A recipe for baking states that the almond powder must be dried in an oven at 60°C for 2 hours. After that, the almond powder must be allowed to “rest” at room temperature and then baked with it. My concern now is: if you dry the almond powder, you will remove all the moisture, but if you just let it “rest” at room temperature, it will just absorb moisture from the air itself, right? The amount of water in the air will be much higher than in the powder, so an equilibrium will be created again between the powder and the air, with the result that water will go from the air to the powder. Or is almond powder highly hydrophobic and won’t absorb moisture?
Answer
Dear Jens,
moisture absorption by food is mainly determined by four factors. I start with the simplest and end with the most complex.
(1) time: moisture release or absorption is actually a process that is diffusion-driven and therefore time-dependent: the longer you wait, the more diffusion and thus water exchange can occur; concretely in your case: if you wait a very long time after drying, there is a greater chance of water exchange
(2) the relative humidity of the air (but I need to combine this with the following factor): this is actually the relative pressure of water in an air mixture; it depends on the amount of water in the air (the more, the higher the relative humidity) and the temperature (the higher, the lower the relative humidity)
(3) the ‘relative humidity of the food’; we actually call the latter the water activity of a food. It is a measure of the availability of water in the food for chemical reactions, micro-organisms, as a transport medium, etc. You can actually consider the water activity as the relative humidity of a layer of air that is present just above the food and therefore actually in balance with the food.
You can actually see it as follows: if you place a food with a water activity lower than the relative humidity of the ambient air, the food will lose moisture to the environment and will therefore dry out. This lasts until equilibrium is reached, ie when the water activity of the food becomes equal to the relative humidity of the air. This is also the reason why the product dries at 60°C: due to the higher temperature of the air, it has a lower relative humidity and can therefore absorb moisture from a product that has a higher water activity.
If the dry product (with a low water activity) then ends up in a humid environment, it can indeed absorb moisture again: think of a crispy baked biscuit that remains on the cupboard and loses its crispness.
(4) Then I come to the most difficult point. In the scenarios above I have assumed for simplicity that the water activity in the food is the same everywhere in this food. This does not always have to be the case. For example, think of a pizza with a low-moisture baked cake and moisture-rich cheese on top. In the specific example of the dried almond powder, we may have to consider a film formed on the surface of each powder particle that is drier than the inside of the particle. This outermost layer can become particularly hard, especially with moisture-rich products, and also form a barrier for the moisture (which either wants to get in or out).
In your specific case, this could mean that the moisture absorption after drying can be somewhat delayed. I think the chance of getting such an impermeable layer is higher if your drying process is done abruptly: i.e. at high temperatures, which would induce rapid drying at the surface. In your specific case, however, the drying conditions seem to be quite mild.
So in summary: the dry powder will be able to absorb moisture again if you don’t store it in a sealed environment. But a lot depends on how long you keep it, of course.
Hopefully this has made things clearer.
Regards
Bruno DeMeulenaer
Answered by
Prof. Dr. Ir. Bruno De Meulenaer
Food Science Food Chemistry
http://www.ugent.be
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