10.2.9   Definition of moisture sensitivity
"Hygroscopicity" should not be confused with "moisture sensitivity".
A product is sensitive to moisture if even slight water vapor adsorption rapidly causes severe changes, especially if these changes occur at low relative humidities, such as the loss of crispness in rusks which occurs at a relative humidity of only 50%; see also point XK2 in Fig. 5 for long-life cakes and cookies.
Figure 5: Types of sorption isotherms
Heiss [16]

Crystalline goods such as sugar, salt, fertilizer are slightly hygroscopic, but very sensitive to moisture at the point at which the flow moisture point is reached and the product deliquesces.
Coconut fat is absolutely not hygroscopic, but even slight traces of water cause the coconut fat to become soapy.
Ferrous and other metals are not hygroscopic, but they are extremely sensitive to moisture, with corrosion rapidly increasing from a relative humidity of < 50%.

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