Abstract
During thermal treatments such as drying, food products undergo deformations that can be characterized by changes in volume, shape, porosity, density, shrinkage, or collapse phenomena. These modifications are closely linked with product quality and the characteristics of mass and heat transfer. Indeed, these structural modifications very often result in poor efficiency of dehydration and rehydration processes, leading to a deterioration in the quality of the final products. Because both heat and mass transfer mechanisms are simultaneously involved during dehydration, optimization of these processes is essential for developing efficient drying technologies. Knowledge concerning pore formation and evolution in foods during drying is vital for process design and for the characterization of the quality of final products.
Access provided by Autonomous University of Puebla. Download chapter PDF
Similar content being viewed by others
Keywords
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
1.1 Background
Porosity is defined as the ratio of free spaces occupied within a material to the total volume of the material. What defines a material as porous or a nonporous remains to be determined, although a few studies suggest that a nonporous material is one that has a porosity of less than 0.25, with a porous material having a porosity greater than 0.4 (Karathanos et al. 1996; Goedeken and Tong 1993; Waananen and Okos 1996; Lozano et al. 1983; Zogzas et al. 1994; Krokida and Maroulis 1997).
Knowledge of pore formation in food during the drying process can be useful in predicting transport properties, such as thermal conductivity, thermal diffusivity, and mass diffusivity (Rahman 2001), as well as dried-food quality (Xiong et al. 1992; Mayor and Sereno 2004; Mavroudis et al. 1998). This is because most of the factors that contribute to transport properties are significantly affected by the structure of the food material.
Drying methods and drying conditions affect the porosity of the final dried product. Thus, the same raw material may possess different pore characteristics at the conclusion of the drying process, depending on the drying method and conditions (Sablani et al. 2007). There is a scarcity of thorough research in the literature concerning the characteristics of pores in food products. Depending on the end use of the dried food materials, high porosity may be either desirable or undesirable. For example, if a long bowl life is required for a cereal product, a crust (lower porosity) product that prevents moisture absorption may be preferred. On the other hand, dehydrated foods, such as fruits, vegetables, and instant noodles, should possess high porosity to facilitate their fast rehydration.
In addition, porosity also gives an indication of the extent of shrinkage that a food material undergoes during drying, which in turn determines the size and shape of the finished product (Ayrosa and Pitombo 2003). Furthermore, it is well established that a more porous structure of the final product is an indication that less structural damage took place over the time of drying. Finally, knowledge of the porous structure of dehydrated materials may help in the accurate modeling of mass and heat transfer processes, including the prediction of water diffusivity in various foods (Datta 2007).
It has also been found that porosity has a direct effect on other physical properties, such as the mass diffusion coefficient, thermal conductivity, and thermal diffusivity. The mechanical and textural properties of food are also correlated to porosity. Vincent (1989) found that torsional stiffness (0.5–7 MPa) varied with porosity (0.83–0.54) in the case of fresh apples. In addition to these, porosity plays a significant role in the agglomeration of cell strength in dried foods. Furthermore, variations in pore characteristics have a significant effect on the textural and sensory properties of foods (Vickers and Bourne 1976; Christensen 1984). Despite the importance of the effects of porosity on food properties, little work has been done to establish a relationship between porosity and dried-food qualities.
In brief, the prediction and control of porosity are complex tasks faced by food scientists and engineers. Porosity is one of those physical parameters that is required to build food behavior models; it is particularly relevant in the case of porous solid materials and as such plays a paramount role in the modeling and understanding of drying operations, cold and controlled-atmosphere storage, and other food-treatment processes (Lozano et al. 1983).
The outline of this book is as follows. The pore formation mechanism is described first, followed by a discussion of the factors that affect porosity during drying and a description of heat and mass transfer mechanisms through porous media. Pore development in various drying processes is then discussed. The effects of porosity on the other physicochemical properties of dried food materials are then reviewed. Finally, a hypothesis is proposed concerning the relationship between process parameters and product quality through an examination of pore characteristics.
References
Ayrosa AMIB, Pitombo RNM (2003) Influence of plate temperature and mode of rehydration on textural parameters of precooked freeze-dried beef. J Food Process Preserv 27(3):173–180
Christensen CM (1984) Food texture perception. Adv Food Res 29:159–199
Datta AK (2007) Porous media approaches to studying simultaneous heat and mass transfer in food processes. II. Property data and representative results. J Food Eng 80(1):96–110
Goedeken DL, Tong CH (1993) Permeability measurements of porous food materials. J Food Sci 58(6):1329–1331
Karathanos VT, Kanellopoulos NK, Belessiotis VG (1996) Development of porous structure during air drying of agricultural plant products. J Food Eng 29(2):167–183
Krokida MK, Maroulis ZB (1997) Effect of drying method on shrinkage and porosity. Drying Technol 15(10):2441–2458
Lozano JE, Rotstein E, Urbicain MJ (1983) Shrinkage, porosity and bulk density of foodstuffs at changing moisture contents. J Food Sci 51:113–120
Mavroudis NE, Gekas V, Sjöholm I (1998) Osmotic dehydration of apples. Shrinkage phenomena and the significance of initial structure on mass transfer rates. J Food Eng 38:101–123
Mayor L, Sereno AM (2004) Modelling shrinkage during convective drying of food materials: a review. J Food Eng 61(3):373–386. doi:10.1016/s0260-8774(03)00144-4
Rahman MS (2001) Towards prediction of porosity in food foods during drying: a brief review. Drying Technol 19(1):3–15
Sablani SS, Rahman MS, Al-Kuseibi MK, Al-Habsi NA, A-BR H, Al-Marhubi I, Al-Amri IS (2007) Influence of shelf temperature on pore formation in garlic during freeze-drying. J Food Eng 80(1):68–79. doi:10.1016/j.jfoodeng.2006.05.010
Vickers Z, Bourne MC (1976) A psychoacoustical theory of crispness. J Food Sci 41(5):1158–1164. doi:10.1111/j.1365-2621.1976.tb14407.x
Vincent JFV (1989) Relationship between density and stiffness of apple flesh. J Sci Food Agric 47:443–463
Waananen KM, Okos MR (1996) Effect of porosity on moisture diffusion during drying of pasta. J Food Eng 28(2):121–137
Xiong X, Narsimhan G, Okos MR (1992) Effect of composition and pore structure on binding energy and effective diffusivity of moisture in porous food. J Food Eng 15(3):187–208
Zogzas NP, Maroulis ZB, Marinos-Kouris D (1994) Densities, shrinkage and porosity of some vegetables during air drying. Drying Technol 12(7):1653–1666
Author information
Authors and Affiliations
Rights and permissions
Copyright information
© 2016 The Author(s)
About this chapter
Cite this chapter
Joardder, M.U.H., Karim, A., Kumar, C., Brown, R.J. (2016). Introduction. In: Porosity. SpringerBriefs in Food, Health, and Nutrition. Springer, Cham. https://doi.org/10.1007/978-3-319-23045-0_1
Download citation
DOI: https://doi.org/10.1007/978-3-319-23045-0_1
Publisher Name: Springer, Cham
Print ISBN: 978-3-319-23044-3
Online ISBN: 978-3-319-23045-0
eBook Packages: Chemistry and Materials ScienceChemistry and Material Science (R0)