Due to scientific enhances inside the temperatures controllers and you will control manage procedure, it has become it is possible to to put drying air heat manage steps (nonisothermal drying out), making it possible to go after sinusoidal wave profiles (Figure 1a), square-wave otherwise container function pages (Contour 1b), broadening and you can p users, saw-enamel revolution profiles, and trapezoidal wave pages (Contour 1c), among others. Nonisothermal drying out made it you’ll to acquire situations out-of large quality plus faster drying minutes as opposed to those obtained which have constant convective drying ( Chua and others 2002 ). Regardless of if various quality attributes were read into the material submitted to nonisothermal drying out, structural functions have not been advertised, neither has actually equations become proposed to spell it out its decisions ( Chua while some 2000 ; Chua although some 2002 ). At this point, no equations were suggested to help you associate changes in new architectural attributes out of material confronted by changeable external criteria.
It has been noticed there is zero significant difference ranging from the fresh new conclusion from true occurrence according to the X off the information presented during the convective drying out and you will shortly after getting rehydrated to various moisture content; but not, differences was indeed observed anywhere between noticeable density according to the X of the situation through the lyophilization and you may then rehydration to different wetness contents ( Krokida and you will ).
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 ( si puÃ² provare questo fuori Karathanos and others 1996 ).
Genuine occurrence (?p) models
Some article writers provides suggested correlations acquired as a consequence of nonlinear regression having forecasting correct thickness and you can visible occurrence due to the fact a purpose of moisture blogs. not, the outcomes perhaps not getting widespread and therefore are only legitimate towards the thing, geometry, and you may drying standards included in those individuals work (Dining table 3).
It observed linear decisions out-of occurrence a variety of suspensions out-of absolute ingredients, instance proteins, fats, carbohydrates, fibers, and you can ash. Such article writers opposed its habits having fresh density opinions to own milk, orange juice, and you can bratwurst sausage, acquiring an optimum mistake of 1.45%, and in search of a quadratic features getting liquid.