UV Plus Heat Treatment Unable to Eliminate Food Safety Risk of Clostridium in Honey
A new study published in the Journal of Food Safety has found that ultraviolet (UV) radiation, applied alone or in combination with mild heat treatment, significantly reduced vegetative cells of Clostridium sporogenes (used as a surrogate for C. botulinum) in honey, but was ineffective at inactivating bacterial endospores. The findings highlighted the need for additional processing strategies to control resistant Clostridium spores while preserving honey quality.
Unlike vegetative bacterial cells, which are metabolically active and generally more susceptible to antimicrobial treatments, endospores are dormant structures that can survive adverse conditions and germinate into vegetative cells when conditions become favorable. This is particularly concerning for C. botulinum, as its endospores can survive in honey and, if ingested by susceptible infants, germinate and colonize the intestines, potentially causing botulism in vulnerable populations, especially infants.
Conducted by researchers from the Federal University of Lavras and the Federal University of Ouro Preto in Brazil, the study investigated the effectiveness of ultraviolet C (UV-C) radiation, alone and combined with ultraviolet B (UV-B) radiation or heat treatment, against C. sporogenes endospores in two types of honey. Because of the pathogenicity of C. botulinum, the researchers used C. sporogenes as a surrogate.
UV Radiation Reduced Vegetative Cells but Not Endospores
The researchers evaluated two types of Brazilian honey: orange blossom honey and wildflower honey. Samples were inoculated with standardized C. sporogenes endospore suspensions at a concentration of 10⁵ colony-forming units per milliliter (CFU/mL).
Three treatments were tested:
- UV-C radiation for 30 minutes
- Combined UV-C and UV-B radiation for 30 minutes
- Heat treatment at 60 °C for 15 minutes, followed by UV-C radiation for 30 minutes.
In orange blossom honey, all three treatments significantly reduced C. sporogenes vegetative cell counts compared to untreated controls, with no significant differences among treatments.
In wildflower honey, only UV-C radiation alone and combined UV-C and UV-B radiation significantly reduced vegetative cell counts. Adding heat treatment did not produce a statistically significant reduction compared to the control.
Importantly, none of the treatments significantly reduced C. sporogenes endospore counts in either honey type.
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The researchers explained that endospore resistance may be attributed to protective proteins that shield spore DNA from UV-induced damage. Additionally, honey's high viscosity, sugar concentration, and optical density can limit UV penetration, potentially shielding microorganisms from sufficient radiation exposure.
Effects on Honey Quality and Processing Recommendations
Physicochemical analyses showed that the treatments generally decreased water activity and moisture content while increasing soluble solids and viscosity, although the researchers found no changes in the samples' fundamental flow behavior. Additionally, all treated honey samples maintained hydroxymethylfurfural (HMF) concentrations below the Codex Alimentarius limit of 40 milligrams per kilogram, indicating that the treatments did not cause unacceptable increases in this indicator of heat-related honey quality deterioration.
The researchers suggested that optimizing UV-C processing through thin-film systems, continuous-flow reactors, improved mixing, or repeated exposure cycles could help overcome limitations associated with radiation penetration.
Overall, the findings demonstrated the potential of UV-C radiation for reducing vegetative microbial contamination without compromising essential honey quality parameters. However, the researchers emphasized that additional or more intensive combined preservation strategies would be necessary to achieve effective Clostridium endospore inactivation.
