The potential for destructuring of food processing waste 173 Molecular basis of mechanical behaviour

Polymers are classically described as amorphous (e.g. polystyrene) or semi-crystalline (e.g. polyethylene) structures, the properties of which change dramatically with increasing temperature at the glass transition and crystalline melting temperatures, respectively.

The glass transition marks the change in molecular mobility and is often identified by differential scanning calorimetry and dynamic mechanical thermal analysis (Levine and Slade, 1993). The low-strain mechanical properties, such as the modulus, fall dramatically at the glass transition temperature. The high-strain mechanical properties are not the same but they often undergo a brittle-ductile transition which maps onto the glass transition for synthetic amorphous polymers (Ward, 1983). The origin of the brittle-ductile transition is in the intersection of the weakly temperature-dependent brittle fracture stress and the more strongly temperature-dependent yield stress, such that the latter mode of failure is favoured above a certain temperature. Polymers have time-dependent properties but the yield stress increases more rapidly with strain rate than the fracture stress. This means that the intersection of the two stresses shifts to high temperatures such that substances will remain brittle to higher temperatures when tested at higher rates; this has ramifications for both texture and processing.

Plasticisers, especially water, affect these transitions in biopolymers extensively, in addition to the other role of water in undamaged cells whereby it will affect the properties of materials through turgor pressure. In an example of a plasticised starch, Ollett et al. (1991) observed a gradual change from a glassy, shiny broken surface to one with rough morphology as moisture content increased (Fig. 8.3(a) and (b)). Meat also provides an example of the operation of the brittle-ductile transition where a fibre-filled composite model might be applicable. Dobraszczyk et al. (1987) tested frozen meat and the scanning electron micrographs of fracture surfaces showed brittle fracture of the matrix giving way to some fibre debonding and pullout. Tensile testing of notched samples of cooked meat (Purslow, 1985) showed debonding between muscle fibre bundles and then fracture (Fig. 8.3(c) and (d)) without debonding or pullout of individual fibres.

Toughness is the mechanical engineering property defined as the energy used to propagate a crack per unit area. Measurements can be made through cutting experiments (Hiller and Jeronimidis, 1995). Toughness increases with turgor and it takes more energy to cut tissues made turgid by hydration (Fig. 8.4(a)). Cooking generally softens vegetables (Fig. 8.4(b)), but this can be due to the cell walls becoming more easily broken or due to a change of failure mode to cell separation, as shown in Fig. 8.2, which requires less energy. In the examples in Fig. 8.4(b), a cell breakage to separation change occurs in potato heated for 20 and 30 min. These data are relevant to various size-reduction processes where energy must be expended to create new surfaces (see below).

Fig. 8.3 (a) and (b) Failure in samples of starch/glycerol (80 : 20 weight/weight of non-aqueous components); water content of sample is (a) 6.5% (wet weight basis) and (b) 9.9% (wet weight basis) (from Kirby et al., 1993). (c) and (d) Failure in cooked meat (from Purslow, 1985).

Because fracture mechanics involves a dimension, as in the Griffith criterion for semi-infinite plates and the Ashby formula for notched foams, it is not surprising in considering the brittle-ductile transition that the brittle fracture stress depends on the size of the specimen. The observation that it is harder to break objects as they become progressively smaller is in accordance with analysis that there is a brittle-ductile transition indepen-

Fig. 8.3 cont'd

dent of temperature. This is elegantly described by Roberts et al. (1989) in studies on salt (Fig. 8.5). This science has implications for size-reduction processes such as grinding and milling (see below).

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