Rheological measurements help improve product texture.

Peanut flour is a dry ingredient prepared after the partial extraction of oil from roasted peanut seed. In food applications, these flours are often dispersed in water before being further processed. This additional processing usually includes a heating step.

Scientists at North Carolina State University wanted to characterize the rheological changes associated with the heating and cooling of peanut flour dispersions. They found that a range of rheological properties are achievable with the dispersions. Rheological measurements will greatly help technologists improve the texture of products containing the dispersions.

In their experiments, the researchers used commercially available peanut flours. The flours had a fat content of 12% and 28% and were light and dark roast. These were dispersed in deionized water (20% by weight). The researchers adjusted the pH to 8.0.

Then the investigators heated the dispersions from 40 C to 90 C at 1 C per minute. These were subsequently cooled to 40 C while any rheological changes were monitored. Large strain measurements were conducted using continuous rotation at shear rates from 0.1 to 100 l per second. Small strain measurements also were undertaken. These involved oscillations at a stress of 1.5 Pa.

The small strain data indicated that the gel point occurred at lower temperatures for the 12% fat flours--about 65 C to 72 C--compared to the 28% fat flours, for which the gel point occurred at greater than 80 C. Additionally, the light roast dispersions had lower gel points than the dark roast dispersions. Similar patterns were observed during large strain testing, in that the apparent viscosity (50 1/s) increased more rapidly and at lower temperatures for the 12% fat flours--at about 70 C--than for the 28% fat flours, at more than 80 C.

Large strain shear rate data indicated that all of the dispersions were shear-thinning. Higher amounts of soluble protein in the 12% fat flour dispersions possibly contributed to differences in viscosity. However, the insoluble portions of the dispersions, when they were centrifugally separated and reconstituted, had rheological characteristics that were similar to the original dispersions.

Further information. Tim Sanders, USDA-ARS Market Quality and Handling Research Unit, 236-A Schaub Hall, Department of Food Science, North Carolina State University, Raleigh, NC 27695; phone: 919-515-6312; email: tim_sanders@ncsu.edu.