A model of moisture‐induced plasticization of crunchy snacks based on fermi' distribution function

Abstract

The irregular force-deformation relationships of commerical cheese balls and Zwiebacks at nine levels of water activity in the range 0.11-0+3 were recorded with a computer-interfaced Universal testing machine. They were characterized by two empirical stiffness parameters, the force at 10 and 20% deformation, and two jaggedness measures, the apparent fractal dimension of the normalized force-deformation curve and the mean magnitude of its power spectrum. The plots of all four parameters versus the water activity had a stable region followed by a substantial drop at a characteristic water activity level, of the kind expected when a material undergoes a glass transition. The phenomenon over the entire experimental water activity range, could be described by a model whose mathematical format is a slightly modified version of Fermi's distribution function that is P(a,) = PJ{ 1 + exp[(a, -uwcp)]/bp] + c, where P(a,)is any of the stiffness or jaggedness parameters, P, and c, constants whose sum is the magnitude of the parameter when the material is in the dry state (c, is the residual level after plasticization), awcp is a characteristic water activity level representing the region where the major textural changes take place, and b, a constant representing the steepness of the relationship at the transition region.

Effect of water activity on the mechanical properties of selected legumes and nuts

“…a w Its magnitude was steady at the low region but it a w dropped as the level increased. The toughness a w measure vs relationships of the two beans clearly had a w a sigmoid shape of the kind previously reported in certain cereal products (Wollny and Peleg 1994 ;Harris and Peleg 1996). It could be described by a modiÐed Fermian model whose Ðt is shown in Figs 6 and 7 and Table 2.…”

“…They clearly indicate that the jaggedness and toughness losses occurred at signiÐcantly di †erent levels and at di †erent rates with respect to a w the That di †erent mechanical properties need not a w . change in unison as a result of moisture sorption has also been found in a variety of baked and extruded cereal products (Wollny and Peleg 1994 ;Harris and Peleg 1996.) The observation of the same in natural products of very di †erent composition and structure suggests that it might be a general phenomenon, and that plasticisation by water has di †erent manifestations in di †erent materials.…”

“…Provided that the dimensions and geometry of seeds of a certain type do not vary by much, tests performed on them while intact can be sufficient to establish the e †ect of moisture, for example, on their texture. Moreover, at least when dry, certain seeds are brittle to such an extent that their forceÈdisplacement curves can be irregular and irreproducible, a situation encountered in the mechanical analysis of brittle snacks and particles of cereal origin (Barrett et al 1992 ;Wollny and Peleg 1994 ;Harris and Peleg 1996). In these it was shown that useful information can still be derived from such relationships provided that the general shape of the curve and its jaggedness are treated separately.…”

Understanding the texture of low moisture cereal products: mechanical and sensory measurements of crispness

“…The underlying idea is that the fractal analysis of the force-deformation curve describes not only the overall jaggedness but also any individual feature of the fracture pattern (peak shape and height). In order to characterize the contribution of the different frequencies as a whole, the pattern of the normalized length versus sampling frequency (Fig 6a) was, for each water activity, ®tted to a sigmoõ Èd plot (Fermi 17,29 ) as described in eqn (5): where P 1 corresponds to the initial value of the plotted property (here the normalized line length), X to the logarithm of sampling frequency, P 2 to the X value corresponding to P 1 /2 (also called critical X value) and P 3 describes the steepness of the decrease. In this case the parameter P 1 was ®xed equal to 1, since the line length was normalized to its initial value (at a 2 mm resolution).…”

Análise das propriedades mecânicas de cereais matinais com diferentes fontes de amido durante o processo de absorção de leite

“…Para faixas de atividades de água baixas, o produto encontra-se crocante e a curva força-deformação apresenta diversos picos até a ruptura efetiva do alimento [36]. Em materiais com estrutura porosa e quebradiça, como cereais matinais "secos", a força oscila muito durante a quebra da parede e a desintegração da estrutura.…”