Investigation of some physical characteristics of plant structures which are used as sources of pectic substances

Summary We have investigated the physical parameters characterizing the porous structure of dried apple and orange peels, which are sources of pectic substances (PS). Mercury and nitrogen porosimetry were used to obtain quantitative data describing the characteristic diameter of the pores (dchar). Normally the diameter is between 0.13 and 0.39 μm for the apple samples, depending on the cultivar and treatment, and dchar˜ 0.27 μm for the orange samples. The total surface area of the capillaries (Stot) for the apple samples is in the range 7.18–25.52 m2g–1, and is 6.86 m2g–1 for the orange samples. The total density (ρtot) for the apple samples is 142.6–661.2 kg.m–3, and 684.4 kg.m–3 for the orange samples. The skeletal density for the apple samples is 226.8–864.4 kg.m–3, and 997.5 kg.m–3 for the orange samples. The porosity (ε) varies from 23.5% to 37.12% for the apple samples, and is 31.4% for the orange samples. The scanning electron microscopy investigation confirmed the conclusions from the mercury porosimetry that both sources of PS have a complex capillary‐porous structure. The latter structure includes pores with different shapes and dimensions, connected with each other in an unspecified manner. We present a suggested description of the kinetics of the water‐binding capacity in dried apple and orange peels, using the exponential equation q = q∞(1−e−kt). The values obtained for the kinetic parameters k and q∞ are: q∞ = (2.78 ± 0.02) g/g and k = (2.60 ± 0.13) × 10–3 s–1 for the apple peels, and q∞ = (4.62 ± 0.17) g/g and k = (3.23 ± 0.16) × 10–4 s–1 for the orange peels. The results obtained enable the development of theoretical models describing the processes of extraction of PS, and the construction of machines to be used in the PS extraction.