Intermediate moisture extrusion enhances enzyme activity.

Although extrusion is usually a low-moisture process, a moisture content of more than 25% may be useful. Georgia scientists have come up with two uses for such intermediate moisture extrusion processing: starch degradation with thermostable alpha amylase and the production of snack products.

The goal of researchers was to dextrinize starch during its extrusion in order to sweeten and reduce its viscosity, as well as to increase the nutrient density of weaning foods. The investigators also wanted to produce third-generation snacks by cooking high-moisture doughs, cooling them in the terminal barrel, and drying and frying the product. Generally, they found that the intermediate moisture extrusion approach makes possible the production of unique products.

First, a weaning food formulation--70% maize and 30% cowpea--was extruded with three amylase preparations in a single-screw extruder at 25%, 30% and 35% moisture levels at screw speeds of 50%, 75% and 100% of full speed. The investigators analyzed the extrudates for dextrose equivalents, viscosity, water absorption and water solubility indices.

The scientists developed third-generation snacks from tapioca or rice flour and fish or partially defatted peanut flour by extruding the product at 40% moisture levels. They studied the effects of the feed rate, barrel temperature and screw speed on product texture, bulk density and sensory quality.

The viscosity of all amylase treatments was less than 3,000 cP and as low as approximately 1,700 cP. The dextrose equivalents ranged from 1.56 to 3.75. The researchers found that doing the extruding at higher moisture levels was more effective than doing it at lower moisture levels. The greater amount of moisture enhanced both enzyme action and non-catalyzed starch hydrolysis.

High amounts of moisture also created a longer residence time in the extruder, allowing hydrolysis to proceed to a greater extent. The optimum conditions for producing tapioca, fish or peanut half-products involved using extrusion temperatures of 94 C to 100 C and screw speeds of 220 rpm to 400 rpm; or temperatures of 95 C to 100 C and screw speeds of 230 rpm to 400 rpm.

Consumer testing showed that more Asian consumers accepted the fish snack products than did American consumers. Peanut products were acceptable to both groups. For rice and peanut snacks, the optimum formulations and extrusion processing conditions were 30% to 38% peanut flour and a feed rate of 4.0 kg per hour to 5.2 kg per hour. Further information. Dick Phillips, Department of Food Science and Technology, University of Georgia, 1109 Experiment St., Melton Building, Griffin, GA 30223; phone: [phone]; fax: [phone]; email: [e-mail].