Effect of starch, sucrose and their combinations on the mechanical and acoustic properties of freeze-dried alginate gels A. Nussinovitch a , M.G. Corradini b , M.D. Normand b , M. Peleg b, * a Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, PO Box 12, Rehovot 76100, Israel b Department of Food Science, Chenoweth Laboratory, University of Massachusetts, Amherst, MA 01003 USA Received 27 February 2000; accepted 3 January 2001 Abstract Alginate gels (2%) with and without incorporated starch and/or sucrose were freeze-dehydrated to produce cellular sponges. The stress–strain relationships of these dried gels were all irregular and jagged, typical of brittle cellular solids. The stiffness was assessed in terms of the fitted (‘smoothed’) apparent engineering stress at two pre-selected engineering strain levels (40 and 60%) using a polynomial model. Their brittleness was assessed in terms of the apparent fractal dimension of their stress-strain curves using the compass and box counting algorithms which yield the Richardson’s and Kolmogorov’s dimensions, respectively. The presence of starch in the dried gels solid matrix invariably increased their stiffness, but not in a manner that could be predicted on the basis of a stochiometric relationship or density increase. Sucrose, in contrast, could have the opposite effect suggesting that its presence in the solid matrix might interfere with its mechanical integrity. All the dried gels had a ‘rich’ acoustic signature as judged by the magni- tude of their apparent fractal dimension, which was determined by the ‘blanket’ algorithm. But if there were differences between the different dried gels’ acoustic signatures, they were too subtle to be detected by the apparent fractal dimension alone. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alginates; Dried gels; Texture; Acoustic signatures; Cellular solids 1. Introduction Freeze-dried gels are a convenient model system in the study of cellular solids of biological origin, at least in principle. Selecting the gels composition and prepara- tion conditions enables the creation of sponges with controlled microstructure and mechanical properties. The sponges themselves can have a potential use as biodegradable absorbents (Nussinovitch, Velez-Silves- tre, & Peleg, 1993) or as the basis of foods with unique (‘‘crunchy’’) textural properties (Nussinovitch, Corra- dini, Normand, & Peleg, 2000). In a previous study the mechanical and acoustic properties of agar, gellan and kappa carrageenan based dried gels were determined and the effect of admixed sucrose and starch assessed. It was found that the influence of these added ingredients on the dried gels density was well beyond their stochio- metric contribution, i.e. their presence had a qualitative effect on the solid matrix properties. As could be expected, low density dried gels having an open cellular structure with thin walls are very brittle and fragile. Consequently, their compressive force-displacement curve, or mechanical signature, is irregular (jagged) and irreproducible. The ‘‘noise’’, however, is not an artifact that ought to be smoothed or discarded, but a record of internal fracture events that should be monitored and analyzed. Several methods to quantify the degree of jaggedness of irregular signatures are now readily avail- able (Peleg, 1997), and it has been demonstrated that, at least in certain cellular food products, the degree of jaggedness can be used as an objective measure of ‘‘crunchiness’’ or ‘‘crispness’’ (Suwonsichon & Peleg, 1998). The stiffness of such materials, i.e. their resis- tance to deformation, can also be assessed indepen- dently and quantitatively from jagged records. The stiffness measure can be the magnitude of the fitted force at two or more preselected deformation levels (Harris & Peleg; 1996; Suwonsichon & Peleg, 1998; Wollny & Peleg 1994). It has been shown that in mon- itoring the effect of moisture on cereal foods, the actual 0963-9969/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0963-9969(01)00111-9 Food Research International 34 (2001) 871–878 www.elsevier.com/locate/foodres * Corresponding author. Tel.: +1-413-545-5852; fax: +1-413-545- 1262. E-mail address: micha.peleg@foodsci.umass.edu (M. Peleg).