Separation, biomass harvesting

Harvest the cellulase-containing transgenic plants while they are green, macerate them, and separate the solid to produce an enzyme concentrate, which can later be used in the enzymatic hydrolysis of pretreated biomass. Depending on the production level of this enzyme in transgenic plants, the need for externally added cellu-lases in the enzymatic hydrolysis step might be avoided or minimized. 

The classical way to determine biomass concentration is typically an off-line method, namely to harvest a known aliquot of the culture suspension, separate cells by centrifugation, wash the cells and dry them to constant weight at a few degrees above the boiling point of the solvent (i.e. aqueous medium, usually... 

It is sometimes desirable to physically separate potential biomass feedstocks into two or more components for different applications. The subject is quite broad in scope because of the wide range of biomass types processed and the variety of separation methods that are used. Even the harvesting of virgin biomass involves physical separation technologies. Examples are the separation of agricultural biomass into foodstuffs and residues that may serve as fuel or as a raw material for synfuel manufacture, the separation of forest biomass... 

Straw and Other Crop Residues Crop residues are another source of renewable feedstock that do not find their way into milling processes, especially the residues which are already separated from the crop during or directly after harvesting. For most cereals and oil crops, this Hgnocellulosic biomass is called straw, for example, wheat straw, rice straw, or barley straw. The crop residues of maize are known as corn stover. 

After fermentation, the cells were separated from the fermentation broth by centrifugation (4000 g, 10 min, 4 °C) and concentrated to -10 cfii/ml. After harvesting, the cells were washed twice with phosphate buffer saline (0.1 M K2HPO4/KH2PO4 with 0.15 M NaCl, pH 7) in order to remove residual fermentation broth. As protectant, D(-)Sorbitol (Merck, Darmstadt, Germany) was added to the cell suspension. The concentration was 25 % related to dry biomass. 

Since most industrial enzymes are produced extracellularly, only unit operations used for recovering these types of enzymes are considered. For intracellularly produced enzymes, additional steps must be included to harvest and open the cells (e.g., by a bead mill or high-pressure homogenizer). This complicates the process and increases the costs hence this type of processing is avoided, if possible. A recovery process typically consists of a pretreatment, followed by a primary separation of the enzyme from the biomass. Later, the enzyme is concentrated by removal of water, and unwanted impurities are removed in a purification step. Figure 27.4 illustrates an example of a simplified recovery process. 

On the other hand, understanding different techniques for cell immobilization may help develop new applications of a certain approach. For example, self-flocculation is commonly used to separate yeast cells from culture broth in the brewery industry, which provides a clue for developing a cost-effective harvesting technology for microalgae biomass production. 

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