Modification of water vapour transfer rate of low density polyethylene films for food packaging
Introduction
Fresh fruits and vegetables harvested seasonally in large amounts from different areas are mostly stored in suitable environments until marketed and consumed. Among the chemical and physical preservation methods, the physical ones are preferred as they necessitate the minimal processing of the produce so that it resembles its natural features to the maximum extent.
Modified atmosphere (MA) storage together with packaging are of prime importance for fresh produce (Brecht et al., 2003; Exama, Arul, Lencki, Lee, & Toupin, 1993; Fennema, 1975; Floros, 1990; Jayas & Jeyamkondan, 2002). Packaging a food product is aimed to prevent most of the possible kinds of degradation that may render it a lower quality or make it unsuitable for consumption. As a feature of proper sealed packaging films, a fresh fruit or vegetable is capable of creating a MA within the package due to the respiratory gases. The quality affecting gases oxygen, carbon dioxide, ethylene and water vapour can permeate through the packaging material at different rates (Fonseca, Oliveira, Lino, Brecht, & Chau, 2000; Kader, Zagory, & Kerbel, 1989; Mathlouthi & Leirtis, 1990).
Polyethylene is the mostly used polymer film for packaging as it offers the advantages of being inert, permeable to oxygen, carbon dioxide, ethylene and, comparatively less permeable to water vapour. Therefore, in many applications it is considered more like a barrier for water and/or its vapour on either side of the film. The water that is retained in the package may be due to a washing operation or is due to the moisture content of the food itself. If such moist foods are packaged at around room temperature, upon chilling, some of the water vapour in the package may condense (Esin, 1993).
Water activity, aw, expressed as the equilibrium relative humidity (RH) of the ambient atmosphere in contact with a food, is an easier measure for the moisture content of the food. Thus, the entire role the moisture of foods as deteriorative effects has been accounted through this parameter (Karel, 1975a; Labuza, 1980; Morillon, Debeaufort, Blond, & Voilley, 2000; Rockland & Nishi, 1980). Especially, for the industrial or household chilled storage of some foods, tight packaging films are used. These are mostly inert polymer films having some permeability to respiration gases but very low permeability to water vapour. Thus, it is expected that if the food is of intermediate moisture type, the water activity approaches to a value close to 1.0, which becomes definitely 1.0 for a high moisture food with the excess of water condensed in the package. Therefore, for the high and the intermediate moisture foods, the packaging material needs a compromise between the naturally created MA in the package and the moisture content to extend the shelf-life.
Theoretically, the steady-state water vapour transfer rate through the films is according to the Fick’s first law of diffusion written for 1-D (Treybal, 1981). Hence, in z-direction, for the fixed conditions of the pressures, temperature and humidity, the water vapour transfer will be given byHere, M is molecular weight, N is molar flux, pw, pAlm, PT is the partial, log-mean and total pressures respectively, D is the diffusivity, A is transfer area and, R and T have the usual meaning: universal gas constant and temperature.
The derivative term on the right-hand-side of Eq. (3) is the slope of the line that can experimentally be determined from the water loss with time from the system under steady-state conditions. Thus, combining Eqs. , , the permeability or transfer ability of the film represented by D will be directly proportional with the slope determined.
The study presented is aimed to modify the water vapour permeation properties of low-density polyethylene film. For these purpose two film types were prepared, (i) the solid–polyethylene composite films by using natural highly porous solids as zeolites, (ii) perforated polyethylene films. These films were tested for water vapour transfer rates and compared with that of the low density polyethylene film.
Section snippets
Materials and methods
The types of the films used in this study are given in Table 1. The solid–low density polyethylene composite films were prepared by extrusion of polyethylene beads (Petkim, Turkey) coated with hot zeolite particles of a definite size range in an industrial extruder (Dirim, Esin, & Bayındırlı, 2003). Zeolite was received from Etibank’s Bigadiç natural mineral resources region in an unprocessed form. In the region the purity is quite high, containing about 80% ore and the rocks are mainly
Results and discussion
It is apparent that for packaging of the foods in the range from low to high moisture content, a modification in the water vapour permeability of the films will be offering a wider selection spectrum to the users. To serve the purpose two different types of modified films were produced using the low-density polyethylene.
The temperature of the needle was adjusted by the trials just to be slightly greater than the melting temperature of the PE for the production of perforated PE films. This is
Conclusions
The requirements to decrease food spoilage and minimal processing with the increasing varieties of unprocessed and processed foods of different moisture contents have been effective on making packaging materials a challenging area. That is, the material spectrum available should be widened by both introduction of the new materials and modification of the existing ones for better results. In the light of this study it seems that water vapour transfer rate through one of the most widely used
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