2023 Volume 29 Issue 5 Pages 423-432
This study aimed to investigate the textural properties and cellular structure of three frozen/thawed Japanese radish cultivars (‘Kenka aokubi’, ‘Hoshi riso’, and ‘Ryujin miura No. 2’) after freezing and thawing. The impacts of the cell wall components, cell membrane parameters, and total soluble solids on the textural properties were also studied. The greatest firmness and least drip loss were found in the frozen/thawed ‘Ryujin miura No. 2’, followed by the ‘Hoshi riso’ and ‘Kenka aokubi’ cultivars. The cellular structure of frozen/thawed ‘Ryujin miura No. 2’ was well preserved while cell damage was found in the other cultivars. ‘Ryujin miura No. 2’ contained the highest amount of Na2CO3 soluble pectin and the lowest amounts of water soluble pectin in the alcohol-insoluble solids, indicating the greater cell wall rigidity compared to the other cultivars. However, cell membrane parameters were completely lost and no relationship between total soluble solids and textural properties was detected for all frozen/thawed samples. These findings highlight the importance of cultivar selection and cell wall rigidity for improvement of the quality of frozen/thawed Japanese radish.
Freezing is widely known as an effective preservation method for maintaining the original quality of food and prolonging its shelf life. However, frozen/thawed fruits and vegetables often suffer from texture softening and high drip loss. To overcome this challenge, considerable attention has been devoted to improving manufacturing processes such as pre-treatment (Ando et al., 2012; Ando et al., 2016), blanching (Olivera et al., 2008), freezing (Tu et al., 2015; Wang et al., 2019), frozen storage (Vicent et al., 2018) and thawing (Holzwarth et al., 2012). However, the practical application of these developed processes in commercial production may be restricted by increased production costs or potential side effects such as the formation of a fibrous texture or unpleasant taste in the product. Alternatively, recent studies have shown that differences in raw material quality resulting from the cultivar and ripening stage have an impact on the quality of some frozen vegetables and fruits such as cabbage (Viriyarattanasak et al., 2023), carrot (Takahashi et al., 2001), corn, and broccoli (Barrett et al., 2000), apple (Chassagne-Berces et al., 2010), mango (Rimkeeree and Charoenrein, 2014) and strawberry (Kurokawa et al., 2022).
Previous studies have highlighted the significant role of cultivars and cell wall components in determining the resistance of fruit and vegetables to freezing damage. Rimkeeree and Charoenrein (2014) suggested that the difference in firmness of two frozen mango cultivars might be due to the difference in amounts of alcohol insoluble solids (AIS) comprising the cell wall materials. Furthermore, they also demonstrated that increasing water soluble pectin (WSP) (i.e., pectin polymers loosely bound to other cell wall polysaccharides via non-covalent and non-ionic bonds) during ripening caused texture loss in frozen mango, regardless of cultivar. On the other hand, Kurokawa et al. (2022) revealed that among pectin fractions, only HCl soluble pectin (i.e., pectin polymers tightly bound to cellulose or hemicellulose) regulated the amounts of drip loss in 10 frozen/thawed strawberry cultivars. For frozen vegetables, Takahashi et al. (2001) proposed carrot cultivars suitable for the freezing process based on the amounts of total pectin (TP), WSP, and calcium; however, the correlation between these components and the texture of frozen carrots was unclear. The findings of these previous studies demonstrate that key cell wall components and mechanisms that explain texture changes during freezing appear to be diverse and dependent on the type of fruits and vegetables examined. In addition, the effect of other potential factors such as solute concentrations and cell membrane parameters due to cultivar variation on the quality of frozen fruits and vegetables has not yet been investigated.
Japanese radish (Raphanus sativus L.) is a major vegetable crop in Japan and East Asia. Japanese radish is commonly consumed raw, dried, pickled, or simmered in a soup. In Japan, certain Japanese radish cultivars have been recommended for specific culinary purposes. For example, the riso-daikon cultivar, characterized by its dense flesh and high fiber content, is considered ideal for the pickling (Takuan) process. To date, research efforts have been made to elucidate the distinctive characteristics of Japanese radish flesh. Kaneko et al. (1980) reported higher levels of TP and hemicellulose in ‘Riso daikon’ than in ‘Miura daikon’, which may explain the textural differences between the raw flesh of the two cultivars. In addition, Komiyama et al. (2009) demonstrated a significant correlation between the compressive force needed to fracture samples of 21 raw Japanese radish cultivars and their AIS and TP contents. However, it appears that key cell wall components undergo changes when Japanese radish is subjected to heat. At temperatures of 80 °C or higher, an increase in WSP and a decrease in sodium hexametaphosphate soluble pectin (ionically bound pectin polymers) due to pectin solubilization were observed and assumed to be responsible for softening of Japanese radish flesh (Fuchigami, 1987; Tamura, 1989). Nevertheless, the effect of cultivar on textural changes of Japanese radish due to freezing has not been previously reported.
Currently, the demand for frozen cut vegetables, including Japanese radish, is increasing due to a labor shortage in the restaurant industry and the needs of consumers with increasingly fast-paced lifestyles. Commercially available frozen cut vegetables, including Japanese radish, are typically intended for further cooking processes. Consequently, maintaining the textural quality of frozen vegetables, such as firmness, at levels similar to those of their fresh counterparts, rendering them suitable for further cooking, is a primary industry target. However, a comprehensive understanding of the factors (e.g., cultivars and key components) influencing changes in texture of Japanese radish during freezing is far from complete. This study therefore aimed to investigate the ability of three cultivars of Japanese radish (‘Kenka aokubi’, ‘Hoshi riso’, and ‘Ryujin miura No.2’) to resist freezing damage. Changes in textural attributes (firmness and fracturability), drip loss, and cellular structure were examined. In addition, the impacts of cell wall components (AIS, TP, and pectin fractions), cell membrane parameters (as determined by electrical impedance analysis), total soluble solids (TSS), and pH were also evaluated.
Sample preparation Three Japanese radish cultivars (‘Kenka aokubi’, ‘Hoshi riso’, and ‘Ryujin miura No.2’) were grown in experimental fields under identical environmental conditions at the Institute of Vegetable and Floriculture Science, NARO in Mie prefecture, Japan. The seeds were sown on September 9, 2019, and the radishes were harvested when the roots reached approximately 7 cm in diameter. The growth periods for ‘Kenka aokubi’, ‘Hoshi riso’, and ‘Ryujin miura No.2’ were 77, 86, and 86 days, respectively. Following harvest, 5–7 roots for each cultivar were immediately transferred to Tokyo Innovation Center, Nissui Corporation in Tokyo, Japan, and then stored at 2±1 °C for a maximum of 4 days before further analysis.
The roots were divided into 3 portions as depicted in Figure 1 and the middle portion was used as the sample. The roots were cut into 1-cm thick round pieces and then vertically cut into quarters (Fig. 1). The cut samples were blanched in boiling water for 5 min and then immediately cooled in waterice for 5 min to prevent excess heating that could compromise the quality of the vegetables. This blanching procedure was chosen based on commercial manufacturing conditions and was sufficient for inactivation of the catalase enzyme. Excess water on the sample surface was removed with paper towels, and the samples were subsequently frozen at −25 °C for 2 h in an air-blast refrigerator (RB121A2-CL, Orion Machinery Co., Ltd., Nagano, Japan). The frozen samples were packed into polyethylene bags, and then kept at −25 °C until further use. Thawing was performed in an incubator (LTE-500, Tokyo Rikakikai Co., Ltd., Tokyo, Japan) at 20 °C for 4 h. Most of the experiments, except for the electrical impedance measurement, were carried out at Tokyo Innovation Center, Nissui Corporation in Tokyo, Japan.
Preparation of the cut radish's root sample and direction for texture analysis and SEM observation.
The electrical impedance measurement was conducted at the Institute of Vegetable and Floriculture Science, NARO in Mie prefecture, Japan. The cutting and blanching procedures were performed as per the conditions mentioned above. After blanching, the samples were vacuum-packed in polyethylene film and brine-frozen in ethylene glycol solution controlled at −25 °C using a cooling bath thermostat (KISS-K6, Huber, Germany). They were subsequently thawed in an incubator (FCI-280G, AS ONE Corp., Osaka, Japan) at 25 °C for 4 h before the measurement.
Textural analysis Compression and puncture tests were performed using a Tensipresser (TTP-50BX II, Taketomo Electric Inc., Tokyo, Japan) equipped with a 10 kg load cell. Both tests were performed in the lengthwise direction of the Japanese radish (Fig. 1). Prior to the compression test, the samples were cut into pieces measuring 5 × 15 × 15 mm. The compression test was performed with a 36-mm diameter aluminum compression plate at a test speed of 1 mm s−1 until the sample reached 40 % of its original thickness. Firmness was determined as the maximum force at the peak of the compression curve (Fig. 2A). In addition, a puncture test was performed using a hollow cylindrical plunger (Ø5 mm). The plunger was inserted into the center of the sample at a test speed of 1 mm s−1 until it reached a depth of 70 % of the sample thickness. The displacement of the probe required to fracture the sample was measured as the distance from the sample surface to the fracture point relative to the thickness of the sample (Fig. 2B). Higher displacement values indicated lower fracturability of the samples. The results reported in the present study represent the mean of at least five determinations.
Force-distance curves of ‘Kenka aokubi’ obtained during compression (A) and puncture tests (B). The displacement of the probe required to fracture the sample (%) = (L1/sample's thickness) × 100.
Determination of drip loss Drip loss was determined as the liquid loss following centrifugation. Approximately 2 g of the samples were cut into cubes with sides measuring approximately 5 mm and placed over Advantec No. 2 filter paper. Placing the samples on the filter paper facilitated the measurement of the total amount of the drip flowing out from the samples. In addition, the filter paper also prevents tiny pieces of the sample, which are likely to break during centrifugation, from falling into the centrifuge tube and mixing with the drip. The samples were then centrifuged at 300 × g for 6 min using a universal refrigerated centrifuge (Model 5922, Kubota Corporation Co. Ltd., Tokyo, Japan). Drip loss was calculated as the percentage of liquid loss from the samples during centrifugation based on the initial sample weight. The determination of drip loss was performed with at least four replicates for each sample.
Scanning electron microscopy (SEM) observation The frozen/thawed samples were cut into pieces measuring 5 × 5 × 8 mm, and directly subjected to analysis using a scanning electron microscope (S-3400 N, Hitachi Co. Ltd., Tokyo, Japan) equipped with a backscattered electron detector. Observations were conducted along the lengthwise direction of the Japanese radish (Fig. 1). The samples were observed at magnifications of ×42 and ×100, with a voltage of 15 kV, under a low vacuum of 30 Pa.
Electrical impedance spectroscopy Electrical impedance spectroscopy (EIS) has been used to assess the cell membrane status in various biological tissues (Zhang and Willison, 1992; Zhang et al., 1993; Ando et al., 2014). In the present study, EIS was applied to evaluate cell membrane injury during the processing of fresh, blanched, and blanched-frozen samples in each variety. Steel needle electrodes spaced 10 mm apart were inserted into the samples to a depth of 5 mm, and the impedance magnitudes |Z| (Ω) and phase differences θ (rad) were then measured at 81 points (with logarithmic frequency intervals) over a frequency range from 100 Hz to 10 MHz, using an impedance analyzer (E4990A, Keysight technologies, Santa Rosa, CA, USA). The measured impedance data were analyzed using an equivalent circuit model for cellular tissues, as previously described (Ando et al., 2014, 2017). The resistance of the extracellular fluid, Re, the resistance of the intracellular fluid, Ri, and the capacitance of the cell membrane, Cm, were individually calculated using this model.
Determination of alcohol insoluble solids and pectin fractions
Isolation of cell wall components Cell wall material was isolated as alcohol insoluble solids (AIS) according to the method of Christiaens et al. (2011) with slight modifications. The samples (15 g) were homogenized in 96 ml of 99.5 % ethanol using an ACE AM-3 homogenizer (Nihonseiki Kaisha Ltd.) at a rotation speed of 10 000 rpm for 30 s, with subsequent filtration through Advantec No. 2 filter paper. The residue was re-homogenized first in 48 ml of 99.5 % ethanol and then in 48 ml of acetone under the same homogenization conditions. The residue was then dried overnight at 40 °C, and the resulting product was defined as AIS. The AIS content was calculated as the weight of dried AIS relative to the sample weight before isolation. AIS measurements were performed in triplicate.
Determination of pectin fractions Extraction of pectin fractions was performed according to the method of Christiaens et al. (2011) with slight modifications. 250 mg of AIS was suspended in 45 ml of distilled water, and incubated in hot water at 90–95 °C for 5 min. The suspension was then immediately cooled in an ice bath for 10 min, and subsequently filtered through Advantec No. 5A filter paper. The filtrate volume was adjusted to 50 ml with distilled water and the resulting solution was defined as the water soluble pectin (WSP) solution. The residue was re-suspended in 45 ml of 0.05 M cyclohexane-trans-1,2-diamine tetra-acetic acid in 0.1 M potassium acetate (pH 6.5) for 5 h at 28 °C. After filtration, the volume of the filtrate was adjusted to 50 ml with the same resuspension solution and the resulting solution was defined as chelator soluble pectin (CSP) solution. The residue was re-suspended in 45 ml of 0.05 M Na2CO3 containing 0.02 M NaBH4 at 4 °C for 16 h, and subsequently incubated at 28 °C for 6 h. Finally, the volume of the filtrate was adjusted to 50 ml with the same resuspension solution and the resulting solution was defined as the sodium carbonate soluble pectin (SCSP) solution. The fractionation of pectin for each sample was performed in triplicate.
Quantitative measurement of D-galacturonic acid (GalA) in pectin fractions was performed following the method described by Melton and Smith (2001) with slight modifications. Each pectin fraction (400 ul) was vigorously vortexed with 40 μl of a 4 M sulfamic acid/potassium sulfamate solution (pH 1.6) and 2.4 ml of a 75 mM solution of sodium tetraborate in sulfuric acid. The resulting solution was then heated in boiling water for 20 min and subsequently cooled in an ice bath for 10 min. 80 μl of m-hydroxydiphenyl solution was then added as a substrate and subsequently subjected to absorbance measurement at 525 nm using a microplate spectrophotometer (Multiskan Skyhigh, Thermo Fisher Scientific Inc., Waltham, Massachusetts, USA). A control reagent, with substitution of each pectin fraction with distilled water, and a control sample, with substitution of the substrate solution with 0.5 % NaOH, were used for subtraction of their respective absorbance values from the corresponding sample absorbance. A GalA standard with a concentration range of 0.1–1.0 g/l was concurrently measured with the sample. Finally, the contents of each pectin fraction were determined according to the GalA standard curve and expressed in terms of percentage of AIS. The GalA measurement was carried out in triplicate for each fractionated solution. TP concentrations were calculated by summing the concentrations of WSP, CSP, and SCSP.
Determination of moisture content, pH, and total soluble solids The samples were homogenized with a mill mixer (MR-280-W, Yamazen Corporation Co. Ltd., Japan). Half of the homogenized samples were subjected to measurement of moisture content according to AOAC (1995). The other half of the homogenized samples was filtered through cheesecloths and centrifuged at 8 590 g for 20 min at 4 °C. The supernatant was recovered for determination of pH and total soluble solids (TSS). TSS (°Brix) was measured with a pocket refractometer (PAL-1, Atago Co. Ltd., Japan). The experiment was performed at room temperature (23 ± 2 °C) in triplicate.
Statistical analysis The results are reported as means ± standard deviations. Significant differences in the results among the cultivars were calculated using analysis of variance (ANOVA) with post hoc Tukey's HSD test, with a significance level of 0.05.
Texture, drip loss, and microstructure An example of changes in firmness and fracturability of Japanese radish due to blanching and freezing is shown in Fig. 2. The firmness of the Japanese radish samples decreased when they were blanched and the subsequent freezing process significantly further softened the radish samples (Fig. 2A). Similarly, the results of the puncture test showed that the displacement value increased (indicating lower fracturability) when the samples were blanched, with a further significant increase when the samples were subsequently frozen (Fig. 2B). Fig. 2B shows the presence of a fluctuating line in the force-distance curve for fresh and blanched samples, while this fluctuating line disappears for the frozen/thawed samples. Previous studies have reported similar fluctuating lines in the forcedeformation curve during pressing of a plunger into cucumber flesh (Horie et al., 2004; Yoshioka et al., 2009). The softening of vegetable tissues caused by the splitting of pectin chains via the β-elimination reaction is widely known as a phenomenon that occurs during heating at high temperatures (Sila et al., 2009). The smooth force-deformation curve for frozen/thawed samples can be attributed to the loosening of cell wall adhesions due to the β-elimination of pectin during blanching and the subsequent separation of cell walls due to the formation of ice crystals during freezing, resulting in lower resistance during probe insertion into the sample tissue in the puncture test. Therefore, our results depicted in Fig. 2 suggest that blanching reduces the firmness and fracturability of radish samples, with a more pronounced effect when the samples undergo subsequent freezing/thawing.
Comparisons of the textural properties and drip loss of Japanese radish samples among the cultivars are shown in Figures 3 and 4, respectively. There were no significant differences in firmness or fracturability of fresh Japanese radish flesh among the cultivars (Fig. 3). For drip loss, ‘Kenka aokubi’ showed the lowest drip loss among fresh samples. However, after blanching, ‘Kenka aokubi’ had the highest drip loss with lower firmness and fracturability when compared with the other cultivars. The lowest drip loss and the greatest fracturability were found in the blanched ‘Hoshi riso’ while the highest firmness was found in the blanched ‘Ryujin miura No. 2’. These results suggest that the texture of raw Japanese radish may not be related to the texture after blanching. A similar lack of relation between the textural properties of fresh and boiled Japanese radish has been reported previously by Sawada et al. (2022), who demonstrated the possibility that the texture of processed Japanese radish could be influenced by cultivar. After subsequent freezing, differences in the texture and drip loss of frozen/thawed Japanese radish samples among cultivars were also identified. The highest firmness and fracturability as well as the lowest drip loss were found in ‘Ryujin miura No. 2’. On the other hand, the textural properties and drip loss for the frozen/thawed ‘Hoshi riso’ and ‘Kenka aokubi’ samples were quite similar in that they both showed lower firmness and higher drip loss than did ‘Ryujin miura No. 2’. These results revealed the outstanding resistance to freezing damage of the ‘Ryujin miura No. 2’ radish cultivar. These results demonstrate that there is a difference in the freezing damage resistance among cultivars of Japanese radish.
Firmness (A) and the displacement of the probe necessary to fracture the sample (B) for the three Japanese radish cultivars. Mean results with different letters (a, b) compared among cultivars are significantly different (p ≤ 0.05).
Drip loss of the three Japanese radish cultivars. Mean results with different letters (a-c) compared among cultivars are significantly different (p ≤ 0.05).
Cellular structures of the frozen/thawed Japanese radish samples were observed using SEM as depicted in Fig. 5. Destruction of cell structure and the expansion of cell size were observed throughout the frozen/thawed ‘Kenka aokubi’ and ‘Hoshi riso’ samples (Fig. 5KL, 5HL). In addition, high magnification images (×100) of the ‘Kenka aokubi’ and ‘Hoshi riso’ samples revealed that the cells appeared to be elongated with cell wall breakage and folding (Fig. 5KH, 5HH). In a previous study, similar destruction of the cellular structure and cell wall folding were also observed in frozen carrots, although a well-defined cellular structure was observed in blanched carrots before freezing (Takahashi et al., 2001). It has been suggested that cell rupture and the cell wall folding during freezing are caused by the growth of large ice crystals from the intercellular space concurrent with water movement from within cells as well as changes in osmotic pressure (Li et al., 2018). In contrast, the microstructure of the frozen/thawed ‘Ryujin miura No.2’ samples remained well defined with well-organized individual cell shapes, although cell expansion was also partly observed (Fig. 5RL, 5RH). This indicates that the cell wall rigidity of ‘Ryujin miura No.2’ might be high enough to prevent water migration and ice growth between cells. The differences in cellular structure among these frozen-thawed samples could contribute to the variations in their texture and drip loss. For the frozen/thawed Japanese radish samples, substantial cell damage may contribute to a low firmness and fracturability with higher drip loss. The significant role of cell wall rigidity in determining the textural properties of frozen/thawed Japanese radish samples is thus highlighted.
Scanning electron microscopy (SEM) images of the three frozen-thawed Japanese radish cultivars; ‘Kenka aokubi’ (K); ‘Hoshi riso’ (H); ‘Ryujin miura No.2’ (R). The samples were observed at x42 (L) and x100 (H) magnification. Arrows show the folding of cell wall.
Cell membrane parameters, cell wall components, moisture content, pH, and total soluble solids Cell membrane parameters, as evaluated by electrical impedance measurements, are shown in Table 1. Comparing among the cultivars, fresh ‘Hoshi riso’ exhibited significantly lower cell membrane capacitance (Cm) and extracellular fluid resistance (Re) than did the other cultivars. This is presumably due to differences in the structure or composition of the tissue, such as cell size and electrolyte concentration. After blanching, a decrease in Cm and Re and an increase in intracellular fluid resistance Ri, were observed in all cultivars. Similar changes in these parameters due to heating have also been reported in the literature (Watanabe et al., 2017; Ando et al., 2017), suggesting that the changes in these electrical impedance parameters due to heating are attributable to thermal denaturation of cell membranes and leakage of high concentration electrolyte solutions from the cells. The impedance of the samples after freezing was substantially decreased for each cultivar, and the equivalent circuit parameters could not be calculated. This decrease in impedance is thought to be due to physical damage to the cell membrane caused by the formation of ice crystals during freezing (Ando et al., 2019; Zhang and Willison, 1992). Increased water permeability due to cell membrane damage has been reported to lead to a decrease in the elastic properties of vegetable tissue after freezing/thawing (Ando et al., 2012). This fact explains the significant decrease in impedance and increase in displacement after freezing/thawing that occurred in all samples in the present study. However, significant differences in the textural parameters and drip loss following freezing/thawing were observed among the cultivars, suggesting that cell membrane damage is not the main factor explaining the difference in freezing damage resistance.
| Cm (nF) | Re (Ω) | Ri (Ω) | ||
|---|---|---|---|---|
| Kenka aokubi | Fresh | 3.27a ± 0.16 | 13613b ± 1181 | 577b ± 82 |
| Blanched | 0.26c ± 0.04 | 902d ± 44 | 1721a ± 146 | |
| Blanched-frozen | N.D. | N.D. | N.D. | |
| Hoshi riso | Fresh | 2.75b ± 0.30 | 10793c ± 1121 | 582b ± 55 |
| Blanched | 0.35c ± 0.09 | 1048d ± 52 | 1558a ± 274 | |
| Blanched-frozen | N.D. | N.D. | N.D. | |
| Ryujin miura No.2 | Fresh | 3.24a ± 0.50 | 14875a ± 866 | 794b ± 91 |
| Blanched | 0.36c ± 0.11 | 1179d ± 154 | 1610a ± 243 | |
| Blanched-frozen | N.D. | N.D. | N.D. |
Cm = capacitance of cell membrane; Re = extracellular fluid resistance; Ri = intracellular fluid resistance. Mean results with different letters (a–d) in the same column are significantly different (p ≤ 0.05). N.D. = not detected.
The AIS, pectin fractions, moisture content, pH, and TSS of the three frozen/thawed Japanese radish samples are summarized in Table 2. The amounts of cell wall components in the present study are similar to those previously reported in raw Japanese radish. Komiyama et al. (2009) reported AIS, TP and WSP values for fresh Japanese radish in the range of 1473–2065, 298–479, and 23–122 mg/100 g sample, respectively, depending on cultivar and harvesting date. In the present study, there were no significant differences in AIS, TP, and CSP contents for the frozen/thawed samples of Japanese radish among the cultivars (Table 2). However, the amounts of WSP and SCSP varied depending on the cultivar. ‘Ryujin miura No.2’ exhibited the lowest WSP and the highest SCSP content, which is consistent with its higher firmness and lower displacement values (higher fracturability) (Fig. 3), lower drip loss (Fig. 4), and less pronounced damage to cellular structure (Fig. 5RL, 5RH). Conversely, ‘Kenka aokubi’ showed contrasting results. These results with respect to textural properties, cellular structure, and the amounts of WSP and SCSP in frozen/thawed Japanese radish can be explained as follows. SCSP is a pectin fraction that is predominantly covalently bound to other cell wall polysaccharaides, while WSP is a low molecular weight pectin that is loosely bound to other cell wall polysaccharides. Based on their chemical properties, it can be assumed that plant cells with a high proportion of SCSP and a low proportion of WSP have a more rigid cell wall. Theoretically, sufficiently rigid cell walls can resist cell collapse with corresponding reductions in the extent of physical damage from ice formation and expansion during the freezing process (Rajashekar and Burke, 1996). Therefore, ‘Ryujin miura No.2’, which has the highest SCSP and lowest WSP fraction as compared to the other cultivars, would be expected to have the highest cell wall rigidity and thus possess excellent freezing damage resistance. This could explain why the cell structure in frozen/thawed ‘Ryujin miura No.2’ was well-preserved and showed less damage, resulting in its high firmness and fracturability. Thus, these results confirm that the cell wall rigidity of frozen/thawed Japanese radish, regulated by the amounts of WSP and SCSP, plays a crucial role in determining their textural properties.
| Kenka aokubi | Hoshi riso | Ryujin miura No.2 | |
|---|---|---|---|
| AIS (mg/100g sample)ns | 2161 ± 143 | 2421 ± 20 | 2164 ± 155 |
| TP (% in AIS)ns | 21.47 ± 0.06 | 21.99 ± 0.45 | 21.60 ± 0.55 |
| WSP (% in AIS) | 4.42a ± 0.26 | 4.24a ± 0.19 | 2.96b ± 0.24 |
| CSP (% in AIS)ns | 9.39 ± 0.09 | 9.56 ± 0.26 | 9.73 ± 0.14 |
| SCSP (% in AIS) | 7.66b ± 0.30 | 8.20a ± 0.26 | 8.91a ± 0.42 |
| Moisture content (%w.b.) | 94.7a ± 0.1 | 94.5b ± 0.0 | 94.8a ± 0.1 |
| Total soluble solid (°Brix)ns | 4.0 ± 0.0 | 4.0 ± 0.1 | 4.1 ± 0.1 |
| pH | 6.41b ± 0.01 | 6.49a ± 0.02 | 6.45a,b ± 0.01 |
AIS = Alcohol insoluble solids; WSP = Water soluble pectin; CSP = Chelator soluble pectin; SCSP = Sodium carbonate soluble pectin;
TP = Total pectin. Mean results with different letters (a, b) in the same row are significantly different (p ≤ 0.05). ns = not significant.
A decrease in cellular water content or an increase in the amount of solute in the cell leads to a lowering of the freezing point, a decrease in the amount and size of ice crystals, and thus less damage to cell structures (van der Sman, 2020). Previous studies reported that the texture of frozen/thawed carrot was successfully preserved when a dehydration pretreatment was applied prior to the freezing process (Ando et al., 2012; Ando et al., 2016). However, a varying effect of moisture content or solute concentration by cultivar on the quality of frozen/thawed vegetables has not been reported. The moisture content and TSS for frozen/thawed Japanese radish by cultivar are shown in Table 2. The frozen/thawed ‘Hoshi riso’ had the lowest moisture content (p ≤ 0.05) whilst there was no significant difference in the TSS content among the cultivars (p > 0.05). These results therefore imply that moisture content or solute concentration is not a factor affecting the freezing damage resistance of Japanese radish among cultivars.
pH is one factor affecting the rate of pectin solubilization during heat treatment, reflecting the softening of vegetable texture. Previous studies reported that cooking at pH 4 could enable maintenance of the firmness of vegetables, whilst cooking at either above pH 5 or below pH 3 resulted in a rapid decrease of the vegetable's firmness (Fuchigami, 1983, Fuchigami et al., 1993). Fuchigami (1987) revealed a positive relationship between the pH of the solution after boiling (pH 6–7) and the degree of softening of Japanese radish roots. However, the impact of pH variation on the quality of frozen vegetables with respect to cultivar has not been studied to date. Table 2 reveals that the highest and lowest pH values for our frozen/thawed Japanese radish samples were found in the ‘Hoshi riso’ and ‘Kenka aokubi’ samples, respectively. These pH values do not appear to correlate with the textural properties or cellular structure of frozen/thawed Japanese radish.
The aim of the present study was to assess the freezing damage resistance of three Japanese radish cultivars. The results revealed that frozen/thawed ‘Ryujin miura No. 2’ showed the highest firmness and fracturability, the lowest drip loss, and the least damage to cellular structure compared to the other cultivars. These results, at the macroscopic level, can be explained by cell wall rigidity as indicated by the high SCSP and low WSP remaining in the frozen/thawed ‘Ryujin miura No. 2’. On the other hand, cell membrane parameters and solute concentrations (as determined by TSS) did not appear to be related to the textural deterioration of Japanese radishes during freezing. This information can be used as a guideline for selecting suitable radish cultivars for the production of frozen cut radish, as a basis for the development of processed radish products with a soft texture, and for further research on the mechanism of changes in the texture of other frozen vegetables.
Acknowledgements This study was conducted as a joint research project between the National Agriculture and Food Research Organization (NARO) and NISSUI corporation. The authors would like to thank Dr. Takayoshi Ohara for his contribution to this study and his valuable comments on this manuscript.
Conflict of interest There are no conflicts of interest to declare.
