Moisture regulatio T.H. Powers and w.J. Calvo, Multisorb Technologies, USA 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, mineral 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 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 worlds 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
9.1 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
Moisture regulation 173 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, detectable fraction of crystalline silica, which is of concern as an There is still some residual salt, typically about 0.5% and mostly Na2S 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 pecify ing 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 25C 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 Regeneration 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 150C 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
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 Na2SO4. 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
174 Novel food packaging techniques Cao Molecular sieve Casos Fig 9.1 Adsorption isotherms@ 25C 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 Cla 9.3. 1 Nomenclature and sourees 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
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 nonwovens (coated and uncoated) to permeable or microperforated films. Semirigid capsules of many constructions are available with varying degrees of porosity and permeability. Occasionally, silica gel is filled into injectionmoulded 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
Moisture regulation 175 SILICA GEL Moisture adsorption rate and capacity 25'C and 20, 40, 80% RH MTI@80%RH·MII@40%RH·MII@20% Fig 9.2 Adsorption rate and capacity at 20, 40, and 80% RH 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 35C 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 034P2: P choice for many industrial and particu山时yh applications. Packaging may be in adhesive coated, reinforced kraft paper, non- wovens, or perforated film laminates. Larger sized packages of lkg 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 nother and indeed from one desiccant to another, the US military established a standard unit of adsorption. In November 1963, the Department of Defence
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, nonwovens, 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
176 Novel food packaging techniques sture adsorption of montmorillonite clay at low humidity 432 Adsorption time(hr) at 25C L-Oklahoma-1-20%RH +-Oklahoma-1-40% RH Fig 9.3 Adsorption vs time 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 ource 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- um-- and magnesium-aluminum silicates. These form orderly pores of a consistent size 9.4.2 Common types and nomenclature Molecular sieves are usually designated by their pore size expressed Angstrom units(10m). The most frequently encountered is type 4A with a
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 (10ÿ10m). The most frequently encountered is type 4A with a Fig. 9.3 Adsorption vs. time. 176 Novel food packaging techniques
Moisture regulation 177 nominal pore size of 4 Angstroms. Other common grades are 3A and 5A.A peciality 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 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 molecular sieve Ly pe4∧ watcr vapour adsorption at 25"C 086420 lInc MrI ( a 80: RH MTe4n保RH MTT G 20 RH Fig 9.4 Adsorption isotherms of common types of molecular sieve
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
178 Novel food packaging techniques Molecular sieve type 13X Moisture vapour adsorption at 25C Time(hours) 上10%-20%-30%—4% Fig 9.5 Adsorption isotherms of common types of molecular sieve 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 MgCl2 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
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 MgCl2 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
Moisture regulation 179 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 2 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 compound. The reaction may be reversed but only with an input of sufficient to decompose the compound Since decompositic ars at 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, CaCO3. CaO is principally used as agricultural lime and is known as quicklime. Calcium oxide reacts with water as follows CaO+H2O→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
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 ½ 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 temperature 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, CaCO3. CaO is principally used as agricultural lime and is known as quicklime. Calcium oxide reacts with water as follows: CaO H2O ! 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
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 mould
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
Moisture regulation 181 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 emperature 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 aintain 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
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
