**3.2 LEDs induce disease resistance in fresh produces**

Nowadays, LEDs are becoming increasingly popular as a practical tool for protecting fruits from pathogen attacks. The specific wavelengths of light, especially red and *Green Technology for Reducing Postharvest Losses and Improving the Nutritional Quality… DOI: http://dx.doi.org/10.5772/intechopen.109938*

#### **Table 2.**

*Effect of LEDs in the inducing disease resistance in fresh produces.*

blue LEDs, can induce disease resistance in plants against a wide range of microorganisms (**Table 2**). The mechanism of LED inactivation against pathogens is direct damage to DNA and cell membrane caused by reactive oxygen species (ROS) produced by LED light [54]. Blue light enhanced the production of phytoalexin scoparone thus, preventing the postharvest decay caused by *Penicillium digitatum* and *P. italicum* in citrus fruits [55]. Moreover, it was also reported that blue light stimulated the formation of phospholipase A2 and octanal, which resulted in a reduction of *P. digitatum* and *P. italicum* activities [50]. Red light inhibited disease development by increasing the expression of defense-related genes and promoting the production of phytoalexin stilbenenic [56]. LED lights affect the preservation of fresh produce after harvest by a dual effect; 1) the inhibition of microbial growth and 2) stimulation of plant defense responses. Therefore, further investigation is required to truly understand the mechanism, which may potentially lead to the development of LEDs that are effective tools for reducing postharvest disease.

#### **4. Ultrasound**

Treatment of fruit and vegetables with ultrasound is a relatively new method of maintaining fruit and vegetable quality after harvest. Ultrasound is a type of vibrational energy in the frequency above 20 kHz, which is higher than the range of human hearing. Based on the frequencies, ultrasound waves are divided into three categories, including diagnostic ultrasound (1–500 MHz), high-frequency ultrasound (100 kHz–1 MHz), and low-frequency ultrasound (20–100 kHz). The applications of ultrasound are classified into low and high energy depending on the sound power (W), sound intensity (W m−2), or sound energy density (W s m−3). High-energy ultrasound with low frequencies (20–100 kHz) refers to as 'power ultrasound,' which is widely used in the food industry to preserve food quality and provide food safety

[57]. The principle of ultrasound on microbial inactivation is to produce a cavitation phenomenon through the liquid medium. During the sonication process, longitudinal ultrasound waves pass through liquid media and produce alternating regions of compression and rarefaction to promote the cavitation phenomenon. A certain amount of gas can be trapped in cavities and form cavitation bubbles in the regions of different pressure. During rarefaction, the negative pressure increases the bubble volume, while the positive pressure decreases the bubble volume as the sound wave changes to compression. During treatment, very localized high temperatures (5500 K) and pressures (up to 50–100 MPa) are created by shock waves, which also accelerate bubble collapse. When the bubble collapses, the breakdown of water molecules inside the bubble explosion produces free radicals such as hydrogen atoms [H+ ] and hydroxyl radicals [OH<sup>−</sup> ]. Hydrogen peroxide [H2O2] is formed when OH radicals combine with water [H2O], which is a cause of disruption of the cell wall, thus inhibiting the growth of microorganisms [58].

Microbial spoilage is a major cause of postharvest waste and is caused by fungi, bacteria, yeast, and molds. Fresh produces are normally protected from microbial attack by using synthetic chemicals, but the application of green technologies such as ultrasound also reduce postharvest losses. Cao et al. [59] reported that ultrasound treatment at 40 kHz and 250 W for 10 min could be applied to reduce mesophilic aerobic mold and yeast by 1.49 log CFU g−1 and 1.73 log CFU g−1, respectively, and maintain the quality of strawberry at 5°C for 8 days. Birmpa et al. [60] reported that ultrasound treatment at 37 kHz and 30 W for 45 min was effective in reducing the growth of *E. coli*, *Listeria innocua*, *Salmonella enteritidis*, and *Staphylococcus aureus* inoculated on lettuce and strawberries. Similarly, Alexandre et al. [61] showed that the number of *L. innocua* inoculated on red bell pepper was reduced by 1.9 log CFU g−1 after being treated with ultrasound treatment at 35 kHz and 350 W for 2 min. Gani et al. [62] observed that the use of ultrasound treatment of strawberries at 33 kHz and 60 W for 40 min contributed to a reduction in the number of bacterial from 5.91 to 3.91 log CFU g−1 and yeast and mold from 4.80 to 3.58 log CFU g−1. Supapvanich and Kijka [63] demonstrated that the ultrasound treatment at 40 kHz and 150 w for 10 min maintained visual appearance, inhibited decay incidence, and reduced weight loss of 'Kim Ju' guava fruits during storage at 28 ± 1°C for 6 days. Furthermore, Guerrero et al. [64] confirmed that ultrasound treatment damages the cell wall and membrane integrity of the microbial pathogen. Thus, the ultrasound treatment reduces postharvest microbial growth due to cellular disruption by shear force or increasing temperature and pressure during bubble collapse, which creates hydroxyl radicals and leads to severe cell wall damage [65]. The produced highly reactive radicals can destroy the cell wall and membrane of microorganisms [66], which helps in the prevention of postharvest disease in fresh produces.

While ultrasound treatment has been shown to reduce the levels of microbes in fruits and vegetables, its application in combination with other green technology treatments, such as sanitizers, organic acids, essential oils, and other antimicrobials, has been studied to enhance the efficiency of this technique. For example, Chen and Zhu [67] reported that the combination of ultrasound at 40 kHz and 100 W for 10 min with chlorine dioxide at 40 mg L−1 reduced initial microflora and maintained the quality of Japanese plum. Similarly, the population of *Salmonella and E. coli* O157: H7 contaminated on the surface of apples was reduced by 2 and 1.5 log CFU g−1 in combination with ultrasound at 170 kHz and chlorine dioxide at 40 mg L−1 for 10 min [68]. Sagong et al. [69] observed that the effect of 2% lactic acid, 2% citric acid, and 2% malic acid combined with ultrasound treatment at 40 kHz for 5 min was effective

*Green Technology for Reducing Postharvest Losses and Improving the Nutritional Quality… DOI: http://dx.doi.org/10.5772/intechopen.109938*

to reduce *E. coli, S. typhimurium*, and *L. monocytogenes* on organic fresh lettuce. Yang et al. [70] also reported that the combination of ultrasound at 40 kHz for 10 min with salicylic acid mM could reduce blue mold caused by *Penicillium expansum* in peach fruit. Millan-Sango et al. [71] demonstrated that the combined treatment of ultrasound at 26 kHz and 200 W for 5 min with thyme essential oil at 0.018% reduced *Salmonella enterica* in lettuce. These examples demonstrate the effectiveness of ultrasonic combination treatments to reduce food safety and fruit pathogens; however, more work is required to optimize these treatments to kill pathogens and have minimal effects on product quality.

#### **5. Ultraviolet (UV) radiation**

UV radiation is widely used as a microbial sterilizing treatment in a range of diverse applications such as the treatment of wastewater, process water, surface disinfection, and air disinfection in food processing, packing, pharmaceuticals, and hospitals [72, 73]. In the postharvest treatment of fresh produce, UV radiation has been used for more than four decades. The main uses of postharvest UV treatments in postharvest include the following: (1) its use as defect sorting, that is, separating defective fruit (discolored and wounds) from perfect fruit, (2) control of spoilage and pathogenic microorganisms (i.e., *Alternaria* sp., *B. cinerea*, *Colletotrichum* sp., *Lasiodiplodia theobromea, Monilinia* sp., *Fusarium* sp., *Rhizopus* sp., and *Penicillium* spp.), (3) enhancement of bioactive compounds, (4) delayed fruit ripening and senescence, and (5) control of human pathogens (i.e., *E. coli*, *Salmonella*, and *Listeria*) that can contaminate fresh produce [72, 73]. However, this review will focus on the application of UV-C in the control of plant pathogens that cause postharvest diseases and improving the quality of fresh produce. UV radiation is commonly considered non-ionizing radiation where the wavelength of UV radiation is 100–400 nm, and its light region is between X-ray and visible light spectrum. The electromagnetic spectrum of UV is commonly divided into three regions: UV-C is a short wavelength of 100–280 nm, UV-B is a medium wavelength of 280–315 nm, and UV-A is a long wavelength of 315–400 nm [73]. Among these different UV spectra, UV-C shows the highest antimicrobial effect [72]. The most efficient wavelength for damaging genetic materials (DNA) of microbial and all biological tissues and eliciting antimicrobial compounds is 254 nm [72, 74]. Microbial death caused by UV-C is not only by genetic material damage but also by the overproduction of ROS, which can oxidize membrane lipids and inactivate the activity of cellular enzymes. It has been shown that gramnegative bacteria are more sensitive than gram-positive bacteria, followed by yeasts and eukaryotic organisms [73].

#### **5.1 UV-C treatment in the control of postharvest disease**

Postharvest disease is one of the major economic losses of fresh produce where the most postharvest diseases are caused by fungi, yeasts, and bacteria. Synthetic fungicides are currently used to prevent disease in fresh fruit and vegetables, but consumers and regulators are seeking safe and effective alternative treatments to manage postharvest decay. Treatments such as UV-C are ideal alternatives to synthetic chemicals as it is a physical treatment and does not leave chemical residues [72]. The UV-C desired dose required controlling postharvest diseases, and improving produce quality is dependent on a range of factors such as the type of produce, maturity stage,

cultivar, and harvest season, but generally ranges between 0.2 kJ m−2 and 20 kJ m−2 [72, 73].

The antimicrobial effect of UV-C treatment on the control of postharvest diseases is thought to occur through two different mechanisms: (1) a direct germicidal effect on biomolecules of plant pathogens such as nucleic acid, membrane, and proteins [75] and (2) *via* an indirect effect on plant pathogens through the induction of the plant defense mechanisms following UV-C treatment such as plant defense enzymes, plant pathogenesis-related proteins (PRs), and secondary metabolite accumulation (i.e., antimicrobial agents—phenolic compounds and phytoalexins) [73, 76]. The limitation of a direct effect of UV-C treatment against pathogens is the low penetration of this radiation treatment into plant tissue, where only pathogens present on the surface can be killed due to the poor penetration ability of UV-C into the tissue (only 50–300 nm) [72]. Thus, to get complete coverage fresh produce must be rotated during treatment to ensure that all surfaces are exposed to UV-C light.

In general, plant defense responses can be induced by biotic and abiotic elicitors such as pathogens, antagonistic microorganisms, chemical treatments, and physical treatments like UV-C irradiation. Several studies have shown that UV-C treatment can elicit a range of responses such as the accumulation of antimicrobial compounds, the increase in the activities of various enzymes associated with plant defense, and plant pathogenesis-related proteins (PRs) such as phenylalanine ammonia-lyase (PAL), chitinase (CHI), β-1,3 glucanase (GLU), and peroxidase (POD). PAL is the key enzyme that plays a role in the biosynthesis of phenolics and other secondary metabolites phytoalexins (phenol), and even salicylic acid, which is a crucial signal molecule involved in plant protection from pathogen attack [75, 77]. UV-C radiation has been shown to induce plant defense mechanisms in a range of different fresh produce such as strawberry [74], pear [75], mangosteen [76], mango [78], peach [79], tomato [80, 81], and banana [82], thereby extending the shelf life of these fruits.

#### **5.2 UV-C treatment in postharvest nutritional values**

UV-C has been shown to affect a range of postharvest quality attributes including changes in plant pigments, antioxidants, nutrition, firmness, flavor, and aroma of fresh produce. These physiological changes may be desirable or undesirable depending on the type of produce as these changes directly affect eating quality. UV-C treatment has been shown to enhance antioxidant systems for both antioxidant compounds and oxidative enzyme activities in various fresh produce pear [75], mangosteen [76], mangoes [78], broccoli [83, 84], leaf vegetables [85], garlic [86], pepino fruit [87], and tomato [88, 89]. While many studies have shown that UV-C radiation treatment can induce many bioactive compounds, but some studies have shown that UV-C treatment did not enhance polyphenols, β-carotene, ascorbic acid, chlorophyll contents in persimmon and cucumber [90], and total phenolic content, anthocyanin compounds, and antioxidant activity in grape [81].

#### **5.3 Combined UV-C treatment with other postharvest technologies**

UV-C treatment can combine with other postharvest treatments such as chemical, physical, and biological treatments to improve postharvest quality. For example, Sripong et al. [78] demonstrated that UV-C irradiation (6.16 kJ m−2) combined with hot water treatment at 55°C for 5 min significantly reduced the incidence and severity of postharvest anthracnose disease development in mango fruit compared with UV-C

*Green Technology for Reducing Postharvest Losses and Improving the Nutritional Quality… DOI: http://dx.doi.org/10.5772/intechopen.109938*

or hot water treatment alone. The reduction of disease was related to an increase in plant defense-related enzyme activities and their related gene expressions. In addition, the combined treatments could retard fruit ripening by maintaining firmness, slowing color change, retarding the increase of total soluble solids, and the decrease of titratable acidity (TA). This result could be explained that the treatments may slow down the activity of the enzymes associated with plant cell wall degradation and the hydrolysis of starch to sugar. The acidity of fruit has been known that it is used as a respiratory substrate. Thus, low respiration rate is a cause of slowing down the decrease in TA.

#### **5.4 The advantage and disadvantages of UV-C treatment**

As a physical treatment to improve the storage life of fruit produce, UV-C treatment has numerous practical advantages that include the following: (1) fast and simple handling, (2) low cost and investment, (3) low maintenance, (4) ability to operate at low temperatures and not requiring additional water, (5) less space for treatment, (6) it is non-ionizing treatment, (7) it is non-thermal technology which does not induce heat in the tissue of fresh produce, (8) fewer regulatory restrictions as compared to other irradiation methods such as gamma irradiation, and (9) approved by food control agencies [72, 78]. However, the disadvantages of UV-C radiation include the following: (1) low penetration power, and the potential penetration into plant tissue is only 50–300 nm. Thus, only pathogens on the plant surface are inactivated after exposure to this radiation, (2) UV-C rays are dangerous a workplace health and safety (WHS) issue and can damage the eyes and skin [72]. Thus, during UV-C treatment application even on the laboratory scale and a commercial scale, the treatment must be conducted with complete protection to all users, (3) generation of ozone gas during the operation with UV radiation, particularly at a wavelength below 260 nm, will produce ozone gas. Ozone is hazardous to the human health and can cause the air pollution in the working atmosphere, and thus, air ventilation or treatment may be necessary. The U.S. Environmental Protection Agency (EPA) states that the national ambient air quality standard for ozone is 70 ppb for an 8-hour average [82]. However, activated carbon filters are being used for ozone removal in the atmosphere [83, 84]. In summary, UV-C technology is a potential physical treatment that has many benefits for the storage of fresh fruit and vegetables. UV-C treatment has been shown to induce defense systems within the fruit through the signaling molecules and antimicrobial compounds to protect against plant pathogens, improve the nutritional quality of some fresh produce, and delay senescence resulting in extending the shelf life of fresh fruit and vegetables.

#### **6. Plasma technology**

#### **6.1 Types of plasma and plasma generation**

Plasma technology is a novel approach to preserving the quality and inactivating spoilage microorganisms and pathogens of food processes and maintaining the postharvest quality of fresh produce. It is an accepted technology for industry because of its relatively low input cost [85]. Plasma is commonly known as the fourth state of matter, after gases, liquids, and solids. Plasma is quite like a gas than a solid or liquid, but its property differs from solid, liquid, and gas [86]. Two major types

of plasma are differentiated based on thermodynamics: thermal plasma and nonthermal plasma (referred to cold plasma, temperature < 60°C) [86–88]. Thus, cold plasma (CP) is selected to apply to various foods including fruit and vegetables to avoid the loss of nutrition and sensory contributions. CP can be generated by many techniques including corona discharge (CD), dielectric barrier discharge (DBD), gliding arc discharge (GAD), microplasma, radio frequency (RF) discharge, plasma spray, plasma needle, and atmospheric pressure plasma jet [86, 88]. In all plasma techniques, the feed gas is required to energize into plasma [88]. CP is commonly generated by energizing matter with an electric current (energy), and the solid state of matter changes from liquid to gas and finally to a plasma state, respectively. When the initial gas is treated with sufficiently high energy, the structure of molecules and intra-atomics is broken resulting in the formation of free electrons, neutrals, and other reactive ion species. The initial feed-in gas for plasma generation can be either a single gas or a mixture of various gases, i.e., carbon dioxide, oxygen, nitrogen, argon, neon, helium, or even air [86, 87]. In addition, moisture can be mixed with the feed-in gases [88]. The treatment plasma state consists of a mix of excited atoms and molecules (ionization), positive and negative charged particles, UV photons, ozone, ROS (such as superoxide [O2 − ], hydroxyl [OH˙·], hydroperoxyl [HO2]), hydrogen peroxide [H2O2]), and reactive nitrogen species (RNS such as nitric oxide [NO], nitrogen dioxide [NO2], dinitrogen pentoxide [N2O5], nitrate anions [NO3 − ], and nitrite anions [NO2 − ]) [86–88].

Sufficient concentrations of the plasma compositions in the environment (atmosphere or liquid plasma) have been shown to inactivate microbes such as bacteria, fungi, spores, and viruses without hazardous chemical residues [86, 89]. CP can be subdivided into non-thermal atmospheric plasma (NTAP) and plasmaactivated water (PAW). PAW is conducted by applying plasma discharge in water (plasma generated directly in the water) or on the water surface (plasma generated over the water surface) [90, 91]. An increase in many chemically reactive molecules (reactive oxygen and nitrogen species, RONS) created in PAW is correlated with high oxidation-reduction potential (ORP), electrical conductivity (EC), and low acidity pH levels [92] that are considered to have high antimicrobial effects. However, the use of some oxidizing chemical solutions instead of water for plasma generation is called the plasma-activated solution (PAS) and can be used to increase the antimicrobial mechanism of the treatment, as indicated by the increases in ORP and EC values [92]. ORP, EC, and low pH values provide synergistic inactivation effects along with RONS. In addition to anti-microbial effects, CP can significantly slow the loss of fresh produce quality deterioration *via* the slowdown ripening and senescence processes, thereby maintaining the nutrition and organoleptically appearance [91].

#### **6.2 Mode of action of cold plasma**

There are large numbers of studies examining the mechanisms of CP treatment against microbes. It is generally thought that the mode of action of CP involves the interaction of the active chemical species, radicals, and reactive molecules generated by CP with microbial cell membranes and cellular functions through lipid peroxidation, protein denaturation, and genetically material degradation leading to cell damage, pore formation, cell lysis, and cytoplasm shrinkage [85]. However, the long lifespan of RONS such as hydrogen peroxide, nitrate, nitrite, and ozone may

*Green Technology for Reducing Postharvest Losses and Improving the Nutritional Quality… DOI: http://dx.doi.org/10.5772/intechopen.109938*

help maintaining the inactivation effect on microbial for a longer period than shortlifespan species such as hydroxyl radicals, superoxide, singlet oxygen, nitric oxide, peroxynitrite, and peroxynitric [92].

#### **6.3 Cold plasma for food-borne pathogens**

There are many studies on application of cold plasma treatment for inhibiting food-borne pathogens contamination, postharvest disease infection, and maintaining the quality of fruit and vegetables. For example, the inactivation effects of cold plasma on food-borne pathogens have been reported in a large number of produce varieties: *Salmonella typhimurium* in radish sprouts [93], *Salmonella stanley* and *E. coli* O157: H7 in apples [94], *E. coli* O157:H7 *Salmonella* sp.*, L. monocytogenes* in apples, cantaloupe, and lettuce [95], etc.

As a case study, the antimicrobial efficiency of PAS treatment on cell structure of the food-borne pathogen and the contaminated fresh lettuce was investigated and showed that a 3% (v/v) of H2O2 solution treated with DBD plasma and incubated with *Staphylococcus aureus* for 10–30 min resulted in a decrease in the PAS-treated *S. aureus* population, which was correlated with the increase in PAS incubation time. Although the cell wall of gram-positive *S. aureus* is thick, it is composed of a peptidoglycan (PG) layer, which is less sensitive to ROS. It is thought that the high ROS formation by PAS attacks the PG layer, membrane, and DNA leading to cell death. Higher leakage of intracellular components DNA, proteins, and K+ of PAS-treated *S. aureus* were also found in longer incubation time than in short incubation time. *In vitro* tests where soaking lettuce samples with PAS for 10 min have been shown to reduce the total bacteria counts by 1.25 log CFUg−1, and this treatment also retained the nutrient quality of lettuce, as indicated by the stability of green color and vitamin C and chlorophyll contents when compared with the control (deionized water) [96].

#### **6.4 Cold plasma for postharvest quality and disease control**

Postharvest decays caused by microbial contamination and infection are the major factors of postharvest deterioration. The effects of plasma treatments against various postharvest diseases in a range of different fresh produce have been reported. For example, the application of CP treatment on postharvest fungal pathogens and spoilage fungi has been reviewed in *Aspergillus niger* in date palm [97], *P. italicum* in Satsuma mandarins [98]*, P. digitatum* in lemon [99], *Alternaria alternata, A. niger,* and *P. italicum* in wash water from cherries, grapes, and strawberries [100].

PAW treatment for 45 min has been shown to inhibit postharvest decay of kumquat fruit caused by *Penicillium italicum* and maintained the fruit firmness, while color, vitamin C, total flavonoid, and carotenoids were not affected by PAW treatment [101]. The results of decontamination of strawberries inside closed packages with different gas mixtures (O2, CO2, and N2), and then treated with DBD plasma at the ambient temperature had a similar antimicrobial effect to decontaminate total aerobic mesophiles and yeasts and molds by 3.0 log reductions. Strawberry fruit treated with plasma at high oxygen gas mixtures (65% O2 + 16% N2 + 19% CO2) had lower fruit respiration and higher firmness and brightness than plasma treatment at a high nitrogen gas mixture (90% N2 + 10% O2) [102]. The quality of the mushroom (*Agaricus bisporus*) after soaking with PAW for 1, 10, and 15 min showed that PAW treatment reduced bacterial and fungal contamination by 1.5 and 0.5 log and also

delayed the mushroom softening. In addition, mushroom color, pH, and antioxidant properties were not affected [103].

The effects of plasma treatment on either postharvest disease inhibition or quality maintenance in tropical fruit have been reported. For example, the plasma-activated solution (PAS) obtained by treating with a dielectric barrier discharge (DBD) in the phosphate buffer showed a decrease in spore germination and spore viability of *Colletotrichum asianum,* a causal agent of mango anthracnose disease. Indeed, the incidence of *Colletotrichum* in PAS-treated mango fruit was 48% lower than in nontreated fruit. SEM microstructure of the treated *Colletotrichum* spores showed that the subcellular structure was damaged by plasma treatment [104].

Browning of fresh and minimally processed fresh produce is an undesirable characteristic that is involved with two major browning enzymes: polyphenol oxidase (PPO) and peroxidase (POD). In postharvest trials with treated banana fruit, the PPO and POD activities of the atmospheric plasma-treated banana slices were 70% and 100%, respectively, lower than untreated fruit. In addition, the levels of phytonutrient compounds (total phenol and flavonoid), antioxidant activity, and vitamin B6 also increased following treatment. However, the plasma treatment produced porous structures in the fruit leading to a decrease in hardness [105]. A similar observation resulted in other studies with banana slices has shown that CP treatment inactivated the enzymatic browning activities (PPO and POD) resulting in low quinones and less browning. However, plasma treatment also induced ROS levels in cells and morphological surface changes (rougher, fissures, and cracks) with the increase in treatment time [106].

The shelf life of fruit and vegetables is usually limited by plant pathogens and senescence as indicated by their respiration rate. Cold storage and modified atmosphere packaging (MAP) are commercial preservation technologies, which are commercially used to prolong the shelf life of fresh produce [107]. The combined effect of cold storage, MAP, and plasma treatments on the safety and quality of cherry tomatoes has been studied. Cherry tomatoes treated with PAW and packed in the polypropylene (with perforated-oriented polypropylene film to generate the equilibrium MAP condition) and then stored were found to have reduced the microbial load on the surface of the tomato, whereas MAP and low storage temperature could prevent water loss and maintain the total soluble solids content resulting in shelf life extension [108]. However, there is less information about the effect of cold plasma on the activation of the natural defense mechanisms in fresh produce, and more work is required in this area.

#### **6.5 Advantages and disadvantages of plasma technology**

Cold plasma is heat-free technology that is suitable for fresh produce as it prevents postharvest losses by inactivating plant pathogens, reducing microbial contamination, slowing the loss of freshness and firmness, and delaying nutritional losses: vitamins, colors (i.e., chlorophyll, carotenoids, and anthocyanin), various phytochemical compounds, antioxidant properties, without no harmful synthetic chemical residues [86]. However, the efficiency of cold plasma technology for decontamination and quality maintenance is dependent on the variety of gas, gas flow rate, and process time of treatment. Undesirable conditions related to these factors may provide negative effects such as loss of color, nutrition, and bioactive compounds. For example, the decline in brightness (*L*\* value) of strawberry fruit treated with atmospheric cold plasma under a high nitrogen environment is probably *Green Technology for Reducing Postharvest Losses and Improving the Nutritional Quality… DOI: http://dx.doi.org/10.5772/intechopen.109938*

due to superficial bleaching [102]. The total color difference (Δ*E*<sup>∗</sup> ) value of the carrot slices treated with atmospheric pressure cold plasma treatment is unacceptable and may be caused by high oxidation of the surface carotene and the loss of moisture on the surface. A major disadvantage of the use of CP is the high investment in a stateof-the-art cold plasma system [86].


**4. Conclusions**

The present work presents a comparative study of coating end-mills used to machine Inconel 718 alloy, known for its low machinability and excellent mechanical properties. These cause elevated levels of tool wear, even after only a few meters of cutting.

From the conducted and presented study, the following conclusions can be drawn:

• The obtained results, in terms of machined surface roughness, can be considered quite satisfactory, even when tool wear is quite high. However, the tools' coatings suffered a considerable amount of wear only after a few meters of cutting, not being recommended for longer operations that require many meters of cutting.

These results highlight the advantages of using TiAlN coatings to obtain a satisfactory surface roughness quality for finishing operations applied to Inconel 718. Furthermore, the inclusion of coating sublayers can bring many advantages regarding the wear behavior of the cutting tools, especially when machining a material such as Inconel 718, known for its low machinability and capability of inducing high levels of wear on cutting tools. These results can be used as a basis for future work, especially when aiming to optimize/improve the outcomes of finishing operations applied to Inconel 718, particularly for milling applications.

**Author Contributions:** Investigation, V.F.C.S. and F.J.G.S.; Conceptualization, F.J.G.S. and F.F.; Supervision, F.J.G.S., F.F. and R.C.M.S.-C.; Data collection, V.F.C.S., R.C.M.S.-C., F.F. and N.S.; Formal analysis, F.J.G.S., F.F., R.C.M.S.-C., N.S. and R.D.F.S.C.; Methodology, V.F.C.S., R.C.M.S.-C. and N.S.; Visualization, R.C.M.S.-C., R.D.F.S.C., F.F. and N.S.; Data curation: F.J.G.S., N.S. and R.D.F.S.C.; Writing—draft, V.F.C.S. and F.J.G.S.; Writing—reviewing, R.C.M.S.-C., R.D.F.S.C., F.F. and N.S.; Resources, V.F.C.S., F.J.G.S. and R.C.M.S.-C.; Main research, V.F.C.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Rui Rocha from CEMUP for his crucial collaboration in taking SEM pictures and commenting on some phenomena. The authors would also like to thank CETRIB/INEGI/LAETA, namely Jorge Seabra and Carlos Fernandes, for their continuing support in experimental activities and work dissemination. Moreover, Luís Durão and Victor Moreira from Mechanical Technology Lab of ISEP are acknowledged for their commitment and support in all machining experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.


**2. The role of OTN switching in transport networks**

Technological advancements have seen different node architectures being proposed, each having their pros and cons. Transponders and muxponders still provide the simplest approach to getting traffic on and off a 100G transmission interface, by multiplexing one or more client interface to a single high-speed line interface. On one hand, this approach incentivizes simplicity and relatively low CAPEX, while on the other hand, penalizes the operator because of its limited ability to groom traffic, inability to perform remote configuration, and being inefficient to combine add/drop traffic with pass-through traffic from other line interfaces.

#### **2.1. First-generation OTN switch**

The present day traffic represents a blend of packet traffic and extensively installed legacy TDM traffic. To address this varied traffic pattern and mix, the modern day network architectures requires an OTN switch to cater service-agnostic switching where the multiple client service types can be mapped into ODU frames and the same can be switched at the ODU level. This will not only allow subwavelength services to be aggregated at their source and destination nodes but also allow them to be groomed at intermediated nodes and thus finally contribute to a reduction in the number of expensive WDM line interfaces in use. **Figure 1** depicts the transponder/muxponder and OTN switch architectures. The later architecture introduces a digital OTN switch, which is separated from the WDM box using short-reach optics or integrated with the WDM box to reduce footprint and power consumption [7]. According to Infonetics, the majority of service providers (86%) are choosing OTN switching as the technology best suited to fill 100G optical channels, because it enables efficient aggregation of diverse services and protocols over a single optical link. Noteworthy, it is being embraced and deployed by network providers throughout Asia, Europe, and North America [8].

Another key feature enabled by OTN switching is fast shared protection and restoration schemes with fine granularity, which cannot be achieved with a transponder or muxponder solution. Other than contributing to a reduction of CAPEX, OTN switching does include several other benefits. Among them, high scalability, fast end-to-end service provisioning, multiple traffic type support, subwavelength switching, router port offloading, and client service level mapping are a few to name [7]. But then, certain specific traffic patterns and network topology can result in the switchless architecture to have a CAPEX advantage over

**Figure 1.** Transponder/muxponder-based approach vs. OTN-based approach.

the switched architecture. The same is witnessed when client services data rates match with the WDM channel data rates or even when the traffic type is not a mix and only comprises of packet data, and in these cases, the overall node architecture can be simplified to a switchless one with simplified equipment spanning L3-L0. Another perspective also challenges network operators while including OTN switch and that is contributed by the additional power and space consumption increasing the OPEX.

The traditional or first-generation OTN switch focuses only on switching ODUs and involves dedicated cards for circuit-based switching and dedicated cards for packet-switching, as illustrated in **Figure 2**. Consequently, switching is restricted within the same switch-type domain (packet/circuit). Furthermore, within the same switching domain, interworking between different technologies, such as the OTN and the synchronous digital hierarchy (SDH)/synchronous optical network (SONET), may be also blocked. Essentially, traditional OTN switches fail to deliver "universal client port" functionalities, which would allow to support any mix of client interfaces in a single switching domain, further improving the ability to efficiently utilize the transport network resources.

#### **2.2. The universal OTN switch**

**2.1. First-generation OTN switch**

214 Optical Fiber and Wireless Communications

America [8].

The present day traffic represents a blend of packet traffic and extensively installed legacy TDM traffic. To address this varied traffic pattern and mix, the modern day network architectures requires an OTN switch to cater service-agnostic switching where the multiple client service types can be mapped into ODU frames and the same can be switched at the ODU level. This will not only allow subwavelength services to be aggregated at their source and destination nodes but also allow them to be groomed at intermediated nodes and thus finally contribute to a reduction in the number of expensive WDM line interfaces in use. **Figure 1** depicts the transponder/muxponder and OTN switch architectures. The later architecture introduces a digital OTN switch, which is separated from the WDM box using short-reach optics or integrated with the WDM box to reduce footprint and power consumption [7]. According to Infonetics, the majority of service providers (86%) are choosing OTN switching as the technology best suited to fill 100G optical channels, because it enables efficient aggregation of diverse services and protocols over a single optical link. Noteworthy, it is being embraced and deployed by network providers throughout Asia, Europe, and North

Another key feature enabled by OTN switching is fast shared protection and restoration schemes with fine granularity, which cannot be achieved with a transponder or muxponder solution. Other than contributing to a reduction of CAPEX, OTN switching does include several other benefits. Among them, high scalability, fast end-to-end service provisioning, multiple traffic type support, subwavelength switching, router port offloading, and client service level mapping are a few to name [7]. But then, certain specific traffic patterns and network topology can result in the switchless architecture to have a CAPEX advantage over

**Figure 1.** Transponder/muxponder-based approach vs. OTN-based approach.

Conversely to the traditional or first-generation OTN switch, next-generation universal OTN switch as shown in **Figure 3** can aggregate different protocols on the client side and enable transparent multiplexing of packet and TDM traffic, allowing a single device to be used in multiple applications efficiently.

The universal OTN switch is backed by a universal transport platform which is capable of switching traffic flows based on any L1-L2.5 protocol and on every port, including multiple protocols on the same port simultaneously. Hence, it can offer network operators the best of both worlds by dynamically controlling every flow on every port as a circuit or packet and providing the most efficient, future-proof solution for virtually all applications [9].

These next-generation OTN switches employ universal cards to handle TDM, OTN, carrier Ethernet (CE), and multiprotocol label switching-transport protocol (MPLS-TP) traffic and

**Figure 2.** Traditional vs. universal OTN switch.

**Figure 3.** Universal OTN switch.

provide grooming efficiency, granularity, and service classification of a packet switch along with scalability, operational efficiency, and performance of an OTN switch, as illustrated in **Figure 3**. Furthermore, the universal OTN switch can also assist in reducing router ports by distributing services from aggregated router hand-offs with virtual local area network (VLAN) to ODU mapping, as shown in **Figure 4**.

Naturally, several other benefits, common to first-generation OTN switches, are also present and include fast end-to-end service provisioning, rapid restoration, high scalability, subwavelength level switching, and easy support of new/multiple traffic types. Likewise, if the service data rates are the same as the data rates of the WDM wavelength channels or when only packet traffic is present, then the universal switch might not have a CAPEX advantage over traditional OTN switch or even the conventional transponder/muxponder with OTN encapsulation.

Depending on the location in the network and the traffic matrix, switching at the packet or STS-1/VC-4 level can have efficiency benefits over switching OTN at the ODU0 (1.25G) level or above, as long as one of two conditions are met. Either there must be a significant number of client interfaces below 1G or there must be the potential for large statistical gains from

**Figure 4.** Universal switching fabric [1].

multiplexing uncorrelated bursty traffic flows. These conditions are more likely to occur in a metro network versus the long haul network where statistical gains have often already been achieved at the IP/MPLS layer and client interface speeds are likely to be above 1G.

Importantly, universal switching platforms enable OTN, SONET/SDH, and packet switched traffic to share the same high-speed interface, with SONET/SDH mapped to ODU2, packet traffic mapped to ODU flex, and the remaining capacity available for OTN switched traffic, thus making the most efficient use of each high-speed interface and the optical spectrum it consumes. In addition, universal switching provides investment protection against changes in traffic patterns and client types. With universal fabrics, and the ability to define in software interfaces and virtual interfaces for OTN, CE (Bridging, VLAN cross-connect), or MPLS-TP/ virtual private LAN service (VPLS), and without impacting the capacity of the switch, operators can easily evolve from first-generation to universal OTN switching.