MECHANICAL MEASUREMENTS IN PUFFED RICE CAKES

Abstract

ABSTRACT Puffed rice cakes of a national brand, plain, chocolate and cinnamon, were compressed with an Instron Universal Testing Machine (Canton, MA) to 80% deformation, dry and after moisture equilibration at 85% relative humidity. The dry cakes' engineering stress–strain relationships all had a sigmoid shape characteristic to solid cellular materials. They could be described mathematically by a double‐power model, which had been originally developed for bread crumb and soft synthetic foams. At strains above about 50%, the stress had noticeable random fluctuations, whose amplitude progressively increased. These stress fluctuations were most probably records of closed cells bursting. Upon moisture sorption, the prominent shoulder in the stress–strain relationship almost completely disappeared, and the recorded overall stress levels dropped considerably. This plasticizing effect of water was also manifested in the stress‐relaxation pattern of the compacted cakes. The wet samples' stress decay rate was much faster than that of the dry, and their residual stresses reached a much lower level. These observations suggest that the compressive properties of rice cakes can be determined and analyzed by the same general methods that are used for synthetic cellular solids, and that the effect of moisture on their texture can be assessed in terms of the overall stress level at comparable strain, modulus of deformability of the “intact specimen” and the compact's “degree of solidity,” which can be derived from its stress relaxation pattern. PRACTICAL APPLICATIONSPuffed crunchy cereals and snacks, including rice cakes, are popular foods in many countries, “crunchiness” being their unique attraction. Its objective characterization by instrumental methods and how it might be affected by composition and moisture are of great interest to numerous food companies. The compressibility of rice cakes is governed, simultaneously, by solid foam deformation and multifracture mechanics. The result is force–displacement curves that are both irregular and irreproducible, but which still have certain idiosyncratic characteristics. Their identification and interpretation are crucial to quality control and product development. The special tools to do that include mathematical models to characterize the mechanical signature's overall shape, in order to quantify the cakes' stiffness, and time series analysis, in order to assess the role of brittleness. For the methodology implementation, other factors, such as the “averaging effect” on random events and “glass transition,” also had to be taken into account as demonstrated.