Abstract
Legumes are cultivated and consumed all over the world as a staple food. Legumes are an economical source of protein, slow-release starch, fibre, (un)saturated fatty acids, vitamins and minerals. In the face of a growing world population, which demands an economical and nutritious food source, legumes are an important crop. So far, legumes have not yet been exploited to their fullest potential. Legumes have a volatile profile that can result in overall undesirable odours. Legumes, even undergoing prolonged processing, can have a relatively lower starch and protein digestibility (compared to other crops and meat). Compounding the issue is the presence of deleterious antinutrients in legumes, such as enzyme inhibitors, lectins, tannins, and phytate, which must be inactivated and/or removed by processing prior to consumption. Thus, processing is very important in the utilisation of legumes, both for minimising undesirable volatile compounds, as well as increasing starch and protein digestibility. As such, this thesis aims to fill the gap in the literature of a thorough investigation into the effects of hydrothermal processing.
To do so, this thesis characterised eleven types of commercially available legumes in New Zealand according to their volatile and fatty acid profiles through headspace solid phase microextraction gas chromatography mass spectrometry and gas chromatography flame ionisation detection, respectively. Thereafter, cowpeas, chickpeas and kidney beans were selected to be processed using boiling, a commonly used hydrothermal processing technique. Changes in headspace volatile and fatty acid profile were analysed after the above-mentioned processing technique. Both open system and closed system approaches were conducted to investigate the changes in legume’s volatile profile after boiling of legumes in an open system and processing of legumes in a sealed container. Finally, starch and protein digestibility of legumes processed for varying durations were determined through in vitro oral-gastro-intestinal digestion.
Results showed that integrating volatile fingerprinting and fatty acid analysis is effective in characterising the volatile and fatty acid profiles of dried legumes, with discriminant volatile compounds identified for each legume. The detected volatile profile consisted of aldehydes, alcohols, ketones, terpenes, esters and lactones, furans and hydrocarbons. The lipid profiles comprised of five major fractions consisting of palmitic, stearic, oleic, linoleic and α-linolenic acids. Hydrothermal processing (boiling) was shown to be suitable for the reduction of undesirable volatile compounds, mostly in the aldehyde and alcohol chemical groups, hypothesised to be through the inactivation of endogenous enzymes such as lipoxygenase. Fatty acid profiles changed very little during hydrothermal processing. Simultaneously, an inverse relationship between decreasing legume texture and increasing rate of starch and protein digestion was observed with increasing hydrothermal processing time in cowpeas, chickpeas and kidney beans. Prolonged processing duration (≥60 min) increased the extent of starch digestion but decreased the extent of protein digestion. This could be partially due to the degradation of amino acids (arising from protein) participating in Maillard reactions and thermal degradation; the presence of products such as furans, pyridines and sulphur-containing compounds detected at prolonged processing durations supported this hypothesis. A closed system approach (complementary to the open system approach) provided additional evidence towards the confirmation of the aforementioned hypothesis, as the most volatile compounds (e.g., sulphur-containing compounds) were able to be captured because they were not lost into the environment during processing.
The knowledge gained in this thesis has extended the fundamental understanding of the legumes processing using conventional hydrothermal technique. The knowledge gained in this thesis creates a foundation for future research and can be used to advise consumers and the industry on how to select processing intensity to maximise legume’s utilisation and benefits.