Inactivation by osmotic dehydration and air drying of Salmonella, Shiga toxin-producing Escherichia coli, Listeria monocytogenes, hepatitis A virus and selected surrogates on blueberries

https://doi.org/10.1016/j.ijfoodmicro.2020.108522Get rights and content

Highlights

  • No detectable survivors after osmotic dehydration of blueberries except for Enterococcus faecium.

  • Both osmotic dehydration and air-drying were needed to achieve an > 6 log reduction of bacterial strains and MS2.

  • The combined treatment achieved reductions of 2.6 log10 TCID50/g for HAV .

Abstract

Osmotically dehydrated and air dried berry fruits are used as ingredients for the production of yoghurts, chocolates, cereal bars and mixes, ice creams and cakes and these fruits are often subjected to mild thermal treatments only, posing questions around their microbiological safety. As osmotic dehydration methods and parameters vary considerably within the industry and minimally processed high quality fruits are increasingly sought, the scope of this study was to determine which temperatures are required for the inactivation of relevant bacteria and viruses during osmotic dehydration of berries, using blueberries as a model berry in a thawed state to mimic common industrial practices. Additionally, we studied the inactivation of osmotic dehydration at 23 °C, sometimes referred to “cold infusion” followed by air drying at 100 °C to determine the microbiological safety achieved by this combined treatment. Four pathogens (Salmonella enterica, Escherichia coli O157:H7, Listeria monocytogenes and hepatitis A virus (HAV)) and five surrogates (Enterococcus faecium, Escherichia coli P1, Listeria innocua, murine Norovirus (MNV) and bacteriophage MS2) were inoculated on blueberries and reductions were measured after different treatment combinations. After osmotic dehydration of bacterial strains at 40 °C no survivors were detected on blueberries, with the exception of E. faecium. Inactivation of the viruses at 45 °C showed no survivors for MS2 and mean reductions of 1.5 and 3.4 log10 median tissue culture infectious dose (TCID50)/g for HAV and MNV, respectively. Similarly, in the sugar solution at 40 °C, no survivors were observed, with the exception of E. faecium and the three viruses. The combined process (osmotic dehydration at 23 °C followed by air-drying at 100 °C) achieved an >6 log reduction of all tested bacterial strains and MS2. For HAV and MNV, 2.6 and >3.4 log10 TCID50/g were measured. In summary, the present study shows that osmotic dehydration appears an efficient control measure for the control of L. monocytogenes, S. enterica and E. coli O157:H7 if carried out at 40 °C or at 23 °C and followed by air-drying at 100 °C. Based on the results generated with MNV, the combined treatment is also expected to reduce human Norovirus (NoV) but does not appear to be sufficient to fully control HAV. The results contribute to a better management of the microbial safety of osmotically dehydrated and dried berries and especially the results generated for the viruses emphasize that within a robust food safety management system, safety must be assured through the entire food supply chain and therefore must start at primary production with the implementation of Good Agricultural Practices (GAP).

Introduction

Blueberries are a rich source of anthocyanins which have strong antioxidant effects, making blueberries a very popular health food for their good taste and their protective attributes against various diseases (Mallik and Hamilton, 2017; Prior et al., 1998; Shi et al., 2009). As a result, the per capita consumption of blueberries has increased in many countries, and the world blueberry production grew at an annual rate of 4.95%, from 230,769 t in 2002 to 356,533 t in 2011 with the top five blueberry producers being the United States, Canada, Poland, Mexico and Germany (Evans and Ballen, 2014).

However, fresh and frozen berry fruits have recently been associated with several outbreaks of foodborne infections linked to viruses and bacteria (Chatziprodromidou et al., 2018; Palumbo et al., 2013). From 2013 to 2014, 1444 cases of hepatitis A infections were reported by 12 EU/European Economic Area (EEA) countries and berries were identified as the highest associated risk factor (EFSA, 2014; Li et al., 2018). The consumption of fresh blueberries was associated with outbreaks of hepatitis A virus (HAV) and Salmonella Newport in New Zealand and the United States, respectively (Calder et al., 2003; Miller et al., 2013). The risk factors for the contamination of berry fruits with viruses such as human Norovirus (NoV) and HAV at primary production include environmental factors, in particular the use of sewage-contaminated agricultural water for irrigation and application of agricultural chemicals such as fungicides and cross-contamination by harvesters, food handlers and equipment at harvest or post-harvest (EFSA, 2014; Jacxsens et al., 2017). For the contamination with bacterial pathogens such as Salmonella, additional risk factors were identified, in particular contact with animal reservoirs (domestic or wild life) gaining access to berry fields and the use of untreated or insufficiently treated manure or compost (EFSA, 2014). At primary processing, contamination and cross-contamination of berries with Salmonella and NoV via equipment, water and food handlers represent the main risk factors (EFSA, 2014; Franck et al., 2015).

In order to extend the short shelf-life of berries, a big proportion of harvested berries are washed and individual quick-frozen (IQF). Washing of berries using sanitizers cannot fully remove pathogenic viruses and bacteria from the fruit surface, but generally allows a 1–2 log10 removal and lowers the risk of cross-contamination during washing if disinfectants are properly used within the washing tank (Butot et al., 2008; EFSA, 2014; Zhou et al., 2017). To further extend the shelf life of the fruits and improve the flavor of the final product, fruits are often subjected to osmotic dehydration, also sometimes referred to as sugar infusion or candying (Chandra and Kumari, 2015; Yadav and Singh, 2014). Osmotic dehydration is a process that uses concentrated sugar solutions to remove large amounts of water from the fruits via osmosis, either without heat or by applying mild temperatures to the sugar solution and the fruits, resulting in a high yield of tasty product (Shi et al., 2008; Shi et al., 2009; Torreggiani, 1993). However, osmotic dehydration is a time-consuming process. To accelerate the process of mass transfer and reduce dehydration time, simple and inexpensive methods such as freeze-thawing are taken advantage of and all-year available frozen fruits are widely used as raw material for osmotic dehydration by the industry (Falade and Adelakun, 2007). To remove excess moisture and reach a shelf-stable product, the osmotic dehydration is generally followed by drying. Osmotically dehydrated and dried berries are used as ingredient in various products such as yoghurts, chocolates, cereal bars and mixes, ice creams and cakes (Shi et al., 2009). A wide range of temperatures is applied in the industry during osmotic dehydration and drying, depending on the type of treated fruit and the desired quality of finished product (Teles et al., 2006). Not only the temperatures and times applied during the osmotic dehydration process have a great influence on taste and physical characteristics of the finished product, but also the drying method and its process parameters, e.g. hot air, drum, microwave and infrared drying. Drying has previously been shown to have an effect on the level of anthocyanins (Lohachoompol et al., 2004) and thus industry is interested in a compromise allowing an efficient dehydration process keeping the anthocyanin and antioxidant level of the treated berries high while ensuring the microbiological safety of the product.

Osmotic dehydration of fresh or frozen product takes place under ambient to moderate temperatures and thus it is generally considered as a mild treatment with limited microbial reduction. However, microbial studies looking at the inactivation or removal achieved during osmotic dehydration are scarce (Bourdoux et al., 2016). When treating potato samples for 3 h using sucrose solution at 50°Brix and 40 °C, a reduction of 1–2 log10 units of total Aerobic Plate Count (APC) was observed, possibly due to a microbial “wash-out” effect which could lead to the contamination of other batches if the wash solution is recycled (Mitrakas et al., 2008). This phenomenon was also observed during osmotic dehydration of blueberries in 65 °C Brix sucrose solution for 120 min at 40 °C by monitoring the microbiological status of the osmotic solution. The total numbers of mesophilic bacteria and osmotolerant yeast and moulds raised from <10 colony forming units (CFU)/ml to approximately 102 CFU/ml and 103–104 CFU/ml, respectively, indicating that recycling the osmotic solution may lead to cross-contamination of products (Bourdoux et al., 2016; Kucner et al., 2013). Thermal inactivation experiments carried out at 85 °C in strawberry puree showed that inactivation times of HAV rose dramatically with increasing sucrose concentration. The D value (Decimal reduction time) at 85 °C was 0.98 and 4.98 min for a 28 and 52°Brix solution, respectively (Deboosere et al., 2004). It is also known that the ability of Shiga toxin-producing Escherichia coli and Salmonella to survive as desiccated bacteria is remarkably enhanced in the presence of sucrose (Hiramatsu et al., 2005). Salmonella and Listeria monocytogenes have been reported to survive at 25 °C on dried tomatoes for at least 28 days and dried peaches for 14 days, respectively (DiPersio et al., 2004; Yoon et al., 2004). The reduction of Salmonella on dried strawberries (pH 3.3) was 3.7 log10 CFU/g after 42 days of storage at 25 °C (Beuchat and Mann, 2014). The composition of the food matrix, especially the pH may explain the differences seen in different fruit matrices, as also shown by Deboosere and coauthors for HAV (Deboosere et al., 2004). Results of these studies suggest that a better understanding of the inactivation achieved of both pathogens and representative surrogates during the osmotic dehydration of fruits with or without subsequent drying step is needed, especially taking the requirements of the U.S. Food and Drug Administration (FDA) Food Safety Modernization Act into consideration which asks growers and processors to adopt effective control measures to reduce microbial risks in fresh produce and to ensure the safety of processed products (FDA, 2018).

As osmotic dehydration methods and parameters vary considerably within the industry and minimally processed high quality fruits are increasingly sought, the scope of this study was to determine which temperatures are required for the inactivation of relevant bacteria and viruses during osmotic dehydration of berries, using blueberries as a model berry in a thawed state to mimic common industrial practices. Additionally, we studied the inactivation of osmotic dehydration at 23 °C, sometimes referred to “cold infusion” followed by air drying at 100 °C to determine the microbiological safety achieved by this combined treatment. A total of four pathogens (Salmonella enterica, Escherichia coli O157:H7 Listeria monocytogenes and HAV) and five respective surrogates (Enterococcus faecium, Escherichia coli P1, Listeria innocua, murine Norovirus (MNV) and bacteriophage MS2) were tested to search for adequate surrogates which could be used in validation studies to help the industry to adjust existing osmotic dehydration and drying processes for the production of high quality fruits which are microbiologically safe.

Section snippets

Preparation of blueberries and sugar solution

Frozen European blueberries (Vaccinium myrtillus) were purchased at a local distributor in Lausanne, Switzerland, and stored at −20 °C. The diameter size of the blueberries was approximately 0.5–0.6 cm. Each sample was prepared by weighing 25 g of frozen blueberries in a 90 mm sterile petri dish. The frozen blueberries were left at 23 °C under laminar flow to thaw for at least 3 h and any emerged juice was removed by pipetting.

White sugar (sucrose) was purchased at a local distributor in

Osmotic dehydration profile

Sucrose concentrations targeted by the industry at the end of the osmotic dehydration process of various types of berries vary between 34 and 45% corresponding to 26 to 31°Brix which was chosen as the reference range in the present study. To simulate common industrial practices (Shi et al., 2009), a constant sugar concentration was applied at a 1:1 ratio of sugar solution (25 ml) and blueberries (25 g). Preliminary trials showed that osmotic dehydration of thawed blueberries in a 45% sucrose

Discussion

The results of this study show that the level of bacterial reduction achieved during osmotic dehydration of blueberries at 23 °C was very limited (<2 log10 units for S. enterica, E. coli O157:H7, E. coli P1 and E. faecium) and higher (close to 4 log10 units) for the Listeria strains tested (Fig. 1A). It is known from survival studies in different juice concentrates that °Brix level, pH and intrinsic compounds play an important role in the bactericidal effect of different juice concentrates (

Acknowledgments

We thank Mireille Moser for providing statistical support for the data analysis.

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