ATTRITION EVALUATION FOR SELECTED AGGLOMERATED FOOD POWDERS: THE EFFECT OF

AGGLOMERATE SIZE AND WATER ACTIVITY

HONG YAN and G.V. BARBOSA-CANOVAS'

Biological Systems Engineering Department Washington State University Pullman, WA 99164-6120

Accepted for Publication November 10, 2000

ABSTRACT

Investigation of the attrition of agglomerates is very important for assessing the agglomerate strength, compaction characteristics, and quality control. A one-ten exponential attrition index model and the Hausner ratio were used to study the ejJects of agglomerate size and water activity on the attrition kinetics of some selected agglomerated food powders. It was found that the agglomerate size and water activity played significant roles in affecting the attrition: the larger the agglomerate size and the higher the water activity, the higher the attrition index under the same tap number. The Hausner ratio was well correlated with the attrition index at high tap numbers and might be used as a simple index to evaluate attrition severity for agglomerates. Knowing the efsects of agglomerate size and water activity is very useful to minimize the attrition phenomenon during the handling and processing of agglomerated powders.

INTRODUCTION

The powder size enlargement technique by agglomeration is widely used in many industries to improve such important physical properties of powders as flowability, instant property, bulk density, appearance, and dust behavior (Hoornaert et al. 1998). However, many agglomerated powders are fragile, brittle, and tend to disintegrate upon exposure to mechanical impact among themselves or with a solid surface and vibration during handling, transportation, and consumption (Peleg and Nonnand 1987; Malave-Lopez and Peleg 1986). Thus, the unwanted particles breakdown, known as attrition, occurs in many processes where agglomerates are used or transported (Shipway and Hutchings

I To whom correspondence should be addressed. TEL: 509-3354188; Email: barbosa@ mail. WSll.edU

Journal of Food Process Engineering 24 (2001) 37-49. ,411 Rights Reserved. ~Copyrighr 2001 by Food & Nutririon Ptess, Inc., Rimbull, Connecticut. 37

38 H. YAN and G.V. BAREOSA-CANOVAS

1993). In fact, it takes place whenever there is relative movement between particles in contact with each other or the container wall (Gundepudi ef af. 1997). Industries facing this attrition problem are from the chemical, cement, agricultural, mineral, and food sectors.

Many food agglomerates (such as instant coffee and milk) are generally less hard and have lower mechanical strength than inorganic or polymeric materials, which make them more susceptible to attrition (Bemrose and Bridgwater 1987). The most remarkable consequences of attrition include the changes in the bulk properties of agglomerates (such as bulk density, flowability, and internal friction angle), reduction of the product’s instant properties (i.e., dispersibility and wettability), and segregation which causes the product’s composition un- uniformly distributed and results in functional or nutritional problems to consumers. For example, the segregation of fines to the container’s bottom is one of the most often used factors to remove instant coffee from supermarket shelves due to its adverse effect on product appearance and nutritional functionality (Barbosa-Chovas er af. 1985; Barletta er al. 1993a).

The attrition phenomenon is more widespread and not properly understood because the attrition process is a very complex one. However, it is certain that it is influenced by variables involving both the particles and environment system. The former includes particle size and shape, moisture content and porosity, particle surface condition and hardness, and composition while the latter includes fluid or gas velocity during processing, environmental temperature and pressure, and mechanical impact of friction intensity (Bemrose and Bridgwater 1987; Gundepudi ef al. 1997; Peleg and Normand 1987; Shipway and Hutchings 1993).

As described by Chraibi and Flamant (1989), the stresses causing attrition may be systematically summarized as thermal, chemical, static mechanical, or kinetic. Thermal stresses come from rapid temperature changes in particles, which cause uneven particle expansion or liquid evaporation. Chemical stresses are caused by particle phase changes or chemical reactions. External loads on particle surfaces will cause static mechanical stresses, while impacts among particles and/or with the container walls are associated with kinetic stresses.

Three main mechanisms governing the particle attrition process have been distinguished: abrasion, breakage, and the combination of the former two. i.e., chipping (Menacho 1986). In the first, a great number of very fine particles are removed from the surface layer and edges or comers of the parent particles, resulting in the production of fine particles with much smaller size than the parent particles and others of slightly smaller than the initial. Abrasion occurs when the applied energy is insufficient to cause significant particle fracture. When the applied energy is in excess of that required for fracture, particle breakage occurs, producing several mid-sized particles as compared to the parents. The third attrition mechanism is characterized by fine particle

EVALUATION FOR SELECTED FOOD POWDERS 39

production plus those that are chipped and near the parent sizes (Menacho 1986; Neil and Bridgwater 1994; Biscans ef al. 1996).

The many different types of experimental tests that have been designed to study the attrition tendency of particulate systems can be mainly divided into two groups: those dealing with single particles and those with multi-particles (Bemrose and Bridgwater 1987). Several approaches have been used to characterize the attrition process for some agglomerated food powders, especially instant coffee. These include measuring particle size distribution variations (Peleg and Normand 1987; Popplewell et al. 1988), kinetics of the attrition process (Popplewell and Peleg 1989; Barletta and Barbosa-Chnovas 1993a), compaction characteristics (Barletta ef al. 1993b), and morphology of the particles by fractal analysis (Peleg and Normand 1987; Barletta and Barbosa- Chnovas 1993b; Gerhards ef al. 1998).

Attrition curves are one of the most convenient ways to present data in attrition kinetic studies. The attrition curve shows the relationship between the percentage of agglomerates which retain their original size as a function of a certain attrition time or tap number (Barletta et al. 1993a). Several kinetic models have been used to characterize the attrition curves; these include a single exponential decay model by Austin (Austin et al. 1981), two-term exponential and nonexponential models by Malave-Mpez and Peleg (1986), and a one-term exponential form proposed by Barletta and Barbosa-Chnovas (1993a).

The objectives of this paper were to study the attrition kinetics of some typical food agglomerates affected by different agglomerate sizes and water activities by using the one term exponential attrition model proposed by Barletta and Barbosa-Chnovas (1993a) and try to correlate attrition with the widely used Hausner ratio. The one term exponential model has been shown to successfully describe the attrition extent of an agglomerated particulate system with known or theoretical initial particle size distribution. It is expressed as:

A,=Ct"

where A, is the attrition index, C and OL constants, and t the number of taps. A, is defined as the ratio of the weight fraction of fines generated in the attrition test to that of the agglomerates retaining their original particle size. The latter can be calculated by the following formula:

H. YAN and G.V. BARBOSA-CANOVAS

where Wi is the amount of material retained on each sieve and Si the normalized sieve opening size.

The Hausner ratio, which calculates powder volume changes in a graduated cylinder after a certain period of time or number of tapping, is widely used to evaluate the flow properties of particulate solids (Hayes 1987). It is defined as:

where po and V, are the loose density and volume of the material, and P, and V, the tapped density and volume, respectively.

MATERIALS AND METHODS

One brand of commercial instant coffee and instant nonfat milk were purchased in a local grocery store. Each powder was sieved by a set of RX-29 sieve screens (W.S. Tyler Inc., U.S.A.) for 4 min to obtain materials of five or six fractions with narrowed particle size ranges. As different powders had different particle size distribution ranges, two sieve sets were used to classify them: #8, 12, 16,20,30,40, and 50 for instant spray dried coffee and #20, 30, 40, 50, 70, and 100 for instant skim milk. Sieve numbers #8, 12, 16, 20, 30, 40,50,70, and 100 correspond to sieve opening sizes: 2400, 1700, 1180,850, 600,425, 300, 212, 150, and 106 pm, respectively. Coffee fractions retained on sieve #12, 16.20, and 30 and milk fractions retained on#l6,20,30, and 40 were used in the experiments because other fractions were not sufficient enough for the tests. For simplicity, agglomerates passing through a #8 sieve and retained on a #12 one were referred to as the sample with '1700' particle size, and so on.

The size classified agglomerates were conditioned to two water activities (i.e., 0.20 or O M ) , either by equilibrating on dry silica gel or saturated solution of K2C03 in evacuated glass desiccators at an ambient temperature of about 25C. The dry silica gel was used to maintain the low original water activity of the

EVALUATION FOR SELECTED FOOD POWDERS 41

powders while the saturated solution of K,CO, to produce equilibrium relative humidities (ERH) of approximately 44%. In order to allow enough time for equilibration under the different ERH conditions, samples were stored for 72 h over the dry silica gel dehydrates or 48 h over the saturated solution before testing.

Treated single particle size ranged samples from each product were confined to a graduated cylinder at a fixed height (40 mm in all experiments) and exposed to attrition on a Tap Density Tester (Vankel Industries, Edison, NJ). The selected numbers of taps were 100, 500, 1,000, 5,000, and 10.000. The “number of taps” order was set randomly to avoid any influence of environmen- tal conditions to the results and each test was run in triplicate.

After each attrition test, the total sample weight and tapped volume were recorded. For each sample with different particle size range, a corresponding sieve (ATM Corp., Milwaukee, WI) was used to gently sieve the sample to determine the weight fraction of the agglomerates that maintained their original size (i.e., with little attrition). Therefore, after the agglomerates underwent the attrition process, the fines generated by abrasion, breakage, or both passed through the sieve with their initial size. The recorded amounts of particles retained on the sieve and passed through it were used for calculating the attrition index, which was then used to calculate the constants C and a in Eq. 1 by using the least squares method for nonlinear regression (Microsoft Excel ‘97).

RESULTS AND DISCUSSION

Tables 1 and 2 show the estimated parameters C and a in Eq. 1 by nonlinear regression for both instant coffee and milk with different particle size ranges at two water activities, as well as the correlation coefficient R2. The C values may be taken as an initial attrition tendency after the agglomerates were subjected to tapping. By comparing all the C values for both materials in Tables 1 and 2, it was found that in most cases they decreased as the particle size decreased, and the higher the water activity, the higher the C values for the same material. It is also clear that different materials had different initial attrition tendencies although they had the same agglomerate size ranges and water activities. Instant coffee of 850 and 600 pm sieve opening sizes had lower initial attrition values than the same size ranged instant milk had. They also had lower overall attrition indexes than instant milk had, as listed in Tables 3 and 4. These results proved the influence of material itself on the attrition process.

42 H. YAN and G.V. BARBOSA-CANOVAS

TABLE 1.

FOR INSTANT COFFEE WITH FOUR DIFFERENT S E E RANGES AND TWO WATER ACTIVITIES

Sieve opemg s u e a, = 0.44 a, = 0.20

ESTIMATED PARAMETERS C AND u IN EQ. 1 AND CORRELATION COEFFICIENT

bm)

C U R2 C a R2

1700 3.28.10’ 0.672 0.99 l.lO*lO’ 0.789 0.99

1180 l.80*103 0.714 0.99 5.35*10‘ 0.826 0.99

850 7.50*104 0.769 0.99 6.36*104 0.726 0.99

600 1.83*10-’ 0.577 0.99 5.99*104 0.662 0.99

TABLE 2. ESTIMATED PARAMETERS C AND u IN EQ. 1 AND CORRELATION COEFFICIENT R‘

FOR INSTANT MILK WITH FOUR DIFFERENT SIZE RANGES AND TWO WATER ACTIVITIES

Sieve opemg s u e a, = 0.44 a, = 0.20

C U RZ C a R2

850 1.56*10’ 0.783 0.99 1.49*10’ 0.760 0.99

600 1.38*10’ 0.755 0.99 1.26*10-’ 0.722 0.99

425 1.31*10” 0.675 0.99 2.02*101 0.553 0.99

300 J.18*10’ 0.581 0.99 l.99*103 0.456 0.99

TABLE 3. TYPICAL VALUES FOR THE ATTRITION INDEX A, AND HAUSNER RATIO AFTER 1O.OOO TAPS FOR INSTANT COFFEE WITH DIFFERENT SIZE RANGES AT TWO

WATER ACTIVITIES a. = 0.44 a,& = 0.20

Sieve opening Amition Hausner ratio Amition Hausner size (pm) index A, index A, ratio

1700 2.236 1.818 1.586 1.818

1180 1.518 1.818 1.076 1.739

850 0.869 1.739 0.578 1.667

600 0.367 1.600 0.267 1.519

EVALUATION FOR SELECTED FOOD POWDERS 43

TABLE 4. TYPICAL VALUES FOR THE ATTRITION INDEX A, AND HAUSNER RATIO AFTER

10,OOO TAPS FOR INSTANT MILK WITH DIFFERENT SIZE RANGES AT TWO WATER ACTIVITIES

a, = 0.44 aW = 0.20 Sieve opemg Attrition Hausner Attrition Hausner

size (pm) index A, ratio index A, d o

850 2.116 1.818 1.640 1.818

600 1.440 1.739 0.976 1.667

425 0.660 1.622 0.329 1.429

300 0.249 1.481 0.133 1.412

Equation 1 can be rewritten to have its linear form by taking logarithms on both sides. It becomes as:

Log A, = Log c + ar Log n (4)

It is a straight line with slope of ct and intercept of Log C. The relationship between the tapping number and attrition index in logarithms for instant coffee and milk at two water activities are shown in Fig. 1 and 2. The attrition index in logarithm scale is proportional to the tapping numbers in all cases. As assumed before, the agglomerate size is an important factor related to the attrition process: at the same tapping conditions, the larger the size, the bigger the attrition index for both coffee and milk. For instant coffee, the attrition index for agglomerates with ‘1700’ sieve opening size was 2.236, while for the ‘600’ ones, the attrition index was 0.376 after they both underwent 10,OOO taps at the same water activity a, = 0.44. The particle size effect on attrition can be explained by the larger agglomerates having more edges or comers on their surface and more deficient crystal bonds inside than the smaller ones, which results in more abrasion, breakage, and chipping.

As shown in Fig. 1 and 2, the attrition process was also affected by water activity for all of these two materials. The attrition severity was always less at a, = 0.20 than that at a, = 0.44, as indicated by the dashed lines and solid lines in these figures. It was found that the lower the activity, the smaller the attrition index for materials with the same agglomerate size at the same tapping numbers. This may be explained by the mechanisms for agglomeration. For all the particles in an agglomerated state, it is well known that forces causing primary particles to stick together are: solid bridging, liquid bridging, inter-

44 H. YAN and G.V. BARBOSA-CANOVAS

particle attraction forces, and interlocking (Schubert 1981). The solid bridging force is dominant for materials in agglomerated state, and is mainly due to the crystallization of dissolved materials (lactose for instant milk and coffee extract for instant coffee, for example). As described by Moreyra and Peleg (1981), powders will generally have two kinds of changes when they are exposed to moist atmosphere: (1) the solid material softens and becomes plasticized, and (2) the particle surface is altered by solubilization, liquid layer formation, or local plasticization. At higher water activity, the added water molecules apparently penetrate the outer layers of the agglomerates, weakening the solid bonds between the fine particles and thus resulting in the increased attrition tendency for agglomerates at higher water activities.

1

0.1

0.01 I 100 1000 1Oooo

Tapping Number

FIG. 1. A’ITRITION INDEX, TAPPING NUMBER RELATIONSHIP. AND CURVE FITTING BY EQ. 4 FOR INSTANT COFFEE AT TWO WATER ACTIVITIES:

SOLID LINE a, = 0.44, DASHED LINE & = 0.20

EVALUATION FOR SELECTED FOOD POWDERS 4s

10

1 3 3 d B c(

L: 0 :z 4 0.1

0.01

r 0 300

100 1000 10000

Tapping Number

FIG. 2. ATTRITION INDEX, TAPPING NUMBER RELATIONSHIP, AND CURVE FI'ITING BY EQ. 4 FOR INSTANT MILK AT TWO WATER ACTIVITIES:

SOLID LINE: a,, = 0.44, DASHED LINE a,, = 0.20

Remarkable bulk density changes were noticed for all the tapping tests, especially at high tapping numbers. Under low tapping numbers, the Hausner ratio changes is mainly caused by particle rearrangement or reorientation in the test cylinder, therefore, it is not reasonable to correlate them with attrition seventy. Typical attrition index values and Hausner ratios of instant coffee and milk at two water activities after 10,OOO taps are listed in Tables 3 and 4. The Hausner ratio was found very useful for evaluating the agglomerate attrition tendency by correlating its value changes with the attrition index. It could be noticed that the larger agglomerates had larger attrition index values and high Hausner ratios meant that they suffered bigger density changes for both materials.

46 H. YAN and G.V. BARBOSA-CANOVAS

The relationship between the tapping number and Hausner ratio for instant coffee and milk with different size ranges at two water activities are shown in Fig. 3 and 4. The effect of particle size and water activity was not significant (P < 0.95) at low tap numbers (i.e., 100,500, and 1,OOO). After 5,000 taps the Hausner ratios went up and this was mainly due to the attrition that took place in the form of abrasion, breakage, or chipping of the agglomerates. The smaller particles passed through the powder bed, promoting the reduction of the bed volume with the consequent Hausner ratio increase. It was also found that the larger the agglomerates the higher the Hausner ratios, and this relationship followed the same trend as the attrition index. As shown in Fig. 3 and 4, for low tap numbers, differences in the Hausner ratios were not significant (P < 0.95), i.e., differences in water activity or agglomerate size did not play such a relevant role as for high tap numbers. These findings could be related to initial packing irregularity.

1.9 i 1.8

1.7

I .6 X .g 1.5 2 L.

d 1.4 1.3

1.2

1.1

_ _ - - -

-m- I 180

-A- 850

1 4 0 2000 4000 6000 8000 10000 12000

Tapping Number

FIG. 3. RELATIONSHIP BETWEEN HAUSNER RATIO AND TAPPING NUMBER FOR INSTANT COFFEE WITH DIFFERENT SIZE RANGES AT TWO WATER ACTIVITIES:

SOLID LINE a, = 0.44. DASHED LINE a, = 0.20

EVALUATION FOR SELECTED FOOD POWDERS 47

X

1 9

1 8

1 7

1 6

I 5

1 4

1 3

1 2

1 1

1 J 1

0 2000 4000 6000 8000 10000 I2000

Tapping Number

FIG. 4. RELATIONSHIP BETWEEN HAUSNER RATIO AND TAPPING NUMBER FOR INSTANT MILK WITH DIFFERENT SIZE RANGES AT TWO WATER ACTIVITIES:

SOLID LINE: a,, = 0.44, DASHED LINE a,, = 0.20

CONCLUSIONS

The attrition kinetics for both instant coffee and milk were greatly affected by agglomerate size and water activity in such a way that the larger the particle and the higher the water activity the severer attrition they suffered. The one- term exponential model proposed by Barletta and Barbosa-Chovas was very successful for the attrition kinetic studies for the selected agglomerated food powders. It was very helpful to evaluate the effects of agglomerate size and water activity on the attrition tendency for selected food agglomerates used in our experiments. The Hausner ratio had good correlation with the attrition index at high tapping numbers and might be used as a simple index to evaluate attrition

48 H. YAN and G.V. BARBOSA-CANOVAS

severity: the higher the Hausner ratio, the higher the attrition suffered by the agglomerates.

It was found that under the same attrition conditions, the attrition index was larger for the agglomerates of larger size and higher water activity and different kinds of agglomerates (i.e., coffee or milk) had different attrition tendency. These results may serve as useful guidelines for selecting product formulation, processing, transportation, and storage conditions for agglomerated foods where attrition effects are concerned. The attrition of agglomerated food powders can beminimized by properly controlling the particle size, the water activity of products, and the environment relative humidity.

REFERENCES

AUSTIN, L.G., BAGGA, P. and CETIC, M. 1981. Breakage properties of some materials in a laboratory. Powder Tech. 28, 235-239.

BARBOSA-CANOVAS, G.V., MALAVE-LdPEZ, J. and PELEG, M. 1985. Segregation in food powders. Biotech. ProgrJ(2). 140-146.

BARLETTA, B.J. and BARBOSA-CANOVAS, G.V. 1993a. An attrition index to assess fines formation and particle size reduction in tapped agglomerated powders. Powder Tech. 77, 89-93.

BARLETTA, B.J. and BARBOSA-CANOVAS, G.V. 1993b. Fractal analysis to characterize ruggedness changes in tapped agglomerated food powders. J. Food Sci. 58(5), 1030-1035, 1046.

BARLElTA, B.J., KNIGHT, K.M. and BARBOSA-CANOVAS, G.V. 1993a. Review: Attrition in agglomerated coffee. Rev. Esp. Cienc. Tecnol. Aliment 33(1), 43-58.

BARLE'ITA, B.J., KNIGHT, K.M. and BARBOSA-CANOVAS, G.V. 1993b. Compaction characteristics of agglomerated coffee during tapping. J . Texture Studies 24, 253-268.

BEMROSE, C.R. and BRIDGWATER, J. 1987. A review of attrition and attrition test methods. Powder Tech. 49, 97-126.

BISCANS, B., CHEMINI, R., GUIRAUD, P. and LAGUERIE, C. 1996. Design of an attrition experiment to simulate the effects of crystal-wall or crystal-stirrer impacts occurring in a crystallizer. Powder Tech. 86,

CHRAIBI, M.A. and FLAMANT, G. 1989. Kinetic, thermal and chemical attrition of Manganese Chloride particles in a fluidized bed. Powder Tech.

GERHARDS, C.H., ULBRICHT, D.M. and PELEG, M. 1998. Mechanical characterization of individual instant coffee agglomerates. J. Food Sci.

155-161.

59, 97-107.

63(1), 140-142.

EVALUATION FOR SELECTED FOOD POWDERS 49

GUNDEPUDI, M.K., SANKAR, B.V., MECHOLSKY JR., J .J . and CLUPPER, D.C. 1997. Stress analysis of brittle spheres under multiaxial loading. Powder Tech. 94, 153-161.

HAYES, G.D. 1987. Food Engineering Duru Handbook, pp. 83, John Wiley & Sons, New York.

HOORNAERT, F., WAUTERS, P.A.L., MEESTERS, G.M.H., PRATSINIS, S.E. and SCARLETT, B. 1998. Agglomeration behavior of powders in Lodige mixer granulor. Powder Tech. 96, 116-128.

MALAVE-L6PEZ, J. and PELEG, M. 1986. Mechanical attrition rate measurement in agglomerated instant coffee. J. Food Sci. 52(3), 687-690, 697.

MENACHO, J.M. 1986. Some solution for the kinetics of combined fracture and abrasion breakage. Powder Tech. 49, 87-96.

MOREYRA, R. and PELEG, M. 1981. Effect of equilibrium water activity on the bulk properties of selected food powders. J. Food Sci. 46, 1918-1922.

NEIL, A.N. and BRIDGWATER, J . 1994. Attrition of particulate solids under shear. Powder Tech. 80, 207-219.

PELEG, M. and NORMAND, M.D. 1987. Computer simulation of the attrition patterns of particulated and agglomerated foods. J. Food Sci. 52(4), 943-947.

POPPLEWELL, L.M., CAMPANELLA, O.H. and PELEG, M. 1988. Quantitative characterization of the particle size distribution of instant coffee during mechanical attrition. J. Food Sci. 53(3), 877-881.

POPPLEWELL, L.M. and PELEG, M. 1989. An ‘Erosion index’ to character- ize fines production in size reduction processes. Powder Tech. 58,

SCHUBERT, H. 1981. Principles of agglomeration. Intern. Chem. Eng. 21(3),

SHIPWAY, P.H. and HUTCHINGS, L.M. 1993. Attrition of brittle spheres by fracture under compression and impact loading. Powder Tech. 76, 23-30.

145- 148.

363-377.