Food Microbiology and Safety

Abstract

Strawberry is a healthy fruit with numerous health-benefit compounds. Unfortunately, it is highly perishable and occasionally can be contaminated with foodborne pathogens. The overall goal of this study was to evaluate pulsed light (PL) processing for disinfection of strawberries, extension of shelf life, and preservation of quality attributes and compounds that are beneficial to health. Preliminary screening of PL conditions based on visual appearance of strawberries was conducted, and 3 PL treatments were identified for full evaluation. Salmonella inoculum was artificially deposited onto the skin of strawberries via spot-inoculation or dip-inoculation.

The 3 PL treatments slightly reduced the level of inoculated Salmonella on strawberries, ranging from ~ 0.4 to 0.8 log reduction.

They also slowed down the visible mold development on strawberries by 2 – 4 days compared with the untreated control. Regarding the counts of natural yeasts and molds, the quality attributes (weight loss and firmness), and the health-benefit compounds (total anthocyanin, total phenolics, and total antioxidant activity), The 3 PL treatment showed no significant or negligible difference comparing to the control group.

Overall, the 3 PL treatments demonstrated potential in extending the shelf life of strawberries. Their impact on the quality attributes and health-benefit compounds of strawberries was either no significant or minimal.

Practical Application: Pulsed light processing for strawberry decontamination and shelf life extension was evaluated. Results demonstrated that pulsed light processing could have potential in extending the shelf life of strawberries without significantly affecting the quality attributes and health-benefit compounds of strawberries.

Introduction

The production of strawberries in the U.S. increased from 1901 to 3176 million pounds from 2000 to 2018 (USDA-National Agricultural Statistics Service, 2019).

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However, the strawberry industry faces severe losses in the post-harvest stages, including processing, storage, and transportation (Brecht et al., 2014). The perishable nature of strawberry makes it highly sensitive to handlings and atmospheric changes. Another cause of the losses is fungal decay, and Botrytis cinerea is the major pathogen, which would result in grey mold (Hadenburg, Watada, & Wang, 1990). Strawberries were also identified as the transmission vehicle of several foodborne illness outbreaks (FAO & WHO, 2008). In 2012, a batch of frozen strawberries contaminated with human norovirus caused 10950 illness in 390 facilities in Germany and 38 people were hospitalized (Mäde, Trübner, Neubert, Höhne, & Johne, 2013). In 2011, one people in Oregon died of E. coli O157:H7 infection due to tainted strawberries and 7 other people were hospitalized in this outbreak (Laidler et al., 2013).

Washing with disinfectants such as chlorine is usually adopted in the fresh produce industry to decontaminate and clean fresh produce, but it is highly unfavorable for berries since the presence of moisture would encourage mold growth and decrease their shelf life. Therefore, berries destined for fresh market are usually not washed before sale. To extend the shelf life of berries and improve their food safety, various food processing technologies have been studied. These technologies include heat (Civello, Martínez, Chaves, & Añón, 1997), short-wave UV (UV-C) irradiation (Nigro, Ippolito, Lattanzio, Di Venere, & Salerno, 2000), edible coating (Del-Valle, Hernández-Muñoz, Guarda, & Galotto, 2005), vapor (Hu et al., 2012) and combined treatments (Allende, Marín, Buendía, Tomás-Barberán, & Gil, 2007; Pan, Vicente, Martínez, Chaves, & Civello, 2004). However, these technologies either showed undesirable side effects on sensory properties or had limited effectiveness. Therefore, effective preservation technologies were needed.

Pulsed light (PL) is a nonthermal food processing technology that utilizes short-pulsed and high-energy light of a broad spectrum (200 – 1100 nm) (Cao, Huang, & Chen, 2017; Elmnasser et al., 2007), PL with a dose < 12 J/cm2 has been approved for treatments of food (21 C.F.R. § 179.41). Several studies demonstrated that PL exhibited microbial inactivation effects on different kinds of vegetables and fruits, such as spinach and bell pepper (Gómez-López, Devlieghere, Bonduelle, & Debevere, 2005), raspberries (Bialka & Demirci, 2007) and strawberries (Marquenie, Geeraerd, et al., 2003). For the application on fresh produce, one problem with PL is that it could induce excessive heat on the surface and damage the quality (Y. Huang & Chen, 2014).

Therefore, the PL intensity and exposure time need to be optimized to achieve a reasonable level of pathogen inactivation while preserving food quality. A few studies were conducted to investigate the impact of PL on quality attributes of vegetables and fruits. Agüero, Jagus, Martín-Belloso, & Soliva-Fortuny (2016) showed that a PL treatment of 4 J/cm2 could increase the total polyphenolic content and antioxidant capacity of spinach without affecting its color. F. Charles, Vidal, Olive, Filgueiras, & Sallanon (2013) demonstrated that a PL treatment of 8 J/cm2 maintained firmness, color, carotenoid content, phenol and total ascorbic acid contents of fresh-cut mangoes.

In this study, we aimed to evaluate PL processing for the extension of the shelf-life of strawberries and achieve a certain level of pathogen inactivation. Specifically, the effects of PL treatments on Salmonella inactivation, shelf-life, quality characteristics and health-benefit compounds of strawberries during subsequent storage were determined.

Materials and methods

Strawberries

Strawberries, kindly provided by a producer, were harvested in near full mature conditions and transported overnight from fields to the laboratory (packed with ice packs; overnight shipping). Damaged or rotted strawberries were discarded when they were delivered, and strawberries used for this study were chosen based on the criteria of similar size and color. Since each shipment of strawberries varied in size, different numbers of strawberries were placed in each PET clamshell (Highland packaging solutions, Plant City, FL) to achieve similar weight among different shipments (4, 6, or 8 fruits per clamshell). The packed strawberries were then cooled at 2 – 4 °C for 2 h before PL treatments.

PL treatments

The strawberries were treated by PL with a home-built PL system as described in R. Huang & Chen (2018). During the PL treatments, strawberries were put at the center of a meshed metal shelf. Different intensities of PL were achieved by altering the distance between the strawberry sample and the PL lamp. The method of PL intensity measurement followed the protocol described by R. Huang & Chen (2018) with slight modifications. Before each PL treatment, the energy emitted per pulse was recorded with a 10-s treatment at the center of the shelf, and the total dose of each treatment was calculated based on the energy emitted per pulse and the number of pulses.

A preliminary screening of PL conditions via observation of visual appearance was carried out, and these 3 PL treatments were selected for full evaluation, ~ 3 J/cm2 of PL exposure (0.1 J/cm2 per pulse; ~35 cm from PL lamp; 10 s) on the berries (high intensity and low dose; denoted as H3), ~ 5 J/cm2 of PL exposure (0.1 J/cm2 per pulse; ~35 cm from PL lamp; 16 s) on the berries (high intensity and high dose; denoted as H5), and ~ 3 J/cm2 of PL exposure (0.05 J/cm2 per pulse; ~55 cm from PL lamp; 20 s) on the berries (low intensity and low dose; denoted as L3). One side of the strawberries was first treated by PL for half of the treatment time (for example, 5 s for the H3 treatment).

Then, a 1.5-min ozone removal process as described in (Cao et al., 2017) was used to ensure operator safety before opening the door (an ozone destruction unit attached the PL treatment chamber removed the air from the chamber at a flow rate of 1.13 m3/min and decomposed the ozone through a catalytic process). The berry samples were then flipped over to expose the other side of the berries to PL for the same amount of time. Another 1.5-min ozone removal process was used before taking the strawberries from the chamber. After the PL treatment, strawberries were packed in clamshells (4, 6, or 8 fruits per package depending on the size of strawberries), which were stored in corrugated boxes (12 clamshells in each box). The boxes were stacked together and stored at 2 – 4 °C and 30 – 40% RH for up to 28 days.

Destruction of Salmonella inoculated on strawberries by PL and subsequent storage

The inoculation of Salmonella on strawberries and microbiological analysis were conducted with the method described by Y. Huang & Chen (2015) with modifications. Four nalidixic-acid resistant strains of Salmonella enterica (S. Newport, S. Montevideo, S. St. Paul and S. Stanley) were used and information of the strains was provided in the paper by Y. Huang et al. (2013). To prepare the Salmonella inoculum, each strain was inoculated in tryptic soy broth with 0.6% yeast extract and 50 µg/mL of nalidixic acid and incubated at 35 °C for 24 h. After the incubation, cultures were mixed, centrifuged, and resuspended with 0.1% peptone water to final concentrations of ~ 4 × 109 CFU/mL (spot-inoculation) or ~ 5 × 108 CFU/mL (dip-inoculation). For spot-inoculation, each sample (6 strawberries) was inoculated with 1 mL of inoculum on the skin. For dip-inoculation, 150 strawberries were dipped in 3200 mL of inoculum for 2 min, drained, and kept in clamshells (6 strawberries per clamshell). For both inoculations, each sample was measured to keep a similar weight and placed in a clamshell. The inoculated strawberries were dried in a biosafety hood for 2 h and then stored at 4 °C overnight for subsequent treatments.

PL treatments and subsequent storage were conducted as described in Section 2.2. For each sample, 6 strawberries were separated, weighed, and put in 3 filter bags. Dey-Engley neutralizing broth (D/E broth; four times of the strawberry weight) was added into each bag. The samples were then crushed with a stomacher at 260 rpm for 2 min, diluted in 0.1% peptone water, and plated on tryptic soy agar with 0.6% yeast extract and 50 μg/mL of nalidixic acid. The plates were placed in a 35 °C incubator and grown for 48 hours. Typical Salmonella colonies were counted and converted to log CFU/g of strawberry to calculate log reduction. Media and antibiotics were purchased from Becton, Dickinson and Company and Thermo Fisher Scientific.

Effects of PL and subsequent storage on the shelf life of strawberries

Fungal decay evaluation

For each test, five clamshells of strawberries were treated by PL and stored as described in Section 2.2. Each clamshell was examined visually every two days for signs of fungi. When fungal decay was identified, the clamshell of strawberries was disposed to avoid cross-contamination. The evaluation continued until fungi appeared in all five clamshells of strawberries.

Weight measurement

Right before PL treatments and every two days afterwards, each clamshell of strawberries was weighed and recorded. If fungi were spotted in a clamshell of strawberries, the clamshell was removed. To ensure even air flow in the box, the clamshells were rearranged after each weight recording.

Influence of PL and subsequent storage on natural yeasts and molds, quality characteristics and bioactive compounds of strawberries

Six clamshells of strawberries were treated by PL and stored as described in Section 2.2. On day 0 (right after PL treatments), day 10 and day 20, 2 clamshells of strawberries were taken out for analyses of natural yeasts and molds, quality attributes and health-benefit compounds of strawberries. All analyses followed the methods developed in our group previously with slight modifications. Details of these methods were described in Cao et al. (2017).

To analyze the natural yeasts and molds, two strawberries from the two clamshells, weighed, and mixed with 4 times (v/w) of D/E broth in a filter bag. Samples were then stomached, diluted in 0.1% peptone water, and quantified by spread plating onto potato dextrose agar plates.

To analyze the surface color and firmness, 5 strawberries were used for each sample. First, the color of the 5 strawberries were recorded with a colorimeter at 5 random spots. Then the same strawberries were cut in halves and the firmness of each strawberry half was measured at two spots near the equator with a texture analyzer.

To analyze the pH, total soluble solids, and titratable acidity, 5 strawberries were blended into puree and 45 mL of the puree was centrifuged at 4696 × g for 10 min. The supernatant was collected for analyses of pH (with a pH meter), total soluble solids (with a refractometer), and titratable acidity (with 0.1 M NaOH titration).

To determine the total anthocyanin content, total phenolic content and total antioxidant activity, strawberry puree was prepared as described above and used for metabolites extraction (Cao et al., 2017). The metabolites of puree (2 g) was extracted twice with 10 mL of acidified methanol solution and separated with centrifugations at 4696 × g for 10 min. The supernatant from the centrifugations was then used for the analyses of total anthocyanin content (pH differential method), total phenolic content (Folin-Ciocalteau method), and total antioxidant activity (DPPH method).

Statistics

Independent trials were repeated three times for all the experiments except for the fungal decay evaluation, which had four independent trials. One-way analysis of variance and Tukey’s multiple comparisons were used for identifying significant difference (P < 0.05) among treatments. All analyses were conducted with JMP Pro 13.

Results and Discussion

Fig. 1 shows the survival of Salmonella inoculated on strawberries after PL treatments and during storage. Limited inactivation of Salmonella on strawberries was achieved by the 3 PL treatments, ranging from ~ 0.4 to 0.8 log reduction. For spot inoculation, all PL-treated strawberries had significantly (P < 0.05) lower Salmonella counts than the control on day 0 (right after treatment) and during the 20 days of storage. For dip inoculation, no significant (P > 0.05) difference in Salmonella counts between the control and the 3 PL treatments was observed on day 0 or during the 20 days of storage.

The reason why the inactivation of Salmonella on strawberries was limited (< 1 log) might be due to the low penetration depth of PL and the rough surface of strawberries, which provided shields for Salmonella. Krishnamurthy, Demirci, & Irudayaraj (2004) tested the inactivation of Staphylococcus aureus in suspended cells with PL (5.6 J/cm2 per pulse) and demonstrated that the inactivation efficacy decreased as the sample depth increased. Huang, Ye, Cao, & Chen (2017) showed similar results that PL treatments (5.9 – 22.5 J/cm2 in total fluence) could achieve significantly higher reduction of Salmonella on blueberry surfaces (smooth) than on strawberry surfaces (rough), indicating that the decontamination efficacy of PL would be diminished with rough surfaces.

A previous study also showed that a 1-min water-assisted PL could inactivate a significantly higher amount of Salmonella on blueberries (> 5.6 log) than on raspberries (3.0 log) (Y. Huang, Sido, Huang, & Chen, 2015). In addition, no significant (P > 0.05) difference in inactivation of Salmonella by the PL treatments between the two inoculation methods was observed. A previous study demonstrated that UV treatment of blueberries achieved better inactivation effect of E. coli O157:H7 with spot-inoculation than with dip-inoculation (C. Liu, Huang, & Chen, 2015). However, in this study, the rough surface of strawberries provided shields for both spot- and dip-inoculated Salmonella from PL, leading to similar inactivation effect.

For both spot and dip inoculation, the efficacies of 3 PL treatments for Salmonella inactivation were not significantly (P > 0.05) different on day 0 and during the 20 days of storage. Previous studies showed that the microbial inactivation effect of PL increased with higher dose (Y. Huang & Chen, 2014). A portion of the inoculated Salmonella populations was shielded by the rough surface of strawberry (near achene and stamen) from PL. Only Salmonella cells that were directly exposed to PL could be inactivated and increasing the dose of PL would not be able for PL to reach the shielded ones. Similar results were reported by Huang and Chen (2014) who found that inactivation of E. coli O157:H7 and Salmonella on the calyx of blueberries were not significantly increased when the total dose increased from 28.2 to 56.1 J/cm2. C. Liu et al. (2015) also showed that as the time of UV treatment increased from 5 to 10 min, the reduction of E. coli O157:H7 on the calyx of blueberries did not significantly increase.

During the 20 days of storage, Salmonella on the control and PL-treated strawberries gradually died off for both inoculation methods. However, it died off at a higher rate with the dip inoculation than with the spot inoculation. One possible reason for this phenomenon might be that Salmonella with spot-inoculation was more condensed on strawberries and had higher initial counts comparing to Salmonella with dip-inoculation. The more condensed distribution of Salmonella on strawberries would result in less free water and aid in the survival of Salmonella (Knudsen, Yamamoto, & Harris, 2001).

Flessa, Lusk, & Harris (2005) demonstrated that the decrease of Listeria monocytogenes on strawberries during storage at 24 °C was lower with lower inoculation levels. In that study, after 48 h storage at 24 °C, L. monocytogenes on strawberries decreased by 3.3 and 1.4 log with inoculation levels of 106 and 108 log CFU/sample, respectively. In addition, although Salmonella populations decreased during the 20 days of storage for the control and the 3 PL treatments, it still survived pretty well on dry strawberry surface during the cold storage. A previous study also showed that Salmonella inoculated on strawberry surface did not significantly decrease after forced-air cooling and cold-storage at 4 °C for 7 days (Sreedharan, Tokarskyy, Sargent, & Schneider, 2015). Due to the fragility of strawberries, juice (around pH 3.5) might leak to the surface and created an acidic environment (Sreedharan et al., 2015). However, Salmonella survived well around this pH level (Lanciotti, Sinigaglia, Gardini, Vannini, & Guerzoni, 2001).

Effects of PL and subsequent storage on spoilage of strawberries

The results for the effect of PL and subsequent storage on the development of visible mold on strawberries were shown in Fig. 2. Green and white molds started to appear on day 16. All the control samples showed fungal decay on day 24. Most of the PL-treated samples showed fungi appearance 2 – 4 days later than the control samples. On day 28, all the PL-treated samples showed visible mold. No significant (P > 0.05) difference in delaying fungal development among the 3 PL treatments was observed.

Effects of PL and subsequent storage on weight loss of strawberries

As shown in Fig. 3, no significant difference (P > 0.05) in weight loss among the control and 3 PL treatments was found from day 0 to day 10. For both control and PL-treated samples, > 3% and > 10% of weight losses were found on day 2 and day 8, respectively. Weight loss is a crucial factor for the shelf life of strawberries, which is mainly caused by moisture evaporation from skin and respiration (Cao et al., 2017). Comparing with some previous storage studies of strawberries (Shin, Liu, Nock, Holliday, & Watkins, 2007; Shin, Ryu, Liu, Nock, & Watkins, 2008; Vicente et al., 2002), our study showed higher weight loss, which was probably due to the lower RH used. Lower RH usually results in higher moisture evaporation and leads to higher weight loss. Sample heating caused by PL treatments might lead to higher moisture loss of strawberries, which is undesirable (Y. Huang et al., 2015). With the PL conditions adopted in this study, comparable weight loss of strawberries were observed between PL treated groups and the control group. Similar results were reported by Duarte-Molina, Gómez, Castro, & Alzamora (2016) who showed that PL treatments for up to 20 s did not significantly affect the weight loss of strawberries during storage.

Effects of PL and subsequent storage on natural yeasts and molds of strawberries

The counts of natural yeasts and molds of strawberries during storage were shown in Table 1. The 3 PL treatments did not cause any noticeable reduction of the counts of yeast and mold in strawberries. In addition, PL-treated strawberries did not show significant lower (P > 0.05) counts of yeasts and molds than the control ones. From day 0 to 20 of the storage, the total counts of natural yeasts and molds increased by 0.53 and 0.34 log CFU/g for control and PL-treated (average of three PL treatments) samples, respectively. It should be noted that visible molds developed on day 20 even though the count of yeasts and molds did not show much increase.

Marquenie, Geeraerd, et al. (2003) reported that the PL inactivation of conidia of Botrytis cinerea and Monilia fructigena had a lag phase of ~ 40 s. This result corroborated with our result that no direction inactivation of natural yeasts and molds was found in this study, which was probably due to the low intensity and dose of PL used in our study. In addition, the natural microbiota was very likely to reside in fleshes or under rough surface and the calyx that PL could hardly reach due to its limited penetration ability.

In the fungal development study (Fig. 2), the PL treatments slowed down the visible mold development by 2 – 4 days compared with the control. However, no difference in the counts of natural yeasts and molds was found between the control and PL-treated samples (Table 1). This indicated that other mechanism existed for the delayed spoilage of PL-treated strawberries. One reason might be that the yeasts and molds were not inactivated by PL, but instead injured, which led to a slower growth at low temperature.

Effects of PL and subsequent storage on color of strawberries

The surface colors of strawberries during storage were shown in Table 1. No significant (P > 0.05) difference of L value was found between the control and the 3 PL treatments from day 0 to day 20. During the 20-day storage, all samples showed decreasing trends of L value. The red color of strawberries, an important factor affecting the appearance of strawberries, is reflected by the a value. Strawberries treated by H5 showed slightly higher a value than the control on day 0. However, on day 20, strawberries treated by H5 showed lower a value than the control, which indicated that the H5 PL treatment stimulated the degradation of red pigment. The other 2 PL treatments, H3 and L3, did not significantly (P > 0.05) affect the a value comparing with the control on day 0 and through the 20 days of storage. In general, the a value showed slightly increase on day 10, but a more obvious drop on day 20. The b value, which influences the overall surface color, is not an important evaluation factor for strawberries comparing to L and a values. No significant (P > 0.05) difference between control and PL-treated samples was found on day 0 and overall change of b value, showing a trend similar to the a value.

Previous studies demonstrated that PL or UV-C treatment did not affect the skin color of strawberries (Bialka & Demirci, 2007; Pan et al., 2004). In this study, we found that the L, a and b values of strawberries treated by H5 treatment were in general slightly affected, but the other two lower doses treatments, H3 and L3, did not affect the color values. This demonstrated that higher dose of PL treatment could lead to a greater change of strawberry surface color.

Effects of PL and subsequent storage on firmness of strawberries

Higher firmness would make strawberries more resistant to mechanical injuries and have better storage capacity, which is favored in cultivation (L. Liu et al., 2016). The 3 PL treatments did not significantly (P > 0.05) affect the firmness of strawberries on day 0 and through the 20 days of storage (Table 1). Overall, firmness values of strawberries slightly increased from day 0 to 10 and then slightly decreased on day 20, which were still higher than those on day 0. It should be noted that some of the strawberry samples showed softening during the storage while other showed hardening due to the loss of water. The highly uneven surface of strawberries also introduced lots of variation into the firmness measurement. Luksiene, Buchovec, & Viskelis (2013) showed a similar result that a PL treatment of 3.9 J/cm2 did not affect the strawberry firmness. Other studies also reported that no significant difference of firmness was found between control and PL-treated strawberries (Duarte-Molina et al., 2016; Marquenie, Michiels, Van Impe, Schrevens, & Nicolaï, 2003).

 Effects of PL and subsequent storage on total anthocyanin content, total phenolic content and total antioxidant activity of strawberries

As shown in Table 1, the anthocyanin content of strawberries showed an increasing trend from day 0 to 20. Only on day 10 did H3 treatment show significantly (P < 0.05) lower anthocyanin content than the control group. This might be an indication of PL-induced delayed anthocyanin synthesis. A few UV-C studies also demonstrated that anthocyanin accumulation in strawberries was impacted after UV-C treatments (Erkan, Wang, & Wang, 2008; Pan et al., 2004).

Markaris, Livingston, & Fellers (1957) reported that the red color of strawberry was highly related to the anthocyanins in it. However, we could not identify similar relationships between the anthocyanin content and the a value in this study (Table 1). One probable explanation might be that the total anthocyanin content in this study was reflected merely by pelagonidin-3-glucoside, which could not represent the constitution and transitions of anthocyanins in strawberries. When pelagonidin-3-glucoside transits to (orange brown color) to cyanidin-3-glucoside (red color), the color of strawberries usually became darker (Nunes, Brecht, Morais, & Sargent, 2006).

As shown in Table 1, the total phenolic content reached a peak on day 10 and then reduced on day 20. No significant (P > 0.05) difference in total phenolic content was found among the control and PL-treated samples from day 0 to day 20. During the 20-day storage, the total antioxidant activity showed a slight increasing trend. No significant (P > 0.05) difference in total antioxidant activity was found among the control and PL-treated samples from day 0 to day 20 (Table 1). Luksiene et al. (2013) reported that a high power PL treatment did not impact the total phenolic content of strawberries, nor the total antioxidant activity. However, a study reported that strawberries treated by UV-C (0.043 – 0.43 J/cm2) showed ~ 30% higher total phenolic content and ~10% higher total antioxidant activity than the control group on day 15 of storage (Erkan et al., 2008).

Conclusion

The 3 PL treatments used in this study were selected based on the FDA regulation and a screening study. Overall, the 3 PL treatments demonstrated potential in decontamination and shelf life extension of strawberries with little or no impact on their quality attributes. All 3 PL treatments showed slight Salmonella inactivation effect from day 0 to 20 for both inoculation methods. Visual mold development was delayed by the PL treatments. Weight loss, firmness and color of strawberries were comparable to the control group after three PL treatments. Furthermore, in general the 3 PL treatments did not significantly affect the total anthocyanin content, total phenolic content and total antioxidant activity.

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