9.1 Introduction Drying is probably the oldest form of preservation. Wrapping things that have been dried to protect them from moisture may well have been the earliest form of packaging. Even today a lot of technological development resources are expended to find new ways to package things to keep them dry. Some of the oldest materials used to control moisture are still used today: clay, salt, minerals, and plant extracts that have a greater affinity for water than the material being protected. Clay has been used for centuries; moist clay to keep things moist and dried clay to keep things dry. Likewise the importance of salt is legendary, whether added to foodstuffs and plant materials to bind moisture or used in the dry form to adsorb moisture. Economic losses due to moisture – not to mention the threat to life – in some areas of the world (due in part to spoilage of foodstuffs) attest to the importance of keeping things dry. It has been estimated that up to 25% of the world’s food supply is lost each year due to spoilage mostly from failure of packaging, ravages of moisture and lack of refrigeration. One of the earliest sorbents, still widely used today, is clay. It is inexpensive, widely available and requires a minimum of processing. Silica gel is the most popular sorbent due to its availability and purity as well as its whiteness, which connotes purity. Other silicates are likewise widely used in the form of natural zeolites and the synthesised forms called molecular sieves. These are used for their selectivity and their ability to keep things very dry. Many other minerals and salts are also described below. 9 Moisture regulation T. H. Powers and W. J. Calvo, Multisorb Technologies, USA 9.2 Silica gel 9.2.1 Origins The origins of silica gel lie on every beach and river bottom in the world. Sand is the raw material. Sand is relatively pure crystalline silicon dioxide. In order to manufacture silica gel, sand is first put into solution with a strong alkali. Then after filtration, precipitation, neutralisation, repeated rinsings and drying, amorphous silica is obtained. This is silica gel, needing only to be milled and classified to make it ready for use. 9.2.2 Composition Silica gel manufactured in this way is completely amorphous, lacking any detectable fraction of crystalline silica, which is of concern as an irritant. There is still some residual salt, typically about 0.5% and mostly Na 2 SO 4 . The pH is near neutrality. The usual specification is pH 4–8. There is little if any titratable acidity. Evaluation may routinely be accomplished by preparing a 10% slurry of silica gel in distilled water, extracting for two hours, and measuring the supernatant for conductivity, pH, and titratable acidity. 9.2.3 Purity and compliance: EU, FCC, USP Silica gel is permitted for use as a desiccant with foods and pharmaceuticals under EU regulations. The US Food Chemicals Codex contains a monograph specifying silica gel for food use and the US Pharmacopoeia describes silica gel for pharmaceutical use. 9.2.4 Adsorption profile Silica gel adsorption, as with any sorbent, is proportional to the equilibrium relative humidity (ERH) and the temperature of its environment. In order to view adsorption characteristics it is customary to plot an adsorption isotherm at 25oC as in Fig. 9.1. As may be seen, the adsorptive capacity of silica gel is only 3–4% at an ERH of 10% rising to a capacity of over 30% at an ERH of 90%. The rate at which silica gel approaches its capacity at differing ERH is illustrated in Fig. 9.2. Though capacity varies greatly, the rate at which silica gel approaches its capacity does not. 9.2.5 Regeneration Silica gel may be regenerated and used indefinitely. With repeated adsorption and regeneration, some particle attrition occurs which eventually diminishes its usefulness. Complete regeneration is possible to < 2% moisture at 150oC for three hours. Moreover, 75–80% of capacity may be regained at 115–120oC for six hours. Microwave regeneration at low power (<400W) is also possible. Moisture regulation 173 Regeneration of packaged silica gel may be limited by the temperature tolerance of the package material itself. 9.2.6 Packaging and applications Silica gel, as with other desiccants, is often packed in pouches or sachets. Materials range from adhesive coated papers and paper based laminates to non- wovens (coated and uncoated) to permeable or microperforated films. Semi- rigid capsules of many constructions are available with varying degrees of porosity and permeability. Occasionally, silica gel is filled into injection- moulded packages or incorporated directly into resins for moulding or extrusion. The uses of silica gel are too numerous to list. They include foodstuffs, pharmaceuticals, medical and diagnostic devices, textiles, leather goods, sealed electronics and many more. 9.3 Clay 9.3.1 Nomenclature and sources Nearly all sources and types of clay, when fully dried, have some adsorptive properties. Clays used commercially fall into the category of Bentonite. The most frequently used is Montmorillonite. Clays are principally composed of metal silicates with some sulfates and phosphates present. Fig. 9.1 Adsorption isotherms @ 25oC. 174 Novel food packaging techniques 9.3.2 Capacity and conditions of use Mined clay is activated for use through careful drying. Adsorption capacities are in the range of 25–30% of dry weight at normal room temperature and below. Above 35oC clay will begin to desorb moisture. As a result the utility of clay is greatest under temperate conditions. 9.3.3 Adsorption/desorption Adsorptive capacity varies with composition and the source. Fig. 9.3 illustrates the adsorptive characteristics of clay from a particular mine located in Oklahoma known as Oklahoma 1. Adsorption of moisture by clay is relatively rapid even at low relative humidity. As can be seen in Fig. 9.3 clay will adsorb its full complement of moisture in a matter of hours. 9.3.4 Packaging and applications Clay is the desiccant of choice for many industrial and particularly bulk applications. Packaging may be in adhesive coated, reinforced kraft paper, non- wovens, or perforated film laminates. Larger sized packages of 1kg or more may be in fabric bags, filled, then sewn closed. As noted above, the adsorption of moisture is more rapid than necessary for some applications. For operational expediency and to permit more open or working time, a package material with somewhat restricted permeability may be chosen. In order to account for the differences between one source of desiccant clay and another and indeed from one desiccant to another, the US military established a standard unit of adsorption. In November 1963, the Department of Defence Fig. 9.2 Adsorption rate and capacity at 20, 40, and 80% RH. Moisture regulation 175 released MIL-D-3464C, covering the use of bagged desiccants for packaging and static dehumidification. Three years later, MIL-D-3464D served to update the original specification, creating a uniform standard of comparison in a wide variety of areas: adsorption capacity and rate, dusting characteristics of the package, strength and corrosiveness of the package and particle size of the desiccant. In 1973, the DOD followed with specifications for cleaning, drying, preserving, and packaging of items, equipment and materials for protection against corrosion, mechanical and physical damage and other forms of deterioration. MIL-D-3464 and MIL-P-116 have long been the only objective source for packaging engineers. The strength of these specifications lies in their determination of a uniform unit of drying capacity, enabling one to compare desiccant effectiveness on a common scale. 9.4 Molecular sieve 9.4.1 Composition and purity The composition of molecular sieves are sodium-, potassium-, calcium-, and magnesium-, aluminum silicates. These form orderly macrostructures with rigid pores of a consistent size. 9.4.2 Common types and nomenclature Molecular sieves are usually designated by their pore size expressed in Angstrom units (10 10 m). The most frequently encountered is type 4A with a Fig. 9.3 Adsorption vs. time. 176 Novel food packaging techniques nominal pore size of 4 Angstroms. Other common grades are 3A and 5A. A speciality grade is type 13X with a nominal pore size of ~10 Angstroms. Figures 9.4 and 9.5 illustrate adsorption of types 4A and 13X at selected relative humidity conditions. 9.4.3 Selective and preferential adsorption Retention and selectivity among various adsorbed compounds is proportional to their polarity and effective molecular size. This is the rationale for the name ‘molecular sieve’. The ability of a particular type of molecular sieve selectively to adsorb a particular molecular species depends on what other compounds are present, e.g., more polar will displace less polar compounds in the same size range. 9.4.4 Moisture adsorption Thus in a moist environment, water will displace nearly every other compound. The adsorption capacity of a molecular sieve in a moist environment will therefore be mostly taken up by water. As a result, desiccant applications most often utilise a type 4A molecular sieve since its pore size is suitable for water and its polarity favours water. These properties combine to give type 4A a significant affinity for moisture even at low ERH. As can be seen in Figs 9.4 and 9.5, a type 4A molecular sieve can adsorb 10% of its weight in moisture at less than 10% relative humidity. This permits the moisture of a closed environment to be maintained at less than a fraction of a percent with a suitable quantity of molecular sieve. Fig. 9.4 Adsorption isotherms of common types of molecular sieve. Moisture regulation 177 9.5 Humectant salts A common and economical method of controlling humidity in moist environments is the use of humectant salts. Such salts will adsorb moisture until they go completely into solution. As this occurs, what will be seen is a mixture of solid salt and a salt solution. This solution will be saturated and will have an equilibrium relative humidity characteristic of the particular salt used. 9.5.1 Sodium chloride As an example, a saturated solution of common table salt, sodium chloride, will have an ERH of ~75%. A corollary of this is that sodium chloride will not adsorb moisture if the ambient humidity is less than 75%. 9.5.2 Magnesium chloride Similarly, a saturated MgCl 2 solution has an ERH of about 35%. Numerous tables can be found in handbooks, that list the ERH of many salts and common ambient temperatures. Wexler defined a relationship between ERH and temperature; constants for Wexler’s equation for some commonly used humectant salts have been published. Therefore it is usually possible to select a salt that will control humidity in a range suitable for the system or packaged product being protected. Fig. 9.5 Adsorption isotherms of common types of molecular sieve. 178 Novel food packaging techniques 9.5.3 Calcium sulfate Certain salts, sulfate salts in particular, will take on water of hydration in fixed mole proportions. Some salts such as calcium sulfate have multiple hydration states. Anhydrous calcium sulfate can take on ? mole of water which becomes the commonly known ‘Plaster of Paris’. Likewise it can take on two moles of water; this is known as gypsum. Many salts then can take on water of hydration. Their utility as moisture sorbents depends on the kinetics of adsorption under the conditions of use. Some experimentation may be required to make the right choice. 9.6 Irreversible adsorption Closely related to water of hydration is the addition of water to alkali metal and transition metal oxides. Here water reacts with the oxide to form a separate compound. The reaction may be reversed but only with an input of energy sufficient to decompose the compound. Since decomposition occurs at a tem- perature well above any possible conditions of use, adsorption is considered irreversible. 9.6.1 Calcium oxide The most frequently encountered example of this is calcium oxide. It is the product of high temperature decomposition of limestone, CaCO 3 . CaO is principally used as agricultural lime and is known as quicklime. Calcium oxide reacts with water as follows: CaO H 2 O!Ca(OH) 2 The resulting product, calcium hydroxide, is likewise used for agricultural purposes and is known as slaked lime. To reverse this reaction requires raising the temperature to nearly 600oC. Its use as a desiccant depends on the same reaction. The great amount of energy required to reverse the reaction makes this irreversible in a practical sense. As may be inferred, the product is quite alkaline. Although it is technically a corrosive product and irritating to the skin, it is not particularly hazardous in use due to its very low solubility (about 0.06g/100cc of water). Calcium oxide is used as a desiccant principally where extremely low residual moisture is required. In a closed system it is possible to reduce moisture to a few parts per million with a suitable amount of calcium oxide. And it is a fairly efficient desiccant as well, capable of absorbing about 28% of its dry weight as water. In addition it is specific for water. Calcium oxide is used for very demanding packaging applications but, more particularly, for sealed electronic devices that are expected to last for years. It must be kept in mind that calcium hydroxide is also a good carbon dioxide absorber. Therefore if calcium oxide is used as a desiccant in a carbon dioxide environment it will absorb carbon dioxide as well, releasing water in the process. Moisture regulation 179 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 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 mildew and fungus. For example, if a solid is very water soluble (such as a sugar coating), dissolution into the adsorbed layer can trigger irreversible water uptake and subsequent deliquescence, given the appropriate conditions. Selection of the proper desiccant can be inexpensive insurance for protecting packaged food products, thus resulting in improved quality. 9.7.1 Sources of moisture in packaging Those involved in food packaging applications face a confusing array of variables when selecting moisture adsorbents (desiccants), as moisture regulation is a multi-faceted challenge. More specifically, sources of water permeation into a closed package or container can be attributed to moisture from (i) the product itself, (ii) any material (such as felt, foam, paper, etc.) used to support or retain the product, and (iii) permeation through the protective barrier of the package. With the goal of selecting the appropriate desiccant, moisture contributed by the product environment (ambient moisture) and the package (bound moisture) must be considered independently. Ambient moisture Temperature and relative humidity are two of the most influential environmental factors affecting product integrity and must be controlled to match the conditions of optimum product preservation and performance. Before selecting the correct desiccant, it is imperative to know the conditions surrounding the shipment and storage of the product. Furthermore, at the time of packaging, it must be noted that the product is sealed in the conditions of the packaging room. The moisture content of the air can be defined by its relative humidity, equal to the ratio (expressed as a percentage) of the partial pressure of water vapour present in the air to the saturated vapour pressure. The most useful combined measure of temperature and relative humidity is the dewpoint, that is, the temperature at which the actual vapour pressure equals the saturated vapour pressure. As the temperature drops, the saturation water vapour pressure decreases. Any additional drop in temperature will give rise to condensation, as the amount of water in the air has then exceeded the saturation point. Condensation provides the most dramatic visual observation of the effects of moisture damage. An effective desiccant will adsorb water vapour from the air in a package, lowering the relative humidity to the point where condensation will no longer occur or the threshold relative humidity is never exceeded under the conditions to which the package will be exposed. As a general rule of thumb, designing the package and the desiccant to maintain an internal relative humidity of 10–12% at normal room temperature conditions (70oF) will provide adequate protection. It is strongly suggested that the desiccant supplier be contacted to discuss the elements of the package and the level of protection required. Moisture regulation 181 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 activity’ and denoted A w . More specifically, water activity can be defined by the equation A w P=P sat where P denotes the partial pressure of water vapour at the product’s surface and P sat 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 A w 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 surface area) and the length of time in storage are all known, the moisture ingress can be determined. This value represents the amount removed by the desiccant to protect against possible condensation within the package or damage due to prolonged exposure in a high humidity environment. The permeability coefficient is a material constant which specifies the volume of gas which will pass through a test material of known surface area and thickness in a fixed time, with a given partial pressure difference. The coefficient varies with temperature. Permeability coefficients for packaging materials can usually be obtained from the material supplier or, more appropriately, permeability data on the actual package may be available. Published material permeability values can be found not only for the packaging material, but also for desiccant packaging (a sachet, for example) as well. Permeability coefficients can be converted into permeability rates defined by the origin of the driving force involved (whether it be water concentration, partial pressure or mole fraction). Water vapour partial pressure data are acquired through thermodynamic steam tables. These rates may have to be adjusted to reflect actual shelf-life conditions. Moisture ingress (mg water/day, for example) across a package wall for given relative humidity conditions inside the package is determined by the use of a general material balance for water, namely Flow of H 2 O A pkg P pkg C out C in where A pkg surface area of package P pkg permeability rate of package material C out concentration of water C in concentration of water in headspace. The permeability rate of the package material needs to be adjusted both for the thickness of the material and the partial pressure differential. It is important to note that the reaction rates are assumed to be relatively rapid compared to the rate of diffusion, so that the process is diffusion controlled. The amount of water permeating into a package during shelf-life depends largely on the type of packaging (e.g., flexible, rigid) and the packaging materials that are used. For example, various flexible packaging materials exhibit different MVTRs. Water ingress into flexible packaging can only be roughly estimated from such data. This is because the contribution of moisture ingress from the seal is not included in material permeability data. Desiccants, which address moisture concerns, are usually chosen by directly running tests under the intended application. However, more rigorous testing can become time-consuming and expensive. 9.7.2 Desiccant selection process In order to maximise success in the selection of a desiccant, the properties of both the commercially available desiccant and the package material(s) must be considered. The conditions involving optimum product preservation and performance are analysed experimentally and/or by seeking published literature values. Moisture regulation 183 In a porous desiccant, water is removed from the headspace either by multilayer adsorption, where thin layers of water molecules are attracted to the surface of the desiccant or by capillary condensation, where the smaller pores become filled with water. In multilayer adsorption, the surface area is high due to the extensive porosity and significant amounts of water can be attracted and adsorbed. In contrast, however, capillary condensation occurs because the saturation water vapour pressure in a small pore is reduced by the effect of surface tension. Capillary condensation also may occur in any pores within the packaged product. The harmful results of moisture condensation may occur in some products at humidity levels below those that would be predicted from examination of the bulk package headspace. Moreover, the container in which the product will be packaged, shipped and stored is vital in determining how much of a particular desiccant is needed and in what packaging form. Before the adsorbent selection process itself, the size of the container based on the flexibility of the package wall structure must be determined. The three most important parameters to consider in the desiccant selection process are relative humidity, adsorbent capacity and adsorbent rate. 1 Relative humidity Knowledge of the maximum and minimum humidity levels that should be maintained can further enhance the correct choice of desiccant. Product stability tests by the food manufacturer usually establish the maximum moisture level that should not be exceeded during shelf-life. In addition, the maximum relative humidity in the packaging allowed by the product manufacturer during shelf-life is an important consideration. This is because dividing the total amount of moisture to be adsorbed by the adsorption capacity of pure desiccant (at the maximum allowable relative humidity) yields the minimum amount of pure desiccant required. The effect of residual moisture, which is usually specified by the desiccant supplier, also needs to be taken into account. There is usually a minimum moisture level to be maintained during shelf-life in order to avoid problems arising as a result of moisture loss in the product. 2 Adsorption capacity Adsorption capacity is calculated by a mass balance on the various sources of water involved. The total moisture (W tot ) is calculated by taking the sum of headspace air humidity (W hs ), available residual moisture in the packaged product (W rm ) and the amount of water ingress into the package during shelf-life (W in ). W tot W hs W rm W in W hs is the product of headspace volume and absolute air humidity in the environment during the packaging operation. The contribution of W hs to W tot is usually negligible, because headspace is minimized during packaging design and when packaging is done under controlled low-humidity conditions. W rm is the 184 Novel food packaging techniques amount of water that can be desorbed from the packaged product by the desiccant during shelf-life. This amount depends on several factors including moisture desorption thermodynamics and kinetics (originating from the packaged product) as well as the overall moisture content of the product after manufacturing. 3 Adsorption rate The third parameter to note when considering desiccants is adsorption rate. For example, silica gel reduces the relative humidity in a closed container from 20% to virtually 0% in about one hour. Under actual manufacturing conditions, the product being packaged contains residual moisture. This moisture has to be adsorbed by the desiccant, thereby increasing the reaction time. Note also that as residual moisture increases, adsorption capacity decreases. In most cases, the desorption kinetics for water from the product are slower than the adsorption kinetics of the desiccant. Consequently, the desorption reaction is the rate- limiting step for overall dehumidification. This phenomenon eliminates the difference between the desiccant reaction rates and leads to more prolonged reaction times. 9.8 Future trends Moisture management is taking on a broader meaning as the whole field of active packaging expands. It is no longer adequate simply to keep things ‘dry’. They must be dry enough to attain stability but not so dry as to affect the structure on a macro or on a molecular level. Therefore moisture control becomes water activity control. There is a growing recognition that in some sealed systems moisture is not the only volatile substance which can degrade the active components of the system. Volatile organics, particularly tri-halo methanes as well as volatile acids, aldehydes and alcohols may be present as well. Desiccants then may need to be blended with other sorbents or impregnated with compounds that can selectively bind molecules other than water. Finally, greater importance is being placed on the selectivity of sorbents. A wider selection of molecular sieves is becoming available. New synthetic techniques are being developed; some mimicking biosynthetic mechanisms are being studied. Developments are moving in the direction of more specialised sorbents and multifunctional sorbents. This trend is sure to continue and will most likely accelerate. Moisture regulation 185