Shedding UV light on peppercorns: impact on microbial load, chemical composition and sensory properties

• Autores: Victor Arcos Limiñana
• Directores de la Tesis: María Soledad Prats Moya (dir. tes.), Salvador Enrique Maestre Pérez (dir. tes.)
• Lectura: En la Universitat d'Alacant / Universidad de Alicante ( España ) en 2025
• Idioma: inglés
• Número de páginas: 208
• Tribunal Calificador de la Tesis: Ana Belén Ropero Lara (presid.), Arantzazu Valdés García (secret.), Łukasz Woźniak (voc.)
• Enlaces
  • Tesis en acceso abierto en: RUA
• Resumen

  • Pepper (Piper nigrum) has historically occupied a central role in global spice trade, valued not only for its distinctive sensory attributes but also for its economic significance within the food industry. In 2023, global production reached approximately 855,000 tonnes, with a commercial value of 2.61 billion USD, underscoring its widespread culinary and industrial relevance. The principal commercial varieties—black, white, green, and pink pepper—are distinguished by their respective processing methods, which significantly alter their chemical composition and organoleptic properties. Black pepper is derived from unripe berries subjected to blanching and sun-drying; white pepper is obtained from fully ripened berries that undergo decortication following water immersion; green pepper is stabilized through rapid drying or brining; and pink pepper, although commonly grouped with the others, originates from a different botanical species (Schinus terebinthifolius). While these technological processes are essential for developing the desired sensory profiles, they simultaneously introduce microbiological risks. Pepper is particularly susceptible to contamination due to factors such as open-air drying, the use of untreated water, manual handling, and inadequate storage conditions. Pathogenic microorganisms including Salmonella spp., Escherichia coli, and Bacillus cereus are frequently detected in pepper samples, making it one of the most recurrent spices in international food safety alerts, particularly within the European Union’s Rapid Alert System for Food and Feed (RASFF). The microbiological safety of pepper is of paramount importance, especially considering its frequent use in raw or minimally processed food products such as spice blends, dressings, cured meats, and ready-to-eat meals. Unlike ingredients that undergo thermal processing prior to consumption, pepper must reach the consumer in a hygienically safe state, as it is often incorporated directly into food without further microbial reduction steps. Foodborne illnesses represent a significant public health concern, particularly for vulnerable populations including children, the elderly, pregnant individuals, and immunocompromised patients. In 2023, the European Union recorded 5,791 outbreaks of foodborne diseases, resulting in 53,285 reported cases, 3,019 hospitalizations, and 75 deaths. Salmonella remains one of the most prevalent pathogens, with its incidence rising to levels comparable to those observed in the early 2000s. Beyond public health, the presence of microbial contaminants in pepper has substantial economic and commercial repercussions. The detection of pathogens in exported batches can lead to border rejections, product recalls, regulatory sanctions, and erosion of consumer and buyer confidence. In the United States alone, the annual economic burden associated with foodborne illnesses is estimated at 17.6 billion USD. To mitigate these risks, the spice industry has traditionally employed thermal disinfection methods, such as steam pasteurization and high-temperature air treatment. Although effective in microbial load reduction, these approaches often compromise the sensory and nutritional quality of the product. Heat treatments can degrade volatile aromatic compounds, alter natural pigments, and diminish antioxidant content, thereby reducing the overall quality of the spice. In response to the limitations of thermal methods, non-thermal technologies have garnered increasing attention. Among these, ultraviolet (UV) radiation has emerged as a promising alternative for microbial decontamination. UV radiation is widely utilized in diverse applications, ranging from wastewater treatment to surface sterilization in clinical environments, owing to its efficacy and non-thermal nature. The principal advantages of UV radiation include preservation of sensory and nutritional properties due to the absence of heat, no introduction of chemical residues, lower energy consumption compared to conventional methods, and elimination of water usage, aligning with sustainability goals. UV treatment has demonstrated efficacy in various food matrices, including milk, fruit juices, plant-based beverages, fresh produce, meat products, and spices. Furthermore, it can be integrated with other technologies—such as cold plasma, infrared radiation, and ozone-modified atmospheres—to enhance microbial inactivation. The antimicrobial mechanism of UV radiation is primarily based on its ability to damage microbial DNA, thereby inhibiting replication and inducing cell death. UV radiation is categorized by wavelength into UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (200–280 nm), with UV-C being the most effective for microbial inactivation. DNA exhibits peak absorbance near 265 nm, making this range particularly potent for disinfection. Microorganisms may activate various DNA repair mechanisms in response to UV-induced stress, including photoreactivation, excision repair, and the production of protective compounds such as carotenoids and melanins. Sensitivity to UV radiation is influenced by multiple factors, including cell wall structure, growth phase, spore formation, and the optical properties of the surrounding matrix. The application of UV radiation to solid foods such as peppercorns presents unique technical challenges. Unlike liquids, which can be agitated to ensure uniform exposure, solid foods have irregular geometries that create shadowed areas, reducing treatment efficacy. Peppercorns, in particular, exhibit quasi-spherical morphology and microscale topography that hinder homogeneous irradiation. To address these limitations, two strategies have been proposed: dual-point irradiation, which requires quartz supports to allow transmission from multiple angles, and controlled movement of the sample, typically achieved through mechanical reactors or fluidized beds. Mechanical reactors are more compatible with existing industrial processing lines, whereas fluidized beds offer improved exposure but entail higher development and maintenance costs. Although pepper is relatively stable under UV exposure, certain compounds—such as fatty acids, tocopherols, and phenolics—may degrade under prolonged or high-intensity treatment. Additionally, volatile compounds may be lost due to mechanical agitation during processing. These changes can adversely affect aroma, flavor, and nutritional value, necessitating careful optimization of treatment conditions to balance microbial efficacy with quality preservation. Historically, low-pressure mercury lamps emitting at 254 nm have been the standard for UV disinfection. While effective, they present several drawbacks, including warm-up time requirements and the use of mercury, which is increasingly regulated due to environmental concerns. In contrast, UV light-emitting diodes (UV-LEDs) offer several advantages: mercury-free operation, longer operational lifespan, higher energy efficiency, rapid attainment of full emission intensity, modulation of wavelength and intensity, and compact design facilitating integration into continuous processing systems. Despite these benefits, the application of UV-LEDs in food disinfection remains underexplored, particularly regarding their impact on chemical composition and sensory attributes. In light of the aforementioned considerations, the primary objective of this doctoral research is to evaluate the disinfection capacity of UV radiation on peppercorns using scalable mechanical reactors, and to assess its impact on nutritional and sensory quality. The specific objectives are as follows: to conduct a critical review of the state of the art regarding UV disinfection in spices; to determine the microbial reduction efficacy of UV radiation across different pepper types, wavelengths, exposure times, and reactor configurations; to analyze the chemical changes induced by UV treatment, including alterations in phenolic content, antioxidant capacity, and volatile profiles; to evaluate the sensory implications of UV-induced chemical changes, focusing on color, aroma, and flavor; to compare the responses of various bacterial species and pepper varieties to UV treatment; and to establish optimal treatment conditions that maximize microbial inactivation while preserving product quality. The experimental phase of the thesis was structured around two distinct UV treatment systems: System A, which utilized a conventional low-pressure mercury lamp emitting at 254 nm, with peppercorns irradiated for 30, 60, and 90 minutes inside a rotating drum reactor across four pepper types (black, white, green, and pink); and System B, which employed UV-LEDs emitting at 280 nm, 300 nm, 365 nm, and a combined 280+300 nm configuration, with treatments conducted for 10, 20, and 30 minutes using a vibrating shaker reactor exclusively on black peppercorns. Both systems aimed to evaluate microbial inactivation efficacy and assess changes in chemical composition, including total phenolic content (TPC), antioxidant capacity, volatile compounds, piperine, organic acids, fatty acids, tocopherols, and color. Additionally, sensory analyses were conducted to determine perceptible changes in odor, flavor, and appearance. The 254 nm mercury lamp treatment demonstrated significant microbial reductions. E. coli was reduced by 2.5 log CFU/g in black and green peppercorns, 2.0 log CFU/g in white peppercorns, and 1.2 log CFU/g in pink peppercorns. Despite shorter exposure times, UV-LED treatments achieved comparable disinfection levels: E. coli reductions reached 1.6 log CFU/g with combined 280+300 nm wavelengths, 1.2–1.3 log CFU/g with individual 280 or 300 nm, and 0.7 log CFU/g with 365 nm. Salmonella enterica exhibited greater resistance, with reductions ranging from 1.15 to 1.35 log CFU/g across all wavelengths, and 1.0 log CFU/g at 365 nm. Bacillus cereus spores were the most resistant, showing 0.9–1.0 log CFU/g reduction at 280 and 280+300 nm, 0.5 log CFU/g at 300 nm, and negligible inactivation at 365 nm. The 280 nm wavelength yielded the highest individual efficacy, while the 280+300 nm combination was most effective against E. coli. A viability study using DyeTox13 Green C-2 Azide and PCR analysis revealed that S. enterica cells, although unculturable post-treatment, retained viability, suggesting that UV radiation may induce a viable but non-culturable (VBNC) state rather than complete inactivation. However, interference from pepper compounds may have affected the accuracy of the viability assay. Regarding chemical and sensory impact under System A, minimal changes were observed in phenolic and antioxidant content, with some increases in TPC, particularly in green pepper. Gallic and quinic acids increased in several pepper types, while caffeic acid decreased. No significant changes were detected in piperine, preserving the pungency of P. nigrum varieties. Slight reductions were observed in fatty acids, with tetracosanoic acid most affected; white pepper showed a 22% decrease in total fatty acids, while pink pepper remained unaffected. Alpha-tocopherol decreased in black and white pepper, and gamma-tocopherol decreased in green pepper. Volatile compounds showed reductions of up to 15% in black and green pepper, up to 60% in white pepper (e.g., pinene, 3-carene), and even greater losses in pink pepper, likely due to exposed seed cores. Color changes were minor in P. nigrum varieties and imperceptible to the human eye, though ΔE values reached 4.4 in pink pepper. Under System B, phenolic and antioxidant content showed no significant changes or slight increases. Organic acids presented more pronounced alterations, with succinic and ferulic acids decreasing by 40% across all samples, and vanillic acid notably reduced by 280 and 300 nm radiation. Changes in volatile compounds were primarily attributed to reactor agitation, with sabinene being the only compound affected solely by UV radiation. Total fatty acids decreased by 20% due to reactor action and by 35% under 280+300 nm treatment, with palmitic and stearic acids being most affected, while 365 nm was least aggressive. Alpha-tocopherol decreased by 32% due to agitation, whereas gamma-tocopherol increased by nearly 50% under 280 nm radiation. Overall, UV-LED treatments induced minor or non-significant compositional changes, comparable to those observed with mercury lamp treatments. Sensory evaluation using triangular tests revealed no statistically significant perceptible changes in color, odor, or flavor across all treated samples. Despite theoretical perceptibility at ΔE > 1, no color differences were detected due to the complex visual texture of peppercorns. For odor, the complexity of volatile profiles likely masked any changes in key aromatic compounds. Flavor assessment was challenging due to pepper’s pungency and the need for a neutral matrix; however, the stability of piperine and minimal changes in organic acids and fatty acids contributed to the preservation of flavor perception. The findings of this research demonstrate that UV radiation—both from conventional mercury lamps and UV-LEDs—is effective in reducing microbial loads in peppercorns without significantly compromising chemical composition or sensory quality. The 280 nm wavelength and its combination with 300 nm proved most effective for microbial inactivation, particularly against E. coli, while maintaining product integrity. The study also highlights the importance of reactor design and sample movement in achieving uniform exposure and minimizing quality degradation. Mechanical reactors offer a scalable and practical solution for industrial implementation. Furthermore, the potential induction of VBNC states in bacteria suggests that conventional culture-based methods may overestimate disinfection efficacy, warranting further investigation using advanced viability assays. This thesis contributes valuable experimental data and methodological insights toward the development of safe, scalable, and quality-preserving UV-based decontamination processes for spices, particularly peppercorns.