Apart from mechanical injury and improper harvest time, water loss is the most essential cause of short shelf life and reduced quality of postharvest horticultural products (S. P. Burg, 2004). Fruits and vegetables lose freshness when they transpire more than 3-10% of their weight, but the exact value depend on the type of commodity. The acceptable weight loss ranges from as low as 5% for apple (Pieniazek, 1942) and oranges (Kaufman et al 1956) and as high as 37% for green beans (Hruschka, 1977).
Application of low pressure inside ventilated chambers will increase water loss in the fresh produce (Burg and Kosson, 1983). However, maintaining high RH in the air, overcomes water loss of produce stored under hypobaric conditions (Lougheed, Murr, & Berard, 1978). For that reason, water loss was termed as a limiting factor of hypobaric storage although a few examples of high water loss are reported in the literature when RH is not maintained near saturation. For example, (McKeown & Lougheed, 1980) observed highest weight loss of asparagus under low pressure and saturated RH. Vapour pressure deficit (VPD) regulates water loss patterns under low pressure and will be greatly affected by the air exchange inside the storage room and the RH of the surrounding air. Higher water loss was reported by in Hibiscus cuttings stored under hypobaric conditions than cuttings stored under normal atmospheric pressure (H. G. Kirk, Anderson, Veierskov, Johnsen, & Aabrandt, 1986).
Water loss and quality
Water loss is one of the most significant parameters in postharvest life of horticultural commodities.
Water is responsible for turgor pressure inside plant cells, which gives diverse freshness to commodities. Water loss is vital for fresh produce marketing because the loss of weight is directly associated with a loss of revenue (Laurin, 2006). Textural quality and visual characteristics of fresh produce are adversely affected by water loss, resulting in appearance of symptoms like wilting, floppiness and loss of juiciness (Kader, 2002). Water loss is influenced by two parameters: water potential and transpiration.
Water potential
Loss of turgor pressure within the cells is one of the most noticeable effects of water loss (Berg, 1987). After harvest, transpiration of commodities dehydrates the plant tissues due to low water potential of warm and dry air (Willis, Lee, Graham, McGlasson, & Hall, 1989). The water potential determines the movement of water into or out of the plant cells. The difference in water potential across the membrane will determine the flow of water and is calculated as follows:
ψw = ψs + ψp
ψw : Water potential (MPa)
ψs : Solute potential indicating the number of particles dissolved in water (MPa)
ψp: Forces exerted on water by its environment (turgor) (MPa)
Burg and Kosson (1983) stated that reduced air pressure around plant tissue will reduce the cellular hydrostatic pressure, leading to low cellular water potential. However, due to engagement of water in many biochemical reactions which involve cellular activity, reaction rate will only be slightly altered by pressure reduction. Changing the water potential by 20 atmospheres will only change the water activity by 3 or 4% (S.P. Burg & Kosson, 1983).
Transpiration and vapour pressure deficit
Harvested commodities lose water mainly through the process of transpiration. Different environmental factors such as RH, temperature and possibly even atmospheric pressure will affect the transpiration rate during postharvest handling and storage of fresh produce (Laurin, 2006). In fresh produce transpiration occurs through a mass transfer process, where water vapor moves from the surface of produce to the surrounding air (Ben-Yehoshua, 1987). Diffusion of gas in porous materials like plant tissues can be better understood by the simplified version of Fick's law:
J = (Pi - Pa) At/ (RD*T)*r
Where:
Pi = Partial pressure in the intercellular spaces
Pa= Partial pressure at the ambient air surrounding the produce
At= Fruit's surface area (cm2)
RD= Gas constant per gram
T = Absolute temperature
r = Resistance (s cm-1)
J = Flux in g s -1 cm-1
This law states that the movement of a gas across a fluid membrane at the ambient atmosphere is proportional to partial pressure gradient in the intercellular spaces of produce, the surface of the membrane and is inversely proportional to the resistance of the barrier to diffusion. Water vapor molecules will diffuse in and out of the porous materials from the higher concentration to the lower concentration until equilibrium is achieved. Therefore, the driving force of transpiration is the difference in water vapor pressure between the tissues and the surrounding air (Ben-Yehoshua, 1987). This difference between the vapour pressure of the intercellular spaces in the product (Pi), which is at or near saturation, and the vapor pressure (Pa) of the surrounding air is termed as water vapor pressure deficit (VPD) (Van-Beek, 1985). The water VPD is a function of temperature and RH where RH expresses the ratio of water vapor pressure in the air to the saturation vapor pressure possible at a given temperature (Ben-Yehoshua, 1987). Thus, RH is a direct function of pressure.
RH = Vapor pressure of air / Saturation vapor pressure of air
Diffusion of water vapor in plant tissues
Water vapor transport occurs through the epidermis (Fockens & Meffert, 1972) and is regulated by various plant components i.e. stomata, lenticels, cuticles and epidermal cells (Kader, 2002). Diffusion of gases in plant tissues occurs in three steps. First, gases exchange from the cell sap to the intercellular space. Molecules like ethylene and CO2 have to diffuse across the organelle, its outer membranes, the cytosol, the plasmalemma, the cell wall and penetrate into the internal air channels. Secondly the diffusion of the gas phase from the intercellular space to the outer dermal system. Finally, gas will diffuse from the dermal system, through the boundary layer and into the atmosphere (Laurin, 2006). This process may occur in stomata, lenticels or cuticles (Burton, 1982; Kays, 1991).
Mechanism of stomata
The function of stomata is to allow gas exchange during photosynthesis, respiration and transpiration activities (Collin and Folliot, 1990; Salisbury and Ross, 1991). In most plants, water vapor and other gases diffuse out of the tissues through the stomata pores, because the waxy cuticle of most plant surfaces restricts water diffusion out of the tissues. Taiz & Zeiger (2002) stated that humid surrounding air, presence of light and reduced CO2 concentration triggers the opening of stomata. On the other hand, closure of the stomata pore is stimulated by dry climate, dark environment, and high CO2 concentration.
Behavior of Stomata in plants and harvested commodities under low pressure
During a study on the effect of low-pressure (LP) stress during air cargo transportation, Laurin (2006) observed that exposure of Beit Alpha-type cucumbers to a low pressure of 0.7 atm for 6 hours shorten the shelf life from 1 to 1.5 days due to increase water loss. He concluded that the increase in water loss is the result of induced stress response expressed in term of failure of stomata to close. Though the same author suggested that the primary effect of LP might be due to the outward diffusion of enhanced intercellular CO2 causing the stomata to remain open till the restoration of intercellular CO2 concentration. Similarly, Kirk & Skytt Anderson (1986) have reported the opening of stomata in cuttings of ornamentals, stored under LP, they explained that the opening of stomata is probably caused by the low concentration of O2. Storage of banana under low pressure resulted in damaged stomata, but the fruit retaines normal position after placed back under normal atmospheric pressure (Collin & Folliot, 1990).
Kirk et al, (1986) analyzed that low pressure storage of Hibiscus cuttings resulted in opening of stomata, which closed under normal pressure. The open stomata caused water loss from cuttings.
Under low pressure conditions, the concentration gradient between liquid phase of water within tissues and the gaseous phase of water in surrounding increases, resulting in outward flow of water vapours, which will tend to equilibrate the internal pressure and the external pressure (Kays, 1991). Hence, the equilibrium will tend to be reached faster under reduced pressure in a sealed container, than in a flow-through system.
When gases are produced by harvested commodities under a reduced pressure, the steepness of the concentration gradient from the site of origin of the gas to the exterior will increase, accelerating the outward diffusion rate out of the tissues (Kays, 1991) for gaseous water vapour. Same author stated that for every 10% decrease in pressure, the rate of water loss increases by 10%.
