effect of agglomeration on the course of isotherms of

CHEMICAL AND PROCESS ENGINEERING 29, 99–112 (2008)

EWA DOMIAN, MONIKA JANOWICZ, ANDRZEJ LENART

EFFECT OF AGGLOMERATION ON THE COURSE OF ISOTHERMS OF WATER VAPOUR SORPTION BY SELECTED FOOD POWDERS Warsaw University of Agriculture, Faculty of Food Technology, Department of Food Engineering and Production Management, ul. Nowoursynowska 159 C, 02-787 Warsaw The effect of wet agglomeration of selected food powders has been analysed in terms of its influence on the isotherms of adsorption and desorption of water vapour. General physical properties of the examined materials have been investigated in the form of powder and agglomerates as well as their sorption capacities based on the course of isotherms of adsorption and desorption of water vapour. Zbadano wpływu aglomeracji nawilżeniowej na izotermy adsorpcji i desorpcji pary wodnej wybranych proszków spożywczych. Analizowano ogólne właściwości fizyczne badanych materiałów w postaci proszku i aglomeratów oraz ich właściwości sorpcyjne na podstawie przebiegu izoterm adsorpcji i desorpcji pary wodnej.

1. INTRODUCTION The production of foodstuffs and food additives in the form of powder involves such technological processes as disintegration, drying, crystallization and precipitation, agglomeration and granulation, microencapsulation and coating as well as mixing powdered components [16, 38]. Food powders may by amorphous (e.g., milk, some whey powders, instant coffee, instant tea, protein preparations), crystalline (e.g., refined sugar, organic acids, salts), or mixed (e.g., some whey powders, starches, icing sugar). Physical state and structure of food powders are determined by the type and conditions of their processing as well as by the presence of other components, mainly water, in the system [40]. Highmolecular substances (starches and proteins) form amorphous structures, whereas salts form only crystalline powders. Low-molecular carbohydrates, including lactose, glucose and saccharose, as well as organic acids and polyols may occur both in crystalline and amorphous states [20, 34, 41].

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The physical state of a powder affects its functional properties. Stability of the crystalline form exceeds that of the amorphous state. Striving for equilibrium, metastable amorphous products may be subject to structural relaxation or crystallization under conditions of increased humidity and/or temperature [2, 9, 14, 53]. The composition, type and number of amorphous components, temperature and air relative humidity as well as time have a significant impact on water vapour adsorption capacity of a material, its hygroscopicity. In such systems, water sorption is a complex process and may require the knowledge of kinetics as well as sorption equilibrium [17, 40, 53]. The character of links between water and food and its components determines the availability of water and, thus, water activity of a product. In food material, water is retained by various physical and chemical mechanisms. The shape of a water vapour sorption isotherm, depicting the dependence between equilibrium water content and water activity at constant temperature and pressure, reflects the mechanism of water binding in the material [12, 25, 26]. Practical application of sorption isotherms in food technology includes predicting the shelf life of foodstuffs. The content of water corresponding to the capacity of a monomolecular layer is the minimal and, simultaneously, the optimal moisture content of a product. Excess of water in respect of the monomolecular layer leads to reaching critical moisture content, exceeding of which evokes various undesirable changes in the product that deteriorate its quality and enable the growth of microorganisms. Water activity of a food product determines its physical properties, affects the course of chemical reactions, activity of an enzymatic complex as well as microbiological stability [36, 46]. Dusting, caking, difficulties in reconstitution in water, flow inhibition at dosage are typical problems with powders characterized by particle size of up to 100 μm [18, 49]. Those problems may be solved by means of agglomeration consisting in combining small particles into larger structures [44, 45]. It results in homogeneity of product composition, improvement of flowability and an increase in the rate of reconstitution in liquid through an increase of its wettability and dispersibility. Combining small particles into larger, more stable agglomerates, through surface wetting and drying, facilitates the formation of crystalline bridges in the case of thermoplastic food powders. The formation of stable bridges as a result of crystallization of amorphous saccharides is a typical process [15, 16]. It seems to be interesting, therefore, to investigate sorption capacity of amorphous food powders with attention paid to the effect of agglomeration evoking changes in the structure and size of their particles. A study was undertaken to analyze the effect of wet agglomeration on the course of isotherms of water vapour adsorption and desorption by selected food powders. The scope of the research included an analysis of general physical properties of the examined materials in the form of powder and agglomerates as well as their sorption capacities based on the course of isotherms of water vapour adsorption and desorption.

Isotherms of water vapour sorption

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2. MATERIALS AND METHODS Experimental raw materials included: skimmed milk powder (OSM Radomsko), granulated skimmed milk powder (SM Gostyń), whey powder (Lacma), and powdered soybean protein isolate SUPRO 670 (the Solae Company. Experimental materials were the following products: • skimmed milk powder (SMP), • skimmed milk in the form of agglomerate, a product of agglomeration through wetting of milk powder in a fluidized bed (SMPf), (laboratory agglomerator STREA 1), • skimmed milk in the form of agglomerate, a product of agglomeration through wetting of milk powder with mechanical stirring (SMPm), (laboratory agglomerator LÖDIGE MIXER, • skimmed milk in the form of agglomerate, a product of agglomeration as a result of spray-drying of milk in a two-stage drying installation (SMPi), (industrial agglomerator, • mixture of whey and soybean protein isolate (66 : 34) in the form of powder (M), • mixture of whey and soybean protein isolate (66 : 34) in the form of agglomerate, a product of agglomeration through wetting of the mixture in a fluidized bed (Mf), (laboratory agglomerator STREA 1, • mixture of whey and soybean protein isolate (66 : 34) in the form of agglomerate, a product of agglomeration though wetting of the mixture with mechanical stirring (Mm), (laboratory agglomerator PLUGHSHARE MIXER, Milk in the form of powder (SMP) and agglomerates (SMPf, SMPm, SMPi) as well as a mixture of whey and soybean protein in the form of powder (M) and agglomerates (Mf, Mm) were characterized by a similar content of basic components: lactose 52%, protein 35%, ash 8%, fat 1% and water >300 15±4 12±2 14±3 >>300 7±1 1±1

For non-agglomerated powders the mean diameters d50 differ between 53 and 56 μm. Agglomeration in a fluidized bed increased the characteristic dimensions of the particles 7–8 times on average, agglomeration through mechanical stirring – 11 and 17 times, whereas agglomeration of milk powder during spray drying – 27 times. Once particle diameter exceeds 200 μm, powders may be characterized by good flowability, whereas fine-grained powders are claimed cohesive ones and their dosage is more difficult [6, 10, 37]. Densities of particles ρ of the analyzed powders before and after agglomeration were observed to reach similar values, yet the powders agglomerated through mechanical stirring were characterized by a few per cent higher density, which implies the existence of a packed structure of those granules. Bulk densities of powders, granulates or other loose materials depend on particle packaging in a bed [15, 45]. Tapped bulk densities ρT of the non-agglomerated powders reached 616 and 538 kg/m3, respectively. Agglomeration of the powders, irrespective of the method, substantially decreased their bulk densities to range from 278 to 477 kg/m3 (Table 1). The significantly lower bulk densities of the agglomerates obtained in a fluidized bed may be explained by higher porosities of their particles [15, 44]. Bulk density and particle density are linked with porosity of a bed of a loose material that includes outer intergranular porosity, i.e. a system of empty spaces between single particles, and inner porosity, i.e. a network of open pores and capillaries within single particles [44]. Fine-grained cohesive loose materials in the form of powders are characterized by high porosities, yet it refers only to beds of loosely poured powder as a result of the formation of large free intermolecular spaces [49]. Porosities of a packed bed εT of the examined powders achieved values ranging from 0.38 to 0.46. Agglomeration in a fluidized bed yielded a 46–92% increase of porosity εT, whereas

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that in the mechanically-generated bed resulted in a 32–67% increase of that parameter (Table 1). The IS parameter of a powder characterizes flowability of a material under dynamic conditions similar to those of pouring and mechanical transport of loose materials. According to NIRO criterion, pouring time of powder IS up to 20 s enables classifying the loose material as free flowing, whereas that over 60 s determines the material as sparingly flowing [18, 48]. The examined materials in the form of powders were poorly loose, whereas those in the form of agglomerates (irrespective of agglomeration method) demonstrated very good flowability according to the IS parameter (Table 1). The parameter characterizing reconstitution properties is wettability Z described as an index of powder reconstitution rate in a liquid [14, 15]. Wettabilities of the nonagglomerated powders were low, since a portion of powder was maintaining on the surface of liquid for a period longer than 3 min (Table 1). Those powders were subject to dispersion in water already during intensive stirring. As a result of agglomeration, the powders displayed wettabilities enabling instant solubility (wettability time < 15 s). According to Ziajka [39], the quality of agglomerated milk powder is affected to a significant extent by conditions of technological processing before drying as well as by such factors as wetting degree, time and temperature of agglomeration, and redrying. In properly agglomerated powder, the content of crystallized α-lactose should exceed 50% of the total content of that carbohydrate. Amorphous lactose is characterized by more rapid solubility, a result of which in the wettability in water is the formation of a supersaturated solution with high viscosity at the interface. In the case of crystalline lactose, the formation of such a solution proceeds slowlier, what is reflected in improved wettability. The degree of crystallization of α-lactose increases along with a wetting degree and time which powder spends in the agglomeration zone. The wetting of powder to a water content of 17–18% has a positive effect on wettability, but simultaneously affects negatively solubility and dispersibility. The most favourable parameters of those characteristics are obtained when the moisture content of powders reaches 7–10%. Extension of the agglomeration time results in obtaining larger particles of powder and a higher content of α-lactose as compared to that of β-lactose, yet it simultaneously decreases dispersibility and deteriorates solubility of powders. Particle size and physical properties of milk powder are also determined by agglomeration temperature. An increase of temperature to 55 ºC results in a decrease dispersibility and solubility. Temperature of the final re-drying of powder should not exceed 55–60 ºC [16, 39]. 3.2. ISOTHERMS OF WATER VAPOUR SORPTION

Isotherms of water vapour adsorption and desorption for milk powder (SMP) were characterized by a similar course to isotherms plotted for agglomerated milk (SMPf, SMPm, SMPi) (Fig. 1). Simultaneously, for each type of milk the desorption isotherm had a different course as compared to the isotherm of water vapour adsorption. Usu-


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ally in food, at a given water activity, the content of water is higher during desorption than during adsorption, yet the size and shape of hysteresis is likely to differ considerably as affected by the type of food, temperature, rate and degree of dehydration [11, 13, 31, 47]. The observed occurrence of a hysteresis loop, with higher water contents during adsorption than desorption at water activity range from 0.5 to 0.8, in the case of milk powder and its agglomerates should be explained by changes in the crystalline form of lactose as a result of water adsorption.

Fig. 1. Isotherms of water vapour adsorption and desorption for skimmed milk powder (SMP) and its agglomerates (SMPf, SMPm, SMPi)

The above explanation is confirmed by the results of investigation of water vapour re-adsorption and re-desorption for samples of milk powder and agglomerated milk previously subject to the process of adsorption and desorption (Fig. 2). In the second cycle, isotherms of re-adsorption were characterized by smooth, sigmoidal course, without any inflexions at a water activity range of 0.5–0.8 as well as by lower values of water content in the entire water activity range as compared to re-desorption. Water adsorbed in the first cycle, bound in a multi-layer mode according to the BET theory, led to an increase in mobility of lactose particles contained in milk powder and enabled the transition of that saccharide from a metastable amorphous state to a crystalline state [5, 12, 19]. The above observations are consistent with findings of other authors who investigated dried dairy products [8, 24, 32]. The phenomenon of lactose crystallization as a result of water vapour adsorption and water release, corresponded with the interrupt of the course of adsorption isotherm [1, 7, 22, 33]. Water activity at which isotherm course was interrupted reached values ranging from 0.4 to 0.6 and depended on temperature, type of powder, its composition or processing technology

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[19, 31]. Lactose crystallization in dairy powders accompanied by water vapour sorption is a complex process that is determined by a number of factors, including: moisture content, temperature, time, pH, mutarotation rate, and presence of other components, especially of salt, protein and fat [42, 43]. According to Linko et al. [32], during water vapour adsorption in milk powder at aw < 0.2, casein seems to be the main adsorber, whereas at aw between 0.2 and 0.6 adsorption is predominated by the transformation of the physical state of lactose and at aw > 0.6 adsorption is affected by salts.

Fig. 2. Isotherms of water vapour adsorption and desorption and isotherms of re-adsorption and re-desorption for: a) skimmed milk powder (SMP), b) agglomerated milk (SMPm)

Agglomeration changes the sorption capacity of a mixture of lactose powders and soybean protein isolate. Isotherms of water vapour adsorption for the mixture and in the form of powder (M) and agglomerates (Mf, Mm) had a course consistent with that of type II BET isotherms with a sigmoidal shape typical of protein-carbohydrate products (Fig. 3) [21, 28, 35, 50]. A lack of inflexions and interruptions as well as a flat course of the adsorption isotherm for the non-agglomerated mixture M may indicate both a crystalline form of lactose in whey and interactions between mixture components during sorption [4, 27, 52]. The content of water was higher during water desorption than during adsorption in the case of mixture both in the form of powder (M) and agglomerates (Mf, Mm). Subjecting the mixture (M) to the agglomeration process in the fluidized bed and through mechanical stirring resulted in a remarkable decrease in the course of adsorption and desorption isotherms. Also a decrease of the hysteresis loop was found, which is likely to indicate irreversible structural changes in the material as a result of processing by means of wet agglomeration [3, 25, 31].


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The modified properties of the adsorbent, in the aspect of agglomeration, were attempted to be elucidated by a theoretical analysis of water sorption isotherms through mathematical description of experimental isotherms with BET and GAB equations.

Fig. 3. Isotherms of water vapour adsorption and desorption for a mixture of whey and soybean protein isolate in the form of powder (M) and agglomerates (Mf, Mm)

There are a vast number of proposals of the mathematical description of sorption isotherms [3, 13, 25, 29]. The model of multi-layer adsorption of vapours by Brunauer, Emmett and Teller (BET) is most commonly applied in practice and enables calculating the content of water bound in the monomolecular layer at the assumption of homogeneity of adsorption surface and a lack of interaction between adsorbed water molecules. It is useful in the description of some types of isotherms in a restricted range of water activity. An equation of Guggenheim, Andersen and De Boer (GAB) describes isotherms in the whole range of water activity and, additionally, enables calculating water content in the monolayer. However, the GAB equation may be used only when its constants k and C are contained in the flowing ranges: 0.24 < k ≤ 1 and 5.67 < C ≤ ∞ [30]. The description of experimental isotherms of water vapour adsorption and desorption for the examined powders and their agglomerates required introducing restrictions referring to the water activity range. The BET equation was used to describe adsorption isotherms at aw range of 0.05–0.3 for all materials examined; whereas the GAB equation was applied in the whole aw range, i.e. 0.05–0.9, to describe experimental adsorption data for a mixture and its agglomerates as well as experimental desorption data for milk and a mixture in the form of powder and agglomerates. Once using the GAB equation for the description of adsorption of milk powder and its agglomerates, calculations were performed at aw range of 0.05–0.5. A similar step-wise description of adsorption isotherms for dairy powders may been found in literature [3, 30, 32].

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Table 2. Water content um and constants of BET and GAB equations for the examined food powders and their agglomerates Adsorption Material

BET equation um

SMP SMPf SMPm SMPi M Mf Mm

Desorption

C

GAB equation 2

r

um

3.13* 5.6706 0.9890 3.27* 2.87* 5.6710 0.9793 3.15* 3.26* 5.6701 0.9799 3.39* 2.31 6.1356 0.9939 3.17 4.19 17.0358 0.9935 5.54 2.78 5.6702 0.9701 3.23 3.31 6.9324 0.9961 3.12

C

k

5.6710 5.6710 5.6702 5.6727 8.1808 5.6764 9.0771

0.9538 0.9064 0.9613 0.7044 0.8621 0.8917 0.8994

r2

um

0.9917 5.26* 0.9764 5.40* 0.9849 5.34* 0.9982 5.15 0.9987 7.75 0.9975 3.03 0.9953 3.17

C

k

r2

14.1877 6.5878 8.6085 10.9480 27.7880 23.5979 26.3104

0.8670 0.8625 0.9691 0.9733 0.8066 0.9445 0.9423

0.9711 0.9718 0.9691 0.9733 0.9855 0.9997 0.9999

*

Values differing insignificantly between one another in the groups at an assumed significance level of p = 0.05.

Adopting the above assumptions and considering conditions provided by Lewicki [30], the coefficient of correlation obtained at the description of isotherms with BET and GAB equations ranged from 0.95 to 0.99.

4. CONCLUSIONS Water content um of the monomolecular layer calculated for all materials examined with the use of BET and GAB equations is presented in Table 2. Depending on the type of material and equation applied, the um values ranged from 2.31 to 7.75 (g water/100g d.m.). The values of water content of the monolayer um were significantly lower during adsorption than during desorption as a result of the occurrence of sorption hysteresis. During adsorption and desorption, the values of um accounted for 2.31–3.38 (g water/100g d.m.) and 5.15–5.40 (g water/100g d.m.) in the case of milk powders SMP and its agglomerates, as well as for 2.78–5.54 (g water/100g d.m.) and 3.03–7.75 (g water/100g d.m.) in the case of a mixture M in the form of powder and its agglomerates. The calculated water content corresponding to the monomolecular capacity is minimal and, simultaneously, the optimal moisture content in respect of product stability. Standard spray-dried skimmed milk powder should contain less than 4% of water. The water content recommended for milk powder protection against crystallization at a room temperature reaches 5.7% [39]. Water content exceeding 5% facilitates the induction of multiple unfavourable changes in physical properties and stimulates reactions between components. It is when lactose links with proteins in Maillard’s reactions which lead to product’s browning, its diminished solubility and wettability, which in turn decreases its nutritive value and evokes losses of exogenous amino acids, mainly of lysine. The content of water in the monomolecular layer de-


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termined for some dairy powders in other investigations was reported to range from 4 to 5.5 (g water/100g d.m.) depending on the conditions and parameters of the water vapour sorption process, preparation of the material, its composition and equations applied for description of isotherms. Jouppila and Ross [22], while determining water content of the monomolecular layer for skimmed milk powders at relative humidity of environment ranging from 11.5 to 44.4%, obtained 5.47 g water/100g d.m. based on BET equation and 5.10 g water/100g d.m. based on GAB equation, respectively. It was also found that water content of monomolecular layer determined based on desorption isotherms of milk powder with various contents of fat was changing along with a change in temperature, i.e. from 6.16 to 6.89 g water/100g d.m. [22, 31, 33]. The wetting of milk powder during agglomeration both in the fluidized bed and by mechanical stirring had no significant impact on the capacity of monolayer um. The values of um for agglomerates SMPf and SMPm as well as for SMP milk powder, irrespective of the equation applied were found to be statistically significantly different (Table 2). Out of powdered milk agglomerates examined, the lowest, statistically different capacity of monolayer during adsorption was observed for SPMi agglomerate obtained during spray drying. Most probably, it is linked with an increased moisture content of milk powder during agglomeration in a dryer chamber that facilitates lactose crystallization. The mixture M prepared on the basis of whey and soybean protein isolate was characterized by a significantly higher capacity of monolayer as compared to the SMP milk powder which was most likely due to a higher capacity for water adsorption by soybean protein than by milk proteins, mainly casein [13, 23]. Simultaneously, that type of material was demonstrated to be significantly affected by the agglomeration process carried out by means of wetting both in the fluidized bed and by mechanical stirring. Agglomeration, especially under conditions of mechanical stirring, had a significant effect on a decrease in water content of the monomolecular layer. For agglomerates Mf and Mm and a powdered mixture M, values of um were statistically significantly different, irrespective of the equation model used for its determination (Table 2). Due to a significant change in the structure of milk powder as a result of agglomeration, that method of processing was expected to exert a distinct influence on a change in the sorption capacity of the material examined. • Wet agglomeration of skimmed milk powder and a mixture of whey powders and soybean protein isolate carried out under conditions of fluidized bed or mechanical stirring enables obtaining a product with altered structure, instant reconstitution in water and very good flowability. • The effect of changes in the sorption capacity of food powders examined as a result of agglomeration depended on the type of material and the applied method of agglomeration. • The effect of agglomeration towards a significant decrease of water vapour adsorption capacity was observed in the case of milk agglomerated under conditions of

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spray drying and in the case of a mixture of whey powders and soybean protein agglomerated both in a fluidized bed and by means of mechanical stirring. • The course of isotherms of water vapour adsorption and desorption by milk in the form of powder and agglomerates is affected by water activity, which is linked with crystallization of lactose dependent on water availability. SYMBOLS d50 – ρT – ρ – εT – IS – Z – u – um – aw – k, C –

mean diameter of particles, μm tapped bulk density, kg/m3 particles density, kg/m3 tapped porosity, flowability as pouring time, s wettability, s water content of the feed material (g water/100g d.m.), water content of the monomolecular layer (g water/100g d.m.), water activity constants REFERENCES

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steczkowe tworzą tylko amorficzną strukturę. Sole formują tylko krystaliczne proszki. Niskocząsteczkowe węglowodany, takie jak laktoza, glukoza i sacharoza oraz kwasy organiczne i poliole mogą być zarówno w stanie krystalicznym, jak i amorficznym. W warunkach szybkiego suszenia żywności cząsteczki zestalają się w postaci amorficznej lub w mieszanej – krystaliczno-amorficznej. Łączenie drobnych cząstek w większe stabilne aglomeraty przez powierzchniowe nawilżenie i suszenie sprzyja tworzeniu krystalicznych mostków w przypadku termoplastycznych proszków spożywczych. Przejściu ze stanu amorficznego do stanu krystalicznego przez adsorpcję pary wodnej towarzyszy zjawisko uwalniania nadmiaru wody przez powstające struktury krystaliczne. Sorpcja wody w takich układach jest skomplikowana i może wymagać znajomości zarówno kinetyki, jak i równowagi sorpcyjnej. Skład, rodzaj i liczba amorficznych składników, temperatura i wilgotność względna powietrza oraz czas mają istotny wpływ na zdolność adsorpcji pary wodnej materiału, jego higroskopijność. Celem pracy była analiza wpływu aglomeracji nawilżeniowej na przebieg izoterm adsorpcji i desorpcji pary wodnej wybranych proszków spożywczych. Zakres pracy obejmował analizę ogólnych właściwości fizycznych badanych materiałów w formie proszku i aglomeratów oraz ich właściwości sorpcyjnych na podstawie przebiegu izoterm adsorpcji i desorpcji pary wodnej. Materiał badawczy stanowiły: odtłuszczone mleko oraz mieszanina serwatki i białka sojowego w postaci proszku i aglomeratów. Aglomerację prowadzono przez pneumatyczne generowanie złoża fluidalnego w aglomeratorze STREA 1, Niro-Aeromatic AG oraz mechanicznie generowane złoża fluidalnego w laboratoryjnym mieszalniku lemieszowo-płużącym PLUGHSHARE MIXER L5, Lödige. Analiza ogólnych właściwości fizycznych badanych materiałów obejmowała wyznaczenie wielkości cząstek, gęstości nasypowej, porowatości, sypkości i zwilżalności w wodzie. Do pomiarów sorpcji pary wodnej w celu wyznaczenia izoterm adsorpcji i desorpcji użyto automatycznego urządzenia Hydrosorb 1000, Quantachrome Instruments. Zmodyfikowane właściwości badanych proszków w aspekcie aglomeracji próbowano wyjaśnić teoretyczną analizą izoterm sorpcji wody, poprzez matematyczny opis eksperymentalnych izoterm równaniami BET i GAB. Aglomeracja nawilżeniowa odtłuszczonego mleka w proszku oraz mieszaniny proszków serwatki i izolatu białka sojowego prowadzona zarówno w warunkach suszenia rozpyłowego, jak i złoża fluidalnego czy mieszania mechanicznego, umożliwia otrzymanie produktu o zmienionej strukturze. Efekt zmian właściwości sorpcyjnych badanych proszków spożywczych na skutek aglomeracji zależał od rodzaju materiału i zastosowanej metody aglomeracji. Istotne zmniejszenie zdolności adsorpcji pary wodnej zaobserwowano w przypadku mleka aglomerowanego w warunkach suszenia rozpyłowego oraz mieszaniny proszków serwatki i białka sojowego aglomerowanej zarówno w złożu fluidalnym, jak i przez mieszanie mechaniczne. Received 10 October 2007