Prediction of porosity of food materials during drying: Current challenges and directions

ABSTRACT

Pore formation in food samples is a common physical phenomenon observed during dehydration processes. The pore evolution during drying significantly affects the physical properties and quality of dried foods. Therefore, it should be taken into consideration when predicting transport processes in the drying sample. Characteristics of pore formation depend on the drying process parameters, product properties and processing time. Understanding the physics of pore formation and evolution during drying will assist in accurately predicting the drying kinetics and quality of food materials. Researchers have been trying to develop mathematical models to describe the pore formation and evolution during drying. In this study, existing porosity models are critically analysed and limitations are identified. Better insight into the factors affecting porosity is provided, and suggestions are proposed to overcome the limitations. These include considerations of process parameters such as glass transition temperature, sample temperature, and variable material properties in the porosity models. Several researchers have proposed models for porosity prediction of food materials during drying. However, these models are either very simplistic or empirical in nature and failed to consider relevant significant factors that influence porosity. In-depth understanding of characteristics of the pore is required for developing a generic model of porosity. A micro-level analysis of pore formation is presented for better understanding, which will help in developing an accurate and generic porosity model.

KEYWORDS:

• Pore formation
• drying conditions
• food properties
• porosity prediction models
• microstructure

Nomenclature

Nomenclature	=	Meanings Unit
M	=	Mass Kg
V	=	Total Volume m3
W	=	Water content Kg of water/kg of sample
X	=	Water content Kg of water/kg of solid
T	=	Temperature K
M	=	Molecular weight of waterkg/mole
a, b, c, d, e, f, j	=	Fitting parameters involved in empirical models

Greek symbols

$\varphi$	=	Shrinkage expansion function
$ϵ$	=	Porosity
$\delta$	=	Collapse function
$\rho$	=	Density kg/m3
$\beta$	=	Density ratio, volume shrinkage coefficient
φ,δ	=	Volume-shrinkage coefficient

Subscripts

0	=	Initial
A	=	Air
B	=	Bulk
S	=	Solid
W	=	Water
P	=	Particle
G	=	Glass transition
M	=	Material

Acknowledgments

This research was conducted at the experimental facilities of the Queensland University of Technology (QUT) – Brisbane, Australia. The research studies were financially supported by QUTPRA Scholarship.