Effect of forced convection roasting on physicochemical and antioxidant properties of whole grain maize (Zea mays L.) and optimisation of roasting conditions

[摘要] ENGLISH ABSTRACT: Maize (Zea mays L.) is the most cultivated cereal and grain crop in the world and it is used as astaple food in developing countries such as Nigeria, South Africa, Mexico and economically lessprivileged countries. The grains of maize are processed into intermediate products (flour and meal)which are utilised for the production of different types of ready to eat foods. In most cases, the flouror meal used is refined (bran and germ removed) and not pregelatinised. Heat-processingmethods of maize that uses dry heat reduced the nutritional quality of intermediate and endproducts. Forced convection roasting (FCR), a novel heat-processing technique, that has anadditional advantage of using superheated steam was used to study the changes inphysicochemical, proximate composition and antioxidant properties of Nigerian (S28, S33) andSouth African (H2G1, H7D1) maize varieties. Roasting temperature and rotating speed(determining roasting time) limits (150 to 220 °C) and (30 to 90 Hz), respectively were used for theroasting in a forced convection continuous tumble roaster (FCCTR). Roasting conditions(temperature/rotating speed) of the maize varieties were optimised for the production of wholegrain flour or meal. Comparison of the proximate composition, antioxidant and pasting properties ofthe optimally processed whole grain flour or meal with raw whole grain flour or meal and anunroasted refined commercial maize meal (CMM) was made.The nutritional quality and antioxidant properties (content and activity) of the Nigerian maizeS28 (yellow kernel) and S33 (white kernel) were not negatively affected by FCR. For the SouthAfrican maize varieties, FCR did not show a negative effect on the proximate composition andantioxidant properties except the increase in total phenolics content of H2G1. Variations in thephysicochemical properties such as bulk density, kernel hardness, colour and pH of the roastedmaize varieties did not compromise the quality of the optimally processed whole grain flours. Theprediction models of moisture content, whiteness index (WI), yellowness index (YI), total essentialamino acids (TEAA) and total amino acids (TAA) had good fit (R2 >0.8) with the experimental dataand non-significant (p ≤0.05) lack-of-fits. The desirability profiling of moisture content, WI and YIindicated 189.9 °C/90 Hz and 140.9 °C/49.8 Hz as the mean optimum roasting conditions of S28and S33 maize varieties, respectively for the production of high quality whole grain flour or meal.Similarly, the desirability profiling of moisture content, WI, YI, TEAA and TAA showed the meanoptimum roasting conditions of H2G1 and H7D1 white maize varieties to be 185.0 °C/65.5 Hz and182.6 °C/55.0 Hz, respectively.The carbohydrate, crude protein, fat and fiber, ash, total phenolics and flavonoids as well asfree radical scavenging capacity of the optimally processed whole grain flours did not significantlydiffer from those of the raw whole grain flours of each of the maize varieties. Both whole grainflours of the raw and roasted maize grains had higher proximate composition, total phenolics, totalflavonoids and antioxidant activity than CMM, except the carbohydrate content which was found tosignificantly higher in the later. The optimally processed whole grain flour of each maize variety had a non-significantly lower pasting temperatures and significantly higher pasting viscositiescompared to the raw whole grain flour. This indicated better pasting characteristics of the optimallyprocessed whole grain flours with reference to the raw whole grain flours of the maize varieties.However, CMM had significantly lower pasting temperatures and higher pasting viscosities thanthe whole grain flours which indicated better pasting properties of the former.Considering the non-negative effect on proximate composition and antioxidant properties, andbetter pasting characteristics of whole grain flours of the roasted maize varieties, it could beconcluded that FCR is a good alternative for roasting maize grains in the process of producingwhole grain flours with the best quality for human consumption. It was also observed that thewhole grain flours had better nutritional and antioxidant properties, but poorer pasting propertiescompared to CMM.

[发布机构] Stellenbosch University