The appeal of the approach lies in its elegant simplicity. Polyphenol oxidase, the copper-containing enzyme responsible for the browning that occurs when an apple is sliced, catalyzes the two-electron oxidation of o-diphenols such as chlorogenic acid into their corresponding o-quinones. These quinones are highly electrophilic and readily react with nucleophilic amino acid residues, including lysine, cysteine, and tryptophan, on proteins and peptides, forming covalent adducts. In other words, the very reaction that food processors spend fortunes trying to prevent becomes, under controlled conditions, a precise and environmentally friendly tool for building novel peptide-phenolic conjugates.

Coffee itself provides an exceptionally rich reservoir of phenolic substrates. Chlorogenic acids, esters of caffeic and quinic acids, can account for up to twelve percent of the dry weight of a coffee bean and are the principal contributors to the beverage’s antioxidant capacity. Using HPLC coupled with UV/Vis detection and mass spectrometry, the team profiled the phenolic composition of green Uganda coffee beans and identified eight major chlorogenic acid derivatives. Three caffeoylquinic acid isomers, 3-CQA, 4-CQA, and 5-CQA, were detected with a common molecular ion at m/z 353, while two feruloylquinic acid isomers appeared at m/z 367 and three dicaffeoylquinic acids at m/z 515. Among these, 5-caffeoylquinic acid emerged as the dominant phenolic constituent, confirming its central role in coffee chemistry and its suitability as the primary substrate for conjugation reactions.

The choice of enzyme source proved equally consequential. Five fresh apple varieties purchased from a German market, Jonagold, Boskoop, Golden Delicious, Cox Orange, and Braeburn, were screened for both polyphenol oxidase activity and chlorogenic acid content. The results revealed striking cultivar-dependent differences. Braeburn juice exhibited by far the highest enzyme activity, measured at 6262.4 units per liter using a 4-methylcatechol-proline adduct assay based on the Lambert-Beer law, with excellent linearity over the reaction period. Jonagold and Golden Delicious showed intermediate activities of roughly 3600 and 3400 units per liter, while Boskoop and Cox Orange lagged dramatically behind, with reductions of approximately 94 and 91 percent relative to Braeburn. Chlorogenic acid content also varied widely, with Boskoop containing the most at 325.3 micrograms per milliliter of juice and Cox Orange the least at 63.6 micrograms per milliliter.

Because the primary goal was to identify an efficient natural source of the enzyme, Braeburn was selected as the biocatalyst for the modification experiments, a choice the authors justified by noting that its chlorogenic acid content remained sufficiently high to support enzymatic oxidation while its catalytic potential far exceeded that of the other cultivars. The researchers also explored how storage conditions affect enzyme performance, and the findings carry practical significance for any future food application. Apples frozen at minus 20 degrees Celsius before juicing yielded juices with remarkably high polyphenol oxidase activity, reaching 15208.6 units per liter, compared with only 1780.7 units per liter for apples held at room temperature. The authors attribute this effect to ice crystal formation during freezing, which disrupts cellular membranes and releases membrane-bound enzyme into the juice matrix. Subsequent storage of the juices at minus 20 degrees Celsius for up to two weeks produced only minor losses in activity, indicating that frozen apple juice could serve as a stable, practical reservoir of the biocatalyst.

With the phenolic donor and the enzymatic catalyst characterized, the team turned to the conjugation reactions themselves. Ground green coffee was infused in boiling water to prepare an extract, which was then combined with the milk peptides BioZate 1 and glycomacropeptide under two different conditions: an alkaline pathway at pH 9, a common reference method for driving phenolic-protein conjugation, and an enzymatic pathway in which freshly pressed Braeburn apple juice supplied approximately 18.8 units of polyphenol oxidase per reaction at a natural pH of 4.7. Both reactions ran for 24 hours at room temperature with continuous stirring and exposure to air, after which the products were dialyzed, lyophilized, and stored for analysis.

Quantification of free chlorogenic acid derivatives by HPLC revealed substantial depletion of the phenolic pool over the reaction period. At the start, 5-CQA predominated at roughly 47 to 52 milligrams per gram of protein across all treatments, with the other isomers present at considerably lower levels. After 24 hours, total CQA content fell significantly in every sample. The alkaline-modified BioZate 1 conjugate showed the greatest decline, dropping from 67.8 to 26.1 milligrams per gram of protein, while the corresponding glycomacropeptide sample fell from 69.0 to 40.8. Dicaffeoylquinic acids were nearly eliminated in the alkaline BioZate 1 sample. The authors caution, however, that this reduction reflects the overall conversion of phenolic substrates, encompassing oxidation, isomerization, self-polymerization, and condensation, rather than a direct measure of covalent binding efficiency alone. Notably, alkaline conditions promoted isomerization of 5-CQA into its 3- and 4-CQA forms, a well-documented side reaction that the enzymatic route largely avoids.

Structural evidence came from MALDI-TOF mass spectrometry. The unmodified BioZate 1 peptide displayed characteristic peaks at m/z 7089.2, 7581.3, 12463.5, and 14173.7, while both modified forms exhibited new signals at approximately m/z 9300 and 18600, consistent with the incorporation of coffee-derived phenolics and, in the alkaline case, possible cross-linking or partially degraded species. Glycomacropeptide, which shows a sharp native peak at m/z 6802.6, gained new peaks at roughly m/z 9300 and 9500 after treatment under either condition, indicating the formation of higher-molecular-mass peptide-polyphenol complexes. The authors are careful to note that mass spectrometry alone cannot distinguish true covalent attachment from cross-linking or aggregation, and that definitive identification of binding sites will require high-resolution LC-MS/MS and peptide mapping in follow-up studies.

To estimate the extent of stable phenolic association, the researchers employed reverse-phase HPLC with 8 molar urea, which disrupts non-covalent interactions such as hydrogen bonding and hydrophobic association, leaving behind a fraction considered consistent with covalent modification. The alkaline-treated BioZate 1 conjugate retained the highest bound CQA level at 14.95 micrograms per milligram of protein, significantly exceeding its enzymatic counterpart, and the same pattern held for glycomacropeptide, where the alkaline sample reached 10.14 micrograms per milligram. The authors attribute the superior alkaline performance to the greater nucleophilicity of lysine and cysteine residues at high pH, which facilitates stronger interactions with oxidized phenolic intermediates.

Yet the enzymatic route retains compelling advantages despite its somewhat lower conjugation yield. Alkaline treatment promotes non-specific oxidation, chlorogenic acid isomerization, and potential damage to susceptible amino acid residues, whereas polyphenol oxidase-mediated oxidation proceeds under mild, food-relevant conditions and generates reactive quinones through a naturally occurring enzymatic mechanism. The use of apple-derived enzyme also aligns with clean-label processing strategies that consumers increasingly demand. The authors acknowledge a limitation: because a heat-inactivated apple juice control was not included, the specific contribution of the enzyme cannot be fully separated from other heat-sensitive components of crude juice, and future experiments with purified or inactivated enzyme preparations will be needed. Even so, the study establishes a credible foundation for a new class of peptide-phenolic conjugates built from coffee and milk, two of the world’s most familiar food matrices. Before these hybrid molecules reach functional food formulations, their antioxidant activity, digestibility, bioaccessibility, stability, and safety must be rigorously evaluated, work the researchers indicate is already underway in dedicated follow-up investigations.

Subject of Research: Enzymatic conjugation of coffee chlorogenic acids with milk bioactive peptides using apple-derived polyphenol oxidase

Article Title: Conjugation of Coffee Phenolics With Milk Bioactive Peptides: Characterization of Coffee and Apple Phenolics and Evaluation of Polyphenol Oxidase as a Natural Biocatalyst

Article References: Hamadneh, B. N., Mustafa, E. D., Alkafafy, M. E., Khojah, E., Samra, B. N., Wapet, D. E. M., Mahmoud, M. M., Badawy, W. Z., Khalil, M., Salama, M. A., Abdin, M., & Ali, M. (2026). Conjugation of Coffee Phenolics With Milk Bioactive Peptides: Characterization of Coffee and Apple Phenolics and Evaluation of Polyphenol Oxidase as a Natural Biocatalyst. Food Science & Nutrition, 14(10), Article e72435. https://doi.org/10.1002/fsn3.72435

Image Credits: AI Generated

DOI: 10.1002/fsn3.72435

Keywords: chlorogenic acid, polyphenol oxidase, milk peptides, coffee phenolics, BioZate 1, glycomacropeptide, apple juice, biocatalyst, MALDI-TOF mass spectrometry, HPLC-MS, functional foods, protein-phenolic conjugation

The review, conducted according to PRISMA guidelines and published in September 2026, searched ScienceDirect, Scopus, and Web of Science for studies published between January 2015 and June 2025. From 327 initial records, the researchers screened 218 full-text articles and ultimately distilled 52 studies that directly addressed UV-C treatment of fresh apples, minimally processed fresh-cut apples, and apple juice. The bibliometric analysis reveals a field that has grown steadily and shifted geographically. Spain leads the world with 11 publications, followed by the United States with 9 and China with 7, and the journal Food Control stands out as the dominant venue, contributing nine articles over the decade. Publication activity peaked in 2015, 2020, and 2021, a rhythm the authors link to real-world shocks: a deadly listeriosis outbreak tied to caramel apples in the United States in 2014 and 2015, and heightened food safety anxieties during the COVID-19 pandemic.

The science behind the technology is deceptively simple. Ultraviolet light spans wavelengths from 100 to 400 nanometers, and the UV-C band, from 100 to 280 nanometers, is the most germicidal because microbial DNA absorbs photons most efficiently in this range, peaking near 260 nanometers. Conventional low-pressure mercury lamps emit roughly 85 percent of their energy at 253.7 nanometers, almost perfectly matching that absorption maximum. When photons strike microbial DNA, they forge pyrimidine dimers, kinks in the genetic strand that block replication and render pathogens harmless. The US Food and Drug Administration has approved UV-C at 254 nanometers as an alternative to thermal pasteurization for juices, provided processors achieve a 5-log reduction, meaning a 99.999 percent kill of target pathogens.

But microbes fight back. Many organisms repair UV damage through photoreactivation, which requires light between 300 and 500 nanometers, or through light-independent nucleotide excision repair. This is where combining UV-C with other treatments, so-called hurdle technology, becomes powerful. Mild heat between 50 and 60 degrees Celsius fluidizes bacterial membranes and disables DNA repair machinery, producing synergistic lethality. In one study, adding the antimicrobial dimethyl dicarbonate before combined UV-C and heat treatment at 55 degrees achieved a 5-log reduction of E. coli in apple juice in just 1.8 minutes, cutting processing time and dose requirements by 44 percent.

On fresh apples themselves, the results are striking. Because apple skins are relatively smooth and less hydrophobic than those of berries or melons, radiation reaches microorganisms more directly. Researchers recorded reductions of 2.9 log units for E. coli O157:H7 and 1.6 log units for Listeria monocytogenes on apple surfaces after applying just 0.92 kilojoules per square meter. Low doses also trigger hormesis, a beneficial stress response in the fruit itself: irradiated apples showed increased phenolic compounds and antioxidant activity while maintaining firmness, soluble solids, and acidity through weeks of storage. Even more remarkably, UV-C fluences of 8.8 and 35.1 kilojoules per square meter degraded patulin, the dangerous mycotoxin produced by P. expansum, eliminating 99.9 percent of the contaminant at the higher dose.

Apple juice presents a tougher optical problem. The liquid is extraordinarily opaque to UV-C, with transmission below 0.01 percent, and 90 percent of radiation is absorbed within the first 0.67 millimeters. Suspended solids, fibers, pigments, and polyphenols scatter and soak up photons, shielding microbes in the liquid’s depths. Industry solutions rely on thin-film reactors that pass juice through gaps of just 2 to 3 millimeters, or on Dean vortex technology, which uses helical tube geometry to create secondary flows that continuously cycle particles toward the lamp. With these engineering fixes, studies report up to 5-log reductions of heat-resistant Alicyclobacillus acidoterrestris spores and significant kills of spoilage yeasts, alongside shelf-life extensions of at least three weeks.

A technological revolution is now underway in the light sources themselves. Ultraviolet light-emitting diodes, or UV-LEDs, are displacing mercury lamps with compelling advantages: they switch on instantly rather than requiring up to 30 minutes to stabilize, last more than 100,000 hours compared with 4,000 to 10,000 for lamps, resist breakage, and contain no mercury, a toxic metal whose disposal is increasingly restricted under the international Minamata Convention. Intriguingly, LEDs at wavelengths near 277 nanometers have inactivated P. expansum spores more effectively than traditional 253.7-nanometer lamps, and 222-nanometer far-UV sources outperformed 254 nanometers against E. coli O157:H7 in juice, possibly because their spectra overlap better with microbial action spectra or because they damage viral proteins faster.

The economics are equally persuasive. Recent analyses suggest UV-C consumes only 6 to 8 percent of the energy of conventional thermal, ohmic, microwave, or high-pressure treatments, and continuous UV-C systems use roughly 39 percent of the energy of thermal pasteurization. Because it is a dry disinfection method, it eliminates the vast wastewater streams generated by chlorine washing, and its low capital cost makes it accessible to small-scale juice producers who could never afford high-pressure processing equipment. Treated juices also retain more flavor, color, and vitamin C than heat-pasteurized counterparts, adding commercial value. Combined approaches amplify the benefits: UV-C with ultrasound achieved a 5-log kill of E. coli O157:H7 in juice in 51.5 minutes versus 77.5 minutes for ultrasound alone, while UV-C paired with peracetic acid completely inactivated E. coli O157:H7 and Listeria on apples in just 5 minutes without harming color or firmness.

Regulation, however, lags behind the science. The United States explicitly permits UV-C at 254 nanometers under 21 CFR Part 179.39, but the European Union has no framework for non-ionizing UV decontamination of food; EU Regulation 2017/2470 restricts UV-C approval to vitamin D enrichment of mushrooms, bread, yeast, and milk. This regulatory asymmetry, the review’s authors argue, hampers trade and slows adoption in a major apple-producing region. They call for internationally harmonized dose standards, validated protocols, and industrial-scale trials to close persistent gaps in the literature, where exposure conditions vary wildly between studies and real processing environments remain underexplored.

The verdict after ten years of research is that UV-C radiation stands as one of the most promising sustainable tools in the apple supply chain, capable of replacing or complementing chlorine while boosting, rather than stripping, the fruit’s natural defenses. Excessive doses do carry risks, from cell membrane rupture and browning to accelerated oxidation, so precision matters. But with LEDs making the technology cheaper, safer, and more flexible than ever, and with consumer demand for minimally processed, additive-free food only growing, the invisible light that unzips microbial DNA may soon be as standard in apple processing as refrigeration itself.

Subject of Research: Use of ultraviolet C radiation for sanitizing fresh apples, minimally processed apples, and apple juice

Article Title: Global Overview of the Use and Effectiveness of Ultraviolet C Technology in Fresh, Minimally Processed Apples and Apple Juice Over the Last 10 Years

Article References: Silva, M. D. P. E., de Azevedo, C. R., Coelho, C. C. D. S., Belas, J. P. R., Soares, A. G., Rosenthal, A., & Freitas‐Silva, O. (2026). Global Overview of the Use and Effectiveness of Ultraviolet C Technology in Fresh, Minimally Processed Apples and Apple Juice Over the Last 10 Years. Food Science & Nutrition, 14(9), Article e72316. https://doi.org/10.1002/fsn3.72316

Image Credits: AI Generated

DOI: 10.1002/fsn3.72316

Keywords: UV-C radiation, apples, apple juice, food safety, Listeria monocytogenes, Penicillium expansum, patulin, UV-LEDs, hurdle technology, non-thermal processing, bibliometric review, food decontamination