Shrinkage, effective diffusion coefficient, surface transfer coefficients and their factors during solar drying of food products – A review

•Shrinkage, effective diffusion coefficient (De), surface transfer coefficients on solar drying are reviewed.•Their influencing factors were thoroughly discussed.•The main factors volume of water, moisture content, air temperature, air velocity are discussed.•Different mathematical models involved in the above parameters are reviewed.•Important results are listed and recommendations are given. Drying of food materials leads to an increase in shelf-life and gives quality end products. Drying with solar energy is an economical and environmentally friendly method than other drying methods. The food products are dried using solar energy in different methods such as open sun drying, direct, indirect, mixed-mode and hybrid solar drying. There are some parameters such as shrinkage, effective diffusion coefficient (De), surface transfer coefficients, etc. majorly influence the solar drying of food products and these elements are extensively reviewed in this work. The shrinkage of food products affects the drying rate and quality of the food products as it involves surface cracking, volume, porosity and hardness changes. The De gives the moisture transfer rate in the food products during solar drying. The values of surface transfer coefficients including mass transfer coefficient (hm) and heat transfer coefficient (h) give the mass and heat transfer rate from the surface of the food product as these parameters are surface parameters. The parameters such as shrinkage, De, hm, and h of food products and their influencing factors need to be analyzed to understand more about the physics of the solar drying process. An extensive review is made on the factors of shrinkage, De, hm and h such as volume of water, moisture content, air temperature, air velocity, the thickness of product and geometry of food. The values of shrinkage, De, hm and h of different solar drying processes are tabulated and discussed. A brief review was performed on different mathematical models used to determine the values of shrinkage, De, hm, and h during solar drying of food products. The influencing factors such as moisture content, air temperature, velocity, thickness and geometry of the product on these three factors shrinkage, De, hm, and h were discussed. Important results and recommendations of this topic are mentioned at the final because it is an innovative review trial in the solar drying area.