Studies of high-fiber foods: I. The effect of a pinto bean diet on plasma cholesterol in hamsters. II. The effect of freeze-drying and heating during analysis on dietary fiber in cooked and raw carrots

The effect of a diet including cooked pinto beans on plasma total cholesterol in hamsters was investigated. The legume diet had 8.1% total fiber (1.0% soluble, 7.1% insoluble), all from pinto beans. Animals initially consumed a hypercholesterolemia-inducing diet. One control group remained on this feed for the duration of the study, and another consumed a diet with 8.6% total fiber (cellulose).

The pinto bean diet lowered plasma cholesterol significantly (13%) only in hypercholesterolemic hamsters (approximately 75% of the animals). When all animals were considered, the legume diet had no effect on cholesterol. Variability among hamsters in plasma cholesterol levels and changes was large. Results indicate the importance of including only hypercholesterolemic animals in studies of dietary modifications to decrease blood cholesterol and also that a large sample size may be required to detect significant effects.

Soluble and insoluble dietary fiber in raw and cooked carrots were measured by a typical enzymatic-gravimetric procedure. Analyses were done with and without freeze-drying and the starch gelatinization treatment (heating 1.25 hrs at 121°C). The gelatinization procedure caused a 3- to 5-fold increase in soluble fiber. In general, insoluble fiber decreased in the same proportion as the soluble fraction increased, suggesting a conversion of the former to the latter. The differences were greatest for cooked carrots and freeze-dried raw carrots. Compositional analyses indicated that heating increased arabinose, galactose, and uronic acids in soluble fiber fractions.

Cooked carrots had 3-9% more soluble fiber than raw carrots, with the difference being greater when the analysis included the gelatinization procedure. However, freeze-dried raw samples analyzed with gelatinization had the same level of soluble fiber (ca. 10%) as their boiled counterpart, suggesting an interaction between freeze-drying and the heat treatment in raw carrots.

Freeze-drying and heating to gelatinize starch are an integral part of many standard dietary fiber methods. Results of the present study suggest these treatments may mask differences between the amount of fiber in cooked and raw foods, and that dietary fiber values may not accurately represent the level of this component in foods as they are eaten. The results also raise the possibility of increasing soluble fiber by cooking modifications.