2022 Volume 28 Issue 2 Pages 179-185
Salted radish root (takuan-zuke) turns yellow naturally; however, light-induced color fading reduces its commercial value. In this study, we evaluated strategies to protect the color of salted radish root and prevent fading in response to light, by evaluating the effects of various fluorescent lamps and light-emitting diodes (LED). Higher irradiance LEDs promoted the pale coloration of takuan-zuke; however, blocking shorter wavelengths, particularly those below 490 nm, suppressed color fading. The b* value calculated from reflectance was significantly reduced by white or blue LEDs. Therefore, LED irradiation could enable easy and efficient achievement of pale coloration of salted radish roots. This study provides a theoretical basis for the development of a new method for the production of takuan-zuke without the use of food colorants.
Salted radish roots (takuan-zuke) are one of the most popular traditional foods in Japan. Color is one of the defining characteristics of takuan-zuke, with yellowing progressing over long-term aging under high salt and normal temperature conditions. Takuan-zuke is first soaked in liquids for desalting and seasoning followed by processing into various pickle types, such as fukujin-zuke and sakura-zuke. The latter can be manufactured at low cost and it consists of salted radish roots decolorized using irradiation with sunlight, desalinized, and dyed pink. Radish roots naturally turn bright yellow during salting and fermentation over several months by a mechanism that is characteristic of takuan-zuke, which has been elucidated at the molecular level. Specifically, we have shown that 4-methylthio-3-butenyl isothiocyanate (MTBITC), which imparts pungency, is a key compound in the yellowing reaction of salted radish roots (Ozawa et al., 1990a). MTBITC is unstable in aqueous media; it is easily converted into 2-thioxo-3-pyrrolidinecarbaldehyde (TPC) and 3-methylthiomethylene-2-thioxopyrrolidine in the aqueous phase (Uda et al., 1990; Matsuoka et al., 1997). TPC is converted to 1-(2-thioxopyrrolidine-3-yl)-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (TPCC), which, through Pictet-Spengler condensation with l-tryptophan, is a precursor of the yellow pigment. This compound is generated by microorganisms or through self-maturation during fermentation (Ozawa et al., 1990b; Ozawa et al., 1990c). TPCC is converted to 2-[3-(2-thioxopyrrolidin-3-ylidene)methyl]-tryptophan (TPMT), the main yellow pigment at a neutral pH (Matsuoka et al., 2002). TPMT is a photochromic compound; light yellow (E)-TPMT is isomerized to deep yellow (Z)-TPMT by long-wave ultraviolet (UV) radiation, while the reverse isomerization from the (Z)- to the (E)-form is induced by irradiation with visible light (Matsuoka et al., 2008). (Z)-TPMT is more sensitive to light irradiation than (E)-TPMT. The quantum yields at the optimal wavelength for photoisomerization in the presence of nitrogen are 0.24 and 2.53 for (E → Z) and (Z → E) conversion, respectively. The isomerization of (Z)- to (E)-TPMT is considered a chain reaction because the quantum yield exceeds 1.0. Both TPCC and TPMT are present in commercial takuan-zuke (Takahashi et al., 2009).
We previously found that salted radish samples consistently turned yellow during dehydration and long-term salting, and their b* values increased with time and temperature. Salted radish roots that were sun-dried and pickled at 10–20 °C turned the brightest yellow, and TPMT production was confirmed (Takahashi et al., 2015). However, the takuan-zuke color fades easily under light irradiation, and the development of uneven coloring in commercial display cases or refrigerators ultimately results in a loss of commercial value. The use of light-emitting diodes (LEDs), instead of fluorescent lamps, worsens the color fading of takuan-zuke and, consequently, reduces its shelf life. In these circumstances, a common practice is to add a colorant to commercial takuan-zuke. Many consumers have the misconception that the color of takuan-zuke is only due to color additives; and therefore, tend to prefer lightly salted pickles (asa-zuke) that look like fresh vegetables, rather than pickles prepared using the traditional long-term salting method. White takuan-zuke is produced under conditions of slow yellowing (i.e., low temperatures). Manufacturers are continuously searching for the best approach to achieve color stabilization and avoid the use of color additives. TPMT is a photochromic compound; therefore, it may be possible to produce value-added pickles by accelerating the induction of pale coloration by light. Furthermore, color stabilization in takuan-zuke by inducing pale coloration could enable uniform coloring during different pickling processes.
Light promotes the degradation of food nutrients, aromas, and pigments. The effects of light on plant growth (Goins et al., 1997; Wu et al., 2007) and postharvest storage is studied extensively (Ma et al., 2012; Loi et al., 2019); however, few studies have considered the quality of processed foods exposed to light over long periods. Goto et al. (2015) observed color fading in processed foods stored in transparent containers and irradiated with fluorescent or LED light. Furthermore, Tanabe et al. (2006) found that part of the visible light spectrum suppressed the discoloration of green tea in polyethylene terephthalate bottles. These results highlight the importance of determining the influence of light on food quality, as most foods are exposed to fluorescent or LED light in commercial settings.
In this study, we investigated the effects of light irradiation treatments on takuan-zuke color, to identify strategies for preventing color fading, through controlling the radiation wavelength of various fluorescent lamps and LEDs. Our findings will aid in the development of techniques for maintaining the color tone of takuan-zuke by controlling the light source; therefore, contributing to the production of takuan-zuke free of color additives.
Materials Salted radish root samples were provided by Shin-Shin Foods Co., Ltd. (Tokyo, Japan). Samples were cut to a size of 3.5 cm × 3.5 cm × 2-3 cm (thickness), for further experiments.
Color change of salted radish under fluorescent light The experimental light irradiation equipment was prepared in-house using 10 W fluorescent lamps. The fluorescent light bulbs used were daylight (FL10N; NEC Lighting, Ltd., Tokyo, Japan), red (FL10P; Mitsubishi Electric Corporation, Tokyo, Japan), blue (FL10B; Mitsubishi Electric Corporation), and yellow (FL-10Y; Kyokko Denki Co., Ltd., Tokyo, Japan). These fluorescent lamps were positioned such that the light covered the entire surface of the sample. The colorimetric change was measured using a CM-3500d spectrophotometer (Konica Minolta, Osaka, Japan) with illuminant D65, 10° standard observer, and a measurement area of φ8 mm. The results are expressed as b* values in the L*a*b* system according to the Commission Internationale de l'Eclairage (1976). In this system, b* defines the position of the sample on the blue-yellow axis, where a positive value indicates a yellow color. The rate of decline of b* as an index of color fading was calculated as follows:
Rate of decline of b* (%) = {1 − (b* after irradiation/b* before irradiation)} × 100
The irradiance of the fluorescent lamps and emission spectra were measured using a multichannel spectrophotometer (MCPD-3700; Otsuka Electron Co., Ltd., Osaka, Japan).
Color change of salted radish under fluorescent light with sharp-cut filters Daylight fluorescent-light wavelength was controlled using four sharp-cut filters (L-42, Y-45, Y-47, and Y-49; AGC Techno Glass Co., Ltd., Shizuoka, Japan). Salted radish root samples were placed in a transparent polyethylene bag, and a filter was attached to the skin. The samples were irradiated from a distance of 5 cm. The part of the root that was not attached to the filter was covered with aluminum foil. A sample wrapped in aluminum foil was used as the control. Illuminance and emission spectra of the light transmitted through the sharp-cut filter were measured as described above.
Color change of salted radish under LED irradiation Molded, salted radish-root samples were placed in a laminated packing bag with low oxygen permeability (i-Pack ONY15/PE15/LDPE40; Maruai Corporation, Yamanashi, Japan). The headspace of the bag was replaced with nitrogen gas, and the bag was sealed using a heat sealer. The LED light irradiation equipment was prepared in-house. The LED lights used for the experiments were white (NSPW500GS-K1; Nichia Corporation, Tokushima, Japan, λP = 448 nm), blue (NSPB513A, Nichia Corporation, λP = 464 nm), and UV (NS375L-5RLM; Nitride Semiconductors Co., Ltd., Tokushima, Japan, λP = 380 nm). Radish samples were placed in a fume hood at 20 ± 2 °C and irradiated with LED light. Illuminance of the LEDs was controlled by varying the distance between the samples and the light source. The duration of irradiation was 0–120 min. The irradiance at the peak wavelength of each LED was adjusted to 20 µW/(cm2·nm).
Effect of blue LED irradiation on yellow pigment in salted radish pickling liquid For the pickling liquid, 33.4 g of sucrose and 31.4 g of high-fructose corn syrup were mixed and adjusted to 100 g with water. Salted radish root samples (20–25 g) and a pickling solution of 50% of the sample weight were placed in a polyethylene bag and sterilized by heating at 80 °C for 90 seconds using a water bath. After equilibration at 4 °C for 24–36 h, in the dark, the samples were irradiated with blue LED. The irradiance of blue LED was adjusted to 20 µW/(cm2·nm). The b*value and TPMT of the pickling solution were analyzed at 60 and 120 min after irradiation.
Analytical HPLC was performed with an Agilent 1100 system with a Inertsil ODS-4 HP (3 µm, φ3.0 × 150 mm, GL science Inc., Tokyo, Japan). The flow rate was set at 0.42 mL/min, and the column temperature was set to 40 °C. Elution was achieved using a gradient of two eluents: 20 mM phosphate buffer (pH 6.5) as eluent A and acetonitrile as eluent B. The gradient program was set at 5.9% B rising to 80% B at 20 min, and held for 6 min. The detection wavelength was 400 nm for TPMT using a diode array detector.
Statistical analysis All measurements were recorded in triplicate and the average values were calculated along with the corresponding standard deviation (SD). Data were statistically analyzed with Student's t-test, using GraphPad Prism ver. 9 for Macintosh (GraphPad Software, Inc., CA, USA). p < 0.05 was considered significant.
Effect of fluorescent light on the color of salted radish roots Preliminary experiments confirmed that color fading occurs when commercial takuan-zuke and yellowed salted radish are irradiated with fluorescent light. Daylight fluorescent lamps, which are widely used in Japanese grocery stores, had a greater fading effect than the other white light sources. Therefore, a daylight fluorescent lamp was used to examine the influence of fluorescent light on the discoloration of salted radish roots. The b* values of salted radish roots ranged from 25 to 35, because of variations among the product batches; therefore, changes in b* were normalized against values for unirradiated samples. Fig. 1 shows the rate of decrease in the b* value of salted radish roots when irradiated with various fluorescent lamps with characteristic emission spectra (i.e., daylight, red, blue, and yellow). In samples subjected to blue and daylight fluorescent lamp illumination over 60 min, b* values decreased significantly by 30% and 20%, respectively (p < 0.05). In contrast, the decrease in b* value was < 10% under treatment with red or yellow fluorescent lamps. Visible-light within the 400–500 nm range decreases in the order of blue > daylight > red > yellow light; therefore, our results indicated that the decrease in b* was suppressed as the wavelength decreased within the 400–500 nm range. Therefore, color fading of salted radish roots was clearly affected by irradiation with visible light (i.e., 400–500 nm).
E Effects of fluorescent light on color fading of salted radish root. Values are expressed a mean ± SD (n = 3). Symbols: ● blue; ○ daylight; ■ yellow; □ red.
Color change of salted radish roots under fluorescent light with sharp-cut filters The effect of daylight fluorescent-light irradiation on the b* value of salted radish was studied by controlling the wavelength using four optical filters. Filters L-42, Y-45, Y-47, and Y-49 block light below 420, 450, 470, and 490 nm, respectively. Fig. 2 shows a comparison of the rate of decrease in the b* value with the application of these four types of sharp-cut filters. When samples were illuminated for 60 min with filters Y-45, Y-47, and Y-49 attached, the rate of decrease in b* value was 14.7%, 7.4%, and 3.6%, respectively. Wavelength control with these filters significantly (p < 0.05) suppressed the decrease in the b* value, compared to that with no wavelength control. When L-42 was attached, the decrease in b* did not differ from that with no wavelength control. Suppression of fading was not observed. Filters Y-45, Y-47, and Y-49 block the 435 nm wavelength typical of fluorescent lamps, whereas filter L-42 (or no filter) does not. These results suggest that the 435 nm wavelength significantly affects the decrease in b* value and that this decrease could be effectively suppressed by blocking light at wavelengths below 470 nm. In particular, shading at wavelengths below 490 nm prevented color fading in takuan-zuke. Our previous studies showed that the optimal wavelength for isomerization of (Z)-TPMT to (E)-TPMT was 440–460 nm (Matsuoka et al., 2008); therefore, photoisomerization of TPMT (Z → E) is related to the color fading of takuan-zuke. Moreover, these results suggest that the use of containers and packaging materials that block light below 490 nm could prevent the discoloration of takuan-zuke (Kwon et al., 2018).
Effect of daylight fluorescent lighting equipped with sharp-cut filters on color fading of salted radish roots. Values are expressed as mean ± SD (n = 3). Symbols: ● dark; ○ no filter; ■ Y-49; □ Y-47; ▲ Y-45; △ L-42.
Color change of salted radish roots under illumination by LED TPMT is a photochromic compound; therefore, it could be possible to produce “white” takuan-zuke by controlled light irradiation. We examined pale coloration of takuan-zuke subjected to illumination with LEDs instead of fluorescent lamps. Nitrogen gas packing systems are used in the production of low-salt takuan-zuke (Kato, 1991), which results in a clear yellow color. We examined the change in takuan-zuke color under both nitrogen and air atmospheres. When stored at 4 °C in the dark for 30 d, salted radish roots placed in a bag filled with nitrogen exhibited a bright yellow color, even after the storage period; however, all salted radish roots placed in a bag filled with air turned brown. Therefore, to match actual product conditions, takuan-zuke samples were sealed in a nitrogen gas atmosphere, in low oxygen-permeability packages. The effects of various LED light sources on the coloration of takuan-zuke were investigated. Irradiance was adjusted to 20 µW/(cm2·nm) with UV, blue, and white LED emission spectra (Fig. 3). The rates of decrease in b* value after irradiation for 120 min were 26.0%, 48.2%, and 49.0% for UV, white, and blue LEDs, respectively (Fig. 4). White and blue LEDs promoted the pale coloration of pickled radish, and there was no significant difference in the rate of decrease in b* value between these two treatments.
Spectral distribution of experimental LEDs.
Effects of illumination using LED on pale coloration of salted radish roots. Values are expressed as mean ± SD (n = 3). Symbols: ● UV-LED; ○ white LED; ■ blue LED.
The pale coloration of takuan-zuke induced by a blue LED was investigated. Irradiance was adjusted by changing the distance between the blue LED and the sample. Upon irradiation for 120 min, b* values decreased with the increase in the irradiation time. In addition, the rate of decrease in b* value at spectral irradiances of 10, 20, and 30 µW/(cm2·nm) were 39.7%, 49%, and 53.7%, respectively. Pale coloration increased as spectral irradiation increased. However, no difference was found in the rate of decrease of the b* value between 20 and 30 µW/(cm2·nm), suggesting that the light energy reached a sufficiently high level.
Blue and white LED wavelengths peak near 400–500 nm, which is a wavelength range that exerts a significant influence on the pale yellow coloration of the takuan-zuke. There was a strong correlation between the radiant energy at 465 nm and the rate of decrease in b* value (y = −45.1e−0.021x − 50.46, r2 = 0.964). In addition, the photoreaction was significant when irradiance was high (Fig. 5); a radiant energy of 50–100 (mJ/cm2/nm) was sufficient to effectively brighten the yellow color of takuan-zuke. Fig. 6 shows a color change to pale yellow following irradiation with a blue LED (20 µW/cm2/nm) for 3 h, corresponding to a b* value decrease from 26.6 to 11.9.
Radiant energy at 465 nm and the corresponding rate of decrease in b*. Blue LED was used for irradiation.
Takuan-zuke irradiated with blue LED. Notes: (A) unirradiated (b* = 26.6); (B) after 3 h of blue LED irradiation (b* = 11.9).
We made takuan-zuke under conditions that simulated the preparation of commercial products, and examined the effects of blue LED irradiation on the b* value of takuan-zuke and the TPMT levels of the pickling liquid. The b* value and the ratio of (Z)-TPMT in (E, Z)-TPMT decreased with time, indicating that the isomerization from (Z)- to (E)-form was responsible for the pale coloration of takuan-zuke (Fig. 7). Isomerization of (E)-TPMT to (Z)-TPMT under UV-LED irradiation is believed to be responsible for the darkening of takuan-zuke; however, the b* value decreased (Fig. 4) unexpectedly, possibly due to a mechanism other than the isomerization of TPMT. The quantum yield of photoisomerization of (E)- to (Z)-TPMT, which is involved in the darkening, is lesser than that of (Z)- to (E)-form photoisomerization, which is involved in the lightening (Matsuoka et al., 2008). In addition, components that absorb light in the UV-A region, such as 3-methylthiomethylene-2-thioxopyrrolidine (wavelength of absorption peak: 300–325nm), produced by the degradation of MTBITC (Uda et al., 1990; Matsuoka et al., 1997), could reduce the irradiation efficiency of UV-LED. This is a disadvantage to the darkening reaction of takuan-zuke. Therefore, further studies are required to examine the relationship between the photoisomerization reaction of TPMT and color fading in takuan-zuke. Overall, our results revealed that irradiation of takuan-zuke with white or blue LED for 120 min significantly reduced the b* value. LEDs have high wavelength selectivity and high irradiance; therefore, they can be controlled more easily than fluorescent lamps.
Changes in the b* value of salted radish (a) and the levels of TPMT in pickling liquid (b) in response to blue LED irradiation.
LED irradiation efficiently promoted pale coloration. We further evaluated its effect on takuan-zuke components. Fatty acids are easily degraded by light; therefore, the levels of palmitic, oleic, vaccenic, linoleic, and linolenic acids, the main fatty acids in takuan-zuke, were analyzed. Linoleic and linolenic acids decreased in light-irradiated takuan-zuke placed in a bag without nitrogen gas, but not in takuan-zuke placed in a bag filled with nitrogen. The levels of other fatty acids remained unaltered, irrespective of the presence or absence of nitrogen gas (data not shown).
To the best of our knowledge, this is the first study to report that it is possible to control color fading in takuan-zuke salted radish roots by controlling light irradiation. Compared to daylight and blue fluorescent lamps, red and yellow fluorescent lamps suppressed takuan-zuke color fading. The rate of decrease in b* value was reduced when the fraction of light in the visible-light region of 400–500 nm became smaller. In particular, color fading in takuan-zuke was almost negligible when light below 490 nm was blocked using a sharp-cut filter. Therefore, it is possible to prevent takuan-zuke color fading by using packaging films that shield food from light of wavelengths below 490 nm. In addition, during irradiation of takuan-zuke with LEDs, an increase in the irradiance level promoted pale coloration. Furthermore, b* values were markedly reduced upon irradiation for 2 h with blue and white LEDs, peaking at wavelengths within 400–500 nm. Therefore, pale coloration of salted radish roots could be effectively achieved easily and efficiently using LEDs with high wavelength selectivity and irradiances, instead of fluorescent lamps. Therefore, the combination of LED irradiation and desalting treatment could enable the production of “white” takuan-zuke. Our results provide information for the establishment of a production technology for takuan-zuke that does not depend on color additives. Further studies are required to investigate the effects of light irradiation on the components of takuan-zuke.
Acknowledgements We gratefully acknowledge Mr. Masahiro Hasegawa of Shin-Shin Foods Co., Ltd. for providing the takuan-zuke samples. We also wish to thank Editage (www.editage.com) for English language editing.
Conflict of interest There are no conflicts of interest to declare.
