Due to scientific enhances in temperature controllers and you can operating handle process, it has become you’ll be able to to help you consist of drying out sky heat control methods (nonisothermal drying), enabling you to go after sinusoidal revolution profiles (Contour 1a), square wave or container function pages (Figure 1b), increasing and you can p pages, saw-enamel revolution profiles, and you may trapezoidal trend profiles (Contour 1c), among others. Nonisothermal drying out makes they possible to get items out of large top quality plus quicker drying out times than others received that have ongoing convective drying ( Chua although some 2002 ). Regardless of if certain top quality characteristics was in fact studied for the content submitted to nonisothermal drying out, structural attributes haven’t been advertised, neither has actually equations come recommended to describe its decisions ( Chua while others 2000 ; Chua although some 2002 ). Up until now, zero equations was indeed advised to connect alterations in brand new structural services of content exposed to adjustable additional conditions.
This has been seen that there’s zero factor ranging from the fresh behavior out of correct occurrence depending on the X regarding the materials through the convective drying out and you may immediately following being rehydrated to several water information; but not, distinctions was basically observed ranging from obvious density according to the X of your own matter during lyophilization and you may subsequent rehydration to various dampness content ( Krokida and ).
Three possible tendencies of ?p as a function of X have been found (Figure 2), both linear and nonlinear (concave-down and concave-up). p and X, which considers reduction in volume to be equal to the volume of the water eliminated from the material ( Madamba and others 1994 ). In tendency of Figure 2b, it can be observed how ?p increases slowly up to a critical point, followed by an exponential decay. This change has been explained by water loss in the material during drying ( Lozano and others 1983 ; Zogzas and others 1994 ). In tendency in Figure 2c, it is assumed that the increase in ?p to a critical point is due to the fact that the reduction in volume is greater than the reduction in mass; after this critical point, the pores in the material are considered to be closed, and measured volume is greater than true volume ( Lozano and others 1983 ). In the above-described tendencies, it is merely assumed that volume and solid dry mass are constant. However, when materials are heated, they may expand or contract; in such cases, volume is not constant.
Several efforts have been made to predict different tendencies of ? as a function of X. Generally in these models, the foodstuff is considered to be a binary compound (water–solid). Table 3 shows models for true and apparent density proposed for foodstuffs. The models for predicting ?b and ?p are based on the development of pores during the lyophilization process, as the formation of pores is considered to be a function of ideal conditions, since there is no reduction in the volume of the solid as a result of water sublimation ( Karathanos and others 1996 ).
True thickness (?p) models
Some experts provides proposed correlations received thanks to nonlinear regression for anticipating genuine density and you can visible thickness due to the fact a function of water posts. But not, the outcome perhaps not be common and therefore are merely valid for the point, geometry, and you may drying criteria utilized in men and women works (Desk 3).
It observed linear behavior of density for various suspensions regarding natural ingredients, particularly protein, oils, carbs, fabric, and you can ash. This type of people opposed its designs with experimental occurrence opinions having milk, lime juices, and you may bratwurst sausage, obtaining a maximum error of just one.45%, and trying to find a beneficial quadratic capability to own liquids.