Crumbs permeability and cellular structure of baked products Chaunier L. 1 , Della Valle G. Unité BIA - Equipe MC2. INRA, 44316 Nantes Cedex 3 1 Email : chaunier@nantes.inra.fr Abstract The permeability of a medium is defined by its aptitude for being crossed by a fluid when submitted to a gradient of pressure. Widely employed for porous media such as soils, this concept is less current for solid food foams (Bird et al., 1960; Dullien, 1979). The objective of this work was first to set up a device dedicated to the study of baked products for the measurement of their gas permeability, k (m -2 ). Secondly, to enhance the knowledge of relationships between their physical properties and their cellular structures, particularly the connectivity of their gaseous cells, created during baking (Lostie et al., 2002; Rouillé et al., 2005). In the case of expanded cereal-based products, its measurement should supplement their textural evaluation. To overcome a wide range of crumbs densities, various recipes based on wheat flours were baked, sometimes added with sugar, or submitted to increasing strains during k measurements. The sample (L, from 0.02 to 0.1 m) is taken using a punch (Ø = 0.035 m), then placed in an hermetic cell crossed by a gas flow of viscosity µ (Q, about 0.2 l/min). In laminar flow, the measurement of the differential pressure, dP, between the entry and the exit of the cell (from 1 to 75 hPa) makes it possible to determine the coefficient of permeability, k, following the Darcy’s law : k = Q/A. L/dP. µ. After checking the application of this law in present experimental conditions, k was found from 1 to 500 Darcies (1 D = 0.987.10 -12 m 2 ), depending on the end-products cellular structure. As found on highly porous materials, such as expanded graphite (ρ from 50 to 200 kg.m -3 ), relationships were found between k and τ (Mauran et al., 2001). Furthermore, following the Carman-Kozeny law (see Figure), k was linked to crumbs porosity, ε, as previously found on expanded pre-gelatinized dough : k = c.ε 3 /(1-ε) 2 , c depending on the tortuosity, τ, the gaseous cells shape factor and the gas features (Goedeken et al., 1993). Figure. Carman-Kozeny relationship applied to native and compressed crumbs Thus, crumbs permeability was linked to their main structural features (ε, ρ and τ). In prospect, k should be introduced into models of transfer of matter and heat during baking stages, as fermentation and cooking. y = 13,3x R 2 = 0,77 y = 10,6x R 2 = 0,80 0 100 200 300 400 500 0 5 10 15 20 25 30 35 40 p 3 /(1-p) 2 (-) k (x 10 -12 m 2 ) Native bread crumbs Compressed crumbs P_0.20g/cm3 P_0.22g/cm3 P_0.40g/cm3 P_0.50g/cm3 Baking flour water- Baking flour sugar+ Baking flour sugar++ Baking flour Panif Biscuit flour Baking flour CNS