Relative humidity and moisture.
Almonds can pick up or lose moisture depending on their initial moisture content and the relative humidity (rH) of the surrounding environment — called moisture migration. Unwanted moisture migration in almonds may affect texture, microbial stability, and the rate of various reactions that impact shelf life. When almonds pick up moisture (adsorption), they may lose some of their crunch, mold may start to grow, and lipid oxidation increases. Moisture loss (desorption) may lead to some desirable changes, such as more crunch, but at very low moisture lipid, oxidation also increases.
Moisture migration occurs until equilibrium within the system is reached; almonds in high-humidity environments will generally pick up moisture, especially at ambient and higher temperatures. Stopping moisture migration requires either a moisture-barrier package and/or reducing the humidity of the environment.
The effects of environmental rH on almond moisture levels are expressed by water sorption isotherms. As shown from almond isotherms, at a range from 20 to 65% rH, almonds will retain moisture levels from 3.0 to 6.0; at these levels, almonds are less prone to biological or chemical reactions. More ideal moisture levels for almonds are 3.0 to 5.0%, which can be achieved at environmental conditions of 20 to 55% rH. During storage, managing environmental humidity is a key to preserving almond quality. It is critical to maintain a steady environmental rH so the moisture levels in almonds will not fluctuate over storage.
Studies at the University of California, Davis, indicate that different varieties or sizes of whole almond kernels and pasteurized or unpasteurized almonds interact similarly with environmental rH, but roasted and blanched almonds interact differently.
Relative humidity fluctuation will affect almond moisture changes, which will impact texture quality. This online moisture and texture model demonstrates the effects of environmental rH on almond moisture content and the impact on texture properties.