Most of the vegetables and fruits contains high amounts of water .
All food materials have some porosity (air space) in them. These internal pores are of three types i.e closed pores, where the pore space is enclosed from all sides, blind pores ,where one end is closed and through pores where flow can take place. The porosity of the materials can be found out by air comparison pychnmeter working on helium gas as it is the lightest inert gas. Porosimeters based on liquid extrusion method and image analysis are also employed to find out porosity.
Porosity is of two types: apparent/internal, as has been described by Debandya Mohapatra in a previous reply; and external/interparticle/bulk which accounts for space between particles when stacked in bulk. Indirectly apparent and bulk porosities can be computed from the apparent and bulk densities of the materials. Apparent Porosity = 1 - (apparent density/solid or true density); bulk porosity = 1 - (bulk density/apparent density). The total porosity of the material when packed in bulk would be a summation of apparent (internal) and bulk (interparticle) porosities. This is how it is definded in an excellent text book on physical properties of foods: S. Sahin and S.G. Sumnu (2006), Physical Properties of Foods), Springer Science+Business, New York ISBN 0-387-30780. See also M.J. Lewis (1996), Physical Properties of Foods and Food Processing Systems, Woodhead Publishing Limited, Cambridge, ISBN 0895733994. However in most published journal articles, porosity will be reported in a generic sense but will have been calculated using the equation for bulk porosity given above. Search for journal articles on physical properties of foods or agricultural materials for more information. You will find mention of applications of these properties being discussed in most of those publications. Hope this helps. Cheers!
porosity is volume of the air space present in the food product. for frutis and veg, it can be measured by changes in volume by liquid displacement method. refer this article for the method of determining porosity in fruits n vegs.
Structural changes in apple tissue during glucose and sucrose osmotic dehydration: shrinkage, porosity, density and microscopic features
Journal of Food Engineering
Volume 61, Issue 2, February 2004, Pages 269–278
To measure the porosity, you have to measure Bulk density and True density of the sample. Then You can calculate porosity by following equation
Porosity (%) = 1-(Bulk density)/Density×100
Read some good article on porosity especially by one of my friends MS Rahman. He has a book entitled Food Properties Handbook and he write a nice chapter on porosity. Good reading.
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