Accepted Manuscript Condensation and moisture regulation in packaged fresh-cut iceberg lettuce Stefania Volpe, Pramod Mahajan, Guido Rux, Silvana Cavella, Elena Torrieri PII:
S0260-8774(17)30333-3
DOI:
10.1016/j.jfoodeng.2017.08.015
Reference:
JFOE 8990
To appear in:
Journal of Food Engineering
Received Date: 28 November 2016 Revised Date:
1 August 2017
Accepted Date: 2 August 2017
Please cite this article as: Volpe, S., Mahajan, P., Rux, G., Cavella, S., Torrieri, E., Condensation and moisture regulation in packaged fresh-cut iceberg lettuce, Journal of Food Engineering (2017), doi: 10.1016/j.jfoodeng.2017.08.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Condensation and Moisture Regulation in Packaged Fresh-cut Iceberg Lettuce
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Stefania Volpe, 2Pramod Mahajan, 2Guido Rux, 1Silvana Cavella, 1Elena Torrieri
Department of Agricultural Sciences, University of Naples Federico II, Portici (NA), Italy
Department of Horticultural Engineering, Leibniz Institute for Agricultural Engineering and Bioeconomy, Potsdam, Germany
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Abstract
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The aim of this work was to understand the transpiration rate of fresh-cut iceberg lettuce and
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select appropriate packaging material for regulating moisture and minimising condensation
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inside the package. Experiments were conducted by conditioning the sample at 2, 6 and 10°C
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and 76, 86, 96 and 100% RH. TR was recorded during 7 days of storage. Packaging design
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optimization (with a cellulose-based film window on polymeric film) was performed using
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TR predicted at temperature of 6°C and 98 and 99% of RH, respectively, in order to establish
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the desired WVTR of packaging materials. TR ranged from 0.04 to 2.36 g kg-1 h-1 over all the
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combinations of temperature and RH tested. Based on package design optimization both pure
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materials (polymer or cellulose-based) didn’t satisfy WVTR requirement for fresh-cut iceberg
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lettuce. Among combined packages, the use of a surface ratio between 5% and 15% could prevent
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moisture condensation inside the package. Results from validation experiment confirmed the
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goodness of the package design procedure and showed that the package film with 15% of
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cellulose film window area on polymeric film was the only one that prevent water vapour
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condensation inside the package and avoid an excessive weight loss.
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Keywords: fresh-cut produce; transpiration rate; modelling; packaging design.
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*Corresponding author: Department of Agricultural Sciences, University of Naples Federico II, Via Università
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100, 80055, Portici (NA), Tel. 0039-0812539327; E-mail address:
[email protected]
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ACCEPTED MANUSCRIPT 1. Introduction
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Iceberg lettuce (Lactuca sativa L.) is a highly perishable product and it is considered one of
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the most popular fresh-cut vegetables. However, commercially available iceberg lettuce has a
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short shelf-life of 7 days at 7 °C. Appearance attributes for fresh products, such leaf turgidity,
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depend mainly on product weight loss or moisture condensation in the package headspace. As
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reported by Dinella et al. (2014), liking and freshness of fresh-cut salad were positively
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related to appearance attributes (green colour, salad assortment and leaf turgidity). Among
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appearance attributes, leaf turgidity depends mainly on product weight loss or moisture
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condensation in the package headspace. Normally, a weight loss greater than 5-10 % is not
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acceptable because the product wilts and soon becomes unusable (Ben-Yehoshua & Rodov, 2003).
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Moreover, an excessive moisture accumulation in the package can create favourable
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environment for microbial growth (Caleb et al., 2013).
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Transpiration and water condensation are the main phenomena that affects product weight
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loss or moisture accumulation in the package. This involves water transfer from the inter and
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intracellular space to the skin of the product, water evaporation from the outer surface layer
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and convective mass transfer of the moisture to the surroundings. Under saturated relative
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humidity (RH) condition, transpiration is caused by internal heat generation due to respiration
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(Chau and Gaffney, 1990; Bovi et al., 2016). Whenever the packaging material has lower
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water vapour transmission rate than product transpiration rate, water molecules evaporated
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from the product remain in the package enhancing the water vapour pressure inside the
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package leading to water condensation (Linke & Geyer, 2013). Temperature is one of the
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main critical factors for transpiration (Mahajan et al., 2008). Even small temperature
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fluctuation might influence the formation of water droplets underneath the film packaging
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leading to deterioration of packaged product (Linke & Geyer, 2013).
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Most polymeric films normally used to package fresh-cut vegetables have lower water vapour
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transmission rate compared to the transpiration rates of fresh products (Bovi et al., 2016).
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Therefore, the excess condensed water accumulated inside the package is still unresolved
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problem for MA packaged products, where the optimum number and size of perforations
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needed to maintain equilibrium gas atmosphere are not enough to control humidity and
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condensation. In this work a properly packaging system for fresh-cut lettuce iceberg is
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designed by choosing the film material on the basis of product transpiration. To this aim, the
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specific objectives of the present study were (i) to study the transpiration rate of fresh-cut
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iceberg lettuce, (ii) to develop a mathematical model to describe transpiration rate as a
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ACCEPTED MANUSCRIPT function of temperature and humidity and (iii) to integrate the model into the engineering
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packaging design concept.
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2. Materials and methods
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2.1. Materials
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Iceberg lettuce heads (Lactuca sativa) of Spanish origin were purchased from a local
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supermarket (Postdam, Germany). Propafilm (30 µm) and Naturflex NVS (30 µm) were
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furnished by Innovia Films (Cumbria, UK).
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2.2. Fresh-cut Iceberg lettuce processing
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Iceberg lettuce heads were transported to the laboratory in 30 minutes and equilibrated for 1 h
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at the test temperature (2, 6 and 10 °C). Before the experiment started, the external leaves and
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the central stern were removed. Then the lettuce heads were cut in pieces of 4 x 3 cm wide
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using a sharp knife and washed in cold tap water for 5 min. Finally lettuce was centrifuged in
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a manual kitchen basket type centrifuge for 2 minutes to remove the excess of water.
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2.3. Transpiration rate measurements
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To evaluate the transpiration rate, a weight loss technique was used. Cut iceberg lettuce of an
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initial average weight of 10g were stored in three containers, located in walk-in cooling room,
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maintained at different temperatures (2, 6 and 10 °C). Relative humidity within the test
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containers was controlled by using saturate salt solutions of sodium chloride, potassium
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chloride, potassium nitrate and distilled water, giving 76, 86, 96 and 100% RH, respectively.
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Temperature and relative humidity were continuously monitored using an air humidity sensor
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FHA 646R (Ahlborn, Holzkirchen, Germany). The weight loss was measured daily for 7 days
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using an electronic balance (Pharmacy PHS, Metter Toledo, Switzerland). Transpiration rate
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(TR) was calculated from the changes in weight of iceberg lettuce over time:
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=
×(
(1)
)
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where TR is the transpiration rate expressed in g kg-1 h-1, Mi is the initial weight of the lettuce
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(g), M is the weight of lettuce (g) at time t (h). The flow of water vapour through a fruit or a
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vegetable is proportional to the difference in water activity (RH/100) between the surface of a
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commodity and the surrounding air (Chau et al., 1990). Transpiration rate model as a function
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of temperature and RH was developed by modifying the model developed for strawberries
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(Sousa-Gallagher et al., 2013) by adding an empirical term for transpiration rate at 100% RH
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(Eq 2): 3
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TR= (b × ecT ) + ki × (1-aw) (1- e– aT)
(2)
where aw is the water activity (RH/100) of the container; T is the temperature (°C); and ki (g
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kg-1 h-1) is a coefficient related to the mass transfer coefficient, and a (°C-1), b (g kg-1 h-1) and
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c (°C-1) are empirical parameters.
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2.4. Water vapour transmission rate measurements
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The water vapour transmission rate (WVTR) of Naturflex and Propafilm was evaluated. The
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WVTR was measured by a gravimetric method by means of bottle containing deionized
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water. Films were placed on the top of the bottle and sealed by rubber rings and lids with a
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hole of 3 cm of diameter; the film area was 7.07 cm2. The bottles were weighted and then
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placed in a box containing a saturated magnesium nitrate solution which provided a constant
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water activity of 0.57 at 6°C; the bottles were weighted every day. The WVTR through the
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film was estimated by the linear portion of the diagram obtained by plotting the weight
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increment of the bottle as function of the time per unit of surface. It was assumed that the
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steady state was reached once the linear regression analysis based on the last four data points
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resulted in R2 ≥ 0.998. The WVTR was expressed as g m-2 h-1. Three replicates for each film
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sample were performed.
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2.5. Packaging design and experimental validation
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To select the proper film for fresh-cut iceberg lettuce, the water vapour flux (WVF) through
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the film (g h-1) and the rate of water loss from the product due to transpiration (WTR) (g h-1)
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were calculated at 6 °C with relative humidity range of 98 to 99 %. This humidity range is a
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compromise to prevent excessive weight loss of fresh-cut lettuce and also avoid moisture
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condensation inside the package. The ideal composite film material should have WVF
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matching with the target WTR of the product. Moreover, a maximum weight loss of 5 % has
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been considered as further criteria for selection of the packaging film. To obtain a package
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material with the desired properties in terms of WVTR, Propafilm was used as a base
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packaging material whereas high water vapour permeable film Naturflex was used to develop
115
a window for water vapour transfer. Natureflex film windows were attached onto the
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Propafilm film using double sided hermetic sealing tape. The surface area covered by the
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sealing tape was much smaller compared to the total surface area of the package. Moreover,
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the covered area is composed of 3 layers: PP, sealing tape and NF, therefore it has high barrier
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for water vapor permeability.
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The surface area of windows (A) were 2, 5, 10, and 15 % of total surface area of the bag
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(Table 1). The effective WVTR of the composite structure were calculated using Eq. (3):
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×
=
×
(3)
Where the subscripts C, PP, NF denote composite, propafilm and natureflex, respectively.
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Six bags from composite films of size 240 x 180 mm were used for validation study. Fresh-
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cut lettuces (150g) were packed and heat sealed by using a manual heat sealer (hand sealer
127
HZ, Kopp Verpackungssyteme, Reichenbach/Fils, Germany). Samples were stored at 6°C and
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57% of relative humidity for 7 days. The package surface area available for gas exchange was
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about 864 cm2. The water vapour absorption of the bag was gravimetrically determined by
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measuring mass uptake of the bag at the end of the storage time. The amount of water
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transmitted over the package was gravimetrically measured by weighing the package at the
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start and at the end of storage period. The amount of condensation was measured by weighing
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the bag before and after wiping off the water accumulated on the film.
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2.6. Experimental design and statistical analysis
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TR experiments were performed according to a full factorial design. The experimental were
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replicated three times for a total of 36 samples. Transpiration rate data obtained for all
137
combination of temperature and RH were fitted by the model reported in the Eq. (2) by non-
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linear regression analysis to estimate the value of constants a, b, c and ki. Statistica software
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(Statistica 7.0, Statsoft, Tulsa, Oklahoma, USA) was used to estimate the model parameters
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and also to analyse the effect of temperature and RH on transpiration rate by using Pareto
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analysis at 95% significant level. The effect of package on the weight loss, water absorbed by
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the film, water condensed in the package and water transmitted through the package were
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studied by ANOVA and Duncan’s test by using SPSS v 17.0 for Windows (SPSS, Milan,
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Italy). Significant differences were defined at p<0.05.
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3. Results and Discussion
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3.1 Transpiration behaviour
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Transpiration rate (TR) of fresh-cut iceberg lettuce is shown in figure 1. TR ranged from 0.04
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to 2.36 g kg-1 h-1 over all the combinations of temperature and RH tested. The results showed
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that TR decreased by increasing RH from 76 to 100 %, at fixed temperature. Moreover, TR
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increased by increasing temperature from 2 to 10 °C, at fixed RH. Pareto analysis showed
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that, in the range of conditions studied, both temperature and relative humidity affected
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ACCEPTED MANUSCRIPT significantly the transpiration rate (p<0.05). However the effect of humidity was more
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pronounced than that of temperature. Also the interactive effects of temperature and humidity
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were significant (data not shown). These results are in agreement with those obtained on
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mushroom, strawberry and pomegranate arils (Mahajan, et al., 2008; Sousa-Gallagher et al.,
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2013; Caleb et al., 2013). TR of fresh cut iceberg is of the same order of magnitude of the TR
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mushrooms (range from 0.29 to 5.2 g kg-1 h-1) (Mahajan, et al., 2008), but higher than TR of
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strawberry (range 0.24 to 1.16 g kg-1 h-1) (Sousa-Gallagher et al., 2013) at the similar range of
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temperature and relative humidity. Our values are higher than those reported for cut onion, cut
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green onion (0.447 and 0.369 g kg-1 h-1, respectively, at 10°C and 82% RH in normal air),
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grape tomatoes (0.018 to 0.107 g kg-1 h-1) (Xanthopoulos et al., 2014), arils and aril sacs
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(range from 1.42 to 15.23 g kg-1 day-1 and 0.63 to 9.95 g kg-1 day-1, respectively) (Aindongo et
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al., 2014). The higher TR of fresh-cut lettuce than other fruit and vegetables can be justified
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considering that the water loss rate varies with the type of produce. In fact, leafy green
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vegetables such as spinach and leaf lettuce, lose water more quickly than the spherical ones
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because they have a high surface-area to-volume ratio and a thin waxy cuticle with many
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pores (Bai & Plotto, 2012).
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Moreover the high stress due to the cutting operation increases the TR. It is important to
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underline that even under water saturated conditions of 100 % RH, TR showed a positive
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value equal to 0.04 and 0.14 g kg-1 h-1 at 2 and 10 °C, respectively. This loss of moisture
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occurred due to respiratory heat generation which increased the product surface temperature,
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thereby creating a gradient for water vapour pressure deficit between the product surface and
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the ambient air (Rux et al., 2015; Mahajan et al., 2016). Therefore, transpiration rate model as
174
a function of temperature and RH was modified by adding a term for transpiration rate at
175
100% RH (Eq 2). Figure 2 shows the relationship between observed and predicted value of
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TR of fresh-cut lettuce for all the experimental data. The value of TR predicted by Eq. (2) was
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in close agreement with those obtained experimentally (R2 > 0.98), therefore, the model
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described the TR process adequately. The value of constant coefficients a, b, c, and ki were
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0.42 °C-1, 0.031 g kg-1 h-1, 0.151 °C-1 , and 8.92 g kg-1 h-1, respectively.
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3.2 Integrated packaging design
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The model reported in the Eq. 2 was used to estimate the TR as a function of temperature-
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humidity conditions. The measured values WVTR of NF and PP films and their combinations
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(Table 1) were used for packaging design. The WVTR of PP and NF film were equal to 0.29
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g m-2 h-1, and 2.3 g m-2 h-1, respectively. In order to simulate the normal package condition, it
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was assumed to pack 150 g of produce in a bag of size 240 x 180 mm with surface area of 864
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ACCEPTED MANUSCRIPT cm2. In this condition, the rate of water loss from the product (WTR) was calculated by
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multiplying the TR by the weight of product. At 6 °C, the WTR of fresh-cut lettuce was equal
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to 0.036 g h-1 at 98 % RH and to 0.024 g h-1 at 99 % RH.
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To avoid moisture condensation inside the package, the flux of water vapour through the
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package should be as close as possible to the WTR of fresh-cut lettuce. Results reported in
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table 1 showed that PP had a WVF, of 0.017 g h-1, lower than the required one, whereas NF
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had a WVF equal to 0.2 g h-1, much more higher than the required one. Thus, both materials
193
didn’t satisfy the product requirements. This means that for PP film the WVF is too low and
194
would accelerate water vapour condensation inside the package. Moreover this condition
195
could increase the microbial growth and produce decay at sub-optimal temperatures (Linke &
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Geyer, 2013; Ayala-Zavala et al., 2008). On the other hand, WVTR for NF film alone is too
197
high, leading to an excessive loss of water and consequently to a loss of turgor and saleable
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mass. Thus, a combination of PP and NF with a fixed window was used to achieve the proper
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WVF in order to satisfy the WTR requirements and regulate the RH inside the package. These
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results were in agreement with Caleb et al., (2016) who reported that the optimal package for
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minimally processed broccoli was obtained by a combination of polypropylene and naturflex
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film. By increasing the surface ratio of NF from 2% to 15%, the WVF increased. Since the
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requirement in terms of WTR of fresh-cut lettuce is between 0.024 gh-1 and 0.036 gh-1, the
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films with transmission rate in these range were PP95NF05, PP90NF10 and PP85NF15 (figure 3).
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3.3 Packaging performance
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Experimental package validation was performed by measuring the amount of water loss by
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fresh-cut iceberg lettuce, water absorbed by the film, water transmitted through the film and
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water vapour condensed inside packages (Figure 4). ANOVA showed that the package type
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had a significant effect on all parameters studied (p<0.05). The predicted weight loss of fresh-
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cut iceberg lettuce (WTR x time) ranged from 4 to 6 g after 7 days at 6°C with a relative
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humidity between 98 and 99%. The measured water loss from the product was 0.8, 4.1, 7.4
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and 9.5 g for PP100NF0, PP95NF05, PP90NF10, and PP85NF15, respectively. The lower water loss
213
in PP100NF0 was due to low WVTR of packaging film, therefore, it attained 100% RH inside
214
the package. The higher water loss in PP85NF15 corroborated to a stable headspace relative
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humidity (<98%). The values of water loss for PP95NF05 and PP90NF10 were well predicted by
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the model in the range 98-99% RH since the WVF of packaging film falls into the range of
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WTR of product. These bags allowed to maintain an optimal RH of 98-99% inside the package.
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Overall, the bags were designed to reach optimal RH that led to reduced condensation. Thus,
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ACCEPTED MANUSCRIPT the model fits well due to low condensation. These results are in agreement to Caleb et al. (2016)
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who reported the lowest mass loss (0.9%)in non-perforated bi-axially oriented polypropylene.
221
This value is comparable to that found for PP100NF0 (0.6%). In contrast micro-perforated and
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non-perforated polypropylene packages fitted with 20% cellulose-based film window showed
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the highest mass loss of 14.8% and 9.4% respectively.
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Even if the rate of water produced by lettuce in PP100NF0 was lower than the other packages,
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it was still enough to have moisture condensation inside the package; in fact 75% of water
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loss by the product represents the condensed water. Moreover, fresh-cut lettuce continues to
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produce moisture even at 100% RH. Since the WVTR of the polypropylene is very low, water
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molecules cannot escape and thus they remain inside the package. On the other hand, NF
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windows effectively prevent water vapour condensation inside the package. This could be
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attributed to the higher WVTR of NF which leads in lower water vapour pressure and RH
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inside the package (Caleb et al., 2016). However the weight loss of 6.4% in PP85NF15 is not
232
acceptable because causes deterioration of lettuce.
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Regarding the transmitted water, Duncan's test (p<0.05) showed that PP100NF0 was
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significantly different from the other samples, while there was no significant differences in
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water transmission of PP90NF10 and PP85NF15 packaging types. The differences in predicted
236
and experimental transmitted water were less obvious for PP95NF05, PP90NF10 and PP85NF15
237
but more pronounced for PP100NF0. Concerning the absorbed water, the general trend was an
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increase of water absorption when the surface area of NF increased. Natureflex film material
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absorbed water due to its hydrophilic properties. Experimental results confirmed that both
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PP95NF05 and PP90NF10 offered a valid composite material to maintain in the package an
241
optimum RH of 98-99% required for fresh-cut iceberg lettuce and reduce the moisture
242
condensation inside the package. However, PP90NF10 was more able than PP95NF05 to
243
reduction of condensed water inside the package from 8.5 to 3.3%. Thus, the best package for
244
fresh-cut iceberg lettuce was PP90NF10.
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4. Conclusions
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Transpiration rate of fresh-cut iceberg lettuce was found to be in the range of 0.04 - 2.36 g kg-
247
1
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mathematical model that showed a good predictability for transpiration rate of fresh-cut
249
lettuce. A packaging validation confirmed that in order to design a proper package, the
250
requirements in terms of water vapour flux through the film and the rate of water loss from
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the product must be taken into account. The results revealed that the use of a PP film with
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h-1. The influence of RH in the range 76 % - 100 % and temperature was well described by a
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ACCEPTED MANUSCRIPT 10% of NF surface ratio effectively prevent water vapour condensation inside the package
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compared to the PP control package. Moreover an optimum humidity inside the package is
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maintained. Such model will be useful for the fresh produce packaging industry.
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Acknowledgements
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This work was supported by the MIUR in the frame of the Prin project “Long Life, High
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Sustainability” - "Shelf Life Extension come indicatore di sostenibilità", 2012; The first author
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acknowledges a Ph.D. scholarship from the University of Naples ’FedericoII, Napoli, Italy.
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Aindongo, W. V., Caleb, O. J., Mahajan, P. V., Manley, M., Opara, U.L. (2014). Effects of
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storage conditions on transpiration rate of pomegranate aril-sacs and arils. South African
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Journal of Plant and Soil, 31(1), 7–11
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Ayala-Zavala J. F., Del Toro-Sanchez L., Alvarez-Parilla E., Gonzalez-Aguilar G. A. (2008).
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High relative humidity in-package of fresh-cut fruits and vegetables: advantage or
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disadvantage considering microbiological problems and antimicrobial delivering systems?
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Journal of Food Science, 73: R41-47.
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Bai J., Plotto A. (2012). Coatings for fresh fruits and vegetables. In E. A. Baldwin, R.
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Hagenmaier, J. Bai (Eds.), Edible Coatings and Films to Improve Food Quality (pp. 192-193).
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moisture evolution in packaged fresh horticultural produce and the role of integrated
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Caleb O. J., Ilte K., Frohling A., Geyer M., Mahajan P. V. (2016). Integrated modified
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atmosphere and humidity package design for minimally processed Broccoli (Brassica oleracea
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L. Var. italica). Postharvest Biology and Technology 121:87-100.
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ACCEPTED MANUSCRIPT Table 1. Surface ratio of packaging films used in composite material Symbol
Propafilm (PP)
g h-1
PP100NF0
100
0
0.2±0.4
0.017
PP98NF2
98
2
0.24
0.021
PP95NF05
95
5
0.31
0.026
PP90NF10
90
10
0.41
0.035
PP85NF15
85
15
0.52
0.045
PP0NF100
0
100
2.3±0.2
0.20
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315 316 317
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309
313
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g m-2 h-1
307
312
WVF
%
306
311
WVTR
%
305
310
NatureFlex (NF)
SC
304
11
ACCEPTED MANUSCRIPT 3.0 10°C
6°C
2°C
RI PT
2.0
1.5
1.0
SC
Transpiration rate, g kg -1 h-1
2.5
0.0 76%
86%
M AN U
0.5
96%
100%
Relative humidity, %
318 319
Figure 1. Transpiration rate of fresh-cut lettuce at 10, 6 and 2°C for RH of 76, 86, 96 and
321
100%. Error bars represent the standard deviation.
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EP
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AC C
322
TE D
320
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ACCEPTED MANUSCRIPT 2.0
RI PT SC
1.0
0.5
M AN U
Predicted transpiration rate, g kg -1 h-1
1.5
0.0 0.0
0.5
1.0
1.5
2.0
Experimental transpiration rate, g kg-1 h-1
332
TE D
333
Figure 2. Relationship between experimental and predicted values of transpiration rate (TR)
335
of fresh-cut lettuce
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AC C
336
EP
334
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ACCEPTED MANUSCRIPT
347
Figure 3. Water vapour flux (WVF) through the film (g h-1) and rate of water loss (WTR) of
349
150 g of fresh-cut iceberg lettuce (g h-1) calculated at 6 °C with a relative humidity from 98 to
350
99 %.
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EP
352
AC C
351
TE D
348
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M AN U
SC
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ACCEPTED MANUSCRIPT
354 355
Figure 4. Distribution of water loss in different packages after 7 days of storage at 6°C.
AC C
EP
TE D
356
15
ACCEPTED MANUSCRIPT Highlights -Transpiration rate of iceberg lettuce was 0.04 to 2.36 gkg-1h-1 -Transpiration rate was used to estimate water vapour transmission rate -Package combining low and high water vapour permeable films was optimum
AC C
EP
TE D
M AN U
SC
RI PT
-Combined film led to reduction in moisture condensation for fresh-cut lettuce