EFFECT OF WATER ACTIVITY ON THE MECHANICAL-PROPERTIES OF SELECTED LEGUMES AND NUTS

Two legumes (kidney beans and chickpeas) and two nut kernels (almonds and hazelnuts) were compressed intact with a Universal testing machine at various a(w) levels in the range of 0.11-0.85 at an ambient temper ature of 22 degrees C. Mechanical characterisation of the four was don e through four parameters; the slope of the initial part of force-disp lacement curve, considered a measure of stiffness, the failure force, considered a measure of strength, the deformation at failure, consider ed a measure of brittleness or deformability and the area under the fo rce-displacement curve corresponding to 20% compression, considered a measure of toughness. Among these parameters only the latter could be determined and used effectively in all four seeds and along the whole water activity range. The jaggedness of the force-displacement curve w as also determined in terms of its apparent fractal dimension calculat ed with two different algorithms for verification. There was a noticea ble difference between the mechanical behaviour of the two groups. The nut kernels, apparently because of their high oil content, had a much smoother force-displacement curve, and the effect of absorbed moistur e on their texture was generally more moderate than on that of the leg umes. But in all the four types of seeds tested, each mechanical prope rty had its own specific moisture dependency, an observation consisten t with previous reports on cereal-based products.