Ultraviolet (UV) radiation has been used in the food industry during processing, and with increased demands for safer and higher quality foods, UV-A and UV-B are being explored as antimicrobial treatments. This project consisted of three studies: the first study investigated the production of reactive oxidative species (ROS) by the action of UV radiation on fructose. The second study focused on evaluating the impact of UV-A irradiated chitosan-gallic acid (CH-GA) antimicrobial film on the quality of strawberries. The third study evaluated the effects of using UV-C individually or in combination with UV-A and UV-B to improve the fruit color and safety, respectively, of Honeycrisp apples. It is known that fructose can generate ROS under thermal treatments and UV-C (254 nm) exposure. However, it is unknown whether UV-A or UV-B exposure can generate similar effects. For the first study, fluorescein, a fluorescent dye, was used as an indicator due to its known loss of fluorescence when exposed to ROS. Varying concentrations of fructose solutions combined with fluorescein were exposed to up to 1 J/cm2 of UV-A or UV-B radiation. Ascorbic acid (AA), a known ROS scavenger, was added to the fructose-fluorescein solutions prior to UV exposure to verify ROS generation. The fluorescence was measured at 485 nm (excitation) and 510 nm (emission), respectively. A storage study was done to determine whether ROS continued to generate following UV exposure. Fructose-fluorescein solutions were exposed to 0.1 J/cm2 of UV-B radiation and stored at 4°C or 37°C. The UV-B exposure of fructose-fluorescein showed a dose-dependent fluorescence decay, whereas UV-A did not elicit this response. Fluorescein degradation followed first-order kinetics, as indicated by the rate constants. The rate constants in the presence of 10-, 50-, and 100- mM fructose were 0.7±0.01 J/cm2, 4.3±0.6 J/cm2, and 0.3±0.03 J/cm2, respectively. However, in the presence of AA, fluorescein degradation deviated from first-order kinetics. The storage study indicated no significant difference between the UV-B exposed and control solutions, indicating ROS generation ceased after UV-B exposure. The results of the studies using control solutions were extrapolated to coconut water, a commonly consumed beverage. UV-B exposure did have a degradation effect on AA, but the ROS generated did not affect the AA. The ROS was produced only when fructose was exposed to UV-B. ROS can have adverse effects on the organoleptic properties of foods containing fructose, and the addition of AA can help quench ROS in a concentration-dependent manner. The second study evaluated quality parameters such as color, texture, pH, total soluble solids, and titratable acidity of strawberries coated with an edible chitosan-gallic acid (CH-GA) coating. The strawberries were dipped in the CH-GA solution and allowed to dry. The coated strawberries were exposed to UV-A with appropriate, unexposed controls also being used for comparison. Previous studies have indicated that the coating can exhibit moderate antimicrobial activity when irradiated with UV-A at 360nm. A 180-minute exposure reduced Escherichia coli (E. coli) O157:H7 on CH-GA coated strawberries by ~2-3-log CFU/mL. However, when the quality parameters were evaluated, it was found that the UV irradiated strawberries may have been initially affected with respect to color and texture, but the loss in quality slowed down over a 14-day refrigerated storage period. It was also seen that no significant differences were observed in color and firmness between the control and experimental groups on day 14. The third study (appendix 1) aims to evaluate UV-C radiation's efficacy on the inactivation of Listeria monocytogenes (L. monocytogenes) on apple surfaces. This study was performed within the broader aim of evaluating the effects of UV-A, UV-B, and UV-C and their combinations on the quality and safety of Honeycrisp apples. UV-C radiation can serve as an antimicrobial agent, while UV-A and UV-B radiations can affect the quality parameters such as color through the hormetic effect. Therefore, our goal was to identify optimum UV-A, UV-B, and UV-C radiation doses that can be applied to Honeycrisp apples to improve their coloration and microbial safety as the marketability of apples often depends on the redness of the fruit. The UV-C dose of 7.5 kJ/m2 resulted in a 1.2±0.06 log CFU/sample inactivation of L. monocytogenes on the apple surface. Interestingly, the additional UV-C dose exposure did not result in additional inactivation. This observed lack of dose-dependence could be the result of a) UV-C penetration interference from previously inactivated microbial cells resulting in a shadowing effect, b) the formation of a biofilm during ambient air drying and 4°C incubation that provided some protection during treatment, or c) higher resistance of L. monocytogenes sub-population against UV-C inactivation. This data will allow for future exploration of a synergistic treatment that can improve the color and appearance of Honeycrisp apples and improve their safety at the same time.
UV radiation has shown promising antimicrobial activity and, through the studies carried out in this project, demonstrated potential beneficial or deleterious effects on food quality. The results from the first study showed the significance of understanding the interaction of food ingredients with UV radiation. The strawberry and apple studies show that UV radiation, when used at the correct dosage, can increase, or maintain the visual appearance of the fruits, making them more marketable. When used at the correct wavelength and for the appropriate duration, UV radiation can mitigate the prevalence of foodborne pathogens and contribute to food products' quality and shelf life.