180 Novel food packaging techniques Moisture absorption by Cao at low humidity 0%RHat25°C 8a 5030205 司10 0200400600800100012001400160018002000 Absorption time(hr) 100mg Cao powder 100mg Cao desimax label Fig. 9.6 Adsorption profile of calcium oxide In most applications this is not an issue since the package or device is typically sealed in a normal atmosphere, containing no more than 0.03% carbon dioxide Nevertheless, in Fig. 9.6, the calcium oxide powder line can be seen to rise to about 27% moisture absorbed, then display an additional weight gain to over 40%. This second step corresponds to carbon dioxide absorption. Note also that CaO absorbs its full complement of moisture even in a 10% Rh environment The companion curve represents calcium oxide in a self-adhesive label structure that is sealed but which has a microperforated face that allow diffusion of moisture through to the calcium oxide the restriction of the microperforated material slows diffusion resulting in a constant slope, rather than the more typical asymptotic absorption curve characteristic of a desiccant In an open environment. 9.6.2 Magnesium oxide and barium oxide Similar behaviour may be seen with magnesium oxide and barium oxide as well Both are somewhat slower to react but barium oxide has the capability of reducing moisture in a closed system to less than I ppm. Its use is limited to these very special circumstances. 9.7 Planning a moisture defence Moisture trapped within a food product package or leaking into it during storage and shipping can cause many harmful effects. If not removed, this moisture will be adsorbed by the product or condensate will form, causing growth of mouldIn most applications this is not an issue since the package or device is typically sealed in a normal atmosphere, containing no more than 0.03% carbon dioxide. Nevertheless, in Fig. 9.6, the calcium oxide powder line can be seen to rise to about 27% moisture absorbed, then display an additional weight gain to over 40%. This second step corresponds to carbon dioxide absorption. Note also that CaO absorbs its full complement of moisture even in a 10% RH environment. The companion curve represents calcium oxide in a self-adhesive label structure that is sealed but which has a microperforated face that allows diffusion of moisture through to the calcium oxide. The restriction of the microperforated material slows diffusion resulting in a constant slope, rather than the more typical asymptotic absorption curve characteristic of a desiccant in an open environment. 9.6.2 Magnesium oxide and barium oxide Similar behaviour may be seen with magnesium oxide and barium oxide as well. Both are somewhat slower to react but barium oxide has the capability of reducing moisture in a closed system to less than 1 ppm. Its use is limited to these very special circumstances. 9.7 Planning a moisture defence Moisture trapped within a food product package or leaking into it during storage and shipping can cause many harmful effects. If not removed, this moisture will be adsorbed by the product or condensate will form, causing growth of mould, Fig. 9.6 Adsorption profile of calcium oxide. 180 Novel food packaging techniques