Inmovilizado material

Psychrometrics explains the relationship between air and water vapour based on the physical and thermodynamic properties of moist air (Thompson, 2002). Psychrometric properties allow the analysis of processes involving air humidity gradients, being important to understand moisture loss conditions and their management in horticultural products (Grierson and Wardowski, 1975). The

application of psychrometric concepts in the storage of fresh commodities is often insufficient, which limits the efficacy of produce preservation (Talbot and Baird, 1991).

Cold storage at high relative humidity (RH) reduces produce moisture loss resulting in higher turgor, less wilting and improved marketable life (Wills et al., 2007). Blueberries are recommended to be stored at 90-95% RH to reduce their moisture loss (Mitcham et al., 2011; Perkins-Veazie, 2004). However, high RH enhances pathogen development for this commodity, constituting an important factor of quality loss.

In this section, psychrometric concepts determining fresh produce moisture loss are reviewed, in order to provide information for the further analysis of weight loss of this study. Moreover, RH standard used in blueberry operations and the risks associated to its management are also outlined.

1.3.1.1 Psychrometrics and fresh produce moisture loss

Water loss from fresh produce is determined by the difference in moisture content between the product and the surrounding air (Wills et al., 2007). Moist air is comprised of dry air and water vapour (Grierson and Wardowski, 1978). The capacity of air to retain water increases as temperature increases and ambient pressure decreases (Grierson and Wardowski, 1978). The content of moisture in the air can be described in different ways. The absolute humidity (or humidity ratio) represents the proportion between the weight of the water contained in an air volume and the weight of the dry air of the same volume (Talbot and Baird, 1991). Water vapour movement is driven by the gradient of absolute humidity between two air conditions, allowing the movement from high to low vapour concentration until the equilibrium is reached (Taiz and Zeiger, 2006). Absolute humidity is often expressed as vapour pressure in horticulture management, with both terms providing the same information (Thompson, 2002). The absolute humidity gradient between the intercellular air spaces of plant tissue and the surrounding air is the driving force for water loss, defining the rate at which fresh produce is dehydrated (Wills et al., 2007).

Other psychrometric variables used in analysis of produce storage conditions are dry bulb temperature, wet bulb temperature, dew point temperature and relative humidity (RH) (Talbot and Baird, 1991). Dry bulb temperature is the temperature of the air, which combined with the wet bulb temperature, can be used to calculate the absolute humidity (Thompson, 2002). Wet bulb temperature indicates the cooling of the bulb due to water evaporation (i.e. evaporative cooling), as measured from a moist- covered bulb of a regular thermometer (Talbot and Baird, 1991). It represents the minimum temperature to which an air volume can be cooled just by increasing its moisture content (Talbot and Baird, 1991). For a given environmental pressure, the air decreases its capacity to hold water as temperature decreases, starting to condensate at a temperature referred as the dew point temperature (Grierson and Wardowski, 1975). RH indicates the percentage of the ratio between the absolute humidity and the maximum humidity which can be retained by an air mixture without varying its temperature (Wills et al., 2007). RH can be directly measured by using a hygrometer (Wills et al., 2007). RH is the most known and poorest applied psychrometric variable, being often miss utilised to indicate humidity gradient in postharvest environments (Talbot and Baird, 1991). RH must be always provided together with air temperature, in order to evaluate the moisture loss potential of fresh commodities in a given storage condition (Thompson, 2002).

The relationships between psychrometric variables are represented by the psychrometric chart (Figure 1-7). The horizontal axis indicates the dry bulb temperature, whereas the wet bulb temperature is shown as an axis sloping diagonally upwards from right to left (Talbot and Baird, 1991). The vertical right axis shows the moisture content of the air expressed as absolute humidity or vapour pressure, which increases as dry bulb temperature and RH increase. RH is indicated as left most upward-curved lines, with 100% RH representing the maximum moisture that can be held by an air volume at a given temperature (Thompson, 2002). The psychrometric chart is valid for a specific ambient pressure, being normally reported at atmospheric pressure. The chart allows obtaining all the properties of a given moistened air, as long as at least two variables are known. For instance, absolute humidity (humidity ratio) can be easily calculated either from the dry and wet bulb temperatures, or from the RH and dry bulb temperature. The psychrometric chart is a useful tool for packhouses and storage facilities, which facilitates the

application of psychrometric concepts on postharvest management (Talbot and Baird, 1991).

Figure 1-7. Psychrometric chart in SI (metric) units for atmospheric pressure conditions. Reproduced from Thompson (2002)

In the postharvest management of fresh commodities, the rate of moisture loss is highly related to product temperature. The temperature of fresh produce is affected by respiration heat and evaporative cooling, although it is largely determined by the ambient temperature (Grierson and Wardowski, 1975; Talbot and Baird, 1991). The temperature of the product defines its absolute humidity since RH within the tissues can be assumed to be invariably close to saturation (Grierson and Wardowski, 1978). The gradient of absolute humidity between the product and the environment increases as the product temperature increases, leading to increased moisture loss (Thompson, 2002). Consequently, a rapid product cooling is an effective management to reduce the moisture loss from fresh produce (Thompson, 2002).

1.3.1.2 Relative humidity management in blueberry

Appropriate management of humidity conditions during postharvest is a primary issue for blueberry quality. To reduce weight loss and shrivelling in blueberries, it is recommended to keep RH in the range of 90-95% during storage, together with temperatures close to 0°C (Mitcham et al., 2011; Perkins-Veazie, 2004). Nevertheless, ideal air moisture contents are also optimal for the growth, infection and spore germination of main blueberry postharvest pathogens (Caruso and

Ramsdell, 1995; Snowdon, 1990). Microbial spoilage is frequently observed along blueberry supply chains, being the major reason for fruit losses (Gough, 1994). Accordingly, environmental conditions which may lead to increased RH must be avoided in order to favour the quality maintenance of this commodity.

Temperature variations and RH management in the blueberry cold chain favour condensation, resulting in free water on the produce surface which may enhance pathogen growth (Ehlenfeldt, 2002; Forney, 2009). However, literature is not consistent about the effects of condensation on blueberry decay. While Cappellini et al. (1983) found no influence of condensation on the decay of inoculated (Botrytis sp. and Alternaria sp.) blueberries after moving fruit from storage at 2°C and holding them for 4 d at 21°C, Cline (1997) reported greater development of Alternaria sp. on wet fruit after holding dry and wet inoculated blueberries for 7 d at 21°C. These different results could have resulted from different conditions of each experiment, such as pathogen specie inoculated, amount of inoculum, cultivar resistance or incubation period.