Antiplasticization of cereal-based products by water. Part I. Extruded flat bread Agata Marzec, Piotr P. Lewicki * Faculty of Food Technology, Department of Food Engineering and Process Management, Warsaw Agricultural University (SGGW), Nowoursynowska 159c, 02-776 Warsaw, Poland Received 29 July 2004; accepted 9 December 2004 Available online 13 September 2005 Abstract Flat extruded wheat and rye bread was equilibrated to variable water activities using saturated salt solutions. At those water activities the bread was subjected to compression and three-point breaking tests. Compression test showed that water antiplasticizes both investigated breads. In the range of water activities from 0 to 0.53 for wheat bread, and from 0 to 0.59 for rye bread, the com- pression force reached 600 N at true strain 0.8. Compression proceeded in two steps; hence the mechanical resistance of the surface layers is larger than that of the center of the slice. Above the critical water activities water plasticizes the breads, and decrease of compression force with increasing water activity is recorded. Three-point breaking test shows that water affects mechanical properties of extruded breads in a complicated fashion. At water activities lower than 0.15, water plasticizes the material. At higher water activities antiplasticizing effect of water is evident. The cri- tical water activities at which the plasticizing effect of water is evident are lower than those in compression test. The respective values are 0.49 and 0.44 for wheat and rye bread, respectively. Three-point breaking test is more sensitive to the kind of investigated mate- rial and shows larger differences between wheat and rye bread than the compression test. Critical water activities correspond to hydration levels at which internal dynamics of macromolecules begins. Hence, it is sug- gested that rotational and translational movements of macromolecules caused by adsorption of water are pronounced by the plastic deformation of the material. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Compression force; Breaking force; Compression work; Breaking work; Plastic flow; Sorption isotherms; Critical water activity 1. Introduction Consumption of crispy bread, breakfast cereals, wa- fers,crackers,biscuitsandsnacksiscontinuouslyincreas- ing in many countries and in Poland as well. Besides of such attributes as dietetic properties, convenience and safety, consumer acceptance is dependent on the taste and texture of this cereal-based products. Sensory panels and consumer survey have shown that texture of dry cer- eal products is the most important in quality assessment. Texture, as a complex quality attribute contains features liked and disliked by consumers (Surmacka-Szczesniak, 1988). In dry cereal products crunchiness and crispness are considered by consumers as the signs of freshness, proper technology and good quality while gumminess, plasticity or sogginess caused by increased moistness of the material (Hsieh, Hu, Huff, & Peng, 1990; Knorr & Kleemair, 1975) disqualify the product. Dry cereal products are hygroscopic due to chemical composition, porosity and presence of starch in amorphous state (Colonna, Doublier, Melcion, De Monredon, & Mercier, 1984) and undergo substantial quality changes associated with water adsorption. Water content correlated with water activity affects crispness, fragility and plasticity of cereal products. 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.12.002 * Corresponding author. Tel./fax: +48 22 8434 602. E-mail address: lewicki@sggw.waw.pl (P.P. Lewicki). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 73 (2006) 1–8