Preservation processes

Higher quality of plant material and meat is normally synonymous with less destructuring, therefore the requirements of the waste utilisation processes

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80-

Yield stress \

60-

\ Fracture

40-

stress

20

-1-1-1 Mill

T-1-1—i—i iri

10 50 100 200 500

Particle diameter (|m)

Fig. 8.5 Brittle-ductile transition in salt (from Roberts et al., 1989).

10 50 100 200 500

Particle diameter (|m)

Fig. 8.5 Brittle-ductile transition in salt (from Roberts et al., 1989).

may or may not be aligned with protocols for maintenance of quality. This is a general comment that pervades the transfer of rules from other industries.

Before destructuring, the waste stream may need to be preserved or to have the time scale of any change in properties lengthened. This is more important than it would be for raw material input to a food factory because of possible damage and mixing and removal of preservation methods after input to the food process line. The selection of the preservation technology to stabilise waste streams depends on their structure, moisture content and composition, and also the throughput and the processes downstream that derive components from the waste streams. Proper stabilisation of waste streams, before storage or transportation, is a necessary condition for the valorisation and possible upgrading of the streams, and the separation of valuable components.

Physical methods are preferred and elevated-temperature drying is the most common procedure for stabilisation and storage of these materials, although this process affects the quality of the extracted material. High-pressure treatment is often cited as preserving quality attributes better than thermal treatments but chilling and freezing are also options. In addition, chemical preservatives and sterilising filtration are further candidates for preservation protocols (Loncin and Merson, 1979). However, a barrier or 'hurdle' approach (Alakomi et al., 2002), such as where low pH, high pressure and refrigeration are used, has been observed to produce greatest longevity.

Different process conditions are applicable including freeze-drying (often used to control the stability of the material) and drying using physical techniques such as pressing, vacuum concentration and modified atmospheres. Heating, cooling and drying are described below.

178 Handbook of waste management and co-product recovery 8.4.1 Heating methods

Heating animal tissues is necessary for microbiological safety and to bring about desirable eating textures. Whereas destruction of microorganisms is achieved with rapid, high-temperature treatment, the inactivation of enzymes may require a lower temperature, and longer treatment time. Cooking of meat involves denaturation of structural proteins, and the activation and subsequent deactivation of proteolytic enzymes. Lipids and water are also excluded and there are structural changes in sarcomere tissue and myofibrils, and coagulation of proteins (Aguilera and Stanley, 1990). During heating, myosin, collagen and actin undergo successive endother-mal transitions with increasing temperature (Findlay et al., 1986).

Cooking of fruits and vegetables 'softens' them. The mechanisms can be loss of turgor, degradation of cell wall polysaccharides and gelatinisation of starch. In the extraction of useful ingredients, the thermal method may be engineered to yield either intact granules or to release cell contents including starch.

In the controlled dehydration of potato, starch swelling is minimised because cell rupturing is not desirable and separate intact granules are required. Respiration rates also increase with increasing temperature, which has a negative effect on storage life and the rate of loss of quality attributes (Aguilera and Stanley, 1990). Mild heating, such as pre-cooking or blanching, leads to cell walls of enhanced mechanical properties which gives improved texture. An alternative strategy used with heating is to use calcium to crosslink pectic substances which again tends to improve the mechanical properties of the tissues.

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