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Article Study of Postharvest Quality and Antioxidant Capacity of Freshly Cut Amaranth after Blue LED Light Treatment
Siyuan Jin 1,2, Zhaoyang Ding 1,2 and Jing Xie 1,2,3,*
1 College of Food Science and Technology, Shanghai Ocean University, Shanghai 201306, China; [email protected] (S.J.); [email protected] (Z.D.) 2 National Experimental Teaching Demonstration Center for Food Science and Engineering, Shanghai Ocean University, Shanghai 201306, China 3 Shanghai Professional Technology Service Platform on Cold Chain Equipment Performance and Energy Saving Evaluation, Shanghai 201306, China * Correspondence: [email protected]; Tel.: +86-021-6190-0391
Abstract: Freshly cut vegetables are susceptible to microbial contamination and oxidation during handling and storage. Hence, light-emitting diode technology can effectively inhibit microbial growth and improve antioxidant enzyme activity. In this paper, the freshly cut amaranth was treated
with different intensities of blue light-emitting diode (LED460nm) over 12 days. Chlorophyll content, ascorbic acid content, antioxidant capacity, antioxidant enzymes activity, the changes in microbial count, and sensorial evaluation were measured to analyze the effects of LED treatment on the
amaranth. Blue LED460nm light irradiation improved the vital signs of the samples and extended the shelf life by 2–3 days. The AsA–GSH cycle was effectively activated with the irradiation of 2 30 µmol/(m ·s) blue LED460nm light. According to the results, the LED460nm light could retard the growth of colonies and the main spoilage bacteria, Pseudomonas aeruginosa, of freshly cut amaranth.
Citation: Jin, S.; Ding, Z.; Xie, J. Study of Postharvest Quality and Keywords: freshly cut amaranth; light-emitting diode; antioxidation ability; microbial community Antioxidant Capacity of Freshly Cut Amaranth after Blue LED Light Treatment. Plants 2021, 10, 1614. https://doi.org/10.3390/plants 1. Introduction 10081614 Amaranth (Amaranthus dubius L.)is rich in ascorbic acid and other nutrients [1]. Freshly cut vegetables are ready-to-use products made from fresh vegetables after sorting, cleaning, Academic Editors: Jianye Chen and and other treatment, which are convenient and fresh for consumers [2]. Cutting could Zhongqi Fan cause mechanical damage to vegetables that would speed up their respiration rate. In addition, the cut wounds are susceptible to microbial invasion, which accelerates the Received: 14 July 2021 deterioration of plant quality. There is a great demand for effective preservation techniques Accepted: 3 August 2021 Published: 6 August 2021 to maintain the quality of freshly cut commodities at this stage. Light energy is a necessary condition to maintain plant growth [3]. LEDs have characteristics such as low cost, high
Publisher’s Note: MDPI stays neutral efficiency, and environmental protection. They have a wide range of applications in with regard to jurisdictional claims in freshly cut vegetable preservation as well as in other areas with high research prospects [4]. published maps and institutional affil- Bhavya et al. [5] found that blue light could inhibit the proliferation of Escherichia coli iations. and Staphylococcus aureus, and effectively improved the antioxidant enzyme activity of freshly cut pineapple slices. Zhai et al. [6] confirmed that UVC–LEDs could effectively sterilize Escherichia coli inoculated on freshly cut dragon fruit, while maintaining the quality of the freshly cut dragon fruit. Bian et al. [7] agreed that red and blue LEDs scavenged free radicals by enhancing the antioxidant capacity of lettuce while reducing Copyright: © 2021 by the authors. µ 2· Licensee MDPI, Basel, Switzerland. nitrate levels. Maroga et al. [8] showed that 100 mol/(m s) blue LED450nm light extended This article is an open access article the shelf life of red freshly cut bell peppers, and improved antioxidant capacity and distributed under the terms and phenolic compounds. Chang et al. [9] confirmed that blue LED light irradiation enhances conditions of the Creative Commons L-ascorbic acid content while reducing reactive oxygen species accumulation in Chinese Attribution (CC BY) license (https:// cabbage seedlings. Samuoliene˙ et al. [10] showed that blue LED irradiation of baby leaf creativecommons.org/licenses/by/ lettuce had a significant positive effect on its DPPH scavenging capacity and enhanced its 4.0/). antioxidant properties.
Plants 2021, 10, 1614. https://doi.org/10.3390/plants10081614 https://www.mdpi.com/journal/plants Plants 2021, 10, x FOR PEER REVIEW 2 of 14
Plants 2021, 10, 1614 2 of 14 lettuce had a significant positive effect on its DPPH scavenging capacity and enhanced its antioxidant properties. In this paper, the physiological characteristics of freshly cut leafy vegetables treated withIn blue this light paper, were the studied, physiological and the characteristics bacterial colonies of freshly in the cut vegetables leafy vegetables were assayed treated as withwell. blue It is lightshort wereof research studied, on andantioxidant the bacterial performance colonies inand the antibacterial vegetables effect weres assayed on freshly as well.cut fruits It is shortand vegetables of research irra ondia antioxidantted with monochromatic performance and light antibacterial (460 to around effects 470 on freshlynm). In cut fruits and vegetables irradiated with monochromatic light (460 to around 470 nm). this study, blue LED460nm light was used to treat the freshly cut amaranth, and the changes In this study, blue LED light was used to treat the freshly cut amaranth, and the in microbial total bacteria460nm and antioxidant capacity, combined with physiological and bi- changes in microbial total bacteria and antioxidant capacity, combined with physiological ochemical indexes were measured to check the effects. and biochemical indexes were measured to check the effects.
2.2. MaterialsMaterials andand MethodsMethods 2.1.2.1. TreatmentTreatment MethodMethod ofofAmaranth Amaranth and and LED LED Equipment Equipment Diagram Diagram AmaranthAmaranth plants were purchased purchased from from Shanghai Shanghai Duoli Duoli farm farm fruit fruit and and vegetable vegetable co- cooperative.operative. After After amaranth amaranth plants plants were were picked picked,, they they were were quickly quickly sent sent to the to the laboratory laboratory for fortreatment. treatment. Amaranth Amaranth samples samples were were cut about cut about 5 cm away 5 cm from away the from stem the of stem the leaf of, the wash leaf,ed washedwith ultrapure with ultrapure water, water,and dr andied driednaturally. naturally. Samples Samples were were chosen chosen without without yellowing yellowing or ormechanical mechanical damage damage and and freshly freshly cut cutwith with a 1-cm a 1-cm scalpel. scalpel. Samples Samples were placed were placedon a steri- on alized sterilized tray (80 tray g/plate), (80 g/plate), sealed sealedwith polyvinyl with polyvinyl chloride chloride (PVC) high (PVC)-transmittance high-transmittance anti-fog anti-fogfilm, and film, stored and at stored 4 °C for at 412◦C days. for 12 days. ThisThis experimentexperiment waswas divideddivided intointo fourfour groups,groups, eacheach groupgroup havinghaving10 10 samples. samples. TheThe ◦ samplessamples were were irradiated irradiated with with different different intensities intensities of blueof blue LED LED460nm460 nmlight light at 4at C4 and°C and relative rela- humiditytive humidity of 90 of± 905%. ± The5%. irradiationThe irradiation height height was aboutwas about 30 cm. 30The cm. irradiationThe irradiation time wastime 12was h per12 h day. per Asday. shown As shown in Figure in Figure1, the T31, groupthe T3 hadgroup a 30 hadµmol/(m a 30 μmol/(m2·s) 4602·s) nm 460 LED nm light. LED Similarly,light. Similarly, a 10 µmol/(m a 10 μmol/(m2·s) 460 nm2·s) LED 460 nmlamp LED was lamp installed was in installed group T1, in a 20 groupµmol/(m T1, a2 · 20s) 460μmol/(m nm LED2·s) 460 lamp nm was LED installed lamp was in installed group T2, in and group no T2 LED, and lamp no LED was installedlamp was in ins grouptalled 2 CK.in group The intensities CK. The intensities were 0, 10, were 20, 30 0, µ10,mol/(m 20, 30 ·μmols), and/(m related2·s), and indicators related indicators were tested were at two-daytested at intervals two-day forintervals 12 days. for The 12 days. detailed The treatments detailed treatments for each group for each are showngroup are in Table shown1. Thein Table physiological 1. The physiological and biochemical and biochemical indexes, total indexes, bacterial total count, bacterial and antioxidant count, and capacity antioxi- ofdant vegetables capacity under of vegetables the three under light conditionsthe three light were conditions evaluated, were where evaluated, antioxidant where capacity anti- wasoxidant assessed capacity mainly was by assessed peroxidase, mainly superoxide by peroxidase, dismutase, superoxide and oxidative dismutase stress-related, and oxida- enzymes,tive stress and-related the correspondingenzymes, and datathe corresponding were processed. data were processed.
FigureFigure 1.1. LEDLED equipmentequipment diagram.diagram.
TableTable 1. 1.Freshly Freshly cut cut amaranth amaranth of of each each experimental experimental group. group.
GroupGroup Light IntensityLight Intensity (μmol/(m (µmol/(m2·s)) 2·s)) LEDLED Band Band (nm) (nm) CK CK- --- T1 T110 10460 460 T2 20 460 T3 30 460
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2.2. Organoleptic Properties Twelve consumers formed an evaluation group. The color, shape, and smell of the experimental samples were evaluated, reasonably and objectively. The full score of the three indicators was 10 points, and the specific scoring standards are shown in Table2. Table 2. Sensory evaluation project.
Score Color Form Smell 10 Full and bright color Crisp Refreshing fragrance The color is a little dim, but It’s brittle, but it doesn’t No fragrance, no peculiar 8 not brown shrink smell No fragrance, slightly Overall acceptable, with 6 Slight atrophy peculiar smell after occasional browning careful smelling Obvious atrophy, but not Obvious odor, but not 4 Browning rate < 1/3 serious serious 2 Browning rate ≥ 1/3 Atrophy serious Severe odor All browning and the color All severely atrophied 0 Stench of mildew spots can be seen and moldy
2.3. Soluble Solids The soluble solids method referred to the method of Zhang et al. [11]. The samples were fully ground, and centrifuged at 4000 r/min for 10 min. The supernatant scale was added to the detection mirror to set the content of soluble solids, expressed by mass fraction (%), and repeated 3 times.
2.4. Weight-Loss Rate The water weight-loss rate was calculated according to Formula (1).
M − M Weight − loss rate/% = 0 t × 100% (1) M0
2.5. Water Distribution and Migration The water distribution and migration referred to the method of Bimal et al. [12].
2.6. Chlorophyll Content Chlorophyll content was calculated according to the experimental method of Hasperué et al. [13]. The content of total chlorophyll was calculated according to Formula (2), the content of total chlorophyll, and the content of demethylated chlorophyll and carotenoid was calculated by:
(20.29 ∗ A + 8.05 ∗ A ∗ V ∗ n) G/% = 645nm 663nm T (2) 1000 ∗ m
where G, the content of chlorophyll in a 1 g sample, mg/g; A645nm, the absorbance of the extract was measured at 645 nm; A663nm, the absorbance of the extract was measured at 663 nm; VT, total volume of extract, mL; n, dilution ratio of extract; m, fresh weight of freshly cut amaranth, g.
2.7. Ascorbic Acid Content Ascorbic acid content referred to the method of Young et al. [14], with some modifica- tions. A 1 g sample of amaranth leaf tissue was weighed and ground in 5 mL 0.05 mol/L oxalic acid. The supernatant was extracted by centrifugation at 4000 r/min for 10 min. All experiments were performed three times.
2.8. Ascorbate Peroxidase (APX) Activity APX activity referred to the method of Zhao et al. [15]. Plants 2021, 10, 1614 4 of 14
2.9. Glutathione Reductase (GR) Activity GR activity referred to the method of Giacomo et al. [16].
2.10. Peroxidase (POD) Activity POD activity referred to the method of Zhao et al. [17].
2.11. Superoxide Dismutase (SOD) Activity SOD activity was determined by monitoring the inhibition of photochemical reduction by nitroblue tetrazolium (NBT) [15].
2.12. Malondialdehyde (MDA) Content MDA content refers to the method of Young et al. [18], with some modifications. A5 mL measure of 100 g/L trichloroacetic acid solution was added to a 1 g sample of amaranth. After homogenizing, samples were centrifuged at 10,000× g for 20 min at 4 ◦C. Next, 2 mL of supernatant was added to 2 mL of 0.67 g/100 mL thiobarbituric acid solution. After mixing, the solution was boiled for 20 min in a boiling water bath. The solution was centrifuged at 10,000× g, cooled with water. The experiment was repeated 3 times.
2.13. Aerobic Plate Count and Specific Spoilage Organism (SSO) Count The aerobic plate-counting method referred to the method of Zhang et al. [11]. A total 10 g of freshly cut amaranth samples were weighed on the sterile operating platform, and homogenization solution with a ratio of 1:10 was prepared in the sterile bag according to the gradient dilution method. The homogenization solution was evenly placed on the plate- counting agar medium (PCA), and the colonies were tested by the inverted plate method. The specific spoilage organism(SSO)count referred to the method of Amal et al. [19]. Where M0 is the quality of amaranth before storage; Mt is the quality of amaranth during storage time t.
2.14. DNA Extraction and PCR Amplification Microbial DNA was extracted using the HiPure Soil DNA Kits (Magen, Guangzhou, China) according to the manufacturer’s protocols. The 16S rDNA V5-V7 region of the ribo- somal RNA gene was amplified by PCR using primers799F: AACMGGATTAGATACCCKG; 1193R: ACGTCATCCCCACCTTCC [20].
2.15. Illumina Novaseq 6000 Sequencing Amplicons were extracted from 2% agarose gels and purified using the DNA Gel Extraction Kit according to the manufacturer’s instructions and quantified using ABI StepOnePlus Real-Time PCR System. Purified amplicons were pooled in equimolar and paired-end sequenced on an Illumina platform according to the standard protocols.
2.16. Data Analysis Method Three parallel measurements were performed in all experiments. All data were expressed as average values ± standard error (n = 3); All data were performed by one-way analysis of variance (ANOVA) and the differences among the means were compared by Duncan’s multiple range test with a significance of p < 0.05 using the SPSS 17.0 statistical program (SPSS Inc., Chicago, IL, USA).
3. Results and Discussion 3.1. Changes in Organoleptic Properties and Shelf Life Sensory evaluation shows the freshness of freshly cut amaranth most intuitively and objectively [21]. In Figure2, the color, appearance, and odor of freshly cut amaranth decreased during storage time. Significant differences were present in the irradiated and control groups. Onwards of the 8th day, the control group scored below 6 marks on each item. This indicated the deterioration of the quality of freshly cut amaranth Plants 2021, 10, x FOR PEER REVIEW 5 of 14
2.16. Data Analysis Method Three parallel measurements were performed in all experiments. All data were ex- pressed as average values ± standard error (n = 3); All data were performed by one-way analysis of variance (ANOVA) and the differences among the means were compared by Duncan’s multiple range test with a significance of p < 0.05 using the SPSS 17.0 statistical program (SPSS Inc., Chicago, IL, USA).
3. Results and Discussion 3.1. Changes in Organoleptic Properties and Shelf Life Sensory evaluation shows the freshness of freshly cut amaranth most intuitively and Plants 2021, 10, 1614 objectively [21]. In Figure 2, the color, appearance, and odor of freshly cut amaranth5 ofde- 14 creased during storage time. Significant differences were present in the irradiated and control groups. Onwards of the 8th day, the control group scored below 6 marks on each item. This indicated the deterioration of the quality of freshly cut amaranth in the control group.in the controlThe leaves group. of each The treatment leaves of group each treatment presented group various presented degrees variousof water degrees loss, ap- of water loss, appearing wrinkled and yellowed on the 10th day. The overall score was: pearing wrinkled and yellowed on the 10th day. The overall score was: T3 > T2 > T1 > CK. T3 > T2 > T1 > CK. During storage, the sensory scores of the treatment groups were much During storage, the sensory scores of the treatment groups were much higher than those higher than those of the control group. Blue LED460nm light can slow down the aging of of the control group. Blue LED460nm light can slow down the aging of freshly cut amaranth freshly cut amaranth and prolong the shelf life of freshly cut amaranth. Aiamlaor et al. [22] and prolong the shelf life of freshly cut amaranth. Aiamlaor et al. [22] demonstrated that demonstrated that blue light irradiation was effective in delaying broccoli floret yellowing blue light irradiation was effective in delaying broccoli floret yellowing to extend the shelf to extend the shelf life. Shelf-stage differences showed a close correlation with chlorophyll life. Shelf-stage differences showed a close correlation with chlorophyll content. The chlo- content. The chlorophyll content in the CK group appeared to decrease significantly at the rophyll content in the CK group appeared to decrease significantly at the 6th day. Yellow- 6th day. Yellowing was also evident at the 6th day during the shelf period. In all, the T3 ing was also evident at the 6th day during the shelf period. In all, the T3 treatment group treatment group had the best effects. had the best effects.
Figure 2. Changes in smell (A), form (B) and color (C) sensory characteristics, and sample shelf-life changes (D) during storage.
3.2. Changes in Soluble Solids Content Sugars, vitamins, and minerals are the prime soluble solids in plants. Soluble solids can corroborate the consumption rate and maturity of plant nutrients. Certainly, an important indicator measures the freshly cut fruit and vegetable preservation effects [23,24]. In Figure3, the soluble solids of the T2 and T3 treatment groups were able to remain in a stable range at the beginning of storage, which might be due to the enhanced respiration and vital activity of freshly cut amaranth, causing the balance between the consumption and formation rate of soluble solids. The plant vital signs decreased with storage time, resulting in increased consumption of soluble sugars and a decreased trend in the later stages of storage. Dramatic differences were observed between the T2 and T3 treatment groups and the control group (p < 0.05). It may happen that higher light intensity can better Plants 2021, 10, x FOR PEER REVIEW 6 of 14
Figure 2. Changes in smell (A), form (B) and color (C) sensory characteristics, and sample shelf-life changes (D) during storage.
3.2. Changes in Soluble Solids Content Sugars, vitamins, and minerals are the prime soluble solids in plants. Soluble solids can corroborate the consumption rate and maturity of plant nutrients. Certainly, an im- portant indicator measures the freshly cut fruit and vegetable preservation effects [23,24]. In Figure 3, the soluble solids of the T2 and T3 treatment groups were able to remain in a stable range at the beginning of storage, which might be due to the enhanced respira- tion and vital activity of freshly cut amaranth, causing the balance between the consump- tion and formation rate of soluble solids. The plant vital signs decreased with storage time, Plants 2021, 10, 1614 resulting in increased consumption of soluble sugars and a decreased trend in the6 later of 14 stages of storage. Dramatic differences were observed between the T2 and T3 treatment groups and the control group (p < 0.05). It may happen that higher light intensity can better stimulate photosynthesis in amaranth and further synthesize more organicorganic matter.matter. No distinct differences were observed between the T1 treatment group and the control control group, group, which might be due to insufficient insufficient light intensity to reach the light supplementation point of the freshly cut amaranth. The CK CK group group was unable unable to to photosynthesize photosynthesize under sheltered conditions in a more effective way. NoeliaNoelia etet al.al. [[25]25] alsoalso confirmedconfirmed thethe results.results.
Figure 3. Changes in soluble solids content The means with differentdifferent lowercase letters (a–c)(a–c) in the figurefigure differ significantlysignificantly (p < < 0.05). 0.05).
3.3. Changes in Weight Loss and Moisture Migration The loss raterate ofof plantplant weight weight is is mainly mainly reflected reflected in in water water loss loss and and nutrient nutrient consumption. consump- tion.In Figure In Figure4A, the 4A, weight-loss the weight rate-loss was: rate T3was >:T2 T3 > > T1T2 >> CK.T1 > This CK. resultThis result indicated indicated that blue that light could effectively enlarge the stomata of plant leaves, and stomatal conductance blue light could effectively enlarge the stomata of plant leaves, and stomatal conductance increased with the increase of blue light intensity [26]. Sander et al. [27] also proved that increased with the increase of blue light intensity [26]. Sander et al. [27] also proved that blue light could trigger the qualitative signal effect of plants and photosynthesis again. blue light could trigger the qualitative signal effect of plants and photosynthesis again. Moreover, blue light could effectively accelerate the transpiration rate, further increasing Moreover, blue light could effectively accelerate the transpiration rate, further increasing the mass loss rate [28]. The samples were wrapped by PE film during the experiment, so the mass loss rate [28]. The samples were wrapped by PE film during the experiment, so the experimental results were controlled in a reasonable range. the experimental results were controlled in a reasonable range. In Figure4B, the water migration of bound water (0–2 ms), immobilized water In Figure 4B, the water migration of bound water (0–2 ms), immobilized water (2–20 (2–20 ms), and free water (20–100 ms) in the leaves of the samples at the early stage ms)of storage, and free (0 d) water and the(20– end100 ofms) storage in the (12leaves d) was of the obtained samples by at inversion the early and stage calculation. of storage (0 d) Waterand the content end of was storage observed (12 d) by was peak obtaine valued andby inversion peak area. and In thecalculation. first 12 d, the bound water and immobilized water content for the treatment groups showed an upward trend, at the same time as the content of free water fell. This may be due to the mechanical damage caused by self-healing of freshly cut amaranth and the synergistic effect of blue light irradiation to stimulate the stress response of the amaranth [29]. The increase in the bound water content showed that the stress resistance of the treated group was enhanced to a certain extent [3] The bound water content for the control group decreased at the end of storage, with significant differences among the other groups (p < 0.05). The possible reason behind the difference is due to the high metabolic reaction caused by light, which feeds freshly cut amaranth with energy in an efficient way. Combined with sensory evaluation index, the quality of freshly cut amaranth deteriorated significantly on the 12th day. This indicated that the metabolic system of freshly cut amaranth collapsed and could not maintain the plant body signs. At the end of storage, the free water content for each experimental group decreased. The consequence was: CK > T1 > T2 > T3. After blue light irradiation, freshly cut amaranth had vigorous life activity, metabolic activity, and oxidation Plants 2021, 10, x FOR PEER REVIEW 7 of 14
Water content was observed by peak value and peak area. In the first 12 d, the bound water and immobilized water content for the treatment groups showed an upward trend, at the same time as the content of free water fell. This may be due to the mechanical dam- age caused by self-healing of freshly cut amaranth and the synergistic effect of blue light irradiation to stimulate the stress response of the amaranth [29]. The increase in the bound water content showed that the stress resistance of the treated group was enhanced to a certain extent [3] The bound water content for the control group decreased at the end of storage, with significant differences among the other groups (p < 0.05). The possible reason behind the difference is due to the high metabolic reaction caused by light, which feeds freshly cut amaranth with energy in an efficient way. Combined with sensory eval- uation index, the quality of freshly cut amaranth deteriorated significantly on the 12th day. This indicated that the metabolic system of freshly cut amaranth collapsed and could Plants 2021, 10, 1614 7 of 14 not maintain the plant body signs. At the end of storage, the free water content for each experimental group decreased. The consequence was: CK > T1 > T2 > T3. After blue light irradiation, freshly cut amaranth had vigorous life activity, metabolic activity, and oxida- tionreactions, reaction whichs, which consume consume a lot a oflot water. of water. The The results results of moisture of moisture migration migration experiments experi- mentscorresponded corresponded to the to weight-loss the weight rate.-loss rate.
FigureFigure 4. 4. ChangesChanges in in weight weight loss loss (A (A) and) and moisture moisture migration migration (B (B).). The The means means with with different different lowercase lowercase letters letters (a (a–c)–c) in in figure figure differdiffer significantly significantly (p (p << 0.05).
3.4.3.4. Changes Changes in in Chlorophyll Chlorophyll Content Content ChlorophyllChlorophyll is is an importantimportant indicatorindicator of of plant plant vital vital signs signs in thein the photosynthesis photosynthesis of plant of plantcells. cells. Chlorophyll Chlorophyll can can absorb absorb light light energy energy to synthesizeto synthesize carbohydrates, carbohydrates, carbon carbon dioxide, diox- ideand, and water. water. Light Light can can affect affect photosynthesis photosynthesis of plantof plant cells cells by changingby changing the the absorption absorption and andconsumption consumption of light of light energy energy and electronand electron transport transport [30]. In[30] Figure. In Figure5, the chlorophyll 5, the chlorophyll content contentof the treatmentof the treatment groups groups irradiated irradiated by blue by blue LED 460nmLED460nmlight light show show a trenda trend of of increasing increas- ingfrom from 0–6 0– days6 days and and then then decreasing decreasing after after the the 6th 6th day, day, with with peak peak value value occurring occurring at at day day 6 6( (pp< < 0.05)0.05).. The peak value of the the T3 treatment treatment group group reached 41.18 mg/g. mg/g. The The chlorophyll chlorophyll ofof the the control control group group decreased decreased continuously continuously w withith storage storage time. time. Bukhov Bukhov et et al. al. [31] [31 ]found found thatthat barley barley leaf leaf seedlings seedlings could could synthesize synthesize chlorophyll chlorophyll more more effectively effectively under under blue blue light light irradiationirradiation;; and and the the quantity quantity of of carotenoids carotenoids,, which which could could consume consume the the energy energy of of excessive excessive excitationexcitation of of chlorophyll chlorophyll and and maintain maintain the the balance balance of of physi physiologicalological activities activities of of the the plants plants,, increasedincreased under under blue blue LED LED light. light. Light Light stimulated stimulated the the activity activity of of magnesium magnesium chelatase, chelatase, thenthen increased increased the the content content of of chlorophyll. chlorophyll. The The results results showed showed that that the the treatment treatment of of freshly freshly µ 2· cutcut amaranth amaranth with with 30 30 μmolmol/(m/(m2·s) blues) blue LED LED460nm460nm light couldlight couldeffectively effectively improve improve the chlo- the rophyllchlorophyll content. content. The sensory The sensory score scorewas consistent was consistent with withthe result, the result, which which could could inhibit inhibit the qualitythe quality deterioration deterioration of freshly of freshly cut amaranth cut amaranth for 2 foror 3 2 days. or 3 days.
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Figure 5. Changes in chlorophyll content. The means with different lowercase letters (a–d) in figure FigureFigure 5. 5.ChangesChanges in inchlorophyll chlorophyll content content.. The The means means with with different different lowercase lowercase letters letters (a (a–d)–d) in infigure figure differ significantly (p < 0.05). differdiffer significantly significantly (p ( p< < 0.05). 0.05). 3.5.3.5. ChangesChanges inin AsAAsA ContentContent andand OxidativeOxidative StressaseStressase ActivityActivity 3.5. Changes in AsA Content and Oxidative Stressase Activity AsAAsA cancan removeremove ROSROS andand HH22OO22 indirectlyindirectly throughthrough APX,APX, becausebecause ofof thethe reducibility.reducibility. AsA can remove ROS and H2O2 indirectly through APX, because of the reducibility. AsAAsA synthesissynthesis pathway pathway in in plants plants is is called called the theSmirnoff Smirnoff Wheeler Wheeler cycle. cycle. TheThe oxidativeoxidative stress stress AsA synthesis pathway in plants is called the Smirnoff Wheeler cycle. The oxidative stress processprocess ofof AsAAsA is is shown shown in in Figure Figure 6.6 .APX APX and and GR GR were were particularly particularly important important in the in AsA the - process of AsA is shown in Figure 6. APX and GR were particularly important in the AsA- AsA-GSHGSH cycle. cycle. APX APX could could scavenge scavenge reactive reactive oxygen oxygen species species (ROS (ROS)) and and prevent oxidative oxidative GSH cycle. APX could scavenge reactive oxygen species (ROS) and prevent oxidative damagedamage in in plants.plants. PlantPlant stressstress resistanceresistancecould couldalso alsobe be corroboratedcorroborated by by the theincrease increase of of APX APX damage in plants. Plant stress resistance could also be corroborated by the increase of APX activity.activity. GRGR promotedpromoted AsAAsA indirectlyindirectly byby regulatingregulating thethe dynamicdynamic balancebalance ofof Glutathion Glutathion activity. GR promoted AsA indirectly by regulating the dynamic balance of Glutathion (GSH),(GSH), which which was was particularly particularly important important in in the the ROS ROS removal removal process. process. (GSH), which was particularly important in the ROS removal process.
Figure 6. Mechanism diagram of AsA–GSH. FigureFigure 6. 6.MechanismMechanism diagram diagram of ofAs AsA–GSH.A–GSH. AsAs shownshown inin FigureFigure7 A,7A, AsA AsA content content in in the the treatment treatment group group increased increased and and decreased decreased As shown in Figure 7A, AsA content in the treatment group increased and decreased late,late, andand reachedreached thethe peakpeak inin the the T3 T3 experimental experimental groupgroup on on the the 6th 6th day day in in addition. addition. The The late, and reached the peak in the T3 experimental group on the 6th day in addition. The controlcontrol groupgroup occurredoccurred aa continuouscontinuous decline.decline. OhashiOhashi etet al.al. [[32]32] confirmed confirmed thatthat blue blue light light control group occurred a continuous decline. Ohashi et al. [32] confirmed that blue light cancan effectivelyeffectively improveimprove thethe contentcontent ofof AsAAsA inin leafy leafy vegetables. vegetables. CombinedCombined withwith thethe APXAPX can effectively improve the content of AsA in leafy vegetables. Combined with the APX activityactivity andand GRGR activityactivity inin FigureFigure7 B,C,7B,C, APX APX activity activity of of all all samples samples rose rose then then decreased. decreased. activity and GR activity in Figure 7B,C, APX activity of all samples rose then decreased. InIn addition,addition, thethe APXAPX activityactivity ofof thethe treatmenttreatment groupgroup waswas significantlysignificantly higherhigher thanthan thatthat ofof In addition, the APX activity of the treatment group was significantly higher than that of thethe controlcontrol group. group. GRGR activityactivity inin thethe treatmenttreatment group group remained remained at at a a stable stable level, level, while while GR GR the control group. GR activity in the treatment group remained at a stable level, while GR activityactivity decreaseddecreased for the control group. group. Blue Blue LED LED460nm460nm lightlight could could effecti effectivelyvely regulate regulate the activity decreased for the control group. Blue LED460nm light could effectively regulate the theAsAAsA–GSH–GSH cyclecycle and and increase increase AsA AsA content content in in the the end. end. Blue Blue light light was more effectiveeffective inin AsA–GSH cycle and increase AsA content in the end. Blue light was more effective in increasingincreasing AsAAsA contentcontent comparedcompared toto other other light light sources, sources, as as was was demonstrated demonstrated by by Mishra Mishra increasing AsA content compared to other light sources, as was demonstrated by Mishra etet al. al. [ 33[33]].. Rabelo Rabelo et et al. al [.34 [34]] confirmed confirmed that that high high light light intensity intensity could could promote promote the productionthe produc- et al. [33]. Rabelo et al. [34] confirmed that high light intensity could promote the produc- oftion photosynthetic of photosynthetic products products and further and further increase increase the accumulation the accumulation of AsA. of The AsA. activities The activ- of tionAPX of andphotosynthetic GR were positively products correlated and further with increase AsA content the accumulation under different of AsA. environmental The activ- stresses [35]. In conclusion, the freshly cut amaranth irradiated by blue light could increase
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ities of APX and GR were positively correlated with AsA content under different environ- mental stresses [35]. In conclusion, the freshly cut amaranth irradiated by blue light could ASA content,increase enhance ASA content, the enhance activity the activity of GR of and GR and APX, APX, and and further excite excite the AsA– the AsA–GSH cycle, 2 2 especiallyGSH 30 cycle,µmol/(m especially· 30s) μ LEDmol/(m460nm·s) LEDblue460nm blue light light had hadthe the best best effect. effect. Commented [M3]: Please eliminate the shading in Figure 7(A) or privide a new figure without shading.
Figure 7. Effects ofFigure blue 7. LEDEffects460nm of bluelight LED460nm with light differentwith different intensities intensities on on AsA AsA content content (A); APX ( Aactivity); APX (B); activityand GR activity (B); ( andC) GR activity (C) of freshly cut amaranth.of freshly cut amaranth. 3.6. Changes in Antioxidant Enzyme Activity 3.6. ChangesIt in can Antioxidant be seen from Figure Enzyme 8A,B that Activity the antioxidant enzymes of freshly cut amaranth It canirradiated be seen by blue from LED Figure460nm light8A,B ascended that then the descended. antioxidant In the enzymesascending phase, of freshly blue cut amaranth LED460nm light induced an increase in POD and SOD activities. The activities of SOD and irradiatedPOD by in blue the T3 LED treatment460nm grouplight reached ascended the peak on then the 6th descended. day, increased In by the 27.33% ascending and phase, blue 58.49%, respectively, compared with the control group. With the increase of storage time, LED460nm light induced an increase in POD and SOD activities. The activities of SOD the photosynthesis and respiration activities in plants weakened, and the antioxidant en- and PODzymes in the activity T3 treatmentbegan to decline. group On the reached 8th day, the the antioxidant peak on enzymes the 6th of the day, control increased by 27.33% and 58.49%,group respectively, were lower than the compared initial value. with Combined the with control the sensory group. indexes, With the thecontrol increase of storage time, thegroup photosynthesis had lower antioxidant and respiration enzymes than activitiesthe original values in plants on the weakened, 8th day. Figure and 8 the antioxidant revealed that blue LED460nm light can effectively enhance the activity of antioxidant en- enzymeszymes. activity SOD beganmainly disproportionates to decline. On O2- theand H 8th+ to day,form O the2 and antioxidant H2O2. When the enzymes rate of of the control group wereO2- scavenging lower than by SOD the was initial lower than value. the rate Combined of O2- production, with as the dynamic sensory equilib- indexes, the control rium was broken, freshly cut amaranth accelerated its aging process [36]. POD and H2O2 group hadwere lower oxidized antioxidant to produce phenolic enzymes free radicals than to scavenge the original oxygen free values radicals on [37]. the The 8th day. Figure8 revealedaccumulation that blue of LED phenolic460nm freelight radicals can led effectivelyto the increase enhanceof chlorophyll the content activity and the of antioxidant en- degradation of chlorophyll in freshly cut amaranth.− Blue light+ could effectively activate zymes. SOD mainly disproportionates O2 and H to form O2 and H2O2. When the rate − the antioxidant defense system of plants，such as Stevia rebaudiana [38],− lettuce [39], and of O2 scavengingcarnation [40]. The by increase SOD of was antioxidant lower enzyme than activity the rate not only of helped O2 production,to remove ROS, as the dynamic equilibriumbut reduced was broken, the irreversible freshly damage cut caused amaranth by photooxidation accelerated [41]. its aging process [36]. POD and H O were oxidizedMDA content to is produce an important phenolic signal of peroxidation free radicals of plant to scavengesomatic cell membrane oxygen free radicals [37]. 2 2 during storage. In Figure 8C, MDA content was decreasing. On the 6th day, the measure- The accumulationment was: T3 of< T2 phenolic < T1 < CK. The free results radicals were consistent led to thewith increasethe data of POD of chlorophyll and SOD content and the degradationactivities on of the chlorophyll 6th day, indicating in freshly that the cut freshly amaranth. cut amaranth Blue treated light couldwith blue effectively activate the antioxidantLED460nm light defense could effectively system reduce of plants, the degree such of oxidative as Stevia damage, rebaudiana minimize the [dam-38], lettuce [39], and Plants 2021, 10, x FOR PEER REVIEW age of cell membrane and cell structure, and lessen the accumulation of MDA content. Li 10 of 14 carnationet[40 al.]. [42] The confirmed increase that ofChinese antioxidant Kale irradiated enzyme by blue LED activity460nm light not could only effectively helped to remove ROS, but reduceddelay thethe accumulation irreversible of MDA. damage caused by photooxidation [41].
460nm FigureFigure 8. EffectsEffects of blue LEDLED460nm withwith different different intensities on SODSOD activityactivity ((AA);); POD POD activity activity ( B(B);) and; and MDA MDA contents contents (C ()C of) offreshly freshly cut cut amaranth. amaranth.
3.7. SSOMDA Count content and Aerobic is an important Plate Count signal of peroxidation of plant somatic cell membrane duringIn Figure storage. 9A, In the Figure total 8numberC, MDA of content colonies was in the decreasing. sample without On the any 6th pretreatment day, the mea- is 4.2surement (log CFU was:/g) T3at
Figure 9. Changes in aerobic plate count (A) and Pseudomonas spp. count (B). The means with differ- ent lowercase letters (a–d) in figure differ significantly (p < 0.05).
3.8. Changes in Microbial Community RDP classifier and blast analysis were used to classify the OTUs. A total of 43 genera, 32 families, 5 phyla, 6 classes, and 20 orders were identified. Figure 10 showed the relative abundance distribution map of freshly cut amaranth samples under the condition of phyla and genus classification standards. Under the condition of phylum classification standard,
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Figure 8. Effects of blue LED460nm with different intensities on SOD activity (A); POD activity (B); and MDA contents (C) of freshly cut amaranth. Li et al. [42] confirmed that Chinese Kale irradiated by blue LED460nm light could effectively delay the accumulation of MDA. 3.7. SSO Count and Aerobic Plate Count 3.7. SSO Count and Aerobic Plate Count In Figure 9A, the total number of colonies in the sample without any pretreatment is 4.2 (logIn FigureCFU/g)9A, at the the total 0th number day. During of colonies storage, in the the sample total bacterial without anycounts pretreatment of samples is 4.2in- (logcreased CFU/g). On the at the 12 0thth day, day. the During aerobic storage, plate the count total was: bacterial T3 (6.6 counts log CFU of samples/g) < T2 increased. (7.4 log CFUOn the/g) 12th< T1 day, (7.8 thelog aerobicCFU/g) plate < CK count (9.1 log was: CFU T3/ (6.6g). The log CFU/g)antibacterial < T2 (7.4effect log was: CFU/g) T3 >
Figure 9. 9. ChangesChanges in in aerobic aerobic plate plate count count (A) ( Aand) and PseudomonasPseudomonas spp. spp.countcount (B). The (B). means The means with differ- with entdifferent lowercase lowercase letters letters (a–d) (a–d)in figure in figure differ differ significantly significantly (p < 0.05). (p < 0.05).
3.8. Changes in Microbial Community RDP classifier classifier and blast analysis were used to classify the OTUs. A total of 43 genera, 32 families, 5 phyla, 6 classes, and 20 orders were identified. identified. Figure 1010 showed the relative abundance distribution distribution map map of offreshly freshly cut cut amaranth amaranth samples samples under under the condition the condition of phyla of phylaand genus and classification genus classification standards. standards. Under the Under condition the condition of phylum of classification phylum classification standard, standard, the flora of fresh samples mainly include cyanobacteria, Proteobacteria, and so on. Among them, Proteobacteria was the dominant species during the whole storage period, and the relative abundance increased from 0.04% to 77.45%. Under the condition of genus classification standard, the flora of fresh samples mainly includes Pseudomonas, Shewanella, Sphingobacterium, and so on. During storage, the abundance of Pseudomonas increased from 0.04 to 41.83%, and finally to 56.93%, so the dominant spoilage bacteria of freshly cut amaranth was Pseudomonas. The relative abundance of other flora decreased gradually. The main reason was that the growth of dominant bacteria was too fast, which inhibited the growth of other microorganisms. P. fluorescens had been confirmed to be the dominant spoilage bacteria for many vegetables [49]. The physiological properties of P. fluorescens were adapted to the growth environment of vegetables, and its abundance accounted for Plants 2021, 10, x FOR PEER REVIEW 11 of 14
the flora of fresh samples mainly include cyanobacteria, Proteobacteria, and so on. Among them, Proteobacteria was the dominant species during the whole storage period, and the relative abundance increased from 0.04% to 77.45%. Under the condition of genus classi- fication standard, the flora of fresh samples mainly includes Pseudomonas, Shewanella, Sphingobacterium, and so on. During storage, the abundance of Pseudomonas increased from 0.04 to 41.83%, and finally to 56.93%, so the dominant spoilage bacteria of freshly cut amaranth was Pseudomonas. The relative abundance of other flora decreased gradually. Plants 2021, 10, 1614 The main reason was that the growth of dominant bacteria was too fast, which inhibited11 of 14 the growth of other microorganisms. P. fluorescens had been confirmed to be the dominant spoilage bacteria for many vegetables [49]. The physiological properties of P. fluorescens were adapted to the growth environment of vegetables, and its abundance accounted for about 5 50–90%0–90% of spoilage bacteria. In In this this genus, genus, P.P. fluorescens fluorescens is the main bacteria causing soft rot and yellowing of freshly cut amaranth [[50]50]..
Figure 10. Changes in percent of co communitymmunity abundance on phylumphylum (A) and genusgenus (B)) levels.levels.
3.9. Correlation Correlation Analysis As shown in Figure 11,11, the AsA content of freshly cut amaranth during storage was significantlysignificantly and and positively positively correlated correlated with with APX APX activity, activity, SOD SOD activity activity,, and and POD POD activity. activity. In addition, AsA content was positively correlated with physiological indices of soluble 2 2 solids and and chlorophyll chlorophyll content. content. The The results results showed showed that that 30 30μmolµmol/(m/(m ·s) blue·s) blue LED LED460nm460nm light treatmentlight treatment of freshly of freshly cut amaranth cut amaranth AsA AsAcontent content was wasclosely closely correlated correlated with with AsA AsA–GSH–GSH cy- clecycle activity, activity, which which demonstrated demonstrated strong strong oxidative oxidative stress stress properties properties and and enhanced enhanced the thein- creaseincrease of ofAsA AsA content. content. The The physiological physiological indicators indicators were were also alsoelevated elevated in freshly in freshly cut am- cut aranthamaranth under under blue blue LED LED460nm460nm lightlight irradiation irradiation to mainta to maintainin its vital its vital signs. signs. Blue Blue LED LED460nm460nm light couldlight could align alignthe antioxidant the antioxidant capacity capacity of freshly of freshly cut amaranth cut amaranth and and prolong prolong the theshelf shelf life life of freshlyof freshly cut cut amaranth amaranth by byincreasing increasing the the activity activity of ofantioxidant antioxidant enzymes. enzymes.
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FigureFigure 11. Correlation 11. Correlation of A ofsAAsA content content with withAPX APXactivity, activity, GR activity, GR activity, SOD SOD activity, activity, POD POD activity, activity, MDAMDA content, content, soluble soluble solids solids content, content, weight weight loss loss,, and andchlorophll chlorophll content. content.
4. Conclusion4. Conclusionss In conclusion, the shelf life of freshly cut amaranth treated with blue LED light In conclusion, the shelf life of freshly cut amaranth treated with blue LED460nm460nm light irradiationirradiation could could be effectively be effectively prolonged prolonged by 2 by–3 2–3days, days, and and sensory sensory scores scores were were increased increased to satisfy the consumers. Blue LED light improved the content of chlorophyll and to satisfy the consumers. Blue LED460nm light460nm improved the content of chlorophyll and sol- ublesoluble solids, solids,ascorbic ascorbic acid, and acid, antioxidant and antioxidant capacity. capacity. The rise The in rise POD, in POD, SOD, SOD, APX APX,, and andGR GR activitiesactivities advanced advanced the theantioxidant antioxidant capacity capacity of freshly of freshly cut cutamaranth. amaranth. It could It could effectively effectively inhibit colony reproduction and the growth of dominant spoilage bacteria Pseudomonas. inhibit colony reproduction and the growth of dominant spoilage2 bacteria Pseudomonas. The best preservation effect was obtained by 30 µmol/(m ·s) blue LED460nm light on freshly The best preservation effect was obtained by 30 μmol/(m2·s) blue LED460nm light on freshly cut amaranth compared with the other groups. However, the water loss of freshly cut cut amaranth compared with the other groups. However, the water loss of freshly cut amaranth irradiated by blue LED460nm was not good enough. In a follow-up study, red amaranth irradiated by blue LED460nm was not good enough. In a follow-up study, red light, ultraviolet light, or other different light sources could be used on freshly cut amaranth light, ultraviolet light, or other different light sources could be used on freshly cut ama- for further exploration. ranth for further exploration. Author Contributions: Conceptualization, J.X.; writing—original draft preparation, S.J.; writing— Author Contributions: Conceptualization, J.X.; writing—original draft preparation, S.J.; writing— review and editing, J.X. and Z.D.; supervision, J.X.; funding acquisition, J.X. All authors have read review and editing, J.X. and Z.D.; supervision, J.X.; funding acquisition, J.X. All authors have read and agreed to the published version of the manuscript. and agreed to the published version of the manuscript. Funding: This research was funded by Shanghai Green Leafy Vegetables Agriculture Research Funding: This research was funded by Shanghai Green Leafy Vegetables Agriculture Research Sys- System (postharvest preservation group), and Public Service Platform Project of Shanghai Science tem (postharvest preservation group), and Public Service Platform Project of Shanghai Science and and Technology Commission (20DZ2292200). Technology Commission (20DZ2292200). Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: We did not report any new data in this review. Data Availability Statement: We did not report any new data in this review. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Hoang, L.; De Guzman, C.; Cadiz, N.; Tran, D. Physiological and phytochemical responses of red amaranth (Amaranthus tricolor L.)
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