Hightemperature shorttime extrusion

Extrusion cooking is arguably a combination process in its own right and has been demonstrated to be a useful process for the production of foods and feeds. The process converts biopolymers, e.g. proteinaceous and starchy materials, into a 'melt' for forming through dies (Smith, 1992). In the case of starchy materials the breadth of processing possibilities dictates that the extruder may merely compact the flour, grit or starch, or bring about disruption and degradation. Water solubility increases with specific mechanical energy (SME) input for maize grits (Kirby et al., 1988), although above a certain value of SME the maize comprises completely disrupted granules. With increasing SME, the water absorption index reaches a peak before decreasing with further solubility and SME increase. Similar studies on maize starch show that as the SME increases, the intrinsic viscosity, which is proportional to molecular weight, decreases (Parker et al., 1990). Figure 8.8 shows microstructure and molecular size changes in pure starch extrusion cooking (in different equipment).

Extrusion cooking was quickly exploited to produce textured protein from defatted soybean flour and isolates (Frazier et al, 1983; Stanley, 1989). Denaturation and association steps lead to the formation of a 'melt' as with starchy materials. Texturised proteins can be produced as meat analogues by extrusion cooking and formation through an appropriate die assembly. Conventionally, spinning processes are used to form textured proteins. Cheese and fat analogues have been formed from caseinate (Cavalier et al., 1990; Queguiner et al, 1992).

Another use of the extruder has been in fruit leathers (McHugh and Huxsoll, 1999). Using fruit as an input in this type of process indicates what could be done with the factory-grade off-cuts from fruit and vegetable processing. Extrusion cooking has also transformed materials that would otherwise be of limited usefulness such as hard-to-cook beans (Stanley and Aguilera, 1985); it is now possible to produce an extrudate that can be used to make an 'instantised' food (Martin-Cabrejas et al., 1999).

Mitchell and Areas (1992) summarised the effect of extrusion cooking on different animal proteins and compared this with effects on soya isolate. Their observations have implications for the upgrading of meat or mixed waste. Proteins from animal by-products have been texturised after partial defatting by extrusion; Areas and Lawrie (1984) showed that phospholipids stabilise the proteins.

Another extrusion area relevant to co-product use is the extrusion of pet foods and fish feeds (Rokey, 1994). Fish feeds comprise fishmeal, wheat and various vitamins, minerals and binding agents (Oliveira et al., 1992). The extruded product needs to have a structure and density and be able to absorb fat or oil. Its water absorption can be modulated so that the particle sinks at a predetermined time.

Onion waste example

In a complementary experiment to the pressure cooking of onion waste (section 8.6.3), extrusion cooking was carried out on cell wall material from

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

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

Feed throat

Screw Conformation

Filling

200

100

100

100

52

35

35

Lower part of the chambers

-Completely

End product

Starch

Physical state

Intrinsic viscosity (ml/g)

Granular state with birefringence 240 238 242

Continuous solid phase

"white spots" 176 152 143 132 120

Amorphous expanded phase

Fig. 8.8 Changes in (a) microstructure and (b) molecular size in extrusion cooking of maize starch ((b) from Colonna et al., 1983).

Zone the same white, fleshy outer leaves. The solubility of pectic polymers and hemicelluloses was increased and there was increased swelling and some depolymerisation of the cell wall material (Ng et al., 1999). The difference compared with pressure cooking was that extrusion cooking degraded the arabinose side chains more and the galactose side chains less.

The potential for destructuring of food processing waste 189 Potato outgrades example

Waste includes outgrades that do not meet retail standards for size and shape. They are often not even collected from the field, a problem that was recognised in the 1980s. One approach is to use a twin-screw extruder to deal with the outgrades as a second feed stream with potato granules as the primary and more conventional feedstock. Potato granules are used as the source for domestic mashed potato and for industrial extrusion of potato-based snack products. They are, however, produced by energy-intensive processes that involve cooking and drying. The extrusion of potato granules rehydrates and heats them again to form snack pellets. By this time any original potato flavours have been largely lost. One concept is to use the potato outgrades as a source of unprocessed potato to be cooked once rather than twice and to supply its intrinsic water content to the dehydrated granules (Ferdinand et al., 1989). Granules are conventionally fed gravi-metrically into the feed end of the extruder but the addition of diced potato was achieved with an auger device feeding in part-way along the barrel against the pressure developed at that stage. An alternative is to configure the twin-screw extruder so that the pressure falls to atmospheric levels; however, some form of forced pumping would still be required.

Destructuring of Brewer's spent grain (BSG) example The extrusion cooking process is often used to incorporate dietary fibre into expanded cereal-based products (Smith, 2003). Lue et al. (1991) added sugar beet fibre and Hu et al. (1996) added soy fibre to maize in both cases. Spent grain was used in extrusion cooking by Wampler and Gould (1984). Brewers' spent grain (BSG) is a major by-product of the brewing industry and is mainly used as cattle feed. The composition of BSG includes arab-inoxylan, protein and residual starch, and therefore it could be more economically used. To investigate this, BSG has recently been subjected to milling and sieving to produce palea and lemma rich in arabinoxylans, and also to produce protein- and starch-rich material (Jay et al., 2006).

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