Foaming Properties of Egg Albumen with a Bubbling Apparatus Compared with Whipping

Neither standard method nor instrument exists to accurately measure foaming properties in food. Usually, whipping methods are used but they are only moderately reproducible. We used an original apparatus in which foam was generated by bubbling and then analyzed by conductimetry (liquid in the foam) and a uniline camera (foam volume). The foaming properties of a series of egg-white powders were studied. Among parameters describing foam formation and destabilization, three were repeatable and discriminating. They were correlated with those obtained with whipping methods. Easy to use and rapid, the bubbling method is an alternative for controlling and measuring the foaming properties of egg-white.     

MODELISATION D'UN SECHOIR SOLAIRE A POISSONS FONCTIONNANT EN CONVECTION NATURELLE

A mathematical model of the behaviour of a fish solar dryer has been described. This model has allowed an optimization of the design of this dryer (solar-chimney area and height, insolator area, etc...). The natural convection had to pulse drying air inside apparatus, whose capacity should be 30 kg of fresh fish.

Then the dryer has been built in National Fishing School of Anjouan (Comores Island). The first results mean a good fit between theoretical and experimental results.

The air-insolator, used for air heating, is a porous type, made with blacked coco fibers, crossed by the drying air. This local material is confirmed as a good thermic interface.

The most sensitive parameter, on the drying time, is the thickness of the fish slices. The choice of the fish quality is also very important: each different fish is characterized by one or several specific value of the water diffusivity constant, D_m. The model uses experimental values for the calculation.

Then the Fick's diffusion equation may be solved by decomposition of the moisture content inside fish slice (Fourier analysis).

Un modeéle matheématique du comportement d'un seéchoir solaire, destineé aà seécher du poisson, a eété eécrit. II a permis d'optimiser au mieux la geéomeétric de ce seéchoir qui a pu eêtre construit sur le site de I'eécole Nationale de peêche d'Anjouan aux Comores. II s'agit d'un seéchoir fonctionnant grace aà la convection naturelle, et dont la capacite est d'environ 30 kg.

L'air de seéchage est chauffeé par un capteur solaire de type absorbeur poreux, reéaliseé en bourre de noix de coco noircie. Le choix de ce mateériau local est bien suÛr volontaire; il s'est par ailleurs aveéreé comme un excellent milieu d'eéchange thermique.

Le parameètre le plus sensible sur la dureée de sechage est l'eépaisseur des filets de poisson. La qualiteé du poisson est aussi un choix important, chacun posseédant une constante de diffusiviteé de l'eau, D_m, bien speécifique. Le modeèle utilise les valeurs expeérimentales de cette constante. L'eéquation de Fick, qui reégit la migration de l'eau dans un mateériau poreux, peut alors eâtre reésolue en utiiisant une deécomposition en seérie de Fourier, de la reépartition de l'eau dans les tranches de poisson.     

OPTIMIZATION METHODS FOR SOLVING NON-LINEAR EQUATIONS IN VISCOELASTIC FLOW PROBLEMS

Numerical simulation of complex flows of viscoelastic fluids requires the use of efficient algorithms for solving the non-linear governing equations. In this paper, optimization algorithms, associated to a non-linear least squares methods are considered. The governing equations are written in the context of stream-tube analysis. This method is particularly adapted to handle memory-integral constitutive equations and permits calculation of flows by considering elementary sub-domains in a mapped computational domain where the transformed streamlines are parallel and straight. The procedure used to solve the equations is the Trust Region optimization algorithm, which satisfies a global convergence property. Using an integral codeformational equation, various tests involving the influence of parameters of the algorithms are applied to different flow geometries and underline the robustness and efficiency of the Trust Region algorithm.     

Gas exchange in modified atmosphere packaging. 2: Experimental results with strawberries

Strawberries were used to test a new model describing gas transport through micro-perforated polypropylene films and fruit respiration involved in modified atmosphere packaging. Some experiments were conducted with empty packs initially filled with either 100% N₂ or 100% O₂. Simulations agreed very well with experiments only if we replaced the cross-sectional area of the micro-perforations by areas of approximately half the actual areas in order to account for the resistance of air around the perforations. It is also possible to fit the model to gas concentration changes in packs filled with strawberries, although deviations have been due to the contamination of strawberries by fungi. the model was used to quantify the consequences of the variability of pack properties (number of micro-perforations per pack and cross-sectional area of these perforations) on equilibrium gas concentrations and to define minimum homogeneity requirements for modified atmosphere packaging.     

Gas exchange in modified atmosphere packaging. 1: A new theoretical approach for micro-perforated packs

Gas transport and exchange through the perforations in micro-perforated packs used in modified atmosphere packaging were numerically modelled by using Stephan-Maxwell laws, and, outside these perforations, by Fick's law. the model accounts simultaneously for oxygen, carbon dioxide, nitrogen, argon and water vapour. Fruit and vegetable respiration was approximated by Michaelis-Menten kinetics depending on oxygen concentration with an uncompetitive inhibition due to carbon dioxide. Concentration equilibrium was reached after 2 to 3 days, depending on the void volume and on the diffusion properties of the packs. the equilibrium concentrations were dependent on the number of perforations, their diameter, the thickness of the film and the temperature. However, a pack which makes 10% oxygen possible within the pack at 10°C might induce anaerobiosis at 20°C.     

Whey Changes during Processing Determined by Near Infrared Spectroscopy

We assessed the potential application of near infrared (NIR) spectrophotometry using a remote optic fiber probe for the development of calibrations. We evaluated the heat denaturation of whey proteins, the chemical composition of a whey protein concentrate (WPC) during production, and the degree of hydrolysis (DH) of whey proteins during trypsin hydrolysis. Partial least square (PLS) calibrations were developed for these applications and were related to a variety of products and processing conditions, so as to increase their reliability. NIR has potential applications in real time analysis for whey processing lines.     

STUDY OF FOAM STABILITY BY VIDEO IMAGE ANALYSIS: RELATIONSHIP WITH THE QUANTITY OF LIQUID IN THE FOAMS

Foaming properties of three different protein solutions [casein ca, soybean isolate si, deamidated gluten dg] were studied. A video image camera, coupled to an apparatus designed for the study of the formation and the stability of foams, made it possible to follow the evolution of the foam textures at the same time as physical characteristics, such as the foam volume and the liquid in the foam. The texture of the images was characterized by an original mathematical procedure derived from the two-dimensional Haar transform. A principal component analysis was then applied to the assessed texture characteristics, in order to emphasize the similarities and the differences between the foam textures. Video image analysis was found to be complementary to the physical characterization of the foams. According to the regularity and the stability of their foams, the proteins could be ranked in the following order (from the more to the less stable foams): ca, dg, si.     

Displacement fields using correlation methods as a tool to investigate cell migration in 3D collagen gels

Living cells embedded in a complex extra-cellular matrix migrate in a sophisticated way thanks to adhesions to matrix fibres and contractility. It is important to know what kind of forces are exerted by the cells. Here, we use reflectance confocal microscopy to locate fibres accurately and determine displacement fields. Correlation techniques are used to this aim, coupled with proper digital image processing. Benchmark tests validate the method in the case of shear and stretching motions. Finally, the method is tested successfully for studying cancer cells migrating in collagen gels of different concentration.