Abstract
The effect of roasting temperature and rotating speed (determining roasting time) during forced convection roasting on physicochemical and antioxidant properties of two South African white maize hybrids (H2G1; H7D1) was investigated and roasting conditions optimised. Minimum and maximum temperatures (136 and 234 °C) and rotating speeds (20 and 90 Hz) were selected to generate experimental runs using central composite design. The results showed that variation in temperature and speed did not significantly alter kernel hardness, protein content, total flavonoids and antioxidant activity of both hybrids. There was a significant decrease in whiteness index (WI) and increase in yellowness index (YI) of H7D1 meal with increase in temperature and decrease in speed. Total amino acids (TAA) content of H2G1 meal significantly decreased while that of H7D1 increased with increase in temperature and decrease in speed. Desirability profiles of prediction models of WI, YI and TAA suggested the following mean optimum roasting conditions: 185.0 °C, 65.5 Hz for H2G1 and 182.6 °C, 55.0 Hz for H7D1.
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The total global production of maize (Zea mays L.), the most cultivated cereal crop in the world, was about 990 million metric tons (MMT) in 2014 and South Africa was ranked the 8th largest
producer with approximately 13.5 MMT (USDA, 2015). Most of the maize produced in developed countries such as USA is used for the production of animal feed and bio-ethanol with some used as raw materials for food products. Whereas in developing countries such as South Africa most of the maize cultivated is consumed as staple food by humans.
‘Mielie pap’ (maize porridge) is one of the major food products prepared using maize meal and consumed in Southern Africa. The maize meal used is not pregelatinised and does not contain bran and germ. Gelatinisation of native starch results in structural changes of the starch granules due to increase in temperature in the presence of water (Waigh et al., 2000). The process is irreversible and needed in most industrial and culinary applications. Maize bran contains dietary fibre [hemicellulose (75%), cellulose (25%) and lignin (0.1%)] and phytochemicals (phenolics, phytic acid, tocotrienols, lignans and flavonoids), minerals and vitamins while the germ provides protein, oil (unsaturated fat), vitamins and minerals. Consumption of whole grain (including bran and germ) provides better diet quality and nutrients intake in adults (O'Neil et al., 2010). Whole grain consumption was reported to significantly reduce obesity (Good et al., 2008), stroke and hypertension (Steffen et al., 2003), certain types of cancer (Schatzkin et al., 2007), cardiovascular diseases (Jensen et al., 2004) and type 2 diabetes mellitus (de Munter et al., 2007). Phytochemicals in whole grains are believed to directly contribute to reducing and preventing diseases due to their antioxidant properties (Kennedy & Knill, 2003).
Heat-processing methods that can convert maize into preprocessed and finished food products include boiling, baking, nixtamalisation, extrusion and roasting. The selection of a heat- processing method depends on ingredients, environment, economy and traditions of people across the world. Roasting is a heat-processing technique that involves the use of dry hot-air (Oliviero et
al., 2008) resulting in various physicochemical, nutritional and phytochemical changes which can
be desirable or undesirable. Enhancing the content and activity of antioxidants (Chung et al., 2011), increasing flavour of end products (Lee & Lee, 2009), improving product shlef life and quality (Cämmerer & Kroh, 2009), removal of harmful substances (Yazdanpanah et al., 2005) and efficiency of subsequent processing steps (Chung et al., 2011) are some of the desirable changes. The undesirable changes are associated with decrease in nutritive values caused by the loss of essential amino acids through Maillard reaction, decrease in digestibility of proteins and carbohydrates and loss of vitamins (Özdemir et al., 2001).
A novel grain processing technique [forced convection roasting (FCR)], involves forcing superheated steam (dry steam) through the grains while continuously being mixed and moved through a roasting chamber (Fritz, 2013). The FCR was developed with an option of using superheated steam to replace dry heat during the process of roasting. Continuous tumbling of the roasting chamber and the superheated steam generated by the forced convection continuous
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increase in moisture (tempering) before roasting. However, other crops such as some legumes with high oil and protein content do not require tempering. The physicochemical, nutritional, functional and antioxidant properties of marama bean (Tylosema esculentum) dry-heated (roasted) using FCCTR were extensively studied (Maruatona et al., 2010; Kayitesi et al., 2012; Nyembwe et
al., 2015). Significant increase in in vitro protein digestibility, water absorption capacity, and
reduction in protein solubility and emulsifying capacity of the marama bean were reported. FCR with superheated steam did not show significant changes in physicochemical and antioxidant properties of Nigerian soft maize cultivars (S28, S33) (Chapter 3). However, for the South African hard maize hybrids (H2G1, H7d1), there was significant increase in total phenolics content of H2G1, but the physicochemical properties and antioxidant activities were not significantly affected.
Response surface methodology (RSM) is a statistical technique used for studying complex processes and it has been successfully used in different food processing optimisation procedures using central composite design (Mendes et al., 2001; Reyes-Moreno et al., 2003; Milán-Carrillo et
al., 2004; Kahyaoglu, 2008; Youn & Chung, 2012). RSM is used for studying the effect of and
relationship between experimental factors (independent variables) and response (dependent) variables. It can also be used in empirical building of models for the optimisation of response variables using second-degree polynomial equation and desirability profiling. RSM saves experimental materials and time by providing useful and reliable information with the lowest possible number of experiments. The aim of this study was to investigate the effect of FCR on physicochemical and antioxidant properties of South African maize hybrids and to optimise the roasting conditions using RSM, for the production of whole grain meal.