Transpiration Rates and Epidermal Permeabilities of Grapes Based on an Unsteady-State Mass-Transfer Analysis

Abstract

Transpiration rates of grapes as measured by the convective mass-transfer coefficient were evaluated from experimental measurements and a lumped-capacity unsteady-state mass-transfer analysis. Parameters to be used in the diffusion equation to describe the flow of moisture through the epidermis were determined. Effects of mechanical polishing and chemically disturbing the cuticle on the mass-transfer coefficient of grapes were found. Fruits were placed in a small container of dry air, and the dew point of the air was observed by circulating the air through a hygrometer. The dew points were converted to vapor pressure, and the convective mass-transfer coefficient and an apparent equilibrium relative humidity were determined by iteratively fitting the vapor-pressure ratio to an exponential regression. Permeabilities of the epidermis were determined from the thickness of the tissue and apparent and true convective mass-transfer coefficients. The true convective mass-transfer coefficient was considered to be the mass-transfer coefficient of the peeled fruit, while the apparent convective mass-transfer coefficient was the mass-transfer coefficient from the unpeeled fruit. Air flows over a range of one to ten air changes per minute had no significant effect on the convective mass-transfer coefficients. Values of the mass-transfer coefficients were of the order of 0.4 x 10^-[unknown] to 0.9 x 10^-8 grams of H₂O per (minute) (square mm) (mm of mercury). For purposes of predicting epidermal mass transfer, fruits were modeled after a one-dimensional slab. The relationship between vapor pressure and time in a lumped capacity unsteady-state mass-transfer system as predicted by these equations was in close agreement with experimental values. The procedure developed in this study is expected to be quite valuable in developing systems in which the moisture loss of fresh fruits will be reduced.