Original papers
Effectiveness of sodium hypochlorite and peracetic acid for spoilage-causing molds on sweet potato slices
2023 Volume 29 Issue 3 Pages 257-267
Details
2023 Volume 29 Issue 3 Pages 257-267
In the present study, the effectiveness of sterilization of peracetic acid (PA) solution and sodium hypochlorite (SH) solution for several fungi (Penicillium sp., Cladosporium sp., and Fusarium sp.) attached on the cut surface of sweet potato (Ipomoea batatas) was evaluated. The mixture of each strain isolated from rotten sweet potatoes was inoculated on the cut surface of presterilized sweet potato blocks. Further, each inoculated cut potato was washed with water, 200 mg/L SH, a mixture of SH, and 100 mM acetic acid (SH+AA), or 20–80 mg/L PA for 5 min at room temperature. Comparing with the untreated ones (4.2–5.1 log CFU/mL), the 80 mg/L PA washing could reduce 2.2, 2.2, and 2.0 log CFU/mL of Penicillium, Fusarium, and Cladosporium species, respectively. The increasing concentration of PA resulted in an increase the fungicidal efficiency. The use of 80 mg/L PA solution was suggested to be more highly effective than the one of (pH controlled) SH solution to control the spoilage fungi on cut sweet potato.
Sweet potato (Ipomoea batatas) is a major crop harvested and consumed around the world. In 2020, 89.5 million tons of this crop were produced around the world, and Japan produced 0.77 % of themi). In Japan, most of this crop was consumed domestically, while the others were exported. Since 2020, the Japanese government has enhanced exports of domestic crops, including sweet potatoes. Sweet potato exports from Japan increased significantly from 237 tons (with an export value of 70 million yen) in 2006 to 5 603 tons (export value of 2.3 billion yen) in 2021, according to trade statistics from the Ministry of Finance Japan ii). Fungal and bacteriological spoilage during storage and transportation is a major problem both in the domestic use and exportation of this crop.
Sweet potatoes can be stored for months after harvest in the optimal storage condition (13 °C–15 °C, 80 %–90 % humidity) (Scruggs and Quesada–Ocampo, 2016; Krochmal–Marczak et al., 2020). However, once fungi grow on a small portion of potatoes, it spreads quickly entire storage area. Penicillium sp., Cladosporium sp., and Fusarium sp. are frequently detected in potato storage (Meno et al., 2021). Especially, Penicillium sp. and Fusarium sp., typical filamentous fungi (molds), have been reported to be associated with this sweet potato spoilage (Ogawa and Watanabe, 1988; Shimada, 2021). Penicillium expansum rots crops (citrus fruits, apples, pears, etc.) and causes serious economic losses (Tournas and Katsoudas, 2019; Chand–Goyal and Spotts, 1997; El-Dawyet al., 2021). Controlling the growth and spread of fungus mycelium and spores is necessary to solve this issue. Moreover, in the case of sweet potatoes, it is difficult to reduce the growth of these fungi or other spoilage microorganisms by temperature control because cold damage (chilling injury) occurs at less than 5 °C (Nagata et al., 2021).
Surface washing of crops with chemical sanitizers can partially eliminate surface-attached harmful or spoilage microorganisms. This operation can increase the commodity value of agricultural products such as cut vegetables and ready-to-use foods (Rico et al., 2007). In Japan, sodium hypochlorite, hypochlorite water, sodium chlorite, chlorite water, peracetic acid preparations, and chlorinated lime are approved as designated food additives that can be used for wash sterilization of raw vegetables iii). Sodium hypochlorite (SH) is commonly used for fresh-cut vegetable production (Ölmez and Kretzschmar, 2009). Hypochlorous acid (HClO) existing in SH solutions is considered a major bactericidal compound, and it is also active against viruses and fungi (Gomes et al., 2016). The organic compounds in the washing solution or on the surface of washing crops are known to reduce the effectiveness of SH or other HClO-containing sanitizers (Xi and Yen-Con, 2017).
Peracetic acid (PA), a nonchlorinated disinfectant, can be an alternative to chlorine-based agents (Petri et al., 2021). A stable concentration of PA exists in the PA formulation, a mixture of peracetic acid (12 %–15 %), acetic acid (30 %–50 %), hydrogen oxide (4 %–12 %), and hydroxyethylidene phosphonic acid (less than 1 %)iv). The PA formulation was approved for use in the surface sterilization of foods in Japan in 2016. The hydroxyl radicals in the PA solution are thought to disrupt bacterial cell membranes, inactivate metabolic enzymes, and denature proteins, thereby demonstrating bactericidal effects against a wide range of microorganisms, including spoilage fungi (Malchesky, 2000; Iwasawa and Nakamura, 2000).
Fungi are thought to grow more readily in the injured area because sweet potatoes with scratches significantly decay (Ogawa and Watanabe, 1988). An excessively oxidizing environment during surface washing may enhance the injury of potatoes and cause a negative effect. Therefore, it is crucial to confirm whether the product can be stored after washing.
The purpose of this study was to determine the fungicide effectiveness of two chemical sanitizers (SH and PA) and the shelf life of sweet potato slices after washing. In this study, molds associated with sweet potato spoilage (Penicillium sp., Cladosporium sp., and Fusarium sp.) were isolated from spoiled sweet potatoes and used for the study.
Measurement of the number of surfaces-attached molds on sweet potatoes Domestic sweet potatoes (Ipomoea batatas) harvested in the autumn of 2020 were purchased and stored for less than 3 months at 13 °C ± 1 °C with soil. The potatoes without fungi on the surface were selected and used for this study.
The number of molds on the selected sweet potatoes was evaluated by the plate counting method. Nine sweet potatoes taken from nine different lots were washed under running water to remove surface-attached soil. All the peel (a thickness of 0.8 mm) of each sample was shaved off, emulsified with nine times the weight of phosphate buffer solution (PBS, pH 7.2), and smeared on potato dextrose agar (PDA) (Eiken Chemical Co., Ltd.) containing 100 mg/L chloramphenicol. The developed colonies were counted after 5 days of incubation at 25 °C. The number of filamentous fungi (molds) on the sample was calculated from the numbers of typically developed colonies on the agar plates that had 15 to 150 colonies. Since some yeast can grow on PDA, a few typical colonies on the plates were selected and examined under a microscopic to confirm whether or not they were molds.
The colonies developed on the PDA plate were classified based on their colony morphology and some strains were identified by the method shown below. The ratio of the counts of each kind of mold to the total counts was calculated.
Isolation of molds from sweet potato and preparation of inoculum Molds used in the inoculation tests were isolated five times from different lots of several spoiled sweet potatoes during storage. The mold was isolated by puncturing the mold-developing site with a sterilized needle or loop and smearing the mold on a PDA plate containing 100 mg/L chloramphenicol. Developed typical mold-like colonies were purified by streaking on the same agar plates. The isolated strains were observed by microscopic morphology and identified by molecular phylogenetic analysis based on nucleotide sequences for each fungus. To amplify the fungal ribosomal DNA-internal transcribed spacer (rDNA-ITS) region sequence for Penicillium sp. (Hinrikson et al., 2005), rDNA-ITS, TEF, and ACT region sequence for Cladosporium sp. (Zhou et al., 2022), and rDNA-TEF region sequence for Fusarium sp. (O'Donnell et al., 1998) were employed. The resultant sequences were subjected to similarity searches via the NCBI GenBank databasev). Three strains of Penicillium expansum, two strains of Cladosporium sp. (Cladosporium tenuissimum and Cladosporium anthropophilum), and two strains of Fusarium cugenangens were selected for the use of inoculation tests.
Each of the strains was streaked on the surface of PDA plates and incubated at 25 °C for 5 to 7 days. The surface-developed mold was scraped off by a sterile scraper after pouring 5 mL of sterilized distilled water with 0.01 % (w/v) polyoxyethylene (20) sorbitan monolaurate (Fujifilm Wako Pure Chemical Industries, Ltd., Osaka). Each of the scraped solutions was collected in a sterile test tube and diluted appropriately to a concentration of 8–9 log CFU/mL. These mold solutions were stocked at 4 °C until further use. A cocktail method was used for inoculation (Inatsu et al., 2011). Inoculum solutions were prepared by mixing equal volumes of mold solutions of the same species (three strains of Penicillium sp., two strains of Cladosporium sp., and two strains of Fusarium sp., respectively). Each of mixtures used for inoculum on the surface of sweet potato slices.
Effectiveness of sanitizers against molds dispensed in the washing solution The fungicidal effect of sanitizers against three kinds of molds dispensed in sweet potato washing solution was evaluated. The sanitizers used in the study were 200 mg/L SH solution (200 SH) (Fuji Film Wako Pure Chemical Industries, Ltd., Osaka), 200 mg/L SH solution with 100 mM acetic acid (200 SH+AA, pH 4.1), and 80 mg/L PA (80PA, pH 4.1) (Diapower FP, Mitsubishi Gas Chemical Co., Ltd., Tokyo) solution. Sterile distilled water (DW) was used as the control. The 100 mM acetic acid was omitted based on the results of preliminary experiments.
Stocked sweet potatoes without fungi on the surface were selected and washed with tap water to remove surface-attached soil. Later, the sweet potatoes were soaked in an 80 mg/L PA solution for 10 minutes and then washed with tap water again to reduce the number of natural contaminated microorganisms (Pre-sterilization). After wiping the sweet potatoes with paper towels, they were sliced into 3–5 mm of thicknesses (30 to 35 mm diameter, 3 to 4 g in weight) using a knife. Thirty sweet potato slices were soaked in 2.5 L of DW for 5 minutes to remove the water-soluble components. The microorganisms in this solution were removed by passing it through a 5 µm filter (MILLEX® SV, Millipore) and a 0.22 µm filter (MILLEX® GV, Millipore). Each of the sanitizers was mixed with the filter-sterilized cut sweet potato washing solutions after inoculating 4–5 log CFU/mL of each mold. The solution was diluted 10-fold after 5 min of sterilization at room temperature, and the number of molds was enumerated by the plate counting method. Samples were smeared at 0.1 mL on PDA medium and incubated at 25 °C. Colonies were counted after 48 hours of incubation and continued until day 5 for the confirmation of mycelial color. The number of molds in the wash solution was calculated from the number of colonies that appeared after incubation. The experiment was repeated twice to confirm the results.
Surface sterilization of inoculated sweet potatoes Stocked sweet potatoes were sliced after pre-sterilization and washing, as described in the previous section. The slices were air-dried on a clean bench for 20 min at room temperature. A 0.05 mL of inoculum was spread on the each of dried slices and dried again for 1 h at room temperature before use. The initial numbers of Cladosporium sp. and Fusarium sp. on the slices of sweet potato were set to be smaller than those of Penicillium sp. to reflect real conditions.
The surface sterilization (washing) effectiveness of SH, SH+AA, PA, and DW was evaluated. In the case of PA, the effectiveness of various concentrations (20, 40, 60, and 80 mg/L) of the solution was also evaluated. Thirty pieces of inoculated sweet potato slices were put into 2.5 L of washing solution and washed for 5 min with occasional stirring. Later, ten pieces of washed slices were picked up and submerged and stirred for 1 min. in 200 mL of PBS. Recovered molds in 0.1 mL of this solution were enumerated by the plate counting method. The PDA plates that had 10 to 100 colonies after 48 h of incubation at 25 °C were subjected to colony counting. This experiment was repeated three times, and a sample taken from each washing batch was analyzed. Tukey–Kramer multiple comparison tests were used to compare the effectiveness of four kinds of washing solutions.
The change in viable cell counts of washed slices of the sweet potatoes during storage was evaluated. Each of the ten pieces of the surface-sterilized inoculated slices was air-dried in a safety cabinet for 10 min, put into a plastic film bag (P-Plus®, Sumitomo Bakelite Co., Ltd., Tokyo), and heat-sealed. Later, the bags were stored in a refrigerator at 13 °C. All the slices in each of the bags were taken out after 4 or 8 days, and the number of molds was measured. The logarithm of the relative values of the evaluated mold number of washed samples on days 0, 4, and 8 against the prewashed one (on day 0) was calculated. This experiment was repeated three times, and a sample was used for each experiment.
Similarly, experiments were performed to evaluate the effectiveness of washing with a concentration of PA. A 20, 40, 60, and 80 mg/L peracetic acid-containing solution was used for the experiments. Dunnett's comparison test was used to compare the viable cell counts between the DW-washed sample (0 mg/L) and each concentration of the peracetic acid-washed sample.
Evaluation of the color change of sweet potato's cross section during storage Sterilized and washed sweet potatoes were sliced and washed with sanitizers or DW without inoculation. The slices were put into a heat-sealed plastic bag and stored for 8 days.
The L*, a*, and b* values (ISO/CIE 11664-4:2019) of the recommended color parameters indicated by the International de l'Eclairage (CIE) were measured the center of sweet potato's cross section (edible part) on days 0, 4, and 8 by a portable color reader (model CR-20, KONICA MINOLTA). The L* represents lightness, and the a*b* represents chromaticity. The positive and negative values of a* represent red-violet and blue–green, and b* represent yellow and blue, respectively. Ten slices in the same bag were measured. The effectiveness of storage or washing solutions on the change of the values of L*, a*, and b* was assayed by a two-way factorial ANOVA analysis. Each of the average values of L*, a*, and b* of the washed samples was compared with the ones of the unwashed samples by Duncan's multiple comparison test.
Surface-attached fungi on the sweet potatoes The numbers of surface-attached fungi on the nine sweet potatoes picked up from nine different lots were counted. The average viable fungal number was estimated to be between 3.5–4.3 log CFU/g (Table 1). These colonies isolated from the PDA plate were classified based on their colony morphology and microscopic morphology (Hamada, 1989), and typical strains in the classified group were identified as Penicillium sp., Cladosporium sp., Fusarium sp. by molecular phylogenetic analysis based on nucleotide sequences. Penicillium sp. was the most major species, accounting for more than 90 % of all detected colonies on the peel. Cladosporium sp. and Fusarium sp. were observed in some specimens, but the ratio in all colonies was less than 10 %. The other white colonies with no mycelial (yeast) were also detected in two samples (Table 1).
| Viable cells counts (log CFU/g) | Total counts (CFU) (%) | The counts (CFU) and the ratio (%) of each fungi | ||||
|---|---|---|---|---|---|---|
| Penicillium sp. | Cladosporium sp. | Fusarium sp. | Yeast | |||
| sample A | 3.7 | 47 (100) |
47 (100) |
0 | 0 | 0 |
| sample B | 4.2 | 147 (100) |
147 (100) |
0 | 0 | 0 |
| sample C | 3.9 | 75 (100) |
74 (99) |
0 | 1 (1) |
0 |
| sample D | 3.7 | 53 (100) |
50 (94) |
0 | 3 (6) |
0 |
| sample E | 4.3 | 21 (100) |
20 (95) |
0 | 0 | 1 (5) |
| sample F | 3.9 | 73 (100) |
73 (100) |
0 | 0 | 0 |
| sample G | 4.0 | 102 (100) |
102 (100) |
0 | 0 | 0 |
| sample H | 3.8 | 58 (100) |
57 (98) |
1 (2) |
0 | 0 |
| sample I | 3.5 | 33 (100) |
29 (88) |
0 | 1 (3) |
3 (9) |
Comparison of the fungicidal effect of sanitizers The fungicidal effectiveness of SH and PA against 3 kinds of molds dispended in the filtrated liquid of the slices of sweet potato was evaluated. The viable cell counts of inoculated Penicillium sp. (5.1 log CFU/mL), Cladosporium sp. (4.6 log CFU/mL), and Fusarium sp. (5.5 log CFU/mL) were less than 3.0 log CFU/mL after 5 min exposure with 200 mg/L sodium hypochlorite solution (200 SH), 200 mg/L sodium hypochlorite solution with 100 mM acetic acid (200 SH+AA) (pH 4.1), and 80 mg/L peracetic acids solution (80 PA) (pH 4.1) at room temperature. Hence, all the sanitizers could reduce the tested three kinds of molds in the liquid by at least 2 log CFU/mL. This result remained unchanged when the filtered washing solution for the sweet potatoes was replaced with distilled water (Data was not shown).
The fungicidal effectiveness of SH and PA against 3 kinds of molds inoculated on the slices of sweet potatoes was evaluated (Table 2). Penicillium sp. and Cladosporium sp., 0.5 to 0.8 or 0.2 to 0.5 log CFU/mL, respectively, displayed a reduction of the viable counts by DW, 200 SH, or 200 SH+AA washing, and no significant difference in the log reduction among the three washing solutions was detected (p > 0.05). An 80 PA exhibited significantly higher effectiveness (2.2 log CFU/mL) compared with the three washing solutions. Furthermore, DW washing reduced viable cell counts of Fusarium sp. by only 0.2 log CFU/mL. The 200 SH and 200 SH+AA exhibited significantly (p < 0.05) higher effectiveness (0.9 and 1.1 log CFU/mL, respectively) than that of the DW. The effectiveness of 80 PA (2.0 log CFU/mL) was significantly (p < 0.05) higher than that of the other 3 washing solutions.
| Untreated | Viable cell counts (log CFU/mL) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Penicillium sp. | Cladosporium spp. | Fusarium sp. | ||||||||||
| 5.1 | ± | 0.1 | 4.2 | ± | 0.4 | 4.2 | ± | 0.0 | ||||
| Treated | Log reduction compared with untreated samples (log CFU/ml) | |||||||||||
| DW | 0.8 | ± | 0.3 | a | 0.2 | ± | 0.3 | a | 0.2 | ± | 0.1 | a |
| 200 SH | 0.6 | ± | 0.1 | a | 0.4 | ± | 0.2 | a | 0.9 | ± | 0.2 | b |
| 200 SH+AA | 0.5 | ± | 0.1 | a | 0.5 | ± | 0.3 | a | 1.1 | ± | 0.3 | b |
| 80 PA | 2.2 | ± | 0.1 | b | 2.2 | ± | 0.4 | b | 2.0 | ± | 0.2 | c |
Figure 1 depicts the change of viable cell counts of surface-washed inoculated sweet potato slices during 13 °C storage. The prewashed inoculated sample count on day 0 was compared to the logarithmic relative value (ratio) of each of the counts of washed samples on days 0, 4, and 8. In addition, the difference between any two of the logarithmic ratios of the same mold is equal to the difference in the logarithmic value of viable cell counts of the two samples. Additionally, no significant difference (p > 0.05) in the ratio of Penicillium sp. was detected among DW, 200 SH, and 200 SH+AA on day 0 and 4. The viable cell counts of the 80 PA-washed sample were 1.7 to 1.9 log CFU/mL (day 0) and 1.0 to1.2 log CFU/mL (day 4), respectively, lower than those of the other three washing solutions. The mold continued to grow after the period, but the viable cell counts were kept lower (0.7 to 1.2 log CFU/mL) than the unwashed inoculated samples on day 8. Similarly, no significant difference (p > 0.05) in the ratio was observed among all tested wash solutions including 80 PA. In the case of Cladosporium sp., no significant difference (p > 0.05) in the logarithmic ratio was detected among DW, 200 SH, and 200 SH+AA on day 0 and day 4. The viable cell counts of the 80 PA-washed sample were 1.8 to 2.2 log CFU/mL (day 0) and 0.6 to 0.8 log CFU/mL (day 4) lower than those of the other 3 washing solutions, respectively. The viable cell counts were kept lower (0.9 to 1.5 log CFU/mL) than the unwashed inoculated sample on day 8. Simultaneously, no significant difference in the logarithmic ratio (= log reduction) was observed among all tested wash solutions including 80 PA. Moreover, no significant difference (p > 0.05) in the logarithmic ratio was detected for Fusarium sp. between 200 SH and 200 SH+AA on day 0. Comparatively, a slightly higher (0.9 to 1.1 log CFU/mL) and lower (0.7 to 0.8 log CFU/mL) level of log reduction was achieved by PA 80 and DW washing, respectively, on day 0. Furthermore, the viable cell counts of 80 PA or 200 SH+AA washed samples were 1.7–1.8 and 0.7–0.8 log CFU/mL lower than that of DW-washed one and 200 SH-washed ones, respectively. Finally, on day 8, the viable cell counts of DW and 200 SH-washed sample (almost) exceeded the of prewashed level (logarithmic ratio = 0.0). Therefore, washing by 200 SH+AA or 80 PA could keep viable cell counts 0.6 to 0.7 log CFU/mL lower from the prewashed level.
Change in the relative value of mold, which grow on sweet potato slices after washing-treatment. These figures showed the result of Penicillium sp. (A), Cladosporium sp. (B) and Fusarium sp. (C). Each symbol indicated the following legend: (●) treated by water, (♦) treated by 200SH, (♢) treated by 200SH+AA and (△) treated by 80PA. Samples washed by each solution were stored at 13 °C. Relative value of mold were the difference between the viable counts on each storage date and the untreated. Different alphabets in the graph mean significant differences by Tukey-Kramer's method on the respective storage date(p < 0.05). All values are average of three replicate tests and error bars represent the standard deviation of the mean (n = 3).
Effectiveness of the concentration of peracetic acid Figure 2 shows the effectiveness of different concentrations (20, 40, 60, and 80 mg/L) of the PA solution. In Penicillium sp., significantly (p < 0.05) higher effectiveness (1.8 log CFU/mL higher log reduction by 80 mg/L PA) than that of DW was observed on day 0. Furthermore, 80 mg/L tended to suppress increases of molds number on day 4 (1.1 log CFU/mL lower than DW). Moreover, on day 8, there was no difference in the effectiveness between DW and 80 mg/L washing. The use of 80 mg/L of PA produced better results than the DW for at least 4 Days.
Viable number of Penicillium sp.(A), Cladosporium sp.(B) and Fusarium sp.(C) in emulsion sample of sweet potato slices after washing solutions of various peracetic acid concentrations. (
) indicates Day 0 of storage, (
) indicates Day 4 of storage, and (
) indicates Day 8 of storage. Different alphabets in the graph mean significant differences by Tukey–Kramer's method compared between five concentrations of PA on the respective storage date(p < 0.05). All values are average of three replicate tests and error bars represent the standard deviation of the mean (n = 3).
PA was more effective than DW in treating Cladosporium sp. by 0.6 log CFU/mL (20 mg/L PA) to 2.4 log CFU/mL (80 mg/L PA) on day 0. On day 4, it was observed that 1.0 log CFU/mL (40 mg/L PA) to 1.4 log CFU/mL (80 mg/L PA) had higher effectiveness than DW. Furthermore, on day 8, a 1.3 log CFU/mL (60 mg/L PA) and 1.4 log CFU/mL (80 mg/L PA) higher effectiveness than DW was observed. The use of 60 mg/L of PA was required to get significant result than DW washing after 8 Days storage.
Similarly, PA was more effective against Fusarium sp. on day 0 than DW by 0.8 log CFU/mL (20 mg/L PA) to 1.0 log CFU/mL (80 mg/L PA). On day 4, a 0.9 log CFU/mL (40 mg/L PA and 80 mg/L PA) higher effectiveness than that of DW was observed. Furthermore, on day 8, a 0.6 log CFU/mL (40 mg/L PA) to 1.0 log CFU/mL (80 mg/L PA) higher effectiveness than that of DW was observed. The results indicated that the use of 40 mg/L of PA displayed better result than DW washing after 8 days of storage.
Effectiveness of washing on the color change of samples The color change of washed and unwashed samples was measured. On days 0, 4, and 8 no significant (p > 0.05) difference in L*, a*, and b* was observed by ANOVA analysis (Table 3). A significant (p < 0.05) difference among the types of samples was observed by the analysis.
| L* | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| day 0 | day 4 | day 8 | |||||||
| Untreated | 82.3 | ± | 2.6 | 80.3 | ± | 2.0 | 81.5 | ± | 3.3 |
| Water | 80.3 | ± | 1.5 | 82.8 | ± | 2.1 | 81.2 | ± | 2.7 |
| 200SH | 82.4 | ± | 2.6 | 82.0 | ± | 2.3 | 82.9 | ± | 2.2 |
| 200SH+AA | 80.4 | ± | 3.7 | 80.1 | ± | 3.2 | 79.7 | ± | 2.3 |
| 80PA | 79.5 | ± | 3.0 | 81.7 | ± | 3.2 | 80.1 | ± | 4.1 |
| a* | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| day 0 | day 4 | day 8 | |||||||
| Untreated | 4.7 | ± | 0.8 | 4.7 | ± | 0.9 | 4.4 | ± | 1.1 |
| Water | 4.2 | ± | 0.7 | 4.5 | ± | 0.8 | 4.7 | ± | 1.0 |
| 200SH | 3.8 | ± | 0.5 | 4.8 | ± | 1.0 | 4.6 | ± | 0.9 |
| 200SH+AA | 4.5 | ± | 1.0 | 5.3 | ± | 1.4 | 4.8 | ± | 1.1 |
| 80PA | 4.9 | ± | 1.1 | 4.7 | ± | 1.0 | 5.4 | ± | 1.6 |
| b* | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| day 0 | day 4 | day 8 | |||||||
| Untreated | 25.1 | ± | 1.0 | 22.1 | ± | 2.0 | 20.5 | ± | 2.4 |
| Water | 23.4 | ± | 1.2 | 21.1 | ± | 1.9 | 22.9 | ± | 1.2* |
| 200SH | 23.7 | ± | 1.2 | 20.8 | ± | 2.3 | 21.1 | ± | 1.8 |
| 200SH+AA | 24.1 | ± | 2.3 | 21.0 | ± | 1.9 | 20.1 | ± | 2.8 |
| 80PA | 24.4 | ± | 1.7 | 22.1 | ± | 2.3 | 20.2 | ± | 2.0 |
The colors of washed samples were compared with the unwashed sample. The L* and a* values of unwashed samples was compared each day and there was no significant difference (p > 0.05) in the values of DW, 200 SH, 200 SH+AA, and 80 PA-washed samples. The b* value of DW on day 8 was 1.8 to 2.8 higher than it was for the other three washed samples (p < 0.05).
The optimal temperature for mold growth is 15°C–30°C and temperatures less than 5°C are effective for inhibiting mold growth (Morozumi, 2008). However, storage of sweet potatoes at low temperatures is not appropriate as it causes chilling injury. Furthermore, the reduction of the initial level of the surface-attached molds instead of storage at a lower temperature prevents the growth of the molds on the sweet potato. Hence, we attempted to reduce the growth of molds in sweet potato slices by washing them with a fungicide solution.
We evaluated the presence ratio of mold groups on the sweet potato surface (peel) to fix the target mold species. Three groups of molds (Penicillium sp., Cladosporium sp., and Fusarium sp.) were isolated from rotten parts of decayed and skins of undecayed sweet potatoes. Ogawa and Watanabe (1988) reported that injured sweet potatoes tend to decay easier than non-injured ones since the hyphae of the mold can invade the crop from the injured parts. Some of the growing hyphae on the injured or rotten area may contaminate the surface of the other crops. According to several reports, surface washing with sanitizer was considered effective to suppress the surface growth of molds. Bernardi et al. (2018) reported that SH was the most effective of the five agents: benzalkonium chloride (0.3 %, 2.5 %, 5 %), biguanide (2 %, 3.5 %, 5 %), PA (0.15 %, 1.5 %, 3 %), quaternary ammonium (0.3 %, 2.5 %, 5 %), and SH (0.1 %, 0.5 %, 1.5 %) against Penicillium sp. and Cladosporium sp. (fixed on stainless steel disks). In addition, Beatriz et al. (2008) reported that Penicillium expansum spores attached to apple surfaces were reduced by up to 2.6–4.2 log CFU/g in 100 mg/L SH solutions acidified by 0.25 N phosphoric acid (pH 6.5, 25 °C). Moreover, we evaluated the fungicidal effectiveness of SH or PA as a washing solution to reduce the growth of three spoilage-causing molds of sweet potatoes (Penicillium sp., Cladosporium sp., and Fusarium sp.).
Inatsu et al. (2011) reported that fungicides are more effective when molds are diffused in liquid (medium or washing solution) than when they are attached to vegetable surfaces and disinfected under the same conditions. Furthermore, we dispended each of the three molds (ranging from 4.4 to 5.5 log CFU/mL) into the water that contained water-soluble components of sweet potato slices and then added a tested fungicide solution (200 SH, 200 SH+AA, and 80 PA), and all the numbers of molds were reduced to less than the detective limit (3.0 log CFU/mL). However, the effectiveness of 200 SH or 200 SH+AA to the cut surface-attached molds was comparably lower than that of liquid dispensed (Table 2). This is consistent with earlier research on potato strips, where microbial populations (including mesophilic bacteria, psychrotrophic bacteria, yeasts, lactic acid bacteria, and coliforms) were not controlled by 80 mg/L SH adjusted to pH 6.5 with citric acid (Beltrán et al., 2005). The interaction between SH and the high concentration of organic compounds on the surface of the sweet potato slice was one of the possible causes of the reduction of the fungicidal effect (Xi and Yen-Con, 2017). The same variety of sweet potatoes used in our study contains 25–30 g of starch per 100 g of raw sample (Ando et al., 2018). It was hypothesized that the reaction between crude components of sweet potatoes and hypochlorous acid reduced the fungicidal effect. However, in the test where the agents were added to a solution containing eluted components of sweet potato, a reduction in the mold count of about 2 log was observed, indicating that this concentration of organic components did not affect the effectiveness of SH. The amount of starch diffusing into the filtered washing solution is estimated to be between 3.8 and 6% (w/w) based on the weight of the sample, which is less than the amount of starch that would be present in the slices. The difference in starch concentration surrounding the molds was therefore presumed to have a strong influence on the difference in fungicidal effect on the slice and the filtered washing solution.
However, PA washing demonstrated a fungicidal effect on all tested molds (> 2 log CFU/mL) and a statistically significant difference (p < 0.05) in the efficacy between DW, 200 SH, and 200 SH+AA. SH and SH+AA did not show sufficient fungicidal activity against molds on sweet potato slices (Table 2). Figure 2 shows that PA treatment significantly reduced the viable counts in comparison to DW at the following concentrations for each species: 1.8 log for Penicillium sp. (80 mg/L), 1.7 log, and 0.8 log for Cladosporium sp. and Fusarium sp. (40 mg/L) on 0 Day. These results were almost as similar to the ones that had previously been reported. Salomao et al. (2008) reported that a 30-sec immersion wash with PA solutions at concentrations of 80 mg/L (25 °C) reduced Penicillium expansum spores on apples' surface by 1.12–1.93 log spore/g.
In the Japanese commercial supply chain, fresh-cut vegetables, including cut sweet potatoes were transported at 4 °C–8 °C and consumed after 3–5 days of production; however, storing sweet potatoes at low temperatures damages them from chilling, so storage at 13 °C is recommended for a longer duration (Ohashi and Uritani, 1972). The growth of molds at 13 °C was evaluated since one of the objectives of this study was to prevent the spoilage of whole sweet potatoes for an extended period. The number of molds did not exceed that of the untreated samples in any of the washing tests for Penicillium sp. and Cladosporium sp. when stored at 13 °C after washing. On the contrary, rapid growth was observed for Fusarium sp. between days 4 and 8 and washing with water resulted in the growth of attached molds, exceeding the number of molds in the untreated sample (Figure 1). The rapid increase in the number of Fusarium sp. could be strongly influenced by the optimum humidity for growth. Furthermore, molds can be divided into three main types based on their different requirements for humidity, i.e., hydrophilic fungi that prefer high humidity, and optimal water activity (Aw) is 1.00, but can grow at the lowest Aw of 0.88; the medium-hydrophilic fungi, which have optimal Aw between 0.95–1.00 but can growth at the lowest Aw of 0.80; and xerophilic fungi that prefer dryness, which have optimal Aw is 0.95, but can grow at the lowest Aw is 0.65–0.75 (Hamada, 1989; Christensen and Kaufmann, 1969). The lowest Aw values for the growth of test species (Penicillium sp., Cladosporium sp., and Fusarium sp.) are 0.82–0.83 (Mislivec and Tuite, 1970), 0.86 (Hocking et al., 1994), and 0.89 (Schneider, 1954), respectively. Hence, all the species examined in this study were hydrophilic or medium-hydrophilic fungi, except Fusarium sp. which especially prefers high humidity. Magan et al. (2010) reported that Fusarium sp. significantly increased dry matter loss in humid wheat (Aw 0.95) compared to low-humidity wheat (Aw 0.88) at 15 °C–30 °C. In the present study, the Aw of sweet potato slices during this period remained at 0.98, 0.99, and 0.98 on days 0, 4, and 8, respectively. It was assumed that the moist environment after washing provided favorable conditions for growth, particularly for Fusarium sp. The rapid multiplication on day 8 was thought to have been brought on by this environment.
The DW, 200 SH, and 200 SH+AA washes caused decreases of 0.36–1.18 and 0.4–1.13 log for Penicillium sp. and Cladosporium sp., respectively, and the 200 SH, 200 SH+AA washes induced a decrease of 0.16–1.45 log for Fusarium sp. by day 4 (Figure 1). These were the characteristic results of the preservation tests in the study. Molds injured by SH or PA exposure to a dry environment, which are unfavorable for mold growth, were assumed to be the potential causes for the decrease in mold counts. However, throughout all time periods, the Aw of sweet potato slices remained in the proliferative range for all tested species. In addition, the reduction of molds was also observed with and without PA and SH in the wash solution. Washing helps to remove dirt, microorganisms responsible for quality loss, and cell exudates that may support microbial growth (Zagory, 1999). Thus, the decrease in mold over the 4 days period was attributed to the leaking of nutrients (e.g., starch and carbohydrates) by washing and the lack of nutrients in molds rather than to residual SH, PA, or storage conditions in the sliced sweet potatoes.
This report showed that the PA wash had a stronger bactericidal effect than SH on mold attached to sweet potato cross-sections. On the negative side, it showed that it was difficult to maintain low mold counts over a storage period of several days. There are few reports on fungi attached to agricultural products, and the majority of previous studies evaluating the bactericidal potential of PA on vegetables and fruits have focused on bacteria (Davidson et al., 2017; Joerger et al., 2021), and there are few reports on fungus. This study investigated the effectiveness of SH and PA for washing sweet potatoes to kill molds that may be attached to sweet potatoes and cause quality loss at the slicing surface (injury areas). Furthermore, PA was the most effective in reducing the number of molds that remained, but the concentration at which it achieved a higher fungicidal effect than water washing ranged from 40 to 80 mg/L, depending on the type of mold. However, since this study was conducted on the sliced surface, a different study should be conducted for mold adhering to the outer skin of sweet potatoes. Though the use of agents may be recommended for a temporary reduction in mold, the high-humidity environment may cause a significant increase in mold if stored after washing. For hydrophilic molds such as Fusarium sp., the degree of drying of the crop after washing or the humid environment during storage is an important factor to be considered in future studies.
Conflict of interest There are no conflicts of interest to declare.
