Determination of film requirements and respiratory behaviour of fresh produce in modified atmosphere packaging

Postharvest Biologyand Technology Postharvest

ELSEVIER

Biology and Technology 6 (1995) 41-54

Determination of film requirements and respiratory behaviour of fresh produce in modified atmosphere packaging G.B.Y. Christie a**,J.I. Macdiarmid a*‘, K. Schliephake at2, R.B. Tomkins b a Division

of Material Science and Technology C.S.I.R.O., Private Bag 33, Rosebank MDC, Clayton, EC. 3169, Australia b Institute for Horticulture Development, Knoxjeld, Victorian Depaflment of Agriculture and Rural Affairs, I?O. Box 174, Femtree Gully, Vie. 3156, Australia Accepted 1 December 1994

Abstract

A simple technique was developed for determining the respiration rate of produce stored in modified atmosphere packaging (MAP) over the entire storage period including the initial time when the produce responds to the modified atmosphere. It is based on a material balance equation that relates the package film permeability and produce metabolism to the in-package gas concentrations. The equation is particularly useful for determining basic metabolic processes of produce such as respiration and ethylene biosynthesis in actual modified atmosphere packages, even when steady state conditions do not exist. Alternatively, when the respiration rate is known, the film permeability required for MAP can be determined. The work reported here shows there is significant difference between permeability values measured by the ASTM Dow cell method and the mixed gas cell method. It was found that the film permeability must be measured by a mixed gas cell method for the technique to give realistic predictions. When film permeability is determined using this method at the conditions under which the produce is to be stored, it can be used directly in the equation without any need for corrections. The material balance and permeability measurement were tested by determining the metabolic activity of broccoli (cv. ‘Marathon’) stored under MAP (6% CO*, 1.5% Or) at 1S”C. The steady state oxygen consumption, carbon dioxide production and ethylene biosynthesis were 8.8 ml kg-’ h-l, 9.0 ml kg-’ h-’ and 0.04 ~1 kg-’ h-i, respectively. These measurements agree with published and measured values. Keywords:

Modified-atmosphere

packaging; Overwrapping;

Film technology; Broccoli; Pack-

aging * Corresponding author. Fax: 03 544 1128. ’ Present address: Dept. of Psychology, Univ. of Leeds, Leeds LS2 9JT, UK * Present address: Dept. of Chemistry, Swinburne Inst. of Technology, Hawthorn, Vie. 3122, Australia Elsevier Science B.V. SSDI 0925-5214(94)00053-O

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G.B.Y Christieet al. /PostharvestBiologyand Technology6 (1995) 41-54

1. Introduction

The accumulation of carbon dioxide and depletion of oxygen to beneficial levels by the application of modified atmosphere packaging (MAP) is known to extend the post-harvest life of many horticultural products (Zagory and Kader, 1988; Kader et al., 1989). MAP can be difficult to implement commercially because of inconsistent produce respiration rates, temperature changes during the handling-delivery chain and non-ideal packing leading to variability around the average gas atmosphere where some packages may be anaerobic and others safe. MAP can be achieved by use of polymeric films where gas transmission rate through the film, product respiration, carbon dioxide and oxygen levels within the package are related by a simple material balance (Henig and Gilbert, 1975). There have been several mathematical models based on this material balance developed for MAP applications (Henig and Gilbert, 1975; Deily and Rizvi, 1981; Zagory and Kader, 1988; Cameron et al., 1989; Mannapperuma et al., 1989; Zagory et al., 1989). Tailoring the polymer film properties, predicted by this material balance, to match the respiration behaviour of produce subjected to changes in modified atmosphere and temperature during storage, is the challenge presented by this field (Zagory and Kader, 1988). Alternatively, a fully characterised polymer film can be used in conjunction with the material balance equation to calculate directly the respiration behaviour of produce under MAP. Polymer film permeability is dependent on many factors including gas-polymer solubility, temperature (Rogers, 1985) and microporosity (Mannapperuma et al., 1989). Significant variation in film permeability occurs within individual polymer types (Cambellick, 1985). Therefore, permeability determination of the specific film under the envisaged storage conditions is a requirement for effective use of the material balance equation and models based upon it. The objectives of this work were to test the material balance using actual storage data and determine the best film permeability measurement technique. Under cold storage conditions the time-dependent permeability behaviour of two polyethylene packaging films were characterised. These films were used to package broccoli and determine its metabolic behaviour using the material balance. This allowed oxygen consumption, carbon dioxide production and ethylene biosynthesis to be monitored throughout the storage period, including the initial period in which the produce succumbs to the MAP control. One film allowed aerollc and the other anaerobic processes to be monitored. 2. Materials and methods

Film permeability Two films were tested. The films were not micro-perforations or ventilation holes. They pm film, Cl, and a 25 f 3 pm experimental polyethylene (LDPE) films impregnated with

microporous and did not contain any were a commercially available 35 f 3 film, DF155. Both were low density inorganic particles. They were tested

G. B.Y Christie et al. /Postharvest Biology and Technology 6 (1995) 41-54

43

for oxygen, carbon dioxide and ethylene permeability. The films were shown not to scavenge any ethylene. The permeabilities of the films were measured by two techniques. A Dow cell method described in ASTM D1434-82 (Storer, 1990) was used to determine the film permeability at 22°C and in dry conditions. The applied gas pressure used in the Dow cell was 9.30 x lo3 Pa. A second technique which used a mixed gas cell was developed to measure the film permeability under storage conditions of high humidity and low temperature. It was also used at 22°C and 60% relative humidity (RH) to determine the initial permeability for comparison with the Dow cell measurement. The technique involved placing a piece of film over the open face of a 300-ml hemispherical glass dome. The apex of the dome had a conical B24 ground glass opening to allow both sampling through a septum fitting and flushing of the atmosphere inside the dome. The film was sealed against the ground glass flange (I.D. = 10.7 cm) using ample silicone grease. The seal was inspected regularly and any data indicating a suspected leak were discarded. Three replicates were performed for each permeability measurement. Before measurement, the container was flushed with a carbon dioxide, oxygen, ethylene and nitrogen mixture, typically 15%, l%, 3 ,~l 1-l and 84%, respectively. Over a period of 6 h the depletion of carbon dioxide and ethylene and accumulation of oxygen were measured. Gas samples were taken from the cell at l-h intervals by removing 1 ml of the sample gas and simultaneously replacing this with 1 ml of air. Oxygen and carbon dioxide concentrations were determined by thermal conductivity detector (TCD) gas chromatography. Argon was used as the carrier gas so as to avoid argon interference with the oxygen determination. The carrier gas flow was 45 ml min- I. The injector and column temperatures were 100 and 8o”C, respectively. The column was an Alltech@ CRT1 dual column with molecular sieve 5A and Porapak Q 80/100 mesh. Ethylene concentrations were determined by flame ionisation detector (FID) gas chromatography. The carrier gas was nitrogen. The flow rate was 50 ml mm-‘. The injector and column temperatures were 180 and 150°C respectively. The column was 2 m x 3.125 mm stainless steel tube packed with alumina Fl 80/100 mesh. The transmission rates of the gases through the film were calculated by the differential of the change in gas concentration with time, using the numerical method of differentiation developed by Savitzky and Golay (1964). The mixed gas cells were used under simulated storage conditions. The conditions external to the cells were 1.5 f 0.5”C and 70 f 10% RH. To simulate the high RH generated within produce packages during storage, 10 ml of free water was introduced into the cell. Time-dependent behaviour of the films was measured by storing the films on the gas cells for one, two, three and eight days before permeability was measured. This provided information on the time period required for the film permeability to reach a steady state under typical storage conditions where the film adjusts to the temperature and the water environment. Broccoli metabolism under MAP

Broccoli (cv. ‘Marathon’) was hand-harvested, the stems trimmed to approximately 12 cm in length, forced air cooled to approximately 2°C overnight and stored

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G.B.X Christie et al. /Postharvest Biology and Technology 6 (1995) 41-54

at 0°C for two days before being packaged into the films Cl and DF155 (described in the section above) and stored at 1.5 f 05°C for 27 days. The films were used as sealed bags (lay-flat 900 x 1000 mm) inside waxed boxes (300 x 300 x 500 mm). Each bag contained 6.2 f 0.8 kg of broccoli and some air was squeezed out of each bag in a consistent manner before it was sealed air-tight to provide 1.0 f 0.05 m2 of area and 19.5 f 0.6 1 of void volume. The void volume was later determined by subtracting the volume of broccoli from the total volume within the film as measured by water displacement. There were five replicates of each treatment. During the storage period, gas samples were taken using a l-cm3 air-tight syringe through septa secured to the films (Ton&ins and Cumming, 1988), daily for the first four days then every three days for the remaining storage time. Gas relative concentrations were determined by gas chromatography. Error analysis All error limits quoted and shown on the diagrams are the 90% confidence limits calculated using the sample standard deviation and the Student’s t critical point. Mathematical model for MAP To determine the production rate of gases under MAP the basic material balance given in Eq. 1 can be used (Henig and Gilbert, 1975). Production rate = Accumulation

+ GTR

(1)

Production rate is the rate gas is produced or consumed by the produce in the package. Accumulation is the concentration increase or decrease within the package. Gas transmission rate (GTR) is the rate gas transfers through the package wall. Production rate, accumulation and GTR all have the units of mol s-l. The GTR is calculated from the experimentally determined film permeability and gas concentration in the package. It is defined by the permeability, film thickness, film area and the concentration difference of gas across the film as given in Eq. 2. GTR __Px(C,-C,)xAx

Const.l L

(2)

where P is the permeability (mol m-l s-l Pa-‘), C, is the concentration of the gas in the atmosphere surrounding the package (%), C, is the concentration of the gas in the package (%), A is the area of the film (m2), L is the thickness of the film (m), and Const. 1 is calculated from the units to be 1013.3 Pa. Accumulation is defined by Eq. 3. Accumulation

= $$

x V x Const. 2

(3)

where dC,/dt is the change of gas concentration in the package (%) with time (h), V is the free volume in the package (l), and Const. 2 is calculated from the units to be 1.24 x 1O-7 mol 1-l. From the above equations the production or consumption rate of the gases in the packages can be calculated in the units of mol s-l. This can be converted into

G.B.Y Christie et al. /Postharvest Biology and Technology 6 (1995) 41-54

45

ml kg-* h-’ by th e multiplication of the constant 8.0676 x lo7 ml s mol-’ h-’ and division by the mass of the product (kg). 3. Results and discussion Mixed gas cell method The mixed gas cell technique allowed the permeability

of CO?, 02 and C2H4 to be measured simultaneously. The basis of the permeability measurement is the gas concentration in the cell which changes as gas permeates in and out. Fig. la, b shows typical cell concentration curves for CO2 and C2H4 permeating through film DF155 into the surrounding air. It is expected that this technique will provide realistic measurements for MAP calculations because the gas, temperature and humidity conditions can be adjusted to match actual MAP conditions. The initial gas compositions used for the measurements were similar to the compositions expected in the packages, but within reasonable limits (i.e., 5-20% COz, l-10% 02, and 0.5-10 ppm C2H4) this was shown not to be an important

(a)

(b)

0.6

0.4

4

6

12

Time (hours)

Fig. 1. Permeant concentration with time in mixed gas cell used to calculate film permeability at storage conditions(l.YC, 100%RH). (a) CO2 using three initial concentrations and the DF155 film after one day; (b) C2H4 using one initial concentration and the Cl film after eight days.

G.B. Y Christieet aL I PosthamestBiologyand Technology6 (1995) 41-54

46

criterion. Fig. la shows the CO2 concentration curves for three different initial concentrations at 15°C in the mixed gas cell, immediately after flushing and using DF155 film conditioned by one day simulated storage at 1.5”C. The same sample of film was used to generate the three curves by differing the initial flushing time with CO2. The differential of these curves determined numerically at 2 h gives film permeabilities of 6.32 x 10-15, 5.97 x 1O-15 and 5.81 x lo-l5 mol m-l s-l Pa-’ (ave. = 6.03 f 0.44 x lo-l5 mol m-l s-l Pa-‘) indicating that the effects of initial gas composition are small. The decrease of ethylene concentration at 1.5”C in the mixed gas cell immediately after flushing and using the film Cl conditioned by eight days simulated storage at 1.5”C is shown in Fig. lb. Ethylene permeability of Cl was determined from this and similar curves and was found to be 1.83 f 0.29 x 1O-15 mol m-l s-l Pa-‘. The experimental scatter with the mixed gas permeability technique was suitable for calculating respiration rates from the above equations with reasonable confidence limits. The permeability 90% confidence limit was found to vary depending on the film and gas type. For example, for film Cl the 90% confidence limit was f15% for all three gases and for film DF155 the 90% confidence limit was f9% for CO2 and 02, and f6% for ethylene. The CO2 and 02 permeabilities for the two films by two methods (Dow and mixed gas cell) are given in Table 1 and the ethylene permeabilities for the two films are given in Table 2. Table 1 also shows the effect of humidity on film permeability.

Table 1 Comparison of Dow cell and mixed gas cell permeabilities and CO2 to 02 permeability ratio at 22 and 1.5”C (after an eightday equilibration period) under varying humidity conditions Film

Temp. (“C)

Permeability (mol me1 s-l Pa-‘) Mixed gas cell

Dow cell co2/02

co2

co2/02

co2

ratio DF155 Cl DF155 DFl55 Cl

22 22 1.5 1.5 1.5

0% RH: 3.91 rt 0.07 x lo-” 0% RH: 2.38 f 0.10 x lo-l5 -

ratio

3.9 4.7

60% 60% 0% 100% 100%

-

RH: RH: RH: RH: RH:

7.16 f 4.70 f 3.72 f 2.82 f 1.46 *

0.65 0.79 0.34 0.26 0.25

Table 2 Film permeability to ethylene measured using the mixed gas cell method Film

Conditions

Permeability (mol m-l s-l Pa-‘)

DFl55

60% 100% 60% 100%

6.00 f 3.32 f 4.07 f 1.83 f

Cl

RH, 22°C RH, 1.5”C RH, 22°C RH, 1.5”C

0.34 0.19 0.63 0.29

x x x x

lo-” lo-l5 lo-l5 lo-l5

x x x x x

lo-” lo-l5 lo-” lo-” lo-l5

4.1 4.7 3.1 3.4 5.9

G.B.Y Christie et al. I Postharvest Biology and Technology 6 (1995) 41-54

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Dow cell method

The Dow cell technique measures film permeability using a pressure and volume measurement which results in reduced experimental scatter. The 90% confidence limit of the Dow cell technique here was f6%. The drawback is that gases can only be measured singly and only at 0% RH. Therefore, the permeability variation due to mixed gas interactions cannot be measured. The importance of this failing is demonstrated in Table 1, where 0% RI-I is shown to increase the permeability over 100% RH for DF155. Also the permeability values from the Dow cell, measured at 0% RH, were consistently lower than those measured using the mixed gas cell, even though the lower RH would be expected to increase the permeability. This phenomenon is consistently observed in this laboratory and it is proposed here that the mixed gas technique is the better technique to use for determining respiration rate. A possible explanation for the observed permeability increase with the mixed gas cell is that absorbed gases may change the film properties such as plasticity. The adsorption of gas into the polymer is a necessary step in the overall permeation process of gas (Rogers, 1985). In the mixed gas cell the polymer will absorb N2,02, CzH4, Hz0 and CO2, whereas in the Dow cell the polymer will absorb either 02 or CO2 singly. Time-dependent permeability effects

The conditioning of the films significantly affects the permeability. In practice films are taken from room condition, simulated here by 22°C and 60% RH, packed with produce and stored in a cool room, simulated here by 15°C and 100% RH on one side and cool room conditions on the other. Significant time-dependent permeability was observed with the polymers subjected to this simulated storage pattern. Polymer permeability to carbon dioxide and oxygen decreased over the first three to four days until a steady state was reached (Fig. 2). This was due to the film adapting to the low temperature and the high humidity. The effect of humidity only is shown in Table 1 where the permeability of DF155 was shown to decrease in the high humidity conditions. This decrease is not due to condensation on the film surface because this did not occur during measurement. The change in temperature and humidity also affects the CO;! to 02 permeability ratio. For DF155 and Cl, measured in the mixed gas cell under 60% RH at 22”C, the ratios were 4.1 and 4.7, respectively (Table 1). The expected ratio for LDPE is about 5 (Mannapperuma et al., 1989) and the lower value for the DF155 probably arises from internal cavities built into the new film. After eight days in the storage conditions these ratios for DF155 and Cl had changed to 3.4 and 5.9, respectively (Table 1). Based on the activation energy of 02 and CO2 in polyethylene, it would be expected that the ratio for polyethylene would decrease slightly with decreasing temperature. The larger than expected decrease with DF155 may be explained by the flow through the internal cavities (which has a ratio of 0.8; Mannapperuma et al., 1989) because this flow is not significantly affected by the decreased temperature, whereas the flow through the polymer (which has a ratio of 5) is significantly

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G.B.Y. Chrirtie et aL IPosthatvest Biology and Technology 6 (1995) 41-54

DF155.

DFl55,

2 Storage

3

4 Period

5

6

7

CO,

0,

8

(Days)

Fig. 2. Time-dependent CO2 and 02 permeability of DF155 and Cl measured under simulated storage conditions (1.5”C and 100% RH) using mixed gas cell.

reduced, leading to an overall decrease in the ratio. The ratio for DF155 is only marginally affected by the humidity at 15°C. The reason for the large ratio increase of film Cl may be related to the change in humidity and the hydrophilic nature of the film. The anti-fogging properties of Cl result from the high oxygen content of a hydrophilic additive which causes water to form as a film on the polymer rather than as droplets. A second property related to the high oxygen content of the additive is that water is readily absorbed into the polymer as indicated by a 2.5% increase in weight through moisture uptake after three days in 100% RH at 1.5”C. Henry’s law constant for 02 and CO2 in water at 25°C is 2.58 x lo9 and 7.377 x lo7 Pa (mole fraction)-’ respectively, indicating that CO2 is 35x more soluble than 02 in water. The higher solubility in the internal water may be selectively enhancing the transport of CO2 through the polymer. Condensation effects A criticism of the technique developed here is that condensation that forms on the surface of the film during storage might have affected the permeability in an inconsistent and uncontrollable way. However, this is not the case when the polymer is free from ventilation holes or micro-perforations. The rate of diffusion of 02 and CO2 through water is significantly greater than its rate of permeation through polymer. For example, the diffusivities of 02 and CO2 in water at 20°C are 7.39 x lo-l3 and 7.27 x lo-l3 mol m-l s-l Pa-’ (Foust et al., 1980), respectively,

G.B.Y Christie et al. /Postharvest Biology and Technology 6 (1995) 41-54

49

and their permeabilities in LDPE are approximately 6 x lo-l6 and 3 x lo-l5 mol m-l s- ’ Pa-‘, respectively. Using these values, a 50 pm LDPE film with a 1 mm thick water layer on it is calculated to have the same 02 transmission rate as a 50.8 ,um LDPE film without a water layer. Therefore, condensation droplets would not have a significant effect on the overall permeability. When the majority of gas flow is through micro-perforations or ventilation holes, condensation on or near these holes may significantly affect the overall GTR. However, it should be noted that in general it is observed that condensation does not form around ventilation holes greater than 0.5 mm diameter because significant moisture loss occurs through these holes, thus keeping the humidity in the vicinity below the condensing humidity. Therefore, in packages which do not contain micro-perforations or ventilation holes (as is the case with Cl and DF155), the presence of condensation will not affect the material balance. Calculation of broccoli metabolism under MAP

The COz, 02 and C2H4 concentrations for broccoli stored at 1.5”C in the two films are given in Fig. 3a, b. These curves are used along with the mixed gas cell permeability curves (Fig. 2) to calculate the respiration rate and ethylene biosynthesis curves shown in Fig. 4a, b. This involves taking the differential of the gas concentration curves (Fig. 3a, b) at given times and film permeability determined at the corresponding times. The concentration curves in Fig. 3a, b do not display the smooth exponential change as predicted by the existing mathematical models (Henig and Gilbert, 1975; Deily and Rizvi, 1981; Cameron et al., 1989). The CO2 and C2H4 curves presented here display a characteristic maximum similar to those reported by other workers (Henig and Gilbert, 1975; Rij and Stanley, 1987). Similarly, the 02 curves display a minimum at approximately six to nine days. Gas levels in the Cl and DF155 packages (Fig. 3) indicate that the more permeable and thinner DF155 film provides a modified atmosphere of much lower CO2 and slightly higher 02 concentration than the thicker, less permeable Cl film. After four days the CO2 and 02 levels in Cl packages were 16 and 0.75%, respectively. The CO2 concentration continued to increase to over 24% indicating a shift to anaerobic respiration. This was confirmed by high respiratory quotient (RQ) values. The 90% confidence limits for these respiration curves are not shown in Fig. 4 for the sake of clarity. For the CO2 and 02 they ranged from f20% to f25% apart from film Cl where the CO2 ranged from f30% to f50%. For ethylene they ranged from f25% to f50% for film DF155 and from f35% to flOO% for film Cl. The large error associated with CO2 and ethylene in Cl is related to the variation of these gases in the replicate packages and presumably results from inconsistencies in the broccoli synthesising these gases under anaerobic metabolism. Aerobic metabolism

The CO2 production rate of the broccoli stored in DF155 can be seen in Fig. 4a to decrease 36% from 13.8 to 8.8 ml kg-’ h-* over the initial 14-day storage period as the produce adjusted to the modified atmosphere generated in the package,

50

G.B. Y Chktie et al. I PostharvestBiologyand Technology6 (1995) 41-54

,

6

12

18

30

24

Storage Period (days)

(b)

6

72 Storage

18 Period (days)

24

G. B.Y Christie et al. I Postharvest Biology and Technology 6 (1995) 41-54

6

12 Storage

18 Period

24

51

30

(days)

DF155

Storage Period (days)

Fig. 4. Calculated produce metabolism of broccoli in sealed DF155 and Cl packages during storage at 15°C. Calculated using the measured 02, COz, CzH4 levels (Fig. 3), the material balance and the film permeabilities (Fig. 2 and Table 2). (a) CO2 production and 02 consumption; (b) C2H4 biosynthesis.

Fig. 3. (left) Actual gas levels in sealed broccoli packages using DF155 and Cl polymer films during storage at 1.5”C. Each value is mean of five packages. The error bar represents the 90% confidence limit calculated from the sample standard deviation of the five packages. (a) CO2 and 02; (b) C2H4.

52

G.B.Y Christieet al. I PosthutvestBiologyand Technology6 (1995) 41-54

. DF155 I

I

I

I

I

I

2

4

6

6

10

Oxygen Concentration (%I Fig. 5. Respiratory quotient at 1.5”C of broccoli in sealed DF155 and Cl packages plotted against oxygen concentration in the packages.

shown in Fig. 3a to be 1.5% 02 and 5.5% COz. The majority of the 02 production rate decrease had occurred by day 4 (Fig. 4a), which corresponds to 1.5% 02 and 9.1% CO2 (Fig. 3a). However, it took a further ten days for the broccoli respiration to decrease to its steady state level (Fig. 4a) under the effect of the modified atmosphere. The RQ (Fig. 5) fluctuated around 1.1, indicating that anaerobic respiration did not occur in DF155 although Fig. 3a would indicate it was perilously close. Anaerobic metabolism In contrast, the CO2 production rate of the broccoli stored in Cl fluctuates around an initial level of 12.7 ml kg-’ h-l with a significant minimum at three days and maximum at five days (Fig. 4a). In Cl, the broccoli CO2 production rate appears to decrease in response to the modified atmosphere up to day 3, before the shift to anaerobic behaviour increases the CO2 production rate and decreases the 02 consumption rate. The shift to anaerobic behaviour is demonstrated by the RQ increasing from 1.1 to 2.5 on day 3 (Figs. 3a and 5). The RQ continues to increase to a maximum of 6.7, indicating that the anaerobic behaviour was not reversed. Fig. 4b shows that ethylene biosynthesis in Cl is suppressed during anaerobic respiration over that measured for DF155. The RQ for the two films are plotted in Fig. 5 as a function of 02 concentration in the packages. DF155 did not show a break point even at 02 levels around 1%. However, Cl had a break point at an 02 level somewhere below 4%. This indicates that broccoli stored with a higher CO2 atmosphere in Cl had less tolerance for low oxygen.

G. B.Y Christie et al. I Postharvest Biology and Technology 6 (1995) 41-54

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Selecting the bestpermeability measurement technique When the mixed gas cell is used to calculate the permeability under simulated storage conditions, the calculated CO2 production rate for broccoli stored in DF155 drops from 13.8 ml kg-’ h-l, the initial air storage rate, to 8.8 ml kg-’ h-l, the steady state modified atmosphere storage rate. The CO2 respiration of similar broccoli (cv. ‘Marathon’) handled in a similar way has, since these packaging experiments, been independently measured on broccoli heads (0.5 kg lots) with six replicates at 15°C and two days after harvest in air and found to be 12.8 f 15% ml kg-’ h-l. The calculated values are also similar to broccoli respiration rates measured by other workers. For example, 14.4 ml kg-’ h-’ at 2°C in air and 8.6 ml kg-’ h-l at 2°C in 1.5% 02 and 10% CO2 (Kader et al., 1989) and 11.7 ml kg-’ hh’ at 5°C in 8% 02 and 9% CO2 (Forney et al., 1989). When the Dow cell permeabilities at 22 and 1.5”C are used to calculate the CO;! production rate in DF155, the steady state modified atmosphere rates are 12.2 ml kg-’ h-’ and 4.1 ml kg-’ h-‘, respectively. The close agreement of the calculated respiration rate using the mixed gas cell and the poor agreement of the Dow cell method to the measured and literature values, indicates that the mixed gas cell permeability technique provides a more accurate measurement for modelling MAP. 4. Conclusions A simple technique is demonstrated which uses the mixed gas cell permeability measurement, a material balance equation and measured gas concentrations to determine the respiration and ethylene production rates of produce stored in MAP This study shows that the respiration rates determined by the technique agree with measured and published respiration data. The technique can provide data over the entire storage period where temperature, humidity, produce behaviour and produce maturity may vary whereas other techniques require steady state to be obtained (Cameron et al., 1989; Forney et al., 1989). Also, the technique is simpler than those which require the housing of packaged produce in respiration chambers (Fomey et al., 1989), particularly when large package sizes need to be tested. Permeability studies on two LDPE films showed that when selecting polymer films for particular packaging applications, it is important that the film permeabilities be measured under the envisaged storage conditions and using a mixed gas technique. Acknowledgments

The authors are grateful to Yesim Gijztikara for measuring the respiration rate of broccoli (cv. ‘Marathon’) and Lou and Rose Filippin for supplying the broccoli. References Cambellick, W.A., 1985. Barrier Polymers. Goodyear Inc. and Rubber Co. Encyclopedia Science and Engineering. John Wiley and Sons, New York, N.Y., 176 pp.

of Polymer

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Cameron, A.C., Boylan-Pett, W. and Lee, J., 1989. Design of modified atmosphere packaging systems: modelling oxygen concentrations within sealed packages of tomato fruits. J. Food Sci., 54: 14131421. Deily, K.R. and Rizvi, S.S.H., 1981. Optimisation of parameters for packaging of fresh peaches in polymeric films. J. Food Eng., 5: 23-41. Forney, CF., Rij, R.E. and Ross, S.R., 1989. Measurement of broccoli respiration rate in film-wrapped packages. HortScience, 24: 111-113. Foust, A.S., Wenzel, L.A., Clump, C.W., Maus, L. and Andersen, L.B., 1980. Principles of Unit Operations (2nd ed.). John Wiley and Sons, New York, N.Y., 745 pp. Henig, Y.S. and Gilbert, S.G., 1975. Computer analysis of the variables affecting respiration and quality in polymeric films. J. Food Sci., 40: 1033-1035. Kader, A.A., Zagory, D. and Kerbel, E., 1989. Modified atmosphere packaging of fruit and vegetables. Crit. Rev. Food Sci. Nutr., 28: l-30. Mannapperuma, J.D., Zagory, D., Singh, R.P. and Kader, A.A., 1989. Design of polymeric packages for modified atmosphere storage of fresh produce. In: Proceedings of the 5th International Controlled Atmosphere Conference, Wenatchee, Wash. pp. 255-233. Rij, R.E. and Stanley, R.R., 1987. Quality retention of fresh broccoli packaged in plastic films of defined carbon dioxide transmission rate. Packag. lbchnol., 17: 22-23. Rogers, C.E., 1985. Permeation of gases and vapours in polymers. In: J. Comyn (Editor), Polymer Permeability. Elsevier, London, pp. 1l-74. Saltveit, M.E. and Kasmire, RF., 1985. Changes in respiration and composition of different length asparagus spears during storage. HortScience, 20: 1114-1116. Savitzky, A. and Golay, J.E., 1964. Smoothing and differentiation of data by simplified least squares procedures. Anal. Chem., 36: 1627-1639. Storer, AS. (Editor), 1990. Standard Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting. ASTM D1434-82. Annual Book of ASTM Standards Section 8.01. American Society for Testing and Materials, Easton, Md., pp. 611-626. Tomkins, R.B. and Cummins, B.A., 1988. Effect of pre-packaging on asparagus quality after simulated transportation and marketing. Sci. Hortic., 36: 25-35. Zagory, D. and Kader, A.A., 1988. Modified atmosphere packaging of fresh produce. Food lbchnol., 42: 70-74. Zagory, D., Mannapperuma, J.D., Kader, A.A. and Singh, R.P., 1989. Use of computer model in the design of modified atmosphere packages for fresh fruits and vegetables. In: Proceedings of the 5th International Controlled Atmosphere Conference, Wenatchee, Wash. pp. 479-486.