The results obtained in this study provide important information for the optimization of VI process parameters. The difference in pressure between the impregnation of the two materials could be due to the difference in wetting angle and pore size, but impregnation could also be affected by recompression of the expanded leaf. Impregnation of apple tissue was seen almost as soon as restoration of atmospheric pressure was started, while spinach tissue impregnation occurred later. However, the pores in plant tissue have evolved to allow the diffusion oxygen and carbon dioxide, which is much faster in gas than in water, and it would thus not be surprising to fi nd that the wetting angle was high to avoid the plant from being “ drowned ”. The discrepancy between our results and those predicted by the HDM model may be due to the fact that it was assumed in the HDM model that the impregnating solution wetted the pores. Under the experimental conditions used, sample impregnation was only detectable during the restoration of atmospheric pressure. No impregnation was observed during this period either. To verify this, additional experiments were carried out in which the duration of the lowest working pressure was increased to 20 min (step 8 in Fig. Our experiments showed no impregnation under the equilibrium conditions at 150 mbar.
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