Introduction

Destructuring of plant- and animal-derived materials and waste residues provides a basis for exploiting potentially marketable components. Such an approach can be carried out on a number of length scales; e.g. metres, millimetres, microns, depending on the structural entities.

Plant organs comprise tissues, which can be separated although that is not normally a desired final result. Nevertheless, fruit or vegetable skins may be separated for purposes of recovery of hydrophobic waxes. The tissues consist of cells that might, if separated by specialist thermal and dehydration treatments, provide a suitable final product (such as in the production of dehydrated potato granules). Such destructuring of the tissue must be carried out in a specific way to achieve whole intact cells, otherwise the tissue will merely be fragmented into small tissue pieces containing assemblies of intact cells surrounded by broken ones. Having arrived at this point there are a host of structural and storage-related polymeric species in the cell walls and cells including a combination of lignin, cellulose, hemi-celluloses, pectic polysaccharides, starch, protein and lipids. There are also a large number of solutes in the solutions within cells. If the aim is to gain access to the cell contents, the deconstruction strategy would be to prevent cells from separating and to break open as many cells as possible, enhancing juice accessibility.

Although generally much richer in protein, and lacking a polysaccharide-based cell wall, animal cells exhibit a number of similarities with plant cells

(Aguilera and Stanley, 1990). The cell is possibly the most important structural feature in biological tissue.

Processing rarely achieves the maximum benefit from foods in single stages or unit operations. The following operations may be used in isolation or in combination. Heating has an effect on almost all biological materials. Biopolymers have some of the same types of responses as synthetic polymers with respect to temperature, time and stress, although these are usually more complex as a result of irreversible changes as a result of structuring or destructuring at the molecular level. Heat transfer is a key process and a number of options are classically available. Water has many roles and affects the mobility of biopolymers and the processes chosen (dry-milling-type operations are key to separation of flour components and extraction of cell contents in plant materials). The availability of water will also affect the chemical and microbiological stability of food microstructures (Levine and Slade, 1993). Mass transfer, such as water removal, is again a classical operation and can be achieved in a number of ways. Heat and mass transfer are usually undertaken together as in many drying processes.

A destructuring process, which will inevitably involve separation of the biological material, is difficult to deconvolute from the heat and mass transfer associated with the unit processes. Most processes are actually combinations of chemical engineering unit operations and obviously food processes are equally applicable to the treatment of food wastes and co-products that are often sourced in the clean environment of a food factory.

If the food material has already been processed for its primary resources - e.g. grain for brewing, vegetables and fruit for juice, seeds for oil extraction - then a second operation (or often a third in the case of olive oil extraction) is required to deal with the residue, often with the added targeting of chemical or biochemical processes. Brewer's spent grain, oil seed and vegetable pulps can be processed as added bulking ingredients or further processed to remove more components. The challenge in waste utilisation is in using processes in a new way to achieve added-value products, often with the complexity that the time scale is greatly reduced by the onset of spoilage. Enzyme and microbial attack need to be halted or exploited depending on the product(s).

Finally, food wastes can be dealt with for both food or non-food utilisation, the latter opening up a number of additional, but generally low-value, opportunities. Fundamental to process design for destructuring is an understanding of the effect of the base variables, from which decisions can be made to tailor existing processes or their combinations, or to investigate new opportunities for bespoke processes. The economic demands are, however, stringent given the opportunity to use the food factory raw material input directly if a large added-value process is developed, rather than have to deal with damaged, aged or mixed-input material as it is diverted from the factory line.

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