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Time-variable average effective diffusion coefficients of sodium in the skin and flesh were determined using Macroscopic Mass Balances from experimental data obtained during debittering of green olives, variety Arauco at temperatures of 15, 20 and 25C and lye concentrations of 1.50, 2.25 and 3.00% of NaOH. The effective diffusion coefficient of sodium increased with treatment time at both surfaces of the skin, while it varied very little within the flesh, and in many cases, remained almost constant during the debittering process. In general, the diffusion coefficients determined in the present study were within an order of magnitude of 10 -10 m 2 /s for the flesh during most of the treatment time, while for the skin, they were of the order of 10 -12 m 2 /s for the initial treatment times. Macroscopic mass balances are a simple mathematical method that can be used in lieu of more common numerical techniques, to evaluate variable diffusion coefficients providing accuracy and greater detail of the whole phenomena.

The present study reports a simple method, both mathematical andexperimental, to determine variable effective diffusion coefficients for sodiumthrough the skins of olives. Skins removed from green olives, variety Arauco(also known as Criolla), were studied using a lye concentration of 2.25% (w/w) of NaOH at 20C. The diffusion of sodium was evaluated through fresh skinsand previously alkali-treated skins. The measured values of effective diffusioncoefficients for untreated (fresh) olive skins increased two orders of magnitudeduring the processing time, from 10 - 12 to 10 - 10 m 2 /s. Whereas, the effectivediffusion coefficients determined for previously treated olive skins were of theorder of 10 - 10 m 2 /s and increased very little with treatment time. INTRODUCTION In biological systems, the phenomena of solute diffusion can be studiedat different structural levels in order to evaluate their influence in variousagronomic and processing treatments.The diffusion of sodium chloride has been investigated by several authorsin various foodstuffs; in pork meat by Kormendy and Ganter (1958) and Wood(1966); in pickles by Pflug

A diffusion model for a composite flat plate, consisting of a thin skin of thickness δ, and a thicker flesh of thickness L, with constant flesh and skin effective diffusion coefficients (D F and D S ) was successfully adjusted to the experimental data obtained during the debittering of green olives variety Arauco (also known as Criolla) at 15C, 20C and 25C and lye concentrations of 1.50%, 2.25% and 3.00% of NaOH. The D F values ranged between 1.64 x 10 -10 and 9.20 x 10 -10 m 2 /s, and increased with increasing temperature and lye concentration. The D S values ranged between 1.27 × 10 -12 and 5.63 × 10 -12 m 2 /s, and also increased with increasing lye concentration and temperature. Activation Energies (Ea) of diffusion in the flesh and the skin were evaluated with an Arrhenius-type relationship. The Ea values for the flesh were 93.2, 72.7 and 64.8 kJ/mol, and 92.7, 91.46 and 33.67 for the skin for lye concentrations of 1.50%, 2.25% and 3.00%, respectively.