Compressibility

Food powders can be compacted by tapping or by mechanical compression. These processes can occur either unintentionally as a result of handling or transporting, or intentionally as when tableting or agglomerating. In the food industry, unintentional compression is normally undesirable, while operations aimed at obtaining defined shapes are usually required in some processes. Unintentional compression will be discussed in this section, whereas the latter will be included under the general scope of processing operations in a subsequent part of this text.

The theoretical and empirical considerations of vibratory compaction have been mainly focused on nonfood powders (Hausner et al., 1976). Sone (1972) reported the following relationship for food

78 Food Powders

Table 3.5. Effect of anti-caking agents on the bulk density and compressibility of selected food powders. Poured bulk

Powder Agent Concentration density (kg/m3_{) Compressibility}

Sucrose (powdered) None — 700 0.066

Calcium stearate 0.5 870 0.039 Silicon oxide 0.5 750 0.052 Tricalcium phosphate 0.5 760 0.044

Salt (powdered) None — 1,010 0.080

Calcium stearate 0.1 1,140 0.032 Silicon oxide 0.1 1,100 0.045 Tricalcium phosphate 0.1 1,160 0.025

Soup mix None — 700 0.27

Aluminum silicate 2.0 750 0.15 Calcium stearate 2.0 630 0.27

Gelatin (powdered) None — 680 ∼0

Aluminum silicate 1.0 700 0.016 Microcrystalline cellulose None — 350 0.017 Aluminum silicate 1.0 360 0.030

Corn starch None — 620 0.109

Calcium stearate 1.0 590 0.099 Silicon oxide 1.0 670 0.077 Tricalcium phosphate 1.0 610 0.062

Soy protein None — 270 0.040

Calcium stearate 1.0 270 0.041 Silicon oxide 1.0 270 0.036 Tricalcium phosphate 1.0 310 0.024 powders: γn= V0− Vn V0 = abn 1+ bn (3.6)

whereγn is the volume reduction fraction, V0 is the initial volume, Vn is the volume after n taps,

and a and b are constants.

The applicability of Eq. (3.6) was tested through its fitting to the following linear form:

n γn = 1 ab+ n a (3.7)

The constant a in Eqs. (3.6) and (3.7) represents the asymptotic level of the volume change or, in other words, the level obtained after a large number of tapings or a long time in vibration. The constant b is representative of the rate at which this compaction is achieved, i.e., 1/b is the number of vibrations necessary to reach half of the asymptotic change. In general, this form of data presentation is very convenient for systems comparisons, since it only involves two constants.

A very common undesirable aspect of compressibility is its negative influence on flowing capacity. In powder technology, great attention has been paid to the general behavior of powders under compressive stress (Peleg, 1977). Compression tests have been used widely in pharmaceutics, ceramics, metallurgy, civil engineering, as well as in the food powder field, as a simple and convenient technique to measure such physical properties as powder compressibility and flowability. In order to get the pressure–density relationship for a given powder, a set of compression cells (usually a piston in a cylinder) is used. The tested powder is poured into the cylinder and compressed with

the piston attached to the crosshead of, for example, a TA-XT2 Texture Analyzer (Stable Micro Systems, England) or Instron Universal Testing Machine. Normally, the instrument will record a force–distance relationship during a compression test. It is relatively easy to change this relationship into a pressure–density relationship to get the compressibility after data treatment, when the cross section area of the cell and the initial powder weight are known. The compression process takes place in two stages: filling voids with particles of the same or smaller size than the voids by particle movement, and filling smaller voids by the particle’s elastic, and/or plastic deformation, or fragmenta- tion.

The pressure–density for powders in a compression test at a low-pressure range can be described by the following equation (Barbosa-C´anovas et al., 1987):

ρ(σ ) − ρ0

ρ0

= a + b log σ (3.8)

where ρ (σ ) is the bulk density under the applied normal stress σ, ρ0 the initial bulk density,

and a and b are constants. The constant b represents, specifically, the compressibility of a given powder. Compression tests are useful in characterizing the flowability of powders because the inter- particle forces enabling non-flowing open structures stand still in powder beds are crushed under relatively low pressures. As shown in Eq. (3.8), the constant b, representing the change in bulk density by applied stress, is referred to as powder compressibility. It has been found that b can be correlated with cohesion of a variety of powders and, therefore, could be a simple parameter to indicate flowability changes (Peleg, 1977). Generally, the higher the compressibility the poorer the flowability, but if quantitative information about flowability is required, shear tests are necessary (Schubert, 1987).

One of the standard methods for evaluating the flowability of a particulate system is to calculate the Hausner ratio after tapping. As described before, the Hausner ratio is defined as the ratio of a powder system’s initial (loose) bulk density to its tapped bulk density (i.e., the ratio of loose volume to tapped volume). It is easy to calculate the Hausner ratio and evaluate flowability when the loose and tapped volumes of the test material are known. Hayes (1987) has defined different ranges for Hausner ratio (HR) to characterize flowability:

r 1.0 < HR< 1.1, for a free flowing free flowing powder;

r 1.1 < HR< 1.25, for a medium flowing powder;

r 1.25 < HR< 1.4, for a difficult flowing powder;

r HR> 1.4, for a very difficult flowing powder.

Adding a small amount of fine powders, such as anti-caking agents, is often used to improve the flow properties of powdered materials in the chemical, pharmaceutical, and food industries. In this case, fine particles coat the coarser particles of the main constituent and prevent them from sticking together. Damp or sticky solids, which are difficult to handle, may be converted into free flowing after the added fine powders absorb small quantities of liquids. Another way to improve flowability of food powders is by using the agglomeration process, which is accomplished by wetting the fine particles in an atmosphere of water or suitable solvent droplets, causing them to collide and stick together, and then drying the agglomerated material in an air stream. Apart from improving flowability, agglomerated powders may show better wettability and dispersibility in liquids, and tend to be dust-free (Hoseney, 1994). Agglomeration will be discussed further in a subsequent chapter of this text.

80 Food Powders

3.4. STRENGTH PROPERTIES

There are a number of properties of particulate materials that determine particle breakage and attrition. Many solid food materials, especially when dry, are brittle and fragile, showing a tendency to break down or disintegrate. Mechanical attrition of food powders usually occurs during handling or processing, when the particles are subjected to impact and frictional forces. Attrition represents a serious problem in most of the food processes where dry handling is involved, since it may cause undesirable results such as dust formation, health hazard, equipment damage, and material loss. Dust formation may be considered the worst of these aspects, as it may develop into a dust explosion hazard. The topics of attrition and dust explosion are included in Sections 12.1 and 12.4 of this book, respectively. Some procedures to assess strength properties will be discussed as follows.

3.4.1. Abrasion

Abrasiveness of bulk solids, i.e., their ability to abrade or wear surfaces with which they come into contact, is considered a property closely related to the hardness of the material. The hardness of powders or granules is defined, in direct analogy with the definition of hardness of solid materials, as the degree of resistance of the surface of a particle to penetration by another body. Hardness is often considered a relative rather than an absolute property and may be determined by using the well-known Mohs’ hardness scale shown in Table 3.6. In this scale, the ten selected minerals are listed in order of increasing hardness, so that a material of a given Mohs’ number cannot scratch any substance of a higher number, but will scratch those of lower numbers. In a qualitative manner, materials different from those included in the scale are referred to as having an equivalent number of hardness of the ten listed.

Likewise, the abrasiveness of food powders can be assessed in different ways. It can be implied from the relative hardness of the particles and the surface with which they are in contact, using the Mohs’ hardness scale. It can also be described by an abrasion index, which combines the effects of particle hardness, shape, size distribution, and bulk density into one factor, independent of the nature of the contacting surface. The best way to assess abrasiveness is to use the actual bulk material and the contact surfaces in question. There have been some developed tests proposed for specific materials. For example, a test used for coke and coal, consisting of measuring the wear on a standard surface when it is brought into moving and intimate contact with the material under specific conditions, can be adapted to many different materials, including food powders. Abrasiveness and hardness are two major factors that govern the choice and design of different types of equipment, such as size reduction

Table 3.6. Mohs’ scale of hardness. Hardness number Material Notes

1 Talc, graphite Can mark paper powdered by finger 2 Gypsum rock salt Can scratch lead

3 Calcite Can scratch finger nail 4 Fluorospar Can scratch copper coin 5 Apatite

6 Feldspar Can scratch window glass 7 Quartz Can scratch a knife blade

8 Topaz

9 Sapphire, corundum 10 Diamond

machines, air classifiers, mixers, dryers, etc. Hardness, rather than abrasiveness figures and values, are normally found in the literature; from a practical standpoint hardness can be taken as the chief property of this kind when making a decision on design and operating aspects of processes involving equipment units like those mentioned above. As a rule of thumb, considering Mohs’ hardness scale, materials can be generally rated as soft, medium hard, or hard, when they show values between 1 and 3, 3.5 and 5, and 5 and 10, respectively. Many food materials are normally soft according to this criterion and, thus, the problems related to strength of materials normally faced in the food industry have to do with attrition and friability, rather than hardness and abrasion.

3.4.2. Friability

This property is defined as the tendency of particles to break down during storage and handling, while attrition is the actual, unwanted breakdown of particles. The above two definitions imply total breakdown, but attrition usually means particles getting smaller due to their corners or surface irregularities being knocked off. Attrition is a serious, yet little understood problem in handling of food materials, which may be considered responsible for economical losses in the food industry. Friability can be commonly determined using impact, vibration, shear, and tumbler tests.

Breakdown of particles on impact can be tested either on single particles or on a quantity of the bulk solid, and the result is a measure of the material’s friability. The most common types of multi-particle impact test are the drop and shatter tests in which a specified quantity of the material is dropped through a specified height onto a hard surface or into a container. Drop shatter tests are generally used on coarser solids than those within the scope of this text; the fine fraction is in fact removed from the bulk material before the test. With harder materials, like aggregates, an impact test machine can be used that employs a 14-kg hammer which drops from a height of 380 mm, a specified quantity of the sample in a cup. The amount of fines produced by the impact is an indication of shatter resistance. Friability of tablets or granules is tested quite commonly by vibration in a container or on a sieve. A similar procedure also has been used in testing finer materials like catalysts, bone char, or fertilizers, but no standard exists.

Shear cells, as used in testing yield strength of solids, may also be used for testing friability. Since a particle large strain is required in order to produce significant attrition, the annular shear cells, which permit infinite strain, are normally used. Tumbler tests, and, more specifically, drum tests, have probably been the most popular. They involve rotation within a drum with internal flights and analysis of the material for particle size distribution. Friability indices can be derived in terms of percentages of materials retained on specific screens after given numbers of rotations.

3.5. RECONSTITUTION PROPERTIES

Many powdered products produced by spray drying or grinding are difficult to rehydrate. In the context of food drying, reconstitutability is the term used to describe the rate at which dried foods pick up and absorb water, reverting to a condition which resembles the undried material when put in contact with an excessive amount of this liquid (Masters, 1976). Especially in rehydration operations, when water aided by capillary forces penetrates into the narrow spaces between fine particles (i.e., particle size less than 100µm), the particles will start to dissolve and form a thick, gel-like mass that resists further water penetration. Thus, lumps containing dry particles in the middle will be formed requiring strong mechanical stirring to be homogeneously dispersed or dissolved in the liquid (APV, 1989).

In the case of powdered dried biological materials, a number of properties such as wettability, sinkability, and, dispersability may influence the overall reconstitution characteristics. Food powders

82 Food Powders obtained from drying processes are normally reconstituted for consumption. The selected drying method and adjustment of drying conditions can result in a product with good rehydration properties. For example, reconstitution characteristics will not be the same if drying methods such as freeze- drying or osmotic dehydration are used.

The most efficient method to improve the rehydration characteristics of dried food powders is probably the use of agglomeration (Barletta and Barbosa-C´anovas, 1993). In order to agglomerate particles, the powder is treated with steam or warm, humid air such that condensation occurs on the particle surface. Inter-particle contact is promoted, often by swirling the wetted powder in a vortex. By agglomerating fine powders of about 100µm in size into particles with the size of several millimeters, the wetting behavior of the particles is improved and lump formation can be avoided (Schubert, 1987).

3.5.1. Instantizing Processes

The term “instant” is usually used in industries such as food, pharmaceuticals, animal feed, chemicals, and pigments to describe the dispersing and dissolving properties of powders. Some popular commercially available instant powders are milk, coffee, cocoa, baby foods, soups, sauces, soft drinks, sugar mixtures, as well as vitamins and medicated powders. Even though some powders are “naturally” instant, it is a common practice to apply a special treatment, a so-called instantizing process, to powdered materials. This treatment will provide food powders with the “instant” attribute so that they can be dissolved or dispersed more readily in aqueous liquids than when they are in their original powdered forms (Schubert, 1980).

There are two main groups of instantizing processes: agglomeration and non-agglomeration. Agglomeration processes include straight-through agglomeration (e.g., spray drying and agglomer- ation), rewetting agglomeration, spray-bed dryer agglomeration, and press agglomeration. The size enlargement of powders by agglomeration is a technique often used in a wide range of industries and it has had increased demand in recent years. Depending on its application or the area in which it is used in industry, the agglomeration process is also sometimes referred to as granulation or instantizing (APV, 1989). These methods will be covered in detail in Chapter 7. To better understand the instant properties of agglomerated food powders, it is important to have a fundamental knowl- edge of the inter-particle forces or the binding mechanisms that are involved in agglomeration. For all the particles in an agglomerated state, it is well known that the forces causing primary particles to stick together are solid bridging, liquid bridging, inter-particle attraction forces, and mechanical interlocking (Schubert, 1980).

Solid bridging forms as a result of sintering, solid diffusion, condensation, or chemical reaction.

All of these are more likely to happen at an elevated temperature, but ex-solution of soluble material can form solid bridges at room temperature. Liquid bridging results from the presence of a bulk liquid between individual particles. Once a liquid bridge is established, any evaporation of liquid reduces the curvature radii of liquid–gas interfaces and thus increases the forces holding the particles together so that they approach each other more closely. Inter-particle attraction forces can be either electrostatic or Van der Waals forces (short ranged attraction forces between solid surfaces). Electrostatic forces arise through charging by contact with charged particles or friction. Van der Waal forces arise from electron motion within an atom, which protrudes beyond the surface of a particle. Mechanical

interlocking occurs in agglomerates formed by particle interlocking and only if these are fibrous or

plate-shaped particles. These mechanisms are explained in more detail in Section 7.2 when describing particle aggregation fundamentals.

Agglomeration processes can be accomplished between two particles if they are brought together (with or without pressure) and/or at least one of them has a sticky surface. What happens in the

Figure 3.16. Microstructures of some typical agglomerated food powders observed under scanning electron microscopy:

(A) spray-dried nonfat milk; (B) spray-dried coffee; and (C) freeze-dried coffee. Bar length= 60 µm in all cases.

rehydration of an agglomerated powder is that the large passages between the primary fine particles can assist in quickly displacing the air and allowing the water to penetrate before an impenetrable layer is formed. Therefore, the powder can disperse into the liquid and have complete dissolution (APV, 1989). Microstructures of some typical agglomerated food powders observed under scanning electron microscopy (SEM) are shown in Fig. 3.16.

Satisfactory instant properties can also be achieved using non-agglomeration techniques, such as freeze drying, osmotic drying, and drum drying, adding additives like lecithin (in dried whole milk), removing certain components like fat (low fat dried milk), and applying thermal treatment to amorphous materials (APV, 1989; Pietsch, 1999; Schubert, 1980; Schubert, 1981).

Freeze-drying consists of the production of ice crystals and their sublimation at very low pres- sures (Heldman and Singh, 1981). This procedure results in food particles with an open pore structure, which absorb water easily when they are reconstituted. Another alternative is the use of the so-called combined methods, such as osmotic dehydration followed by conventional drying. In osmotic dehy- dration, food particles are immersed in a concentrated solution. By osmotic pressure, the water inside the particles tends to migrate to the solution in order to equate water activities on both sides of the cellu- lar wall (Monsalve-Gonz´alez et al., 1993). This partial dehydration will aid in the final stage of drying, and textural damage of the biological materials will be minimized. In this sense, biological materials dehydrated by combined methods will also have an open pore structure and, similar to freeze dried materials, will present good reconstitution properties. Beltran-Reyes et al. (1996) developed an apple powdered ingredient by grinding dried apples obtained by osmotic dehydration followed by conven- tional heated air drying. They determined that the firmness of the rehydrated mash, measured as an

84 Food Powders extrusion force in a texture analyzer, was a direct function of the particle size. For the same ingredient, Ortega-Rivas and Beltran-Reyes (1997) reported that rehydration improved as particle size decreased. 3.5.2. Instant Properties

Parameters that determine the properties of agglomerates include those related to primary particles and agglomerates. Among the parameters related to primary particles are particle size, size distribution, shape, and surface area. Agglomerates are related to the following parameters: particle size, size distribution and shape of the agglomerate, the apparent and bulk densities, porosity of the agglomerate, pore sizes and their distribution in the agglomerate, and the agglomerate strength