Packaging design for potato chips

Journal of Food Engineering 47 (2001) 211±215

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Packaging design for potato chips M.A. Del Nobile * Institute of Composite Materials Technology, National Research Council, P.le Tecchio, 80-80125 Naples, Italy Received 24 March 2000; accepted 24 July 2000

Abstract For many years modi®ed atmosphere packaging (MAP) has been successfully used to prolong the shelf life of many products. One of the issues that still need to be addressed is the evaluation of the optimal head space gas composition (i.e., the initial head space gas composition that guarantees the longest shelf life to the product). An experimental evaluation of the optimal head space gas composition is usually avoided because it takes long time; instead, a rough estimation is generally made by means of empirical methods. In the present paper an evaluation of the optimal head space gas composition, based on the use of mathematical models able to predict the shelf life of the product, has been made for the speci®c case of potato chips. Two commercially used multilayer ®lms have been analyzed, for both ®lms the potato chip's shelf life can be substantially prolonged by varying the head space gas composition. In particular, replacing the currently used nitrogen with a mixture of nitrogen and water vapor, having a relative humidity ranging from 0.1% to 32%, yields an increase in the shelf life of the product as high as 80% for ®lm 1 and 43% for ®lm 2. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Shelf life; Potato chips; Modeling; Lipid oxidation; Modi®ed atmosphere packaging

1. Introduction Today, potato chips' packaging systems are designed to keep the oxygen and water partial pressures in the package head space as low as possible during storage. This is because the quality of this product depends on two factors (Quast, Karel, & Rand, 1972; Quast & Karel, 1972): the extent of lipid oxidation and the amount of sorbed water (the latter is inversely related to the crispness of the product). To satisfy to the above conditions, potato chips are currently packed using nitrogen as inert gas, and polymeric ®lms characterized by a low permeability to oxygen and water vapor. As will be discussed in more detail later, an increase in the initial value of the oxygen partial pressure in the package head space, increasing the lipid oxidation rate, leads to a decrease in the shelf life of the product. On the other hand, an increase in the initial value of the water vapor partial pressure in the package head space can yield either an increase in the product's shelf life or a decrease, depending on the factor that causes its unacceptability. The shelf life increases if rancidity is responsible for the

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unacceptability of the product, while it decreases in the other case. The objective of the present paper is to make an estimation of the potato chips' optimal head space gas composition (in terms of nitrogen and water vapor) based on the use of a mathematical model able to accurately describe the product's quality decay kinetics. The study has been carried out using two di€erent multilayer ®lms commercially used for this product: the ®rst obtained by laminating a ®lm of polypropylene with a ®lm of metallized polypropylene (from here on it will be referred to as ®lm 1); the second obtained by laminating two ®lms of polyethylene coated with polyvinylidene chloride (from here on it will be referred to as ®lm 2).

2. Modeling Lipid oxidation is a rather complex phenomenon, the rate at which it evolves depends on the oxygen partial pressure, water partial pressure and the extent of the lipid oxidation reaction. To predict the lipid oxidation reaction rate of potato chips, Quast and Karel (1972) have proposed the following empirical equation:

0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 1 1 8 - 7

212

M.A. Del Nobile / Journal of Food Engineering 47 (2001) 211±215

List of symbols pOin2

initial value of oxygen partial pressure in the package head space initial value of water vapor partial pressure in the package head space polypropylene metallized polypropylene polyethylene polyvinylidene chloride time the extent of lipid oxidation reaction at time t the weight of the packed product the water vapor partial pressure in the package head space at time t the water vapor pressure at the test temperature the oxygen partial pressure in the package head space at time t model's constant, they can be determined by ®tting the experimental data sorbed water concentration at time t the gas constant the absolute temperature the head space volume the water molecular weight the oxygen permeability of the packaging ®lm the packaging ®lm thickness the area of the package exposed to the mass ¯ux the water vapor permeability of the packaging ®lm the water vapor partial pressure outside the package the oxygen partial pressure outside the package PVdC's total thickness

in pH 2O

PP PPM PE PVdC t Ext(t) W i pH …t† 2O  pH 2O pOi 2 …t†

Ki 's c(t) R T Vst M PO2 ` A pH2 O o pH 2O pOo 2 `PVdC

1

0



!

H2 O PPP

water permeability of PP

O2 PPP

oxygen permeability of PP

N2 PPP

nitrogen permeability PP

H2 O PPE N2 PPE O2 Pfilm 1

water permeability of PE

H2 O Pfilm 1

water permeability of ®lm 1

nitrogen permeability of PE oxygen permeability of ®lm 1

QW …t†

quality sub-index related to the sorbed water at time t

max pH 2O

i threshold value of pH …t† 2O

QO …t†

quality sub-index related to the extent of lipid oxidation at time t threshold value of Ext(t) in value of pH in correspondence to which the curve 2O in representing the shelf life versus pH has a maximum 2O in shelf life of the product when pH is equal to 2O 62  10ÿ6 atm (RH ˆ 0.1%, i.e., technical nitrogen) in is equal to shelf life of the product when pH 2O in …pH † 2 O opt ……OSL ÿ SL†=SL†100 permeability of the metallized ®lm whose values are reported in Table 1 permeability of a hypothetical metallized ®lm with a thickness of the aluminum coating di€erent from the ®lm whose permeability values are reported in Table 1  metallization level ˆ Pmet =Pmet standard temperature and pressure

Extmax in † …pH 2 O opt SL OSL DSL%  Pmet Pmet a STP

H2 O

pOi 2 …t† : K3 ‡ K4 pOi 2 …t†

…1†

K  5  ‡ K6 : i  100 ln pH2 O …t†=pH 2O

…2†

Making a mass balance on the oxygen and the water vapor contained in the package head space, the following equations are obtained: i o i dpH …t† pH O ÿ pH2 O …t† 2O ˆ APH2 O 2 dt ` 0 1 , K5 W B Vst C ÿ @ h  i2 A; RT i i  100MpH2 O …t† ln pH2 O …t†=pH2 O

‡

pOo ÿ pOi 2 …t† APO2 2 ` 2

! 0 ÿ9

RT 6 W …44:615  10 † B 4 @Ext…t† Vst 3600 1

The same authors have reported that water sorption isotherm of potato chips can be successfully described through the Khun's isotherm (Quast & Karel, 1972): c…t† ˆ

PE's total thickness PVdC's permeability PE's permeability

dpOi 2 …t† RT ˆ dt Vst

dExt…t† K1 ‡ K2 Ext…t† C B ˆ W @Ext ‡ q A dt i  …p …t†=p †100 H2 O

`PE PPVdC PPE

…3†

K1 ‡ K2 Ext…t† C ‡ q A i  …pH2 O …t†=pH2 O †100

pOi 2 …t† K3 ‡ K4 pOi 2 …t†

!

3 7 5: …4†

Eqs. (1), (3) and (4) represent a set of three ordinary di€erential equations with three unknown variables (i.e., i …t†, pOi 2 …t†). Once the initial condition (i.e., Ext(t), pH 2O the packaging condition) is known, the di€erential equations can be numerically solved (Press, Flannery, Teukolsky, & Vetterling, 1989). In this way it is possible to predict the kinetic decay of the two sub-indices during storage, and consequently the shelf life of the product. 3. Materials Potato chips storage simulations have been made for a bag type package with the following characteristics:

M.A. Del Nobile / Journal of Food Engineering 47 (2001) 211±215

the area of the package exposed to the mass ¯ux is equal to 400 cm2 ; the head space volume is equal to 1435 cm3 ; the weight of the product is equal to 65 g. The values of the water and oxygen permeability coecients of the ®lms considered in the present investigation are reported in Table 1. To determine the water vapor permeability of ®lm 1 it has been assumed that H2 O PPP H2 O PPE

ˆ

N2 PPP N2 PPE

and H2 O O2 PPP PPP ˆ : H2 O O2 Pfilm Pfilm 1 1 N2 H2 O O2 N2 O2 , PPE , Pfilm The values of PPP 1 , PPE and PPP are reported in the literature (Del Nobile, Mensitieri, Aldi, & Nicolais, in press; Myers, Meyer, Rogers, Stannett, & Szwarc, 1961; Rogers, Meyer, Stannet, & Szwarc, 1956). In this way only a rough estimation of water vapor permeability of ®lm 1 can be obtained; however, it can be regarded as acceptable considering the objective of the present paper. To determine the water vapor and oxygen permeabilities of ®lm 2 the following equation has been used:   `PVdC 1 `PE 1 ‡ : …5† P ˆ1 ` PPVdC ` PPE

The storage and packaging conditions considered for the simulations are: the storage temperature is equal to 37°C; the oxygen partial pressure outside the package is equal to 0.21 atm; the water vapor partial pressure outside the package is 24:8  10ÿ3 atm (RH ˆ 40%); the gas ¯ushed into the package is composed by humidi®ed nitrogen with a humidity ranging from 0.1% to 32%; the initial value of Ext is equal to zero.

4. Results and discussion

213

uct can be described through two quality sub-indices de®ned as: QW …t† ˆ 1 ÿ

i pH …t† 2O ; max p H2 O

…6†

QO …t† ˆ 1 ÿ

Ext…t† : Extmax

…7†

max is equal As reported in the literature the value of pH 2O to 20  10ÿ3 atm (Quast & Karel, 1972), while that of Extmax is equal to 1200 …ll O2 …STP††=g (Quast & Karel, 1972). The product became unacceptable wherever one of the two sub-indices exceeds its threshold value. In the hypothetical case where the product's quality was related only to QW …t†, the shelf life of the product in (curve A in Fig. 1); on the would decrease with pH 2O other hand if the product's quality was related only to QO …t†, the shelf life of the product would increase with in (curve B in Fig. 1). The curve representing the pH 2O in is obtained, by de®nition, by ``real'' shelf life versus pH 2O superimposing curves A and B, and is shown in Fig. 2. It in equal to 0.0146 shows a maximum (114.5 days) at pH 2O atm. According to the data reported in Fig. 2, and considering that nitrogen, the inert gas currently used to package potato chips, has only small traces of water in lower than 0.0146 atm), vapor (i.e., it has a value of pH 2O substituting nitrogen with a mixture of water vapor and in lower than nitrogen, characterized by a value of pH 2O 0.0146 atm, yields an increase in the product's shelf life. in corresponding to a maximum of the The value of pH 2O curve reported in Fig. 2, represents the optimal head space water concentration. Its value depends on the storage and initial conditions and on the permeability and selectivity to water vapor and oxygen of the packaging ®lm. In order to evaluate the in¯uence of the ®lm in † , its value has been evaluated permeability on …pH 2 O opt for `PVdC ranging from 1 to 6 lm. Increasing the value of `PVdC , the multilayer's permeability decrease, while its

As reported above, the quality of potato chips depends on the amount of sorbed water and on the extent of lipid oxidation reaction (Quast et al., 1972; Quast & Karel, 1972). Henceforth, the total quality of the prodTable 1 Values of the water and oxygen permeability coecients of the ®lms used in the present investigation

a b

Film type

PH2 O (mole cm/cm2 s atm)

PO2 (mole cm/cm2 s atm)

PVdC PE Film 1

3.47 ´ 10ÿ13 a 52.186 ´ 10ÿ12 a 3.48 ´ 10ÿ13 b

5.041 ´ 10ÿ15 1.897 ´ 10ÿ12 15.468 ´ 10ÿ15

Data from Yasuda and Stannet (1975). Data from Del Nobile et al. (in press).

a a b in Fig. 1. Shelf life as a function of pH for ®lm 2; the thickness of PE 2O layers is equal to 40 lm, the thickness of PVdC layers is equal to 4 lm.

214

M.A. Del Nobile / Journal of Food Engineering 47 (2001) 211±215

Fig. 4. SL, OSL and DSL% as a function of `PVdC for ®lm 1.

in Fig. 2. Shelf life as a function of pH for ®lm 2; the thickness of PE 2O layers is equal to 40 lm, the thickness of PVdC layers is equal to 4 lm.

sures can cause either an increase or a decrease in the lipid oxidation rate, as evident from Eq. (3). Henceforth, it is not possible, on the basis of simple intuition, to predict the e€ect of an increase of `PVdC on the lipid oxidation rate. The evidences suggest that for values of `PVdC lower than 2.5 lm the e€ect related to water vapor is dominant, while for higher values of `PVdC the contrary is true.

5. Conclusions Fig. 3. SL, OSL and DSL% as a function of `PVdC for ®lm 2.

mechanical properties and permselectivity to oxygen and water vapor are substantially unmodi®ed. In Fig. 3 are shown three curves: the ®rst represents SL plotted as a function of `PVdC ; the second represents OSL plotted as a function of `PVdC ; the third represents DSL% plotted as a function of `PVdC . As reported in Fig. 3, OSL is always higher than SL; moreover, a value of DSL% as high as 48% has been obtained for a value of `PVdC equal to 4:4  10ÿ4 cm. The potato chips storage simulations made for ®lm 2 have been repeated for the ®lm 1. In Fig. 4 are reported SL, OSL and DSL% as a function of the metallization level. The metallization level is a measure of the thickness of the aluminum coating. Similar to results from ®lm 2, OSL is always higher than SL, in this case a value of DSL% as high as 80% as been obtained. Contrary to what one would expect, the curves representing SL versus `PVdC (Fig. 3), and SL versus a (Fig. 4), have a minimum. In fact, increasing the value of `PVdC (or alternatively a) the water vapor and oxygen permeabilities are reduced by about the same amount. This reduces the rate at which both substances penetrate into the package and consequently reduce the rate at which their respective partial pressures increase inside the package during storage. As reported previously, a reduction in the water vapor and oxygen partial pres-

The use of predictive mathematical models has been reported to be particularly advantageous to design packaging systems. In order to show the e€ectiveness of such an approach, the determination of the optimal head space gas composition of a bag type package for potato chips has been addressed in the present paper. In particular, two commercial multilayer ®lms have been analyzed. For both multilayer ®lms a substantial increase in shelf life can be obtained by replacing pure nitrogen with a mixture of water vapor and nitrogen having the appropriate composition. An increase in the shelf life as high as 80% has been obtained, demonstrating the advantages that can be obtained when the design of a given packaging system is based on the use of a predictive mathematical model.

References Del Nobile, M. A., Mensitieri, G., Aldi, & A., Nicolais, L. (1999). The transport mechanism of gases through metallized ®lms intended for food packaging application. Packaging Technology and Science, 12, 261±269. Myers, A. W., Meyer, J. A., Rogers, C. E., Stannett, V., & Szwarc, M. (1961). Studies in the gas and vapor permeability of plastic ®lms and coated papers. Tappi, 44(1), 58±64. Press, W. H., Flannery, B. P., Teukolsky, S. A., & Vetterling, W. T. (1989). Numerical Recipes in Pascal (pp. 602±607). Cambridge: Cambridge University Press. Quast, D. G., Karel, M., & Rand, W. M. (1972). Development of a mathematical model for oxidation of potato chips as a function of

M.A. Del Nobile / Journal of Food Engineering 47 (2001) 211±215 oxygen pressure, extent of oxidation, and equilibrium relative humidity. Journal of Food Science, 37, 673. Quast, D. G., & Karel, M. (1972). Computer simulation of storage life of foods undergoing spoilage by two interacting mechanisms. Journal of Food Science, 37, 679±683. Rogers, C., Meyer, J. A., Stannett, V., & Szwarc, M. (1956). Studies in the gas and vapor permeability of plastic ®lms and coated papers.

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Part I: Determination of the permeability constant. Tappi, 39(11), 737±741. Yasuda, H., & Stannett, V. (1975). Permeability coecients. In J. Brandrup, & E. H. Immergut (Eds.), Polymer Handbook. New York: Wiley.