NOTE
Modified atmosphere packaging of onion bulbs (Allium cepa L.)
Noureddine BENKEBLIA1, 2 *, Patrick VAROQUAUX2
RÉSUMÉ L’emballage de l’oignon (Allium cepa L.) en atmosphères modifiées.
La conservation d’oignons en emballages à atmosphères modifiées avec le MV3000 (film hydrophile, 22µm), le P+, 35 Pa 90 (film microperforé) et l’OPP (polypropylène orienté, 35µm) a été étudiée. La pression partielle en oxygène et en dioxyde de carbone atteint l’état d’équilibre après 6 jours dans le MV3000 et le P+, 35 Pa 90 à 2 et 3 kPa respectivement. Dans l’OPP, la teneur en CO2a augmenté durant 8 jours et elle a atteint une valeur d’équilibre de 10 kPa après 12 jours. Un taux de germination de 25 % a été noté dans le MV3000 en comparaison avec les 50 % observés dans le P+, 35 Pa 90 après 6 mois. Le taux de pourriture a été de 23 % dans le MV3000 et le P+, 35 Pa 90 et n’était pas différent des bulbes témoins ; dans l’OPP tous les bulbes pourri- rent durant les deux premiers mois.
Mots clés : atmosphères modifiées, oignon, conservation, germination, pourri- ture.
SUMMARY
The modified atmosphere storage of onions in MV3000 (hydrophilic film, 22µm), P+, 35 PA 90 (microperforated film) and OPP (oriented polypropylene, 35µm) at 20°C was investigated. Oxygen and carbon dioxide partial pressures reached steady state after 6 days in MV3000 and P+, 35 PA 90 with 2 and 3 kPa respectively. In OPP, CO2level increased for 8 days and reached an equilibrium of 10 kPa after 12 days. There was 25% sprouting of bulbs in MV3000 in comparison with 50% in P+, 35 PA 90 after 6 months. Rotting was 23% in MV3000 and P+, 35 PA 90 and did not differ from the untreated control sample; with OPP, all bulbs rotted within two months.
Key words: modified atmosphere; onion; storage; sprouting, rotting.
1 Department of Biology, Faculty of Science, Mostaganem University, PO Box 931, 27000 Mostaganem, Algeria.
2 Institut national de la recherche agronomique, Station de technologie, St-Paul, Agroparc, 84914 Avignon cedex 9, France.
* Correspondence benkeblia@hotmail.com
1 - INTRODUCTION
The use of Modified or Controlled Atmospheres (MA/CA) storage of whole vegetables and beneficial effects of extending shelf-life in improving commercial qualities is well documented (KADERet al., 1989; VAROQUAUXet GOUBLE, 1997).
Nevertheless, applications of MA/CA to onion bulbs as well as to other Allium crops, have not been extensively investigated.
The outer scales of onion reduce gas transfer and the main gas exchanges take place through the neck; little gas diffusion takes place through the root plate. (HOFTUN, 1993). In low oxygen partial pressures (1-5 kPa) CA storage, onions decayed by neck rot fungi after 15 weeks (MIKIZEL et al., 1993). CO2 injury decreased and was accompanied by a similar decrease in black mould (SMITTLE, 1988); other authors reported successful CA storage for some culti- vars of onions (ADAMICKIand KEPKA, 1974; SALTVEIT, 1989).
In order to provide some data for the further development of this technique, the present study evaluates the behaviour of whole onion bulbs during storage in modified atmosphere packaging, and especially the O2and CO2concentra- tions that inhibited sprouting and fungal attacks, and that favoured improved market quality.
2 - MATERIAL AND METHODS
2.1 Onions
Onion bulbs Allium cepa L. (produced without preharvest or postharvest chemical treatments, “organic produce”) cv Rouge Amposta were obtained from the local market (Avignon, France), sorted for uniformity and absence of defects, packed in commercial plastic (PVC) trays in quantities of 12 kg and sto- red at 18°C prior to experimental treatments.
2.2 Modified atmosphere packaging
The modified atmosphere packaging films evaluated were OPP (Oriented Polypropylene, 35µm. Danisco Flexible, Bristol, UK ), MV3000 (hydrophilic film, 25µm, polyether polyamide Pebax. Elf Atochem, La Défense, France) microper- forated film (P+, 35 PA 90. Danisco Flexible, Bristol, UK) and macroperforated film (Grace Cryovac, SM 570 SW. Epernon, France).
The gas transfer rate (permeance) of the films to O2and CO2were respecti- vely 900 and 3 500 (OPP), 6 500 and 56 000 (MV3000), 10 000 and 13 500 (P+, 35 PA 90), expressed in mL·m–2·24 h–1·atm–1 at 23°C. Permeance to water vapour was 3 300 g·m–2·24 h–1(standard norm ASTM E 96 E) for MV3000 and negligible for OPP and P+, 35 PA 90 films (data from suppliers). Permance of MV3000 to CO2increases as the film hydrates (7). A 20µm thick macroperfora- ted film (SM 570 SW) was used for comparison. The permeance of macroperfo-
rated film to O2 and CO2 was infinite and permeance to water was 570 g·m–2·24 h–1. Onions were first allowed to come to equilibrium at 20°C (room temperature), then transferred in quantities of 650 ± 20 g (ca 4 onions) into 0.11 m2pouches of the four films previously described and left at the same temperature (20°C).
2.3 Measurement of O2and CO2
Measurement of O2and CO2partial pressures was done by using a MTI gas chromatograph analyser (model M200, Fremont, USA). The analyser comprised two manifolds: one fitted with an MS-5A, 4-meter capillary column set at 80°C with argon as carrier gas at a pressure of 110 kPa, and the other fitted with a capillary Poraplot 4, 6-meter column set at 110°C with helium as carrier gas at a pressure of 193 kPa. Both manifolds were coupled with katharometric detec- tors. In such conditions argon does not appear in the O2 peaks. A sample (50µL) was taken through a silicone septum set in the film and injected into the MTI chromatograph.
2.4 Statistical analysis
Experiments were carried out on three pouches of each film, and all experi- ments were run in triplicate. Data were analysed statistically by determination of least significant difference (LSD at P < 0.05) using XLStat. Pro® statistical soft- ware.
3 - RESULTS AND DISCUSSION
3.1 Internal atmosphere within film pouches
Variations in CO2composition within the packs are shown in figure 1. With MV3000 and P+, 35 PA 90, the partial pressure of CO2increased and that of O2 decreased (data no shown) during the three first days of storage. Thereafter, the CO2partial pressure stabilised at 2 kPa and 3 kPa for MV3000 and P+, 35 PA 90, respectively. With both films a steady state was reached on day 6thand the- reafter the CO2composition in the pouches remained unchanged during sto- rage. In contrast, with OPP, CO2 production increased during the first eight days thereafter becoming relatively stable. A sharp decrease in O2consumption was noted (from 20.43 kPa to 14.7 kPa) during the first three days of storage, followed by relative stability for four days with a second decrease in O2 consumption to a steady state of 10 kPa. The second decrease may have been due to fermentation induced by a low O2 partial pressure (less than 5 kPa) (LADEINDE and HICKS, 1988) and high CO2 partial pressure (higher than 5 kPa) thus substantiating the findings of SITTON et al. (1997). SITTON et al. (1997) reported that CO2partial pressure higher than 2 kPa was phytotoxic. The decay and necrosis of bulb tissues may have been due to a high relative humidity in OPP and P+, 35 PA 90 which are hydrophobic, whereas MV3000 is hydrophilic.
Tissue damage at high (over 5 kPa) CO2partial pressure was reported by others (SMITTLE, 1988; ADAMICKI, 1977).
Nevertheless, the gaseous compositions within packages are considerably influenced by relative humidity (MUJICA-PAZand GONTARD, 1997; ROY et al., 1996). The permeance of the films to O2and CO2may be recalculated from the experimental conditions and data using computer programmes (DOYON et al., 1996; GOUBLEand VAROQUAUX, 1999). Thus, the calculated CO2permeance of MV3000 at 23°C is 55 000 mL·m-2·24 h-1·atm-1 as compared to 56 000 as mea- sured by the supplier. Similarly, the calculated permeance to O2 was 34 500 mL·m-2·24 h-1·atm-1in comparison with 6 500 as described by the sup- plier.
Table 1
Sprouting and rotting (%) of onion bulbs in MA films during storage at 20°C
Months OPP P+, 35 PA 90 MV3000 MacroPerf
1 0 0 0 0
2 0 0 0 0
3 0 10 10 7
Sprouting 4 0 20 15 30
5 0 35 17 80
6 0 50 23 85
LSD 35.7
1 25 0 0 0
2 100 0 10 10
3 – 7 12 13
Rotting 4 – 15 15 17
5 – 20 20 20
6 – 28 22 24
LSD 32
Figure 1
Variation of CO2in Modified Atmosphere Packaging of onion bulbs at 20°C Films: ◆OPP, ■P+, 35 PA 90, MV3000, ×Macroperforated ; LSD at P < 0.05.
3.2 Sprouting and rotting
Onion bulbs in macroperforated, P+, 35 PA 90 and MV3000 started to sprout after two months of storage, and the sprouting reached 85%, 50% and 23% res- pectively after six months storage (table 1). The low incidence of sprouting rate with MV3000 (which is hydrophilic) was likely due to a low relative humidity in the pouches which inhibits sprouting. A shrivelling of external skins of the bulbs was also observed, but in macroperforated and P+,35 PA 90 film onions were more shrivelled, even though in MV3000 they were slightly affected.
A beneficial effect of a moderate relative humidity in packaging was pre- viously reported for mushroom by ROY et al. (1996) and for tomato by SHIRAZI and CAMERON (1992). However, rots of bulbs packed in these films started within one month of storage, and were similar throughout the whole period of storage. After 6 months storage, the incidence of rotting was an average of 20%, but in OPP, all bulb rotting occurred within one month of storage. SITTON et al. (1997) noted similar effects of CO2on the decay of onion bulbs during CA storage. GOFFINGSand HERREGODS(1989) reported conflicting results on modi- fied atmosphere storage of leek under various atmospheres (2% O2-2% CO2; 2% O2-2% CO2-5% CO) at 0°C during 8 weeks, with a decrease in firmness and in the severity of fungal attacks. Nevertheless with MV3000 opportunity could exist for MA storage of whole onions with less reliance upon refrigeration or chemical pesticides (e.g. maleic hydrazide) to reduce losses and decay, and would extend their marketability after the normal marketing season.
4 - CONCLUSION
Extended modified atmosphere of onion bulbs (cv Rouge Amposta) appears to be dependent upon CO2concentration and relative humidity within the packs, which are regulated by the characteristics of the films. The data showed that sto- rage of bulbs in MV3000 could be considered as a good choice for MA storage of onions. The beneficial effect of the hydrophilic films may have been due to their high permeance to CO2and H2O. P+, 35 PA 90 was not efficient at inhibi- ting sprouting. OPP proved not to be suitable for MA storage of onion bulbs at room temperature due to the low permeances to gases including moisture.
An optimal atmosphere (CO2below 5 kPa) and humidity control, are essential for the successful long term storage of onion bulbs, and the results suggest that some films, such as MV3000, have the potential to delay sprouting and rotting in store.
ACKNOWLEDGEMENTS
The authors are grateful to Dr Andrew ENTWISTLE(Leamington Spa, Warwick, UK) for his critical review and editing of the paper.
REFERENCES
ADAMICKI F., 1977. Respiration rate of onion bulbs depending on the temperature, O2and CO2concentrations of the atmosphere. Acta Hortic., 62, 15-22.
ADAMICKI F., KEPKA A.F., 1974. Storage of onions in controlled atmosphere. Acta Hortic., 8, 53-74.
DOYON G., PRUDHOMME J.C., FRANCOIS S., REMY L., CASTAIGNE F., 1996. Estima- tion rapide des perméabilités au gaz carbo- nique et à l’oxygène des matériaux plastiques utilisés pour les produits horti- coles frais. Calculatrice HP et PC compatible.
Ind. Alim. Agric., 4, 229-234.
GOFFINGS G., HERREGODS M., 1989. Sto- rage of leeks under controlled atmospheres.
Acta Hortic., 258, 481-484.
GOUBLE B., VAROQUAUX P., 1999. Choix du film d’emballage des végétaux prêts à l’emploi (4egamme). Ind. Alim., avril, 65-70.
HOFTUN H., 1993. Internal atmosphere and watery scales in onion bulbs (Allium cepa L.).
Acta Hortic., 343, 135-140.
KADER A.A., ZAGORY D., KERBEL E.L., 1989. Modified atmosphere packaging of fruits and vegetables. Crit. Rev. Food Sci.
Nut., 28, 1-30.
LADEINDE F., HICKS J.R., 1988. Internal atmosphere of onion bulbs stored under various oxygen concentrations and tempera- tures. HortScience, 23, 1035-1037.
MIKIZEL L.J., PATTERSON M.E ., WEALTI H., 1993. The effect of CA storage on the postharvest life of sweet onions. Proceedings of the VI International CA Research Confe-
rence, Ithaca (New York), 15-17 June 1993, 789-796.
MUJICA-PAZ H., GONTARD N., 1997. Oxy- gen and carbon dioxide permeability of wheat gluten film. Effect of relative humidity and temperature. J. Agric. Food Chem., 45, 4101-4105.
ROY S., ANANTHESWARRAN R.C., BEEL- MAN R.B., 1996. Modified atmosphere and modified humidity packaging of fresh mush- room. J. Food Sci., 61, 391-397.
SALTVEIT M.E.A., 1989. Summary of CA and MA storage requirements and recommenda- tions for the storage of harvested vegetables.
Proceedings of the V International CA Research Conference, Wenatchee (Washing- ton), 14-16 June 1989, 800-818.
SHIRAZI A., CAMERON A.C., 1992. Control- ling relative humidity in modified atmosphere packages of tomato fruit. HortScience, 27, 336-339.
SITTON J.W., FELLMAN J.K., PATTERSON M.E., 1997. Effects of low- oxygen and high- carbon dioxide atmospheres on postharvest quality, storage and decay of “Walla Walla”
sweet onions. Proceedings of the VII Interna- tional CA Research Conference, Davis (Cali- fornia), 13-18 July 1997, 20-25.
SMITTLE D.A., 1988. Evaluation of storage methods for “Granex” onions. J. Am. Soc Hort Sci., 113, 877-880.
VAROQUAUX P., GOUBLE B., 1997. Films à perméabilité ajustable aux gaz pour la conservation en frais des fruits et légumes.
Ind. Alim. Agric., 3, 133-136.
