- Email: [email protected]
Use of biodegradable films for prolonging the shelf life of minimally processed lettuce
Available online at www.sciencedirect.com
Journal of Food Engineering 85 (2008) 317–325 www.elsevier.com/locate/jfoodeng
Use of biodegradable films for prolonging the shelf life of minimally processed lettuce M.A. Del Nobile a,b,*, A. Conte a, M. Cannarsi a, M. Sinigaglia a,b b
a Department of Food Science, University of Foggia, Via Napoli, 25 – 71100 Foggia, Italy Istituto per la Ricerca e le Applicazioni Biotecnologiche per la Sicurezza e la Valorizzazione dei Prodotti Tipici e di Qualita`, Universita` degli Studi di Foggia, Via Napoli, 25 – 71100 Foggia, Italy
Received 22 February 2007; received in revised form 22 May 2007; accepted 5 June 2007 Available online 11 September 2007
Abstract The ability of biodegradable films to prolong the shelf life of minimally process lettuce stored at 4 °C was addressed. Four different films were tested: two polyester-based biodegradable films (NVT1, NVT2), a multilayer film made by laminating an aluminum foil with a polyethylene film (All-PE), and an oriented polypropylene film (OPP). Package headspace, microbial load and colour of the packed lettuce were monitored for a period of 9 days. A simple mathematical model was used to calculate the senescence level, whereas the reparameterized Gompertz equation was used to calculate microbial shelf life of the fresh product. The fastest quality decay kinetic was observed for the lettuce packed in OPP, whereas the slowest one was detected for that in All-PE. Results suggest that the gas permeability of the investigated films plays a major role in determining the quality of the packed produce. Moreover, it was observed that biodegradable films guarantee a shelf life longer than that of OPP. Ó 2007 Published by Elsevier Ltd. Keywords: Biodegradable film; Minimally processed food; Lettuce; Shelf life
1. Introduction Minimally processed vegetables are ready to use products developed in the 1980s to respond to the emerging consumer demand for both convenience and high quality aspects. Their production process includes washing, cutting and packaging with sealed polymeric films or trays (Saracino, Pensa, & Spiezie, 1991). The gas composition within the package is modified through the respiration of the vegetable tissue. As a consequence, the oxygen (O2) from the headspace is consumed and the carbon dioxide (CO2) evolves until the creation of an equilibrium-modified atmosphere that depends on both temperature changes and gas permeability of the packaging film (Del Nobile, Baiano, * Corresponding author. Address: Department of Food Science, University of Foggia, Via Napoli, 25 – 71100 Foggia, Italy. Tel./fax: +39 881 589 242. E-mail address: [email protected] (M.A. Del Nobile).
0260-8774/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.jfoodeng.2007.06.040
Benedetto, & Massignan, 2006; Jacxsens, Devlieghere, & Debevere, 2002; Mathooko, 1996). The main spoilage mechanisms affecting the shelf life of the fresh-cut products are oxidation phenomena, due to the enzymatic activity of the cut leaves, moisture loss and proliferation of spoilage and pathogenic microorganisms (Gimenez et al., 2003; Watada & Qi, 1999). Good manufacturing handling practices and treatments with sanitizing agents in combination with proper storage conditions can concur to the extension of their shelf life (Bolin & Huxsoll, 1991; Pirovani, Piagentini, Guemes, & DiPentima, 1998). Warm chlorinated water, for example, has been shown to extend the shelf life of shredded lettuce in more than one laboratory study (Delaquis, Stewart, Toivonen, & Moyls, 1999; McKellar et al., 2004; Odumeru, Boulter, Knight, Lu, & McKellar, 2003). Also the use of modified atmosphere packaging (MAP) was reported inhibiting the detrimental phenomena, as well as reducing the respiration rate of vegetables (Fonseca,
318
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
Oliveira, & Brecht, 2002; Watkins, 2000). However, MAP often produces high levels of CO2 with a consequent development of off-flavours, due to the change to anaerobic fermentative metabolism and stimulation of potential pathogens (Jacxsens, Devlieghere, Ragaert, Vanneste, & Debevere, 2003; Kakiomenou, Tassou, & Nychas, 1996; Nguyen-the & Carlin, 1994; Szabo, Scurrah, & Burrows, 2000). Predictive modelling was used in literature for designing fresh-cut produce packaging by studying the effects of storage conditions and film permeability on the phenomena responsible for the unacceptability of the ready-to-use products (Jacxsens, Devlieghere, & Debevere, 1999; Jacxsens, Devlieghere, De Rudder, & Debevere, 2000; Jacxsens et al., 2002; Lee, Haggar, Lee, & Yam, 1991). Due to the large waste problems, in alternative to the traditional plastics used in MAP (Guilbert, Gontard, & Gorris, 1996), biodegradable films and coatings could be used (Makino & Hirata, 1997); however, due to their water vapour and gas barrier properties the application of the biodegradable films to food packaging is still limited. The effects of alginate-based edible coatings and laminate of chitosan-cellulose and polycaprolactone on minimally processed lettuce were studied to retard moisture loss and to better retained product crispness (Makino & Hirata, 1997; Tay & Perera, 2004). In this work the suitability of biodegradable films to be used as packaging materials for fresh-cut lettuce was addressed. In a previous work the authors explored the influence of low environmental impact packaging materials on the respiration rate of minimally processed lettuce (Del Nobile et al., 2006). The present study was aimed to evaluate the total quality of the product packaged in green polymers. To this purpose, one traditional package film (OPP) and two different new biodegradable polymeric matrixes (NVT1 and NVT2) were applied to fresh-cut lettuce, without modifying the packaging headspace compositions to better understand the ability of the films in preventing detrimental phenomena of the packaged product. A multilayer film (All-PE) was also used to evaluate separately the respiration activity of the packed produce, and the oxygen and carbon dioxide permeability coefficient of the investigated films in the real working conditions. The tests on quality decay kinetics of the packed produce were conducted on lettuce stored at 4 °C. Respiration rate, microbial count and colour were evaluated during storage and analyzed to calculate the product shelf life. 2. Materials and methods 2.1. Sample preparation The lettuce (Iceberg) was purchased on a local market immediately after harvesting and it was directly transported to the laboratory. After manual removing of the external leaves and core, the lettuce was shredded by knife
according to the typical size of packed salad (about 2 cm 4 cm). The shredded lettuce was then quickly washed with tap water to remove residuals, treated for 1 min with cold chlorinated (0.25 g/l) water and rinsed by immersion for another minute in tap water. The excess water was removed by centrifuging in a manual salad spinner. The investigated lettuce was packaged using four types of bags made respectively by an oriented polypropylene film (OPP), a monolayer film based on a blend of biodegradable polyesters (NVT1), a multilayer co-extruded film based on a blend of biodegradable polyesters (NVT2) and a multilayer film made by laminating an aluminum foil with a polyethylene film (All-PE). All films were kindly provided by Novamont (Novara, Italy), and were used as received. 2.2. Packaging procedure The bags were hermetically sealed and stored at 4 °C for 9 days. Samples were withdrawn three times a day for the first 2 days, two times a day for further two days and then daily analysed. The package headspace volume was about 118 cm3, the weight of packed lettuce was about 100 g, and the package area available for gas exchange was about 525 cm2. 2.3. Headspace gas composition O2 and CO2 contents of all packaged lettuce were measured using an O2 and CO2 analyser (PBI Dansensor, Checkmate 9900, Rønnedevej 18, DK-4100 Ringsted, Denmark). The volume taken from the package headspace for gas analysis was about 10 cc. To avoid modifications in the headspace gas composition due to gas sampling, each package was used only for a single determination of the headspace gas composition. 2.4. Microbiological analyses For the microbiological analyses, about 25 g of lettuce from each bags were homogenised in a Stomacher (mod. 4153-50, International PBI, Milan, Italy) for 2 min with 225 ml of sterile peptone water. Serial dilutions (1:10) of each homogenised sample were made in the same diluent and surface spread in duplicate. Total aerobic mesophiles and psychrotrophs were determined using Plate Count Agar (PCA, Merck); plate were incubated at 30 °C for 48 h and 7 °C for 10 days, respectively. The number of Enterobacteriaceae was determined using Violet Red Bile Agar (VRBA, Oxoid); plates were incubated at 37 °C for 24 h. Pseudomonades were determined by surface plating onto Pseudomonas agar base with selective supplement (Oxoid) followed by aerobic incubation at 25 °C for 48 h. In addition, Sabouraud dextrose agar (Oxoid) incubated at 25 °C for 48 h or 5 days, respectively, was used for yeasts and moulds.
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
2.5. Product appearance Images of the lettuce, packaged in the four different bags, were acquired daily with a scanner (Epson Perfection 1250). Image analysis was then performed on the scanned images at 0, 3 and 6 days of storage, to determine hue, saturation and intensity parameters by using Image Pro Plus 6 (Media Cybernetics, Silver Spring, MD, USA). Hue parameter represents the type of colour (e.g. red or yellow), saturation refers to the relative purity or the amount of grey in a colour and intensity indicates brightness. 2.6. Carbon dioxide transmission rate The CO2 permeability was measured using a gas-chromatograph HP 5890 with a Thermal Conductivity Detector (TCD), as described below. Eight bags were conditioned in a CO2 saturated ambient at 23 °C and 0% RH (Relative Humidity), according to the following conditions: three bags at 5 days; three bags at 7 days; two bags at 28 days. At the end of the conditioning time, 1 ml of the gas contained in the bags was sampled and injected in the GCTCD system. 2.7. Shelf life calculation The approach proposed by Corbo, Del Nobile, and Sinigaglia, 2006 was adopted to calculate the shelf life of packed lettuce. The following re-parameterized Gompertz equation was fitted to the experimental data: logðcfu g1 Þ ¼ ½logðcfu g1 Þmax k S:L: A exp exp ðlmax 2:71Þ þ1 A kt þ1 þ A exp exp ðlmax 2:71Þ A ð7Þ
where [log(cfu g1)]max is the decimal logarithm of the acceptability limit, S.L. is the shelf life of the packed product, A is the decimal logarithm of the maximum microbial growth attained at the stationary phase, lmax is the maximal specific growth rate, k is the lag time, and t is the time. The advantage of using Eq. (7) for estimating the shelf life of packed food instead of using the Gompertz equation as modified by Zwietering, Jongenburger, Roumbouts, and Van’t Riet (1990) is that by fitting Eq. (7) to the experimental data it is possible to directly estimate the shelf life and its confidence interval. On the other hand, by using the other approach it is not possible to estimate the shelf life confidence intervals (Corbo et al., 2006). 2.8. Statistical analysis The experimental data on respiration rate of the product, as well as the microbiological analyses, were carried
319
out in duplicate. The media and standard deviations were calculated. In fact, the data shown in the figures are the average of all repetitions, whereas the error bars are the standard deviation. The confidence intervals of model’s parameters were evaluated as follow: first, a fit was run with the original data; then, using the data points standard deviation, 100 additional fits were run on artificial data sets, which were generated by randomly varying the data around the fitted function. From these additional fits, a distribution of values for each parameter was obtained. The sets of data obtained for each parameter was statistically treated to obtain the 95% confidence interval. The image analysis was carried out for five times on each image of lettuce sample to obtain an average value for each parameter. All obtained data were submitted to one-way analysis of variance (ANOVA) and Duncan’s test (p < 0.05) and to a cluster analysis (the Euclidean distance was used as the amalgamation method) through the statistic package Statistica for Windows (Statsoft, Tulsa, USA). 2.9. Modeling In previous papers (Del Nobile et al., 2006; Del Nobile, Licciardello, Scrocco, Muratore, & Zappa, 2007) a mathematical model that predicts the respiration rate of minimally processed produce was presented. This model is summarized as follows. The Michaelis–Menten model can be simplified to the following expression: rO2 ¼ A ½O2 where rO2
ð1Þ i mL is oxygen consumption rate expressed as kg ,A h h
is the kinetic of the entire respiration process exh constant i mL pressed as kg h , [O2] is the percent oxygen concentration. A depends on temperature and, in some cases, on carbon dioxide concentration. In this work it has been assumed that A depends on carbon dioxide concentration through an exponential type expression: A ¼ A1 expfA2 ½CO2 g
ð2Þ
where [CO2] is the percent carbon dioxide concentration, h i mL A1 is the pre-exponential term expressed as kg , and it h is the maximum oxygen consumption rate, A2 is the exponential factor and accounts for the carbon dioxide induced respiration inhibition, it is dimensionless. Substituting Eq. (2) in Eq. (1) one obtain: rO2 ¼ A1 expfA2 ½CO2 g ½O2
ð3Þ
To describe the carbon dioxide consumption rate in this work it is assumed that the ratio between carbon dioxide produced and oxygen consumed that is the respiratory quotient (RQ) is constant, but not necessarily equal to one: rCO2 ¼ K 1 fA1 expfA2 ½CO2 g ½O2 g
ð4Þ
320
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
where rCO2 is carbon dioxide production rate expressed as h k mL , K1 is the ratio between the moles of carbon dioxide kg h produced and the moles of oxygen consumed, it is dimensionless. To describe the time course during storage of oxygen and carbon dioxide concentration inside the package, the mass balance on these two substances in the package headspace is written: j k nO2 ðtÞRT est p þO V st dðnO2 ðtÞÞ 2 ¼ S P O2 mp 4:615 dt ‘ 106 fA1 ½O2 expfA2 ½CO2 gg ð5Þ j k n ðtÞRT CO2 pest CO2 V st dðnCO2 ðtÞÞ ¼ S P O2 þ mp 4:615 dt ‘ 106 fK 1 fA1 ½O2 expfA2 ½CO2 ggg ð6Þ where nO2 ðtÞ is the mole of oxygen in the package head space at time t, S is the area of the package surface, P O2 is the package oxygen permeability, pest O2 is the external oxygen partial pressure, mp is the mass of the packed product, Vst is the volume of the package headspace, T is the temperature expressed in K, R is the universal gas constant, k is the film thickness, nCO2 ðtÞ is the mole of carbon dioxide in the package head space at time t, P CO2 is the package carbon dioxide permeability, pest CO2 is the external carbon dioxide partial pressure. Eqs. (5) and (6) are a set of two ordinary differential equations that was integrated numerically using the fourth-order Runge–Kutta formula (Press, Flannery, Teukolsky, & Vetterling, 1989). 3. Results and discussions
Fig. 1. Time course during storage of headspace gas composition for Iceberg lettuce packed in aluminum bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
Fig. 2. Time course during storage of headspace gas composition for Iceberg lettuce packed in OPP bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
As reported above, the quality of the investigated product depends on several quality sub-indices, among which respiration activity, microbial growth, nutritive compounds and appearance are the most important. To determine the influence of using biodegradable films with different permeability on the quality decay kinetics of fresh minimally processed lettuce the variation during storage time of the above quality sub-indices was determined. The results obtained are presented in the following. 3.1. Respiration activity Figs. 1–4 show the variation of package headspace composition during storage for the lettuce packed using the four investigated films. As expected a decrease in the headspace oxygen concentration along with an increase in the headspace carbon dioxide concentration was detected. The curves shown in the above figures were obtained by fitting the set of ordinary differential Eqs. (5) and (6) to the experimental data. It is worth noting that the set of ordinary differential Eqs. (5) and (6) was simultaneously fitted to
Fig. 3. Time course during storage of headspace gas composition for Iceberg lettuce packed in NVT2 bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
both sets of data available, i.e., oxygen and carbon dioxide headspace concentration. The calculated values of model’s parameters are listed in Tables 1 and 2, along with their
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
Fig. 4. Time course during storage of headspace gas composition for Iceberg lettuce packed in NVT1 bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
95% confidence interval, and they were obtained according to the following procedure. The model was first fitted to the data regarding the lettuce packed using All-PE film. In this case the oxygen and carbon dioxide permeability of the film was set to zero and the values of K1, A1 and A2 were determined. Afterwards, the above model was fitted to the data regarding the lettuce packed using the remaining three films investigated in this work. In this case the values of K1, A1 and A2 were set to that evaluated previously, whereas the oxygen and carbon dioxide permeability were evaluated. As can be inferred from the data showed in the above figures the simple model proposed in this work satisfactorily fits the experimental data. The above procedure gives a double advantage: a separate evaluation of the respiration activity of the packed pro-
321
duce, and the determination of the oxygen and carbon dioxide permeability coefficient of the investigated films in the real working conditions. As also reported in the literature, the determination of the actual mass transport properties of the packaging film is not a simple task to accomplish (Del Nobile, Fava, & Piergiovanni, 2002; Del Nobile, Buonocore, Dainelli, & Nicolais, 2004; Del Nobile, Buonocore, Altieri, Battaglia, & Nicolais, 2003a; Del Nobile, Buonocore, La Notte, & Nicolais, 2003b). The above method could be proposed as a simple way for evaluating the ability of the packaging film to control the gas exchange between the inside and the outside of the package. Fig. 5 shows the variation during storage of the ratio between the amount of oxygen consumed by the packed
Fig. 5. Variation during storage of the ratio between the moles of oxygen consumed and the moles of oxygen initially present in the package headspace, as predicted by means of Eq. (3) using the data listed in Table 1 for Iceberg lettuce. (—) OPP, (- -) NVT2, (– –) NVT1, (– –) aluminum.
Table 1 Values of model’s parameters obtained by fitting Eqs. (5) and (6) to the lettuce experimental data along with their calculated values of E% Film
A1 a
A2 b
K1 c
E%d
Aluminum
3.9804 [3.4576–37.6996]
0.2142 [0.1840–0.6132]
0.7685 [0.5168–0.8107]
5.66120
h
k .
a
Maximum oxygen consumption rate expressed as
b
Carbon dioxide respiration inhibition (dimensionless). Ratio between moles of carbon dioxide produced and moles of oxygen consumed (dimensionless). Relative percent difference, or mean relative deviation modulus (Boquet, Chirifie, & Iglesias, 1978).
c d
mL kg h
Table 2 Values of model’s parameters obtained by fitting Eqs. (5) and (6) to the lettuce experimental data along with their calculated values of E% Film
PO2a
PCO2b
E%c
OPP
4.4519e7 [2.7512e–22 to 4.7856e7]
4.1144e7 [2.0915e7 to 4.1752e7]
0.67730
NVT1
7.9678e8 [5.0486e9 to 8.4583e8]
2.3373e7 [1.3738e7 to 2.3985e7]
1.61510
NVT2
8.7487e8 [2.5115e22 to 9.2560e8]
2.4409e7 [1.7823e7 to 2.5000e7]
1.29700
a b c
Package oxygen permeability. Package carbon dioxide permeability. Relative percent difference, or mean relative deviation modulus.
322
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
produce and the initial amount of oxygen in the package headspace as predicted by means of Eq. (3) using the data listed in Tables 1 and 2 for the four investigated films. It is worth noting that the total amount of oxygen consumed by the packed produce is directly related to extent of metabolic activity (senescence level) associated to its respiration (Bottcher, Gunther, & Kabelitz, 2003). As can be inferred from the data shown in Fig. 5, the senescence level of the lettuce packed using All-PE film is constantly lower than that of the lettuce packed using the other three films; whereas, the senescence level of the lettuce packed using OPP is always the higher one. The senescence level of the lettuce packed in the two biodegradable films investigated in this work is comprised between those detected for AllPE and OPP films, being that of the film NVT1 higher than that of film NVT2. Results suggest that the different performances observed for the four investigated films are strictly related to their gas permeability coefficient. Moreover, the lettuce packed using the two biodegradable films has a senescence level lower than that detected for the lettuce packed using OPP, which is the film commercially used to pack minimally processed produces.
Fig. 6. Time course during storage of psychrotrophs in lettuce stored in the different packaging systems. The curves are the best fit of modified Gompertz equation to experimental data. (s) data in NVT1 bags, (—) Gompertz equation to experimental data with NVT1 bags; (N) data in Aluminum bags, (- - -) Gompertz equation to experimental data with Aluminum bags; (j) data in NVT2 bags, (– –) Gompertz equation to experimental data with NVT2 bags; (h) data in OPP bags, (– –) Gompertz equation to experimental data with OPP bags.
3.2. Microbiological stability The cell load of six different microbial groups (mesophilic total count, psychrotrophic total count, Enterobacteriaceae, Pseudomonas spp., yeasts and moulds) was monitored during storage. As an example in Fig. 6 the changes in psychrotrophic counts are plotted as a function of storage time for the four investigated films. The curves shown in the above figure were obtained by fitting Eq. (7) to the experimental data. As can be inferred from the data shown in the above figure, the model satisfactorily fits the data. As reported above, among the parameters appearing in the above equation S.L. is the most meaningful one. The values of S.L. are listed in Table 3 along with their 95% confidence intervals. The values related to moulds are not always reported because moulds were not recovered from the lettuce packaged in aluminum bags, in NVT2 bags and in NVT1 bags. The value of [log (cfu g1)]max used to calculate S.L. was set equal to 6 for all microbial groups; In fact at this value of microbial concentration detrimental phenomena start to occur (Barriga, Tracky, Willemot, & Simard, 1991; King, Magnusson, Torok, & Goodman, 1991). The microbiological shelf life of the lettuce packed in different investigated films was determined as the lowest value of S.L. among that calculated for each of the monitored microbial groups. As can be inferred from the data listed in the above table, the lettuce packed using All-PE has the highest microbiological shelf life, whereas the lettuce packed using OPP has the lowest one. The lettuce packed in the two biodegradable films investigated in this work shown a microbiological shelf life comprised between that of the other two films. In particular, the lettuce packed in NVT2 has a microbiological shelf life higher than that packed in the NVT1. The results reported above are in agreement with that obtained for the respiration activity, suggesting that the gas permeability of the investigated films plays a major rule in determining their quality decay kinetic during the storage of the investigated produce (McKellar et al., 2004). As for the senescence level, also the microbiological shelf life of the lettuce packed using the two biodegradable films is higher than that of the lettuce packed using OPP.
Table 3 Values of calculated shelf life by fitting modified Gompertz equation to the experimental data on spoilage microbial groups Film
Total aerobic mesophiles
Psychrot.
Enterob.
Pseudo.
Yeasts
Moulds
Shelf life
All-PE
79.78 [63.33– 111.19]
6.71 [6.34– 7.01]
55.63 [14.95– 545.32]
6.63 [5.77– 7.35]
7.06 [6.68– 7.59]
–
6.63 [5.77– 7.35]
NVT1
7.95 [7.12– 8.75]
4.58 [4.35– 4.88]
16.31 [13.40– 174.50]
4.98 [4.39– 5.54]
5.66 [5.12– 6.24]
–
4.58 [4.35– 4.88]
NVT2
8.33 [7.91– 8.81]
6.37 [5.71– 6.89]
11.92 [9.17- 17.24]
5.96 [5.67– 6.28]
6.17 [5.32– 6.94]
–
5.96 [5.67– 6.28]
OPP
8.13 [7.29– 9.19]
3.87 [3.36– 4.48]
109.36 [13.19– 3258.76]
4.51 [4.00– 4.84]
3.90 [3.17– 4.59]
131.50 [108.8– 147.25]
3.87 [3.36– 4.48]
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
3.3. Product appearance Images of the lettuce packed in the four investigated films were daily taken for the entire period of observation. As an example Fig. 7 shows the images of the packed lettuce taken at three different storage times: 0, 3 and 6 days. It is worth noting that pictures taken at time longer than 6
323
days are not shown as the longest microbiological shelf life measured is about 6 days. In agreement to the literature data (Li, Brackett, Shewfelt, & Beuchat, 2001), great correspondence was found between shelf life, calculated on microbial load limit, and changes in the product appearance. In fact, after 6 days, together with microbial spoilage, browning phenomena became to occur. Within the
Fig. 7. Images of lettuce at different times during storage.
324
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
Table 4 Hue, saturation and intensity parameters calculated on the scanned images of lettuce packaged in the different materials at zero, three and six days of storage Film
H(T0)
H(T3)
H(T6)
S(T0)
S(T3)
S(T6)
I(T0)
I(T3)
I(T6)
All-PE NVT1 NVT2 OPP
48.17 47.81 46.09 47.16
46.09 44.00 45.15 45.35
46.45 44.06 46.38 42.24
154.15 161.77 125.07 149.43
127.19 127.70 134.64 157.72
148.01 133.24 132.58 124.78
112.88 108.71 146.51 120.94
121.3 120.62 132.40 109.74
137.78 131.35 133.23 132.44
estimated shelf life the general lettuce appearance was considered visually acceptable. In order to reach quantitative information on the differences recorded among the investigated samples, hue, saturation and intensity were analyzed by Image Pro Plus software. The set of selected parameters put in evidence that no significant differences on visual quality were recorded on lettuce stored in the different packaging materials, due to the high scatter of the data (Table 4). 4. Conclusions In this work the performance of two biodegradable films to package minimally processed lettuce at 4 °C is determined. The following lettuce quality sub-indices were monitored for a period of 9 days: oxygen and carbon dioxide concentration in the package headspace, microbial load and appearance. Results indicated that the barrier properties of the investigated films, determining the oxygen concentration in the package headspace, control the rate of all the detrimental phenomena responsible for the unacceptability of the packed lettuce. Moreover, it has been observed that both the senescence level and the microbiological shelf life of the lettuce packed into the two investigated biodegradable films is longer than that observed for the lettuce packed into OPP, which is currently used to pack minimally processed lettuce. At the actual stage of our investigation the above results suggest that both biodegradable films could be advantageously used to pack minimally processed lettuce. Acknowledgement The research was funded under collaboration between University of Foggia, Agricultural Faculty, Department of Food Science and Novamont (Novara, Italy), which kindly provided the biodegradable films. References Barriga, M. I., Tracky, G., Willemot, C., & Simard, R. E. (1991). Microbial changes in shredded lettuce stored under controlled atmospheres. Journal Food Science, 56, 1586–1599. Bolin, H., & Huxsoll, C. (1991). Effect of preparation and storage parameters on quality retention of salad-lettuce. Journal of Food Science, 56, 60–62, 67.
Boquet, R., Chirifie, J., & Iglesias, H. A. (1978). Equations for fitting water sorption isotherms of foods. II. Evaluation of various twoparameters models. Journal Food Technology, 13, 319–327. Bottcher, H., Gunther, I., & Kabelitz, L. (2003). Physiological postharvest responses of common Saint-John‘s worth herbs (Hypericum perforatum). Postharvest Biology and Technology, 29, 342–350. Corbo, M. R., Del Nobile, M. A., & Sinigaglia, M. (2006). A novel approach for calculating shelf life of minimally processed vegetables. International Journal Food Microbiology, 106, 69–73. Delaquis, P. J., Stewart, S., Toivonen, P. M. A., & Moyls, A. L. (1999). Effect of warm, chlorinated water on the microbial flora of shredded iceberg lettuce. Food Research International, 32, 7–14. Del Nobile, M. A., Fava, P., & Piergiovanni, L. (2002). Water transport properties of cellophane flexible films intended for food packaging applications. Journal Food Engineering, 53(4), 295–300. Del Nobile, M. A., Buonocore, G. G., Dainelli, D., & Nicolais, L. (2004). A new approach to predict the water transport properties of multilayer films intended for food packaging applications. Journal Food Science, 69(3), 85–90. Del Nobile, M. A., Buonocore, G. G., Altieri, C., Battaglia, G., & Nicolais, L. (2003a). Modeling the water barrier properties of nylon film intended for food packaging applications. Journal Food Science, 64(4), 1334–1340. Del Nobile, M. A., Buonocore, G. G., La Notte, E., & Nicolais, L. (2003b). Modeling the oxygen barrier properties of nylon film intended for food packaging applications. Journal Food Science, 68(6), 2017–2021. Del Nobile, M. A., Baiano, A., Benedetto, A., & Massignan, L. (2006). Respiration rate of minimally processed lettuce as affected by packaging. Journal Food Engineering, 74, 60–69. Del Nobile, M. A., Licciardello, F., Scrocco, C., Muratore, G., & Zappa, M. (2007). Design of plastic packages for minimally processed fruits. Journal Food Engineering, 79, 217–224. Fonseca, S. C., Oliveira, F. A R., & Brecht, J. K. (2002). Modeling respiration rate of fresh fruits and vegetables for modified atmosphere packages: a review. Journal Food Engineering, 52, 99–119. Gimenez, M., Olarte, C., Sanz, S., Lomas, C., Echa`varri, J. F., & Ayala, F. (2003). Relation between spoilage microbiological quality in minimally processed artichoke packaged with different films. Food Microbiology, 20, 231–242. Guilbert, S., Gontard, N., & Gorris, L. G. M. (1996). Prolongation of the shelf-life of perishable food products using biodegradable films and coatings. Lebensmittel–Wissenschaft und Technologie, 29, 10–17. Jacxsens, L., Devlieghere, F., & Debevere, J. (1999). Validation of a systematic approach to design equilibrium modified atmosphere packages for fresh-cut produce. Lebensmittel–Wissenschaft und Technologie, 32(7), 425–432. Jacxsens, L., Devlieghere, F., De Rudder, T., & Debevere, J. (2000). Designing equilibrium modified atmosphere packages for fresh-cut vegetables subjected to changes in temperature. Journal Food Science Technology, 33, 178–187. Jacxsens, L., Devlieghere, F., & Debevere, J. (2002). Predictive modelling for packaging design: equilibrium modified atmosphere packages of fresh cut vegetables subjected to a simulated distribution chain. International Journal of Food Microbiology, 73, 331–341. Jacxsens, L., Devlieghere, F., Ragaert, P., Vanneste, E., & Debevere, J. (2003). Relation between microbiological quality, metabolite produc-
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325 tion and sensory quality of equilibrium modified atmosphere packaged fresh-cut produce. International Journal of Food Microbiology, 83, 263–280. Kakiomenou, K., Tassou, C., & Nychas, G. (1996). Microbiological, physiochemical and organoleptic changes of shredded carrots stored under modified conditions. International Journal Food Science and Technology, 31, 359–366. King, A. D., Magnusson, J. A., Torok, T., & Goodman, N. (1991). Microflora and storage quality of partially processed lettuce. Journal Food Science, 56, 459. Lee, D. S., Haggar, P. E., Lee, J., & Yam, K. L. (1991). Model for fresh produce respiration in modified atmosphere based on principles of enzyme kinetics. Journal Food Science, 56(6), 1580–1585. Li, Y., Brackett, R. E., Shewfelt, R. L., & Beuchat, L. R. (2001). Changes in appearance and natural microflora on iceberg lettuce treated in warm, chlorinated water and then stored at refrigeration temperature. Food Microbiology, 18, 299–308. Makino, Y., & Hirata, T. (1997). Modified atmosphere packaging of fresh produce with a biodegradable laminate of chitosan-cellulose and polycaprolactone. Postharvest Biology and Technology, 10, 247–254. Mathooko, F. (1996). Regulation of respiratory metabolism in fruits and vegetables by carbon dioxide. Postharvest Biology and Technology, 9, 247–264. McKellar, R. C., Odumeru, J., Zhou, T., Harrison, A., Mercer, D. G., Young, J. C., et al. (2004). Influence of a commercial warm chlorinated water treatment and packaging on the shelf-life of ready-to-use lettuce. Food Research International, 37, 343–354.
325
Nguyen-the, C., & Carlin, F. (1994). The microbiology of minimally processed fresh fruits and vegetables. Critical Reviews in Food Science and Nutrition, 34, 371–401. Odumeru, J. A., Boulter, J., Knight, K., Lu, X., & McKellar, R. (2003). Assessment of a thermal–chemical process to extend the shelf life of ready-to-use lettuce. Journal Food Quality, 26, 197–209. Pirovani, M., Piagentini, A. M., Guemes, D. R., & DiPentima, J. H. (1998). Quality of minimally processed lettuce as influenced by packaging and chemical treatment. Journal Food Quality, 21, 475–484. Press, W. H., Flannery, B. P., Teukolsky, S. A., & Vetterling, W. T. (1989). Numerical recipes in pascal (p. 277). Cambridge: University Press. Saracino, M., Pensa, M., & Spiezie, R. (1991). Packaged ready to eat salads, an overview. Agro Food Industry Hi-Tech, 2(5), 11–15. Szabo, E. A., Scurrah, K. J., & Burrows, J. M. (2000). Survey for psychrotrophic pathogens in minimally processed lettuce. Letters in Applied Microbiology, 30, 456–460. Tay, S. L., & Perera, C. O. (2004). Effect of 1-methylcyclopropene treatment and edible coatings on the quality of minimally processed lettuce. Journal Food Science, 69(2), 131–135. Watada, A., & Qi, L. (1999). Quality of fresh-cut produce. Postharvest Biology and Technology, 15, 201–205. Watkins, C. (2000). Responses of horticultural commodities to high carbon dioxide as related to modified atmosphere packaging. Horticultural Technology, 10, 501–506. Zwietering, M. H., Jongenburger, F. M., Roumbouts, M., & Van’t Riet, K. (1990). Modelling of the bacterial growth curve. Applied Environmental Microbiology, 56, 1875–1881.
Journal of Food Engineering 85 (2008) 317–325 www.elsevier.com/locate/jfoodeng
Use of biodegradable films for prolonging the shelf life of minimally processed lettuce M.A. Del Nobile a,b,*, A. Conte a, M. Cannarsi a, M. Sinigaglia a,b b
a Department of Food Science, University of Foggia, Via Napoli, 25 – 71100 Foggia, Italy Istituto per la Ricerca e le Applicazioni Biotecnologiche per la Sicurezza e la Valorizzazione dei Prodotti Tipici e di Qualita`, Universita` degli Studi di Foggia, Via Napoli, 25 – 71100 Foggia, Italy
Received 22 February 2007; received in revised form 22 May 2007; accepted 5 June 2007 Available online 11 September 2007
Abstract The ability of biodegradable films to prolong the shelf life of minimally process lettuce stored at 4 °C was addressed. Four different films were tested: two polyester-based biodegradable films (NVT1, NVT2), a multilayer film made by laminating an aluminum foil with a polyethylene film (All-PE), and an oriented polypropylene film (OPP). Package headspace, microbial load and colour of the packed lettuce were monitored for a period of 9 days. A simple mathematical model was used to calculate the senescence level, whereas the reparameterized Gompertz equation was used to calculate microbial shelf life of the fresh product. The fastest quality decay kinetic was observed for the lettuce packed in OPP, whereas the slowest one was detected for that in All-PE. Results suggest that the gas permeability of the investigated films plays a major role in determining the quality of the packed produce. Moreover, it was observed that biodegradable films guarantee a shelf life longer than that of OPP. Ó 2007 Published by Elsevier Ltd. Keywords: Biodegradable film; Minimally processed food; Lettuce; Shelf life
1. Introduction Minimally processed vegetables are ready to use products developed in the 1980s to respond to the emerging consumer demand for both convenience and high quality aspects. Their production process includes washing, cutting and packaging with sealed polymeric films or trays (Saracino, Pensa, & Spiezie, 1991). The gas composition within the package is modified through the respiration of the vegetable tissue. As a consequence, the oxygen (O2) from the headspace is consumed and the carbon dioxide (CO2) evolves until the creation of an equilibrium-modified atmosphere that depends on both temperature changes and gas permeability of the packaging film (Del Nobile, Baiano, * Corresponding author. Address: Department of Food Science, University of Foggia, Via Napoli, 25 – 71100 Foggia, Italy. Tel./fax: +39 881 589 242. E-mail address: [email protected] (M.A. Del Nobile).
0260-8774/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.jfoodeng.2007.06.040
Benedetto, & Massignan, 2006; Jacxsens, Devlieghere, & Debevere, 2002; Mathooko, 1996). The main spoilage mechanisms affecting the shelf life of the fresh-cut products are oxidation phenomena, due to the enzymatic activity of the cut leaves, moisture loss and proliferation of spoilage and pathogenic microorganisms (Gimenez et al., 2003; Watada & Qi, 1999). Good manufacturing handling practices and treatments with sanitizing agents in combination with proper storage conditions can concur to the extension of their shelf life (Bolin & Huxsoll, 1991; Pirovani, Piagentini, Guemes, & DiPentima, 1998). Warm chlorinated water, for example, has been shown to extend the shelf life of shredded lettuce in more than one laboratory study (Delaquis, Stewart, Toivonen, & Moyls, 1999; McKellar et al., 2004; Odumeru, Boulter, Knight, Lu, & McKellar, 2003). Also the use of modified atmosphere packaging (MAP) was reported inhibiting the detrimental phenomena, as well as reducing the respiration rate of vegetables (Fonseca,
318
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
Oliveira, & Brecht, 2002; Watkins, 2000). However, MAP often produces high levels of CO2 with a consequent development of off-flavours, due to the change to anaerobic fermentative metabolism and stimulation of potential pathogens (Jacxsens, Devlieghere, Ragaert, Vanneste, & Debevere, 2003; Kakiomenou, Tassou, & Nychas, 1996; Nguyen-the & Carlin, 1994; Szabo, Scurrah, & Burrows, 2000). Predictive modelling was used in literature for designing fresh-cut produce packaging by studying the effects of storage conditions and film permeability on the phenomena responsible for the unacceptability of the ready-to-use products (Jacxsens, Devlieghere, & Debevere, 1999; Jacxsens, Devlieghere, De Rudder, & Debevere, 2000; Jacxsens et al., 2002; Lee, Haggar, Lee, & Yam, 1991). Due to the large waste problems, in alternative to the traditional plastics used in MAP (Guilbert, Gontard, & Gorris, 1996), biodegradable films and coatings could be used (Makino & Hirata, 1997); however, due to their water vapour and gas barrier properties the application of the biodegradable films to food packaging is still limited. The effects of alginate-based edible coatings and laminate of chitosan-cellulose and polycaprolactone on minimally processed lettuce were studied to retard moisture loss and to better retained product crispness (Makino & Hirata, 1997; Tay & Perera, 2004). In this work the suitability of biodegradable films to be used as packaging materials for fresh-cut lettuce was addressed. In a previous work the authors explored the influence of low environmental impact packaging materials on the respiration rate of minimally processed lettuce (Del Nobile et al., 2006). The present study was aimed to evaluate the total quality of the product packaged in green polymers. To this purpose, one traditional package film (OPP) and two different new biodegradable polymeric matrixes (NVT1 and NVT2) were applied to fresh-cut lettuce, without modifying the packaging headspace compositions to better understand the ability of the films in preventing detrimental phenomena of the packaged product. A multilayer film (All-PE) was also used to evaluate separately the respiration activity of the packed produce, and the oxygen and carbon dioxide permeability coefficient of the investigated films in the real working conditions. The tests on quality decay kinetics of the packed produce were conducted on lettuce stored at 4 °C. Respiration rate, microbial count and colour were evaluated during storage and analyzed to calculate the product shelf life. 2. Materials and methods 2.1. Sample preparation The lettuce (Iceberg) was purchased on a local market immediately after harvesting and it was directly transported to the laboratory. After manual removing of the external leaves and core, the lettuce was shredded by knife
according to the typical size of packed salad (about 2 cm 4 cm). The shredded lettuce was then quickly washed with tap water to remove residuals, treated for 1 min with cold chlorinated (0.25 g/l) water and rinsed by immersion for another minute in tap water. The excess water was removed by centrifuging in a manual salad spinner. The investigated lettuce was packaged using four types of bags made respectively by an oriented polypropylene film (OPP), a monolayer film based on a blend of biodegradable polyesters (NVT1), a multilayer co-extruded film based on a blend of biodegradable polyesters (NVT2) and a multilayer film made by laminating an aluminum foil with a polyethylene film (All-PE). All films were kindly provided by Novamont (Novara, Italy), and were used as received. 2.2. Packaging procedure The bags were hermetically sealed and stored at 4 °C for 9 days. Samples were withdrawn three times a day for the first 2 days, two times a day for further two days and then daily analysed. The package headspace volume was about 118 cm3, the weight of packed lettuce was about 100 g, and the package area available for gas exchange was about 525 cm2. 2.3. Headspace gas composition O2 and CO2 contents of all packaged lettuce were measured using an O2 and CO2 analyser (PBI Dansensor, Checkmate 9900, Rønnedevej 18, DK-4100 Ringsted, Denmark). The volume taken from the package headspace for gas analysis was about 10 cc. To avoid modifications in the headspace gas composition due to gas sampling, each package was used only for a single determination of the headspace gas composition. 2.4. Microbiological analyses For the microbiological analyses, about 25 g of lettuce from each bags were homogenised in a Stomacher (mod. 4153-50, International PBI, Milan, Italy) for 2 min with 225 ml of sterile peptone water. Serial dilutions (1:10) of each homogenised sample were made in the same diluent and surface spread in duplicate. Total aerobic mesophiles and psychrotrophs were determined using Plate Count Agar (PCA, Merck); plate were incubated at 30 °C for 48 h and 7 °C for 10 days, respectively. The number of Enterobacteriaceae was determined using Violet Red Bile Agar (VRBA, Oxoid); plates were incubated at 37 °C for 24 h. Pseudomonades were determined by surface plating onto Pseudomonas agar base with selective supplement (Oxoid) followed by aerobic incubation at 25 °C for 48 h. In addition, Sabouraud dextrose agar (Oxoid) incubated at 25 °C for 48 h or 5 days, respectively, was used for yeasts and moulds.
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
2.5. Product appearance Images of the lettuce, packaged in the four different bags, were acquired daily with a scanner (Epson Perfection 1250). Image analysis was then performed on the scanned images at 0, 3 and 6 days of storage, to determine hue, saturation and intensity parameters by using Image Pro Plus 6 (Media Cybernetics, Silver Spring, MD, USA). Hue parameter represents the type of colour (e.g. red or yellow), saturation refers to the relative purity or the amount of grey in a colour and intensity indicates brightness. 2.6. Carbon dioxide transmission rate The CO2 permeability was measured using a gas-chromatograph HP 5890 with a Thermal Conductivity Detector (TCD), as described below. Eight bags were conditioned in a CO2 saturated ambient at 23 °C and 0% RH (Relative Humidity), according to the following conditions: three bags at 5 days; three bags at 7 days; two bags at 28 days. At the end of the conditioning time, 1 ml of the gas contained in the bags was sampled and injected in the GCTCD system. 2.7. Shelf life calculation The approach proposed by Corbo, Del Nobile, and Sinigaglia, 2006 was adopted to calculate the shelf life of packed lettuce. The following re-parameterized Gompertz equation was fitted to the experimental data: logðcfu g1 Þ ¼ ½logðcfu g1 Þmax k S:L: A exp exp ðlmax 2:71Þ þ1 A kt þ1 þ A exp exp ðlmax 2:71Þ A ð7Þ
where [log(cfu g1)]max is the decimal logarithm of the acceptability limit, S.L. is the shelf life of the packed product, A is the decimal logarithm of the maximum microbial growth attained at the stationary phase, lmax is the maximal specific growth rate, k is the lag time, and t is the time. The advantage of using Eq. (7) for estimating the shelf life of packed food instead of using the Gompertz equation as modified by Zwietering, Jongenburger, Roumbouts, and Van’t Riet (1990) is that by fitting Eq. (7) to the experimental data it is possible to directly estimate the shelf life and its confidence interval. On the other hand, by using the other approach it is not possible to estimate the shelf life confidence intervals (Corbo et al., 2006). 2.8. Statistical analysis The experimental data on respiration rate of the product, as well as the microbiological analyses, were carried
319
out in duplicate. The media and standard deviations were calculated. In fact, the data shown in the figures are the average of all repetitions, whereas the error bars are the standard deviation. The confidence intervals of model’s parameters were evaluated as follow: first, a fit was run with the original data; then, using the data points standard deviation, 100 additional fits were run on artificial data sets, which were generated by randomly varying the data around the fitted function. From these additional fits, a distribution of values for each parameter was obtained. The sets of data obtained for each parameter was statistically treated to obtain the 95% confidence interval. The image analysis was carried out for five times on each image of lettuce sample to obtain an average value for each parameter. All obtained data were submitted to one-way analysis of variance (ANOVA) and Duncan’s test (p < 0.05) and to a cluster analysis (the Euclidean distance was used as the amalgamation method) through the statistic package Statistica for Windows (Statsoft, Tulsa, USA). 2.9. Modeling In previous papers (Del Nobile et al., 2006; Del Nobile, Licciardello, Scrocco, Muratore, & Zappa, 2007) a mathematical model that predicts the respiration rate of minimally processed produce was presented. This model is summarized as follows. The Michaelis–Menten model can be simplified to the following expression: rO2 ¼ A ½O2 where rO2
ð1Þ i mL is oxygen consumption rate expressed as kg ,A h h
is the kinetic of the entire respiration process exh constant i mL pressed as kg h , [O2] is the percent oxygen concentration. A depends on temperature and, in some cases, on carbon dioxide concentration. In this work it has been assumed that A depends on carbon dioxide concentration through an exponential type expression: A ¼ A1 expfA2 ½CO2 g
ð2Þ
where [CO2] is the percent carbon dioxide concentration, h i mL A1 is the pre-exponential term expressed as kg , and it h is the maximum oxygen consumption rate, A2 is the exponential factor and accounts for the carbon dioxide induced respiration inhibition, it is dimensionless. Substituting Eq. (2) in Eq. (1) one obtain: rO2 ¼ A1 expfA2 ½CO2 g ½O2
ð3Þ
To describe the carbon dioxide consumption rate in this work it is assumed that the ratio between carbon dioxide produced and oxygen consumed that is the respiratory quotient (RQ) is constant, but not necessarily equal to one: rCO2 ¼ K 1 fA1 expfA2 ½CO2 g ½O2 g
ð4Þ
320
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
where rCO2 is carbon dioxide production rate expressed as h k mL , K1 is the ratio between the moles of carbon dioxide kg h produced and the moles of oxygen consumed, it is dimensionless. To describe the time course during storage of oxygen and carbon dioxide concentration inside the package, the mass balance on these two substances in the package headspace is written: j k nO2 ðtÞRT est p þO V st dðnO2 ðtÞÞ 2 ¼ S P O2 mp 4:615 dt ‘ 106 fA1 ½O2 expfA2 ½CO2 gg ð5Þ j k n ðtÞRT CO2 pest CO2 V st dðnCO2 ðtÞÞ ¼ S P O2 þ mp 4:615 dt ‘ 106 fK 1 fA1 ½O2 expfA2 ½CO2 ggg ð6Þ where nO2 ðtÞ is the mole of oxygen in the package head space at time t, S is the area of the package surface, P O2 is the package oxygen permeability, pest O2 is the external oxygen partial pressure, mp is the mass of the packed product, Vst is the volume of the package headspace, T is the temperature expressed in K, R is the universal gas constant, k is the film thickness, nCO2 ðtÞ is the mole of carbon dioxide in the package head space at time t, P CO2 is the package carbon dioxide permeability, pest CO2 is the external carbon dioxide partial pressure. Eqs. (5) and (6) are a set of two ordinary differential equations that was integrated numerically using the fourth-order Runge–Kutta formula (Press, Flannery, Teukolsky, & Vetterling, 1989). 3. Results and discussions
Fig. 1. Time course during storage of headspace gas composition for Iceberg lettuce packed in aluminum bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
Fig. 2. Time course during storage of headspace gas composition for Iceberg lettuce packed in OPP bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
As reported above, the quality of the investigated product depends on several quality sub-indices, among which respiration activity, microbial growth, nutritive compounds and appearance are the most important. To determine the influence of using biodegradable films with different permeability on the quality decay kinetics of fresh minimally processed lettuce the variation during storage time of the above quality sub-indices was determined. The results obtained are presented in the following. 3.1. Respiration activity Figs. 1–4 show the variation of package headspace composition during storage for the lettuce packed using the four investigated films. As expected a decrease in the headspace oxygen concentration along with an increase in the headspace carbon dioxide concentration was detected. The curves shown in the above figures were obtained by fitting the set of ordinary differential Eqs. (5) and (6) to the experimental data. It is worth noting that the set of ordinary differential Eqs. (5) and (6) was simultaneously fitted to
Fig. 3. Time course during storage of headspace gas composition for Iceberg lettuce packed in NVT2 bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
both sets of data available, i.e., oxygen and carbon dioxide headspace concentration. The calculated values of model’s parameters are listed in Tables 1 and 2, along with their
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
Fig. 4. Time course during storage of headspace gas composition for Iceberg lettuce packed in NVT1 bags. (s) percent oxygen concentration, (N) percent carbon dioxide concentration, (—) best fit of Eqs. (5) and (6) to oxygen data, (– –) best fit of Eqs. (5) and (6) to carbon dioxide data.
95% confidence interval, and they were obtained according to the following procedure. The model was first fitted to the data regarding the lettuce packed using All-PE film. In this case the oxygen and carbon dioxide permeability of the film was set to zero and the values of K1, A1 and A2 were determined. Afterwards, the above model was fitted to the data regarding the lettuce packed using the remaining three films investigated in this work. In this case the values of K1, A1 and A2 were set to that evaluated previously, whereas the oxygen and carbon dioxide permeability were evaluated. As can be inferred from the data showed in the above figures the simple model proposed in this work satisfactorily fits the experimental data. The above procedure gives a double advantage: a separate evaluation of the respiration activity of the packed pro-
321
duce, and the determination of the oxygen and carbon dioxide permeability coefficient of the investigated films in the real working conditions. As also reported in the literature, the determination of the actual mass transport properties of the packaging film is not a simple task to accomplish (Del Nobile, Fava, & Piergiovanni, 2002; Del Nobile, Buonocore, Dainelli, & Nicolais, 2004; Del Nobile, Buonocore, Altieri, Battaglia, & Nicolais, 2003a; Del Nobile, Buonocore, La Notte, & Nicolais, 2003b). The above method could be proposed as a simple way for evaluating the ability of the packaging film to control the gas exchange between the inside and the outside of the package. Fig. 5 shows the variation during storage of the ratio between the amount of oxygen consumed by the packed
Fig. 5. Variation during storage of the ratio between the moles of oxygen consumed and the moles of oxygen initially present in the package headspace, as predicted by means of Eq. (3) using the data listed in Table 1 for Iceberg lettuce. (—) OPP, (- -) NVT2, (– –) NVT1, (– –) aluminum.
Table 1 Values of model’s parameters obtained by fitting Eqs. (5) and (6) to the lettuce experimental data along with their calculated values of E% Film
A1 a
A2 b
K1 c
E%d
Aluminum
3.9804 [3.4576–37.6996]
0.2142 [0.1840–0.6132]
0.7685 [0.5168–0.8107]
5.66120
h
k .
a
Maximum oxygen consumption rate expressed as
b
Carbon dioxide respiration inhibition (dimensionless). Ratio between moles of carbon dioxide produced and moles of oxygen consumed (dimensionless). Relative percent difference, or mean relative deviation modulus (Boquet, Chirifie, & Iglesias, 1978).
c d
mL kg h
Table 2 Values of model’s parameters obtained by fitting Eqs. (5) and (6) to the lettuce experimental data along with their calculated values of E% Film
PO2a
PCO2b
E%c
OPP
4.4519e7 [2.7512e–22 to 4.7856e7]
4.1144e7 [2.0915e7 to 4.1752e7]
0.67730
NVT1
7.9678e8 [5.0486e9 to 8.4583e8]
2.3373e7 [1.3738e7 to 2.3985e7]
1.61510
NVT2
8.7487e8 [2.5115e22 to 9.2560e8]
2.4409e7 [1.7823e7 to 2.5000e7]
1.29700
a b c
Package oxygen permeability. Package carbon dioxide permeability. Relative percent difference, or mean relative deviation modulus.
322
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
produce and the initial amount of oxygen in the package headspace as predicted by means of Eq. (3) using the data listed in Tables 1 and 2 for the four investigated films. It is worth noting that the total amount of oxygen consumed by the packed produce is directly related to extent of metabolic activity (senescence level) associated to its respiration (Bottcher, Gunther, & Kabelitz, 2003). As can be inferred from the data shown in Fig. 5, the senescence level of the lettuce packed using All-PE film is constantly lower than that of the lettuce packed using the other three films; whereas, the senescence level of the lettuce packed using OPP is always the higher one. The senescence level of the lettuce packed in the two biodegradable films investigated in this work is comprised between those detected for AllPE and OPP films, being that of the film NVT1 higher than that of film NVT2. Results suggest that the different performances observed for the four investigated films are strictly related to their gas permeability coefficient. Moreover, the lettuce packed using the two biodegradable films has a senescence level lower than that detected for the lettuce packed using OPP, which is the film commercially used to pack minimally processed produces.
Fig. 6. Time course during storage of psychrotrophs in lettuce stored in the different packaging systems. The curves are the best fit of modified Gompertz equation to experimental data. (s) data in NVT1 bags, (—) Gompertz equation to experimental data with NVT1 bags; (N) data in Aluminum bags, (- - -) Gompertz equation to experimental data with Aluminum bags; (j) data in NVT2 bags, (– –) Gompertz equation to experimental data with NVT2 bags; (h) data in OPP bags, (– –) Gompertz equation to experimental data with OPP bags.
3.2. Microbiological stability The cell load of six different microbial groups (mesophilic total count, psychrotrophic total count, Enterobacteriaceae, Pseudomonas spp., yeasts and moulds) was monitored during storage. As an example in Fig. 6 the changes in psychrotrophic counts are plotted as a function of storage time for the four investigated films. The curves shown in the above figure were obtained by fitting Eq. (7) to the experimental data. As can be inferred from the data shown in the above figure, the model satisfactorily fits the data. As reported above, among the parameters appearing in the above equation S.L. is the most meaningful one. The values of S.L. are listed in Table 3 along with their 95% confidence intervals. The values related to moulds are not always reported because moulds were not recovered from the lettuce packaged in aluminum bags, in NVT2 bags and in NVT1 bags. The value of [log (cfu g1)]max used to calculate S.L. was set equal to 6 for all microbial groups; In fact at this value of microbial concentration detrimental phenomena start to occur (Barriga, Tracky, Willemot, & Simard, 1991; King, Magnusson, Torok, & Goodman, 1991). The microbiological shelf life of the lettuce packed in different investigated films was determined as the lowest value of S.L. among that calculated for each of the monitored microbial groups. As can be inferred from the data listed in the above table, the lettuce packed using All-PE has the highest microbiological shelf life, whereas the lettuce packed using OPP has the lowest one. The lettuce packed in the two biodegradable films investigated in this work shown a microbiological shelf life comprised between that of the other two films. In particular, the lettuce packed in NVT2 has a microbiological shelf life higher than that packed in the NVT1. The results reported above are in agreement with that obtained for the respiration activity, suggesting that the gas permeability of the investigated films plays a major rule in determining their quality decay kinetic during the storage of the investigated produce (McKellar et al., 2004). As for the senescence level, also the microbiological shelf life of the lettuce packed using the two biodegradable films is higher than that of the lettuce packed using OPP.
Table 3 Values of calculated shelf life by fitting modified Gompertz equation to the experimental data on spoilage microbial groups Film
Total aerobic mesophiles
Psychrot.
Enterob.
Pseudo.
Yeasts
Moulds
Shelf life
All-PE
79.78 [63.33– 111.19]
6.71 [6.34– 7.01]
55.63 [14.95– 545.32]
6.63 [5.77– 7.35]
7.06 [6.68– 7.59]
–
6.63 [5.77– 7.35]
NVT1
7.95 [7.12– 8.75]
4.58 [4.35– 4.88]
16.31 [13.40– 174.50]
4.98 [4.39– 5.54]
5.66 [5.12– 6.24]
–
4.58 [4.35– 4.88]
NVT2
8.33 [7.91– 8.81]
6.37 [5.71– 6.89]
11.92 [9.17- 17.24]
5.96 [5.67– 6.28]
6.17 [5.32– 6.94]
–
5.96 [5.67– 6.28]
OPP
8.13 [7.29– 9.19]
3.87 [3.36– 4.48]
109.36 [13.19– 3258.76]
4.51 [4.00– 4.84]
3.90 [3.17– 4.59]
131.50 [108.8– 147.25]
3.87 [3.36– 4.48]
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
3.3. Product appearance Images of the lettuce packed in the four investigated films were daily taken for the entire period of observation. As an example Fig. 7 shows the images of the packed lettuce taken at three different storage times: 0, 3 and 6 days. It is worth noting that pictures taken at time longer than 6
323
days are not shown as the longest microbiological shelf life measured is about 6 days. In agreement to the literature data (Li, Brackett, Shewfelt, & Beuchat, 2001), great correspondence was found between shelf life, calculated on microbial load limit, and changes in the product appearance. In fact, after 6 days, together with microbial spoilage, browning phenomena became to occur. Within the
Fig. 7. Images of lettuce at different times during storage.
324
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325
Table 4 Hue, saturation and intensity parameters calculated on the scanned images of lettuce packaged in the different materials at zero, three and six days of storage Film
H(T0)
H(T3)
H(T6)
S(T0)
S(T3)
S(T6)
I(T0)
I(T3)
I(T6)
All-PE NVT1 NVT2 OPP
48.17 47.81 46.09 47.16
46.09 44.00 45.15 45.35
46.45 44.06 46.38 42.24
154.15 161.77 125.07 149.43
127.19 127.70 134.64 157.72
148.01 133.24 132.58 124.78
112.88 108.71 146.51 120.94
121.3 120.62 132.40 109.74
137.78 131.35 133.23 132.44
estimated shelf life the general lettuce appearance was considered visually acceptable. In order to reach quantitative information on the differences recorded among the investigated samples, hue, saturation and intensity were analyzed by Image Pro Plus software. The set of selected parameters put in evidence that no significant differences on visual quality were recorded on lettuce stored in the different packaging materials, due to the high scatter of the data (Table 4). 4. Conclusions In this work the performance of two biodegradable films to package minimally processed lettuce at 4 °C is determined. The following lettuce quality sub-indices were monitored for a period of 9 days: oxygen and carbon dioxide concentration in the package headspace, microbial load and appearance. Results indicated that the barrier properties of the investigated films, determining the oxygen concentration in the package headspace, control the rate of all the detrimental phenomena responsible for the unacceptability of the packed lettuce. Moreover, it has been observed that both the senescence level and the microbiological shelf life of the lettuce packed into the two investigated biodegradable films is longer than that observed for the lettuce packed into OPP, which is currently used to pack minimally processed lettuce. At the actual stage of our investigation the above results suggest that both biodegradable films could be advantageously used to pack minimally processed lettuce. Acknowledgement The research was funded under collaboration between University of Foggia, Agricultural Faculty, Department of Food Science and Novamont (Novara, Italy), which kindly provided the biodegradable films. References Barriga, M. I., Tracky, G., Willemot, C., & Simard, R. E. (1991). Microbial changes in shredded lettuce stored under controlled atmospheres. Journal Food Science, 56, 1586–1599. Bolin, H., & Huxsoll, C. (1991). Effect of preparation and storage parameters on quality retention of salad-lettuce. Journal of Food Science, 56, 60–62, 67.
Boquet, R., Chirifie, J., & Iglesias, H. A. (1978). Equations for fitting water sorption isotherms of foods. II. Evaluation of various twoparameters models. Journal Food Technology, 13, 319–327. Bottcher, H., Gunther, I., & Kabelitz, L. (2003). Physiological postharvest responses of common Saint-John‘s worth herbs (Hypericum perforatum). Postharvest Biology and Technology, 29, 342–350. Corbo, M. R., Del Nobile, M. A., & Sinigaglia, M. (2006). A novel approach for calculating shelf life of minimally processed vegetables. International Journal Food Microbiology, 106, 69–73. Delaquis, P. J., Stewart, S., Toivonen, P. M. A., & Moyls, A. L. (1999). Effect of warm, chlorinated water on the microbial flora of shredded iceberg lettuce. Food Research International, 32, 7–14. Del Nobile, M. A., Fava, P., & Piergiovanni, L. (2002). Water transport properties of cellophane flexible films intended for food packaging applications. Journal Food Engineering, 53(4), 295–300. Del Nobile, M. A., Buonocore, G. G., Dainelli, D., & Nicolais, L. (2004). A new approach to predict the water transport properties of multilayer films intended for food packaging applications. Journal Food Science, 69(3), 85–90. Del Nobile, M. A., Buonocore, G. G., Altieri, C., Battaglia, G., & Nicolais, L. (2003a). Modeling the water barrier properties of nylon film intended for food packaging applications. Journal Food Science, 64(4), 1334–1340. Del Nobile, M. A., Buonocore, G. G., La Notte, E., & Nicolais, L. (2003b). Modeling the oxygen barrier properties of nylon film intended for food packaging applications. Journal Food Science, 68(6), 2017–2021. Del Nobile, M. A., Baiano, A., Benedetto, A., & Massignan, L. (2006). Respiration rate of minimally processed lettuce as affected by packaging. Journal Food Engineering, 74, 60–69. Del Nobile, M. A., Licciardello, F., Scrocco, C., Muratore, G., & Zappa, M. (2007). Design of plastic packages for minimally processed fruits. Journal Food Engineering, 79, 217–224. Fonseca, S. C., Oliveira, F. A R., & Brecht, J. K. (2002). Modeling respiration rate of fresh fruits and vegetables for modified atmosphere packages: a review. Journal Food Engineering, 52, 99–119. Gimenez, M., Olarte, C., Sanz, S., Lomas, C., Echa`varri, J. F., & Ayala, F. (2003). Relation between spoilage microbiological quality in minimally processed artichoke packaged with different films. Food Microbiology, 20, 231–242. Guilbert, S., Gontard, N., & Gorris, L. G. M. (1996). Prolongation of the shelf-life of perishable food products using biodegradable films and coatings. Lebensmittel–Wissenschaft und Technologie, 29, 10–17. Jacxsens, L., Devlieghere, F., & Debevere, J. (1999). Validation of a systematic approach to design equilibrium modified atmosphere packages for fresh-cut produce. Lebensmittel–Wissenschaft und Technologie, 32(7), 425–432. Jacxsens, L., Devlieghere, F., De Rudder, T., & Debevere, J. (2000). Designing equilibrium modified atmosphere packages for fresh-cut vegetables subjected to changes in temperature. Journal Food Science Technology, 33, 178–187. Jacxsens, L., Devlieghere, F., & Debevere, J. (2002). Predictive modelling for packaging design: equilibrium modified atmosphere packages of fresh cut vegetables subjected to a simulated distribution chain. International Journal of Food Microbiology, 73, 331–341. Jacxsens, L., Devlieghere, F., Ragaert, P., Vanneste, E., & Debevere, J. (2003). Relation between microbiological quality, metabolite produc-
M.A. Del Nobile et al. / Journal of Food Engineering 85 (2008) 317–325 tion and sensory quality of equilibrium modified atmosphere packaged fresh-cut produce. International Journal of Food Microbiology, 83, 263–280. Kakiomenou, K., Tassou, C., & Nychas, G. (1996). Microbiological, physiochemical and organoleptic changes of shredded carrots stored under modified conditions. International Journal Food Science and Technology, 31, 359–366. King, A. D., Magnusson, J. A., Torok, T., & Goodman, N. (1991). Microflora and storage quality of partially processed lettuce. Journal Food Science, 56, 459. Lee, D. S., Haggar, P. E., Lee, J., & Yam, K. L. (1991). Model for fresh produce respiration in modified atmosphere based on principles of enzyme kinetics. Journal Food Science, 56(6), 1580–1585. Li, Y., Brackett, R. E., Shewfelt, R. L., & Beuchat, L. R. (2001). Changes in appearance and natural microflora on iceberg lettuce treated in warm, chlorinated water and then stored at refrigeration temperature. Food Microbiology, 18, 299–308. Makino, Y., & Hirata, T. (1997). Modified atmosphere packaging of fresh produce with a biodegradable laminate of chitosan-cellulose and polycaprolactone. Postharvest Biology and Technology, 10, 247–254. Mathooko, F. (1996). Regulation of respiratory metabolism in fruits and vegetables by carbon dioxide. Postharvest Biology and Technology, 9, 247–264. McKellar, R. C., Odumeru, J., Zhou, T., Harrison, A., Mercer, D. G., Young, J. C., et al. (2004). Influence of a commercial warm chlorinated water treatment and packaging on the shelf-life of ready-to-use lettuce. Food Research International, 37, 343–354.
325
Nguyen-the, C., & Carlin, F. (1994). The microbiology of minimally processed fresh fruits and vegetables. Critical Reviews in Food Science and Nutrition, 34, 371–401. Odumeru, J. A., Boulter, J., Knight, K., Lu, X., & McKellar, R. (2003). Assessment of a thermal–chemical process to extend the shelf life of ready-to-use lettuce. Journal Food Quality, 26, 197–209. Pirovani, M., Piagentini, A. M., Guemes, D. R., & DiPentima, J. H. (1998). Quality of minimally processed lettuce as influenced by packaging and chemical treatment. Journal Food Quality, 21, 475–484. Press, W. H., Flannery, B. P., Teukolsky, S. A., & Vetterling, W. T. (1989). Numerical recipes in pascal (p. 277). Cambridge: University Press. Saracino, M., Pensa, M., & Spiezie, R. (1991). Packaged ready to eat salads, an overview. Agro Food Industry Hi-Tech, 2(5), 11–15. Szabo, E. A., Scurrah, K. J., & Burrows, J. M. (2000). Survey for psychrotrophic pathogens in minimally processed lettuce. Letters in Applied Microbiology, 30, 456–460. Tay, S. L., & Perera, C. O. (2004). Effect of 1-methylcyclopropene treatment and edible coatings on the quality of minimally processed lettuce. Journal Food Science, 69(2), 131–135. Watada, A., & Qi, L. (1999). Quality of fresh-cut produce. Postharvest Biology and Technology, 15, 201–205. Watkins, C. (2000). Responses of horticultural commodities to high carbon dioxide as related to modified atmosphere packaging. Horticultural Technology, 10, 501–506. Zwietering, M. H., Jongenburger, F. M., Roumbouts, M., & Van’t Riet, K. (1990). Modelling of the bacterial growth curve. Applied Environmental Microbiology, 56, 1875–1881.