Factors in the selection of extruded or pelleted diets for research
Graham Tobin
Harlan Teklad, Station Road, Blackthorn, Oxon OX25 1TP, United Kingdom
When selecting whether to use pelleted or extruded diet for particular research purposes, it is important to understand the processes. The major effect of the pelleting and extruding processes is to apply heat to the dietary ingredients. Pelleting and extrusion differ in the temperature rise of the ingredients; the duration of exposure to that temperature; the amount of moisture in the heating phase; and the final forming of the product.
In pelleting, the dry materials are mixed with dry steam in a conditioner, with a modest increase in moisture (to <16%). High cereal products, such as rodent diets, are usually conditioned at 70-80°C: high fibre diets are conditioned at a lower temperature, such as 60°C. Retention time is typically 20-30 secs. The extruder conditioning phase is typically much longer (c.90-120 sec), of a higher temperature (c.90-95°C), and with greater addition of water and steam (to c.20%). Pellet hardness is a key factor in determining the food intake and growth of laboratory animals.
After conditioning, the ingredients are fed into the extruder barrel with more steam and water; the mix pressurised, superheating the water in it. Dwell time is about 30 sec and with the high level of moisture (20-25%) and a die face temperature of upto 150°C, a high degree of cooking takes place. Product leaving the extruder is 'inflated' rather than being compressed. Supposed benefits of extruded products include increased energy intake, digestibility and growth rate. Such benefits depend on the differential in processing conditions between extrusion and pelleting. The perceived improvement with an extruded diet also may disappear if the comparison is with a soft, pelleted diet. Benefits of digestibility will depend on the complexity of the gastro-intestinal tract of the recipient animal. Greatest benefits are to animals such as the dog, cat and ferret with short simple gastrointestinal tracts. Extruded diets retain good physical structure even at fat additions as high as 20%. Destruction of micro-organisms and insects is almost certainly greater with extrusion than with pelleting. However extrusion at typical temperatures and dwell times is insufficient to sterilise the product (cf. autoclaving at 121°C for 20 min) and post-production contamination is a major risk. Destruction of biological oxidants and enzymes by the high temperature processing results in better stability of labile nutrients such as vitamins, which more than compensates for their slightly greater loss in extrusion. Grinding of extruded product is reported to produce a much more uniform particle size, improving the homogeneity of drug addition to ground product.
The gelatinisation of extruded product make them ideal for autoclaving, reducing the clumping and hardening that can be a problem with pelleted diets. The lower density means, however, that food hoppers must be filled more frequently. It also increases packaging, irradiation and transport costs, which with higher equipment and energy costs, increase the cost of extruded products considerably.
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