Strategies for the removal of raffinose family oligosaccharides from navy bean flour : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Bioprocess Engineering at Massey University, Palmerston North, New Zealand

Abstract

Navy beans are legumes with highly nutritious qualities. However they are underutilised in the processed food industry due to the undesirable bloating, abdominal discomfort and excessive flatulence associated with ingestion of the raffinose family oligosaccharides (RFOs) present in them. If a suitable technology were available to sufficiently reduce the concentration of RFOs then navy bean flour could find application in many food production processes. The traditional method for decreasing RFO content in navy beans and other similar legumes has been through soaking in large quantities of water, thereby leaching the RFOs from the bean. However this is a slow process and results in the depletion of all water soluble solids (up to 25% of the dry weight of the bean). The leaching process can be sped up dramatically through the decrease in particle size achieved by milling the beans into navy bean flour prior to the addition of water. However this process makes dewatering of the flour difficult. Rates of moisture uptake and RFO leaching were characterised for navy bean flour and dehulled navy bean cotyledons demonstrating that RFO leaching is slower but of a similar magnitude as moisture uptake and that these rates are dramatically increased with decrease in particle size. The addition of α-galactosidase to the leaching water enables the rapid removal of RFOs from navy bean flour without the need to separate the flour from the leaching water because the RFOs leached out of the flour are hydrolysed into simple sugars. Galactose is a product of the hydrolysis of RFOs and its presence at high concentrations can result in the inhibition of α-galactosidase. However at the concentrations likely to be experienced during RFO reduction in bean flour the inhibition effect is minimal. Rates of hydrolysis were studied for raffinose and stachyose hydrolysis by α-galactosidase in the concentration range expected during processing of navy bean flour. Enzymatic processing using very small amounts of moisture addition was investigated demonstrating partial RFO removal at moisture contents as low as 28% (wet basis). Reduction in RFO content to a level at which the flatus response is negligible can be achieved at moisture contents as low as 38% (wet basis) which significantly minimises the drying required to produce a dry navy bean ingredient or could allow direct application in an extrusion process. The application of α-galactosidase at low moisture content is a novel process for the depletion of RFOs in navy bean flour. The mechanism for this process is explained by preliminary modelling of moisture and RFO diffusion which demonstrates how partial RFO removal is possible. From these mechanisms a series of commercially practical methods for RFO depletion of navy bean flour were explored for several targeted product applications. This process also has potential applications for low moisture enzyme processing in a range of food and other biological systems. The simple processes developed in this work open up the commercial use of navy bean flour for the food industry to develop products that take advantage of its functional and nutritional properties without the negative nutritional problems usually associated with this material.