Thermal properties of cowpea flour: A study by differential scanning calorimetry Folake O. Henshaw 1 , Kay H. McWatters 2 , John O. Akingbala 3 and Manjeet S. Chinnan 2 1 Introduction The functionality of legume flours dictate their suitability as food ingredients in different products [1]. Thermal stability is an important determinant of functional properties. This is because structural parameters that influence functionality are altered chemically during the application of heat. Thermal treatment of varying severity is the most common process step during processing of foods. Differential scanning calorimetry (DSC) is a thermo-analytical technique for monitoring changes in physical and chemical properties of materials as a function of temperature [2]. The change of the state of a substance is accompanied by a change in the energy level. Changes in the energy level can be manifested by the absorption of heat (endothermic reaction) and liberation of heat (exothermic reac- tion). The measuring principle in DSC is to compare the rate of heat flow to the sample of interest and to an inert material, which are heated or cooled at the same rate. Changes asso- ciated with absorption or evolution of heat in the sample cause a differential heat flow, which is recorded graphically as a ther- mogram. DSC has been employed to study effects of heat- induced phase transitions of starch and protein systems [3, 4]. Heat changes associated with such transitions are measured and related to functionality. DSC thermograms for several starches have been obtained at intermediate moisture contents (starch : water 45 : 50% w/w) [5, 6]. Thermally induced order-disorder transitions in other polysaccharides such as carrageenan were studied by Morris et al. [7]. DSC has been used to obtain kinetic and thermody- namic data including temperature of protein denaturation and enthalphy associated with the transition [8–11]. Wright and Boutler [12] studied the behaviour of constituents of food legume meals when subjected to heating. Two thermal transi- tions observed for soybeans were attributed to 11S globulins and 7S globulins, while those observed for cowpea and dry beans were attributed to 7S globulins and starch. More studies have been reported on the thermal behaviour of starch and pro- tein as isolated preparations than on these constituents thermal behaviour as they occur in different food systems. The objec- tive of our study, therefore, was to investigate the thermal properties of cowpea flour, a food system containing starch and protein as major components. 2 Materials and methods 2.1 Materials The 12 varieties of cowpeas used in the study were obtained form various sources in Nigeria and the USA as reported else- where [13]. 2.2 Methods 2.2.1 Flour production The method, which involed dry decortication of conditioned seeds before milling described by Henshaw et al. [13] was used. 2.2.2 Chemical analyses The protein content of flour samples was determined using the Kjeldahl method [14]. Starch and total soluble sugars were determined after extraction by the procedure of Southgate [15]. Flour portions were extracted with methanol, the filtrate was used to determine sugars and the residue to determine starch. The phenol-sulphuric acid method of Dubois [16] was used to determine sugars while starch was determined after dilute acid hydrolysis as hexoses [17]. Amylose content was determined as described by Juliano [18]. The hydration index was determined as described by Henshaw [19]. Nahrung/Food 47 (2003) No. 3, pp. 161 – 165 i 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0027-769X/2003/0306-0161$17.50+.50/0 161 The thermal properties of 12 varieties of cowpea flour were studied by differential scanning calorimetry (DSC). Flour samples were pre- pared to a paste of 60% moisture content and sealed in standard DSC pans. Samples were scanned at a heating rate of 5 8C/min over a scan range of 40–130 8C. Samples exhibited single major endotherms, which occurred over varied temperatures. The transition enthalpy (DH) ranged between 1.4 J/g and 4.7 J/g. Transition onset (T o ) and transition peak (T p ) temperatures ranged between 75–78 8C and 78–82 8C, respectively. All the DSC parameters measured varied significantly among the vari- eties. The transition enthalpy (DH) was the most discriminating param- eter and accounted for 80% of the total variance. The major chemical components of cowpea flour, starch amylose and protein are significant predictors of DH. Protein denaturation appears to be a significant mod- ification which occurs during processing of cowpea seeds to flour. The transition enthalpy DH could become an important functional index of cowpea flour when related to some quality parameters in products that contain the flour. Correspondence: Folake O. Henshaw, Department of Food Science and Technology, University of Agriculture, Abeokuta, Ogun State, Nigeria E-mail: henofcpc@yahoo.com 1 Department of Food Science and Technology, University of Agricul- ture, Abeokuta, Ogun State, Nigeria 2 Department of Food Science and Technology, Center for Food Safety and Quality Enhancement, University of Georgia, Griffin, GA, USA 3 Department of Chemical Engineering, Food Science and Technology Unit, University of the West Indies, St. Augustine Campus, Trinidad and Tobago Abbreviations: DSC, differential scanning calorimetry; PHI, peak height index Keywords: Cowpea flour / Differential scanning calorimetry / Ther- mal properties /