Effect Of UV-C Irradiation Research Articles

Air cleaner prototype: Reduction of airborne viruses and effects of UV-C irradiation on virus concentration and RNA copy numbers considering modeled residence times and doses

Air cleaner prototype: Reduction of airborne viruses and effects of UV-C irradiation on virus concentration and RNA copy numbers considering modeled residence times and doses

Effect of UV-C Irradiation on Growth, Photosynthetic Pigments, and Lipid Profile of Chlorella sorokiniana.

Chlorella sorokiniana holds significant industrial relevance owing to its lipid profile. Consequently, the objective of this investigation was to enhance growth, lipid content, and photosynthetic pigment production through the application of UV-C irradiation. The growth parameters of microalgae demonstrated an increase in response to concentration. After 35days of incubation, cells exposed to UV-C for 8min produced the most biomass at 2.2g/l. Additionally, the chlorophyll content demonstrated a comparable pattern, with the highest concentrations of chlorophyll a (4.99mg/l), chlorophyll b (6.22mg/l), and total chlorophyll (11.21mg/l) observed in cells incubated for 35days and exposed to UV-C for 8min. The lipid profile, nevertheless, demonstrated minor fluctuations. Specifically, the relative abundance of frequently occurring lipid compounds was found to be greater in cells treated with UV-C compared to the control group, and the most significant increase was obtained in 15-day culture exposed to UV-C for 8min. However, after 35days of incubation, this abundance decreased in cells exposed to UV-C for more than 4min. Additionally, the observation of specific lipid compounds presented solely in cells obtained from algal cultures treated with UV-C was made. Consequently, drawing from the results obtained in the current investigation, it is possible to deduce that UV-C can be utilised to augment the overall development and yield of significant metabolites in microalgae. Furthermore, these microalgae can be converted into single-cell bioreactors to facilitate the production of lipids utilised in a variety of applications, a process that could be refined to cater to industrial demands.

Effect of UV-C irradiation on foodborne pathogens in prosciutto and changes in physicochemical properties

Effect of UV-C irradiation on foodborne pathogens in prosciutto and changes in physicochemical properties

Effects of UVC irradiation on polystyrene for healthcare packaging: Study by FTIR and Raman spectroscopy with thermoluminescence

The interaction between ultraviolet C radiation (UVC) and polystyrene (PS) materials has been investigated, particularly in post-packaging irradiation processes for healthcare applications. Effective UVC penetration through PS materials, regardless of their thickness (0.16 and 0.40 mm) has been observed. However, the penetration effectiveness could be affected by the thickness of the PS material. Achieving optimal post-packaging UVC treatment requires a thorough evaluation of chemical composition and material thickness, especially in pharmaceutical and medical packaging industries.Preliminary results reveal minimal degradation in UVC-irradiated PS packaging samples, as supported by FTIR and Raman spectroscopy characterization. Minor variations could be attributed to intrinsic PS materials properties and/or their respective background, rather than the influence of UVC radiation. Consequently, PS materials exhibit resilience under the experimental conditions following UVC irradiation treatment. Furthermore, a comprehensive analysis of thermoluminescence (TL) emissions evaluates several commercial dosimeter materials for UVC radiation detection. The TLD-100 and TLD-200 dosimeters show potential as UVC detectors, displaying distinct responses linked to the non-ionizing component of UVC radiation at 310 °C and in the range of 150–250 °C, respectively. However, the TLD-400 and GR-200 dosimeters are not suitable for UVC detection due to their spread TL emissions considering intensity and curve shape.This UVC-TL analysis consistently detects radiation in the proposed commercial dosimeter materials one-hour post-exposure, providing assurance that healthcare materials have been irradiated. Such analysis enhances reliability during extended UVC exposures, offering valuable insights for industries employing UVC-irradiated materials, particularly in healthcare applications.

The Effects of UV-C Irradiation and Low Temperature Treatment on Microbial Growth and Oxidative Damage in Fresh-Cut Bitter Gourd (Momordica charantia L.)

Fresh-cut fruits and vegetables are convenient and retain maximum nutrients. However, even minimal processing accelerates product deterioration and reduces food safety due to microbial infection. In this study, the effects of UV-C irradiation, low temperature treatment, and their combination on the microbial risk of fresh-cut bitter gourd were evaluated. Firstly, next-generation sequencing technology was utilized to identify microorganisms on the surface of fresh-cut bitter gourd after 12 h of exposure to room temperature, and a total of 34 bacterial species were identified. Subsequently, fresh-cut bitter gourd treated with UV-C or/and 4 °C and then kept at room temperature for 6 h was assessed for its viable bacterial count. The results showed that both 0.5 and 1.5 kJ·m−2 UV-C irradiation significantly inhibited microbial growth compared to 4 °C and the no treatment control. Meanwhile, no significant differences were observed between UV-C and the combined treatments. Lower doses of UV-C irradiation reduced hydrogen peroxide and malondialdehyde content, increased the proline level, and improved the activities of antioxidant enzymes such as superoxide dismutase, ascorbate peroxidase, catalase, and critical enzymes involved in the phenylpropanoid pathway, such as phenylalanine ammonia-lyase and polyphenol oxidase. This suggests that UV-C irradiation alone can effectively reduce bacterial contamination in fresh-cut bitter gourd to an acceptable level.

Effect of UV-C Irradiation and High Hydrostatic Pressure on Microbiological, Chemical, Physical and Sensory Properties of Fresh-Cut Potatoes

UV-C irradiation and high hydrostatic pressure (HHP) successfully reduce the number of bacteria and their growth but can also affect phenolic and sugar content, as well as other physicochemical properties. Therefore, in this work, the effect of UV-C irradiation, HHP, and their combination, UV-C/HHP, on total aerobic mesophilic bacteria count (TAMBC), chlorogenic acid and sugar content, and other physicochemical properties of raw FCP were examined. Acrylamide and polycyclic aromatic hydrocarbons (PAH) were also monitored in treated FCP after frying. Vacuum-packed potato slices pretreated with an antibrowning agent were irradiated with UV-C (2.70 kJ m−2), treated with HHP (400 MPa/3 min) and combined UV-C/HHP, and stored for 15 days. The greatest reduction in TAMBC was achieved in the UV-C/HHP-treated samples, followed by the HHP treatment, and they both resulted in the slowest bacterial growth during storage. All treatments decreased the contents of chlorogenic acid, but the greatest reduction was observed in the HHP-treated samples. All treatments increased the content of reducing sugars, and UV-C/HHP did so significantly, which also led to an increase in acrylamide content in the fried FCP. PAH levels were below the established limits. Acceptable sensory attributes of all samples (raw, boiled, and fried) remained relatively stable during storage.

UV-C treatments extend the postharvest quality of ‘0900 Ziraat’ sweet cherries by protecting the physical and biochemical features of the fruits during the storage

Sustainable practices to extend the postharvest quality of horticultural products lead to cost-effective marketing during the prolonged season. Sweet cherries are among the most commonly consumed and traded fruits worldwide although they necessarily deteriorate after harvest. In the present study, the effects of UV-C irradiation in different application durations along with modified atmosphere packaging (MAP) on quality maintenance of the ‘0900 Ziraat’ sweet cherries were investigated for two years (2018 and 2019). The fruits were divided into six application groups; (1) storage in plastic cups control, (2) using MAP (Xtend®) packages, (3) 5 min UV-C exposure, (4) 10 min UV-C, (5) 20 min UV-C, and (6) 30 min UV-C irradiation. Treatment of UV-C was performed at a 15 cm distance using a metal cabinet equipped with 8 UV-C lamps of 15 watts, 230V/50Hz at 254 nm wavelength. The sweet cherries were stored at 1 °C and 90% RH for 35 days. At the end of the storage, the greatest weight loss occurred in the control fruits, while the lowest loss was obtained from the fruits subjected to 10 min of UV-C for both years. Also, 10 min of UV-C provided the highest firmness value for both years. This treatment also has better effects on maintaining several quality features such as pedicel chlorophyll content, titratable acidity in fruit juice, and total phenolic contents compared to the control fruits. General findings indicated that 10 min UV-C irradiation would be a beneficial practice for extending the general quality features of ‘0900 Ziraat’ sweet cherry during cold storage up to 35 d.

Effects of UV-C Irradiation and Vacuum Sealing on the Shelf-Life of Beef, Chicken and Salmon Fillets.

One-third of the world's food supply is lost, with meat being a major contributor to this loss. Globally, around 23% of all meat and 35% of all seafood products are lost or wasted. Meats and seafood products are susceptible to microbial spoilage during processing, storage, and distribution, where microbial contamination causes significant losses throughout the supply chain. This study examined the efficacy of UV-C irradiation and vacuum-sealing in preventing microbiological deterioration in beef, chicken, and salmon fillets. The samples were sterilized using a constant UV-C irradiation dose of 360 J/m2 and stored under a reduced pressure of 40 kPa. A microbiological analysis was conducted daily to examine the microbial contamination, which included counting the colonies of Pseudomonas spp., aerobic bacteria, lactic acid bacteria (LAB), Salmonella, and Escherichia coli, as well as monitoring the increase in pH levels. The results demonstrated a statistically significant difference (p > 0.05) in the aerobic bacteria counts between the storage conditions and storage days in all samples, which is a primary indicator of microbial spoilage. In contrast, the differences varied in the Pseudomonas spp. and LAB counts between the storage conditions and storage days, and there was no significant difference (p < 0.05) in the pH levels between the storage conditions. The results indicate that the combination of UV-C irradiation and vacuum sealing effectively inhibits microbial growth and extends the shelf-life of beef, chicken, and salmon fillets by 66.6%.

The Effect of UV-C Irradiation on the Mechanical and Physiological Properties of Potato Tuber and Different Products

Amongst the surface treatment technologies to emerge in the last few decades, UV-C radiation surface treatment is widely used in food process industries for the purpose of shelf life elongation, bacterial inactivation, and stimulation. However, the short wave application is highly dose-dependent and induces different properties of the product during exposure. Mechanical properties of the agricultural products and their derivatives represent the key indicator of acceptability by the end-user. This paper surveys the recent findings of the influence of UV-C on the stress response and physiological change concerning the mechanical and textural properties of miscellaneous agricultural products with a specific focus on a potato tuber. This paper also reviewed the hormetic effect of UV-C triggered at a different classification of doses studied so far on the amount of phenolic content, antioxidants, and other chemicals responsible for the stimulation process. The combined technologies with UV-C for product quality improvement are also highlighted. The review work draws the current challenges as well as future perspectives. Moreover, a way forward in the key areas of improvement of UV-C treatment technologies is suggested that can induce a favorable stress, enabling the product to achieve self-defense mechanisms against wound, impact, and mechanical damage.

Effect of UV-C Irradiation on the Shelf-Life of Fresh-Cut Potato and Its Sensory Properties after Cooking

SUMMARYResearch backgroundPotato tissue is damaged during fresh-cut production, which makes fresh-cut potato susceptible to the quality loss and microbiological spoilage. At the same time, such products are desirable due to their convenience; however, they are extremely sensitive and have short shelf life. The main challenge of the fresh-cut potato industry is to find possibilities to overcome these drawbacks. UV-C treatment, known for its antibacterial activity, is a promising technique and it shows a potential to improve shelf life of fresh-cut potato products.Experimental approachThe influence of the UV-C treatment on the safety and quality, as well as sensory traits of fresh-cut potato (Solanum tuberosum L. cv. Birgit) during storage was examined. For this purpose, 0-, 3-, 5- and 10-min UV-C irradiation was applied on vacuum-packed potato slices pretreated with sodium ascorbate solution. During 23 days of storage at (6±1) °C, microbiological, physicochemical and sensory properties of raw samples were monitored, along with sensory properties of boiled and fried fresh-cut potatoes.Results and conclusionsThe 5- and 10-min UV-C treatments significantly reduced microbial growth, increased total solids and lightness (L*), and positively affected odour and firmness of raw potatoes. Cooked UV-C-treated samples were described with more pronounced characteristic potato odour and taste. Overall, UV-C-treated fresh-cut potato retained its good quality and sensory traits up to 15 days at (6±1) °C.Novelty and scientific contributionTo the best of our knowledge, this is the first scientific article dealing with the effect of UV-C light on durability (safety, quality and sensory traits) of fresh-cut potato cv. Birgit and its suitability for boiling and frying. In general, UV-C treatment is a known antimicrobial technique, but its application on fresh-cut potato is poorly explored. Results confirmed that vacuum-packed fresh-cut potato treated only with UV-C and sodium ascorbate as anti-browning agent, without the addition of chemical preservatives, had twofold longer shelf-life at (6±1) °C than the fresh-cut potato not treated with UV-C. Fresh-cut potato treated with UV-C retained good overall quality and sensory properties either raw, boiled or fried. Results of this study could also be useful for producers in terms of potential UV-C application as a strategy for prolonging the shelf-life of fresh-cut potato.

Acaricidal efficacy of ultraviolet-C irradiation of Tetranychus urticae adults and eggs using a pulsed krypton fluoride excimer laser

BackgroundPulsed ultraviolet (UV)-C light sources, such as excimer lasers, are used in emerging non-thermal food-decontamination methods and also have high potential for use in a wide range of microbial decontamination applications. The acaricidal effect of an experimental UV-C irradiation device was assessed using female adults and eggs of a model organism, the two-spotted spider mite Tetranychus urticae.MethodsUV-C light was generated by a pulsed krypton fluoride excimer laser operating at 248-nm emission wavelength. The pulse energy and pulse repetition rate were 5 mJ and up to 100 Hz, respectively. The distance from the light source to the target was 150 mm; the target surface area was 2.16 cm2. The exposure time for the mites and fresh eggs varied from 1 to 4 min at 5–300 mW, which corresponded to UV doses of 5–80 kJ/m2. Post-irradiation acaricidal effects (mite mortality) were assessed immediately and also measured at 24 h. The effects of UV-C irradiation on the hatchability of eggs were observed daily for up to 12 days post-irradiation.ResultsThe mortality of mites at 5 and 40 kJ/m2 was 26% and 92%, respectively. Mite mortality reached 98% at 80 kJ/m2. The effect of exposure duration on mortality was minimal. The effect of irradiation on egg hatchability was even more significant than that on adult mite mortality, i.e. about 100% egg mortality at an accumulated dose of as little as 5 kJ/m2 for each exposure time.ConclusionsA high rate of mite mortality and lethal egg damage were observed after less than 1 min of exposure to 5 mJ UV-C pulsed irradiation at 60 Hz. Pending further developments (such as beam steering, beam shaping and miniaturisation) and feasibility studies (such as testing with mites in real-life situations), the reported results and characteristics of the UV-C generator (modulation of energy output and adaptability to varying spot sizes) open up the use of this technology for a vast field of acaricidal applications that require long-range radiation.Graphical

The effect of UV-C irradiation and EDTA on the uptake of Co2+ by antimony oxide in the presence and absence of competing cations Ca2+ and Ni2+

In nuclear power plants and other nuclear facilities the removal of cobalt from radioactive liquid waste is needed to reduce the radioactivity concentration in effluents. In liquid wastes containing strong organic complexing agents such as EDTA cobalt removal can be problematic due to the high stability of the Co-EDTA complex. In this study, the removal of cobalt from NaNO3 solutions using antimony oxide (Sb2O3) synthesized from potassium hexahydroxoantimonate was investigated in the absence and presence of EDTA. The uptake studies on the ion exchange material were conducted both in the dark (absence of UV-light) and under UV-C irradiation. Ca2+ or Ni2+ were included in the experiments as competing cations to test the selectivity of the ion exchanger. Results show that UV-C irradiation noticeably enhances the cobalt sorption efficiency on the antimony oxide. It was shown that nickel decreased the sorption of cobalt to a higher extent than calcium. Finally, the sorption data collected for Co2+ on antimony oxide was modeled using six different isotherm models. The Sips model was found to be the most suitable model to describe the sorption process. The Dubinin-Radushkevich model was further used to calculate the adsorption energy, which was found to be 6.2 kJ mol−1.

Effect of UV-C Irradiation, Storage and Subsequent Cooking on Chemical Constituents of Fresh-Cut Potatoes.

UV-C irradiation successfully reduces the growth of microorganisms, but it can also affect the content of phenolics and sugars of fresh-cut potatoes (FCP). This could consequently alter antioxidant capacity of FCP or its potential for acrylamide formation. Therefore, this paper investigates the influence of UV-C irradiation on the content of phenolics [chlorogenic acid (CA)] and individual sugars during storage of FCP as well as after cooking. Acrylamide was also monitored in FCP after frying. Potato slices pre-treated with sodium ascorbate solution and vacuum-packaged were UV-C irradiated for 0, 3, 5, and 10 min in order to obtain irradiation doses of 0, 1.62, 2.70, and 5.40 kJ m−2, respectively, stored for 23 days (+6 °C), and subsequently boiled and fried. As the applied dose and storage duration increased, the CA content in raw FCP decreased (it retained for 75.53–88.34%), while the content of sugars as well as acrylamide in fried FCP increased. Although the increase was the most noticeable at the applied dose of 2.70 kJ m−2, the acrylamide content was always below proposed limit. Boiling and frying reduced the content of CA and sugars. In spite of certain alterations, applied doses of irradiation can ensure acceptable product in regard to phenolics and sugars, and acrylamide content particularly.

UV-C irradiation delays the physiological changes of bell pepper fruit during storage

Bell pepper is a model for studying the ripening process of non-climacteric fleshy fruit, during which numerous physiological changes occur. UV-C irradiation is an abiotic stress, which has an important influence on plant growth, development and ripening. We explored the influences of UV-C irradiation on physiological changes in bell pepper fruit during postharvest storage by measuring sensory quality, firmness and mass loss, and the underlying changes at the molecular level by combined transcriptome and metabolome analysis. Effects were detected on transcripts for numerous critical enzymes involved in fruit pigment accumulation, fruit texture change, fruit flavor and aroma production. Moreover, transcripts for several key enzymes and transcription factors involved in plant hormone pathways and their corresponding noncoding RNAs were identified and differential expression profiles were also analyzed. Components of a competing endogenous RNAs (ceRNA) regulatory network changed in response to UV-C irradiation and the relationships between them was analyzed. Thirty-one differentially expressed metabolites were identified. Combined transcriptome and metabolome analysis showed that the genes and metabolites that responded to UV-C were mainly involved in flavonoid, phenylalanine, tyrosine, tryptophan and phenylpropanoid biosynthesis. This study provides insights into bell pepper fruit physiology and the effects of UV-C irradiation on postharvest quality and enhances our understanding of the underlying mechanism whereby UV-C delays postharvest physiological changes in bell pepper fruit.

Effect of UV-C Irradiation and Lactic Acid Application on the Inactivation of Listeria monocytogenes and Lactic Acid Bacteria in Vacuum-Packaged Beef.

The objective of this study was to test the effect of the combined application of lactic acid (0–5%) (LA) and UV-C light (0–330 mJ/cm2) to reduce Listeria monocytogenes and lactic acid bacteria (LAB) on beef without major meat color (L *, a *, b *) change and its impact over time. A two-factor central composite design with five central points and response surface methodology (RSM) were used to optimize LA concentration and UV-C dose using 21 meat pieces (10 g) inoculated with L. monocytogenes (LM100A1). The optimal conditions were analyzed over 8 weeks. A quadratic model was obtained that predicted the L. monocytogenes log reduction in vacuum-packed beef treated with LA and UV-C. The maximum log reduction for L. monocytogenes (1.55 ± 0.41 log CFU/g) and LAB (1.55 ± 1.15 log CFU/g) with minimal impact on meat color was achieved with 2.6% LA and 330 mJ/cm2 UV-C. These conditions impaired L. monocytogenes growth and delayed LAB growth by 2 weeks in vacuum-packed meat samples throughout 8 weeks at 4 °C. This strategy might contribute to improving the safety and shelf life of vacuum-packed beef with a low impact on meat color.

Effect of UV-C Irradiation on Quality from Fresh Grapes var. Bordô

Effect of UV-C Irradiation on Quality from Fresh Grapes var. Bordô

Efficacy of Orange Essential Oil and Citral after Exposure to UV-C Irradiation to Inhibit Penicillium digitatum in Navel Oranges

The effect of UV-C irradiation on antifungal properties of orange essential oil (EO) against Penicillium digitatum in inoculated Navel oranges was examined. The UV-C irradiation of orange EO resulted in a 20% loss of the major constituent, limonene, and the generation of three hydroperoxide oxidation products, (2S,4R)-p-mentha-6,8-diene-2-hydroperoxide,(1S,4R)-p-mentha-2,8-diene-1-hydroperoxide, and (1R,4R)-p-mentha-2,8-diene-1-hydroperoxide. The P. digitatum growth in oranges dipped in non-irradiated orange EO at 1000–4000 µL L−1 was not significantly different to control the fruit. Dipping in UV-C treated orange EO inhibited the growth of P. digitatum with 4000 µL L−1 having the greatest effect. No phytotoxic injury to the rind was observed at any concentration. Citral, as a known antifungal chemical, was included for comparison. The non-irradiated citral (1000 µL L−1) was more effective than irradiated orange EO, but elicited rind phytotoxicity. The irradiated citral was less effective in inhibiting P. digitatum growth with the loss of citral, but not hydroperoxide formation. These results suggest UV-C irradiated orange EO as a potential alternative to synthetic fungicides to inhibit P. digitatum decay. The source of orange EO could be waste flavedo generated by the orange juice processing industry.

Effects of UV Radiation on Egg Hatching Population Growth and Reproductive Parameters of Indianmeal Moth Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae)

In this research, three age groups of Plodia interpunctella (Hübner) eggs (1-, 2- and 3-day-old eggs) were exposed to ultraviolet irradiation (UVC) with 254nm wavelength for 0.5, 1, 1.5, 2, 4, 8, 16, 24, 32 and 40 minutes, under controlled conditions. The effect of UVC-irradiation on reproduction and population growth parameters investigated for eggs irradiated for 0.5, 1 and 1.5 minutes. The percentage of egg hatching decreased with increase in UVC exposure time, while for each exposure time, the older eggs were more sensitive than the younger ones. The results indicated that different exposure times of UVC-irradiation could affect the reproduction and population growth parameters. The highest value of net fertility rates was observed in 1, 2 and 3-day-old eggs which were treated with 0.5 min exposure. Both the intrinsic rate of increase (rm) and the net reproductive rate (R0) decreased with increasing exposure time from 0.5 to 1.5 min while the mean generation time (Tc) and doubling time (DT) increased within this irradiation range. The lowest amount of rm was obtained in 1, 2 and 3-day-old eggs which were treated with 1.5 min exposure time. The results showed that UVC-irradiation is an appropriate technique for controlling P. interpunctella.

Effects of UV-C Irradiation and Thermal Processing on the Microbial and Physicochemical Properties of Agave tequilana Weber var. azul Extracts at Various pH Values

The effects of UV-C irradiation (at doses of 8.16, 10.93, 16.17, and 33.29 mJ/cm2) on the physicochemical and microbiological properties of Agave tequilana Weber extracts at various pH values (4.5, 5.5, and 6.5) were evaluated. Thermal treatment (TT) was used as a control (85 °C for 30 s). Both processed (UV-C or TT) and unprocessed (UP) extracts were investigated. The UV-C dose and the pH significantly (p &lt; 0.05) affected the inactivation of total coliforms (TC), total aerobic mesophiles (TAM), and yeasts and molds (YM). UV-C doses of 10.93 mJ/cm2 at pH 4.5 and 33.29 mJ/cm2 at any agave extract pH completely inactivated the native microbial load compared to TT. The total polyphenols (TP), antioxidant activity (AA), and sugar content did not change in the agave extracts at any dose, but the total flavonoid (TF) content decreased at doses &gt; 16.17 mJ/cm2 at the evaluated pH values. Although the color of the agave extracts (L*, a*, and b*) was significantly affected, the total color difference (∆E) did not change after processing compared to the ∆E in the UP extracts. TT further reduced all the physicochemical properties of the agave extracts compared to UV-C processing. The results suggest that UV-C continuous flow technology can be used to stabilize agave extracts at doses of 10.93 mJ/cm2 and pH 4.5, while preserving their functional properties.

Effect of UVC Irradiation on the Oxidation of Histidine in Monoclonal Antibodies

We oxidized histidine residues in monoclonal antibody drugs of immunoglobulin gamma 1 (IgG1) using ultraviolet C irradiation (UVC: 200–280 nm), which is known to be potent for sterilization or disinfection. Among the reaction products, we identified asparagine and aspartic acid by mass spectrometry. In the photo-induced oxidation of histidine in angiotensin II, 18O atoms from H218O in the solvent were incorporated only into aspartic acid but not into asparagine. This suggests that UVC irradiation generates singlet oxygen and induces [2 + 2] cycloaddition to form a dioxetane involving the imidazole Cγ − Cδ2 bond of histidine, followed by ring-opening in the manner of further photo-induced retro [2 + 2] cycloaddition. This yields an equilibrium mixture of two keto-imines, which can be the precursors to aspartic acid and asparagine. The photo-oxidation appears to occur preferentially for histidine residues with lower pKa values in IgG1. We thus conclude that the damage due to UVC photo-oxidation of histidine residues can be avoided in acidic conditions where the imidazole ring is protonated.