The structure and properties of puffed rice cakes

Puffed cereal grains are common food ingredients and rice cakes are increasingly popular. Relatively little has been published regarding the optimal conditions for the processing of the cakes, the mechanism of puffing or the adhesion of rice grains during processing. Accordingly the objectives of this study have been to examine the quality of rice cakes, the effect of processing variables and to investigate the mechanism of puffing and adhesion involved. A series of trials were run in which rice cakes were made with different levels of tempering moisture (16, 18 and 20%), heating temperature (248, 258 and 268°C), processing time (2, 4 and 6 sec) and the cycle time (5.8, 6.0 and 7.0 seconds). In addition, selected types of rice (brown and white, waxy, low and high protein samples) have been evaluated along with the incorporation of either oil or sugar in the formulation. The physical properties of the resultant rice cakes were measured and it was found that the volume increased with increasing heating temperature and cooking time for brown rice samples at each of the tempering moisture levels evaluated. Similar results were found for white rice and when either oil or sugar was added to the formulation, whereas for cakes made with waxy rice, volume did not change. Textural attributes of the products were measured by a compressive analysis and data expressed in terms of stiffness. Relatively little variation was observed in this parameter except for cakes made with oil where the stiffness values were relatively low. Shorter cycle times during production were found to increase the volume and stiffness characteristics of the cakes. The scanning electron microscope was used to produce images showing the internal microstructure of rice cakes and from these cells size measurements were made. For all treatments the cell size distribution was relatively heterogeneous regardless of the processing variables applied or the type of rice. Differences were noted in the appearance of images depending on the way in which the samples were fractured indicating that different mechanisms applied for strong and weak cakes: in the former the crack develops through the grains, whereas in weak cakes the crack appears to form between the grains. There are two main factors contributing to the overall adhesion of the cake and hence product stiffness: the mechanical strength of adhesion between the surfaces of adjacent puffed rice grains and that of individual grains. The break strength of the cake is determined by whichever of these two is the weaker. In conclusion, the physical characteristics of rice cakes do not alter significantly with different processing variables. Higher processing temperatures appear to accelerate puffing. The present study contributes to an enhanced understanding of the effects of processing and compositional variables on the mechanisms of puffing and adhesion. Further research is recommended to extend our knowledge of the potential of other ingredients that might enhance the adhesion of rice cakes while allowing optimal puffing to occur.<br>