The 'normalized air content': A novel and reliable concept to assess pore formation during dehydration

摘要

Porosity is generally used to monitor the evolution of pore formation during dehydration. However, in some cases of dehydration, porosity could increase without involving pore formation, resulting in misleading interpretation. To avoid this misleading interpretation, this contribution suggests a new concept, which is the normalized air content (NAC). NAC is defined as the volume of air (mL) contained in a matrix per g of dry matter composing the same matrix. Based on this definition, it is possible to calculate the NAC using experimental data of porosity, bulk density, or volume reduction coefficient during dehydration. An extensive validation of this concept was performed by using experimental data. Porosity and NAC profiles as a function of moisture content were compared for some relevant cases. In addition, porosity and NAC values were calculated for more than 80 fresh and dehydrated products obtained by several research groups, diverse drying technologies, and under different operating conditions. The results revealed that, in several cases, porosity increases while NAC decreases. In 75% of the cases investigated, with the exception of freeze-drying technology, the NAC does not increase when the porosity increases. This analysis led to the conclusion that, in some cases, the increase of porosity may not be the consequence of pore formation within the dehydrated products but would rather be due to the removal of water during dehydration. The NAC profiles allowed distinguishing whether there is an actual pore formation within the food products investigated. One practical use of NAC is that it could be exploited to select promising technologies and/or drying conditions for producing porous products.