My sample after filtration was dried in an air-oven for 15 h, at 80 deg C then weighed. Later this was introduced into a vacuum oven at 80 deg for 2 hours. There was a loss of weight compared to the air-dried sample and the electrochemical behaviour was also different from that of the air-dried sample. Can anybody suggest why there is a difference between the two drying processes?
Like Bapi wrote one of the difference between air-oven and vacuum oven is the O2 partial pressure. In vacuum oven the O2 partial pressure is low and therefore the reactions involving O2 decrease their rate, which may explain the electrochemical differences. The other difference depends of oven pressure. The water vapor pressure at 80oC is around 47.4 kPa, if your absolute pressure into vacuum oven is less than 47.4 kPa the remnant liquid water wolud reach its boiling point, which explain your weigh loss. Even if oven pressure was not less than 47.4 kPa, the reduced pressure by vacuum increase the vapor pressure gradient between your product and surroundings which increase the mass transfer of water.
In air oven sample undergo oxidation but in vacuum oven it doesn't happen.
Quantity of oxidation in a product is also a function of time. Oven drying was for 15 h and vacuum drying for 2 h, that explains why you observed a variation in electrochemical behavior in both. Also, it is not unexpected that vacuum drying would lead to removal of more moisture because of minimal change in surface characteristics that could mitigate moisture removal, hence the easy pass for moisture migration even though drying was for 2 h. Under vacuum, you lower the vapor pressure around your product, and more water can evaporate at lower temperature, or faster at higher temperature above the saturation temperature at that pressure.
Ms Ramasami Alamelu,
Besides the possibility of oxidation occurred during (the first) hot air oven drying, your oven dried sample might have had reached the (new) EMC due to it quickly absorbed H2O from the surrounding air. These two mechanisms would certainly caused (additional) weight loss during the later vacuum oven drying and some electrochemical changes in your (further) dried sample. The changes in electrochemical characteristics may be even more significant if your sample is hygroscopic.
In air oven the temperature and humidity control are not maintained, Vacuum oven is used to measure the amount of water and volatile matters present in the product. It requires certain limited temperature. Otherwise the sample may be degraded and it will become unable to pick up the water and volatile impurities, therefore we have to use vacuum oven.
In hot air oven, after some time of moisture removal, your product would reach equilibrium moisture content. After that, no more removal of moisture would take place. But under vacuum, there would be more reduction in moisture.
Higher the drying time of organics, greater the changes in properties occur, which may be the reason of changes in electrochemical behaviour.
Beside this, as stated by Akinbode A. Adedeji, occurance of oxidation may be another factor for changes in electrochemical behaviour.
Haldel et al. (1990) reported about a number of effects which occur during conventional heating, such as starch transition, melting of fats and cell structure changes, are shown to affect the electrical conductivity of foods during ohmic heating
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.1990.tb01055.x/abstract
both methods are used to determine the moisture content of animal feed in my case but the difference is the type of feed to be dried. in oven any dry feeds can be used but in vacuum silage which is fermented feed can be used.
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