|Abstract:||The availability and movement of water inside the food materials play essential roles for food stability by affecting their physical and chemical properties, and microbiological activity. Understanding the moisture transport is a necessary step to control food properties. Food processing unit operations, like drying and sorption, influence the behavior of foods. These processes alter many aspects of foods such as acceptability, nutritional value, quality, and shelf-life. Understanding the water transport in foods and the changes occurring in functional properties has a great importance in describing and modeling their sorption and drying behavior. Due to complexity and dynamic nature of the drying and sorption processes, purely experimental methods are not capable of obtaining information on transport mechanisms. In other words, experimental methods can only be used to express one process condition. Therefore, there is a need for a general modeling approach to express the sorption and drying processes, which would allow predicting not only the water transport but also the quality attributes of the product during the process as a function of process conditions. Primary mechanisms occurring during water transport such as moisture sorption, swelling/shrinkage, and glass transition were incorporated using hybrid mixture theory (HMT)-based multiscale modeling approach (Chapter 2).
It is important to understand the viscoelastic properties of cereals since they affect product texture as a result of moisture transport. The stress relaxation behavior of oat flakes is presented in Chapter 3. The linear viscoelastic region of oat flakes was found to be below 2% strain levels. The relaxation behavior was highly dependent on moisture content. The viscoelastic behavior of oat flakes was represented with two-element generalized Maxwell model. A decreasing trend was observed in stress relaxation coefficients, G_0, G_1 and G_2, as moisture level increased due to changes in oat structure such as alterations in secondary structure of proteins, gelatinization mechanism of starch molecules, and water holding capacity of fibers.
The effect of drying conditions on physical and viscoelastic properties of strawberries and carrots, as representative products for fruits and vegetables, was investigated (Chapters 4 and 6, respectively). For both samples, drying followed falling rate period, denoting that the water transport was limited by internal resistances, especially due to shrinkage effect during drying process. An increase in either drying temperature or drying duration resulted in lower color values, moisture content, and volumes. However, the effect of duration compared to temperature was found to be more important for carrots. The decreasing trend in color parameters can be associated to degradation of carotenoids and anthocyanins, which are responsible from the major color reflection of carrots and strawberries, respectively. The creep and stress relaxation behavior of both samples are expressed using Burger’s (R2 ≥ 0.971) and three-element Maxwell models (R2 ≥ 0.998). The frequency sweep measurements showed that both carrots and strawberries presented solid-like behavior. The findings from these experimental studies were used to solve a multiscale model to estimate the moisture and stress distribution throughout the sample during drying, and to estimate the quality attributes.
The hybrid mixture theory-based multiscale models are able to describe the physico-chemical changes and general transport mechanisms occurring within a porous food matrix. This theory can also be used to predict the quality changes in food products during processing by coupling transport equations with kinetic equations. A multiscale model using hybrid mixture theory was solved for drying of carrots and strawberries (Chapters 5 and 7). The models for carrots and strawberries were validated comparing the experimental results obtained in Chapters 4 and 6. Good agreements were obtained for moisture content, color parameters, and shrinkage. The model results clearly showed the importance of glass transition in water transport. For both samples, the drying in the vicinity of glass transition regime resulted in sharper moisture profiles, indicating non-Fickian transport. Especially the boundaries due to lower moisture content in these regions underwent glass transition during the drying process, which affected the textural properties. Similar to experimental results, it was found that higher drying temperature led to greater shrinkage and deformation throughout the samples, and a greater loss in color values and nutrient content, beta-carotene for carrots and ascorbic acid for strawberries. The developed model for drying of fruits and vegetables would allow finding optimum drying conditions for specific products.