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Soybeans are used as raw material for human nutrition and animal feed because of their high nutritional value. Consumption of raw beans has negative effects on the growth and health of human beings and animals. These effects are caused by antinutritional. factors (ANFs). Trypsin inhibitors (TIs) are generally considered as the most important ANFs in soybeans. TIs can be divided in two main groups: the Kunitz soybean trypsin inhibitor (KSTI) and the Bowman-Birk inhibitor (BBI). TIs and some of the other ANFs are heat labile and are inactivated by heat treatments such as steaming (toasting) and extrusion cooking. Optimal design and optimisation of the heat treatment are necessary because over-processing reduces the protein availability to the animals. In practice, design and optimisation are based on experience and 'trial- and-error'. In some studies 'black-box' models are presented. No explicit models were developed in previous studies to predict the influence of a heat treatment on the feed quality of soybeans.The aim of this research is to develop 'white-box' models to predict the influence of a heat treatment on the product quality of soybeans. Kinetics and process models are developed to predict the change of the product quality during the process. Particular attention is paid to the mechanistic background of the inactivation of Us in soy flour, and to the influence of shear forces on the inactivation of TIs during extrusion cooking.In order to develop an inactivation kinetics model, the inactivation of TIs in soy flour is measured within a large range of temperatures and moisture contents. The inactivation of TIs exhibits a two-phase inactivation behaviour. Six different mechanistic kinetics models are used to describe the experimental data. These models are compared statistically. Two parsimonious models are able to describe the two-phase inactivation behaviour of TIs well with a minimal number of kinetics parameters. One model describes the difference in heat stability of two TI groups, e.g. (KSTI) and (BBI). The second model describes the irreversible inactivation of a native TI to a partially active intermediate TI followed by the denaturation to a complete inactive form of TIThe hypothesis that the two-phase inactivation behaviour of TIs is caused by a difference in heat stability of two TI groups is further examined. The activity of (KSTI) and (BBI). is determined in different heat treated soy samples. The results show that the two-phase inactivation behaviour of TIs cannot be explained by the difference in heat stability of (KSTI) and (BBI). Additional experiments show that the addition of a thiol (cysteine) resulted in a two-phase inactivation behaviour of (KSTI) and (BBI). respectively, in starch. We suggest that TIs in soy flour inactivate by sulphydryl-disulfide interchange during the first inactivation phase, and by heat during the second phase.During extrusion cooking, TIs in soy flour are inactivated by heat and possibly also by the deformation of the TI-molecules due to the shear forces. First, the theoretical influence of these shear forces on the inactivation of TIs is examined. The calculations show that some influence of shear forces on the inactivation of TIs can not be excluded. Furthermore, single screw extrusion experiments are performed to examine to effect of shear experimentally. The decrease of TIA due to heat inactivation during extrusion cooking is calculated by combining the extrusion conditions (temperature profile and residence time distribution) with the inactivation kinetics model of TIs The results show that the measured residual trypsin inhibitor activity TIA values of the extrudates can be predicted properly by only heat inactivation. There is no indication that shear forces are involved in the inactivation of TIs during extrusion cooking.Atmospheric steaming (toasting) is the most used heat treatment of soybeans and flakes. A process model is developed to describe the temperature and moisture profiles in the beans during steaming. In order to evaluate the effect of steaming on the protein availability, the kinetics of nitrogen solubility index (NSI) change is measured and modelled. The kinetics models of TIA and NSI are combined with the process model for steaming to predict TIA and NSI levels in the steamed soybeans. The model predictions are validated with experimental data. The possibility for the optimisation of the product quality of soybeans during steaming is investigated by performing simulations. These simulations indicate that the steaming process can be optimised using TIA and NSI as quality parameters. Initial moisture content rather than steam temperature should be used to optimise the process. |
