The marama bean is an underutilised legume growing wild in the arid and semi arid regions of Southern Africa. Sorghum is an important cereal crop in arid regions of the world. The seed coats of marama beans and bran of sorghum contain antioxidant phenolic compounds with potential health benefits. Aqueous extracts for possible use as antioxidant functional food ingredient were prepared from seed coats of marama beans and bran of condensed tannin sorghum by extracting with water (water extract) or with water acidified to pH 2 (acidified water extract). Aqueous extraction was used in order to obtain an extract free from organic solvents, safe for human consumption, and containing compounds representative of those that are available under aqueous conditions of the gastrointestinal tract. The extracts were analysed for total phenolic content, total flavonoid content, condensed tannin content and protein precipitation capacity using spectrophotometric methods and for individual phenolic compounds using HPLC-MS. Proanthocyanidins were characterised using thiolysis degradation in conjunction with HPLC-MS. The antioxidant activities of the extracts were measured using ABTS, DPPH and ORAC assays. Extracts were also evaluated for protective effect against free radical induced human erythrocytes haemolysis, oxidative DNA damage and human LDL oxidation. Extracts from marama bean seed coats had significantly higher total phenolic content, total flavonoid content, condensed tannin content, protein precipitation capacity and phenolic compound concentration than equivalent extracts from condensed tannin sorghum bran. Three phenolic acids and three flavonols esterified to gallic acid were identified in the extracts from marama bean seed coats. Extracts from condensed tannin sorghum bran had six major phenolic acids, two phenolic aldehydes and three flavanones. Proanthocyanidins in extracts from marama bean seed coats were predominantly highly galloylated prodelphinidins while those in extracts from condensed tannin sorghum bran were procyanidins. Extracts from marama bean seed coats had higher antioxidant activity and protective effects against free radical induced erythrocyte haemolysis and LDL oxidation compared to equivalent extracts from condensed sorghum bran. Extracts from condensed tannin sorghum bran showed some protective effect against oxidative DNA damage. However, extracts from marama bean seed coats gave inconclusive results probably due to prodelphinidins binding to DNA. Extraction of marama bean seed coats under acidic condition resulted in reduction in phenolic compound content, antioxidant activity and protective effect against erythrocyte haemolysis, possibly due to co-precipitation of phenolic compounds with interpolymer complex precipitate formed between highly galloylated condensed tannins and cell wall polysaccharides at pH 2. In contrast, extraction of condensed tannin sorghum bran under the acidic condition resulted in an extract with significantly higher phenolic content and protective effect against erythrocyte haemolysis than the water extract possibly due to enhanced extraction of free and esterified phenolics and release of bound phenolic compounds. Water extracts and acidified water extracts (from marama been seed coats or sorghum bran) did not show significant differences in their protective effect against oxidative DNA damage and LDL oxidation. Extraction under acidic condition may be the preferred method for sorghum bran because it increases recovery of phenolic compounds, but not for marama bean seed coats because it causes reduction in phenolic compound content. The findings of this study show that the extracts have a potential to reduce oxidative stress which is implicated in many chronic diseases such as neurodegenerative diseases, cancer and cardiovascular disease. The extracts can be used in the development of functional foods with potential health benefits. / Thesis (PhD)--University of Pretoria, 2012. / Food Science / unrestricted

Marama bean (Tylosema esculentum (Burch) A. Schreib) is an underutilised, drought-tolerant legume native to the drier parts of Botswana, Namibia and South Africa. The bean is comparable to soya beans in protein content and quality whereas its oil content is comparable to that of peanuts. By adding value to the marama bean through processing into protein-rich flours, its utilisation may be increased. Therefore, one of the objectives of this study was to adopt suitable low-cost processing technologies used for soya processing to produce protein-rich marama bean flours. The effect of dry heating of whole marama beans on lipoxygenase enzymes of its defatted flour was determined since oxidative rancidity catalysed mainly by lipoxygenase enzymes can reduce the shelf-life of the flour. The presence of trypsin inhibitors can affect the protein digestibility of the marama bean flour adversely. The effect of dry heating of whole marama beans on in-vitro protein digestibility and amino acid content of its defatted flour was determined. Lastly, the effect of dry heating of whole marama beans on the protein-related functional properties of the resultant defatted flour was determined. The presence of lipoxygenase iso-enzymes (L-1 and L-2) activity in marama beans was determined by a visual and spectrophotometeric method using unheated soya beans as reference. Lipoxygenase iso-enzymes (L-1 and L-2) activity was not detected in marama beans. This may possibly suggest that these lipoxygenase iso-enzymes are absent or possibly inhibited in marama beans. In an attempt to optimise dry heating to inactivate trypsin inhibitors in marama beans, whole marama beans were dry heated at 100°C, 120°C and 150°C, respectively for 20 min. Defatted flours prepared from the heated marama beans (HMF’s) were analysed for their trypsin inhibitor activity using defatted flours from unheated marama beans (UMF) and soya beans (USF) as control and reference samples, respectively. Trypsin inhibitor activity in UMF was almost four and half times higher than in USF. Dry heating of whole marama beans at 150°C/20 min significantly reduced the trypsin inhibitor activity in its defatted flour to almost zero probably due to inactivation of the trypsin inhibitor. The effect of dry heating of whole marama beans at 150°C/20 min on the physico-chemical, nutritional and protein-related functional properties of defatted marama bean flour was determined. UMF was used as a control while USF and HSF were used as reference samples. HMF had higher protein content but lower fat content than UMF. It is suggested that dry heating disrupted the lipid bodies of the marama beans allowing more oil to be expelled during coarse milling of the flour. Heating significantly reduced the L* values of marama and soya bean flours possibly due to Maillard browning reactions. Heating significantly increased in-vitro protein digestibility of marama and soya bean flours probably due to protein denaturation and inactivation of trypsin inhibitors. Heating generally decreased the amino acid contents of marama and soya bean flours possibly due to chemical modification of the amino acids. UMF and HMF can potentially be used to improve protein quality in marama-cereal composite flours, porridges and breads. Heating significantly decreased the nitrogen solubility index (NSI) and emulsifying capacity (EC) of marama and soya bean flours possibly due to protein denaturation and/or cross-linking. This may make HMF and HSF not suitable for applications in emulsion type meat products such as sausages because emulsion formation is critical during processing of sausages. Heating significantly decreased the foaming capacity of soya flour but did not have an effect on that of marama bean flour probably due to their high residual fat content which may have disrupted protein films during foam formation. UMF has a potential to be used in comminuted meat products because of its relatively high NSI, EC and OAC. The laboratory process used in this study can be modified and adopted by SME’s to produce defatted marama bean flours with potential applications in bakery and meat products and as a protein supplement in composite marama-cereal products. / Dissertation (MSc)--University of Pretoria, 2011. / Food Science / unrestricted