High throughput quality and safety assessment of packaged green produce using two optical oxygen sensor based systems
Highlights
► Assessment of quality and safety characteristics of ready-to-eat salads. ► Applying GreenLight™ and Optech™ systems to TVC and headspace O2 determination. ► Good correlation of the new tests with traditional methods. ► Both systems provide convenience and high sample throughput.
Introduction
The fresh produce market has changed dramatically over the last 2 decades, reflecting the new consumer demands and technological innovations in harvesting, production and packaging. Consumers are eating more fresh produce, purchasing a broader variety and demanding more convenience products such as ready-to-eat salads (Dimitri, Tegene, & Kaufman, 2003). Since fresh produce is still alive and respiring post harvest, it requires rapid processing, adequate packaging and controlled storage. A number of techniques can be applied to increase the shelf life and sensory quality and reduce microbial degradation, including genetic variation (Hayes & Liu, 2008), chilling (Lee, 2008), rational choice of packaging materials (Kim, Luo, Tao, Saftner, & Gross, 2005; Lee, 2008; Seglina, 2009), the use of biodegradable films (Del Nobile, Conte, Cannarsi, & Sinigaglia, 2008),warm chlorinated water treatment (McKellar et al., 2004) and modified atmosphere packaging (MAP) (Allende, Luo, McEvoy, Artés, & Wang, 2004; Jacxsens, Devlieghere, Van der Steen, & Debevere, 2001; Rojas Graü, Oms Oliu, Soliva Fortuny, & Martín Belloso, 2009; Watada & Qi, 1999). MAP, in combination with refrigerated temperature, seems to be the most efficient and well understood strategy to maintaining product quality and enhancing shelf life. Traditionally, reduced O2 (1–5%) and elevated CO2 (5–10%) levels are used to reduce respiration, product transpiration and ethylene production (Rojas Graü et al., 2009). In recent years, elevated O2 levels (>70%) combined with elevated CO2 concentrations (10–20%) (Amanatidou, Smid, & Gorris, 1999; Jacxsens et al., 2001; Van der Steen, Jacxsens, Devlieghere, & Debevere, 2002) have been applied to inhibit growth of naturally occurring spoilage microorganisms, prevent undesired anoxic processes and maintain freshness. Using optimised packaging material it was possible to control tissue browning and senescence by matching the oxygen transmission rate (OTR) of the package and oxygen consumption by the product (Kim et al., 2005). On the other hand, to assure good taste, visual appearance and low microbial load (total viable counts, TVC) of MAP green produce, it is necessary to deploy adequate control systems. In particular, headspace gas composition and TVC are the key quality parameters which can inform on the physico-chemical and microbiological status of individual packs, respectively.
The established methods to control headspace gas include GC and dedicated gas analyzers which are destructive in nature as their usage requires puncturing the pack, removing headspace gas through a needle and determining its CO2 and O2 values using built-in sensors. Knowing the initial headspace gas composition in the pack, to determine malfunctions in MAP tray sealing/gas flushing, pinpointing physical damage to the package or identifying gas mixes or gas interactions between packaging materials and product studied is critical in exerting quality and shelf life control for perishable food products. Recently, optical sensor systems have been described which allow non-destructive headspace analysis of packaged foods, including sensors for O2, CO2 and volatile amine analysis (Bültzingslöwen et al., 2002; O'Mahony, O'Riordan, Papkovskaia, Kerry, & Papkovsky, 2006; Pacquit et al., 2006). Among these, the O2 sensors based on phosphorescence quenching are the most advanced. They have been demonstrated in a number of studies with MAP foods (Fitzgerald et al., 2001; O'Mahony et al., 2006; Papkovsky, Papkovskaia, Smyth, Kerry, & Ogurtsov, 2000; Sandhya, 2010) and in gas permeability testing (Wang, Auty, & Kerry, 2010), operating as portable instruments with disposable, calibration-free sensors, and produced commercially from several vendors. Such systems provide powerful tools for quality control and assessment of MAP products, with the possibility of rapid screening of large number of packs and every pack, and for food research and other applications. This allows for complete inspection of all food packs in a continuous on-line process.
Besides headspace gas analysis, microbiological criteria of all packaged food products are subject to health and safety regulations. When it comes to determining the shelf life of a product, methods of evaluation need to be compliant with the requirements of Regulation (EC) No 2073/2005 (Ireland). Traditionally, TVC in food products are determined by the agar plating method (ISO:4833:2003, 2003) which is time- and labour-consuming, subjective and have limited sample throughput. A number of rapid microbiological methods have been developed in recent years. In particular, optical oxygen respirometry represents a simple, high-throughput, automated micro-assay platform which allows rapid TVC testing and enumeration of bacteria in complex food samples such as raw meat and fish (Hempel et al., 2011; O'Mahony & Papkovsky, 2006; O'Mahony, Green, Baylis, Fernandes, & Papkovsky, 2009). It has recently become available commercially as the GreenLight® system (http://www.mocon.com).
In this study we tested two new O2 sensor systems and benchmarked them against the established methods. For this purpose 3 different types of ready-to-eat salad – Iceberg lettuce, Caesar salad and Italian leave mix salad – were packaged under different conditions (5%, 21%, 45% and 60% O2 and 5% CO2) with the overall aim to control and improve the quality of MAPs. The results show that the new sensor systems provide simple and powerful tools for rapid high throughput quality assessment of packaged products, particularly those related to green produce.
Section snippets
Materials
Sterile peptone buffered water (PBW) was prepared fresh using the ingredients from Sigma–Aldrich Corp. (St Louis, MO) and Milli-Q water (Millipore, Billerica, MD). The stomacher machine and sterile stomacher bags were from Seward, Ltd (London, UK). Sterile 96-well, flat-bottom microplates made of clear polystyrene with lids were from Sarstedt (Nümbrecht, Germany). The GreenLight probe and mineral oil were from Luxcel Biosciences (Cork, Ireland), plate Agar - from Merck (Darmstadt, Germany).
Development of rapid TVC assay for green produce
To assess possible interference of the food matrix on the results of GreenLight TVC assay, crude homogenates of salad samples were analysed at several different dilutions: 1:2, 1:4, 1:8 and 1:16 in PBW by monitoring their respiration profiles and TT which reflect microbial growth. For the different dilutions of the same sample a linear relationship between the threshold time (TT) and logarithm of dilution factor was observed, as seen for the example in Fig. 1 for sweet & crunchy salad (in the
Conclusions
Overall, the new analytical systems GreenLight and Optech demonstrate good working characteristics in the assessment of MAP green produce. The simple, rapid and high throughput TVC test with GreenLight probe exhibits good working performance with a variety of different salad types. This method is well suited for both small and large users as it enables to save on material, space, labour and time, requiring only a conventional stomacher and fluorescent plate reader (already available in many
Acknowledgement
Financial support of this work by the Irish Department of Agriculture, grant 08RDC642, is gratefully acknowledged.
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