182 Novel food packaging techniques Bound moisture The moisture content of the packaged product can be defined as the ratio of water present in the product to its dry weight. Under equilibrium conditions(no exchange of product moisture with the environment), the vapour pressure generated from the moisture content is termed 'water activityand denoted Au More specifically, water activity can be defined by the equation Aw=P/Ps where P denotes the partial pressure of water vapour at the product's surface and Psat denotes the saturation pressure at the product temperature. Equilibrium relative humidity(ErH) is the water activity value expressed as a percentage The advantage of this definition of Aw is that it defines moisture, which can be exchanged between the product and its environment Water activity can provide better information than a products total moisture content when considering the stability of foods. It can be directly compared with the relative humidity of the ambient air to prevent hygroscopic powders(such as powdered sugars or salt) from caking, for example. In a closed package, the desiccant will work to adsorb moisture from all sources. Some plastics(such as nylon), foams paper, wood, felt, cotton and polyester can all contain moisture. Wood, cotton and paper can hold 14% or more, and certain foams up to 10%. The moisture contained in these materials can be released into the air as the desiccant dries the ir space around it or as the temperature increases. The amount of water adsorbed by the desiccant depends upon( i) how strongly the moisture is bound by that source(chemisorption or physical adsorption);(ii) the type of desiccant and quantity used; (iii) how much water has already been adsorbed by the desiccant, and(iv)temperature The main purpose of moisture regulation in foods is to lower water activity, ereby reducing the growth of microorganisms such as moulds and yeast on foods with high water activity(such as meals ready-to-eat, or MREs). Moreover, any change in the temperature of a hygroscopic food product will trigger the product to exchange moisture with the air or gas that surrounds it. Moisture will be exchanged as such until the partial water vapour pressures at the product surface and in the air or gas are equal Moisture ingress: theoretical development Desiccants, which address moisture concerns, are usually chosen by directly running tests under the intended application. However, product stability testing become time consuming and expensive. Consequently, it can be beneficial to know how much moisture theoretically permeates into the package, either ior to or in conjunction with product stability tes The moisture vapour transmission rate or MVTR is defined as the amount of water vapour passing through one square metre of test material under set proportional to the material thickness. If the mvtr (typically in units of grams of water per square metre per hour), the total surface area of the barrier(packageBound moisture The moisture content of the packaged product can be defined as the ratio of water present in the product to its dry weight. Under equilibrium conditions (no exchange of product moisture with the environment), the vapour pressure generated from the moisture content is termed ‘water activity’ and denoted Aw. More specifically, water activity can be defined by the equation Aw ˆ P=Psat where P denotes the partial pressure of water vapour at the product’s surface and Psat denotes the saturation pressure at the product temperature. Equilibrium relative humidity (ERH) is the water activity value expressed as a percentage. The advantage of this definition of Aw is that it defines moisture, which can be ‘actively’ exchanged between the product and its environment. Water activity can provide better information than a product’s total moisture content when considering the stability of foods. It can be directly compared with the relative humidity of the ambient air to prevent hygroscopic powders (such as powdered sugars or salt) from caking, for example. In a closed package, the desiccant will work to adsorb moisture from all sources. Some plastics (such as nylon), foams, paper, wood, felt, cotton and polyester can all contain moisture. Wood, cotton and paper can hold 14% or more, and certain foams up to 10%. The moisture contained in these materials can be released into the air as the desiccant dries the air space around it or as the temperature increases. The amount of water adsorbed by the desiccant depends upon (i) how strongly the moisture is bound by that source (chemisorption or physical adsorption); (ii) the type of desiccant and quantity used; (iii) how much water has already been adsorbed by the desiccant, and (iv) temperature. The main purpose of moisture regulation in foods is to lower water activity, thereby reducing the growth of microorganisms such as moulds and yeast on foods with high water activity (such as meals ready-to-eat, or MREs). Moreover, any change in the temperature of a hygroscopic food product will trigger the product to exchange moisture with the air or gas that surrounds it. Moisture will be exchanged as such until the partial water vapour pressures at the product surface and in the air or gas are equal. Moisture ingress: theoretical development Desiccants, which address moisture concerns, are usually chosen by directly running tests under the intended application. However, product stability testing can become time consuming and expensive. Consequently, it can be beneficial to know how much moisture theoretically permeates into the package, either prior to or in conjunction with product stability tests. The moisture vapour transmission rate or MVTR is defined as the amount of water vapour passing through one square metre of test material under set conditions (temperature and relative humidity) in 24 hours. It is inversely proportional to the material thickness. If the MVTR (typically in units of grams of water per square metre per hour), the total surface area of the barrier (package 182 Novel food packaging techniques