**4. Discussion**

The objective of this study was to determine the effect of ECPTM on fruit quality attributes, surface microbial load, and postharvest diseases on two southern highbush cultivars. ECPTM treatment resulted in a cultivar-specific response on fruit quality. In 'Rebel', ECPTM had no effect on visual appearance, fruit firmness, and skin toughness. In 'Farthing', however, ECPTM at 1.0 kGy, resulted in a reduction in fruit firmness and skin toughness but did not affect the visual appearance of the fruit, which was assessed based on the presence of bruises and defects such as leakiness or dents. The differential cultivar response to irradiation could be due to inherent differences in fruit firmness between the two cultivars. 'Rebel' was softer and had lower firmness and skin puncture force than 'Farthing'. Thus, irradiation may not have decreased firmness further in 'Rebel'. Similar results with differences in responses of blueberry cultivars varying in fruit texture have been observed using previous irradiation studies with various radiation sources [21,27,38]. Cultivars with firmer texture were softened after irradiation, whereas the effect of irradiation on two softer-textured cultivars varied; irradiation further softened fruit of one of the cultivars but had no effect on the other [38]. These data indicate that fruit having inherently firmer texture may be softened by irradiation, whereas the texture of fruit with lower fruit firmness may not be affected.

In this study, fruit softening and a decrease in skin toughness in 'Farthing' occurred only at the highest irradiation dose of 1.0 kGy. These results are consistent with other studies that report a dose-dependent response to irradiation with higher doses resulting in a decrease in firmness in blueberry fruit regardless of the method of irradiation. When conventional electron beam irradiation was used to treat blueberries, doses of 1.1 kGy and higher affected fruit texture resulted in softening [28]. Other studies using gamma irradiation around 0.75 kGy and higher reported increased softening in blueberries [21,27,39]. The effect of higher doses of irradiation on fruit softening has also been observed with other fruits such as raspberries [40], peaches [23,41,42], apricots [23], and grapes [43].

In spite of changes in fruit firmness, irradiation did not change other fruit quality attributes such as total soluble solids content, titratable acidity, and weight. Apart from a few minor differences, our results are consistent with other studies that indicate no effect of irradiation on fruit quality characteristics related to flavor [21,27,28,38]. The overall effect of irradiation on fruit firmness and

quality in terms of consumer acceptability is an important consideration. In this study we did not perform sensory evaluations; only few other studies have conducted post-irradiation sensory analyses, and have shown mixed results related to irradiation induced softening and consumer acceptability [21,28,42] in peaches and blueberries.

In addition to fruit quality attributes, it is important to understand the effect of irradiation on the presence of fruit surface organisms that may cause foodborne illness. Blueberries are produced in open fields and can harbor various human pathogens by route of animal waste, irrigation water, and handling by farm workers. After harvest, blueberries for the fresh market are not washed nor treated for surface pathogens [20,44]. Therefore, it would be an added benefit if irradiation could reduce or eliminate such surface organisms. ECPTM treatment was effective in reducing surface microbial load in both 'Rebel' and 'Farthing'. In 'Rebel' irradiation at smaller doses was more effective in reducing surface pathogen load than in 'Farthing'. This was likely because 'Rebel' harbored a higher load of microbes on the fruit surface than 'Farthing'. In 'Rebel', aerobic bacteria and yeasts were reduced by 0.6–0.7 log units and coliforms by 2 log units at 1.0 kGy irradiation. In 'Farthing', similar reductions were observed for aerobic bacteria and yeasts, but not for coliforms. These results are partially consistent with previous studies suggesting irradiation doses between 0.2–0.8 kGy are sufficient to cause a 1-log reduction in surface bacterial pathogens such as *E. coli* 0157:H7, *Salmonella*, and *Listeria* [32,33]. In another study with blueberries, 0.4-kGy irradiation resulted in a 1-log reduction in *Salmonella* and *Listeria* [34], but those specific taxa were not investigated in the present study. The authors concluded, and we concur, that this level of reduction may reduce risk but not guarantee safety.

Blueberries are affected by various postharvest diseases caused mainly by plant-pathogenic fungi [21,45,46]. In this study, some of the common postharvest pathogens *B. cinerea*, *Alternaria* spp., *Colletotrichum* spp., as well as *Aurebasidium*, *Phomopsis*, and *Cladosporium* were identified after postharvest storage. However, in our study ECPTM treatment did not affect the incidence of symptoms and signs associated with postharvest pathogens. Compared with microbes located on the fruit surface, a much higher dose of irradiation, typically at 1–3 kGy, is necessary to eliminate plant-pathogenic fungi [32]. Further, sensitivity of irradiation also can differ among various plant pathogens. Using an in vitro assay, inactivation of *B. cinerea*, *Penicillium expansum*, and *Rhizopus stolonifer* was observed at irradiation doses of 3–4 kGy and 1–2 kGy, respectively [47]. The maximum dose of irradiation of 1.0 kGy in our study may not have been sufficient to decrease postharvest decay pathogens. In addition, 'Farthing' had an inherently low prevalence of postharvest pathogens; hence, irradiation did not further reduce postharvest disease incidence.

Data from this study with the new ECPTM approach is in agreemen<sup>t</sup> with previous research which recommends a dose between 0.5 and 1.0 kGy for blueberry fruit to avoid undesirable effects on fruit quality [21,28]. While irradiation at this dose may provide protection from insect pests (not tested in this study) and some reduction in surface microbial load, more research is needed on its potential to reduce postharvest rots. In apples, mangoes, peaches, and carrots, irradiation combined with other postharvest treatments, such as cold, heat, fungicides, CaCl2 treatment, or modified atmosphere offered greater benefits in controlling postharvest diseases and maintaining higher fruit quality [48–52]. Importantly, the above studies demonstrate that lower doses of irradiation are more effective when used in combination with other treatments than using irradiation alone. Blueberries are generally not treated after harvest, therefore future studies should focus on preharvest applications such as fungicides or calcium treatments in combination with irradiation and storage with modified atmosphere.

ECPTM is attractive because the method's high dose rates allow the desired irradiation dose to be obtained in a considerably shorter period of time, reducing treatment bottlenecks during operation and potentially improving produce quality through shorter treatment times outside of the cold-chain. However, direct side-by-side comparisons of ECPTM with gamma rays or X-rays at identical irradiation doses (but varying dose rates as dictated by the method) have not been conducted previously, pointing to an important research need. Future research also should address one of the

limitations of our study, the need to ship the fruit to and from the treatment facility after harvest and before postharvest storage, which could have impacted treatment efficacy.

**Author Contributions:** Conceptualization, S.U.N., J.W.D., H.D.C., and C.S.; Formal analysis, S.U.N., J.W.D., H.D.C., and H.S.; Funding acquisition, S.U.N., C.S., and H.S.; Investigation, S.U.N., J.W.D., H.D.C., C.S., and H.S.; Methodology, S.U.N., J.W.D., H.D.C., C.S., and H.S.; Project administration, S.U.N., C.S., and H.S.; Resources, S.U.N., C.S. and H.S.; Supervision, S.U.N., C.S., and H.S.

**Funding:** This project was partially funded by the Georgia Agricultural Commodity Commission for Blueberries (BB1605).

**Acknowledgments:** We thank Renee H. Allen, and the commercial blueberry packing facility for providing fruit required for this study and helping with the shipment of fruit, and Yi-Wen Wang and Laura J. Kraft for assisting with measurement of fruit quality attributes.

**Conflicts of Interest:** The authors declare no conflicts of interest. The Georgia Agricultural Commodity Commission played no role in the design of the study, data collection and analysis, in preparing the manuscript and the decision to publish the results.
