Impact of water rinsing and perforation-mediated MAP on the quality and off-odour development for rucola
Introduction
Eruca Vesicaria ssp. sativa (P. Mill.) Thellung, commonly known as “Ackerrauke” in Germany, “rucola” in Italy, or “arugula” or “garden rocket/rocket salad” in the United States, belongs to the Brassicaceae family (Jirovetz, Smith, & Buchbauer, 2002). Rucola is an edible low calories green leafy vegetable (herb) that has become increasingly popular within the last two decade. It is eaten raw as fresh salad, or as a steamed vegetable, and in sauces (Cavaiuolo and Ferrante, 2014). Major challenges affecting the postharvest quality and shelf life of fresh rucola during storage are: the continued physiological processes as a result of respiration, the development of strong off-odours, tissue flaccidity and sensitivity of cut edges to browning, mechanical damage, and microbial decay associated with liberated nutrients from injured tissues (Cantwell, Rovelo, Nie, & Rubatzky, 1998; Nielsen, Bergström, & Borch, 2008). This results in a shorter shelf life due to leaf yellowing, which is accompanied by other degradation processes. The shelf life of fresh rucola leaves stored at 0, 5 and 15 °C were studied by Amodio, Derossi, Mastrandrea, and Colelli (2015), which described the kinetics of the most important quality attributes of rucola by used Weibullian model. The shelf life significantly changed, depending on storage temperature and thermal quality attributes. When the samples were stored at 5 °C, the appearance limited the shelf life up to 5.8 days.
Rucola is normally sold in plastic trays wrapped with macro-perforated polypropylene film in most of the German supermarkets. Such packaging system prevents wilting and eases of the handling and marketing of the product. However, the macro-perforated packaging system leads to very high gas permeability (Hussein, Caleb, & Opara, 2015; Hussein, Caleb, Jacobs, Manley, & Opara, 2015); therefore, it does not allow to atmosphere modification inside the package resulting in loss of freshness and green chlorophyll pigments. Hence, the major drawbacks associated with packaging leafy green vegetables under modified atmosphere packaging (MAP) systems are; that under sub-optimal MA conditions anaerobic metabolism can be induced and inappropriate packaging allows for accumulation of volatile organic compounds (VOCs) in the package headspace, which can significantly influence the secondary volatiles generated (Nielsen et al., 2008; Caleb, Mahajan, Al-Said, & Opara, 2013; Caleb, Opara et al., 2013; Tudela et al., 2013). Furthermore, fresh rucola has a characteristic pungent, sharp, and peppery flavour attribute, which has been attributed to the breakdown of glucosinolates into simple molecules such as isothiocyanates (Jirovetz et al., 2002; Miyazawa, Maehara, & Kurose, 2002; Bennett, Rosa, Mellon, & Kroon, 2006). Several studies have shown that the characteristic flavour attributes related to VOCs for packaged fresh or fresh-cut produce changes significantly during storage. This results in either loss of freshness odour and/or the accumulation of off-odour VOCs towards the end of storage (Edelenbos, Seefeldt, & Løkke, 2015; Caleb, Ilte, Fröhling, Geyer, & Mahajan, 2016). The development undesirable odours inside packaged rucola during storage are a problem that has been consistently identified by the food processing industry. Therefore, there is still a need to optimize the packaging system for fresh rucola along the cold chain and retail display.
Modified atmosphere packaging (MAP) combined with cold storage is widely used to maintain quality attributes and extend the shelf life of leafy green vegetables, based on the modification in gas composition inside the package. This helps to slow down produce physiological as well as metabolic processes, thereby, preserving the freshness and marketability (Mahajan, Caleb, Singh, Watkins, & Geyer, 2014; Borchert et al., 2014). It is well established that the gas composition inside MAP filled with fresh produce becomes modified due to the gas permeability properties of the packaging material, the respiration rate of the produce, the weight of the produce, and the free volume of the package (Mahajan, Oliveira, Montanez, & Frias, 2007; Hussein et al., 2015, Hussein, Caleb et al., 2015; Belay, Caleb, & Opara, 2016). Fill weight and package size are often pre-determined factors and therefore, the only controllable variable is the gas permeability of the lidding film which can be optimised with suitable micro-perforations in order to achieve equilibrium modified atmosphere.
The use of various antimicrobial rinsing/dipping solutions have been extensively reported in literature for leaf green vegetables (Martínez-Sánchez, Allende, Bennett, Ferreres, & Gil, 2006; Tudela et al., 2013), their industrial application are limited due to existing food regulations and public health implications. For rucola distribution there remains a critical lack of an optimum integrated postharvest treatment step and package design system, which is applicable across the cold chain. This further compounds the identified challenges. Thus, the objectives of this work were to optimise the packaging system for fresh rucola, evaluate its performance under two different time-temperature profiles normally observed in rucola supply chain in Germany, and investigate the impact water rinsing and packaging systems on the development of off-odour and quality of rucola.
Section snippets
Plant material and processing
Rucola (Eruca Vesicaria) was grown and harvested by a commercial grower (Gemüsering, Stuttgart, Germany). After harvest leaves were vacuum-cooled and shortly stored at 4 °C and ≥95% relative humidity until packaging. Leaves were thereafter divided into two groups for unwashed and washed treatment prior to packaging. Washed rucola was dipped in water for 30 s and centrifuged to remove excess water.
Packaging and storage
Rucola leaves were weighed (125 g) into polyethylene terephthalate trays (185 × 145 × 70 mm), which were
Package gas atmosphere
Perforation-mediated packaging systems had a significant impact on the change in headspace gas composition (p < 0.05). In MAP-0 package, the concentration of O2 decreased to levels below 2 kPa and CO2 accumulated above 13 kPa at the end of storage. On the contrary, the gas composition in control packages were not modified and stayed close to air (Fig. 1). The optimized package (MAP-2) achieved an equilibrium modified atmosphere (EMA) of low O2 (2–5 kPa) and high CO2 (<15 kPa), and it effectively
Conclusions
Current macro-perforation used by the industry did not modify gas composition and caused loss of freshness aroma, thus, this practice should be avoided. On the other hand a non-perforated package design for rucola resulted in continuous decline in O2 concentration below critical limit (<2%), and accumulation of off-odour. For longer produce shelf life combined with reduced water/turgor loss, optimized package combined with long-term low temperature is recommended. An optimized package design
Acknowledgments
This work was funded by Gemüsering, Stuttgart, Germany and Georg Forster Postdoctoral Research Fellowship (HERMES) program (Ref. 3.4–ZAF–1160635-GFHERMES-P) from the Alexander von Humboldt Foundation.
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