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