Supercritical fluid extrusion (SCFX) technology, first reported in 1992 by Syed S.H. Rizvi and Steven J. Mulvaney of the Institute of Food Science at Cornell University, combines two high-pressure processes -- supercritical CO2 processing and extrusion -- to extend the capabilities of conventional extrusion technology.

Supercritical fluid extrusion (SCFX) technology, first reported in 1992 by Syed S.H. Rizvi and Steven J. Mulvaney of the Institute of Food Science at Cornell University, combines two high-pressure processes -- supercritical CO₂ processing and extrusion -- to extend the capabilities of conventional extrusion technology.

The patented SCFX process expands the food mass or dough with supercritical carbon dioxide instead of steam, allowing use of dairy ingredients and heat-sensitive flavors which cannot withstand the high temperatures and shear of conventional water-vapor extrusion. "This expansion can occur at temperatures lower than those used in traditional extrusion processes, opening up many opportunities for milk-based ingredients," says Rizvi. By manipulating the properties of whey proteins, for example, manufacturers can improve the sensory characteristics of extruded products. "Whey proteins are particularly useful to improve the texture of extruded products manufactured at higher moisture content and lower temperature," he adds.

SCFX processing creates a composite structure with porous interior and smooth tight exterior, opening new product possibilities such as ready-to-eat cereals that stay crisp in milk, crunchier airy snacks and light fluffy waffles. "Our tests have shown that SCFX-expanded products enhance 'bowl life' by absorbing less liquid," says Rizvi. Whey proteins, calcium and vitamins can be directly added in the SCFX process to boost the nutritional content of foods. Heat-sensitive flavors can be added to instantly soluble powders. Dairy proteins also result in lighter-colored products, which might further boost their use.

R&D underway at Cornell's Northeast Dairy Foods Research Center (NDFRC) in Ithaca, NY and at several undisclosed food labs, using supercritical fluid injection and extrusion equipment supplied by Wenger Manufacturing, Inc., aims at commercializing the SCFX process. Foods being evaluated include cereal, snack, chocolate, confection, bakery, pasta, pet food and powdered products, Rizvi told Food Engineering. Research at NDFRC is funded by U.S. dairy farmers through Dairy Management, Inc. (DMI), which promotes demand for dairy products on behalf of the American Dairy Association, National Dairy Council and the U.S. Dairy Export Council.

New supercritical application

Supercritical fluid technology was first applied in food manufacturing to extract vegetable oils from oilseeds; extract flavors from hops, spices and other plant materials; decaffeinate coffee; and replace distillation in alcohol production. (See: "New Process Tool: Supercritical CO₂," Food Engineering, April 1982.)

As reported by FE, supercritical CO₂ has characteristics of both liquids and gases: it has the density of a liquid, but diffuses as a gas. The process compresses CO₂ under high pressure at a temperature above 31° C, its critical temperature where it cannot be liquified by adding pressure. At high pressures, it behaves like a liquid and functions as a solvent in extraction applications.

SCFX represents a new application for supercritical-fluid technology. According to Rizvi, conventional extrusion cooking -- which applies steam to expand the product -- is inherently a low-moisture, high-temperature process which limits the use of heat-sensitive, shear-sensitive ingredients. Water is both the "plasticizer" for the melt and the "blowing agent" for expansion, or "puffing." Product is expanded by uncontrolled phase change, and the process can be costly in terms of machine wear.

The SCFX process, on the other hand, "decouples" puffing from moisture content to minimize heat and shear damage to feedstock. Expansion occurs by nucleation and diffusion, with no phase change. Decoupling expansion from moisture content also reduces specific mechanical energy (torque) to reduce operating costs and machine wear. By controlling pressure drops, the number and size of microstructure cells in the product can be varied, thus controlling product texture.

According to Rizvi, connecting a supercritical fluid process to an extrusion process is relatively easy and can be accomplished at low additional cost ($30,000 to $40,000) relative to the price of the extrusion system.

Dairy Management, Inc., 10255 West Higgins Road, Suite 900, Rosemont, IL 60018-5616. Tel.: (847) 803-2077; Fax: (847) 803-2077; Website: www.dairyinfo.com

Wenger Manufacturing, Inc., 2405 Grand Ave., Kansas City, MO 64108-2519. Tel.: (816) 221-5084, (800) 833-0174; Fax: (816) 221-5086; Website: www.wenger.com