Analysis of quality protein changes in nixtamalized QPM flours as a function of the steeping time

Abstract

This study showed the protein changes in Quality Protein Maize (QPM H-368C) during the traditional nixtamalization process as a function of the steeping time from 0 to 15 hr. Protein content (N x 6.25), pH, protein fractionation, reactive lysine, essential amino acids, and protein digestibility were analyzed to explain the protein quality modifications in nixtamalized corn flours (NQF). The thermoalkaline process increased significantly (P <= 0.05) the protein content (5.57 +/- 0.86%) in NQF obtained at 3, 5, 7, 9, 11, 13, and 15 hr of steeping time compared with native corn or corn without treatment (NC). The pH values of NQF were not proportional to the steeping time and significantly different (P <= 0.05) between them. At 5 hr critical steeping time, the total lysine and reactive lysine content decreased severely (36 and 32%, respectively) with statistical differences (P <= 0.05) compared with NC. On the other hand, the tryptophan content decreased significantly (P <= 0.05) at steeping times of 5-15 hr (38.70 +/- 6.7%) compared with NC. The changes in the lysine and tryptophan content were not proportional to the steeping time. The protein recovery in the albumin and globulin fraction diminished (P <= 0.05) with respect to raw corn. The protein recovery for gamma-zeins, glutelin-like proteins, glutelins, and residue increased. A significant (P <= 0.05) decrease was found in the essential amino acids in NQF with 3-7 hr of steeping time compared with NC. Equally important was the reduction in protein digestibility observed in NQF steeped at long steeping times (11-15 hr) with significant (P <= 0.05) differences compared with NC. The protein solubility distribution along the steeping step and the essential amino acids location, specifically lysine in corn kernel, could explain partially the protein quality changes observed in this research. Finally, these results contribute to reconciling discrepancies associated with the protein quality modifications in nixtamalized corn reported previously in literature.