Moisture content cell wall

Fiber-saturation point is the moisture content of celhilar materials (e.g., wood) at which the cell walls are completely saturated while the cavities are liquid-free. It may be defined as the equihbrium moisture content as the humidity of the surrounding atmosphere approaches saturation. 

Simulation models describe the various conditions occurring during a press cycle (gradients of the temperature, the moisture content, the steam pressure and the formed bond strengths) which lead both to microbuckling of the wood cell walls by their moisture and temperature-induced densification (Fig. 6) [215-218]. 

Wood is a hygroscopic material, due to the fact that the cell wall polymers contain hydroxyl groups. In an environment containing moisture, dry wood will absorb moisture until it is in equilibrium with the surrounding atmosphere. Similarly, saturated wood, when placed in an atmosphere of lower relative humidity (RH), will lose moisture until equilibrium is attained. If the wood is placed in an environment where the RH is stable, it will attain a constant moisture content (MC), known as the equilibrium moisture content (EMC). At this point, the flux of water molecules into the cell wall is exactly balanced by the outward flux into the atmosphere. 

Wood is a material that has evolved to fulfil a number of structural roles in supporting the tree canopy, such as resisting wind loads, and so on. These mechanical properties can be exploited and used in various structural roles by mankind. It is, however, important to note that wood, as it performs in the natural environment, is fully saturated with water. When used in man-made structures, wood is invariably dried to a relatively low moisture content (below the FSP) and as a consequence exhibits physical properties that are subject to change as the moisture content of the cell wall changes. 

In measurements of the dielectric relaxation of water adsorbed on acetylated wood, a large change in the activation enthalpy and entropy of dielectric relaxation was found to occur at 6 % moisture content (Zhao etal., 1994), this presumably being attributable to the onset of formation of capillary water in the cell wall. 

Chow, S.-Z. (1972). Hydroxyl accessibility, moisture content, and biochemical activity in cell walls of Douglas-fir trees. Tappi, 55(4), 539-544. 

The fact that EMC reduction as a function of acetyl content is the same for many different llgnocelluloslc materials Indicates that reducing moisture sorption and, therefore, achieving cell wall stability are controlled by a common factor. The lignin, hemlcellulose, and cellulose contents of all the materials plotted in Figure 2 are different (Table II). Earlier results showed that the bonded acetate was mainly in the lignin and hemicelluloses (33) and that Isolated wood cellulose does not react with uncatalyzed acetic anhydride ( 4) ... 

Although studies on potato structure had been carried out previously using conventional SEM, van Marie et al [70] used cryo-SEM to advantage in this high moisture material. The fracture planes of cooked and uncooked samples were used to help characterize cell wall adhesion in the four potato cultivars. In particular, differences in cell wall contact area and surface detail were used to explain the mealy versus firm textural attributes in the cultivars. By determining the parameters which contributed to the texture of potatoes, processing conditions and selection of suitable raw materials could be facilitated. Such information would be difficult to obtain with conventional, chemically fixed material due to the high moisture content and the inability of standard chemical fixation to retain carbohydrate-based structures. 

Table I illustrates the range of new treatments introduced during the period of 1930 to 1960 (J ). Some of the monomers are of the condensation type and react with the hydroxyl groups in the wood, while other chemicals react with the hydroxyl groups to form crosslinks. Another group of compounds simply bulk the wood by replacing the moisture content in the cell walls.

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