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A Taguchi approach optimization of date powder production by spray drying with the aid of whey protein-pectin complexes
Journal Pre-proof A Taguchi approach optimization of date powder production by spray drying with the aid of whey protein-pectin complexes Sedighe Moghbeli, Seid Mahdi Jafari, Yahya Maghsoudlou, Danial Dehnad PII:
S0032-5910(19)30847-2
DOI:
https://doi.org/10.1016/j.powtec.2019.10.013
Reference:
PTEC 14761
To appear in:
Powder Technology
Received Date: 6 December 2018 Revised Date:
1 October 2019
Accepted Date: 4 October 2019
Please cite this article as: S. Moghbeli, S.M. Jafari, Y. Maghsoudlou, D. Dehnad, A Taguchi approach optimization of date powder production by spray drying with the aid of whey protein-pectin complexes, Powder Technology (2019), doi: https://doi.org/10.1016/j.powtec.2019.10.013. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A Taguchi approach optimization of date powder production by spray drying with the aid of whey protein-pectin complexes
Graphical Abstract
Scanning Electronic Microscopy (SEM) analysis of date powder microstructures
1
A Taguchi approach optimization of date powder production by spray drying with
2
the aid of whey protein-pectin complexes
3
Running title: Production of date powder by biopolymer complexes
4 5
Sedighe Moghbeli, Seid Mahdi Jafari*, Yahya Maghsoudlou, Danial Dehnad
6 7 8
Faculty of Food Science and Technology, Gorgan University of Agricultural Sciences and Natural
9
Resources, Gorgan, Iran *Corresponding author: [email protected]
10 11 12
Abstract
13
In this research, date powder was produced by drying aids including Tween 80, pectin, and whey
14
protein concentrate (WPC) at different pH values (5.0, 6.5, 8.0, and 9.5) and using a spray drier at
15
different temperatures (160, 170, 180, and 190°C); then moisture content, solubility, hygroscopicity,
16
bulk density, and total phenolic compounds (TPC) were determined and their correlation with structural
17
characteristics of date powder was analyzed by Taguchi method. Interactions of pectin-surfactant, and
18
pectin-WPC were more effective on moisture and bulk density of date powder, respectively. Although
19
hygroscopicity of different treatments was in narrow range of 25-29%, temperature had the highest
20
impact on TPC; beyond 170°C, TPC of date powder decreased. Treatment No. 7, with the highest WPC
21
level and pH values of 5, maintained the highest TPC content (701 mg 100 g-1) in date powder. SEM
22
images revealed that pH=9.5 and lower temperatures led to smaller particles.
23 24
Keywords: Date powder; Biopolymers; Nutritional content; Spray drying; Optimization. 1
25
1. Introduction
26
Date palm (Phoenix dactylifera L.) is considered a major fruit crop in the hot desert regions (semi-arid
27
and arid lands) of the world. The main component of date is carbohydrate (70–80%) most of which are
28
in the form of glucose and fructose; it is estimated that 100 g of this fruit crop can provide > 300 kcal
29
of energy. Date also contains proteins, pectin, lipids, salts, and minerals. The annual per capita
30
consumption of dates in southern provinces of Iran is 25 kg while the approximate value of only 100 g
31
is recorded for the European Union in 2012 [1]. Despite cultivating date from many years ago, date-
32
related industrial activities have not advanced as much as other agricultural produces. On the other
33
hand, consuming sugar-related products enhances the risk of overweight and other prevalent problems
34
e.g. high blood fat, or high blood pressure. Indeed, due to abundance of sugar in different date varieties,
35
substituting it for common white sugar in formulation of food products is one of the various ways for
36
applying date in the food industry.
37
Date paste, date syrup, nectar date, juice date, date seed flour, and fermented date concentrate are the
38
important foodstuffs that can be obtained from dates. Date powder is a new product, obtained from
39
drying and milling of date pulp or its syrup. In fact, date powder could be produced in several ways
40
most of which are based on application of date pulp, mixing it with anti-caking agents, drying of the
41
complex with vacuum oven dryers, and finally, date powder production through milling process.
42
Production of date powder has been the focus of attention from researchers and producers in recent
43
years. The common target of most of these researchers was increasing shelf-life and dwindling date
44
waste although most of commonly-used methods require a long time and result in poor quality
45
products.
46
Spray drying remains to be one of the popular approaches for producing fruit juice powder; in detail,
47
some chief merits of this method are being economical, hygienic, and short contact duration required
48
[2]. The latter advantage fosters retaining nutritional values of original produce as well as high quality 2
49
of final powder. Drying of sticky products, such as fruit and vegetable juices, seems impractical on
50
account of their stickiness to dryer wall and agglomeration, reducing production efficiency; the
51
underlying reason is their low molecular weight sugars e.g. fructose, glucose, sucrose, and some
52
organic acids which have low glass transition temperature [3]. However, this difficulty could be
53
overcome through adding some biopolymers e.g. carbohydrates (starch and maltodextrin), gums,
54
proteins, or their mixture to soluble feed before atomizing it [4]. Actually, these compounds increase
55
glass transition temperature and diminish stickiness of powder by creating a physical barrier among
56
particles and competing with particles for water absorption [5, 6].
57
One of biopolymers used extensively in food industry in view of its film formation abilities and high
58
glass transition temperature is maltodextrin. Glass transition temperatures of maltodextrins are varied
59
according to dextrose equivalent (DE) degree; in fact, the lower the dextrose equivalent degree of the
60
maltodextrin, the higher the glass transition temperature of the complex; the average temperature could
61
be considered around 205°C. Nonetheless, high volume of maltodextrin could affect not only product
62
quality, but also consumer/market acceptance. So, the alternative of modifying surface properties of
63
particles by proteins was put forward. As a matter of fact, considerably lower protein complexes are
64
required to convert sugar-rich products to powder. For instance, only 0.13% calcium caseinate and
65
whey protein is required to powder a rich-sugar food model while higher than 40% maltodextrin
66
(DE=6) should be provided to obtain the same performance [7]. In other words, protein products
67
escalate powder production by reducing surface tension among particles. Other components which
68
could reduce surface tension of the solution considerably are surfactants: emulsifiers which are small in
69
dimension but have high surface activities [8]. Low molecular weight surfactants are smaller than
70
proteins and, as a result, could be lodged in particles surface more extensively [9]. But, due to low glass
71
transition temperature, surfactants could not be applied alone for covering particles surfaces.
3
72
Pectin is a type of polysaccharide composing of galacturonic acid units through α (1-4) bonds and
73
connect protein components to create protein-polysaccharide complexes. Effective deployment of
74
synergistic interactions between proteins and polysaccharides in food systems, such as emulsions and
75
foams, has been attended especially in recent decades [10,11]. Polysaccharides could contribute to
76
higher physical stability, higher viscosity of aqueous phase and changing rheological properties at
77
surface [12]. Size, charge and stability of biopolymers depend on protein-polysaccharide ratios,
78
polysaccharide type, ionic power, and thermal processing conditions. At very higher pH values than
79
isoelectric point, there are electrostatic repulsive forces between proteins and anionic polysaccharides
80
since proteins own high negative charges. On the other hand, at equal pH values to isoelectric point,
81
cationic regions of protein surfaces react with anionic groups of polysaccharide chains, resulting in
82
formation of soluble complexes [13]. pH values below isoelectric point lead to coacervation as the
83
result of electrostatic reactions between protein and polysaccharide molecules. Once reaching lower pH
84
values than pKa point of anionic groups of polysaccharide chains, attractive reactions between proteins
85
and polysaccharides weaken and the complex dissociate [14].
86
Taguchi design is used to find the impact of different factors on product properties and determine
87
optimal conditions of factors in the field of engineering [15]. As an alternative to full factorial design, it
88
decreases the number of experiments required, is more straightforward to use, faster and
89
simultaneously precise, and reliable, saves time and reduce costs [16]. Two tools of analysis of this
90
method are orthogonal arrays and ANOVA. While ANOVA evaluates the impact of a factor on
91
characteristics of the product, orthogonal arrays help to reduce replication of experiments.
92
There are researches dealt which with the issue of producing date powder by various types of driers, the
93
most significant results of which are discussed here. Sahari et al. [17] obtained date powder through
94
drying of date syrup in a vacuum oven and milling of dried date. Three date varieties, having different
95
sugar percentages, were examined to analyze the possibility of their drying under different vacuum 4
96
drying conditions (thickness, temperature, pressure, and time). The results showed that sugar
97
percentage and moisture content of date, date thickness and drying conditions were all effective on
98
powder properties and decreasing their moisture content. Drying at 85°C, 54.6 cm Hg, and 1 cm
99
thickness for 7 h led to achieving powder with desirable color and odor properties. In our previous
100
study, we evaluated microstructural and yield of date powder production through spray drying of date
101
syrup with complexes of whey protein- pectin and our results revealed that it could be a promising
102
approach [11]. Also, Sablani et al. [3] analyzed production of date powder by drying of combined
103
complex of date pulp and maltodextrin in a vacuum oven. They drew a conclusion that when date pulp
104
was mixed in 1:1 ratios to maltodextrin and dried, non-sticky powder with favorable flowability was
105
produced. In another study, physicochemical characteristics and sorption isotherm of date syrup
106
powder, as affected by maltodextrin incorporation, were evaluated by Farahnaky et al [18]. While
107
constant parameters of that research included the type of drier (twin drum drier), drying temperature
108
(130°C), and dextrose equivalent of maltodextrin (19), maltodextrin level was variable: 30, 40, 50 and
109
60%. They found that GAB (Guggenheim, Anderson, and de Boer) and Peleg models were more
110
suitable than BET model to fit moisture sorption data and type ш isotherms, representative of high-
111
sugar foods, were shown by date syrup powder. Besides, maltodextrin addition improved some product
112
characteristics by increasing its lightness and glass transition temperature and decreasing degree of
113
caking. However, as mentioned earlier, these methods require allocation of long time and excessive
114
usage of polysaccharide which, in turn, lessens final consumer acceptance. So, this project aimed to
115
take advantage of proteins, polysaccharides, surfactants and their combinations along with application
116
of spray drying equipment to optimize physicochemical properties of date powder.
117
2. Materials and method
118
Maltodextrin (DE=18), pectin with low methoxylation degree, whey protein concentrate (WPC) and
119
Tween 80 were bought from Sigma Co., Germany. Date of Kalute variety was purchased from palm5
120
grove in Jiroft city (Kerman, Iran) in September and kept in refrigerator (4°C) before preparation of its
121
syrup.
122
2.1. Date syrup preparation
123
First, date was cored, followed by milling via a miller (Kenwood, Japan). After that, hot water was
124
mixed with milled date two times of its weight and they were held in a water bath (Fan Azma Gostar,
125
Iran) at 70 °C for 30 min. Sieving was carried out by a 50 mesh sieve and total soluble solid was
126
increased from 25 to 35 °Brix by a rotary evaporator under vacuum at 70 °C.
127
2.2. Date syrup tests
128
2.2.1. Color measurement
129
Color parameters of date syrup were analyzed using the image analysis method. Plates (with 1 cm
130
height and 6 cm diameter) were filled with samples and their pictures were taken by a scanner (Scanjet
131
G2710, HP, USA) completely shielded by a black cover. The pictures were analyzed by the Image J
132
software (version 1.42e, Wayne Rasband, National Institutes of Health, USA) and RBG parameters of
133
the samples were converted to L*, a* and b* values; Additionally, the ratios of a* to b* values were
134
calculated to evaluate color quality of the samples [19,20].
135
2.2.2. Moisture content
136
Moisture content of date powder and date syrup was determined in the same way. 3-5 g of each sample
137
was placed in a pre-weighed crucible. Then, crucibles were dried in an oven (VO200, Memmert,
138
Germany) at 100-105°C for 24 h to a constant weight. After drying, crucibles were cooled in a
139
desiccator to room temperature, weighed and moisture content was determined according to the
140
following equation [21, 22]:
141
MC =
142
Where ܹଵ , ܹଶ and ܹଷ are sample weight, dish weight + sample prior to drying, and dish weight +
143
sample after drying, respectively.
ଵ(ௐమ ିௐయ ) ௐభ
6
144
2.2.3. Total Soluble Solids (TSS)
145
TSS (°Brix) index of each sample was read through a table refractometer (ABBE, CETI, Belgium) at
146
20°C and in three replications [19].
147
2.2.4. pH
148
This index was measured by a digital pH-meter (BEL, Italy) at 25°C. Before the experiment, the pH
149
meter was calibrated with commercial buffer solutions at pH 7.0 and 4.0 [23]. 10 g of date solutions
150
was weighed, and made to the volume in a 100 mL volumetric flask. Then, it was poured in a beaker
151
and stirred, followed by putting electrodes of pH meter in the beaker.
152
2.2.5. Ash
153
3-5 g of samples was weighed by a balance (Sartorius, Germany) in a pre-weighed crucible. Then, it
154
was dried at 100°C for 1 h and ignited in a muffle furnace (Nabertherm, Germany) at 550-600°C until
155
producing white ash. After cooling and reaching constant weight, ash content was calculated [24]:
156
Ash content (%) =
157
Where ܹଵ , ܹଶ and ܹଷ were weight of crucible plus sample, weight of crucible, and weight of crucible
158
having ash, respectively.
159
2.2.6. Total Phenolic Compounds (TPC)
160
For determination of TPC of date syrup and date powder, the same procedure was followed [25]. 20
161
mL of ethanol 80% was added to 2 g of sample in 50 mL centrifuge tubes and placed inside a water
162
bath at 40oC for 60 min. The samples were mixed using a vortex every 15 min to enhance extraction.
163
The extract was allowed to cool at room temperature and then centrifuged (Centrurion, Scientific Ltd.,
164
Canada) at 10,000 rpm for 15 min. The supernatant was collected and used for the determination of
165
total phenolic content. Total soluble phenolic were determined using the Folin-Ciocalteure reagent. The
166
date extract solution (1 mL) was mixed with 5 mL of the Folin-Ciocalteu reagent 10%; then, after 4
167
min, 4 mL of Na2CO3 7.5% was added and the complex was held in a dark room for 30 min and,
(ௐయ ିௐమ )×ଵ (ௐభ ିௐమ )
7
168
finally, its absorbance was measured at 765 nm using a spectrophotometer (PG Instruments Ltd., UK).
169
A standard curve was plotted using different concentrations of ascorbic acid and the total phenolic
170
compounds were expressed in ascorbic acid equivalents (AAE) in mg per 100g of sample.
171
2.3. Preparation of drying feed
172
Different ratios of pectin and WPC were mixed with each other proportionally, followed by stirring
173
through a heater-stirrer at 70 °C for 30 min and their pH values were adjusted; the solution was left to
174
remain at ambient temperature overnight for complex formation [11].
175
2.4. Spray drying
176
Maltodextrin was added to given weight of date syrup having adjusted TSS by 50% of its weight,
177
followed by adding pectin-WPC solutions and Tween 80 proportionally. Homogenization was carried
178
out at 10000 rpm for 5 min by a homogenizer (D-91126, Heidolph Instruments, Germany) and heating
179
to 70 °C before entering the drier. The size (height×width) of pilot plant spray drier (Azar Tank Tehran,
180
Iran) was 3×1.5 m2 and its atomizer nozzles were of twin fluid pressure type, with the diameter of drier
181
nozzles being 0.5 mm. In this research, temperature (70 °C) of feed stock and drier pressure (2 bar)
182
were kept constant. Different temperatures (the outlet air temperature was kept between 60 and 70 °C)
183
were selected for drying. Drying was carried out at air flow rate of about 0.1 m3/min and feed flow rate
184
of 450 mL/h. The obtained powder was collected from the drier in polyethylene bags, sealed and kept
185
in freezer before experiments.
186
2.5. Experiments on date powder
187
2.5.1. Solubility
188
Sample (0.5 g) was added to 50 mL of distilled water, stirred at 110 rpm for 30 min and then
189
centrifuged (Centrurion, Scientific Ltd., Canada) at 4000 rpm for 5 min. An aliquot (25 mL) of each
190
supernatant was removed, transferred to porcelain dishes, and dried to a constant weight in an oven at
8
191
105°C. The solubility index (%) was calculated as the ratio of the dried supernatant weight to the
192
amount of the original weight of sample (0.5 g) [26,27].
193
2.5.2. Hygroscopicity
194
Samples of each powder (approximately 1 g) were placed at 25°C into a container with NaCl saturated
195
solution (75.29% relative humidity) obtained by adding 268 g of NaCl to 750 mL of distilled water and
196
were weighed until equilibrium (after 1 week) [28]. Hygroscopicity was expressed as g of adsorbed
197
moisture per 100 g of powder (g/100 g).
198
2.5.3. Bulk density
199
The bulk density (ρb) of powder was measured by weighing 2 g of each sample and placing it into a 10
200
mL graduated cylinder. The cylinder was tapped by hand and the bulk density was calculated as the
201
ratio of the mass of powder contained in the cylinder to the volume occupied [28].
202
2.5.4. SEM
203
Date powders were gold-coated and analyzed by a SEM (SU3500, Hitachi, Japan) to investigate their
204
morphology. Accelerating voltage of 5 kV and magnificence of 500× were used to analyze microscopic
205
images of powders [25].
206
2.6. Statistical analysis
207
Taguchi method (total number of 16 treatments) was used for design of experiments, Qualitek software
208
for process optimization and Excel software for depicting figures. In brief, each of 5 variables we
209
intended to optimize its rate was applied in 4 levels and their ranges were as follows: 0.5-2.0% of
210
surfactant, 3-6% of pectin, 8-14% of WPC, pH values of 5.0-9.5, and 160-190°C; experiments were
211
analyzed according to Taguchi method to obtain optimum conditions of process, determine effects of
212
each factor on response, and estimate responses at optimum conditions. Signal to noise ratio,
213
proportion of control to noise factors which are uncontrollable, is the main approach applied in Taguchi
214
method to analyze results of experiments. Analysis of variance is another applied way to analyze the 9
215
results [29]. Lower moisture content and hygroscopicity, and higher bulk density, solubility, and total
216
phenolic compounds were considered as favorable conditions of date powder during optimization.
217
3. Results and discussion
218
Moisture, pH, TPC and ash of date syrup were 31%, 5.6, 723 (mg 100 g-1) and 5.6%; also, the color of
219
date syrup was darkish brown.
220
3.1. Moisture content of date powder
221
Moisture content of final powder plays an important role in its flowability, stickiness, and stability
222
during storage. Final moisture contents of date powder are displayed in Fig. 1A, varied in 1.4-3.4%
223
range, demonstrating effectiveness of the process. Treatment No. 5, with 1% of surfactant, 3% of
224
pectin, 10% of WPC at pH=8 and 190°C, had the highest moisture content whereas treatment No. 15,
225
having 2% of surfactant, 5% of pectin, 10% of WPC at pH=9.5 and 160°C, led to the lowest moisture
226
ratio. Our results showed that all variables were effective on final moisture content of powder (Fig. 1B)
227
although drying temperature, compared with other variables, left behind the highest impact. Attributed
228
to the formation of harsh crust around particles and reducing drying speed at the second drying stage,
229
increasing temperature lowered moisture content of powder. Likewise, Santana et al. [30] reported that
230
increase of temperature or drying speed creates a saturated environment with water vapor around
231
particles, restricting water transfer from particles and, as a result, hindering water removal. Similarly,
232
Frascareli et al. [31] stated that temperature increase culminated in higher moisture content of
233
encapsulated coffee oil obtained by spray drying. The same result was reported by Santhalakshmy et al.
234
[32] for Jamun powder. However, results of the study by Tuyen et al. [33] showed that increasing
235
drying temperature caused a drop in moisture content of the final product owning to more rapidly water
236
removal. These results are in agreement with the analyses of Goula et al. [34] and Chegini and
237
Ghobadian [35] and Osman and Endut [36] on tomato, orange juice, and pineapple juice, respectively.
238
Our results indicated that surfactant inclusion led to a decrease in moisture content of powder because 10
239
hydrophilic head of surfactant absorbs water molecules onto the surface, improving the dehydration
240
process. Also, Kaltsa et al. (2014) [37] expressed that using surfactants minimized particles
241
dimensions, boosting water exhaustion from particles. Jayasundera et al. (2011) [7] stated that non-
242
ionic surfactants (Tween 80), compared with ionic surfactants, made the water depletion from final
243
product more effective. Overall, Qualitek software indicated that optimum conditions for achieving
244
date powder with the least moisture content would be 3% of surfactant, 4% of pectin, 12% of WPC, at
245
pH value of 5 and temperature of 160°C. Fig. 1
246 247
3.2. Solubility of date powder
248
Solubility is an important index of food powder in aqueous environments. In fact, powder used in the
249
food industry should have high solubility. This factor is affected by different factors including main
250
ingredients, feed flow rate, carrier agents, and flow rate of the compressed air [38,34] (Bhandari et al.,
251
1997; Goula et al., 2004). Solubility values of date powder are presented in Fig. 2A. The highest
252
solubility belonged to treatment No. 10 with 1.5% of surfactant, 4% of pectin, 14% of WPC at pH
253
value of 8 and 180°C, and the lowest solubility to treatment No. 1. Analysis of variance showed that
254
pectin was the most effective factor (40.683%) and drying temperature the least one. The effect of
255
pectin could be due to its physical properties and solubility in water. In fact, higher pectin
256
concentrations reduced moisture content of powder, inducing faster water absorption [39]. Goula &
257
Adamopoulos [40] and Grabowski et al. [41] reported that an increase in carrier agent concentration,
258
due to its higher solubility, increased solubility of orange and sweet potato powder, respectively.
259
Fig. 2
260
Our results indicated lack of significant effect of temperature variations on solubility, in line with the
261
observation of Sharifi et al. [42] on barberry. But, Santhalakshmy et al. [32] reported a significant and
262
direct relationship between temperature and solubility, as opposed to the finding of Patil et al. [43] on 11
263
guava dried by spray drier. Higher WPC content than 10% decreased solubility, probably since
264
exposure of protein compounds to high temperature results in formation of non-soluble substances
265
undesirable in powder production. Our study on interactions among variables proved that interaction
266
between surfactant and pectin was more effective on increasing powder solubility than other ones (Fig.
267
2B). Optimum conditions for obtaining the highest solubility of date powder could be 1% of surfactant,
268
5% of pectin, 10% of WPC, at pH value of 8 and 170°C as revealed by Qualitek software.
269
3.3. Hygroscopicity of date powder
270
Lower hygroscopicity could be more favorable when storing or displacing powder as this factor is
271
effective on their flowability. The values are represented in Table 1. The results of analysis of variance
272
showed that temperature had the highest impact on hygroscopicity changes. Increasing the temperature
273
beyond 180°C led to a significant decrease in this parameter (Fig. 3). It could be interpreted that there
274
was a relationship between moisture content and hygroscopicity of powder. Date powder with the
275
lowest moisture content owned the highest hygroscopicity, due to the higher capacity for moisture
276
absorption from the environment [44]. Also, low hygroscopicity could be due to glass transition
277
temperature of the product [45]. Our findings are compatible with the results of Goula et al. [34] on
278
spray drying of tomato pulp. Frascareli et al. [31] concluded that increasing gum Arabic ratio resulted
279
in higher powder hygroscopicity, but temperature didn’t affect it. Manickavasagan et al. [25] reported
280
that the higher the temperature powder was produced at, the lower the moisture content of final powder
281
and the higher their hygroscopicity. Sablani et al. [3] reported hygroscopicity of date powder produced
282
by oven drying method in the range of 4.0-6.2%.
283
Table 1
284
Fig. 3
285
Increasing WPC proportion reduced hygroscopicity of powder, which could be due to film formation
286
characteristics, surface activity of WPC or extending glass transition temperature [46]. Tonon et al. 12
287
[47] and Adhikari et al. [48] reported that increasing protein concentrate ratio decreased powder
288
hygroscopicity. Intensifying pectin level increased hygroscopicity of date powder, due to bonds
289
between hydrogen atoms of water molecules and hydroxyl groups of their amorphous or crystalline
290
regions. Besides, our results indicated that hygroscopicity fell off as surfactant level started to increase,
291
ascribed to its surface activity which causes it to surround particles, impeding water penetration into
292
particles by film forming. Our analysis on effects of variables interactions on hygroscopicity of powder
293
proved that surfactant-WPC interaction was more determining than other interactions on lowering
294
hygroscopicity rate (Figure was not shown). So, suggested conditions to optimize hygroscopicity of
295
date powder would be 1% of surfactant, 3% of pectin, and 10% of WPC, at pH=8 and 190°C.
296
3.4. Bulk density of date powder
297
Bulk density is one of the most important parameters in storage, transportation, packaging and
298
processing of food products. Bulk densities of date powder were in the range of 0.53-0.68 gr cm-3 (Fig.
299
4). Treatments No. 9 (1.5% of surfactants, 3% of pectin, 12% of WPC, pH=9.5 and drying temperature
300
of 170°C), and No. 14 (2% of surfactants, 4% of pectin, 12% of WPC, pH=5 and drying temperature of
301
190°C) resulted in the highest and lowest bulk densities, respectively. Our results revealed that the
302
influences of drying temperature and surfactants on this parameter were higher than other variables
303
(Table 2). In fact, temperature increase led to lower bulk densities due to creating extensive porosity in
304
particles. Goula et al. [34] and Fazaeli et al. [49] reported same observations on tomato and black
305
mulberry powder, respectively. Bazaria, and Kumar [44] emphasized this pattern when drying sugar
306
beet as well. Also, Chegini, & Ghobadian [35], Goula et al. [34] and Farahnaky et al. [18] concluded a
307
decrease in bulk density of food powder after increasing drying temperature. Higher WPC levels
308
caused bulk density augmentation because proteins could be lodged among particles very suitably,
309
occupying less space by particles [50]. Sablani et al. [3] attributed increase in bulk density of powder
310
by higher concentration of carrier agent to lower particles size of date powder obtained in that state. 13
311
Suhag and Nanda [46] reported the same behavior for spray dried honey powder. Optimum conditions
312
for date powder with the highest bulk density could be 0.5% of surfactant, 3% of pectin, 12% of WPC,
313
and pH value of 9.5 at 170°C.
314
Fig. 4
315
Table 2
316
3.5. Total Phenolic Compounds (TPC)
317
TPC of date syrup was 723 mg 100 g-1. TPC proportions of date powder are represented in Table 1.
318
The highest TPC content belonged to treatment No. 7 with 1% of surfactant, 5% of pectin, and 14% of
319
WPC at pH=5 and 170°C, and the lowest level to treatment No. 6 with 1% of surfactant, 4% of pectin,
320
and 8% of WPC at pH=9.5 and 180°C. Analysis of variance showed that temperature had the most
321
major impact on TPC and increasing it beyond 170°C plunged TPC. In fact, high temperature destructs
322
these compounds. Manickavasagan et al. [25] showed that high temperature decreased TPC of date
323
powder. Similarly, increasing temperature decreased TPC of honey powder [46]. Saénz et al. [51]
324
stated that high temperature caused a loss in bioactive compounds of cactus pear powder obtained by
325
spray drier. Bazaria, & Kumar [44] reported intensifying drying temperature caused a decrease in TPC
326
of sugar beet powder produced by spray drying.
327
Our results showed that growth in WPC rate preserved more TPC. Indeed, since WPC is a surface-
328
active compound, it embeds particles and conserves their TPC. Bazaria, & Kumar [44] and Bhusari et
329
al. [52] stated that increasing WPC rate helped to keep more TPC in beetroot juice and tamarind pulp
330
powder. Du et al. [53] drew a conclusion that maltodextrin was more efficacious than protein
331
compounds, e.g. WPC or egg albumin, to preserve TPC in persimmon pulp powder. The results
332
illustrated that interaction between pectin and WPC was more effective on TPC than other kinds of
333
interactions (Fig. 5). In fact, these compounds act as wall materials, surrounding particles and
334
preventing their destruction as they are frequently applied in encapsulation as well. Carneiro et al. [54] 14
335
deployed maltodextrin and WPC for microencapsulation of linseed oil and protected them against
336
oxidation successfully. Mohammadi et al. [55] announced that application of WPC-pectin complex in
337
microencapsulation of TPC of olive leaf extract helped to keep antioxidant activities of these
338
substances. Optimum conditions to prepare powder with the highest TPC rate include: 0.5% of
339
surfactant, 3% of pectin, 14% of WPC at pH=5 and drying temperature of 170°C. In fact, at this pH
340
value as specified in the introduction, WPC (with isoelectric point of 4-6; [56]) owns localized amino
341
groups which react with free carboxyl groups of pectin and form protein-polysaccharide complexes and
342
this complex provides high thermal conservation for nutritional compounds of food product. Fig. 5
343 344
3.6. Microstructural analysis of date powder
345
SEM analysis was carried out from a couple of treatments to compare them and approve or disapprove
346
the results of physical properties; the observations are represented in Fig. 6, which confirmed our
347
previous results. As an example, in Fig. 6, after comparison of treatments 12 and 15 with each other, it
348
was revealed that the microstructure obtained by treatment No. 15 was smaller than that of treatment
349
No. 12; in fact, while the average size of the former was 17.82 µm, the average size of the latter was
350
22.67 µm. On the other hand, according to DOE, WPC levels of two treatments were the same; so, the
351
palpable difference was due to distinctive pH values and temperatures. Indeed, as mentioned in section
352
3.5., since the density of particles at pH value of 9.5 (treatment 15) is higher than other pH values
353
(treatment 12 included) and, on the other hand, higher temperatures (180°C for treatment 12 compared
354
with 160°C for treatment 15) results in more stickiness of particles and, as a consequence, larger
355
particles sizes, it is expected that both conditions of pH value = 9.5 and lower temperature lead to
356
smaller particles sizes that our SEM results proved the same as above-mentioned. Another similar
357
comparison is related to treatments 2 and 15. As mentioned in previous sections, since bulk density is
358
higher at pH=9.5 compared with pH=6.5 and higher surfactant concentrations lead to more moisture 15
359
depletion and lower particles sizes, and, on the other hand, protein ratios are the same in both
360
treatments, it is expected particles obtained by treatment No. 15 to be smaller as SEM results
361
demonstrated this: while particles sizes of treatment No. 15 had 17.8 µm diameter, treatment No. 2
362
resulted in particle sizes of 26.2 µm. Fig. 6
363 364
4. Conclusion
365
This study showed that spray drying, even under different drying conditions, can produce date powder
366
with high shelf life due to low moisture content of powder: in 1.4-3.4% range, demonstrating
367
effectiveness of the process. Analysis of variance showed that pectin was the most effective factor
368
(40.683%) on solubility parameter of date powder and drying temperature the least one. Increasing
369
WPC proportion reduced hygroscopicity of powder, which could be due to film forming characteristic
370
or surface activity of proteins. The influences of drying temperature and surfactants on bulk density
371
were higher than other variables; in fact, temperature increase led to lower bulk densities due to
372
creating extensive porosity in particles. Optimum conditions to prepare powder with the highest TPC
373
rate (e.g. for diet purposes) include: 0.5% of surfactant, 3% of pectin, 14% of WPC at pH=5 and drying
374
temperature of 170°C; in fact, at the highest concentration of WPC and pH value of 5, protein
375
ingredient and protein-pectin complex show thermal conservation effects, respectively. Regarding all
376
properties, to obtain date powder with the highest bulk density, solubility, and TPC, and lowest
377
moisture content, and hygroscopicity (e.g. for easier mixture of powder with high viscosity food
378
products in production lines), 1% of surfactant, 5% of pectin, 10% of WPC, pH value of 8.5 and drying
379
temperature of 170°C is recommended. So, date powder with suitable properties could be obtained by
380
spray drier at the mentioned conditions to replace consumption of white sugar in food formulations.
381
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23
Table 1 Hygroscopicity and Total Phenolic Compounds (TPC) of different treatments for date powder production based on Taguchi approach Treatment
Surfactant
Pectin
WPC
(%)
(%)
(%)
1
0.5
3
8
2
0.5
4
3
0.5
4
pH
Temperature
Hygroscopicity
TPC
(°C)
(%)
(mg 100g-1)
5.0
160
28
673.125
10
6.5
170
29
654.375
5
12
8.0
180
28
432.500
0.5
6
14
9.5
190
27
463.750
5
1.0
3
10
8.0
190
25
488.750
6
1.0
4
8
9.5
180
29
345.000
7
1.0
5
14
5.0
170
28
701.250
8
1.0
6
12
6.5
160
28
532.500
9
1.5
3
12
9.5
170
28
688.750
10
1.5
4
14
8.0
160
28
545.000
11
1.5
5
8
6.5
190
27
445.000
12
1.5
6
10
5.0
180
27
407.500
13
2.0
3
14
6.5
180
29
501.250
14
2.0
4
12
5.0
190
28
391.875
15
2.0
5
10
9.5
160
28
604.375
16
2.0
6
8
8.0
170
28
548.125
1
Table 2 Analysis of variance for bulk densities of date powders Factors Surfactants Pectin WPC pH Temperature
FD
Sum of Squares
3 3 3 3 3
0.006 0.001 0.002 0.001 0.012
2
Contribution Percentage 25.207 5.624 10.833 5.208 52.709
Moisture content of date powders (%)
4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Treatment Number
(A)
(B) Fig. 1(A) Moisture content of date powders for different treatments; (B) Effects of different variables and their levels on moisture content of date powders
1
Solubility (%)
100 90 80 70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Treatment Number
Interaction impact on Solubility
(A) 60 50 40 30 20 10 0
(B) Fig. 2 (A) Solubility rate of date powders for different treatments; (B) Percentage of interactions effects on solubility rates of date powders
2
Fig. 3 Effects of different variables and their levels on hygroscopicity of date powders
3
0.70
Bulk density (g/cm-3)
0.60 0.50 0.40 0.30 0.20 0.10 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Treatment Number
Fig. 4 Bulk densities of date powders
4
80 70
Interaction percentage
60 50 40 30 20 10 0
Fig. 5 Percentage of interactions effects on Total Phenolic Compounds (TPC) of date powders
5
(a)
(b)
(c) Fig. 6 Scanning Electronic Microscopy (SEM) analysis of date powders for treatment Numbers (a) 2, (b) 12, and (C) 15
6
Research Highlights: •
Date powders with low moisture content were produced successfully by spray drier.
•
Total phenolic compounds of date syrup was conserved after converting it into date powder.
•
pH value plays a determining role in particle size of date powder.
•
Temperature was the most important factor in determining different properties of date powders.
.
All authors declare that there is no conflict of interest.
S0032-5910(19)30847-2
DOI:
https://doi.org/10.1016/j.powtec.2019.10.013
Reference:
PTEC 14761
To appear in:
Powder Technology
Received Date: 6 December 2018 Revised Date:
1 October 2019
Accepted Date: 4 October 2019
Please cite this article as: S. Moghbeli, S.M. Jafari, Y. Maghsoudlou, D. Dehnad, A Taguchi approach optimization of date powder production by spray drying with the aid of whey protein-pectin complexes, Powder Technology (2019), doi: https://doi.org/10.1016/j.powtec.2019.10.013. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A Taguchi approach optimization of date powder production by spray drying with the aid of whey protein-pectin complexes
Graphical Abstract
Scanning Electronic Microscopy (SEM) analysis of date powder microstructures
1
A Taguchi approach optimization of date powder production by spray drying with
2
the aid of whey protein-pectin complexes
3
Running title: Production of date powder by biopolymer complexes
4 5
Sedighe Moghbeli, Seid Mahdi Jafari*, Yahya Maghsoudlou, Danial Dehnad
6 7 8
Faculty of Food Science and Technology, Gorgan University of Agricultural Sciences and Natural
9
Resources, Gorgan, Iran *Corresponding author: [email protected]
10 11 12
Abstract
13
In this research, date powder was produced by drying aids including Tween 80, pectin, and whey
14
protein concentrate (WPC) at different pH values (5.0, 6.5, 8.0, and 9.5) and using a spray drier at
15
different temperatures (160, 170, 180, and 190°C); then moisture content, solubility, hygroscopicity,
16
bulk density, and total phenolic compounds (TPC) were determined and their correlation with structural
17
characteristics of date powder was analyzed by Taguchi method. Interactions of pectin-surfactant, and
18
pectin-WPC were more effective on moisture and bulk density of date powder, respectively. Although
19
hygroscopicity of different treatments was in narrow range of 25-29%, temperature had the highest
20
impact on TPC; beyond 170°C, TPC of date powder decreased. Treatment No. 7, with the highest WPC
21
level and pH values of 5, maintained the highest TPC content (701 mg 100 g-1) in date powder. SEM
22
images revealed that pH=9.5 and lower temperatures led to smaller particles.
23 24
Keywords: Date powder; Biopolymers; Nutritional content; Spray drying; Optimization. 1
25
1. Introduction
26
Date palm (Phoenix dactylifera L.) is considered a major fruit crop in the hot desert regions (semi-arid
27
and arid lands) of the world. The main component of date is carbohydrate (70–80%) most of which are
28
in the form of glucose and fructose; it is estimated that 100 g of this fruit crop can provide > 300 kcal
29
of energy. Date also contains proteins, pectin, lipids, salts, and minerals. The annual per capita
30
consumption of dates in southern provinces of Iran is 25 kg while the approximate value of only 100 g
31
is recorded for the European Union in 2012 [1]. Despite cultivating date from many years ago, date-
32
related industrial activities have not advanced as much as other agricultural produces. On the other
33
hand, consuming sugar-related products enhances the risk of overweight and other prevalent problems
34
e.g. high blood fat, or high blood pressure. Indeed, due to abundance of sugar in different date varieties,
35
substituting it for common white sugar in formulation of food products is one of the various ways for
36
applying date in the food industry.
37
Date paste, date syrup, nectar date, juice date, date seed flour, and fermented date concentrate are the
38
important foodstuffs that can be obtained from dates. Date powder is a new product, obtained from
39
drying and milling of date pulp or its syrup. In fact, date powder could be produced in several ways
40
most of which are based on application of date pulp, mixing it with anti-caking agents, drying of the
41
complex with vacuum oven dryers, and finally, date powder production through milling process.
42
Production of date powder has been the focus of attention from researchers and producers in recent
43
years. The common target of most of these researchers was increasing shelf-life and dwindling date
44
waste although most of commonly-used methods require a long time and result in poor quality
45
products.
46
Spray drying remains to be one of the popular approaches for producing fruit juice powder; in detail,
47
some chief merits of this method are being economical, hygienic, and short contact duration required
48
[2]. The latter advantage fosters retaining nutritional values of original produce as well as high quality 2
49
of final powder. Drying of sticky products, such as fruit and vegetable juices, seems impractical on
50
account of their stickiness to dryer wall and agglomeration, reducing production efficiency; the
51
underlying reason is their low molecular weight sugars e.g. fructose, glucose, sucrose, and some
52
organic acids which have low glass transition temperature [3]. However, this difficulty could be
53
overcome through adding some biopolymers e.g. carbohydrates (starch and maltodextrin), gums,
54
proteins, or their mixture to soluble feed before atomizing it [4]. Actually, these compounds increase
55
glass transition temperature and diminish stickiness of powder by creating a physical barrier among
56
particles and competing with particles for water absorption [5, 6].
57
One of biopolymers used extensively in food industry in view of its film formation abilities and high
58
glass transition temperature is maltodextrin. Glass transition temperatures of maltodextrins are varied
59
according to dextrose equivalent (DE) degree; in fact, the lower the dextrose equivalent degree of the
60
maltodextrin, the higher the glass transition temperature of the complex; the average temperature could
61
be considered around 205°C. Nonetheless, high volume of maltodextrin could affect not only product
62
quality, but also consumer/market acceptance. So, the alternative of modifying surface properties of
63
particles by proteins was put forward. As a matter of fact, considerably lower protein complexes are
64
required to convert sugar-rich products to powder. For instance, only 0.13% calcium caseinate and
65
whey protein is required to powder a rich-sugar food model while higher than 40% maltodextrin
66
(DE=6) should be provided to obtain the same performance [7]. In other words, protein products
67
escalate powder production by reducing surface tension among particles. Other components which
68
could reduce surface tension of the solution considerably are surfactants: emulsifiers which are small in
69
dimension but have high surface activities [8]. Low molecular weight surfactants are smaller than
70
proteins and, as a result, could be lodged in particles surface more extensively [9]. But, due to low glass
71
transition temperature, surfactants could not be applied alone for covering particles surfaces.
3
72
Pectin is a type of polysaccharide composing of galacturonic acid units through α (1-4) bonds and
73
connect protein components to create protein-polysaccharide complexes. Effective deployment of
74
synergistic interactions between proteins and polysaccharides in food systems, such as emulsions and
75
foams, has been attended especially in recent decades [10,11]. Polysaccharides could contribute to
76
higher physical stability, higher viscosity of aqueous phase and changing rheological properties at
77
surface [12]. Size, charge and stability of biopolymers depend on protein-polysaccharide ratios,
78
polysaccharide type, ionic power, and thermal processing conditions. At very higher pH values than
79
isoelectric point, there are electrostatic repulsive forces between proteins and anionic polysaccharides
80
since proteins own high negative charges. On the other hand, at equal pH values to isoelectric point,
81
cationic regions of protein surfaces react with anionic groups of polysaccharide chains, resulting in
82
formation of soluble complexes [13]. pH values below isoelectric point lead to coacervation as the
83
result of electrostatic reactions between protein and polysaccharide molecules. Once reaching lower pH
84
values than pKa point of anionic groups of polysaccharide chains, attractive reactions between proteins
85
and polysaccharides weaken and the complex dissociate [14].
86
Taguchi design is used to find the impact of different factors on product properties and determine
87
optimal conditions of factors in the field of engineering [15]. As an alternative to full factorial design, it
88
decreases the number of experiments required, is more straightforward to use, faster and
89
simultaneously precise, and reliable, saves time and reduce costs [16]. Two tools of analysis of this
90
method are orthogonal arrays and ANOVA. While ANOVA evaluates the impact of a factor on
91
characteristics of the product, orthogonal arrays help to reduce replication of experiments.
92
There are researches dealt which with the issue of producing date powder by various types of driers, the
93
most significant results of which are discussed here. Sahari et al. [17] obtained date powder through
94
drying of date syrup in a vacuum oven and milling of dried date. Three date varieties, having different
95
sugar percentages, were examined to analyze the possibility of their drying under different vacuum 4
96
drying conditions (thickness, temperature, pressure, and time). The results showed that sugar
97
percentage and moisture content of date, date thickness and drying conditions were all effective on
98
powder properties and decreasing their moisture content. Drying at 85°C, 54.6 cm Hg, and 1 cm
99
thickness for 7 h led to achieving powder with desirable color and odor properties. In our previous
100
study, we evaluated microstructural and yield of date powder production through spray drying of date
101
syrup with complexes of whey protein- pectin and our results revealed that it could be a promising
102
approach [11]. Also, Sablani et al. [3] analyzed production of date powder by drying of combined
103
complex of date pulp and maltodextrin in a vacuum oven. They drew a conclusion that when date pulp
104
was mixed in 1:1 ratios to maltodextrin and dried, non-sticky powder with favorable flowability was
105
produced. In another study, physicochemical characteristics and sorption isotherm of date syrup
106
powder, as affected by maltodextrin incorporation, were evaluated by Farahnaky et al [18]. While
107
constant parameters of that research included the type of drier (twin drum drier), drying temperature
108
(130°C), and dextrose equivalent of maltodextrin (19), maltodextrin level was variable: 30, 40, 50 and
109
60%. They found that GAB (Guggenheim, Anderson, and de Boer) and Peleg models were more
110
suitable than BET model to fit moisture sorption data and type ш isotherms, representative of high-
111
sugar foods, were shown by date syrup powder. Besides, maltodextrin addition improved some product
112
characteristics by increasing its lightness and glass transition temperature and decreasing degree of
113
caking. However, as mentioned earlier, these methods require allocation of long time and excessive
114
usage of polysaccharide which, in turn, lessens final consumer acceptance. So, this project aimed to
115
take advantage of proteins, polysaccharides, surfactants and their combinations along with application
116
of spray drying equipment to optimize physicochemical properties of date powder.
117
2. Materials and method
118
Maltodextrin (DE=18), pectin with low methoxylation degree, whey protein concentrate (WPC) and
119
Tween 80 were bought from Sigma Co., Germany. Date of Kalute variety was purchased from palm5
120
grove in Jiroft city (Kerman, Iran) in September and kept in refrigerator (4°C) before preparation of its
121
syrup.
122
2.1. Date syrup preparation
123
First, date was cored, followed by milling via a miller (Kenwood, Japan). After that, hot water was
124
mixed with milled date two times of its weight and they were held in a water bath (Fan Azma Gostar,
125
Iran) at 70 °C for 30 min. Sieving was carried out by a 50 mesh sieve and total soluble solid was
126
increased from 25 to 35 °Brix by a rotary evaporator under vacuum at 70 °C.
127
2.2. Date syrup tests
128
2.2.1. Color measurement
129
Color parameters of date syrup were analyzed using the image analysis method. Plates (with 1 cm
130
height and 6 cm diameter) were filled with samples and their pictures were taken by a scanner (Scanjet
131
G2710, HP, USA) completely shielded by a black cover. The pictures were analyzed by the Image J
132
software (version 1.42e, Wayne Rasband, National Institutes of Health, USA) and RBG parameters of
133
the samples were converted to L*, a* and b* values; Additionally, the ratios of a* to b* values were
134
calculated to evaluate color quality of the samples [19,20].
135
2.2.2. Moisture content
136
Moisture content of date powder and date syrup was determined in the same way. 3-5 g of each sample
137
was placed in a pre-weighed crucible. Then, crucibles were dried in an oven (VO200, Memmert,
138
Germany) at 100-105°C for 24 h to a constant weight. After drying, crucibles were cooled in a
139
desiccator to room temperature, weighed and moisture content was determined according to the
140
following equation [21, 22]:
141
MC =
142
Where ܹଵ , ܹଶ and ܹଷ are sample weight, dish weight + sample prior to drying, and dish weight +
143
sample after drying, respectively.
ଵ(ௐమ ିௐయ ) ௐభ
6
144
2.2.3. Total Soluble Solids (TSS)
145
TSS (°Brix) index of each sample was read through a table refractometer (ABBE, CETI, Belgium) at
146
20°C and in three replications [19].
147
2.2.4. pH
148
This index was measured by a digital pH-meter (BEL, Italy) at 25°C. Before the experiment, the pH
149
meter was calibrated with commercial buffer solutions at pH 7.0 and 4.0 [23]. 10 g of date solutions
150
was weighed, and made to the volume in a 100 mL volumetric flask. Then, it was poured in a beaker
151
and stirred, followed by putting electrodes of pH meter in the beaker.
152
2.2.5. Ash
153
3-5 g of samples was weighed by a balance (Sartorius, Germany) in a pre-weighed crucible. Then, it
154
was dried at 100°C for 1 h and ignited in a muffle furnace (Nabertherm, Germany) at 550-600°C until
155
producing white ash. After cooling and reaching constant weight, ash content was calculated [24]:
156
Ash content (%) =
157
Where ܹଵ , ܹଶ and ܹଷ were weight of crucible plus sample, weight of crucible, and weight of crucible
158
having ash, respectively.
159
2.2.6. Total Phenolic Compounds (TPC)
160
For determination of TPC of date syrup and date powder, the same procedure was followed [25]. 20
161
mL of ethanol 80% was added to 2 g of sample in 50 mL centrifuge tubes and placed inside a water
162
bath at 40oC for 60 min. The samples were mixed using a vortex every 15 min to enhance extraction.
163
The extract was allowed to cool at room temperature and then centrifuged (Centrurion, Scientific Ltd.,
164
Canada) at 10,000 rpm for 15 min. The supernatant was collected and used for the determination of
165
total phenolic content. Total soluble phenolic were determined using the Folin-Ciocalteure reagent. The
166
date extract solution (1 mL) was mixed with 5 mL of the Folin-Ciocalteu reagent 10%; then, after 4
167
min, 4 mL of Na2CO3 7.5% was added and the complex was held in a dark room for 30 min and,
(ௐయ ିௐమ )×ଵ (ௐభ ିௐమ )
7
168
finally, its absorbance was measured at 765 nm using a spectrophotometer (PG Instruments Ltd., UK).
169
A standard curve was plotted using different concentrations of ascorbic acid and the total phenolic
170
compounds were expressed in ascorbic acid equivalents (AAE) in mg per 100g of sample.
171
2.3. Preparation of drying feed
172
Different ratios of pectin and WPC were mixed with each other proportionally, followed by stirring
173
through a heater-stirrer at 70 °C for 30 min and their pH values were adjusted; the solution was left to
174
remain at ambient temperature overnight for complex formation [11].
175
2.4. Spray drying
176
Maltodextrin was added to given weight of date syrup having adjusted TSS by 50% of its weight,
177
followed by adding pectin-WPC solutions and Tween 80 proportionally. Homogenization was carried
178
out at 10000 rpm for 5 min by a homogenizer (D-91126, Heidolph Instruments, Germany) and heating
179
to 70 °C before entering the drier. The size (height×width) of pilot plant spray drier (Azar Tank Tehran,
180
Iran) was 3×1.5 m2 and its atomizer nozzles were of twin fluid pressure type, with the diameter of drier
181
nozzles being 0.5 mm. In this research, temperature (70 °C) of feed stock and drier pressure (2 bar)
182
were kept constant. Different temperatures (the outlet air temperature was kept between 60 and 70 °C)
183
were selected for drying. Drying was carried out at air flow rate of about 0.1 m3/min and feed flow rate
184
of 450 mL/h. The obtained powder was collected from the drier in polyethylene bags, sealed and kept
185
in freezer before experiments.
186
2.5. Experiments on date powder
187
2.5.1. Solubility
188
Sample (0.5 g) was added to 50 mL of distilled water, stirred at 110 rpm for 30 min and then
189
centrifuged (Centrurion, Scientific Ltd., Canada) at 4000 rpm for 5 min. An aliquot (25 mL) of each
190
supernatant was removed, transferred to porcelain dishes, and dried to a constant weight in an oven at
8
191
105°C. The solubility index (%) was calculated as the ratio of the dried supernatant weight to the
192
amount of the original weight of sample (0.5 g) [26,27].
193
2.5.2. Hygroscopicity
194
Samples of each powder (approximately 1 g) were placed at 25°C into a container with NaCl saturated
195
solution (75.29% relative humidity) obtained by adding 268 g of NaCl to 750 mL of distilled water and
196
were weighed until equilibrium (after 1 week) [28]. Hygroscopicity was expressed as g of adsorbed
197
moisture per 100 g of powder (g/100 g).
198
2.5.3. Bulk density
199
The bulk density (ρb) of powder was measured by weighing 2 g of each sample and placing it into a 10
200
mL graduated cylinder. The cylinder was tapped by hand and the bulk density was calculated as the
201
ratio of the mass of powder contained in the cylinder to the volume occupied [28].
202
2.5.4. SEM
203
Date powders were gold-coated and analyzed by a SEM (SU3500, Hitachi, Japan) to investigate their
204
morphology. Accelerating voltage of 5 kV and magnificence of 500× were used to analyze microscopic
205
images of powders [25].
206
2.6. Statistical analysis
207
Taguchi method (total number of 16 treatments) was used for design of experiments, Qualitek software
208
for process optimization and Excel software for depicting figures. In brief, each of 5 variables we
209
intended to optimize its rate was applied in 4 levels and their ranges were as follows: 0.5-2.0% of
210
surfactant, 3-6% of pectin, 8-14% of WPC, pH values of 5.0-9.5, and 160-190°C; experiments were
211
analyzed according to Taguchi method to obtain optimum conditions of process, determine effects of
212
each factor on response, and estimate responses at optimum conditions. Signal to noise ratio,
213
proportion of control to noise factors which are uncontrollable, is the main approach applied in Taguchi
214
method to analyze results of experiments. Analysis of variance is another applied way to analyze the 9
215
results [29]. Lower moisture content and hygroscopicity, and higher bulk density, solubility, and total
216
phenolic compounds were considered as favorable conditions of date powder during optimization.
217
3. Results and discussion
218
Moisture, pH, TPC and ash of date syrup were 31%, 5.6, 723 (mg 100 g-1) and 5.6%; also, the color of
219
date syrup was darkish brown.
220
3.1. Moisture content of date powder
221
Moisture content of final powder plays an important role in its flowability, stickiness, and stability
222
during storage. Final moisture contents of date powder are displayed in Fig. 1A, varied in 1.4-3.4%
223
range, demonstrating effectiveness of the process. Treatment No. 5, with 1% of surfactant, 3% of
224
pectin, 10% of WPC at pH=8 and 190°C, had the highest moisture content whereas treatment No. 15,
225
having 2% of surfactant, 5% of pectin, 10% of WPC at pH=9.5 and 160°C, led to the lowest moisture
226
ratio. Our results showed that all variables were effective on final moisture content of powder (Fig. 1B)
227
although drying temperature, compared with other variables, left behind the highest impact. Attributed
228
to the formation of harsh crust around particles and reducing drying speed at the second drying stage,
229
increasing temperature lowered moisture content of powder. Likewise, Santana et al. [30] reported that
230
increase of temperature or drying speed creates a saturated environment with water vapor around
231
particles, restricting water transfer from particles and, as a result, hindering water removal. Similarly,
232
Frascareli et al. [31] stated that temperature increase culminated in higher moisture content of
233
encapsulated coffee oil obtained by spray drying. The same result was reported by Santhalakshmy et al.
234
[32] for Jamun powder. However, results of the study by Tuyen et al. [33] showed that increasing
235
drying temperature caused a drop in moisture content of the final product owning to more rapidly water
236
removal. These results are in agreement with the analyses of Goula et al. [34] and Chegini and
237
Ghobadian [35] and Osman and Endut [36] on tomato, orange juice, and pineapple juice, respectively.
238
Our results indicated that surfactant inclusion led to a decrease in moisture content of powder because 10
239
hydrophilic head of surfactant absorbs water molecules onto the surface, improving the dehydration
240
process. Also, Kaltsa et al. (2014) [37] expressed that using surfactants minimized particles
241
dimensions, boosting water exhaustion from particles. Jayasundera et al. (2011) [7] stated that non-
242
ionic surfactants (Tween 80), compared with ionic surfactants, made the water depletion from final
243
product more effective. Overall, Qualitek software indicated that optimum conditions for achieving
244
date powder with the least moisture content would be 3% of surfactant, 4% of pectin, 12% of WPC, at
245
pH value of 5 and temperature of 160°C. Fig. 1
246 247
3.2. Solubility of date powder
248
Solubility is an important index of food powder in aqueous environments. In fact, powder used in the
249
food industry should have high solubility. This factor is affected by different factors including main
250
ingredients, feed flow rate, carrier agents, and flow rate of the compressed air [38,34] (Bhandari et al.,
251
1997; Goula et al., 2004). Solubility values of date powder are presented in Fig. 2A. The highest
252
solubility belonged to treatment No. 10 with 1.5% of surfactant, 4% of pectin, 14% of WPC at pH
253
value of 8 and 180°C, and the lowest solubility to treatment No. 1. Analysis of variance showed that
254
pectin was the most effective factor (40.683%) and drying temperature the least one. The effect of
255
pectin could be due to its physical properties and solubility in water. In fact, higher pectin
256
concentrations reduced moisture content of powder, inducing faster water absorption [39]. Goula &
257
Adamopoulos [40] and Grabowski et al. [41] reported that an increase in carrier agent concentration,
258
due to its higher solubility, increased solubility of orange and sweet potato powder, respectively.
259
Fig. 2
260
Our results indicated lack of significant effect of temperature variations on solubility, in line with the
261
observation of Sharifi et al. [42] on barberry. But, Santhalakshmy et al. [32] reported a significant and
262
direct relationship between temperature and solubility, as opposed to the finding of Patil et al. [43] on 11
263
guava dried by spray drier. Higher WPC content than 10% decreased solubility, probably since
264
exposure of protein compounds to high temperature results in formation of non-soluble substances
265
undesirable in powder production. Our study on interactions among variables proved that interaction
266
between surfactant and pectin was more effective on increasing powder solubility than other ones (Fig.
267
2B). Optimum conditions for obtaining the highest solubility of date powder could be 1% of surfactant,
268
5% of pectin, 10% of WPC, at pH value of 8 and 170°C as revealed by Qualitek software.
269
3.3. Hygroscopicity of date powder
270
Lower hygroscopicity could be more favorable when storing or displacing powder as this factor is
271
effective on their flowability. The values are represented in Table 1. The results of analysis of variance
272
showed that temperature had the highest impact on hygroscopicity changes. Increasing the temperature
273
beyond 180°C led to a significant decrease in this parameter (Fig. 3). It could be interpreted that there
274
was a relationship between moisture content and hygroscopicity of powder. Date powder with the
275
lowest moisture content owned the highest hygroscopicity, due to the higher capacity for moisture
276
absorption from the environment [44]. Also, low hygroscopicity could be due to glass transition
277
temperature of the product [45]. Our findings are compatible with the results of Goula et al. [34] on
278
spray drying of tomato pulp. Frascareli et al. [31] concluded that increasing gum Arabic ratio resulted
279
in higher powder hygroscopicity, but temperature didn’t affect it. Manickavasagan et al. [25] reported
280
that the higher the temperature powder was produced at, the lower the moisture content of final powder
281
and the higher their hygroscopicity. Sablani et al. [3] reported hygroscopicity of date powder produced
282
by oven drying method in the range of 4.0-6.2%.
283
Table 1
284
Fig. 3
285
Increasing WPC proportion reduced hygroscopicity of powder, which could be due to film formation
286
characteristics, surface activity of WPC or extending glass transition temperature [46]. Tonon et al. 12
287
[47] and Adhikari et al. [48] reported that increasing protein concentrate ratio decreased powder
288
hygroscopicity. Intensifying pectin level increased hygroscopicity of date powder, due to bonds
289
between hydrogen atoms of water molecules and hydroxyl groups of their amorphous or crystalline
290
regions. Besides, our results indicated that hygroscopicity fell off as surfactant level started to increase,
291
ascribed to its surface activity which causes it to surround particles, impeding water penetration into
292
particles by film forming. Our analysis on effects of variables interactions on hygroscopicity of powder
293
proved that surfactant-WPC interaction was more determining than other interactions on lowering
294
hygroscopicity rate (Figure was not shown). So, suggested conditions to optimize hygroscopicity of
295
date powder would be 1% of surfactant, 3% of pectin, and 10% of WPC, at pH=8 and 190°C.
296
3.4. Bulk density of date powder
297
Bulk density is one of the most important parameters in storage, transportation, packaging and
298
processing of food products. Bulk densities of date powder were in the range of 0.53-0.68 gr cm-3 (Fig.
299
4). Treatments No. 9 (1.5% of surfactants, 3% of pectin, 12% of WPC, pH=9.5 and drying temperature
300
of 170°C), and No. 14 (2% of surfactants, 4% of pectin, 12% of WPC, pH=5 and drying temperature of
301
190°C) resulted in the highest and lowest bulk densities, respectively. Our results revealed that the
302
influences of drying temperature and surfactants on this parameter were higher than other variables
303
(Table 2). In fact, temperature increase led to lower bulk densities due to creating extensive porosity in
304
particles. Goula et al. [34] and Fazaeli et al. [49] reported same observations on tomato and black
305
mulberry powder, respectively. Bazaria, and Kumar [44] emphasized this pattern when drying sugar
306
beet as well. Also, Chegini, & Ghobadian [35], Goula et al. [34] and Farahnaky et al. [18] concluded a
307
decrease in bulk density of food powder after increasing drying temperature. Higher WPC levels
308
caused bulk density augmentation because proteins could be lodged among particles very suitably,
309
occupying less space by particles [50]. Sablani et al. [3] attributed increase in bulk density of powder
310
by higher concentration of carrier agent to lower particles size of date powder obtained in that state. 13
311
Suhag and Nanda [46] reported the same behavior for spray dried honey powder. Optimum conditions
312
for date powder with the highest bulk density could be 0.5% of surfactant, 3% of pectin, 12% of WPC,
313
and pH value of 9.5 at 170°C.
314
Fig. 4
315
Table 2
316
3.5. Total Phenolic Compounds (TPC)
317
TPC of date syrup was 723 mg 100 g-1. TPC proportions of date powder are represented in Table 1.
318
The highest TPC content belonged to treatment No. 7 with 1% of surfactant, 5% of pectin, and 14% of
319
WPC at pH=5 and 170°C, and the lowest level to treatment No. 6 with 1% of surfactant, 4% of pectin,
320
and 8% of WPC at pH=9.5 and 180°C. Analysis of variance showed that temperature had the most
321
major impact on TPC and increasing it beyond 170°C plunged TPC. In fact, high temperature destructs
322
these compounds. Manickavasagan et al. [25] showed that high temperature decreased TPC of date
323
powder. Similarly, increasing temperature decreased TPC of honey powder [46]. Saénz et al. [51]
324
stated that high temperature caused a loss in bioactive compounds of cactus pear powder obtained by
325
spray drier. Bazaria, & Kumar [44] reported intensifying drying temperature caused a decrease in TPC
326
of sugar beet powder produced by spray drying.
327
Our results showed that growth in WPC rate preserved more TPC. Indeed, since WPC is a surface-
328
active compound, it embeds particles and conserves their TPC. Bazaria, & Kumar [44] and Bhusari et
329
al. [52] stated that increasing WPC rate helped to keep more TPC in beetroot juice and tamarind pulp
330
powder. Du et al. [53] drew a conclusion that maltodextrin was more efficacious than protein
331
compounds, e.g. WPC or egg albumin, to preserve TPC in persimmon pulp powder. The results
332
illustrated that interaction between pectin and WPC was more effective on TPC than other kinds of
333
interactions (Fig. 5). In fact, these compounds act as wall materials, surrounding particles and
334
preventing their destruction as they are frequently applied in encapsulation as well. Carneiro et al. [54] 14
335
deployed maltodextrin and WPC for microencapsulation of linseed oil and protected them against
336
oxidation successfully. Mohammadi et al. [55] announced that application of WPC-pectin complex in
337
microencapsulation of TPC of olive leaf extract helped to keep antioxidant activities of these
338
substances. Optimum conditions to prepare powder with the highest TPC rate include: 0.5% of
339
surfactant, 3% of pectin, 14% of WPC at pH=5 and drying temperature of 170°C. In fact, at this pH
340
value as specified in the introduction, WPC (with isoelectric point of 4-6; [56]) owns localized amino
341
groups which react with free carboxyl groups of pectin and form protein-polysaccharide complexes and
342
this complex provides high thermal conservation for nutritional compounds of food product. Fig. 5
343 344
3.6. Microstructural analysis of date powder
345
SEM analysis was carried out from a couple of treatments to compare them and approve or disapprove
346
the results of physical properties; the observations are represented in Fig. 6, which confirmed our
347
previous results. As an example, in Fig. 6, after comparison of treatments 12 and 15 with each other, it
348
was revealed that the microstructure obtained by treatment No. 15 was smaller than that of treatment
349
No. 12; in fact, while the average size of the former was 17.82 µm, the average size of the latter was
350
22.67 µm. On the other hand, according to DOE, WPC levels of two treatments were the same; so, the
351
palpable difference was due to distinctive pH values and temperatures. Indeed, as mentioned in section
352
3.5., since the density of particles at pH value of 9.5 (treatment 15) is higher than other pH values
353
(treatment 12 included) and, on the other hand, higher temperatures (180°C for treatment 12 compared
354
with 160°C for treatment 15) results in more stickiness of particles and, as a consequence, larger
355
particles sizes, it is expected that both conditions of pH value = 9.5 and lower temperature lead to
356
smaller particles sizes that our SEM results proved the same as above-mentioned. Another similar
357
comparison is related to treatments 2 and 15. As mentioned in previous sections, since bulk density is
358
higher at pH=9.5 compared with pH=6.5 and higher surfactant concentrations lead to more moisture 15
359
depletion and lower particles sizes, and, on the other hand, protein ratios are the same in both
360
treatments, it is expected particles obtained by treatment No. 15 to be smaller as SEM results
361
demonstrated this: while particles sizes of treatment No. 15 had 17.8 µm diameter, treatment No. 2
362
resulted in particle sizes of 26.2 µm. Fig. 6
363 364
4. Conclusion
365
This study showed that spray drying, even under different drying conditions, can produce date powder
366
with high shelf life due to low moisture content of powder: in 1.4-3.4% range, demonstrating
367
effectiveness of the process. Analysis of variance showed that pectin was the most effective factor
368
(40.683%) on solubility parameter of date powder and drying temperature the least one. Increasing
369
WPC proportion reduced hygroscopicity of powder, which could be due to film forming characteristic
370
or surface activity of proteins. The influences of drying temperature and surfactants on bulk density
371
were higher than other variables; in fact, temperature increase led to lower bulk densities due to
372
creating extensive porosity in particles. Optimum conditions to prepare powder with the highest TPC
373
rate (e.g. for diet purposes) include: 0.5% of surfactant, 3% of pectin, 14% of WPC at pH=5 and drying
374
temperature of 170°C; in fact, at the highest concentration of WPC and pH value of 5, protein
375
ingredient and protein-pectin complex show thermal conservation effects, respectively. Regarding all
376
properties, to obtain date powder with the highest bulk density, solubility, and TPC, and lowest
377
moisture content, and hygroscopicity (e.g. for easier mixture of powder with high viscosity food
378
products in production lines), 1% of surfactant, 5% of pectin, 10% of WPC, pH value of 8.5 and drying
379
temperature of 170°C is recommended. So, date powder with suitable properties could be obtained by
380
spray drier at the mentioned conditions to replace consumption of white sugar in food formulations.
381
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23
Table 1 Hygroscopicity and Total Phenolic Compounds (TPC) of different treatments for date powder production based on Taguchi approach Treatment
Surfactant
Pectin
WPC
(%)
(%)
(%)
1
0.5
3
8
2
0.5
4
3
0.5
4
pH
Temperature
Hygroscopicity
TPC
(°C)
(%)
(mg 100g-1)
5.0
160
28
673.125
10
6.5
170
29
654.375
5
12
8.0
180
28
432.500
0.5
6
14
9.5
190
27
463.750
5
1.0
3
10
8.0
190
25
488.750
6
1.0
4
8
9.5
180
29
345.000
7
1.0
5
14
5.0
170
28
701.250
8
1.0
6
12
6.5
160
28
532.500
9
1.5
3
12
9.5
170
28
688.750
10
1.5
4
14
8.0
160
28
545.000
11
1.5
5
8
6.5
190
27
445.000
12
1.5
6
10
5.0
180
27
407.500
13
2.0
3
14
6.5
180
29
501.250
14
2.0
4
12
5.0
190
28
391.875
15
2.0
5
10
9.5
160
28
604.375
16
2.0
6
8
8.0
170
28
548.125
1
Table 2 Analysis of variance for bulk densities of date powders Factors Surfactants Pectin WPC pH Temperature
FD
Sum of Squares
3 3 3 3 3
0.006 0.001 0.002 0.001 0.012
2
Contribution Percentage 25.207 5.624 10.833 5.208 52.709
Moisture content of date powders (%)
4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Treatment Number
(A)
(B) Fig. 1(A) Moisture content of date powders for different treatments; (B) Effects of different variables and their levels on moisture content of date powders
1
Solubility (%)
100 90 80 70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Treatment Number
Interaction impact on Solubility
(A) 60 50 40 30 20 10 0
(B) Fig. 2 (A) Solubility rate of date powders for different treatments; (B) Percentage of interactions effects on solubility rates of date powders
2
Fig. 3 Effects of different variables and their levels on hygroscopicity of date powders
3
0.70
Bulk density (g/cm-3)
0.60 0.50 0.40 0.30 0.20 0.10 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Treatment Number
Fig. 4 Bulk densities of date powders
4
80 70
Interaction percentage
60 50 40 30 20 10 0
Fig. 5 Percentage of interactions effects on Total Phenolic Compounds (TPC) of date powders
5
(a)
(b)
(c) Fig. 6 Scanning Electronic Microscopy (SEM) analysis of date powders for treatment Numbers (a) 2, (b) 12, and (C) 15
6
Research Highlights: •
Date powders with low moisture content were produced successfully by spray drier.
•
Total phenolic compounds of date syrup was conserved after converting it into date powder.
•
pH value plays a determining role in particle size of date powder.
•
Temperature was the most important factor in determining different properties of date powders.
.
All authors declare that there is no conflict of interest.