Determination of powder flow properties of skim milk powder produced from high-pressure homogenization treated milk concentrates during storage

LWT - Food Science and Technology 97 (2018) 279–288

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Determination of powder flow properties of skim milk powder produced from high-pressure homogenization treated milk concentrates during storage

T

Emin Mercana,∗, Durmuş Sertb, Nihat Akınc a

Faculty of Engineering, Department of Food Engineering, Bayburt University, Bayburt, 69000, Turkey Faculty of Engineering and Architecture, Department of Food Engineering, Necmettin Erbakan University, Konya, 42090, Turkey c Faculty of Agriculture, Department of Food Engineering, Selcuk University, Konya, 42075, Turkey b

A R T I C LE I N FO

A B S T R A C T

Keywords: High-pressure homogenization Powder flow properties Skim milk powder Cohesion Caking

The aim of this study was to determine effects of high-pressure homogenization (HPH) treatment to milk concentrates on powder flow properties of skim milk powders (SMP). For this purpose, SMP samples were produced from skim milk concentrates which were HPH treated at 0 (control), 50, 100 and 150 MPa. SMP samples were stored 180 days at 20 and 40 °C and caking, cohesion and powder flow speed dependency test (PFSD) were performed using a Powder Flow Analyzer. HPH treatment decreased cohesion and caking properties of SMPs. At the 0. day, cake strength of SMPs from HPH treated concentrates varied from 0.43-2.70 mN.m, whereas cake strength of SMP-0 (control) was 8.20 mN.m. Cohesion index (CI) of samples ranged from 6.80-15.74 mm during storage. Based on CI, SMPs from HPH treated concentrates showed free and easy flowing flow behavior during storage at 20 and 40 °C. In addition, cohesion index at four speeds indicated that SMP from HPH treated concentrates were more flowable at even lower flow speeds then SMP-0. PFSD test verified that all SMP samples showed more free-flowing characteristics with increasing flow speeds. Flow stability of samples ranged from 0.95-1.15 and also flow stability of SMP-0 increased during storage at both 20 and 40 °C. The results revealed that HPH treatment to milk concentrates could improve powder flow properties of skim milk powders.

1. Introduction In recent years, utilization of emerging processing technologies has increased in the dairy industry. These technologies include ultrasound (Guimaraes et al., 2018; Monteiro et al., 2018), super critical carbon dioxide (Amaral et al., 2018b, 2018a), ohmic heating (Cappato et al., 2018a, 2018b, 2017), and cold plasma (Coutinho et al., 2018). Among the emerging technologies, high-pressure homogenization has been currently studied for dairy products processing (Lollo et al., 2015; Oliveira, Augusto, Cruz, & Cristianini, 2014). HPH has some advantages due to continuous processing compared to high hydrostatic pressure (HHP) which is a batch processing. In the processing of liquid or semisolid foods, HPH can be preferred due to higher processing capacity. In the dairy industry, HPH treatment, particularly above 150 MPa (inlet temperature > 40∘C), is effective to inactivate bacteria in milk (Hayes, Fox, & Kelly, 2005; Pereda, Ferragut, Guamis, & Trujillo, 2006; Pereda, Ferragut, Quevedo, Guamis, & Trujillo, 2007; Picart et al., 2006; Smiddy, Martin, Huppertz, & Kelly, 2007). Previous studies

∗

reported that shelf life of HPH treated milk is similar to conventionally processed milk (Pereda et al., 2007, 2006; Smiddy et al., 2007). Casein micelles are disrupted by HPH treatment at ∼200 MPa (Hayes, Lefrancois, Waldron, Goff, & Kelly, 2003; Lodaite, Chevalier, Armaforte, & Kelly, 2009; Roach & Harte, 2008; Sandra & Dalgleish, 2005). Previous studies reported that HPH reduces the size of casein micelles up to 200 MPa whereas increases up to 350 MPa (Roach & Harte, 2008). Mohan, Ye, and Harte (2016) revealed that casein micelle dissociation leads to a reduction in particle size at pressures of 100–200 MPa. Prior to spray drying, HPH treatment improves solubility of whey protein powders (Iordache & Jelen, 2003). In addition, HPH treatment of skim milk increases viscosity and modifies the functional properties of caseins (Mohan et al., 2016). In transporting, handling and processing operations, powder flow properties of milk powders are very crucial (Knowlton, Klinzing, Yang, & Carson, 1994) due to the fact that changes in flow properties can result in a poor quality product or stoppages in processing. In order to determine powder flow properties of various food powders, powder

Corresponding author. E-mail address: [email protected] (E. Mercan).

https://doi.org/10.1016/j.lwt.2018.07.002 Received 14 April 2018; Received in revised form 29 June 2018; Accepted 1 July 2018 Available online 04 July 2018 0023-6438/ © 2018 Elsevier Ltd. All rights reserved.

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rheometers (Freeman, 2007) and powder flow analyzers (Abu-hardan & Hill, 2010; Bansal, Premi, Sharma, & Nanda, 2017; Benkovic & Bauman, 2009; Benković, Belščak-Cvitanović, Bauman, Komes, & Srečec, 2017; Benković, Belščak-Cvitanović, Komes, & Bauman, 2013; Janjatović et al., 2012; Wang & Zhou, 2012) have been used due to the fact that they can give sensitive, fast and repeatable measurements with a high degree of automation. To the best our knowledge, there is no study on skim milk powder production from HPH treated milk concentrates. In addition, there is little data about powder flow behavior of milk powders determined by powder flow analyzer in the literature (Bansal et al., 2017; Benković, Srečec, Špoljarić, Mršić, & Bauman, 2013). Therefore, it is believed that this study is the first research attempt to apply HPH treatment on milk concentrate for SMP production. The main goal of this study was to determine the effect of high-pressure homogenization treatment to milk concentrates on powder flow properties of skim milk powders during 180 days of storage at 20 and 40 °C. Caking, cohesion and powder flow speed dependency (PFSD) analyses were performed using a Powder Flow Analyzer (Stable Micro Systems) during storage. 2. Materials and methods 2.1. Material In this study, skim milk concentrate (SMC) containing 42 ± 2% total solids was supplied from Enka Dairy Co. Ltd. (Konya, Turkey). SMC production was carried out according to Fig. 1. 2.2. High-pressure homogenization treatment For each batch of treatment, 25 L of SMC was high-pressure homogenization (HPH) treated using the Panda Plus 2000 lab-scale high-pressure homogenizer (GEA Niro Soavi, Parma, Italy) with a flow rate of 9 L/h. Treatment pressures were 0 (control), 50, 100 and 150 MPa. 2.3. Skim milk powder production After HPH treatment, skim milk concentrates were dried using a Spray-Dryer (GEA Niro Atomizer, GEA Process Engineering A/S, Soeborg, Denmark according to production flow chart in Fig. 1. Processes before HPH treatment were performed at Enka Dairy Co. Ltd. After milk powder production, SMP samples were packed with the 3-ply Kraft paper coated polyethylene bags and stored during 180 days at 20 and 40 °C at 60 ± 5% relative humidity. Samples were stored in temperature-controlled incubators (ES 120, Nüve, Ankara, Turkey) in order to ensure constant storage temperature. Analyses of SMP samples were carried out at 0, 30, 90 and 180. days of storage. The moisture content, tapped density, mean particle size (D3,2) and span (reflects particle size distribution range) of milk powders ranged from 2.38-3.42%, 0.724–0.814 g/cm3, 18.50–31.69 μm and 1.288–1.832, respectively.

Fig. 1. Production flow chart for skim milk powders.

sample (after taring the weight of the vessel) was recorded. After conditioning cycle, caking, cohesion, cohesion at 4 speeds and powder speed flow dependence (PFSD) analyses were performed according to recommended settings from the producer. All results of powder flow analyses were derived by Exponent software (Version 6, Stable Micro Systems).

2.4. Powder flow analyses of skim milk powders For characterization of powder flow behavior in milk powders, TA.XTPlus Texture Analyzer equipped with a Powder Flow Analyzer apparatus (Stable Micro Systems, Godalming, Surrey, UK) was used. The load cell housed in the Powder Flow Analyzer has a 49.0 N capacity. The following attachments were used: a specified rotating helical blade (Rotor no. R48/50/10/2/A, diameter: 48 mm and height: 10 mm) and a standard cylindrical glass vessel with 220 ml total volume (height: 120 mm and internal diameter: 50 mm). Before analyses, force (using 19.6 N force) and height calibration (using height and target calibration disc) of Powder Flow Analyzer was carried out. For each analysis, the vessel was filled to just over 140 ml (approximately 70 mm column height) with skim milk powder sample and the weight of the

2.4.1. Conditioning cycle Before powder flow analyses of skim milk powders, two conditioning cycles were carried out in order to extinguish any user loading difference and to regularize the milk powder column after lifting. Each conditioning cycle consisted of downward movement of the rotating blade at a tip speed 50 mm/s and path angle of 175° (slicing action) and then upward movement at a tip speed 50 mm/s and path angle of 178° (lifting action). 280

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weight (Bansal et al., 2017). Classification of the flow behavior of powders based on cohesion index is presented in Table 1. Cohesion index characterizes the flow behavior of milk powder samples from extremely cohesive to free-flowing.

2.4.2. Caking analysis Firstly, the caking test started with two conditioning cycles. Then the blade moved to a force of 49 mN at 20 mm/s and 2° in order to level off the peak point of the powder column and measure the height of the column. Once column height was measured, it recorded. The rotating blade then went down through the powder column with a speed of 20 mm/s and angle of 20° and it compacted the milk powder to a predefined force of 7.35 N in order to form cake. When the blade reached the target force (7.35 N), it measured the occurred cake height and sliced up through the powder with a tip speed of 10 mm/s and 45°. This compaction cycle was done five times in total. Between cycles the blade shaked and waited for 3 s. When the fifth time the target force (7.35 N) was reached, the rotating blade sliced (20 mm/s and 170°) through the compacted cake of skim milk powders occurred at the bottom of the vessel and measured the force required to do so. This force was recorded as the cake strength (mN.m) and was the work required to cut the cake and the mean cake strength (N) was the average force to cut the cake in grams. The cake height ratio for each cycle was calculated from current cycle powder cake height divided by initial column height (Bansal et al., 2017). The results of caking tests were obtained from force vs distance curve using the macro of Exponent software. Cake height ratios provided information on the extent of formed cake. Strongly increasing cake height ratio of milk powders indicated that powder had a high tendency to cake and thus had higher cake strength and mean cake strength. A typical force vs distance curve of caking analysis for skim milk powder is presented in Fig. 2.

current cycle cake height Cake height ratio for each cycle = initial powder column height

Cohesion index =

cohesion coefficient sample weight

(2)

2.4.4. Cohesion at four speeds analysis This analysis was modified from the standard cohesion and PFSD test. It quantifies resistance of powder sample as a controlled flow is imposed at four different speeds on the upward cycle. Cohesion at four speeds analysis consisted of 1 conditioning cycle followed by 1 cycle at 10 mm/s in both directions, 1 cycle at 20 mm/s, 1 cycle at 50 mm/s, 1 cycle at 100 mm/s and finally 1 cycle at 10 mm/s. The downward parts (compacting action at 5°) of the cycles compacted the powder and the upward stroke (lifting action at 178°) of the cycle used a lifting action. Thus cohesion index at four different speeds (at 10, 20, 50 and 100 mm/s) was calculated by dividing the cohesion coefficient (the upward part of the cycle) by the weight of the sample. Fig. 4 indicates a typical force vs distance curve of cohesion at four speeds analysis for skim milk powder.

Cohesion index at each speed =

cohesion coefficient at each speed sample weight (3)

2.4.5. Powder speed flow dependence (PFSD) analysis The PFSD analysis measures the flow properties of powders depending on flow rate of skim milk powders. Two conditioning cycles were done at the beginning of the PFSD analysis. After conditioning cycles, 5 sets of 2 cycles at increasing speeds (10, 20, 50, 100 mm/s) and then the final 2 cycles at 10 mm/s were performed. First 2 cycle downward action was at a tip speed of 10 mm/s and path angle of 5° (at the bottom of the powder column the blade was programmed to slice through the powder and not to measure data). First 2 cycle return action was at 50 mm/s and 178°. The next 2 cycles were started immediately, in the same format, but with the downward compaction tip speed of 20 mm/s. At the end of these 2 cycles the compaction speed was changed to 50 mm/s, then a further 2 cycles at a compaction speed of 100 mm/s, and finally 2 cycles at 10 mm/s. In the cycles, the downward actions compacted milk powder and the upward actions indicated a lifting action. The positive area under the compaction curves (a typical PFSD curve is shown in Fig. 5) was averaged over the

(1)

2.4.3. Cohesion analysis Cohesion analysis started two conditioning cycles. After the conditioning cycles, cohesion properties of skim milk powders were assessed by a cohesion test in which the blade was moved downwards through the powder column at a tip speed of 50 mm/s and angle of 170° and then it was moved upwards through the powder column at 50 mm/ s and 170°. In downward and upward movements, slicing action was used in order to minimize compaction effect. The upward part of the cycle lifted the powder and the force of the powder on the vessel base was recorded. The force was used in determining cohesion coefficient. Cohesion coefficient (mN.m) was determined from the negative area of the force vs distance curve. Fig. 3 shows a typical force vs distance curve of cohesion analysis for skim milk powder. In the calculation of cohesion index (mm), cohesion coefficient was divided to sample

Fig. 2. A typical force vs distance curve for caking analysis of skim milk powders. 281

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Fig. 3. A typical force vs distance curve for cohesion analysis of skim milk powders.

2.4.6. Morphological characteristics For determining morphological characteristics of samples, scanning electron microscopy (SEM) imaging was used. Prior to SEM imaging, SMP samples attached to double-sided adhesive carbon tape mounted on SEM stubs. Excess particles were removed by gently shaking the stub. Samples were then coated with carbon. The samples were examined with a Nova NanoSEM 450 SEM instrument (FEI, Eindhoven, Netherlands) operated between 2 and 4 kV.

Table 1 Classification of flow behavior of powders based on cohesion index (Benković, Srečec, Špoljarić, Mršić, & Bauman, 2013). Cohesion Index (mm)

Flow Behavior

> 19 19–16 16–14 14–11 < 11

Hardened/Extremely Cohesive Very cohesive Cohesive Easy flowing Free flowing

2.5. Statistical analysis two cycles at each speed and gave the compaction coefficient (mN.m) at each of the speeds tested. The negative area under the upward section of the first 10 mm/s speed curves were averaged and recorded as a cohesion coefficient at 50 mm/s (mN.m). Flow stability was calculated by dividing the compaction coefficient of the last 10 mm/s cycles by the compaction coefficient of the first 10 mm/s cycles (Bansal et al., 2017).

Flow stability average compaction coefficient of last cycles at 10 mm/s = average compaction coefficient of first cycles at 10 mm/s

Duncan's multiple range test at P < 0.05 was done to compare statistical differences among the means (Mercan, Sert, Karakavuk, & Akin, 2018). Moreover, Spearman rank order correlation method was used in order to determine interactions between cake strength, mean cake strength, cohesion index, cohesion coefficient and flow stability as a nonparametric test (Benković, Srečec, et al., 2013). All statistical analyses were performed using SPSS 22.0 (Armonk, NY: IBM Corp.). 3. Results and discussion

(4)

Caking of amorphous food powders with low moisture (< 5%) is a deleterious phenomenon by which powders are firstly turned into

Fig. 4. A typical force vs distance curve for cohesion at four speeds analysis of skim milk powders. 282

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Fig. 5. A typical force vs distance curve for PFSD analysis of skim milk powders.

Table 2 The results of caking analysis results for skim milk powders during storage. Temp.

HPH (MPa)

Storage (day)

Cake Height Ratio 1

Cake Height Ratio 2

Cake Height Ratio 3

Cake Height Ratio 4

Cake Height Ratio 5

Cake Strength (mN.m)

Mean Cake Strength (N)

20 °C

0

0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180

0.142 0.020 0.014 0.008 0.174 0.175 0.109 0.011 0.198 0.178 0.020 0.011 0.151 0.214 0.025 0.012 0.142 0.021 0.010 0.007 0.174 0.168 0.009 0.011 0.198 0.167 0.010 0.012 0.151 0.176 0.009 0.016

0.250 0.117 0.012 0.009 0.283 0.245 0.097 0.011 0.294 0.282 0.102 0.011 0.213 0.278 0.019 0.012 0.250 0.104 0.006 0.007 0.283 0.224 0.009 0.015 0.294 0.238 0.009 0.012 0.213 0.215 0.011 0.017

0.313 0.158 0.097 0.010 0.301 0.281 0.113 0.011 0.344 0.353 0.112 0.012 0.232 0.329 0.031 0.013 0.313 0.178 0.007 0.011 0.301 0.281 0.011 0.013 0.344 0.267 0.009 0.012 0.232 0.246 0.012 0.015

0.363 0.203 0.170 0.007 0.344 0.336 0.173 0.011 0.375 0.384 0.159 0.013 0.241 0.354 0.105 0.015 0.363 0.205 0.007 0.012 0.344 0.302 0.012 0.016 0.375 0.301 0.012 0.014 0.241 0.287 0.013 0.016

0.399 0.960 0.203 0.008 0.355 0.989 0.174 0.012 0.406 0.386 0.203 0.013 0.242 0.361 0.144 0.015 0.399 0.589 0.006 0.010 0.355 0.323 0.009 0.016 0.406 0.325 0.013 0.013 0.242 0.306 0.014 0.015

8.20 1.92 1.95 0.00 2.70 1.87 0.06 0.00 2.43 0.68 0.02 0.00 0.43 0.38 0.00 0.00 8.20 4.12 0.00 0.00 2.70 0.76 0.00 0.00 2.43 0.50 0.00 0.00 0.43 0.11 0.00 0.00

0.57 0.29 0.40 0.00 0.29 0.16 0.06 0.00 0.22 0.10 0.05 0.00 0.09 0.13 0.00 0.00 0.57 0.36 0.00 0.00 0.29 0.10 0.00 0.00 0.22 0.08 0.00 0.00 0.09 0.05 0.00 0.00

50

100

150

40 °C

0

50

100

150

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.003 0.001 0.000 0.001 0.003 0.002 0.129 0.001 0.006 0.002 0.002 0.001 0.004 0.006 0.002 0.001 0.003 0.001 0.001 0.002 0.003 0.006 0.001 0.001 0.006 0.005 0.001 0.001 0.004 0.002 0.003 0.001

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.004 0.004 0.001 0.000 0.006 0.005 0.003 0.001 0.004 0.004 0.002 0.000 0.004 0.012 0.001 0.001 0.004 0.003 0.000 0.001 0.006 0.006 0.001 0.002 0.004 0.008 0.001 0.001 0.004 0.004 0.000 0.001

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.006 0.005 0.005 0.001 0.003 0.005 0.003 0.001 0.004 0.004 0.003 0.001 0.002 0.006 0.004 0.001 0.006 0.002 0.001 0.001 0.003 0.005 0.001 0.001 0.004 0.002 0.001 0.001 0.002 0.007 0.000 0.001

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.004 0.003 0.003 0.000 0.004 0.006 0.004 0.001 0.002 0.006 0.005 0.001 0.004 0.006 0.002 0.001 0.004 0.003 0.001 0.001 0.004 0.004 0.001 0.000 0.002 0.004 0.002 0.001 0.004 0.003 0.001 0.001

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.006 0.008 0.003 0.001 0.006 0.011 0.001 0.001 0.005 0.004 0.001 0.001 0.003 0.006 0.006 0.001 0.006 0.527 0.001 0.001 0.006 0.006 0.000 0.001 0.005 0.004 0.002 0.000 0.003 0.005 0.001 0.001

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.08 0.02 0.02 0.00 0.02 0.04 0.01 0.00 0.02 0.03 0.01 0.00 0.01 0.01 0.00 0.00 0.08 0.04 0.00 0.00 0.02 0.01 0.00 0.00 0.02 0.02 0.00 0.00 0.01 0.01 0.00 0.00

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.02 0.00 0.01 0.02 0.01 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.02 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00

X ± standard deviation, n = 2.

milk powders with increasing cycle number. Steep increases in cake height ratio indicate that powder sample has a high susceptibility to cake formation (Benković et al., 2017). This suggested that all SMP samples were prone to cake formation during first 30 days of storage. These results were in agreement with previous studies who reported that SMP showed increasing trend in cake height ratio (Bansal et al., 2017; Benković, Srečec, et al., 2013). The caking ability of milk powders can be differentiated depending on cake height ratio due to the fact that higher cake height ratio tends to quick and easy cake formation in powders and vice versa (Ramavath, Swathi, Buchi Suresh, & Johnson, 2013). After 30 days, cake height ratios of samples showed a relatively

lumps, then into an agglomerated and sticky undesirable material, resulting in decreasing quality and functionality of powder (Aguilera, del Valle, & Karel, 1995). Occurred materials during caking ranged from small and soft aggregates to hard lumps resulting in loss of flowability. Caking susceptibility of a powder can give a crucial information on the powder flow properties of milk powders. Table 2 shows cake height ratios for each cycle, cake strength and mean cake strength of skim milk powders. Cake height ratios provide data on the extent to which a cake is formed. A powder that has a high tendency to cake would show a strong increase in cake height ratio. First 30 days of storage, there was an increase in cake height ratio of all 283

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Table 3 The results of cohesion and cohesion at four speeds analyses for skim milk powders during storage. Temp.

HPH (MPa)

Storage (day)

Cohesion coefficient (mN.m)

Cohesion index (mm)

Cohesion index 10 mm/s (mm)

Cohesion index 20 mm/s (mm)

Cohesion index 50 mm/s (mm)

Cohesion index 100 mm/s (mm)

20 °C

0

0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180

−6.13 ± 0.08 −10.47 ± 0.09 −11.37 ± 0.08 −8.41 ± 0.18 −4.98 ± 0.07 −10.41 ± 0.05 −7.97 ± 0.04 −5.07 ± 0.04 −9.89 ± 0.03 −10.08 ± 0.04 −9.58 ± 0.04 −5.74 ± 0.06 −6.51 ± 0.04 −12.45 ± 0.04 −8.84 ± 0.04 −6.19 ± 0.09 −6.13 ± 0.08 −13.61 ± 0.03 −9.81 ± 0.04 −10.31 ± 0.05 −4.98 ± 0.07 −8.72 ± 0.04 −6.59 ± 0.04 −5.74 ± 0.06 −9.89 ± 0.03 −9.52 ± 0.09 −8.80 ± 0.10 −6.71 ± 0.06 −6.51 ± 0.04 −11.01 ± 0.08 −6.96 ± 0.13 −6.38 ± 0.05

8.71 ± 0.40 13.06 ± 0.21 15.29 ± 0.08 12.65 ± 0.21 6.80 ± 0.07 11.88 ± 0.17 10.61 ± 0.27 7.41 ± 0.13 11.06 ± 0.20 11.75 ± 0.21 11.38 ± 0.11 7.83 ± 0.11 8.18 ± 0.08 13.72 ± 0.09 10.74 ± 0.34 8.34 ± 0.19 8.71 ± 0.40 16.71 ± 0.03 13.68 ± 0.10 15.74 ± 0.22 6.80 ± 0.07 10.64 ± 0.18 9.03 ± 0.10 8.13 ± 0.11 11.06 ± 0.20 10.79 ± 0.09 10.54 ± 0.09 8.81 ± 0.13 8.18 ± 0.08 12.49 ± 0.16 9.27 ± 0.09 8.52 ± 0.09

8.95 ± 0.22 15.07 ± 0.33 22.65 ± 0.21 15.43 ± 0.11 6.39 ± 0.07 12.04 ± 0.23 11.34 ± 0.09 8.32 ± 0.05 10.57 ± 0.47 11.89 ± 0.15 12.43 ± 0.24 8.95 ± 0.08 7.73 ± 0.06 13.12 ± 0.25 11.82 ± 0.12 9.11 ± 0.16 8.95 ± 0.22 21.90 ± 0.15 15.71 ± 0.13 19.74 ± 0.37 6.39 ± 0.07 10.19 ± 0.16 10.12 ± 0.11 9.35 ± 0.08 10.57 ± 0.47 10.61 ± 0.13 11.81 ± 0.13 9.98 ± 0.03 7.73 ± 0.06 11.77 ± 0.19 10.25 ± 0.13 9.61 ± 0.15

9.14 ± 0.25 15.14 ± 0.22 21.24 ± 0.09 14.74 ± 0.12 6.51 ± 0.09 12.69 ± 0.08 11.71 ± 0.27 8.64 ± 0.15 10.63 ± 0.52 12.18 ± 0.17 12.48 ± 0.16 8.96 ± 0.06 7.84 ± 0.09 13.73 ± 0.24 12.24 ± 0.09 9.31 ± 0.29 9.14 ± 0.25 21.42 ± 0.54 15.70 ± 0.28 17.95 ± 0.21 6.51 ± 0.09 10.39 ± 0.15 10.33 ± 0.17 9.49 ± 0.16 10.63 ± 0.52 10.82 ± 0.12 11.90 ± 0.14 10.05 ± 0.04 7.84 ± 0.09 12.18 ± 0.10 10.29 ± 0.27 9.82 ± 0.17

8.20 ± 0.09 13.83 ± 0.23 16.52 ± 0.11 11.83 ± 0.10 6.38 ± 0.04 12.88 ± 0.16 10.94 ± 0.08 7.78 ± 0.17 10.71 ± 0.12 11.99 ± 0.16 11.77 ± 0.19 8.08 ± 0.17 7.97 ± 0.25 13.84 ± 0.23 11.31 ± 0.13 8.55 ± 0.07 8.20 ± 0.09 18.75 ± 0.21 13.75 ± 0.21 14.69 ± 0.44 6.38 ± 0.04 10.68 ± 0.16 9.49 ± 0.37 8.21 ± 0.24 10.71 ± 0.12 10.56 ± 0.05 10.80 ± 0.15 8.98 ± 0.03 7.97 ± 0.25 12.20 ± 0.14 9.40 ± 0.14 8.83 ± 0.08

8.62 ± 0.10 13.91 ± 0.11 15.33 ± 0.81 10.83 ± 0.09 6.68 ± 0.07 12.94 ± 0.23 10.60 ± 0.14 7.54 ± 0.09 10.81 ± 0.13 11.94 ± 0.22 11.62 ± 0.25 7.75 ± 0.03 8.55 ± 0.10 14.7 ± 0.28 11.59 ± 0.08 8.84 ± 0.19 8.62 ± 0.10 17.67 ± 0.32 12.70 ± 0.29 11.87 ± 0.18 6.68 ± 0.07 10.89 ± 0.16 8.85 ± 0.08 8.21 ± 0.09 10.81 ± 0.13 10.92 ± 0.11 10.40 ± 0.15 8.84 ± 0.23 8.55 ± 0.10 13.09 ± 0.16 9.52 ± 0.17 9.14 ± 0.20

50

100

150

40 °C

0

50

100

150

X ± standard deviation, n = 2.

stable trend when compared to first 30 days. At 90. and 180. days, skim milk powders produced from HPH treated milk concentrates showed no tendency of caking. HPH treatment, storage day and storage temperature strongly affected cake strength and mean cake strength of SMP samples (P < 0.01). Cake strength means that more work was performed by the blade to cut the formed cake in the cylinder. During storage, cake strength and mean cake strength of SMP samples ranged from 08.20 mN.m and 0–0.57 N, respectively. The highest cake strength (8.20 mN.m) was determined in SMP-0 at 0. day, whereas cake strength values of SMP samples produced from HPH treated concentrates at ranged from 0.43-2.70 mN.m at 0. day. Cake strength and mean cake strength values of all samples were at 0. day were significantly higher than other storage periods. In general, cake strength values of samples decreased depending on increasing storage period. In addition, cake did not form at 90. days of storage at 40 °C and 180. days of storage at 20 and 40 °C. The results revealed that HPH treatment decreased caking strength values of SMPs depending on increasing pressure. Also, cake strength values at 40 °C were lower than 20 °C. This may be related that lactose crystalizes at the higher temperature of 40 °C and thus it becomes non-sticky (Fitzpatrick, O'Callaghan, & O'Flynn, 2008). As expected, similar trends were observed in mean cake strength values of SMP samples. Skim milk powders generally show free flowing flow behavior. However, they can lose free flowing behavior during storage, handling or transportation. This situation is generally caused by cake formation. Therefore, cake formation in milk powders is a significant problem in the dairy industry. Results of this study showed that HPH treatment of skim milk concentrates prior to spray drying could reduce cake formation. Therefore, HPH treatment can be used against cake formation of skim milk powders in the dairy industry. The powder particles tendency for cling together and form greater

particles cluster is called cohesiveness. Cohesion coefficient, cohesion index and cohesion index at four speeds of skim milk powders are presented in Table 3. HPH treatment, storage temperature and storage day significantly affected cohesion coefficients of SMP samples (P < 0.01). Cohesion coefficient, which is the necessary effort in order to lift the powder through the powder column by the blade, is calculated from the negative region below the force-distance curve. A more cohesive powder has a largely negative force in the plotted data due to reducing the force exerted on the bottom of the vessel. Therefore, decreasing the cohesion coefficient indicates increasing cohesiveness. At the beginning of the storage, SMP-100 had the lowest cohesion coefficient (−9.89 mN.m) whereas SMP-50 had the highest one (−4.98 mN.m). In general, cohesion coefficients decreased up to 30 days of storage and they increased after 30 days, except SMP-0. Cohesion coefficient is generally affected by the sample weight and bulk density (0.724–0.814 g/cm3) of skim milk powder sample. In quality control of powders, the cohesion index is useful analysis due to removing the effect of sample weight. There is a reverse relationship between the cohesion and flowability of a milk powder sample. As shown in Table 1, flow behavior of powders is classified depending on the cohesion index. Cohesion index values of samples were influenced by HPH treatment and storage day (P < 0.01). However, storage temperature did not significantly affect cohesion index of SMP samples (P > 0.05). During the storage, the cohesion index of SMP samples ranged from 6.8 to 16.71 mm. Therefore, samples had three different flow behaviors including free-flowing, easy flowing and cohesive. The results showed that the lowest and highest cohesion index was determined in SMP-50 and SMP-0 at 0. day, respectively. As expected, cohesion index values of samples showed a similar trend to cohesion coefficients. During storage, cohesion index value of control sample ranged from 8.71 to 16.71 mm. Cohesion index value of SMP obtained by powder flow analyzer was 10.11–13.56 mm (Bansal et al., 284

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Table 4 The results of powder flow speed dependency (PFSD) analysis for skim milk powders during storage. Temp.

HPH (MPa)

Storage (day)

Flow stability

Cohesion coefficient 50 mm/s (mN.m)

Compaction coefficient 10 mm/s (mN.m)

Compaction coefficient 20 mm/s (mN.m)

Compaction coefficient 50 mm/s (mN.m)

Compaction coefficient 100 mm/s (mN.m)

20 °C

0

0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180 0 30 90 180

0.95 1.12 1.07 1.15 0.97 1.00 1.01 1.05 0.97 0.99 0.98 1.02 0.97 0.95 0.98 0.99 0.95 1.07 1.06 1.07 0.97 0.98 1.02 1.04 0.97 0.98 1.00 1.03 0.97 0.96 0.99 1.01

−5.41 ± 0.10 −10.63 ± 0.20 −12.35 ± 0.56 −7.08 ± 0.09 −4.35 ± 0.05 −10.3 ± 0.61 −8.10 ± 0.18 −5.21 ± 0.12 −9.05 ± 0.11 −9.94 ± 0.19 −9.50 ± 0.10 −5.68 ± 0.12 −5.86 ± 0.08 −11.32 ± 0.75 −8.71 ± 0.12 −6.09 ± 0.15 −5.41 ± 0.10 −12.31 ± 2.35 −9.05 ± 0.16 −7.97 ± 0.18 −4.35 ± 0.05 −8.45 ± 0.12 −6.68 ± 0.12 −5.86 ± 0.07 −9.05 ± 0.11 −8.55 ± 0.31 −8.95 ± 0.13 −6.72 ± 0.21 −5.86 ± 0.08 −9.94 ± 0.28 −7.00 ± 0.19 −6.61 ± 0.11

69.83 74.86 71.29 38.74 57.24 86.30 62.07 39.47 86.22 88.33 63.14 45.54 58.97 90.28 56.88 40.36 69.83 80.93 55.21 52.32 57.24 82.91 51.46 46.49 86.22 76.87 63.67 50.27 58.97 85.12 52.25 44.52

65.75 79.65 72.16 36.73 55.21 84.64 59.12 37.66 85.88 87.66 59.78 44.63 56.86 88.31 53.73 38.74 65.75 83.15 52.67 51.01 55.21 80.97 48.38 44.12 85.88 73.83 60.81 49.33 56.86 81.97 48.07 40.70

49.48 61.25 54.59 30.97 44.95 68.87 46.05 30.34 76.05 74.16 48.56 38.36 51.43 77.41 47.33 34.45 49.48 64.83 39.98 39.58 44.95 64.79 37.50 34.19 76.05 59.74 49.78 41.49 51.43 70.15 39.58 32.36

34.12 41.75 37.15 24.12 33.73 49.96 32.67 22.82 60.25 55.62 36.85 30.75 41.40 62.23 37.96 27.61 34.12 45.20 28.05 28.83 33.73 46.88 27.49 24.99 60.25 44.39 39.57 33.51 41.40 55.17 30.74 24.08

50

100

150

40 °C

0

50

100

150

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.07 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.70 0.46 0.42 0.69 0.52 0.28 0.41 0.35 0.37 0.35 0.53 0.41 0.18 0.63 0.69 1.60 0.70 4.02 0.41 0.48 0.52 0.64 0.66 0.57 0.37 0.72 0.10 0.36 0.18 1.11 0.39 0.34

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.34 0.31 0.58 0.29 0.41 0.43 0.40 0.55 0.45 0.56 0.22 0.43 0.09 0.35 0.42 0.42 0.34 5.06 0.40 0.16 0.41 0.46 0.46 0.13 0.45 0.39 0.27 0.42 0.09 0.81 0.30 0.38

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.43 0.29 0.46 0.46 0.26 0.32 0.22 0.19 0.44 0.51 0.86 0.16 0.28 1.29 0.36 0.18 0.43 3.01 0.49 0.50 0.26 0.09 0.33 0.19 0.44 0.44 0.32 0.30 0.28 0.73 0.50 0.22

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.41 0.26 0.16 0.34 0.35 0.48 0.44 0.37 0.35 0.44 0.37 0.36 0.29 0.63 0.13 0.21 0.41 1.50 0.47 0.55 0.35 0.29 0.19 0.34 0.35 0.37 0.76 0.24 0.29 0.35 0.47 0.41

X ± standard deviation, n = 2. Table 5 Spearman rank order correlations for powder flow properties of skim milk powders. Powder flow properties

Cake strength (mN.m)

Mean cake strength (N)

Cohesion coefficient (mN.m)

Cohesion index (mm)

Flow stability

Cake strength Mean cake strength Cohesion coefficient Cohesion index Flow stability

1.000 0.648** 0.184 −0.215 0.025

0.648** 1.000 −0.163 0.052 −0.309*

0.184 −0.163 1.000 −0.943** −0.142

−0.215 0.052 −0.943** 1.000 0.272*

0.025 −0.309* −0.142 0.272* 1.000

**Correlation is significant at the p < 0.01 level. *Correlation is significant at the p < 0.05 level.

food powders (Teunou & Fitzpatrick, 1999). Cohesion directly affects powder flowability of milk powders and powder flow behavior is one of the most important characteristics of milk powders. When a milk powder is cohesive, some problems may occur in handling and transporting in the dairy industry. Results of this study revealed that SMP samples produced from HPH treated milk concentrates had lower cohesion index values after 180 days of storage when compared to control (SMP-0). This situation could be important for decreasing cohesion of skim milk powders in the dairy industry. Cohesion at four speeds analysis measures the flow characteristics depending on flow rate of powders. It quantifies resistance of skim milk powders related to controlled flow, which consist of four different speeds. Cohesion index of skim milk powders was determined at 10, 20, 50, and 100 mm/s flow speeds in cohesion at 4 speeds analysis. These speeds simulate some conditions, which may occur during transportation and handling of milk powders in industry. HPH treatment, storage time and storage temperature significantly affected cohesion index at four different speeds (P < 0.01). Cohesion at four speeds analysis revealed that all milk powders from HPH treated milk concentrates ranged from 6.39-14.7 mm. Therefore, they showed free-flowing and

2017; Benković, Srečec, et al., 2013). However, there was no data obtained by powder flow analyzer on flow characteristics of SMP depending on storage period. Duncan statistical analysis indicated that skim milk powders produced from HPH treated milk concentrates had free-flowing characteristics when compared to control samples. At the end of the storage (180. day), all samples produced from HPH treated milk concentrates showed a free-flowing flow behavior and the lowest cohesion index was determined in SMP-50 at 20 °C. However, SMP-0 at 40 °C (180. day) showed cohesive flow behavior. Cohesion is affected by numerous physical parameters including electrostatic activity, particle size and shape, porosity and hygroscopicity, etc. (Thomas, Scher, Desobry-Banon, & Desobry, 2004). When cohesion index values at 180. day was evaluated, there was an increase in the cohesion index of skim milk powders related to increasing storage temperature. These results were in agreement with Fitzpatrick, Iqbal, Delaney, Twomey, and Keogh (2004) who revealed that increasing storage temperature for SMP decreased flow index due to the increase of lactose thermoplasticity depending on storage temperature. However, it was reported that storage temperature (0–40 °C) of powders generally has no significant effect on the flow behavior of 285

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Fig. 6. Scanning electron micrographs of skim milk powders (a; SMP-0, b; SMP-50, c; SMP-100, d; SMP-150).

than 1, therefore a flow stability higher or less than 1 would show that the powder shows some changes during testing. These modifications may be related to the breaking down of agglomerates or attrition of the milk powder particles themselves. As expected, cohesion coefficients at 50 mm/s determined by PFSD test of SMP samples were in accordance with cohesion coefficient determined by cohesion test. Cohesion coefficients at 50 mm/s of SMP samples decreased up to 30. day of storage and then increased except SMP-0 at 20 °C and SMP-100 at 40 °C. In milk powder samples, an increase in electrostatic forces or cohesiveness results in a higher negative area in force vs distance curves and thus cohesion coefficient of samples increase. HPH treatment, storage day and storage temperature significantly affected compaction coefficients of samples at different speeds (P < 0.01). Compaction coefficients of SMP samples varied from 22.82-90.28 mN.m during 180 days of storage at 20 and 40 °C. Compaction coefficients of samples increased up to 30. day and then decreased, except some situations. Storage temperature significantly affected the compaction coefficients of samples (P < 0.01) and at the end of storage, compaction coefficients of SMP samples stored at 20 °C were lower than those of 40 °C. Significant differences in compaction coefficients were observed between 10 mm/s and 100 mm/s. The results indicated that compaction coefficients of all SMP samples decreased depending on increasing flow speeds. It means that all skim milk powder samples became more free-flowing with increasing flow speeds. It may be advantageous in a production environment of the dairy industry to convey this powder at a higher speed as less work is required to flow at higher speeds. A similar trend in compaction coefficients of SMP was reported by Bansal et al. (2017). According to results, at the highest PFSD test speed of 100 mm/s can be the easiest speed to convey of SMP samples. In order to determine the effect of powder flow properties on each other, the results of Spearman rank order correlation test are presented in Table 5. As expected, there was a significant correlation between cake strength and mean cake strength (P < 0.01). However, cake strength of SMP samples did not significantly affect cohesion coefficient, cohesion index and flow stability (P > 0.05). Opposite to these results, previous studies reported that significant correlation between cake strength, mean cake strength and cohesion index in powder flow analysis of different food powders (Benković et al., 2017; Janjatović et al., 2012). On the other hand, flow stability was negatively influenced by mean cake strength (P < 0.05). There was a negative correlation between cohesion coefficient and cohesion index as expected

easy flowing behavior at four different speeds during all storage period and temperature (Table 3). However, SMP-0 (both 20 °C and 40 °C) showed cohesive and extremely cohesive flow behavior, especially after 30 days of storage at slower rates. While cohesion index values of SMP0 relatively decreased depending on increasing flow rate except for some situations, cohesion indexes of skim milk powders from HPH treated milk concentrates showed almost stabile flow behavior related to flow rate. SMP from HPH treated milk concentrates were more flowable at even lower flow speeds then SMP-0. It is important for the dairy industry. HPH can make easier transporting of skim milk powders. Surprisingly, in some situations, milk powders produced from HPH treated showed more cohesive flow behavior at 20 °C when compared to 40 °C. Powder flow speed dependency (PFSD) analysis determined the flow behavior of skim milk powder samples depending on flow speed. This can be important in a production and transporting. The PFSD analysis can evaluate the suitableness of skim milk powder for different conveying speeds or quantifies a milk powder for differences in speed powder flow properties from batch to batch. In addition, PFSD analysis can provide crucial information on the attrition properties of milk powders. The PFSD analysis was carried out at 20, 20, 50 and 100 mm/s flow speeds since these conditions can simulate various industrial applications such as transporting and handling of milk powders. Flow stability, cohesion coefficient at 50 mm/s, compaction coefficients at 20, 20, 50 and 100 mm/s flow speeds determined by PFSD test are presented in Table 4. While HPH treatment and storage day significantly affected flow stability (P < 0.01), storage temperature did not strongly affect it (P > 0.05). Flow stability of powders provides crucial data on the flow resistance of the milk powder samples and gives an indication of the susceptibility of the product to attrition (breakdown). Flow stability of powder compares required work to move the rotor through the powder at the beginning and at the end of analysis at the same speed. Flow stability of SMP samples ranged from 0.95 to 1.15 during storage. HPH treatment decreased flow stability of SMP samples. Flow stability of SMP-0 increased during storage at both 20 and 40 °C and this increase was higher at 20 °C. However, SMP from HPH treated milk concentrates, especially SMP-100 and SMP-150 showed more stable flow stability compared to SMP-0. When flow stability is close to 1.00, the milk powder does not change significantly throughout the analysis (Benković, Belščak-Cvitanović et al., 2013). Most of the powders had a flow stability that was found to be different 286

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(P < 0.01). In addition, flow stability of skim milk powders significantly affected mean cake strength and cohesion index (P < 0.05). Scanning electron micrographs of skim milk powders are presented in Fig. 6. There was some shallow wrinkles and minute pores on the surface of particles. Also, relatively smooth particle surface was present. Depending on increasing HPH pressure, particle size in micrographs decreased. Beside this, HPH resulted in more homogenous particle structure. Surface morphology could result in an influence on the flow properties of skim milk powders.

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4. Conclusions In the present study, powder flow properties of skim milk powders produced from high-pressure homogenization treated milk concentrates were investigated during 180 days of storage at 20 and 40 °C. Caking, cohesion and powder flow speed dependency analyses of SMP samples were carried out by a Powder Flow Analyzer. HPH treatment significantly affected powder flow properties of SMPs. HPH treatment decreased cohesion and caking properties of SMP samples when compared to the control sample (SMP-0). SMP produced from HPH treated milk concentrates showed free and easy flowing flow behavior during storage at 20 and 40 °C. However, SMP-0 showed cohesive characteristics in some statements. In addition, cohesion index values were determined depending on flow speeds. SMP from HPH treated milk concentrates were more flowable at even lower flow speeds then SMP-0. PFSD test verified that all SMP samples showed more free-flowing characteristics related to increasing flow speeds. The results of present study revealed that HPH treatment to milk concentrates could improve flow properties of skim milk powders. In addition, Powder Flow Analysis was an effective method in order to characterization the powder flow properties of skim milk powders. Conflicts of interest The authors declare that there are no conflicts of interest. Human and animal rights This article does not contain any studies with human or animal subjects. References Abu-hardan, M., & Hill, S. E. (2010). Handling properties of cereal materials in the presence of moisture and oil. Powder Technology, 198(1), 16–24. https://doi.org/10. 1016/j.powtec.2009.10.002. Aguilera, J., del Valle, J., & Karel, M. (1995). Caking phenomena in amorphous food powders. Trends in Food Science & Technology, 6(5), 149–155. https://doi.org/10. 1016/S0924-2244(00)89023-8. Amaral, G. V., Silva, E. K., Cavalcanti, R. N., Martins, C. P. C., Andrade, L., Moraes, J., et al. (2018a). Whey-grape juice drink processed by supercritical carbon dioxide technology: Physicochemical characteristics, bioactive compounds and volatile profile. Food Chemistry, 239, 697–703. https://doi.org/10.1016/j.foodchem.2017.07. 003. Amaral, G. V., Silva, E. K., Costa, A. L. R., Alvarenga, V. O., Cavalcanti, R. N., Esmerino, E. A., et al. (2018b). Whey-grape juice drink processed by supercritical carbon dioxide technology: Physical properties and sensory acceptance. Lebensmittel-Wissenschaft & Technologie, 92, 80–86. https://doi.org/10.1016/j.lwt.2018.02.005. Bansal, V., Premi, M., Sharma, H. K., & Nanda, V. (2017). Compositional, physical, functional attributes and flow characterization of spray-dried skim milk powder enriched with honey. [journal article]. Journal of Food Measurement and Characterization, 11(3), 1474–1485. https://doi.org/10.1007/s11694-017-9526-1. Benkovic, M., & Bauman, I. (2009). Flow properties of commercial infant formula powders. World Academy of Science, Engineering and Technology, 54(6), 495–499. Benković, M., Belščak-Cvitanović, A., Bauman, I., Komes, D., & Srečec, S. (2017). Flow properties and chemical composition of carob (Ceratonia siliqua L.) flours as related to particle size and seed presence. Food Research International, 100(Part 2), 211–218. https://doi.org/10.1016/j.foodres.2017.08.048. Benković, M., Belščak-Cvitanović, A., Komes, D., & Bauman, I. (2013a). Physical properties of non-agglomerated cocoa drink powder mixtures containing various types of sugar and sweetener. [journal article]. Food and Bioprocess Technology, 6(4), 1044–1058. https://doi.org/10.1007/s11947-011-0742-0.

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