Biological molecules

A brief summary is given here as each class of biopolymer has been the subject of numerous studies. In addition, important smaller molecules are lipids comprising glycerides and fatty acids. Phospholipids are important in membranes; these feature strongly in other structures and can be exploited for selective permeability.

Cellular organelles and inclusion bodies

Primary tffff/

cell wall

Plasma membrane

Vacuole

Middle lamella

Plasmodesmata

Intracellular space

Fig. 8.1 (a) Hierarchy of structures in plants (using material from Jackman and Stanley (1995) and Carpita and Gibeaut (1993)). (b) Hierarchy of structures in meat (from Jolley and Purslow (1988)).

Muscle fibre bundle

Epimysium

Muscle fibre bundle

Epimysium

Myofibril

Thin filaments

Thin filaments

Fig. 8.1 cont'd

Proteins

Proteins provide the structural elements of many foods and their amino acids can be arranged in many ways. Globular, temperature-sensitive proteins are illustrated by whey and soya, random-coil, temperature-insensitive proteins are typified by casein (Visser, 1988). Protein-protein and protein-water interactions lead to functionally important structures. Heat leads to protein unfolding and association with other components such as carbo-

170 Handbook of waste management and co-product recovery (a) Crunchy

Hypothetical tooth

Hypothetical tooth

Ruptured cells

Cell walls

Cell walls

Cell walls

Ruptured cells

Cell walls

(b) Soft/mealy

Hypothetical tooth

Intact cells separate

Fig. 8.2 Cell separation/breakage schematic.

Hypothetical tooth

Intact cells separate

Fig. 8.2 Cell separation/breakage schematic.

hydrates and lipids. Kinsella (1978) has reviewed processes for texturing proteins.

Collagen is significant as a connective, rod-shaped protein in meat tissues. Collagen fibrils are formed from helical molecules and in turn the fibrils associate to form collagen fibres. In plant cell walls there are various proteins, glycoproteins and enzymes.

Polysaccharides

Cell wall polysaccharides comprise cellulose, hemicelluloses and pectins (Waldron et al, 2003). Cellulose is the most abundant carbohydrate and the principal structural component of plant tissue and is also fibrillar in structure. Models of the cell wall (Fig. 8.1) show how these polymers interact with proteins (Carpita and Gibeaut, 1993) and these models provide a powerful indicator of the role of composite theory in explaining cell wall deformation (see below). Pectins are also located in the middle lamellae regions where they can be thought of as the glue that sticks cells together.

Starch comprises two polymers: amylose and amylopectin. In an excess of water starch gelatinises at 60-80 °C. With reducing water content, a second melting transition is observed (Donovan, 1979). The amorphous elements of starch have a glass transition that also varies with moisture content and degree of crystallinity of the starch (Zeleznak and Hoseney, 1987). Starch can be transformed to different extents by processing, with granules being partially swollen and gelatinised, or destroyed (Holm et al., 1988).

Lignin

Lignin is a phenolic cross-linked polymer in plant cells; it makes a significant and important contribution to the structure of wood but is less important in food materials (Smith et al., 2003). In thermal analysis, transitions were observed for wood due to lignin and hemicellulose, which are both plasticised by water (Kelley et al., 1987).

Biopolymer mixtures

In addition to their interactions with other molecules such as lipids and fats - as the building blocks of the cell - structures are also fabricated from biopolymers. They and their mixtures with smaller molecules are exploited in fabricated foods so as to produce new structures unlike those of the original ingredients. The simplest fabricated structures are edible films (Krochta, 1992) and packaging films (Shogren et al., 1993), whereas examples of the more complicated structures are the aerated and filled structures such as bread and salami.

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