Compressive Stress-Strain Relationships of Two Puffed Cereals in Bulk

Review: Mechanical properties of dry cellular solid foods / Revisión: Propiedades mecánicas de alimentos celulares sólidos

The compressive stress-strain relationships of most cellular materials and solid foams have a char acteristic sigmoid shape that reflects three deformation mechanisms: primarily elastic distortion under small strains; collapse and/or fracture of cell walls; and densification. The exact shape of the relationship is mainly determined by the materials' composition including: moisture contents; the cells and cell wall size and geometry; and whether the cells are open or closed. Upon repeated compression the nature of the relationship may change as a result of irreversible changes in the solid matrix (especially rupture). When the cell wall material is very brittle the force-displacement curve can be extremely irregular and jagged, often concealing its underlying sigmoid shape. Characterization of such brittle products requires methods of jaggedness evaluation and smoothing procedures. These are now readily available and have been successfully applied to extruded puffed foods. Food particulates tested in bulk can show similar compressibility patterns even if they themselves do not have a cellular structure. Although tensile properties of spongy baked foods, like bread crumbs, can be determined experimentally they have not yet been proven useful as indicators of textural quality, primarily as it appears, because of structural non-uniformity.

Physical properties and microstructural changes during soaking of individual corn and quinoa breakfast flakes

The importance of breakfast cereal flakes (BCF) in Western diets deserves an understanding of changes in their mechanical properties and microstructure that occur during soaking in a liquid (that is, milk or water) prior to consumption. The maximum rupture force (RF) of 2 types of breakfast flaked products (BFP)--corn flakes (CF) and quinoa flakes (QF)--were measured directly while immersed in milk with 2% of fat content (milk 2%) or distilled water for different periods of time between 5 and 300 s. Under similar soaking conditions, QF presented higher RF values than CF. Soaked flakes were freeze-dried and their cross section and surface examined by scanning electron microscopy. Three consecutive periods (fast, gradual, and slow reduction of RF) were associated with changes in the microstructure of flakes. These changes were more pronounced in distilled water than in milk 2%, probably because the fat and other solids in milk become deposited on the flakes' surface hindering liquid infiltration. Structural and textural modifications were primarily ascribable to the plasticizing effect of water that softened the carbohydrate/protein matrix, inducing partial collapse of the porous structure and eventually disintegration of the whole piece through deep cracks.

Uptake of tritiated liquids by individual breakfast cereal flakes

Surface liquid adhesion (SLA) and liquid absorption (LA) of tritiated liquids, including water and skim, low-fat, whole, and fat-enriched milks, by cornflakes (CF) and frosted flakes (FF) were determined by scintillation counting using water-[(3)H] at 0.5 microCi/mL. SLA or the liquid adhering to individual flakes after a short immersion period was the same for CF and FF in the case of water (approximately 0.011 microL mm(-2) of flake) but were always higher for CF than for FF and increased as the fat content in milks augmented. LA of individual flakes, followed for 300 s of soaking, increased with time and was always higher for CF than for FF (for the same liquid), however, data did not follow a regular pattern. Flakes showed quite compact outer surfaces and an internal porous matrix composed of air cells of various sizes separated by dense walls of different thicknesses. This heterogeneous microstructure of individual flakes may be the cause of the lack of a simple kinetics during the soaking process. Previous results obtained by soaking a mass of flakes overestimated the uptake of fluid by individual because they included the liquid occluded between the flakes.

Multi-level approach for the analysis of water effects in corn flakes

The purpose of this work was to analyze the effect of water on thermal transitions, mechanical properties, and molecular mobility in corn flakes (CF), and their relationships. Commercial common (CCF) and sugar-frosted (SCF) corn flakes were studied in a water content (wc) range from 5 to 20 (% dry basis). The slope of (1)H NMR spin-spin relaxation time T 2* (determined by FID) versus temperature plot changed close to T g. Compression force showed a maximum at wc of ca. 12 and 16% (db) for SCF and CCF, respectively. (1)H NMR complemented DSC data in determining the temperature dependence of water and solid mobility, in order to assess quality of laminated corn products. The results of the present work indicate that while the compression force showed a maximum value as a function of water content, T g values determined by DSC or by spin-spin relaxation decreased progressively with increasing water content.

Measures of line jaggedness and their use in foods textural evaluation

The jaggedness of signatures, such as the force-displacement curves of brittle cereal-based foods, can be a manifestation of physical processes. Consequently, it should be regarded as an inherent property of the system rather than noise that should be discarded. Changes in the degree of jaggedness can provide useful information on the system's performance or on how it is affected by external conditions, such as exposure to moisture. Quantitative assessment of the degree of jaggedness can be done by methods that include statistical analysis, conversion of the record into a Fourier power spectrum, and determination of its apparent fractal dimension. Most can be performed with microcomputers and commercially available software. It is recommended that the jaggedness of any experimental curve or signature be determined by at least two methods or algorithms simultaneously for mutual verification. The magnitude of any single measure of jaggedness is determined by the signature's fluctuations amplitude and frequency. While the amplitude effect can be accounted for by normalization, the frequency cannot. Consequently, only curves having about the same number of points at the pertinent interval can be meaningfully compared. There is, however, a mathematical method, at this time still empirical, that allows meaningful comparison of the jaggedness of signatures obtained at different sampling rates. When several or numerous jagged signatures are recorded simultaneously, as in the case of brittle particulates compressed in bulk, the resulting combined signature is considerably smoothed. This is primarily the result of averaging caused by mutual cancellation of peaks in different directions and reducing the relative weight of any large peak. However, there are mathematical methods, some partly empirical, that enable restoration of the jaggedness of the original signatures.