Isoflavone retention during processing, bioaccessibility, and transport by Caco-2 cells: effects of source and amount of fat in a soy soft pretzel

Abstract

The impact of source and amount of lipid used to prepare a soy soft pretzel on the bioaccessibility and transport of isoflavones was investigated using the coupled in vitro digestion/Caco-2 human cell model. Pretzels were prepared without or with 2.9 or 6.0% exogenous lipid from either shortening, canola oil, ground almond, or ground hazelnut. The isoflavone backbone structure was stable during pretzel production, although there was partial conversion from malonyl and acetyl glucosides to simple glucosides and aglycones. Endogenous β-glucosidase activity in ground almond facilitated partial conversion of simple glucosides to aglycones during proofing, resulting in a slight decrease in bioaccessibility of isoflavones as compared with other sources of lipid. Amount and source of lipid did not affect bioaccessibility or uptake and metabolism of isoflavones by Caco-2 cells, although transport across the monolayer was greater with the lesser amount of shortening. These results suggest that the isoflavone structure, but not source or amount of lipid in a soy pretzel, may affect bioavailability of isoflavones.

Figure 1

Representative HPLC chromatograms of isoflavone profiles in extracts from wheat flour, soy flour, soy milk powder, soy soft pretzel with 2.9% shortening, soy soft pretzel with 6.0% lipid from almond, chyme of the pretzel with 6.0% almond digested in vitro, and the aqueous fraction of the chyme. Isoflavones were identified using retention times and absorption spectra of standards from simple glucosides and aglycones and from published literature. Isoflavones eluted in the following order: (1) daidzin, (2) glycitin, (3) genistin, (4) malonyldaidzin, (5) malonylglycitin, (6) acetyldaidzin, (7) malonylgenistin, (8) daidzein, (9) glycitein, (10) acetylgenistin, (11) genistein. Acetylglycitin was not detected.

Figure 2

Addition of almond to pretzel formulation reduced the relative bioaccessibility of isoflavones. Data are the mean ± standard deviation for at least 12 independent digestions of pretzels with indicated source and amount of lipid. NAL, no added lipid; SHO, shortening; ALM, lipid from ground almond; CAN, lipid from canola oil; HAZ, lipid from ground hazelnut. The amount of lipid from these sources added to formulations was either 2.9 or 6.0% lipid, dry weight. Endogenous lipid content from soy components was 1.9 ± 0.2%. Different letters over error bars indicate that differences in means are statistically significant (p < 0.05).

Figure 3

Flux of isoflavones across the differentiated monolayer of Caco-2 cells. Apical media were prepared from simulated digestions of soy soft pretzels made with 2.9% shortening (SHO), 2.9% canola oil (CAN), and 6.0% shortening or 6.0% canola oil. Different letters above bars indicate means are significantly different (p < 0.05).