Obvious density (?b) activities
According to Krokida and you will Philippopoulos (2005) , noticeable density was a function of moisture stuff, sorts of good, and you can heavens volume proportion. A lot of activities getting anticipating obvious density found in the brand new literature was empirical; practical or theoretic patterns only have become proposed by a limited quantity of writers ( Rahman and others 1996 ). The latest visible thickness models was formulated predicated on water stuff (X), instead as a result of the results of drying heat; very few envision diminishing, although it is actually high in the almost all 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 https://datingranking.net/fr/rencontres-de-plus-de-60-ans/, they propose Eq. 19, which considers ? to be a function of , ?w, and X.
Moreira while others (2000) keep that obvious occurrence can be obtained with the apparent occurrence of good ( ) additionally the bulk out-of water ( ) at the beginning of drying while the a guide to possess 1st volume (Eq. 20, Dining table 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)].
Because ?(X) and you can ?(X) are definitely the services off water content, he or she is influenced by handling requirements, the type of your tool being dried, the newest drying out method, plus the drying phase ( Khalloufi while others 2010 ).
The failure picture (Eq. 27) proposed of the Khalloufi and others (2009) pledges the collapse function usually initiate in one and you may stops inside zero.
When the material is actually a good rubbery county, diminishing compensates almost entirely to possess moisture losses, plus the amount of the information presented reduces linearly that have water content ( ). not, low-temperature dehydration of foodstuff suppresses the brand new water stuff regarding cardiovascular system of your own procedure away from ever-being far higher than regarding the exterior, reducing internal worry, and consequently, cracking ( Bai and others 2002 ).
An erroneous formula from ?(X) and you can ?(X) offers increase so you’re able to an error regarding calculation away from apparent occurrence. This occurs quite frequently, as the majority of article authors believe that diminishing is linear ( Zogzas while some 1994 ; Moreira although some 2000 ; ), otherwise they can fit they to help you one minute-degree polynomial ( Khalloufi while others 2010 ).
Coming models should make an effort to depict the fresh new decisions out-of apparent density with deeper accuracy, considering X and the thermal history of the materials being dehydrated.
Results
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.