot and browning increased sharply during the subsequent shelf life test, and the percentage of healthy tubers decreased significantly. In the test groups that combined high-temperature treatment and conditioning treatment (HS+CD1 group, HS+CD2 group), the occurrence of damage was suppressed even after the shelf life test, and a high percentage of healthy tubers was observed. In particular, in the HS+CD2 group, the occurrence of rot and softening after the shelf life test was 0%, and the percentage of healthy tubers was 80%.
[0054] The results from Figures 4 and 5, and Tables 3 and 4, revealed the following: In the test groups that combined high-temperature treatment and conditioning treatment (HS+CD1 group, HS+CD2 group), the occurrence of defects was suppressed not only after low-temperature storage but also after the shelf life test, with defects being particularly suppressed in the HS+CD2 group. Furthermore, low-temperature storage significantly increased the Brix values of both raw and cooked potatoes, and subsequent shelf life tests showed a decrease in sugar content in the CD1, CD2, and HS+CD1 groups, but the sugar content was maintained in the HS+CD2 group even after the shelf life test. These results indicate that by combining high-temperature treatment with a conditioning treatment that involves gradually lowering the temperature during storage, it is possible to suppress the occurrence of defects and the decrease in sugar content even when the potatoes are returned to room temperature after low-temperature storage.
[0055] The present invention is not limited to the embodiments described above, and can be realized in various configurations without departing from its spirit. For example, the technical features in the embodiments and examples corresponding to the technical features in each form described in the summary of the invention can be replaced or combined as appropriate in order to solve some or all of the above-described problems, or to achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be deleted as appropriate.
Claims
1. A method for producing sweet potatoes, The first step involves storing the harvested sweet potato tubers at a temperature between 38°C and 43°C for 1.0 day to 2.5 days, The second step involves storing the tuber after the first step while lowering the temperature, A third step involves storing the tubers after the second step at a second temperature of 3.0°C to 7.0°C for 20 to 40 days, A fourth step involves raising the temperature of the tuber after the third step to 15°C or higher, Includes, The second step includes an acclimatization step in which the temperature is gradually lowered from a first temperature of 8.0°C to 12°C to the second temperature. The aforementioned acclimatization process is a process of lowering the temperature by a range of 0.5°C to 1.5°C per day over a period of 4.0 days to 10 days. Sweet potato production methods.
2. In the method for producing sweet potatoes according to claim 1, The second step includes, before the acclimatization step, a step of lowering the temperature of the tuber to 25°C or below. Sweet potato production methods.
3. In the method for producing sweet potatoes according to claim 1 or claim 2, The first temperature is 9.0°C or higher and 11°C or lower. Sweet potato production methods.
4. In the method for producing sweet potatoes according to claim 1 or claim 2, The second temperature is between 4.0°C and 6.0°C. Sweet potato production methods.
5. In the method for producing sweet potatoes according to claim 1 or claim 2, The duration of the aforementioned acclimatization process is 4.5 days or more and 7.0 days or less. Sweet potato production methods.
6. In the method for producing sweet potatoes according to claim 1 or claim 2, The aforementioned acclimatization process is a process of lowering the temperature by a range of 0.8°C to 1.2°C per day. Sweet potato production methods.
7. In the method for producing sweet potatoes according to claim 1 or claim 2, The temperature of the first step is 39°C or higher and 41°C or lower. Sweet potato production methods.
8. In the method for producing sweet potatoes according to claim 1 or claim 2, The duration of the first step is 1.5 days or more and 2.5 days or less. Sweet potato production methods.