The primary functions of hypoxia (low oxygen) in the storage of perishables are to inhibit ethylene action, to reduce metabolic rate through respiratory inhibition, and to reduce cut surface browning (Beaudry, 1999, 2000; Burg and Burg, 1965). Nitrogen flushing is typically used to achieve the low O2 atmosphere of controlled atmosphere (CA) storage. The O2 partial pressure is maintained mechanically in CA storage using O2 sensors and either adding additional N2 or air as required. In modified atmosphere packaging (MAP), flushing is also used to establish initial atmospheres, but the respiration of the produce maintains the target atmosphere (Day, 2001; Patterson et al., 1999; Tomkins, 1962). In hypobaric storage or LPS, a hypoxic atmosphere is attained through a reduction in absolute pressure, which reduces the partial pressure of O2. Target partial pressures of O2 in LPS can be below those of CA or MAP because LPS improves gas diffusion and minimizes gas gradients between the storage environment and the internal tissues of the stored product (Burg and Burg, 1965). Low pressure improves diffusion by decreasing the frequency of collisions between gas molecules. As a result, the flux of gases (including water vapor) from plant tissues in the LPS environments is enhanced relative to atmospheric pressure.
The O2 partial pressure in LPS is a function of the absolute pressure, humidity, the inlet air flow rate, and the rate of O2 consumption by the commodity. The LPS environment is unique in that, when the structure is properly designed and managed, water vapor is one of the major constituents in the atmosphere (Dilley et al., 1975). For example, at 1.5 kPa (11.3 mm Hg) and 0 °C, if the atmosphere is kept close to saturation, the water vapor partial pressure will be near 0.6 kPa (8.15 mm Hg) or ≈40% of the total pressure. In this case, the O2 partial pressure would be approximately one-fifth of the remaining pressure, or 0.18 kPa, and the corresponding N2 partial pressure would be 0.72 kPa. Thus, in LPS, water vapor comprises a much higher total proportion of the atmosphere than at normal ambient pressure, where, at saturation, the water vapor makes up only 0.6% of the atmosphere (0.6 kPa) at 0 °C. The extremely high proportion of water vapor in the LPS environment means that those factors that alter the humidity of the low-pressure (LP) chamber will markedly alter the concentration of O2. This, in turn, emphasizes the importance of regulating the humidity in an LPS room.
LPS has not been widely used in the United States. In fact, the authors are aware of its commercial use as a permanent installation in only one facility (JA Flower Services, Miami, FL) for the temporary storage of imported floral crops. Dormavac, the Grumman subsidiary company that attempted to commercialize LPS in the 1970s and 1980s was ended in 1982 (The New York Times, 1982). Currently more widespread use is apparently found in China (Zheng, 2018) and in the United States, marketed as RipeLocker (Koger, 2021).
One of the criticisms of LPS is the potential for excessive water loss (Burg, 2004; Hughes et al., 1981; Ilangantileke et al., 1989; Lougheed et al., 1977, 1978). However, Burg (2004) effectively argues that through either moist or “dry” humidification systems, moisture loss can be minimized such that the LPS can “maintain the highest possible humidity.” Humidification of the incoming air is best achieved after the transition to LP. By humidifying the incoming air at LP, rather than before, desiccation of the incoming air due to expansion can be avoided and a near saturated atmosphere maintained. Further, the water added to the incoming atmosphere is sometimes heated to offset the cooling due to evaporation and enhance the water content in the inlet air. In the dry humidification system, water loss that accompanies dissipation of the heat from respiration is leveraged for the humidification of the chamber atmosphere.
Data demonstrating the humidity in properly managed LPS environments are not commonly found in the published literature, in fact, in the peer-reviewed literature, we found humidity measurements in only one study and in that study, produce was not included within the chamber (Jiao et al., 2012). Jiao et al. (2012) reported humidity levels greater than 99% for a commercially built laboratory-scale LPS intended for use as a tool for disinfestation of fruits. We were also unable to find accurate measures of temperature for perishables, objects, chamber, and air in LPS systems to determine if insulation due to the Dewar effect was problematic for heat retention by the product at the pressures being used in this study (Dilley et al., 1975).
Our goal was to monitor the humidity, temperature, and moisture loss in situ for representative perishable horticultural crops in LPS. We constructed four laboratory-scale LPS chambers using recommended design features (Burg, 2004) and evaluated their effectiveness in preserving the quality of root, leaf, fruit, and flower crops represented by bunch carrot (Daucus carota subsp. sativus) with intact crown and leaves, bunch spinach (Spinacia oleracea), strawberry (Fragaria ×ananassa) fruit, and long-stem rose (Rosa ×hybrida ‘Attaché Pink’) flowers. Our objective was to characterize moisture loss from various types of produce under normal atmospheric pressure (NP) and LP to more fully understand those features of LPS that limit its utility and to evaluate the impact of LPS on cut rose quality relative to high-humidity storage at atmospheric pressure and normoxia.
Beaudry, R.M. 1999 Effect of O2 and CO2 partial pressure on selected phenomena affecting fruit and vegetable quality Postharvest Biol. Technol. 15 293 303 doi: 10.1016/S0925-5214(98) 00092-1
Beaudry, R.M. 2000 Responses of horticultural commodities to low oxygen: Limits to the expanded use of modified atmosphere packaging HortTechnology 10 491 500 doi: 10.21273/HORTTECH.10.3.491
Burg, S.P. 2015 Hypobaric storage in food industry: Advances in application and theory Academic Press, Elsevier Amsterdam, The Netherlands doi: 10.1016/C2013-0-09887-8
Capdeville, G.D., Maffia, L.A., Finger, F.L. & Ulisses, G.B. 2005 Pre-harvest calcium sulfate applications affects vase life and severity of gray mold in cut roses Scientia Hort. 103 329 338 doi: 10.1016/j.scienta.2004.06.016
Day, B.P.F. 2001 Modified atmosphere packaging of fresh fruit and vegetables—an overview Acta Hort. 553 585 590 doi: 10.17660/ActaHortic.2001.553.138
Dilley, D.R., Carpenter, W.J. & Burg, S.P. 1975 Principles and application of hypobaric storage of cut flowers Acta Hort. 41 249 267 doi: 10.17660/ActaHortic.1975.41.21
Hardenburg, R.E., Watada, A.E. & Wang, C.Y. 1986 The commercial storage of fruits, vegetables, and florist and nursery stocks U.S. Dept. Agr., Agr. Handbook 66 revised
Hughes, P.A., Thompson, A.K., Plumbley, R.A. & Seymour, G.B. 1981 Storage of Capsicums (Capsicum-annuum (L.) Sendt.) under controlled atmosphere, modified atmosphere and hypobaric conditions J. Hort. Sci. 56 261 266 doi: 10.1080/00221589.1981.11514999
Ilangantileke, S.O., Turla, L.O. & Chen, R.C. 1989 Pretreatment and hypobaric storage for increased storage life of mangos Amer. Soc. Agr. Sci. St. Joseph, MI 15 pps
Jiao, S., Johnson, J.A., Fellman, J.K., Mattinson, D.S., Tang, J., Davenport, T.L. & Wang, S. 2012 Evaluating the storage environment in hypobaric chambers used for disinfesting fresh fruits Biosyst. Eng. 111 271 279 doi: 10.1016/j.biosystemseng.2011.12.003
Koger, C. 2021 RipeLocker containers designed to extend produce shelf life The Packer 14 Jan. 2021. <https://www.thepacker.com/news/packer-tech/ripelocker-containers-designedextend-produce-shelf-life>
Lougheed, E.C., Murr, D.P. & Berard, L.E. 1977 LPS-great expectations Proc. 2nd Intl. Controlled Atmosphere Res. Conf. 5–7 Apr. 1977 Mich. State Univ. 38 44
Mastromatteo, M., Conte, A. & Del Nobile, M.A. 2012 Packaging strategies to prolong the shelf life of fresh carrots (Daucus carota L.) Innov. Food Sci. Emerg. Technol. 13 215 220
Mudau, A.R., Soundy, P., Araya, H.T. & Mudau, F.N. 2018 Influence of modified atmosphere packaging on postharvest quality of baby spinach (Spinacia oleracea L.) leaves HortScience 53 224 230 doi: 10.21273/HORTSCI12589-17
Nunes, M.C.N., Brecht, J.K., Morais, A.M.M.B. & Sargent, S.A. 1998 Controlling temperature and water loss to maintain ascorbic acid levels in strawberries during postharvest handling J. Food Sci. 63 1033 1036 doi: 10.1111/j.1365-2621.1998.tb15848.x
The New York Times 1982 Grumman closing its Dormavac unit New York Times 16 Apr. 1982. 24 Feb. 2021. <https://www.nytimes.com/1982/04/16/business/grumman-closing-its-dormovacunit.html>
Patterson, B.D., Flodin, C. & Bower, J.H. 1999 The oxygen level in relation to oxygen supply and demand within modified atmosphere packages containing fresh plant produce 20th Intl. Congr. Refrigeration, IIR/IIF Sidney 1999 Vol. IV paper 149
Staby, G.L., Cunningham, M.S., Holstead, C.L., Kelly, J.W., Konjoian, P.S., Eisenberg, B.A. & Dressler, B.S. 1984 Storage of rose and carnation flower J. Amer. Soc. Hort. Sci. 109 193 197
Tomkins, R.G. 1962 Film packaging of fresh fruit and vegetables - the influence of permeability 64 69 Inst. Packaging Conf. Guide 1961 Larkfield, Maidstone, Kent, England
U.S. Deparment of Agriculture 2019 FoodData Central USDA-ARS 23 Feb. 2021. <https://fdc.nal.usda.gov/fdc-app.html#/food-details/357642/nutrients>
Vaisala, O. 2013 Humidity conversion formulas: Calculation formulas for humidity Publication No. B210973EN-F, Vaisala Oyj, Helsinki, Finland, 17 pp., 23 Feb. 2021. <https://www.hatchability.com/Vaisala.pdf>
van den Berg, L. & Lentz, C.P. 1974 High humidity storage of some vegetables Canadian Inst. Food Sci. Tech. J. 7 260 262 doi: 10.1016/S0315-5463(74)73924-4
Wagner, W. & Pruß, A. 2002 The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use J. Phys. Chem. Ref. Data 31 387 535 doi: 10.1063/1.1461829
Zheng, H. 2018 Research and application of hypobaric storage technology in agriculture and food industry in China Asian Agr. Res. 10 40 51 doi: 10.19601/j.cnki.isn1943-903.2018.6.09