Modified atmosphere packaging of fresh mushrooms

PACKAGING TECHNOLOGY AND SCIENCE VOL 4 279-286 (1991)

Modified Atmosphere Packaging of Fresh Mushrooms*

1. Beit Halachmy and C. H. Mannheim Department of Food Engineering and Biotechnology, Technion-Israel Institute of Technology, Haifa, 32000, Israel

The storage l i fe o f harvested fruits and vegetables is influenced by their continued respiration and biochemical activity. Lowering the temperature, and modifying the atmosphere surrounding the product, can extend the shelf-life of the produce.

Mushrooms (Agaricus bisporus) were placed into different structural containers covered with PVC fi lms and stored at three temperatures. The effect o f key variables, such as temperature, respiration rate, crit ical and optimal concentrations of 0, and COz, package-weight/area ratio and film permeability, were studied.

It was concluded that a highly perishable product, such as mushrooms, must be wrapped and refrigerated in order t o prevent transpiration and shrivelling. Modif ied atmosphere packaging (MAP) may be beneficial but was no t found t o be essential. If a certain batch o f mushrooms respires faster than predicted, or is exposed t o large temperature fluctuations, MAP could have a damaging effect.

Keywords: Modified atmosphere packaging (MAP); fresh mushrooms; shelf-life; film permeability.

INTRO DU CTlO N

The storage life of harvested fruits and vegetables is influenced by their continued respiration and biochemical activity. Lowering the temperature, and modifying or controlling the atmosphere surrounding the product, reduces respiration and delays senescence, thus extending shelf-life. ' 9 ,

In a sealed package, containing a fresh product, a modified atmosphere (MA) is created by respiratory oxygen uptake and carbon dioxide evolution. Consequently, equilibrium concentrations of oxygen and carbon dioxide are established, so that rates of gas transmission through the packaging material equal the produce respiration rate. The

steady-state equilibrium will depend on several parameters, such as temperature, respiration rate, 0, and CO, permeabilities of the packaging material, fill-weight, free volume in the package, and film surface area.

An optimum atmosphere should minimize respiration rate without danger of metabolic damage to the commodity. However, exposure of fruits and vegetables to 0, levels below their tolerance limits and to CO, levels above their tolerance limits will cause anaerobic respiration and physiological disorders, respectively. 3,4

For short-lived and expensive produce, of which mushrooms are a good example, there is an econ- omic incentive to extend shelf-life by using modified

*Paper presented at the 7th IAPRI World Conference on Packaging Utrecht, APRIL 1991.

0894-32 14/9 1/050279~08$05.00 Q 1YY1 by John Wiley & Sons, Ltd.

Receioed April 1991 Reiiised June 1991

I. B. HALACHMY AND C. H. MANNHEIM

atmosphere packaging (MAP). Mushrooms are highly perishable and deteriorate rapidly after harvesting. Shrivelling, browning and development of the pileus, opening of the cap and stem elongation, are the usual symptoms of senesence and visible ,evidence of de t e r i~ ra t ion .~ ,~ It has been reported' that high CO, Concentration, as well as low 0, concentration and low temperatures, prevented the opening of the mushrooms caps. Aging of mushrooms inside an overwrapped package, as well as deterioration, are retarded, presumably as a result of modified 0,, CO, concentrations and high relative humidity inside the package.8 Critical values, for mushrooms, of 1 % for 0, and 15 % for CO, have been reported.'

In this study the optimum environmental conditions for the preservation of fresh mushrooms, using MAP, were evaluated. The effect of key variables, such as temperature, respiration rate, critical and optimal concentrations of 0, and CO,, package-weight/area ratio and film permeability, were examined. Another objective of this work was to monitor the exchange of gases into and out of packages, at equilibrium, and compare the results with film permeabilities.

MATERIALS AND METHODS

Mushrooms (Agaricus hisporus) were obtained from a local producer immediately after harvesting, and the packaging films used were two types of PVC film supplied by Borden (USA). Their properties are given in Table 1.

Respiration rate in a closed system

Mushrooms (350g) were placed in 2-1 glass jars that were hermetically closed. A septum, for gas

sampling, was fitted into the lid. Gas concentrations were measured periodically.

Respiration rate in a dynamic system

Packed or unpacked mushrooms were placed in 2-1 glass jars or desiccators and flushed with various mixtures of O,/CO,/N, or air. The flow rate and the gas concentrations (at both the entrance and the exit of the container) were measured and used in calculations of respiration rates.

Packaging systems

250g of mushrooms were placed in baskets and wrapped with different PVC films, the properties of which are listed in Table 1. The gas exchange area was 0.08m2. Unwrapped mushrooms, held in air were used as a control. The packages were main- tained at three temperatures. A septum on the film enabled gas sampling to be carried out.

PVC films (VF and MSN) were fitted, by means of special perspex rings that gave well-defined areas, to the top of A2 (560 ml) and A10 (3000 ml) cans. Copper tubes were soldered near the top and bottom of the cans to enable gas sampling. The gas exchange areas of the A2 and A10 cans were, 0.004 m2 and 0.012 m', respectively. Different weights of mushrooms were placed into the cans, which were stored at 3"C, and the gas concentrations were monitored over 8--lO days.

Gas analyses

Oxygen and carbon dioxide concentrations were measured by injecting 0.5 ml of the head space

Table 1. Properties of PVC packaging films

Film Thickness Temp co2 0 2

Permeabilities

type elm) ("C) (ml/rn2.day.atm) (ml/m2.day.atm)

VF 10 3 70000-84000 7 800-1 5 000 10 90 000-1 20 000 12 500-22 000 20 180000-200000 23000-35000

MSN 15 3 18 000 1940 10 22 000 3000 20 26 500 3780

-

H2O (g/m2.day, 90% RH)

27 33

152 15 21 92

MO DI FI ED ATMOSPH ERE PACKAGING 281

gases, from the different containers, into a gas chromatograph (Packard model 406, Becker, Hol- land) equipped with a thermal conductivity detec- tor. The column material was copper, 6 ft x 0.25 in filled with Molecular Sieve 5A, 60/80 mesh for 0, and Poropack Q, 60/80 mesh for CO,. The column temperature was 40°C. The carrier gas flow, helium, was 50 ml/min. The gas chromatograph was connected to a D-2500 Hitachi Integrator, which gave the results as a percentage.

Mixtures of gases

A gas mixer (Witt-Gasetechnik KM 100-3 MESO, Germany) was used to obtain mixtures of O,/CO,/N, for continuously flushing the containers. The nitrogen was used to balance the mixture to 1 atm.

Quality assessment

At the end of the experiments the samples were ranked, visually, according to their appearance. Observations were made by at least eight assessors.

RESULTS AND DISCUSSION

Properties of packaging films

Permeabilities of the films used in this study were evaluated at the different temperatures at which they were applied (Table 1). These results show the importance of obtaining permeability values under the conditions of use. Furthermore, the relatively large range of these values pose a difficulty in predicting gas concentrations at equilibrium when using these films for MAP.

Effect of temperature

In preliminary work with mushrooms no evidence of chilling injury was found even at 1.5"C.

Table 2 shows the respiration rates of mushrooms at three temperatures as measured in a closed system. The significant effect of temperature on respiration is indicated. A t the lowest temperature the appearance of the mushrooms was best and the respiration rate was lowest (Table 2).

Table 2. Respiration rates of mushrooms - O2 consumption and CO, evolution

Temperature CO, evolved 0, consumed ("C) (mg/kg.hr) (rng/kg.hi-)

3 10 20

25 27 65 52

21 5 164

Gas concentrations in a closed system

The change in gas concentrations inside closed jars at 20°C is shown in Figure 1. The evolution of CO, and the consumption of O2 were both very fast. Less than 10 h were needed to achieve anaerobic conditions, at 20"C, whereas it took more than 50 h at cold storage temperatures (data not shown).

The respiratory quotient (RQ) in air, calculated as the ratio of CO, produced (ml/kg.h) to 0, consumed (ml/kg.h), was approximately unity. Its value increased rapidly when the 0, concentration was reduced to 1.5-2%, indicating that this was the critical limit for this commodity (Figure 2).

This break point of the RQ was not observed at the lower tempertures, i.e. 10°C and 3°C (data not shown). Below the critical 0, concentration, respiration led to the accumulation of off-odours. Furth- ermore, a rapid decay of the mushrooms was observed. CO, production was found to increase at very low 0, concentrations, indicating that the metabolism shifted to anaerobiosis.

The critical CO, concentration, as regards physiological disorders, was above 12%. Above this value browning of the caps of mushrooms occurred (Table 3).

Effects of modified atmosphere

Table 3 shows the effect of different gas composi- tions on the respiration rate of mushrooms and their appearance at 3°C and 10°C. The respiration rates were measured in a dynamic flow system.

A reduction in the respiration rate, as compared with the rate in air, was found with only 2% 0, and 0% CO, at 3°C (Figure 3). I t should be noted that this 0, concentration is very close to the critical one, and any slight change in respiration rate due to temperature, or any other variable will cause

282 I. B. HALACHMY AND C. H. MANNHEIM

3

TIME (h) Figure 1 . Gas concentrations in closed jars with mushrooms at 20'C

' I

0 2 4 6 8 10 12 14 16 18

(02) (%) Figure 2. Effect of oxygen concentration on RQ values of mushrooms at 20'C

Table 3. Effect of modified atmospheres on respiration rate and appearance of mushrooms Experiment Temperature Gas Respiration rate Reduction in Better appearance Number "C composition (%) CO, (mg/kg.hr) respiration ratea by rankingb

CO, MA Air

1 3 2 0 15 21 + No difference 2 3 4 0 18 22 + - Air

3 2 15 25 21 - Air 4 3 4 15 19 23 - Air 3

3 4 12 17 18 - no difference 6 10 4 12 46 40 - MA 5

According to ' t ' test: +, p < 0.01 ; + - , p < 0.01 during some days; - , no difference on most days.

0,

b Compared with control in air.

MOD I FI E D ATMOSPHERE PACKAGING 283

A

rx o 1 0 1 2 3 4 5 6 7 0 I

T I M E ( D A Y S ) Figure 3. Respiration rate of mushrooms under MA conditions (2% 0, and 0% CO,) and in air at 3°C. in dynamic flow system (experiment 1, Table 3)

undesirable conditions. No difference in respiration was observed with any other gas mixture at these temperatures (3°C and 10°C).

There was a significant improvement in the quality of mushrooms, under a gas mixture of 4% 0, + 12 % CO, as compared with their quality in air, only at 10°C (Table 3). However, 10°C is not the optimum storage temperature for mushrooms.

According to Kader et a/.,' for most commodities the respiration rate at 5°C is only slightly reduced in various gas mixtures as compared with

the rate in air. Larger differences were observed by these authors at higher temperatures.

Modified atmosphere packaging systems

Overwrapped plastic baskets.

Weight loss. Overwrapping the mushrooms with PVC films reduced the weight loss significantly. Unwrapped mushrooms, held in air at 20"C, lost 40% of their weight within 3 days as compared with only 2-6% in wrapped acks (Figure 4).

45r- &$ 35 W - *I

0 5/ 0 0.5 1 1.5 2 2.5

T I M E ( D A Y S )

+ VF

MSN

CONTROL

--Et

x

Figure 4. Effect of storage time and type of wrapping films on weight loss of mushrooms held a t 20°C


0 rn EZ

VF CO2

VF 02

MSN CO2

MSN 0 2

" . 3 10 20

T E M P ("C) Figure 5. Concentration of 0, and CO, in different packages of mushrooms at three temperatures

Gas concentrations. Within 24 h after the package was sealed, the levels of 0, and CO, in the package reached a steady state and remained fairly constant during storage. The equilibrated atmosphere in the packages varied considerably according to the type offilm used and the storage temperature (Figure 5). At 20°C, with both films, the 0, concentration was below 2"/,, which is very close to the critical value. On the other hand the modification of the internal atmosphere in the packages, stored at 3"C, was negligible.

The gas concentrations, inside baskets overwrapped with two different PVC films, after reaching equilibrium (at lO"C), are listed in Table 4 under the measured column of Internal atmosphere. This table also shows the CO, evolution rate from the

package, and the 0, consumption rate by the package, under the heading respiration rate. The latter data were obtained by measuring the change in the gas composition in a desiccator, in which the film-wrapped mushrooms were placed.

In addition, the 0, and CO, concentrations inside the packages were calculated, based on the permeability data of the packaging films (Table 1, underlined values) and 'respiration rate' (Table 4), using the following equation^'.'^

W a S K , Z = Z R , x 1 0 0 ~ 2 4

Table 4. Respiration rates of mushrooms, and internal gas concentration in two PVC packs at 10°C

Respiration rate (ml/kg.hr) Internal atmosphere (96)

Film Measured Calculated

type 0, uptake CO, evolution 0 2 CO, 0 2 co2 ~ _ _ _ _ _ _ _ _ _ _ _ _ ~ ~

VF 48 68 6.0 7.5 4.6 4.2 MSN 36 36 1.4 12 a 12 Control 80 51

a The value was below the critical value

M 0 D I FI ED ATMOSPHERE PACKAGING 285

Figure 6. Gas concentration in A1 0 cans, containing mushrooms covered with VF and MSN films (0.01 2 m2) at 3°C

where y and z are the 0, and CO, concentrations in the package, at equilibrium (%), y, and z, are the 0, and CO, concentrations in air PA), S the film area (m’), Wthe produce weight (kg), R, and R , the respiration rate (ml/kg.h), and K , and K, the permeabilities of the film at storage conditions (ml/m2.day.atm).

It can be seen (Table 4) that the calculated values, based on these equations, were similar to those measured in the packages. This confirms the assumptions that the rate of produce respiration is equal to the rate of 0, permeation into the package and of CO, permeation out of the package.

Film-wrapped packages were not used for further experiments owing to the high probability of leaks, and because of the difficulty in measuring the package area. Instead, A2 and A10 cans, with well-defined gas exchange areas, were used in further studies.

Gas concentrations in cans.

AIU cans. Figure 6 shows that 300 g of mushrooms in A10 cans covered with MSN or VF led to very similar equilibrium values. The oxygen concentration was close to the critical value (2%) for both films, while CO, values were above critical values, namely 17% for MSN and 14% for VF, after 4 days, and decreased to 10% after 8 days.

The time to achieve equilibrium was found to be dependent upon the type of film used. For MSN the

0, concentration in the cans reached equilibrium after 3 days and for CO, after 5 days, while with the more permeable VF film it took 5 days for 0, and 8 days for CO,, respectively, to reach constant values.

Finally, reducing the fill weight and the free volume of the cans, and using VF film only, led to equilibrium values far from the critical ones. Oxy- gen concentration was 12 % and CO, 4 % with 85 g mushrooms. With 50 g mushrooms, CO, concentration was 2.5 % and no equilibrium was achieved for 0, even after 10 days.

A 2 cans. The use of MSN with 100 g of mushrooms in A2 cans led to 0, and CO, levels beyond the critical limits. Indeed, the R Q was found to be very high (about 4) indicating that the conditions were anaerobic. The same result was found with 50 g of mushrooms and the time to achieve equilibrium was too long for both fill-weights. The equilibrium gas concentrations were found to be connected both to the type of packaging material used and to the fill-weight (Figure 7).

Owing to the large ratio of film area to free volume of cans, the equilibrium was never achieved as quickly as in the film-wrapped baskets. There- fore, a perspex container that had a low free volume and relatively large gas exchange area, enabling a reduction in the time needed to reach equilibrium, was designed. The perspex container was used in further work to check a computer program devel- oped to optimize MA package parameters.


- ~ _ _ _ ~ ~ . _ _ -

10 !L 20

33 50

W E I G II T (g) Figure 7. Effect of weight of mushrooms, in A2 can covered with VF film (0.004 m2) at 3°C. on gas concentrations

CONCLUSION

A highly perishable product, such as mushrooms, must be packaged and refrigerated in order to prevent transpiration and shrivelling. Modified atmosphere packaging may be beneficial in maintain- ing sensory quality, but it was not found to be essential. On the other hand, if a certain batch of produce respires faster than predicted, or it is exposed to large temperature fluctuations, the critical limits for 0, and CO, could be exceeded, and MAP could have a damaging effect.

REFERENCES

I . Kader, A. A,, Zagory. D. and Kerbel, E. L. Modified atmosphere packaging of fruits and vegetables. CRC Crit. Rev. Food Sci. Nurr. 28, 1-30 (1989).

2. Prince, T. A. Modified atmosphere packagng of horticul- tural commodities. In: ControlledlModijied Atmosphere Puckaging of Foods, ed. by A. L. Brody. Food and Nutrition Press, Trumbull, CT. p. 67 100 (1989).

3. Zagory, D. and Kader, A. A. Modified atmosphere packaging of fresh produce. Food Technol. 42(9), 70 77 (1988).

4. Geeson, J. D., Browne, K. M., Maddison, K., Shepherd, J. and Guaraldi, F. Modified atmosphere packaging to extend shelf-life of tomatoes. J . Food Technol. 20, 339- 349 (1985).

5. Nichols, R. and Hammond. J. B. W. Storage of mushrooms in pre-packs: the effect of changes in carbon dioxide and oxygen on quality. J. Sci. Food Agric. 24, 1371 1381 (1973).

6. Burton, K. S., Frost, C. E. and Nichols, R. A combination plastic permeable film system for controlling postharvest mushroom quality. BiotechnoL Lett. 9, 529-534 (1987).

7. Sveine, E., Klougart, A. and Rasmussen, C. R. Ways of prolonging the shelf-life of fresh mushrooms. Mushroom 8 i .

8. Nichols, R. and Hammond, J. B. W. The relationship between respiration, atmosphere and quality in intact and perforated mushroom pre-packs. J. Food. Techno/. 10,

9. Deily, K. R. and Rizvi, S. S. H. Optimization of parameters for packaging of fresh peaches in polymeric films. J . Food Proc. Eng. 5, 23-41 (1981).

10. Cameron, A. C., Pett, W. B. and Lee, J. Design of modified atmosphere packaging systems: modelling oxygen concentrations within sealed packages of tomato fruits. J. Food Sci.

6 463-474 (1967).

427-435 (1975).

54, 1413-1416, 1421 (1989).

Documents

Modified atmosphere packaging of fresh mushrooms