Drying

In addition to preservation, drying brings about structural changes at a range of levels from macroscopic shrinkage, loss of cellular structure to molecular changes (Smith, 1991).

Slow drying reduces mass gradients and uniform shrinkage occurs through the specimen. Moisture gradients induce stresses that can lead to case hardening and trapping of the water, which then escapes by cracking of the surface leading to reduced quality. In general flaws reduce strength

Filtration with filter cake

Filtration with filter cake

A p>osmotic pressure

Micropore filtration and ultrafiltration

A p>osmotic pressure

Reverse osmosis

Gel filtration

Gel filtration

Fig. 8.6 Schematic separation barriers where Ap is the hydrostatic pressure difference applied to the solution (from Loncin and Merson, 1979).

and, again, moisture gradients, e.g. in the case of biscuits, lead to cracks called 'checking', which can lead to easy breakage (Wade, 1987). Controlled changes in moisture and temperature during the drying process - to make the moisture distribution as even as possible - allay these problems and can be 'tuned' to minimise drying times.

In addition, the composite structure of the specimen being dried will affect the structure during and after drying. Although cellular plant structures dry more completely and rapidly if their structures are broken down by heat treatments, this may affect rehydration and post-rehydration structure. While this is important in food texture, the ease of cell content removal will also favour retention of elements of the structure although the economics of drying may favour loss of structural barriers.

Drying may induce crystallisation and aggregation of certain moieties such as sugars, polysaccharides and proteins. The caking temperature, which is related to the glass transition of polymers, will determine the macroscopic behaviour of biopolymeric powders (Levine and Slade, 1993).

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