Cellulosic materials, digestibility

Both pilot-plant and plant-scale processes for cellulose hydrolysis or digestion by acids, enzymes, and microorganisms have been built (I, 7,15,27). Acid and enzyme processes usually have as their objective the production of a sugar syrup, while the microbial process usually results in microbial protein for animal feed. Figure 10 is illustrative of a microbial process (29) that has been developed to convert the unused cellulosic material in manure to recycle feed. Similar processes have been developed... 

Pulverization can reduce the size as well as the crystallinity of cellulosic materials and increase the surface area and bulk density. It is also possible to separate part of the hgnin from carbohydrates which makes it easier for microorganisms to digest cellulose. Various equipment, such as a compression mill, a bead mill, an extruder, a roll mill and disc refiners, etc., can be used for pulverization. Unfortunately these methods tend to be very expensive and too energy intensive. For sohd-state fermentation, if the particles are too fine, the oxygen mass transfer will become a big problem therefore, hghtly crushed or just ground raw material will suffice. 

Alkali treatment of cellulosic materials can remove hgnin as well as part of the hemicellulose and expose cellulose for fungi to digest. In addition, the alkah treatment method can swell and disrupt the structure of cellulose. Commonly used alkalis includes NaOH, Ca(OH)2 or CaO and aqueous as well as gaseous NH3. For solid-state fermentation, after alkah treatment, the cellulosic materials do not need to be washed, because all the solubilized hgnin and hemicellulose can be retained in the raw material [15]. After the addition of other nutrients, the pH has to be adjusted to a value of approx. 6.5 with H2SO4. 

Cellulosic material is treated with steam at 200-240 °C for between about 30 s and 20 min then withdrawn from the reactor quickly to create an explosion. The result is that hemicellulose is decomposed into low-molecular-weight, water-soluble products and hgnin is also degraded both effects are favorable for digestion [17-21]. Steam explosion has been commercially appfied in Canada... 

Table III. Digestibility of Various Cellulosic Materials by the Organism...

Celluloses.—Papers presented at a symposium on the thermal uses and properties of polysaccharides and lignins have discussed the pyrolysis of cellulosic materials. Phenols and carboxylic acids present in condensates of the smoke of cigarettes containing a cellulosic tobacco substitute have been identified and measured. The solubility of holocelluloses in DMSO—paraformaldehyde has provided the basis for a rapid laboratory method for predicting the dry-matter digestibility of cellulosic materials. The amount of dry matter solubilized by this chemical system correlated with that digested by rumen fluid in vitro or by cellulolytic bacteria. 

Historically, dietary fiber referred to iasoluble plant cell wall material, primarily polysaccharides, not digested by the endogenous enzymes of the human digestive tract. This definition has been extended to iaclude other nondigestible polysaccharides, from plants and other sources, that are iacorporated iato processed foods. Cellulose [9004-34-6] (qv) is fibrous however, lignin [9005-53-2] (qv) and many other polysaccharides ia food do not have fiberlike stmctures (see also Carbohydrates). 

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