Effects of Infrared Radiation on Protein Solubility and Antioxidants Content in Maize Flour

The objective of these studies was to observe the effects of the process of micronisation on the protein solubility, as well as, on contents of antioxidants and vitamins in whole maize flour. Maize hybrids differing in the kernel colour developed at the Maize Research Institute, Zemun Polje, Belgrade, Serbia, were selected for these studies. The semi-flint hybrid ZP 633 has pronounced yellow kernels; the pericarp of ZP Rumenka is dark red, while the hybrid ZP 551b is characterised by a white colour of kernels. Grain of selected genotypes was used in the production of flour. Whole flours were produced by milling of intact maize fresh grain in a stone grinder. In order to determine the effect of the process of micronisation on nutritive properties of maize flour, intact kernels of selected ZP hybrids were subjected to the process of micronisation at the temperature of 145°C. Infrared rays are used to cook maize kernels, which were then flaked under the pressure of rolls. Maize flakes were also ground in the stone grinder and produced flour was used for analyses. Differences in nutritive values of produced flours were established by the analysis of standard chemical composition (ash, total proteins, oil, cellulose, starch), soluble proteins (albumin, globulin, zein, glutelin), α- and β+γ tocopherol, total antioxidants, total phenols and β-carotene. According to our results, white maize flour does not contain β-carotene. Raw whole flour of yellow maize, produced by milling of grain of the hybrid ZP 633, had a high content of β-carotene (5.43 μg/g), while this content was somewhat lower in raw whole flour produced from red maize (2.56 μg/g). A micronisation heat treatment caused a significant reduction in the β-carotene content in yellow micronised maize flour (23%). In the present study, the highest γ-tocopherol content was estimated in raw whole flour made from yellow maize (20.94 μg/g). A high temperature of infrared rays caused the reduction of the γ-tocopherol content, hence it amounted to 16.94 μg/100 g in flour made by milling of micronised yellow maize. This reduction was somewhat lower in flour made by milling of micronised red-seeded maize and it amounted to 35%. Flour produced from raw and micronised red maize grain had the highest antioxidant activity. According to our results 3.27 mg of whole flour made from raw red maize grain inhibits 50% activities of DPPH radicals. Some of Maillard reaction products which are formed during process of micronisation have an antioxidant activity, which could be a reason for the increased antioxidant activity in flour produced by milling of micronised white and red maize grain. 2.34 mg of red micronised flour inhibits 50% activities of DPPH radicals. The antioxidant activity in white micronised flour was higher by 40% than of flour produced from raw white kernels. The highest content of total phenols was determined in flour produced by milling of raw and micronised grain of red maize (2.80 mg catechin/g on the average). Heating of plant protein sources leads to decreasing of the protein solubility as well as content of tryptophan.