Optimization of aqueous microgrinding processes for fibrous plant materials

Advanced Powder Technology xxx (xxxx) xxx

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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Optimization of aqueous microgrinding processes for fibrous plant materials Frederik Flach a, Lennart Fries b,⇑, Jana Kammerhofer c, Jutta Hesselbach a, Benedikt Finke a, Carsten Schilde a, Gerhard Niederreiter b, Stefan Palzer b, Stefan Heinrich c, Arno Kwade a a b c

Braunschweig University of Technology, Germany Nestle Research Center, Switzerland Hamburg University of Technology, Germany

a r t i c l e

i n f o

Article history: Received 4 May 2019 Received in revised form 19 August 2019 Accepted 22 August 2019 Available online xxxx Keywords: Food materials Micronization Stirred media bead mill Spray drying Reconstitution

a b s t r a c t Fibrous plant-based materials are characterized by inhomogeneous structure and composition, which further evolve during wet grinding processes and affect the surface functionality of micronized particles. Therefore, the performance of aqueous microgrinding operations in stirred media mills can be optimized by investigating the interaction between process conditions and material properties of heterogeneous fibrous plant materials. In this experimental study it is shown how particle size reduction, tendency of re-agglomeration and stability of the suspension of micronized particles are driven by the specific energy input, residence time, temperature and presence of surfactants during the milling process. A structured experimental approach is described to optimize the achievable particle size reduction, expressed by the top cut diameter d90,3. It was found that the applied wet milling process determines the stability of particle suspensions throughout further downstream processing, making the grinding process the core unit operation with respect to the performance and formulation of food products containing micronized particles. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Motivation for fine grinding of plant materials Milling is a fundamental mechanical process in various industries such as mining (ores, salts), construction (sand, cement), chemistry (pigments, catalysts), paper, pharmaceuticals and food to reduce the size of particles with the aim to increase reactivity, to access inner structures, to improve the homogeneity of mixing and to enhance mass transfer. Common applications in food technology are milling of grains [1], seeds and nuts [2,3], salt and sugar [4], dried herbs and spices [5], coffee [6,7], tea leaves [8], cocoa [9] and legumes [10]. Obtaining small particles in the range between 1 and 100 mm, referred to as micronization in food applications [11], has several advantages in food applications. In liquid products, micronization

⇑ Corresponding author. E-mail addresses: [email protected] (F. Flach), lennart.fries@rdls. nestle.com (L. Fries), [email protected] (J. Kammerhofer), [email protected] (J. Hesselbach), benfinke@tu-braunschwelg. de (B. Finke), [email protected] (C. Schilde), [email protected] (G. Niederreiter), [email protected] (S. Palzer), [email protected] (S. Heinrich), [email protected] (A. Kwade).

enhances the stability of particle dispersions and reduces sedimentation. Spices are traditionally ground to small particle size to enhance the release of flavors and to use the expensive ingredients most effectively. This effect is driven by the increased specific surface area for extraction, avoiding diffusion-limited regimes for transport of volatiles and bioactive molecules. Furthermore, fineness and surface structure of particles affect light diffraction, which is perceived as visibly lighter product colour. Beyond direct physical implications linked to an increased specific surface area, fine grinding is also known to influence the functionality of food particles in their respective applications. Micronized particles can modulate the perception of texture in food products, which is linked to the physiology of the human tongue [12]. Grainy texture in chocolate is a commonly known defect linked to insufficient refining of cocoa and sugar particles [13]. Beyond particle size, texture perception in the mouth also depends on the viscosity and composition of the surrounding medium, the hardness, shape and concentration of the particles themselves and of the presence of bitter compounds acting as mouthfeel antagonists. Micronization of salt and sugar can intensify their delivery and taste perception, if the ingredients are inhomogeneously dispersed

https://doi.org/10.1016/j.apt.2019.08.029 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

2

F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx

over the surface of dry solid food products such as bread, potato snacks, cookies or cereal bars, resulting in more effective stimulation of taste receptors [14,15]. Some food particles contain surface active compounds that preferably move to oil/water or air/water interfaces due to their amphiphilic molecular structure. If they are soluble, micronization enhances their release into the surrounding medium. Insoluble surfactants located on the particle surface will favour the orientation of the entire particle towards the interface, if the mobility is sufficient. Those species are called Pickering stabilizers or particle stabilized emulsions and foams. Several plant particles such as cocoa or coffee are known to contain surface-active compounds that qualify them as candidates for Pickering stabilizers, if they are ground to sufficiently small size [16]. While the mechanical properties of crystalline materials such as sodium chloride or sucrose are well described in the literature, organic materials often consist of complex multi-component structures. Plant-based materials are built of porous cellular networks and often consist of anisotropic fibrous structures. Their complex mechanical characteristics so far limit the ability to predict the breakage and comminution behaviour of plant-based particles. The objective of this work is to improve the understanding of microgrinding processes of fibrous plant materials, such as roast and ground coffee. Building on the energetic description of the grinding process in a stirred media bead mill, operational parameters are varied in a systematical experimental study to optimize the effective size reduction of coffee particles suspended in water. As breakage and re-aggregation phenomena occur simultaneously, the stability and primary particle size of the obtained suspension is dependent on residence time, specific energy input and stress number. Considering the perception of particles in food and beverage applications, it was found that mouthfeel defects, in particular gritty or sandy texture perception can be avoided by micronization. It was reported for confectionery applications that particles smaller than 30 mm are not perceived in mouth, which can be linked to the distance between texture receptors on the human tongue [12,17,18]. Independent of the mean particle size or the overall shape of the PSD it was found that the presence of few larger particles (the coarse tail of the distribution) contributes to sandy texture perception in mouth. Therefore, in the current study the top cut diameter d90,3 was chosen as target criterion for evaluating the micronization performance. The objective function is to minimize the top cut particle diameter x90,3. Quality criteria or constraints are the specific energy input and the stability of the obtained suspension

2. Principle and physical description of stirred media milling The general operating principle of stirred media mills is based on grinding media collisions. Mechanical power is supplied via a stirrer into the grinding chamber in which the media is moved intensively. Finally, the energy has to be transferred to the product particles in order to induce particle fragmentation. The efficiency of particle size reduction is strongly dependent on process variables of stirred media milling. Key operating parameters are the stirrer tip speed and grinding media properties, such as size, material and filling ratio. The variables can be summarized by a stress model in order to describe the impact of parameter changes on the grinding result [19]. Overall, the energy transfer determines the efficiency of grinding, because only a small part of the energy which is introduced into the grinding chamber is transferred to the product particles and used for particle stressing [19]. Different energy dissipation

mechanisms lead to a reduction of the grinding efficiency and a transfer of mechanical energy into heat. The dissipation mechanisms can be categorized as follows: & & & & &

Dissipation by shear into the fluid Friction of grinding media at the mill chamber lining Displacement of suspension by approaching grinding media Grinding media contacts without stressing product particles Energy dissipation due to grinding media wear and deformation

Most of the energy introduced into the grinding chamber is dissipated into heat due to friction. Only a small amount is used for particle stressing which finally is also transferred into heat. A key measure is the specific energy which is transferred to the product, Em;P .

Em;P ¼ mE Em;GC

ð1Þ

According to Kwade this can be estimated from the energy transfer coefficient mE and the specific energy which is transferred to the grinding chamber, Em;GC . The energy transfer coefficient is a function of the different dissipation mechanisms [19], but it is rather difficult to determine the exact value. For optimization tasks of a specific grinding operation also the specific energy input into the grinding chamber Em;GC can be taken as a measure because it is assumed to be proportional to Em;P as long as the energy transfer coefficient is not affected by the varied operating parameters. Another important measure to describe the impact of operating parameters is given by the stress energy of grinding media, SEGM . Kwade derived the stress energy of grinding media from the kinetic energy which is proportional to the mass of grinding media and the square of stirrer tip speed according to Eq. (2) [19].

SE / SEGM ¼ dGM qGM v 2t 3

ð2Þ

SEGM describes a characteristic value for the maximum possible stress energy inside the grinding chamber. It can be applied for process optimization as long as some limiting conditions are respected, which are: & &

&

Low or moderate suspension viscosity The grinding media has a much higher Young´s modulus compared to the product particles Constant mill size and geometry

If milling is linked to a significant increase of the suspension viscosity, the reduction of stress energy has to be taken into account by an energy transfer coefficient mE;g which describes the impact of fluid displacement as shown by Knieke et al. [20]. According to the definition given in Eq. (3) the energy transfer coefficient is a function of suspension viscosity g, grinding media size dGM , density qGM , stirrer tip speed v t and product particle size x.

 SEGM;g ¼ SEGM mE;g ¼ SEGM 1 þ

 2 9g x ln dGM 0:5v t dGM qGM

ð3Þ

It has to be noted that the stress energy inside stirred media mills is a distributed variable which is influenced by geometric aspects of the grinding chamber and the flow properties of the product suspension [21]. However, for practical simplification SEGM can be taken as a measure for process optimization. A relevant measure for grinding is the specific compression energy applied to the product particles [22], this can be expressed in term of the stress intensity which is related to the volume of the stressed particle, SI.

SI /

SEGM x3p

ð4Þ

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx

According to Eq. (4) the particle size can be taken as a simplifying measure to express the volume of stressed particles. It is assumed that only one particle is stressed between two approaching grinding media. On the one hand the breakage result is determined by the stress intensity and on the other hand by the number of stress events acting on the product particles. Therefore, another important measure is given by the stress number of product particles SNP [23].

SNP /

nuGM ð1  eÞx3p t g SNM PS / 2 NP ð1  uGM ð1  eÞÞcv dGM

ð5Þ

SN P is defined according to Eq. (5) where SNM represents the number of grinding media collisions inside the grinding chamber, P S the probability of catching and stressing a product particle and NP the number of product particles. SNP describes how often a product particle of a certain size is stressed during a comminution process [23]. Overall the relation between SN P and SI can be used for the optimization of grinding operations. Kwade showed that the energy utilization is a function of SI, the optimum of SI corresponds to the maximum of energy utilization [24]. That means the specific energy input Em;M can be minimized by optimizing SI for a defined product fineness. Thus, process optimization can be achieved by conducting a few milling experiments under variation of SI. In addition, the production capacity can be increased by maximizing the power input and keeping Em;M at a minimum value [25]. 3. Materials and methods In this chapter, the process chain from the educts consisting of fibrous organic plant material to a final reconstituted beverage is presented. Each process step and its parameters are explained. The term ‘‘fibrous” is used in this work referring to materials primarily consisting of dietary fibers, such as cellulose. It does not necessarily imply an elongated particle shape. Premilled roasted coffee particles (RC) were used as feed material for micronization, their initial particle size distribution is given in Fig. 1. A median particle size x50,3 in the range of 25 mm and a top cut x90,3 of 90 mm are characteristic values of the RC particle size distribution. RC particles were processed in aqueous suspension at different solids mass concentrations in the range of 0.05–0.2. The key step is the microgrinding process in order to obtain micronized roasted coffee particles (MRC). This was performed with a stirred media

3

mill (PM1, Draiswerke), equipped with a Centex disc stirrer process unit (Bühler). The mill was operated in passage mode operation. Six ideal fillings of the grinding chamber were discarded before the samples were taken for analysis or further processing. Representative MRC particles obtained through this process are shown in Fig. 2. During the grinding process, different parameters were varied in order to optimize the micronization. Thus, the material of the grinding media was varied between zirconium oxide, zirconium mixed oxide and zircon silicate, while grinding media of different mean diameters, dGM were used. The grinding media filling ratio was kept constant at 0.8 and the stirrer tip speed, vt was varied in the range of 6–12 m s1. Furthermore, residence time of the product in the grinding chamber was varied by adjusting the volume flow rate from 10 to 30 L/h at a media filling ratio of 0.8 with zirconia dioxide grinding beads of 1.2 mm diameter, a stirrer tip speed of 10 m/s at a solids content of 0.15. Grinding experiments were evaluated by the mass specific energy input into the grinding chamber, Em,GC, which was calculated from the torque measured at the stirrer shaft, M, of the mill according to Eq. (6) for passage mode operation. The calculation is based on the net power input, for which the no load torque was measured after each experiment.

Em;GC ¼

vt

M  M0 _p rs m


ð6Þ

The standard temperature for grinding was set to 30 ± 2 °C. A series of grinding experiments was performed under temperature variation from about 20–60 °C. In a second set of experiments with optimized grinding parameters, the addition of small quantities of additives (soluble coffee, SC and chlorogenic acid, CA) was evaluated. Particle size distributions were measured by laser diffraction using a HELOS device (Sympatec). Measurements were performed with a cuvette system and product suspensions were diluted with deionized water according to the detector signal. The standard measurement procedure was based on 10 s ultrasound sample treatment before measurement in order to disperse agglomerates. Qicpic image analysis was applied additionally to evaluate particle size and shape. Since the investigated MRC particles are mostly compact, no significant shape effect on the particle size distribution was observed. Rheological properties of ground suspensions were characterized by a shear rate controlled test using a coaxial cylinder system

1.0

Cumulative, Q3(x) / -

0.8

0.6

0.4

0.2

0.0 1

10

100

Particle size, x / µm Fig. 1. Volume based particle size distribution of roasted coffee particles before mironization.

Fig. 2. Scanning electron microscopy representation of micronized roasted coffee particles.

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx Table 1 Spray dryer parameter. Parameter

Value

Temperature gas inlet Temperature gas outlet Temperature feed inlet Temperature product outlet Mass flow gas Feed rate

230 °C 100 °C 60 °C 100 °C 73–75 kg/h 5.3 kg/h

at 20 °C. The shear rate was varied in a logarithmic scale ranging from 10 to 1000 s1. Infra-red (IR) spectroscopy (FTIR-ATM) for the detection of oil in the aqueous samples was conducted in a Vertex 70 (Bruker). For this, the suspension was placed directly onto the sampling surface of the device. Transmission at a wavenumber of 1162 1/cm was evaluated because this wavenumber was found to be characteristic to coffee oil. Due to the possible alteration of the spectrum due to the release of other constituents of coffee, the method is considered to be qualitative rather than a quantitative estimation of the oil concentration. After the grinding step the suspension of MRC particles in water was mixed with soluble coffee powder and spray dried in order to obtain a dry powder containing soluble coffee powder and MRC particles. To avoid re-agglomeration of MRC particles after grinding and before drying, the spray dryer feed was treated with a high shear mixer (T50 digital ULTRA-TURRAX with S50N-G45 F dispersing tool, IKA). The shearing step of 4 kg suspension was applied for 2 min at 7000 rpm. Afterwards, the feed was directly transported to the spray dryer. The drying step was performed in a pilot scale spray dryer (Niro Minor, GEA) where the drying parameter were kept constant for all experiments. A summary of all parameters is provided in Table 1. Nitrogen was used as drying gas and the experiment was operated in a closed cycle. The main parts of the spray dryer plant are the drying chamber, the electrical heater, the cyclone, the filter unit and the condenser. Both the drying gas and the feed entered the drying chamber from the top and, thus, the drying process occurred in co-current operation mode. For atomization of the feed, a two-fluid nozzle with a liquid orifice diameter of 0.89 mm was used (liquid nozzle: PF35100-SS and gas cap: PA120-SS, Spraying Systems). The product was mainly collected at the bottom of the spray dryer (coarse fraction), since almost no powder was recovered after the cyclone (fine fraction) due to the nozzle settings. For analysis of the stability of the MRC particles after the drying step, the produced coffee powders were reconstituted in water. Therefore, 2 g of powder were dispersed in 200 mL of hot water (90 °C) and constantly stirred for 2 min. Afterwards, either the stirring continued for further 6 min or the stirring was stopped and a sedimentation time of 6 min followed. The particle size distribution was determined after 2 min of stirring (directly after reconstitution), after 8 min of stirring and after 2 min of stirring and 6 min without stirring by means of a laser diffraction particle size analyser (LS13320 MW, Beckman Coulter) in order to identify reagglomeration and sedimentation. 4. Results 4.1. Impact of process parameters on the grinding result The reduction of the top cut particle size x90,3 was defined as key quality criterion to evaluate the micronization of RC particles. First micronization experiments were performed with suspensions

containing a solids mass concentration of 0.1 using different grinding media types under variation of the stirrer tip speed. Fig. 3 shows the particle size x90,3 as function of the specific energy input for micronization with different grinding media materials under variation of the tip speed after one passage operation. The specific energy input is determined by the stirrer tip speed. At low tip speeds of 6 and 8 m s1 no significant differences in the resulting particle sizes were observed by using different grinding media materials of similar mean sizes. The obtained top cut particle size was approximately 50 mm for all grinding media types. To obtain smaller particles, a higher specific energy input was required. By exceeding the tip speed of 10 m s1 deviations between the different grinding media can be observed. Milling with zirconia grinding media led to a further size reduction of the particles, a top cut particle size below 40 mm was reached. In contrast, at high stirrer tip speeds milling with mixed oxide and glass grinding media resulted in higher particle sizes compared to experiments at lower tip speeds although the specific energy inputs are much higher. Particle size measurements with additional ultrasound dispersion proved that no significant reaggregation took place and, thus, the effect must be a result of changing stressing conditions inside the mill. Increased suspension viscosities might affect the stress energy of grinding media by a higher degree of energy dissipation due to fluid displacement. Table 2 summarizes the stress energies resulting from different sets of operation parameters. Moreover, the suspension viscosities and their impact on stress energies were taken into account. For the stress energy calculation the viscosities at 1000 s1 were applied since inside stirred media mills very high shear rates are active. It can be seen that suspension viscosity increases strongly with increasing stirrer tip speed. The calculation of energy dissipation according to Eq. (3) shows that stress energies of grinding media are reduced by increasing suspension viscosities. Nevertheless, the extent of stress energy reduction was not as significant as reported by Knieke et al. within their study [20]. Even at the highest suspension viscosities the stress energy remains high enough to induce fragmentation of RC particles, because at lower tip speeds a significant size reduction of RC particles was achieved. Therefore, it can be concluded that the decreased grinding efficiency must be a result of reduced stress numbers. On the one hand side the strong increase of viscosity might lower the stress numbers, especially if the mill has zones with low shear rates. On the other hand, the axial grinding media distribution is sensitive to the variation of

100 90 80

Particle size, x90 / µm

4

6 m⋅s-1

8 m⋅s-1

12 m⋅s-1

10 m⋅s-1

70 60 50 40 30 Grinding media:

20 10

dGM [µm] ρ [g⋅cm-3]

Zirconium oxide Mixed oxide Glass

1200 1100 1100

6.1 3.8 2.6

500

1500

2000

V: 9 L⋅h-1 cm: 0.1 vt: 6, 8, 10,12 m⋅s-1

0 0

1000

2500

3000

3500

4000

Specific energy, Em / kJ · kg-1 Fig. 3. Particle size x90,3 as function of the specific energy input for grinding with different grinding media and tip speeds.

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

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F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx Table 2 Power consumption, stress energies and viscosity dependent energy transfer coefficients from milling experiments with different grinding media. Grinding media

Stirrer tip speed, vt [m s1]

Net power draw [W]

Stress energy, SEGM [mJ]

Suspension viscosity at 1000 s1 [mPas]

Energy transfer coefficient, vE,g

Corrected stress energy, SEGM,g [mJ]

Zirconia

6 8 10 12

145 313 581 867

0.38 0.68 1.05 1.52

14.21 35.06 75.14 100.22

0.96 0.93 0.88 0.86

0.36 0.63 0.92 1.31

Mixed oxide

6 8 10 12

115 323 512 969

0.18 0.32 0.51 0.73

14.05 34.41 59.20 89.25

0.94 0.89 0.85 0.82

0.17 0.28 0.43 0.60

Glass

6 8 10 12

83 282 524 916

0.13 0.22 0.35 0.49

15.19 34.77 65.52 95.97

0.91 0.83 0.77 0.76

0.12 0.18 0.27 0.37

80 Grinding media: Zirconia dGM: 1200 µm

70

vt: 10 m⋅s-1

Particle size, x90,3 / µm

stirrer tip speed if the grinding media separation is supported by a deflector wheel at the end of the disc rotor. The deflector wheel generates an axial fluid flow which counteracts to the axial product flow. The counter current increases with increasing stirrer speed. Schons and Kwade have shown that this effect has strong impact on the axial grinding media distribution inside horizontal disc stirrer mills which are equipped with a dynamic classifier for grinding media separation [26]. Especially at high stirrer tip speeds the counter current of the classifier increases the blocking tendency of grinding media at the inlet zone of the mill. In particular smaller grinding media and grinding media with lower densities are sensitive to form packings at the inlet zone since the centrifugal forces resulting from the rotating stirrer are less dominant compared to the axial flow forces. Stehr has shown that packing of grinding media is indicated by an increasing power consumption of the mill [27]. A similar trend was observed within the given results. In general the power consumption of a mill depends mainly on the stirrer tip speed and on the mass of the grinding media filling inside the mill, i.e. its density at constant filling ratio. This physical principle was observed when low tip speeds were applied, milling with zirconia grinding media resulted in the highest power consumption at the tip speed of 6 m s1. In contrast, at a tip speed of 12 m s1 grinding experiments with mixed oxide and glass grinding media resulted in higher power consumptions compared to the experiment with zirconia grinding media. This is the clear indication for the formation of a grinding media packing at the inlet and, thus, an uneven grinding media distribution and a less efficient comminution [27]. Additionally, the residence time of the product suspension for passing the inlet zone of the mill is strongly reduced because the local grinding media filling ratio is increased. Hence, also the overall stress number is reduced, which finally leads to a worse grinding performance of low density grinding media at high stirrer tip speeds. The following experiments were performed at two passage mode operation in order to narrow the residence time distribution and to ensure a sufficient high specific energy input at relatively short process times by applying zirconia grinding media with a nominal diameter of 1200 mm and a stirrer tip speed of 10 m s1. Fig. 4 shows the top cut particle size x90,3 as function of the specific energy input for two passage mode milling experiments with varying solids mass concentrations of RC. The first passage was operated with a suspension volume flow of 25 L h1 and the second passage with 20 L h1. The target fineness x90,3 < 40 mm should be reached after the second passage, which was archived by milling suspensions with solids concentrations of RC up to 15%. In contrast, the milling trial with RC suspension of 20% solids concentration did not lead to the target fineness. On the one hand, increasing solids concentrations can lead to more efficient grinding

60

50

40

30

Solids concentration, cm: V [L⋅h-1] Pass 1 25 Pass 2 20

0.05 0.1 0.15

0.2

20 1000

2000

3000

4000 5000

Specific energy, Em / kJ⋅kg-1 Fig. 4. Particle sizes x90,3 resulting from milling experiments of suspensions containing different solids concentrations plotted as function of the specific energy input.

because the probability of catching and stressing product particles between colliding grinding media increases and therewith, the energy transfer to the product particles increases [25]. But, on the other hand, also the suspension viscosity increases with increasing solids concentration and fineness which can lead to higher energy dissipation due to fluid displacement. Therefore, the grinding operation becomes less efficient by exceeding a certain viscosity and, thus, solids concentration. Fig. 5 shows the viscosities of MRC suspensions with different solids concentrations after two passage milling as function of the shear rate. MRC suspensions reveal distinct shear thinning flow behaviour. With increasing solids concentration a significant increase of the suspension viscosity was observed. Milling of the suspension with the highest RC concentration led to a paste which pushed the grinding unit and connecting pipework almost to the limit of clogging. Additionally, the increased viscosity has also a strong impact on the effective stress energy of grinding media which was significantly lowered and therewith the comminution of RC was limited [20]. Calculations of the energy transfer coefficient according to Eq. (3) proved this phenomenon. Table 3 gives the calculated energy transfer coefficients and values of corrected stress energy for experiments with different solids concentrations. It can be seen that the significant increase of viscosity has a strong impact on the stress energy of grinding. In particular at a solids concentration of 20% most of the stress energy is

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

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F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx

80

100

Viscosity / Pa⋅s

10

1

0.1

cm: 0.1 RC .

70

Particle size, x90,3 / µm

Solids conc., cm 0.05 0.1 0.15 0.2

60

V: 20 L⋅h-1 Grinding media: Zirconia dGM: 1200 µm vt: 10 m⋅s-1

50

40

30

0.01 20

10

100

1000

20

30

40

50

60

Outlet temperature, t / °C

. Shear rate, γ / s-1 Fig. 5. Viscosities of suspensions with different solids concentrations milled in two passage mode operation as function of the shear rate.

Table 3 Viscosities, stress energies and energy transfer coefficients calculated for milling experiments with different solids concentrations. Solids concentration, cm

Suspension viscosity at 1000 s1 [mPa s]

Energy transfer coefficient, vE,g

Corrected stress energy, SEGM,g [mJ]

0.05 0.10 0.15 0.20

11 42 240 1450

0.98 0.93 0.64 0.03

1.03 0.98 0.67 0.04

dissipated by fluid displacement. The best grinding result, i.e. x90,3 < 40 mm, with the lowest specific energy input and the highest production capacity, was reached with a solids concentration of 15%. Moreover, the temperature was identified to have a significant impact on the product quality: The increase of power input leads to an increase of the process temperature which has a negative effect on the grinding result, increasing particle sizes were observed with increasing process temperature. Fig. 6 shows the impact of process temperature on the top cut product particle size x90,3. The mill was operated in one passage mode operation with constant process parameters. The temperature was varied by manual regulation of the cooling system; given values represent outlet temperatures of the mill. In the temperature range from 20 to 30 °C no significant impact of the process temperature can be identified, almost similar particle sizes of 46 mm were reached. With increasing temperature, a linear increase of the particle size in the range from 30 to 60 °C can be identified. As a conclusion the process temperature should be kept <30 °C in order to reach small particle sizes. It is assumed that the dependence of particle size on the temperature is linked to the internal composition of RC particles, especially to the lipids inside. Carbohydrates and Melanoidins are the major components inside RC, but also a fraction of 11–17 wt% of the chemical composition is covered by lipids [28]. These hydrophobic molecules are assumed to be liberated during the micronization process of RC particles. Mechanical stress and the increase of the specific surface area lead to the liberation of lipids. This process is assumed to be intensified at elevated temperatures because of the increased mobility of the oil phase. The melting range of coffee oil is between

Fig. 6. Particle size x90,3 as function of the process temperature for constant process parameters.

5°C and 20 °C [29]. Therefore, the coffee oil is in a liquid state over the observed range of temperatures. The liberation of lipids was demonstrated by staining the lipid phase. The lipophilic colorant Nile red was used as fluorescence marker to visualize the lipid phase. RC particles were ground in the presence of Nile red and a fluorescence microscope was used to image the distribution of the oil phase. Fig. 7, left shows an intense red coloration of the depicted MRC particle which demonstrates that the lipids adhere mainly to the surface. Therewith, hydrophobicity and tendency towards agglomeration increase with increasing specific surface area. The sample was undertaken further processing steps by centrifugation and hexane washing with subsequent redispersion in water in order to remove free oil. The fluorescence micrograph (Fig. 7, right) shows a less intense red coloration of the MRC particle which proves the removal of lipids from the MRC surface. Further Insight on this effect was gained by varying the residence time of the product in the grinding chamber by adjusting the product flow rate. Fig. 8 shows the resulting particle size after a single passage with respect to the specific energy input. The respective flow rates and residence times are depicted in the graph. The previously identified optimum solids content of 0.15 was chosen. It becomes obvious that with increasing flow rate the particle size of the product increases. As the data points can be approximated with a straight line in a double-log-scale diagram the relationship is that of a power function. Furthermore, it becomes obvious, that already at 15 L/h the particle size criterion of 40 mm is no longer met. This underlines the importance of sufficient stress numbers for optimum grinding results. In Fig. 9 the transmission at a wavenumber of 1162 1/cm, which was identified to be characteristic for coffee oil, is plotted respective flow rates along with the resulting viscosity of the experiments displayed above. An initially strong but ceasing decrease in viscosity can be perceived with rising volume flow rate. IRtransmission suggests a decrease in oil release with higher volume flow. Since higher volume flow rates translate into higher residence times and hence more stress events on the product particles, a link between micronization and oil release can be established. Furthermore, the strong increase in viscosity could suggest the formation of an emulsion, even though the extent of this phenomenon could not be assessed quantitatively. Hence, it can be concluded that especially the liberation of lipids leads to agglomeration and the increases of viscosity when

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

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F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx

Fig. 7. Fluorescence micrographs of stained MRC particles, left: original suspension, right: washed suspension.

80

80

30 L/h

70

70

20 L/h

V: 28 L⋅h-1 cm: 0.06

65

Particle size, x90 / µm

Particle size, x90 / µm

60

Grinding media: Mixed Oxide dGM = 1500 µm

75

25 L/h

50

15 L/h 40

Volume flow

10 L/h

Residence time

50

137 s

15 L/h

87 s

20L/h

61 s

vt: 10 m/s

25 L/h

52 s

ρ : 6067 kg/m³ GM

30

30 L/h

41 s

x90, Feed: 108 µm

25

20 500

constant process parameters dGM: 1200 µm

1000

1500

2000

2500 3000 3500

Specific energy, Em / kJ/kg

Run 1 Run 2 Run 3

55

10 L/h

30

vt: 6 m⋅s-1

60

45 40 35

20 0

1000

2000

3000

4000

5000

6000

Specific energy, Em / kJ · kg

Fig. 8. Product particle size of suspensions processed under varied volume flow rates.

7000

8000

-1

Fig. 10. Reproducibility of grinding experiments run in circular operation mode.

4.2. Reproducibility of grinding experiments 0,90

150

Viscosity,η / mPa s

0,89

100 0,88 75 0,87 50 0,86

25

0

Transmission (1162 1/cm), / -

Viscosity Transmission (1162 1/cm)

125

The reproducibility of the grinding experiments was tested in both circular and passage operation mode. In Fig. 10, results of three repeated runs of the same grinding experiments are plotted with respect to the specific energy input. A very similar grinding progress can be observed, which proves the the reproducibility of the grinding experiments. The reproducibility of passage operation mode experiments was tested by repeating an experiment five times. A standard deviation of 3.1 mm was found for the particle size value x90,3, which again proves good reproducability of the experiments. 4.3. Effect of surfactants on reagglomeration

0,85 10

15

20

25

30

Volume flow / L h-1 Fig. 9. Product viscosity and IR Transmission for suspensions with varied volume flow rate during processing.

RC particles are micronized. This process seems to be strongly dependent on operating parameters. In the following it should be evaluated if the agglomeration can be countered by natural surface-active coffee ingredients in order to stabilize MRC particles against agglomeration.

Soluble coffee (SC) and chlorogenic acid (CA) were evaluated as additives for particle surface stabilization. Both were chosen due to their amphiphilic properties in order to modify interfacial properties of MRC particles. SC and CA are known as surface-active, preliminary studies on the impact on the surface tension of water have shown a significant decrease of the surface tension at low concentrations. A representative chemical composition of soluble coffee is reported in [30]. The main surfactant in soluble coffee is chlorogenic acid, which together with other low molecular organic acids represents 12% of the dry matter in soluble coffee. For the solution containg 1% soluble coffee we measured a surface tension of 49.9 mN/m, whereas the solution containing 0.5% chlorogenic acid

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx

is characterized by a surface tension of 47.3 mN/m. Based on these observations, concentrations of 1% SC and 0.5% CA were chosen for the milling experiments. The experimental procedure was the same as the one giving the results presented in Fig. 4: The mill was operated in passage mode at constant process conditions. The volume flow of the first passage was set to 25 L h1 and the second passage was operated either at 10 or 20 L h1. Different additive combinations were chosen in order to show the impact on the agglomeration behaviour of MRC particles. Particle size distributions of these experiments were characterized by two different measurements. The first measurement was performed without ultrasound application in order to demonstrate the agglomeration behaviour of MRC particles and the second measurement was performed after 10 s of ultrasound treatment. Measurement data is given in Fig. 11. The results obtained after the first passage do not show significant differences of the resulting particle sizes. A similar trend was observed with increasing specific energy input up to the range of 1700 kJ kg1. Milling of suspensions with and without additives resulted in similar particles sizes. Only the suspension mixed with SC and CA showed a slightly increased particle size. With increasing specific energy input a further reduction of the particle sizes was achieved. At energy inputs in the range of 3000 kJ kg1 small influences of the additives can be identified. Particle size measurements without ultrasound application show different agglomeration levels. A top cut particle size x90,3 of 39 mm was measured for the MRC suspension without additives. The addition of SC and CA led to a slight reduction of the agglomerate sizes, but significant differences among the additives were not observed. Similar particle sizes in the range of 30 mm were measured by application of ultrasound. This proves that the micronization process on primary particle level is not strongly affected by the different additives. In general, it can be stated that the selected additives do not have a significant impact on the grinding result; the process is mainly determined by operation parameters of the milling setup. In the following it has to be investigated if the different additives have an impact on the spray drying and reconstitution process of MRC powder. 4.4. Performance of suspension after reconstitution In this chapter, the effect of further processing steps and the final reconstitution on the behaviour of the MRC particles was investigated. Therefore, the coffee suspensions after grinding containing micronized particles were mixed with soluble coffee powder and water, processed with the high shear mixer and dried 80 cm: 0.15 RC

70

Grinding media: Zircon oxide dGM: 1200 µm

Particle size, x90,3 / µm

60

vt: 10 m⋅s-1

50

40

30

Stabilizers: SC CA 0.01 0.005 0.01 0.005

20 500

Analysis: no-US US

1000

1500

2000

2500 3000 3500

Specific energy, Em / kJ⋅kg-1 Fig. 11. Impact of different additives on the particle size: Particle size is plotted as function of the specific energy input.

60

after grinding reconstituted in cup Particle size, x90,3 / µm

8

50

40

30

20

no

n

dd oa

add

.+

.

no

hs hig

hea

r ixe rm

0.5

A %C 1%

+ SC

0 .5

A %C

1%

SC

Surfactants / Fig. 12. Comparison of x90,3 values of suspension after grinding and of reconstituted powders in the cup with and without high shear mixing and additives.

by means of the spray dryer as described under material and methods. The measured x90,3-values after the grinding step and after reconstitution in a cup are compared in Fig. 12. The first pair of bars presents the results for the sample containing no additive. It is obvious that the further processing steps after the mill do not favour re-aggregation of the MRC particles, since the x90,3 value in the cup is close to the value after grinding. Comparing the second pair of bars which represents the sample with no high shear mixer treatment before drying, it can be seen that the x90,3-value in the cup increased by 60%. It can be concluded that the high shear mixing step is mandatory for keeping the x90,3-value constant in the cup. Due to intermediate storage time between microgrinding and spray drying, re-aggregated MRC particles have to be separated by shear forces before drying. If completing the full process chain without intermediate storage would allow omitting of the shearing step could not be tested in the course of this study. Furthermore, it was analysed if the additives show an impact on the micronized particles in the reconstituted beverage in the cup. The last three pairs of bars in Fig. 12 present the results of the effect of the addition of 0.5 wt% CA, 1.0 wt% SC + 0.5 wt% CA and 1.0 wt% SC on the behaviour of the MRC particles. The x90,3-value in the three reconstituted samples show an increase of up to 24%. Consequently, the addition of CA and SC do not provide a positive effect on the stability of MRC particles in the cup as it was already seen during grinding. As the stability towards re-aggregation and sedimentation of the MRC particles also has to be ensured during longer times in a cup, two further x90,3-values were determined and compared to the values directly after reconstitution. On the one hand, the reaggregation during stirring was tested by measuring the particle size after an additional 6 min of stirring, while on the other hand, the stirring was switched off after reconstitution and a time of 6 min without stirring followed to detect possible sedimentation. The results are presented in Fig. 13. For all samples including the sample without high shear mixing, an additional stirring of 6 min does not lead to a change of the x90,3-value. Thus, as long as the sample in the cup is slightly stirred, no re-aggregation can be observed. In the case of the sample without high shear mixing, the re-aggregation happened already before the drying step. But when stopping the stirring after 2 min of reconstitution and waiting for 6 min before measuring the particle size again, it is obvious that in the sample without additives an increase of the

Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029

F. Flach et al. / Advanced Powder Technology xxx (xxxx) xxx

from Nestlé Research, we cannot acknoledge ourselves. It is Nestlé’s intention to actively contribute to the scientific progress in Food Science and Technology through publications. Rather than acknowleding for funding, we express the active and collaborative spirit of this scientific project as joint authors.

60

reconstituted in cup after 6 min. of stirring after 6 min. without stirring Particle size, x90,3 / µm

9

50

References 40

30

20

no

n

dd oa

add

.+

.

no

hs hig

hea

r ixe rm

0.5

A %C 1%

+ SC

0 .5

A %C

1%

SC

Surfactants / Fig. 13. Comparison of x90,3 values of reconstituted powders in the cup with and without additives and high shear mixing after reconstitution, after 6 min of additional stirring and after additional 6 min without stirring.

x90,3-value by 16% occurred. In the cups with samples containing additives, the particle size stays constant even if the stirring is stopped. However, the particle size achieved after a stirring break of 6 min in the samples without additives is in the same order as the particle size of the samples with additives. From the reconstitution experiments, it can be followed that the additives CA and SC increase the size of MRC particles directly after reconstitution and do not show any effect during the observation of further 6 min. 5. Conclusions This study investigated the optimization of wet milling processes of aqueous suspensions of fibrous plant-based food particles, taking the example of roast and ground coffee particles. The objectives were to minimize the top-cut particle size x90,3 and to maintain a stable suspension of micronized particles after milling and after further downstream processing. Specific energy input, residence time, particle concentration and temperature were varied in a systematic experimental approach. Unlike for inert particles, the investigated material showed evolving particle surface functionality along the milling process, caused by heterogeneous particle composition. Oil migration to the surface was found to be the main cause for re-agglomeration of micronized particles, up to regimes where the kinetics of re-agglomeration exceed those of the ongoing grinding process. While the effect of surfactant additives on the stability of suspensions of micronized particles was found to be limited, the results show that particle surface functionality is maintained throughout further process steps such as spray drying and reconstitution, if the milling process enabled the formation of a stable suspension. It can therefore be concluded that the milling unit operation is core to the functionality of suspensions of micronized food particles. Acknowledgements This is a joint publication between Nestlé Research, which funded the study, and the universities TUHH and TUBS. As authors

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Please cite this article as: F. Flach, L. Fries, J. Kammerhofer et al., Optimization of aqueous microgrinding processes for fibrous plant materials, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.029