Design of equilibrium modified humidity package for postharvest fruits and vegetables and validation test

Abstract: Fruits and vegetables contain large amounts of water, parts of which are easily lost under low relative humidity conditions because the water vapor pressure at the surface of fresh products is slightly below saturation. Loss of water from fresh products can lead to skin wrinkling, loss of crispiness, and undesirable color changes. Controlling water loss is an important means to maintain quality of postharvest fruits and vegetables. Fruits and vegetables are usually packaged within plastic film and stored at refrigeration temperature in order to prevent excessive water loss and to extend shelf life; however, the low water vapor permeability (WVP) of films combined with the fresh products transpiration rapidly brings about saturation humidity (~100% RH) of the package atmosphere. The saturated in-pack RH condition and inevitable external temperature fluctuation can result in the condensation of water on the inner surface of the packaging film and on the contained products, and lead to defects in the external appearance of commodities and accelerate microbial growth. Equilibrium modified humidity packaging (EMHP) is an effective method for decreasing water loss and avoiding the occurrence of condensed water by modifying the permeability of the packaging film through perforation and optimally regulating the in-package equilibrium water vapor pressure. The water loss of fruits and vegetables packaged in perforated film was influenced by both of transpiration rate of products and the transmission rate of water vapor to the outside of the package. The transmission processes of water vapor in headspace include two ways: permeation through film and diffusion through holes. Water vapor transmission in a perforated package occurs almost entirely through the perforations due to the high permeability of perforated film. Several mathematical models about mass transfer have been proposed, and the majority of them focus on predicting and controlling O2 and CO2 exchange in perforation-mediated modified atmosphere packaging in order to create the desired atmosphere for preservation of fresh fruits and vegetables. Little research has been done so far to measure the equilibrium relative humidity within package. Under certain storage temperature, the above two processes are results of water vapor deficit between the commodity and the gas surrounding the commodity and two sides of film, and follow different mathematical models. If there is a good balance of water vapor press, the constant relative humidity in a package can be obtained. Transpiration affects the main quality characteristics of fresh products, and water loss must be controlled within 4.9%±0.2% to avoid loss of freshness in fresh produce, and it is an important parameter to be considered while designing packaging. So, the area of perforation can be calculated based on the total amounts of water loss and mathematical models about water vapor transmission. The objective of our research is to analyze water vapor transmission rate (WVTR) in different perforation areas and design the equilibrium modified humidity packaging according to mathematical model based on Fick's law of diffusion. The proper relative humidity within each package can be obtained through regulating perforation area, and avoiding excess water loss and production of condensed water. The model was validated with mushrooms (Aagaricus bisporus) packaged in perforated plastic bags at 2℃. The results showed that different perforation areas can affect water loss and shelf life of mushrooms, and the method can guide the design of equilibrium modified humidity packaging for mushrooms storage.