Visible occurrence (?b) activities
Considering Krokida and Philippopoulos (2005) , noticeable density try a purpose of dampness stuff, form of good, and you may air volume ratio. lien vital A lot of models to own anticipating visible occurrence found in the fresh new books is empirical; practical otherwise theoretical activities have only started proposed by the a restricted amount of experts ( Rahman while some 1996 ). Brand new visible thickness designs were created according to wetness stuff (X), as opposed to due to the negative effects of drying out temperature; not many thought shrinking, although it is actually high regarding the most times ( Moreira although some 2000 ; ).
18), where the volumetric shrinking coefficient (?) is a linear function of moisture content. Boukouvalas and others (2006) mention that the parameters ( , ?) included in Eq. 18 of Table 3 depend on the drying method and processing conditions; as such, they propose Eq. 19, which considers ? to be a function of , ?w, and X.
Moreira although some (2000) hold one to obvious thickness can be acquired using the obvious occurrence of the good ( ) while the mass out-of liquid ( ) early in drying as the a reference to own initial regularity (Eq. 20, Desk step 3).
In Eq. 18 and 19, X= 0 is taken as a reference; in Eq. 20, the reference is X0= moisture content at the onset of drying.
The most recent theoretical model (Eq. 21) was put forth by Khalloufi and others (2010) . They consider ?b to be a function of the initial porosity of the material (?), X, ?w, ?s, ?(X), and collapse [?(X)].
Since the ?(X) and ?(X) will be the attributes out-of dampness stuff, he could be influenced by processing standards, the sort of your own product getting dried, the brand new drying out strategy, and the drying out stage ( Khalloufi although some 2010 ).
The new collapse equation (Eq. 27) proposed because of the Khalloufi while others (2009) claims that the collapse function usually starts in one single and you will comes to an end in no.
If matter is actually a good rubbery condition, shrinking makes up nearly entirely having wetness loss, in addition to volume of the materials decreases linearly that have moisture content ( ). However, low-temperatures dehydration of foods suppresses brand new dampness articles in the cardiovascular system of matter off ever being far more than that the outside, reducing inner worry, and therefore, breaking ( Bai while others 2002 ).
An erroneous computation away from ?(X) and you can ?(X) will give go up in order to an error regarding the formula regarding apparent density. This occurs often, as most of article authors believe that shrinking are linear ( Zogzas while some 1994 ; Moreira and others 2000 ; ), otherwise they can fit it in order to a moment-knowledge polynomial ( Khalloufi although some 2010 ).
Future models would be to strive to represent brand new conclusion off obvious density that have deeper precision, looking at X additionally the thermal reputation for the materials becoming dehydrated.
Findings
The tendencies of density with respect to moisture content and drying temperature have been discussed within the framework of currently available theories of second-order phase changes, structural changes, and changes in chemical composition occurring in the mass and heat transference processes. The majority of the empirical and theoretical equations representing true density do not fit the concave-down tendencies of certain materials. Equations for calculating apparent density based on shrinking and collapse are not reliable, as errors arise from failing to consider minimal variations in volume at the end of the drying process. It is recommended that variations in ?s and ?w as a function of the internal temperature of the material, according to the drying conditions (X, T), be included to yield a better fit to the tendencies of density with respect to X.
