Effect of roasting and pressure-cooking on nutritional and protein quality of seeds of mangrove legume Canavalia cathartica from southwest coast of India

Raw and processed (roasted and pressure-cooked) seeds of mangrove wild legume (Canavalia cathartica) of southwest coast of India were evaluated for nutritional and antinutritional qualities. The seeds consist of 28-32% proteins and 1600-1630 kJ/100 g of energy. A significant difference was seen between the proximate composition of raw and pressure-cooked seeds (P<0.05, t-test). Among the minerals, potassium was highest (240-828 mg/100 g) followed by phosphorus (84-120 mg/100 g) and sodium (21-41 mg/100 g). Globulins (18.2%) constituted the bulk of the seed proteins followed by albumins (7.3%) as in most of the legumes. Unlike the pressure-cooked seeds, SDS-PAGE revealed three protein bands in roasted seeds (51.4, 39 and 33.1 kDa) indicating partial or complete denaturation. The essential amino acids (EAA): cystine+methionine of processed seeds exceeded than that of rice; cystine+methionine, tyrosine+phenylalanine and lysine of the roasted seeds and cystine+methionine of pressure-cooked seeds were higher than FAO/WHO pattern. Threonine, valine and isoleucine of roasted seeds were comparable to FAO/WHO pattern, so also valine, isoleucine and lysine of pressure-cooked seeds. The carbohydrates, polyunsaturated/saturated fatty acid ratio and sulphur-amino acids were higher than soybeans. The raw seed flours were devoid of tannins and trypsin inhibitors, in addition, thermal processing decreased total phenolics and hemagglutinins. Growth and nitrogen balance studies in rats were performed to determine food efficiency ratio, protein efficiency ratio, net protein retention (NPR), protein retention efficiency (PRE), true digestibility, biological value (BV) and net protein utilization (NPU) of roasted and pressure-cooked seeds. Pressure-cooked seeds showed better biological indices than roasted seeds. Except for NPR, PRE, BV and NPU, rest of the parameters analyzed for protein quality was significantly different between roasted and pressure-cooked seed diet (P<0.05, t-test). Our study clearly indicated that Canavalia cathartica seeds of mangroves possess high protein and EAA. Even though domestic roasting and pressure-cooking partially detoxified con A-like lectins or hemagglutinins, improved methods of processing are essential to maximize the quality of protein with minimum loss of seed nutrients. This is the first study on the biochemical and protein quality evaluation of mangrove bean, Canavalia cathartica and warrants its conservation and utilization as a future potential protein source for humans and or livestock.