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High-intensity ultrasound processing of kiwifruit juice: Effects on the microstructure, pectin, carbohydrates and rheological properties
Journal Pre-proofs High-intensity ultrasound processing of kiwifruit juice: Effects on the microstructure, pectin, carbohydrates and rheological properties Jin Wang, Jun Wang, Sai Kranthi Vanga, Vijaya Raghavan PII: DOI: Reference:
S0308-8146(19)32273-3 https://doi.org/10.1016/j.foodchem.2019.126121 FOCH 126121
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
7 August 2019 20 December 2019 23 December 2019
Please cite this article as: Wang, J., Wang, J., Kranthi Vanga, S., Raghavan, V., High-intensity ultrasound processing of kiwifruit juice: Effects on the microstructure, pectin, carbohydrates and rheological properties, Food Chemistry (2019), doi: https://doi.org/10.1016/j.foodchem.2019.126121
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1
High-intensity ultrasound processing of kiwifruit juice: Effects on
2
the microstructure, pectin, carbohydrates and rheological
3
properties
4 5
Jin Wang a, *, #, Jun Wang b, #, Sai Kranthi Vanga a, Vijaya Raghavan a
6
a
7 8
Sciences, McGill University, Sainte-Anne-de-Bellevue, H9X 3V9, Canada b
9 10
College of Food Science and Engineering, Northwest A&F University, Yangling, Shaanxi 712100, China
* Corresponding Author E-mail: [email protected]
11 12
Department of Bioresource Engineering, Faculty of Agricultural and Environmental
ORCD ID: https://orcid.org/0000-0003-3117-9173 #
These two authors contributed equally to the work.
13 14 15 16 17 18 19
1
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Abstract: This study aimed to evaluate the influences of high-intensity ultrasound on the
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physiochemical properties of kiwifruit juice. Results reported high-intensity ultrasound
22
processing significantly enhanced the color attributes, cloudiness, and sugars of kiwifruit
23
juice. Further, the shear stress, apparent viscosity, storage and loss modulus was increased
24
with the rise of processing time. However, a significant degradation in the nanostructure of
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water-soluble pectin and suspended particles in ultrasound treated kiwifruit juice was
26
observed. In addition, ultrasound processing resulted in the rupture of cell wall causing the
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dispersion of the intracellular components into juice while higher damage in the cellular
28
structure was observed by increasing the processing time. These structural changes reveal the
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physical mechanism of ultrasound in improving the rheological properties, color attributes,
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cloudiness, and water-soluble pectin of kiwifruit juice. Altogether these findings suggest that
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high-intensity ultrasound has an enormous potential to improve the physical properties of
32
kiwifruit juice.
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Keywords: High-intensity ultrasound processing; Rheological properties; Color attributes;
35
Microstructure; Kiwifruit juice
36 37 38 39 40 41 2
42 43
1. Introduction
44
In recent years, the demand for minimally processed food products has been increasing
45
rapidly due to health concerns and food safety challenges (Ordóñez-Santos, Martínez-Girón,
46
& Arias-Jaramillo, 2017). Ultrasound considered as non-thermal food processing technology
47
has gained significant attention because of its ability to retain original freshness, flavor and
48
nutritional compounds in products, as well as lower energy consumption, compared to the
49
conventional procedures such as pasteurization (Wang, Wang, Ye, Vanga, & Raghavan,
50
2019a). Ultrasound can be classified into low-intensity ultrasound (0-1 W cm-2, > 100 kHz),
51
and high-intensity ultrasound (>1 W cm-2, 20-100 kHz) based on the frequency ranges
52
(Nowacka & Wedzik, 2016). Generally, low-intensity ultrasound is considered as a
53
non-destructive tool to monitor the changes of physicochemical compounds during food
54
processing. High-intensity ultrasound shows many potential applications such as inactivation
55
of enzymes and microorganisms in improving the shelf life of apple juice, pear juice, orange
56
juice, and grapefruit juice (Aadil et al., 2015; Abid et al., 2013). This improvement in the
57
shelf life of juices could be attributed to the inhibition of microbes by ultrasound processing
58
disrupting their cell walls and cell membrane (Roobab, Aadil, Madni, & Bekhit, 2018).
59
Further, ultrasound pre-treatment was considered as one of the most effective techniques to
60
enhance the extraction of bioactive compounds in fluid food systems (e.g., juices) because of
61
its cavitation effects (Ordóñez-Santos, Pinzón-Zarate, & González-Salcedo, 2015).
62
Previously, the extraction of ascorbic acid, total phenolics, and flavonoids were significantly
63
increased in kiwifruit juice after high-intensity ultrasound processing (Manzoor, et al., 2019; 3
64
Wang et al., 2019a; Wang, Vanga, & Raghavan, 2019b). This significant enhancement of
65
bioactive compounds is due to the disruption of fruit tissues when treated with ultrasound
66
causing a higher mass transfer into the liquid.
67
At present, several studies have reported the physical properties of fruit juices were improved
68
under high-intensity ultrasound treatment. In apple juice, a noticeable improvement in cloud
69
stability was obtained when processed with ultrasound at 25 kHz for 30-90 min compared to
70
the untreated juice (Abid, et al., 2013). In cantaloupe melon juice, the color attributes of
71
samples were significantly maintained after high-intensity ultrasound (376 W/cm2) treatment
72
for 10 min compared to the untreated samples (Fonteles, Costa, Jesus, Miranda, Fernandes, &
73
Rodrigues, 2012). Rojas et al. (2016) treated peach juice with high-intensity ultrasound at
74
1000 W, 20 kHz for 0-15 min. The results showed that the apparent viscosity of the juice was
75
significantly enhanced after processing. However, further studies are needed to explain how
76
high-intensity ultrasound processing can promote desirable physical properties in fruit juices.
77
Furthermore, the mechanism of high-intensity ultrasound process improving the physical
78
properties, as well as the correlation between the process and food components structures,
79
and properties are unknown. Therefore, the aim of the present study is to analyze the
80
influences of high-intensity ultrasound treatment on the physical properties of kiwifruit juice,
81
observing the differences in the rheological properties, color attributes, cloudiness, particle
82
size distribution, carbohydrates, and water-soluble pectin properties of kiwifruit juice.
83
2. Materials and methods
84
2.1. Chemicals and reagents
85
Toluidine
blue
and
deuterated
water 4
(containing
0.75%
sodium
86
3-trimethylsilyl-propionate-2,2,3,3-d4) were obtained from Sigma-Aldrich (Quebec, Canada).
87
95% ethyl alcohol, methanol, and high-performance liquid chromatography (HPLC) grade
88
water were purchased from Fisher Scientific (Quebec, Canada).
89
2.2. Juice preparation and processing
90
In the present study, green kiwifruits (Actinidia chinensis, ‘Hayward’) were purchased from a
91
local market of Montreal (Quebec, Canada) and were stored at room temperature until fully
92
ripe (soluble solids, 12-15 °Brix; firmness, 6-8 N) (Wang, MacRae, Wohlers, & Marsh, 2011).
93
As shown in S-Fig.1, the juice was obtained by using a cold centrifugal juicer (BJE430SIL,
94
Breville, Australia). All the juice samples were mixed and were pre-cooled in a refrigerator at
95
4°C. The mixed juice was separated into five groups and each group had six replicates (100
96
mL per replicate). A sonifier (400 W, 20 kHz, CT, USA) was set at 50% of duty cycle. To
97
reduce the formation of heat, kiwifruit juice samples in the glass jar were processed on the ice.
98
According to the previous study, samples were treated at different processing times: 0 min
99
(US0), 4 min (US4), 8 min (US8), 12 min (US12), and 16 min (US16), respectively (Wang,
100
Vanga, & Raghavan, 2019b). After treatments, three of six replicates were capped with
101
polytetrafluoroethylene (PTFE) film and stored at 4°C until further analyses. The leftover of
102
samples was freeze-dried using a freeze dryer (7420020, Labconco Corporation, Kansas City,
103
USA) for 48 hours and the dried samples were stored at -20°C.
104
2.3. Observation of optical microstructure
105
According to the method described by Stratakos et al. (2016), 20 μL of kiwifruit juice was
106
transferred on the glass slide, and then were stained using 0.1% of toluidine blue solution for
107
2 min (Stratakos, Delgado-Pando, Linton, Patterson, & Koidis, 2016). The mixture was 5
108
observed under an optical microscope equipped with a digital camera (Leica DM500, Leica
109
Microsystems Inc., Canada). The images were captured using imaging software (Leica LAS
110
EZ, Leica Microsystems Inc., Canada) at a 10× magnification.
111
2.4. Color attributes of fruit samples
112
In this study, a portable colorimeter (CR-300 Chroma, Minolta, Japan) was applied to
113
evaluate the color changes of samples after a calibration (Y=93.35; x=0.3152; y=0.3212) by
114
an illuminant (D65) and a standard observer (2°). CIELab parameters including L*, a*, and
115
b* of each sample were recorded. The total color difference (∆E), chroma (C), hue angle (h),
116
and yellow index (YI) was obtained from Eqs. (1)-Eqs. (4), respectively (Wang, Wang, Ye,
117
Vanga, & Raghavan, 2019b):
118
E ( L * L0 *) 2 ( a * a0 *) 2 (b * b0 *) 2
119
C
120
h tan 1 (b * / a*)
(3)
121
YI 142.86b* / L*
(4)
122
where a0*, b0*, and L0* represent initial values of untreated samples, while a*, b*, and L*
123
represent values of ultrasound treated samples.
124
2.5. Water-soluble pectin extraction and nanostructure observation
125
The alcohol-insoluble residue of kiwifruit was obtained as described by Wang et al. (2018).
126
Five grams of freeze-dried kiwifruit samples were suspended in 50 mL of 95% ethyl alcohol
127
(v/v) and were stirred for 10 min at 25 °C. The precipitate was collected after being
128
centrifuged at 5000×g, 4 °C, for 5 min. This washing procedure was repeated thrice to
129
remove alcohol-soluble residue. The final alcohol-insoluble residue was dried at 40 °C.
(1) (2)
a *2 b *2
6
130
Water-soluble pectin (WSP) was obtained according to the method described by Christiaens
131
et al. (2012). The alcohol-insoluble residue obtained from the previous procedure was boiled
132
with 100 mL of double distilled water for 10 min. After being centrifuged at 5000×g, for 10
133
min, the supernatant was collected and freeze-dried for 48 h. The yield of water-soluble
134
pectin was calculated using the Eq. (5):
135
WSP(%)
WSP( g) 100 Kiwifruit( g)
(5)
136
The nanostructure of water-soluble pectin was observed using an atomic force microscope
137
(AFM) equipped with a Nanoscope IIIa Controller (Veeco Instruments, Santa Barbara, CA,
138
USA) (Cárdenas-Pérez et al., 2018; Wang, Mujumdar, Deng, Gao, Xiao, & Raghavan, 2018).
139
Water soluble pectin (WPS) was mixed with double distilled water at a concentration of 10
140
μg mL−1. 20 μL of the sample was transferred onto the tip covered with a freshly cleaved
141
mica sheet, and then air-dried at room temperature. An AFM was used to set at a tapping
142
mode with 0.5-2 Hz of scan speeds. During the observation, at least 5 images from each
143
treatment were collected for further analysis.
144
2.6. Fourier Transform Infrared (FTIR) analysis
145
An FTIR spectrometer with deuterated triglycine sulfate (DTGS) detector (Thermo Nicolet
146
Analytical Instruments, Madison, WI) was applied to record the spectra of dried fruit samples.
147
According to the methods described by Wang et al. (2019a), 0.2 g of dried fruit powder was
148
added on the crystal, and 32 scans at the range of 500-4000 cm-1 were taken. The crystal was
149
wiped using 75% methanol at the end of each determination.
150
2.7. Cloudiness and particle size distribution analyses
7
151
Ten milliliters of juice samples from each treatment were centrifuged at 5000×g for 10 min at
152
4 °C using a refrigerated centrifuge (Thermo, USA). The absorbance of the supernatant was
153
determined at 660 nm using a spectrophotometer (Ultrospec 2100pro, Biochrom Ltd.,
154
Cambridge, England)) and the distilled water was used as a blank (Kubo, Augusto, &
155
Cristianini, 2013; Rojas, Leite, Cristianini, Alvim, & Augusto, 2016). The particle size of
156
juice samples was measured using a dynamic light scattering (DLS, Malvern, England). For
157
each treatment, juice samples were filtered by double cheesecloth to remove the big size
158
particles and was diluted by 50 times with distilled water (Kubo, Augusto, & Cristianini,
159
2013). Then, the diluted samples were filled in the cell. The intensity-weighted mean
160
diameter and polydispersity index of kiwifruit juice samples were measured at room
161
temperature.
162
2.8. 1H nuclear magnetic resonance (NMR) analysis
163
Freeze-dried kiwifruit samples (100 mg) were extracted with 700 μL of deuterated water
164
containing the internal standard (0.80 mM, sodium 3-trimethylsilyl-propionate-2,2,3,3,-d4,
165
TSP) (Rosa et al., 2015). After incubation at room temperature for 10 min, the mixture was
166
centrifuged at 5000×g, 4 °C for 15 min. The supernatant was collected for the NMR analysis.
167
A 500 MHz Varian nuclear magnetic resonance spectrometer (VNMRS) (Agilent/Varian,
168
Canada) with a 5 mm probe was operated with VNMRJ 4.2 software. Each measurement was
169
set at 256 scans and 6000 Hz of spectral width. The data was analyzed using MestReNova
170
software (Mestrelab Research, Canada).
171
2.9. Rheological characteristics
172
Rheological analyses were performed using an AR2000 rheometer (TA Instruments, USA) 8
173
with a cone-plate (40 mm diameter). Kiwifruit juice sample (0.5 mL) was transferred on the
174
bottom plate. The gap size and temperature were set at 0.056 mm and 25 ° C, respectively
175
(Huang, Zhao, Zhang, Liu, Hu, & Pan, 2018). In the study of steady flow, the shear rate
176
ranged from 0.1 to 100 s-1 (Wei et al., 2018). Prior to each test, the kiwifruit sample was
177
incubated in the plate for 3 min. After strain sweep tests, 2% of strain was selected to conduct
178
the dynamic frequency sweep analysis. The frequency ranged from 0.1 to 10 Hz to evaluate
179
the behavior of storage modulus(G′) and loss modulus (G″) of kiwifruit samples (Wei et al.,
180
2018). Rheological data analysis was performed using a rheology advantage software (TA
181
Instruments, USA).
182
2.10. Statistical analysis
183
All the data obtained from the study were expressed as mean ± standard deviation for each
184
treatment and were calculated by analysis of variance (ANOVA). The treatments and
185
determination were conducted in triplicates. The significant differences of means were
186
performed by Duncan analyses at p ≤ 0.05. All the figures were obtained using Origin Pro
187
2018 and online analysis software (ProfilmOnline).
188
3. Results and discussion
189
3.1. Microstructure analysis of juice samples
190
The microstructure of kiwifruit juice treated with 0, 4, 8, 12, and 16 min of high-intensity
191
ultrasound processing was visually observed using optical microscopy (Fig.1). The results
192
showed that high-intensity ultrasound processing significantly disrupted the cell walls of the
193
tissue when increased the duration of processing. Specifically, the microstructure of untreated
194
juice samples (US0) presented in integral cells with intact walls. The intracellular 9
195
components can be seen clearly within the cell structure. After 4 min of ultrasound
196
processing, no significant differences in the structures of cells were observed when compared
197
to the control. Whereas a few of kiwifruit tissues started to tear, resulting in a slight release of
198
intracellular components into the juice. Similar results were also observed by Campoli et al.
199
(2018) in guava juice, short time (3 min) ultrasound processing can only cause the movement
200
of these compounds inside of cells without obvious disruption in the fruit tissues.
201
In comparison, the microstructure of US8 and US12 processed juice samples presented a
202
clear difference compared to the control (US0) due to an increase in the processing time.
203
Specifically, more cell disruption and tearing in tissues were observed after US8 treatment,
204
but some intact cells still can be observed in the juice samples. US12 treatment caused a
205
complete breakdown of cell walls resulting in the release of intracellular components to the
206
juice. The large pieces of cell fragments were still clearly present in the juice. In contrast, the
207
clearest disruption in cell structures was observed when the longest duration (US16) was
208
applied. Specifically, US16 treatment completely released the intracellular components and
209
cut the large cell fragments into small pieces, which leads to releasing of enormous amounts
210
of small particles in the juice samples. Similar trends regarding microstructure changes
211
responding to the processing time were observed in guava juice (Campoli, Rojas, Amaral,
212
Canniatti-Brazaca, & Augusto, 2018), peach juice (Rojas, Leite, Cristianini, Alvim, &
213
Augusto, 2016), and strawberry juice (Wang et al., 2019a) when processed with ultrasound.
214
The disruption, tearing and leakage of cell structures can be attributed to the cavitation effects
215
resulted from ultrasound processing (José, Andrade, Ramos, Vanetti, Stringheta, & Chaves,
216
2014). Studies found there are two types of cavitation generated during high-intensity 10
217
ultrasound processing. In respect of the first type, the oscillations of ultrasound waves cause
218
the formation of numerous small bubbles, which rotationally travel through the sonic field
219
leading to the generation of microstreaming. This stable cavitation is associated with certain
220
small-scale effects such as movements and forces, which can be used to explain why US4
221
treatment caused a slight damage on the cell tissues (Cárcel, García-Pérez, Benedito, & Mulet,
222
2012; José et al., 2014). Another type of cavitation, transient cavitation occurs due to the
223
rapid formation and collapse of big-size bubbles within a short time, resulting in a large
224
amount of pressure and stress (José et al., 2014). Altogether, these two cavitation effects
225
provide enough energy to breakdown the cell walls of kiwifruit tissues causing cell torn,
226
leakage, rapture, and loss of tissues (Cárcel, García-Pérez, Benedito, & Mulet, 2012; Wang et
227
al., 2019a).
228
3.2. Color attributes
229
Color attributes are considered as an important standard to evaluate the quality of fruit juice
230
or related products if satisfy the requirements of consumers (Aadil, Zeng, Han, & Sun, 2013).
231
Kiwifruit juice with a bright green color is desired for the market (Tomadoni, Moreira,
232
Espinosa, & Ponce, 2017). The influences of high-intensity ultrasound processing on the
233
color attributes of kiwifruit juice were shown in Table 1. No significant differences in the
234
lightness (L*) values were observed in all treatments, while a* and b* values of juice have
235
significantly improved after processing. Specifically, the highest a* value (greenness) was
236
observed in US12 (-7.64) treated samples, followed by US4 (-7.27) and US16 (-6.45), while
237
no obvious differences were found between US0 (-5.22) and US8 (-5.94). Similarly, the
238
yellowness (b*) of kiwifruit juice increased with the rise of processing time from 0 to 16 min. 11
239
US16 significantly increased the yellowness of samples to 15.13 from an initial level of 4.99
240
(US0), followed by US12 (12.63), US4 (10.04), and US8 (8.50). The total color difference
241
(∆E) ranged from 3.59 to 10.70 after 4-16 min ultrasound treatment, which was higher than
242
the reference (0 < ∆E < 2), and these noticeable changes can be seen by the naked eyes. Thus,
243
ultrasound processing showed a potential application to improve the quality of kiwifruit juice
244
by increasing its yellowness and greenness. Similar results were observed in red grape juice
245
(Tiwari, Patras, Brunton, Cullen, & O’donnell, 2010) and pineapple juice (Costa et al., 2013).
246
These improvements in color attributes (a* and b*) are associated with the release of
247
carotenoids and anthocyanins into the juice resulted from the disruption in fruit tissues under
248
high-intensity ultrasound processing (Wang, Vanga, & Raghavan, 2019b).
249
Chroma, yellow index, and hue angle of the ultrasound treated kiwifruit juice increased
250
during 0-16 min processing (Table 1). Chroma level was enhanced by two-fold in US16
251
(16.45) compared to untreated samples (7.22). The highest yellow index was measured in
252
US16 (19.46), followed by US12 (17.08), US 8 (12.55), US4 (11.91), and US0 (6.44). The
253
hug angle of juice samples was increased to 66.45 from the initial level of 44.66 after 16-min
254
ultrasound processing. These significant increases were due to the increase of a* and b*, the
255
maintenance of L* during ultrasound processing. These findings agree with the results
256
obtained in apple juice and lime juice (Abid, et al., 2013; Bhat, Kamaruddin, Min-Tze, &
257
Karim, 2011). However, the results reported by Ordóñez-Santos et al. (2017) found that the
258
chroma level of Cape gooseberry juice significantly decreased during a 40-min ultrasound
259
processing, which resulted from the oxidation reaction of juice under such a long-time
260
exposure in the air. Thus, the proper processing duration is strongly related to the color 12
261
attributes of juice or products.
262
3.3. Yield and nanostructure of water-soluble pectin
263
Fruit pectin can be considered as an essential dietary fiber to help lower cholesterol and
264
improve the gut health (Mudgil & Barak, 2013). In the present study, the influences of
265
high-intensity ultrasound processing on the extraction of water-soluble pectin were evaluated.
266
As shown in S-Fig.2, the results showed high-intensity ultrasound significantly enhanced the
267
yield of water-soluble pectin in kiwifruit samples. After a 4-min ultrasound, the pectin yield
268
was increased to 25.7% from the initial level of 18.5%. These significant increases were also
269
observed in US8, US12, and US16 compared to the control (US0). However, no significant
270
differences in the yield of water-soluble pectin were found between US12 (36.7%) and US16
271
(37.5%). It suggests that the extraction of water-soluble pectin reached its maximum value
272
after 12-min ultrasound processing. Similarly, Grassino et al. (2016) extracted pectin from
273
tomato waste using ultrasound pre-treatment, the results observed the pectin yield was
274
increased during the first 30 min, while the yield maintained at a similar level after 30-min
275
treatment. These increases in the yield of water-soluble pectin might be due to the
276
degradation of non-water-soluble pectin in fruit tissues (Oliveira, Giordani, Lutckemier,
277
Gurak, Cladera-Olivera, & Marczak, 2016). Further, the rupture of cell structures in kiwifruit
278
samples caused by ultrasound might result in more release of water-soluble pectin compared
279
to the untreated samples (Grassino, Brnčić, Vikić-Topić, Roca, Dent, & Brnčić, 2016). The
280
related mechanisms are not clear and further studies are needed in the future.
281
In the study, AFM was used to characterize the nanostructure of water-soluble pectin in
282
kiwifruit juice. The results showed water-soluble pectin with large branch chains and long 13
283
chains were observed in the untreated kiwifruit samples (Fig.2). After 4-min ultrasound
284
processing, the nanostructure of water-soluble pectin in the samples was broken down into
285
small chains, but still have some large-sized pectin molecules, which can be seen clearly in
286
the samples (Fig. 2). The smaller size of water-soluble pectin was observed when the samples
287
were treated with longer time. The long chains in pectin were completely broken down into
288
many short straight chains after 16-min ultrasound processing (Fig.2). These changes can be
289
attributed to the shear stress caused by the “cavitation effects” under high-intensity
290
ultrasound ( Wang et al., 2019a; Wang, Vanga, & Raghavan, 2019b).
291
3.4. Fourier transform infrared determination
292
FTIR analysis is a nondestructive tool which has been applied on many fruit tissues such as
293
strawberry and lychee, to characterize and identify the functional groups due to its
294
conservative time consumption compared to conventional assays (Wang et al., 2019a). As
295
shown in Fig. 3a, the FTIR spectra of untreated and ultrasound treated kiwifruit samples
296
(freeze-dried tissues) were recorded. A significant peak at 3318 cm-1 was attributed to
297
hydroxyl groups (-OH) indicating carbohydrates such as sugars (e.g., glucose), fibers (e.g.,
298
cellulose) and pectin present in kiwifruit tissues (Alba, Macnaughtan, Laws, Foster,
299
Campbell, & Kontogiorgos, 2018). The prominent peaks at 2926 and 1407 cm-1 in kiwifruit
300
sample represents C-H bonds of aliphatic groups (Yang et al., 2006). In addition, absorbances
301
at 1721 and 1599 cm-1 of kiwifruit samples correspond to acetyl groups or C=O stretching
302
and C=C bonds, respectively, which are associated with p-coumarate, cellulose,
303
hemicellulose, pectin and lignin present in kiwifruit samples (Alba et al., 2018).
304
The peaks at 1234 and 1026 cm-1 of kiwifruit samples are assigned to glycosidic bonds such 14
305
as C-O and C-O-C which might be attributed to tarabinoxylans and xylans present in fruit
306
tissues (Alba et al., 2018). The intensity of these peaks enhanced when increased the
307
processing duration and the strongest intensity of peaks was observed in US16 treated
308
kiwifruit samples among all the treatments. It agrees with the results obtained from grapefruit,
309
strawberry, and tomato tissues when treated with high-intensity ultrasound processing
310
(Grassino et al., 2016; Wang et al., 2019a). These increases of peak intensity might be due to
311
the breakdown of kiwifruit microstructure (Fig.1) resulting in the release of intracellular
312
components such as glucose and pectin during ultrasound processing (Cárcel, García-Pérez,
313
Benedito, & Mulet, 2012). Further, carbohydrates with large molecules might be broken
314
down into small molecules (e.g., glucose) to enhance the intensity of -OH, C-H, C-O, C=C,
315
and C=O bonds resulting from the physiochemical effects generated during ultrasound
316
processing (Gallo, Ferrara, & Naviglio, 2018).
317
3.5. Particle size distribution (PSD)
318
The effects of ultrasound treatment on the PSD of juice samples were illustrated in S-Table 1.
319
It suggests that ultrasound processing caused a significant reduction in the particle size of
320
samples when increased the processing duration from 0 to 16 min. A significant reduction
321
(23%) in the average particle size of juice samples was measured after a 4-min ultrasound
322
treatment (1087.71 nm) compared to untreated samples (1417.67 nm). The particle size of
323
samples continually reduced to 992.98 nm in US8 and 944.93 nm in US12 from an initial
324
level of 1417.67 nm. The highest reduction in the particle size up to 36.24% was observed in
325
US16 compared to that of US0. Further, the polydispersity of kiwifruit samples increased
326
slightly when the longer processing duration was applied, while no significant differences in 15
327
the polydispersity of samples were found between treatments (S-Table 1). Significant
328
reduction in the particles size was also reported in peach juice, orange juice, and tomato juice
329
(Rojas, Leite, Cristianini, Alvim, & Augusto, 2016; Tiwari, Muthukumarappan, O'donnell, &
330
Cullen, 2009). These significant reductions in the particle size are associated with the
331
disruption of microstructure in fruit tissues caused by cavitation effects during ultrasound
332
processing which results in the breakdown of cell walls of fruit samples cutting them into
333
smaller fragments (Cárcel, García-Pérez, Benedito, & Mulet, 2012; Rojas, Leite, Cristianini,
334
Alvim, & Augusto, 2016).
335
3.6. Cloudiness of kiwifruit juice
336
As shown in Fig.3b-c, the cloudiness behavior of kiwifruit juice in relation to various
337
ultrasound processing durations from 0 to 16 min were analyzed. A significant improvement
338
in the cloud stability of kiwifruit juice was observed after ultrasound processing compared to
339
the untreated samples. The cloudiness increased during the first 8-min of processing and
340
reached the maximum threshold, and then decreased with further increase in processing
341
duration. Among all the treatments, the highest cloud value was found in US8 (0.95),
342
followed by US12 (0.93) and US4 (0.92), while no significant differences were observed
343
between them. US16 represented a less cloud value compared to other treatments but was still
344
significantly higher than that of untreated kiwifruit samples. These results agree with the
345
findings described by Rojas et al., (2016) in peach juice and Tiwari et al. (2009) in orange
346
juice when treated with ultrasound at 20 kHz, 1000-1500 W for 0-15 min. The cloudiness of
347
kiwifruit juice is dependent on the ultrasound processing time. In the present study, results
348
found 8-min ultrasound processing improved the cloud value to the maximum threshold due 16
349
to the dispersion stability of macromolecules caused by the formation of physicochemical
350
reactions during high-intensity ultrasound processing. These reactions include the
351
modification of protein conformation and structure (Krešić, Lelas, Jambrak, Herceg, &
352
Brnčić, 2008) and inactivation of cloud-related enzymes such as pectin methylesterase
353
(Tiwari, Muthukumarappan, O'donnell, & Cullen, 2009). Studies have reported that pectin
354
methylesterase could initiate cloud loss of fruit juice by sequential hydrolysis of pectin
355
resulting in protein precipitation (Cameron, Baker, & Grohmann, 1998). In addition, the
356
structure of pectin, particularly water-soluble pectin, can be degraded under high-intensity
357
ultrasonication leading to the enhancement of cloud stability of kiwifruit juice (Tiwari,
358
Muthukumarappan, O'donnell, & Cullen, 2009). Furthermore, the breakdown of tissue cells
359
caused by the cavitation resulted in the release of intracellular compounds (e.g., carotenoid
360
and sugars) during ultrasound processing, which also contributes the increase of cloudiness of
361
kiwifruit juice (Wang et al., 2019a)
362
3.7. Carbohydrates characteristics of ultrasound treated kiwifruit samples
363
As shown in Fig.4, the carbohydrates attributes of ultrasound treated kiwifruit samples were
364
characterized using 1H NMR spectroscopy. As described in the previous studies, the spectrum
365
peaks of sucrose were observed between 5.28-5.32 ppm and 4.06-4.12 ppm (Fig. 4a). The
366
strong signal presented at 5.05-5.15 ppm, 4.48-4.55 ppm, and 3.88-4.05 ppm were related to
367
alpha-glucose, beta-glucose, and fructose, respectively (Cusano, Simonato, & Consonni,
368
2018).
369
The intensity of the sucrose peak in kiwifruit samples decreased when increased the
370
processing duration from 0 to 12 min, while a slight increase was observed in US16 (Fig.4b). 17
371
These obvious reductions might be due to the hydrolysis of sucrose into fructose and glucose
372
during ultrasound processing (Soares et al., 2019). Similar results were also found on
373
ultrasound treated sweet lime juice and orange juice (Khandpur & Gogate, 2015). The slight
374
increase in the intensity of sucrose peak in US16 which is attributed to the release of sucrose
375
from the tissue samples due to the longer ultrasound processing duration. In comparison, the
376
signal intensity of fructose increased with the rise of processing duration from 0 to 16 min
377
(Fig.4c). Specifically, there is no significant increase in the signal intensity of fructose
378
observed in US4 treated samples compared to US0. The highest intensity was found in US16,
379
followed by US12 and US8. Similar increasing trend in the signal intensity was also found in
380
beta-glucose. The increase of beta-glucose and fructose was due to the cell wall damage
381
leading to the release of cell components from kiwifruit tissues under ultrasound processing
382
(Aadil, et al., 2015) (Fig.4e). As mentioned above, the hydrolysis of sucrose also contributed
383
to the enhancement of fructose and glucose in the samples. However, there were no
384
significant differences in the signal intensity of alpha-glucose between each treatment
385
(Fig.4d), which might be because of the stable characteristics of alpha-glucose (Krešić, Lelas,
386
Jambrak, Herceg, & Brnčić, 2008).
387
3.8. Rheological properties
388
3.8.1. Flow behavior of kiwifruit juice
389
The flow properties of ultrasound treated kiwifruit juice were illustrated in Fig. 5. The shear
390
stress of kiwifruit juice increased gradually with the rise of the shear rate (Fig.5a). A
391
significant increase in the shear stress of US8 was observed as compared to the untreated
392
samples, while no obvious differences were observed between US4, US12, and US16. In 18
393
addition, a decreasing trend of apparent viscosity in all treatments was observed when the
394
shear rate increased from 0.1 to 100 s-1 (Fig.5b). The ultrasound treated samples, especially
395
in US16 showed a slower decrease in the viscosity when compared to control (US0), which
396
might be related to the structural changes in kiwifruit juice during ultrasound processing.
397
Similar results were found in mango juice and peach juice, ultrasound processing can
398
improve the viscosity of juice under optimized conditions (Huang et al., 2018; Rojas, Leite,
399
Cristianini, Alvim, & Augusto, 2016).
400
The yield stress increased when treated with ultrasound from 0 to 16 min in both the models,
401
while the flow behavior (n) of kiwifruit juice decreased after processing. The apparent
402
viscosity showed a significant increase after ultrasound processing, especially in US16. These
403
changes in the flow behavior of kiwifruit juice are influenced by a wide range of factors
404
including the disruption of cells structures and breakdown of large molecules under
405
high-intensity ultrasound treatment (Huang et al., 2018; Rojas, Leite, Cristianini, Alvim, &
406
Augusto, 2016). Furthermore, studies have reported that the particle size and particle size
407
distribution of fruit juice decreased after processing and the smaller particle size in the fruit
408
juice can provide a higher total surface area, which can explain the rise in yield stress and
409
apparent viscosity of the juice (Augusto, Ibarz, & Cristianini, 2012). In the study,
410
observations showed high-intensity ultrasound processing, especially US16 broke down the
411
cell structures of kiwifruit tissues into small size (Fig.1), resulting in obvious reductions in
412
the particle size and distribution of juice (S-Table 1), which agrees with this inference
413
mentioned above. However, there are a very limited number of studies that evaluated the
414
effects of ultrasound processing on the flow behavior of kiwifruit juice and further studies are 19
415
recommended.
416
3.8.2. Dynamic rheological characteristics of kiwifruit juice
417
As shown in Fig. 5c-d, the behavior of storage modulus (G′) and loss modulus (G″) was
418
determined at the frequency ranged from 0.1 to 10 Hz using the frequency sweeps model. In
419
comparison to the untreated samples (US0), the kiwifruit samples in other treatments showed
420
an increasing trend in the values of G′ and G″ with the rise of frequency. A significant
421
increase was observed in ultrasound treated kiwifruit juice compared to the untreated samples
422
at the same frequency. The highest G′ and G″ were observed in US16, followed by US12,
423
US8, US4, and US0. However, the differences in G′ and G″ found between US4 and US8
424
were not significant. The other significant increase in the values of G′ and G″ in kiwifruit
425
juice was similar to the flow behavior mentioned above. Furthermore, the results reported
426
that the value of G′ is higher than G″ at the frequency range of 0.1-10 Hz, which are similar
427
with the results obtained in mango juice treated with ultrasound at 20 kHz, 400W for 0-40
428
min (Huang et al., 2018).
429
4. Conclusions
430
In conclusion, high-intensity ultrasound processing significantly improved the color attributes
431
(a*, b*, and YI) and stability of kiwifruit juice compared to the untreated samples. In addition,
432
the yield of pectin, cloudiness, and carbohydrates (fructose and glucose) of kiwifruit samples
433
were obviously enhanced by the increased disruption of cell structures in kiwifruit tissues,
434
especially in US16. These changes mentioned above together resulted in the improvement in
435
rheological characteristics (flow and viscoelastic behavior) of kiwifruit juice. Further,
436
previous studies have reported that high-intensity ultrasound can significantly improve total 20
437
phenolics (e.g., catechin, gallic acid), flavonoids, and antioxidant capacity of fruit juice
438
compared to the untreated samples. Therefore, ultrasound processing can be considered as a
439
potential novel processing technique for improving the quality of kiwifruit juice.
440
Conflict of interest
441
All the authors declared that no conflicts of interest are reported for this work.
442
Acknowledgment
443
The study was supported by China Scholarship Council (CSC) [201506300009] and Natural
444
Sciences and Engineering Research Council of Canada (NSERC) [RGPIN-2014-04190] for
445
supporting this work. Authors also would like to thank Dr. Zhiming Qi in Department of
446
Bioresource Engineering, McGill University for access of dynamic light scattering
447
instrument.
448 449 450
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451
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563 564
Figure captions
565
Fig. 1. Optical microstructure ( × 10) of kiwifruit juice: untreated sample (US0) and those treated by
566
ultrasound for 4 min (US4), 8 min (US8), 12 min (US12) and 16 min (US16).
567
Fig. 2. Nanostructure changes of water-soluble pectin in ultrasound treated kiwifruit samples.
568
Fig. 3. FTIR (a) and cloudiness (b-c) analysis of ultrasound processed kiwifruit samples. Note: values with
569
different letters in various columns are significantly different (p < 0.05) from each other. Note: values with
570
different letters in various columns are significantly different (p < 0.05) from each other. 26
571
Fig. 4. Carbohydrates characteristics of kiwifruit juice under ultrasound processing: 1H NMR spectra of
572
untreated sample (a), signal intensity of sucrose (b), fructose (c), alpha-glucose (d), and beta-glucose (e).
573
Fig. 5. Rheological characteristics of ultrasound processed kiwifruit juice: (a) flow curves; (b) flow
574
viscosity; (c) storage modulus; (d) loss modulus.
575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 27
594 595 596 US0
200 µm
US8
US4
200 µm
US16
US12
597
200µm
200 µm
200 µm
598
Fig. 1. Optical microstructure (×10) of kiwifruit juice: untreated sample (US0) and those treated by
599 600 601 602 603 604 605 606 607 608 609 610 611 612 613
ultrasound for 4 min (US4), 8 min (US8), 12 min (US12) and 16 min (US16).
28
614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657
US0
1
US4
2
3
4
5 (μm)
US12
1
1
US8
2
3
4
5 (μm)
1
2
3
4
US16
2
3
4
5 (μm)
1
2
3
4
5 (μm)
Fig. 2. Nanostructure changes of water-soluble pectin in ultrasound treated kiwifruit samples.
29
5 (μm)
658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683
1026 3318
a
1721
1407 15990
1234
2926
a
c
b US0
US4
US8
US12
a
a
US16
b
684 685
c
686 687 688 689 690 691 692 693
Fig. 3. FTIR (a) and cloudiness (b-c) analysis of ultrasound processed kiwifruit samples. 30
694 695 696 697 698 699 700 701 702 703 704 705
4.10 4.09 4.06 4.05 4.05 4.04 4.01 4.00 3.99 3.99 3.99 3.98 3.98 3.97 3.96 3.95 3.94 3.94 3.93 3.92 3.92 3.92 3.91
4.53 4.52 4.50
5.11 5.11
kiwi-ck2_PRESAT_01 kiwi-ck2 5.30 5.29
707
4.80
706
708 709
β-Glucose
710
Fructose
711
α-Glucose
712
Sucrose
Sucrose
713 714
Residual water
(a) 5.4
715 (b)
5.3
5.2
5.1
5.0
4.9
4.8
4.7
4.6 4.5 f1 (ppm)
(c) 31
4.4
4.3
4.2
4.1
4.0
3.9
3.8 ppm
716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 32
738 739 740 741 742 743
Fig. 4. Carbohydrates characteristics of kiwifruit juice under ultrasound processing: 1H NMR spectra of
744
untreated sample (a), signal intensity of sucrose (b), fructose (c), alpha-glucose (d), and beta-glucose (e).
745 746 747 748 749 750 751
33
b
a
d
c
752 753
Fig. 5. Rheological characteristics of ultrasound processed kiwifruit juice: (a) flow curves; (b) flow
754
viscosity; (c) storage modulus; (d) loss modulus.
755 756 757 758 759 760 761
Table 1. Color attributes changes in ultrasound treated kiwifruit juice. Note: values with different letters in the same column are significantly different (p < 0.05) from each other.
Treatment
L*
a*
b*
∆E
C
YI
h°
US0
107.10 ± 3.32a
-5.22 ± 0.89d
4.99 ± 0.90d
Na
7.22 ± 0.87d
6.44 ± 0.77d
44.66 ± 2.44e
US4
110.10 ± 3.85a
-7.27 ± 0.25ab
10.04 ± 1.05c
6.22 ± 0.97c
12.39 ± 0.62b
11.91 ± 0.45c
51.63 ± 0.50d
US8
107.41 ± 1.92a
-5.94 ± 0.29cd
8.50 ± 1.09c
3.59 ± 0.88d
10.36 ± 0.44c
12.55 ± 1.20c
55.78 ± 0.84c
34
US12
106.50 ± 3.18a
-7.64 ± 0.53a
12.63 ± 0.93b
8.04 ± 1.10b
14.76 ± 0.59ab
17.08 ± 0.67b
60.60 ± 1.33b
US16
110.30 ± 2.84a
-6.45 ± 0.79bc
15.13 ± 1.20a
10.70 ± 2.23a
16.45 ± 0.80a
19.46 ± 1.81a
66.45 ± 2.08a
762 763 764 765 766 767 768 769 770 771 772 773 774 775
Credit Author Statement
776
777
778
The related contributions of each author are described as follows:
779
Jin
780
Writing-Original draft preparation.
781
Jun Wang: Investigation, Data analysis, Software, Writing- Reviewing and Editing.
782
Sai Kranthi Vanga: Writing- Reviewing and Editing.
Wang: Experimental
design
and
operation,
35
Methodology,
Data
analysis,
783
Vijaya Raghavan: Supervision, Reviewing and Editing
784 785
Highlights
786
• High-intensity ultrasound changed nanostructure of pectin into short-straight chains.
787
• US16 significantly increased the yield of pectin by 19%.
788
• Signal intensity of fructose and β-glucose was improved after pretreatment.
789
• Rheological characteristics were enhanced after ultrasound processing.
790
• Microstructure rupture explained why ultrasound improved physical properties.
791 792 793 794 795
Declaration of interests
796 797
☒
The authors declare that they have no known competing financial interests or personal
798
relationships that could have appeared to influence the work reported in this paper.
799 800
☐The authors declare the following financial interests/personal relationships which may be
801
considered as potential competing interests:
36
802 803 804 805 806 807
37
S0308-8146(19)32273-3 https://doi.org/10.1016/j.foodchem.2019.126121 FOCH 126121
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
7 August 2019 20 December 2019 23 December 2019
Please cite this article as: Wang, J., Wang, J., Kranthi Vanga, S., Raghavan, V., High-intensity ultrasound processing of kiwifruit juice: Effects on the microstructure, pectin, carbohydrates and rheological properties, Food Chemistry (2019), doi: https://doi.org/10.1016/j.foodchem.2019.126121
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© 2019 Elsevier Ltd. All rights reserved.
1
High-intensity ultrasound processing of kiwifruit juice: Effects on
2
the microstructure, pectin, carbohydrates and rheological
3
properties
4 5
Jin Wang a, *, #, Jun Wang b, #, Sai Kranthi Vanga a, Vijaya Raghavan a
6
a
7 8
Sciences, McGill University, Sainte-Anne-de-Bellevue, H9X 3V9, Canada b
9 10
College of Food Science and Engineering, Northwest A&F University, Yangling, Shaanxi 712100, China
* Corresponding Author E-mail: [email protected]
11 12
Department of Bioresource Engineering, Faculty of Agricultural and Environmental
ORCD ID: https://orcid.org/0000-0003-3117-9173 #
These two authors contributed equally to the work.
13 14 15 16 17 18 19
1
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Abstract: This study aimed to evaluate the influences of high-intensity ultrasound on the
21
physiochemical properties of kiwifruit juice. Results reported high-intensity ultrasound
22
processing significantly enhanced the color attributes, cloudiness, and sugars of kiwifruit
23
juice. Further, the shear stress, apparent viscosity, storage and loss modulus was increased
24
with the rise of processing time. However, a significant degradation in the nanostructure of
25
water-soluble pectin and suspended particles in ultrasound treated kiwifruit juice was
26
observed. In addition, ultrasound processing resulted in the rupture of cell wall causing the
27
dispersion of the intracellular components into juice while higher damage in the cellular
28
structure was observed by increasing the processing time. These structural changes reveal the
29
physical mechanism of ultrasound in improving the rheological properties, color attributes,
30
cloudiness, and water-soluble pectin of kiwifruit juice. Altogether these findings suggest that
31
high-intensity ultrasound has an enormous potential to improve the physical properties of
32
kiwifruit juice.
33 34
Keywords: High-intensity ultrasound processing; Rheological properties; Color attributes;
35
Microstructure; Kiwifruit juice
36 37 38 39 40 41 2
42 43
1. Introduction
44
In recent years, the demand for minimally processed food products has been increasing
45
rapidly due to health concerns and food safety challenges (Ordóñez-Santos, Martínez-Girón,
46
& Arias-Jaramillo, 2017). Ultrasound considered as non-thermal food processing technology
47
has gained significant attention because of its ability to retain original freshness, flavor and
48
nutritional compounds in products, as well as lower energy consumption, compared to the
49
conventional procedures such as pasteurization (Wang, Wang, Ye, Vanga, & Raghavan,
50
2019a). Ultrasound can be classified into low-intensity ultrasound (0-1 W cm-2, > 100 kHz),
51
and high-intensity ultrasound (>1 W cm-2, 20-100 kHz) based on the frequency ranges
52
(Nowacka & Wedzik, 2016). Generally, low-intensity ultrasound is considered as a
53
non-destructive tool to monitor the changes of physicochemical compounds during food
54
processing. High-intensity ultrasound shows many potential applications such as inactivation
55
of enzymes and microorganisms in improving the shelf life of apple juice, pear juice, orange
56
juice, and grapefruit juice (Aadil et al., 2015; Abid et al., 2013). This improvement in the
57
shelf life of juices could be attributed to the inhibition of microbes by ultrasound processing
58
disrupting their cell walls and cell membrane (Roobab, Aadil, Madni, & Bekhit, 2018).
59
Further, ultrasound pre-treatment was considered as one of the most effective techniques to
60
enhance the extraction of bioactive compounds in fluid food systems (e.g., juices) because of
61
its cavitation effects (Ordóñez-Santos, Pinzón-Zarate, & González-Salcedo, 2015).
62
Previously, the extraction of ascorbic acid, total phenolics, and flavonoids were significantly
63
increased in kiwifruit juice after high-intensity ultrasound processing (Manzoor, et al., 2019; 3
64
Wang et al., 2019a; Wang, Vanga, & Raghavan, 2019b). This significant enhancement of
65
bioactive compounds is due to the disruption of fruit tissues when treated with ultrasound
66
causing a higher mass transfer into the liquid.
67
At present, several studies have reported the physical properties of fruit juices were improved
68
under high-intensity ultrasound treatment. In apple juice, a noticeable improvement in cloud
69
stability was obtained when processed with ultrasound at 25 kHz for 30-90 min compared to
70
the untreated juice (Abid, et al., 2013). In cantaloupe melon juice, the color attributes of
71
samples were significantly maintained after high-intensity ultrasound (376 W/cm2) treatment
72
for 10 min compared to the untreated samples (Fonteles, Costa, Jesus, Miranda, Fernandes, &
73
Rodrigues, 2012). Rojas et al. (2016) treated peach juice with high-intensity ultrasound at
74
1000 W, 20 kHz for 0-15 min. The results showed that the apparent viscosity of the juice was
75
significantly enhanced after processing. However, further studies are needed to explain how
76
high-intensity ultrasound processing can promote desirable physical properties in fruit juices.
77
Furthermore, the mechanism of high-intensity ultrasound process improving the physical
78
properties, as well as the correlation between the process and food components structures,
79
and properties are unknown. Therefore, the aim of the present study is to analyze the
80
influences of high-intensity ultrasound treatment on the physical properties of kiwifruit juice,
81
observing the differences in the rheological properties, color attributes, cloudiness, particle
82
size distribution, carbohydrates, and water-soluble pectin properties of kiwifruit juice.
83
2. Materials and methods
84
2.1. Chemicals and reagents
85
Toluidine
blue
and
deuterated
water 4
(containing
0.75%
sodium
86
3-trimethylsilyl-propionate-2,2,3,3-d4) were obtained from Sigma-Aldrich (Quebec, Canada).
87
95% ethyl alcohol, methanol, and high-performance liquid chromatography (HPLC) grade
88
water were purchased from Fisher Scientific (Quebec, Canada).
89
2.2. Juice preparation and processing
90
In the present study, green kiwifruits (Actinidia chinensis, ‘Hayward’) were purchased from a
91
local market of Montreal (Quebec, Canada) and were stored at room temperature until fully
92
ripe (soluble solids, 12-15 °Brix; firmness, 6-8 N) (Wang, MacRae, Wohlers, & Marsh, 2011).
93
As shown in S-Fig.1, the juice was obtained by using a cold centrifugal juicer (BJE430SIL,
94
Breville, Australia). All the juice samples were mixed and were pre-cooled in a refrigerator at
95
4°C. The mixed juice was separated into five groups and each group had six replicates (100
96
mL per replicate). A sonifier (400 W, 20 kHz, CT, USA) was set at 50% of duty cycle. To
97
reduce the formation of heat, kiwifruit juice samples in the glass jar were processed on the ice.
98
According to the previous study, samples were treated at different processing times: 0 min
99
(US0), 4 min (US4), 8 min (US8), 12 min (US12), and 16 min (US16), respectively (Wang,
100
Vanga, & Raghavan, 2019b). After treatments, three of six replicates were capped with
101
polytetrafluoroethylene (PTFE) film and stored at 4°C until further analyses. The leftover of
102
samples was freeze-dried using a freeze dryer (7420020, Labconco Corporation, Kansas City,
103
USA) for 48 hours and the dried samples were stored at -20°C.
104
2.3. Observation of optical microstructure
105
According to the method described by Stratakos et al. (2016), 20 μL of kiwifruit juice was
106
transferred on the glass slide, and then were stained using 0.1% of toluidine blue solution for
107
2 min (Stratakos, Delgado-Pando, Linton, Patterson, & Koidis, 2016). The mixture was 5
108
observed under an optical microscope equipped with a digital camera (Leica DM500, Leica
109
Microsystems Inc., Canada). The images were captured using imaging software (Leica LAS
110
EZ, Leica Microsystems Inc., Canada) at a 10× magnification.
111
2.4. Color attributes of fruit samples
112
In this study, a portable colorimeter (CR-300 Chroma, Minolta, Japan) was applied to
113
evaluate the color changes of samples after a calibration (Y=93.35; x=0.3152; y=0.3212) by
114
an illuminant (D65) and a standard observer (2°). CIELab parameters including L*, a*, and
115
b* of each sample were recorded. The total color difference (∆E), chroma (C), hue angle (h),
116
and yellow index (YI) was obtained from Eqs. (1)-Eqs. (4), respectively (Wang, Wang, Ye,
117
Vanga, & Raghavan, 2019b):
118
E ( L * L0 *) 2 ( a * a0 *) 2 (b * b0 *) 2
119
C
120
h tan 1 (b * / a*)
(3)
121
YI 142.86b* / L*
(4)
122
where a0*, b0*, and L0* represent initial values of untreated samples, while a*, b*, and L*
123
represent values of ultrasound treated samples.
124
2.5. Water-soluble pectin extraction and nanostructure observation
125
The alcohol-insoluble residue of kiwifruit was obtained as described by Wang et al. (2018).
126
Five grams of freeze-dried kiwifruit samples were suspended in 50 mL of 95% ethyl alcohol
127
(v/v) and were stirred for 10 min at 25 °C. The precipitate was collected after being
128
centrifuged at 5000×g, 4 °C, for 5 min. This washing procedure was repeated thrice to
129
remove alcohol-soluble residue. The final alcohol-insoluble residue was dried at 40 °C.
(1) (2)
a *2 b *2
6
130
Water-soluble pectin (WSP) was obtained according to the method described by Christiaens
131
et al. (2012). The alcohol-insoluble residue obtained from the previous procedure was boiled
132
with 100 mL of double distilled water for 10 min. After being centrifuged at 5000×g, for 10
133
min, the supernatant was collected and freeze-dried for 48 h. The yield of water-soluble
134
pectin was calculated using the Eq. (5):
135
WSP(%)
WSP( g) 100 Kiwifruit( g)
(5)
136
The nanostructure of water-soluble pectin was observed using an atomic force microscope
137
(AFM) equipped with a Nanoscope IIIa Controller (Veeco Instruments, Santa Barbara, CA,
138
USA) (Cárdenas-Pérez et al., 2018; Wang, Mujumdar, Deng, Gao, Xiao, & Raghavan, 2018).
139
Water soluble pectin (WPS) was mixed with double distilled water at a concentration of 10
140
μg mL−1. 20 μL of the sample was transferred onto the tip covered with a freshly cleaved
141
mica sheet, and then air-dried at room temperature. An AFM was used to set at a tapping
142
mode with 0.5-2 Hz of scan speeds. During the observation, at least 5 images from each
143
treatment were collected for further analysis.
144
2.6. Fourier Transform Infrared (FTIR) analysis
145
An FTIR spectrometer with deuterated triglycine sulfate (DTGS) detector (Thermo Nicolet
146
Analytical Instruments, Madison, WI) was applied to record the spectra of dried fruit samples.
147
According to the methods described by Wang et al. (2019a), 0.2 g of dried fruit powder was
148
added on the crystal, and 32 scans at the range of 500-4000 cm-1 were taken. The crystal was
149
wiped using 75% methanol at the end of each determination.
150
2.7. Cloudiness and particle size distribution analyses
7
151
Ten milliliters of juice samples from each treatment were centrifuged at 5000×g for 10 min at
152
4 °C using a refrigerated centrifuge (Thermo, USA). The absorbance of the supernatant was
153
determined at 660 nm using a spectrophotometer (Ultrospec 2100pro, Biochrom Ltd.,
154
Cambridge, England)) and the distilled water was used as a blank (Kubo, Augusto, &
155
Cristianini, 2013; Rojas, Leite, Cristianini, Alvim, & Augusto, 2016). The particle size of
156
juice samples was measured using a dynamic light scattering (DLS, Malvern, England). For
157
each treatment, juice samples were filtered by double cheesecloth to remove the big size
158
particles and was diluted by 50 times with distilled water (Kubo, Augusto, & Cristianini,
159
2013). Then, the diluted samples were filled in the cell. The intensity-weighted mean
160
diameter and polydispersity index of kiwifruit juice samples were measured at room
161
temperature.
162
2.8. 1H nuclear magnetic resonance (NMR) analysis
163
Freeze-dried kiwifruit samples (100 mg) were extracted with 700 μL of deuterated water
164
containing the internal standard (0.80 mM, sodium 3-trimethylsilyl-propionate-2,2,3,3,-d4,
165
TSP) (Rosa et al., 2015). After incubation at room temperature for 10 min, the mixture was
166
centrifuged at 5000×g, 4 °C for 15 min. The supernatant was collected for the NMR analysis.
167
A 500 MHz Varian nuclear magnetic resonance spectrometer (VNMRS) (Agilent/Varian,
168
Canada) with a 5 mm probe was operated with VNMRJ 4.2 software. Each measurement was
169
set at 256 scans and 6000 Hz of spectral width. The data was analyzed using MestReNova
170
software (Mestrelab Research, Canada).
171
2.9. Rheological characteristics
172
Rheological analyses were performed using an AR2000 rheometer (TA Instruments, USA) 8
173
with a cone-plate (40 mm diameter). Kiwifruit juice sample (0.5 mL) was transferred on the
174
bottom plate. The gap size and temperature were set at 0.056 mm and 25 ° C, respectively
175
(Huang, Zhao, Zhang, Liu, Hu, & Pan, 2018). In the study of steady flow, the shear rate
176
ranged from 0.1 to 100 s-1 (Wei et al., 2018). Prior to each test, the kiwifruit sample was
177
incubated in the plate for 3 min. After strain sweep tests, 2% of strain was selected to conduct
178
the dynamic frequency sweep analysis. The frequency ranged from 0.1 to 10 Hz to evaluate
179
the behavior of storage modulus(G′) and loss modulus (G″) of kiwifruit samples (Wei et al.,
180
2018). Rheological data analysis was performed using a rheology advantage software (TA
181
Instruments, USA).
182
2.10. Statistical analysis
183
All the data obtained from the study were expressed as mean ± standard deviation for each
184
treatment and were calculated by analysis of variance (ANOVA). The treatments and
185
determination were conducted in triplicates. The significant differences of means were
186
performed by Duncan analyses at p ≤ 0.05. All the figures were obtained using Origin Pro
187
2018 and online analysis software (ProfilmOnline).
188
3. Results and discussion
189
3.1. Microstructure analysis of juice samples
190
The microstructure of kiwifruit juice treated with 0, 4, 8, 12, and 16 min of high-intensity
191
ultrasound processing was visually observed using optical microscopy (Fig.1). The results
192
showed that high-intensity ultrasound processing significantly disrupted the cell walls of the
193
tissue when increased the duration of processing. Specifically, the microstructure of untreated
194
juice samples (US0) presented in integral cells with intact walls. The intracellular 9
195
components can be seen clearly within the cell structure. After 4 min of ultrasound
196
processing, no significant differences in the structures of cells were observed when compared
197
to the control. Whereas a few of kiwifruit tissues started to tear, resulting in a slight release of
198
intracellular components into the juice. Similar results were also observed by Campoli et al.
199
(2018) in guava juice, short time (3 min) ultrasound processing can only cause the movement
200
of these compounds inside of cells without obvious disruption in the fruit tissues.
201
In comparison, the microstructure of US8 and US12 processed juice samples presented a
202
clear difference compared to the control (US0) due to an increase in the processing time.
203
Specifically, more cell disruption and tearing in tissues were observed after US8 treatment,
204
but some intact cells still can be observed in the juice samples. US12 treatment caused a
205
complete breakdown of cell walls resulting in the release of intracellular components to the
206
juice. The large pieces of cell fragments were still clearly present in the juice. In contrast, the
207
clearest disruption in cell structures was observed when the longest duration (US16) was
208
applied. Specifically, US16 treatment completely released the intracellular components and
209
cut the large cell fragments into small pieces, which leads to releasing of enormous amounts
210
of small particles in the juice samples. Similar trends regarding microstructure changes
211
responding to the processing time were observed in guava juice (Campoli, Rojas, Amaral,
212
Canniatti-Brazaca, & Augusto, 2018), peach juice (Rojas, Leite, Cristianini, Alvim, &
213
Augusto, 2016), and strawberry juice (Wang et al., 2019a) when processed with ultrasound.
214
The disruption, tearing and leakage of cell structures can be attributed to the cavitation effects
215
resulted from ultrasound processing (José, Andrade, Ramos, Vanetti, Stringheta, & Chaves,
216
2014). Studies found there are two types of cavitation generated during high-intensity 10
217
ultrasound processing. In respect of the first type, the oscillations of ultrasound waves cause
218
the formation of numerous small bubbles, which rotationally travel through the sonic field
219
leading to the generation of microstreaming. This stable cavitation is associated with certain
220
small-scale effects such as movements and forces, which can be used to explain why US4
221
treatment caused a slight damage on the cell tissues (Cárcel, García-Pérez, Benedito, & Mulet,
222
2012; José et al., 2014). Another type of cavitation, transient cavitation occurs due to the
223
rapid formation and collapse of big-size bubbles within a short time, resulting in a large
224
amount of pressure and stress (José et al., 2014). Altogether, these two cavitation effects
225
provide enough energy to breakdown the cell walls of kiwifruit tissues causing cell torn,
226
leakage, rapture, and loss of tissues (Cárcel, García-Pérez, Benedito, & Mulet, 2012; Wang et
227
al., 2019a).
228
3.2. Color attributes
229
Color attributes are considered as an important standard to evaluate the quality of fruit juice
230
or related products if satisfy the requirements of consumers (Aadil, Zeng, Han, & Sun, 2013).
231
Kiwifruit juice with a bright green color is desired for the market (Tomadoni, Moreira,
232
Espinosa, & Ponce, 2017). The influences of high-intensity ultrasound processing on the
233
color attributes of kiwifruit juice were shown in Table 1. No significant differences in the
234
lightness (L*) values were observed in all treatments, while a* and b* values of juice have
235
significantly improved after processing. Specifically, the highest a* value (greenness) was
236
observed in US12 (-7.64) treated samples, followed by US4 (-7.27) and US16 (-6.45), while
237
no obvious differences were found between US0 (-5.22) and US8 (-5.94). Similarly, the
238
yellowness (b*) of kiwifruit juice increased with the rise of processing time from 0 to 16 min. 11
239
US16 significantly increased the yellowness of samples to 15.13 from an initial level of 4.99
240
(US0), followed by US12 (12.63), US4 (10.04), and US8 (8.50). The total color difference
241
(∆E) ranged from 3.59 to 10.70 after 4-16 min ultrasound treatment, which was higher than
242
the reference (0 < ∆E < 2), and these noticeable changes can be seen by the naked eyes. Thus,
243
ultrasound processing showed a potential application to improve the quality of kiwifruit juice
244
by increasing its yellowness and greenness. Similar results were observed in red grape juice
245
(Tiwari, Patras, Brunton, Cullen, & O’donnell, 2010) and pineapple juice (Costa et al., 2013).
246
These improvements in color attributes (a* and b*) are associated with the release of
247
carotenoids and anthocyanins into the juice resulted from the disruption in fruit tissues under
248
high-intensity ultrasound processing (Wang, Vanga, & Raghavan, 2019b).
249
Chroma, yellow index, and hue angle of the ultrasound treated kiwifruit juice increased
250
during 0-16 min processing (Table 1). Chroma level was enhanced by two-fold in US16
251
(16.45) compared to untreated samples (7.22). The highest yellow index was measured in
252
US16 (19.46), followed by US12 (17.08), US 8 (12.55), US4 (11.91), and US0 (6.44). The
253
hug angle of juice samples was increased to 66.45 from the initial level of 44.66 after 16-min
254
ultrasound processing. These significant increases were due to the increase of a* and b*, the
255
maintenance of L* during ultrasound processing. These findings agree with the results
256
obtained in apple juice and lime juice (Abid, et al., 2013; Bhat, Kamaruddin, Min-Tze, &
257
Karim, 2011). However, the results reported by Ordóñez-Santos et al. (2017) found that the
258
chroma level of Cape gooseberry juice significantly decreased during a 40-min ultrasound
259
processing, which resulted from the oxidation reaction of juice under such a long-time
260
exposure in the air. Thus, the proper processing duration is strongly related to the color 12
261
attributes of juice or products.
262
3.3. Yield and nanostructure of water-soluble pectin
263
Fruit pectin can be considered as an essential dietary fiber to help lower cholesterol and
264
improve the gut health (Mudgil & Barak, 2013). In the present study, the influences of
265
high-intensity ultrasound processing on the extraction of water-soluble pectin were evaluated.
266
As shown in S-Fig.2, the results showed high-intensity ultrasound significantly enhanced the
267
yield of water-soluble pectin in kiwifruit samples. After a 4-min ultrasound, the pectin yield
268
was increased to 25.7% from the initial level of 18.5%. These significant increases were also
269
observed in US8, US12, and US16 compared to the control (US0). However, no significant
270
differences in the yield of water-soluble pectin were found between US12 (36.7%) and US16
271
(37.5%). It suggests that the extraction of water-soluble pectin reached its maximum value
272
after 12-min ultrasound processing. Similarly, Grassino et al. (2016) extracted pectin from
273
tomato waste using ultrasound pre-treatment, the results observed the pectin yield was
274
increased during the first 30 min, while the yield maintained at a similar level after 30-min
275
treatment. These increases in the yield of water-soluble pectin might be due to the
276
degradation of non-water-soluble pectin in fruit tissues (Oliveira, Giordani, Lutckemier,
277
Gurak, Cladera-Olivera, & Marczak, 2016). Further, the rupture of cell structures in kiwifruit
278
samples caused by ultrasound might result in more release of water-soluble pectin compared
279
to the untreated samples (Grassino, Brnčić, Vikić-Topić, Roca, Dent, & Brnčić, 2016). The
280
related mechanisms are not clear and further studies are needed in the future.
281
In the study, AFM was used to characterize the nanostructure of water-soluble pectin in
282
kiwifruit juice. The results showed water-soluble pectin with large branch chains and long 13
283
chains were observed in the untreated kiwifruit samples (Fig.2). After 4-min ultrasound
284
processing, the nanostructure of water-soluble pectin in the samples was broken down into
285
small chains, but still have some large-sized pectin molecules, which can be seen clearly in
286
the samples (Fig. 2). The smaller size of water-soluble pectin was observed when the samples
287
were treated with longer time. The long chains in pectin were completely broken down into
288
many short straight chains after 16-min ultrasound processing (Fig.2). These changes can be
289
attributed to the shear stress caused by the “cavitation effects” under high-intensity
290
ultrasound ( Wang et al., 2019a; Wang, Vanga, & Raghavan, 2019b).
291
3.4. Fourier transform infrared determination
292
FTIR analysis is a nondestructive tool which has been applied on many fruit tissues such as
293
strawberry and lychee, to characterize and identify the functional groups due to its
294
conservative time consumption compared to conventional assays (Wang et al., 2019a). As
295
shown in Fig. 3a, the FTIR spectra of untreated and ultrasound treated kiwifruit samples
296
(freeze-dried tissues) were recorded. A significant peak at 3318 cm-1 was attributed to
297
hydroxyl groups (-OH) indicating carbohydrates such as sugars (e.g., glucose), fibers (e.g.,
298
cellulose) and pectin present in kiwifruit tissues (Alba, Macnaughtan, Laws, Foster,
299
Campbell, & Kontogiorgos, 2018). The prominent peaks at 2926 and 1407 cm-1 in kiwifruit
300
sample represents C-H bonds of aliphatic groups (Yang et al., 2006). In addition, absorbances
301
at 1721 and 1599 cm-1 of kiwifruit samples correspond to acetyl groups or C=O stretching
302
and C=C bonds, respectively, which are associated with p-coumarate, cellulose,
303
hemicellulose, pectin and lignin present in kiwifruit samples (Alba et al., 2018).
304
The peaks at 1234 and 1026 cm-1 of kiwifruit samples are assigned to glycosidic bonds such 14
305
as C-O and C-O-C which might be attributed to tarabinoxylans and xylans present in fruit
306
tissues (Alba et al., 2018). The intensity of these peaks enhanced when increased the
307
processing duration and the strongest intensity of peaks was observed in US16 treated
308
kiwifruit samples among all the treatments. It agrees with the results obtained from grapefruit,
309
strawberry, and tomato tissues when treated with high-intensity ultrasound processing
310
(Grassino et al., 2016; Wang et al., 2019a). These increases of peak intensity might be due to
311
the breakdown of kiwifruit microstructure (Fig.1) resulting in the release of intracellular
312
components such as glucose and pectin during ultrasound processing (Cárcel, García-Pérez,
313
Benedito, & Mulet, 2012). Further, carbohydrates with large molecules might be broken
314
down into small molecules (e.g., glucose) to enhance the intensity of -OH, C-H, C-O, C=C,
315
and C=O bonds resulting from the physiochemical effects generated during ultrasound
316
processing (Gallo, Ferrara, & Naviglio, 2018).
317
3.5. Particle size distribution (PSD)
318
The effects of ultrasound treatment on the PSD of juice samples were illustrated in S-Table 1.
319
It suggests that ultrasound processing caused a significant reduction in the particle size of
320
samples when increased the processing duration from 0 to 16 min. A significant reduction
321
(23%) in the average particle size of juice samples was measured after a 4-min ultrasound
322
treatment (1087.71 nm) compared to untreated samples (1417.67 nm). The particle size of
323
samples continually reduced to 992.98 nm in US8 and 944.93 nm in US12 from an initial
324
level of 1417.67 nm. The highest reduction in the particle size up to 36.24% was observed in
325
US16 compared to that of US0. Further, the polydispersity of kiwifruit samples increased
326
slightly when the longer processing duration was applied, while no significant differences in 15
327
the polydispersity of samples were found between treatments (S-Table 1). Significant
328
reduction in the particles size was also reported in peach juice, orange juice, and tomato juice
329
(Rojas, Leite, Cristianini, Alvim, & Augusto, 2016; Tiwari, Muthukumarappan, O'donnell, &
330
Cullen, 2009). These significant reductions in the particle size are associated with the
331
disruption of microstructure in fruit tissues caused by cavitation effects during ultrasound
332
processing which results in the breakdown of cell walls of fruit samples cutting them into
333
smaller fragments (Cárcel, García-Pérez, Benedito, & Mulet, 2012; Rojas, Leite, Cristianini,
334
Alvim, & Augusto, 2016).
335
3.6. Cloudiness of kiwifruit juice
336
As shown in Fig.3b-c, the cloudiness behavior of kiwifruit juice in relation to various
337
ultrasound processing durations from 0 to 16 min were analyzed. A significant improvement
338
in the cloud stability of kiwifruit juice was observed after ultrasound processing compared to
339
the untreated samples. The cloudiness increased during the first 8-min of processing and
340
reached the maximum threshold, and then decreased with further increase in processing
341
duration. Among all the treatments, the highest cloud value was found in US8 (0.95),
342
followed by US12 (0.93) and US4 (0.92), while no significant differences were observed
343
between them. US16 represented a less cloud value compared to other treatments but was still
344
significantly higher than that of untreated kiwifruit samples. These results agree with the
345
findings described by Rojas et al., (2016) in peach juice and Tiwari et al. (2009) in orange
346
juice when treated with ultrasound at 20 kHz, 1000-1500 W for 0-15 min. The cloudiness of
347
kiwifruit juice is dependent on the ultrasound processing time. In the present study, results
348
found 8-min ultrasound processing improved the cloud value to the maximum threshold due 16
349
to the dispersion stability of macromolecules caused by the formation of physicochemical
350
reactions during high-intensity ultrasound processing. These reactions include the
351
modification of protein conformation and structure (Krešić, Lelas, Jambrak, Herceg, &
352
Brnčić, 2008) and inactivation of cloud-related enzymes such as pectin methylesterase
353
(Tiwari, Muthukumarappan, O'donnell, & Cullen, 2009). Studies have reported that pectin
354
methylesterase could initiate cloud loss of fruit juice by sequential hydrolysis of pectin
355
resulting in protein precipitation (Cameron, Baker, & Grohmann, 1998). In addition, the
356
structure of pectin, particularly water-soluble pectin, can be degraded under high-intensity
357
ultrasonication leading to the enhancement of cloud stability of kiwifruit juice (Tiwari,
358
Muthukumarappan, O'donnell, & Cullen, 2009). Furthermore, the breakdown of tissue cells
359
caused by the cavitation resulted in the release of intracellular compounds (e.g., carotenoid
360
and sugars) during ultrasound processing, which also contributes the increase of cloudiness of
361
kiwifruit juice (Wang et al., 2019a)
362
3.7. Carbohydrates characteristics of ultrasound treated kiwifruit samples
363
As shown in Fig.4, the carbohydrates attributes of ultrasound treated kiwifruit samples were
364
characterized using 1H NMR spectroscopy. As described in the previous studies, the spectrum
365
peaks of sucrose were observed between 5.28-5.32 ppm and 4.06-4.12 ppm (Fig. 4a). The
366
strong signal presented at 5.05-5.15 ppm, 4.48-4.55 ppm, and 3.88-4.05 ppm were related to
367
alpha-glucose, beta-glucose, and fructose, respectively (Cusano, Simonato, & Consonni,
368
2018).
369
The intensity of the sucrose peak in kiwifruit samples decreased when increased the
370
processing duration from 0 to 12 min, while a slight increase was observed in US16 (Fig.4b). 17
371
These obvious reductions might be due to the hydrolysis of sucrose into fructose and glucose
372
during ultrasound processing (Soares et al., 2019). Similar results were also found on
373
ultrasound treated sweet lime juice and orange juice (Khandpur & Gogate, 2015). The slight
374
increase in the intensity of sucrose peak in US16 which is attributed to the release of sucrose
375
from the tissue samples due to the longer ultrasound processing duration. In comparison, the
376
signal intensity of fructose increased with the rise of processing duration from 0 to 16 min
377
(Fig.4c). Specifically, there is no significant increase in the signal intensity of fructose
378
observed in US4 treated samples compared to US0. The highest intensity was found in US16,
379
followed by US12 and US8. Similar increasing trend in the signal intensity was also found in
380
beta-glucose. The increase of beta-glucose and fructose was due to the cell wall damage
381
leading to the release of cell components from kiwifruit tissues under ultrasound processing
382
(Aadil, et al., 2015) (Fig.4e). As mentioned above, the hydrolysis of sucrose also contributed
383
to the enhancement of fructose and glucose in the samples. However, there were no
384
significant differences in the signal intensity of alpha-glucose between each treatment
385
(Fig.4d), which might be because of the stable characteristics of alpha-glucose (Krešić, Lelas,
386
Jambrak, Herceg, & Brnčić, 2008).
387
3.8. Rheological properties
388
3.8.1. Flow behavior of kiwifruit juice
389
The flow properties of ultrasound treated kiwifruit juice were illustrated in Fig. 5. The shear
390
stress of kiwifruit juice increased gradually with the rise of the shear rate (Fig.5a). A
391
significant increase in the shear stress of US8 was observed as compared to the untreated
392
samples, while no obvious differences were observed between US4, US12, and US16. In 18
393
addition, a decreasing trend of apparent viscosity in all treatments was observed when the
394
shear rate increased from 0.1 to 100 s-1 (Fig.5b). The ultrasound treated samples, especially
395
in US16 showed a slower decrease in the viscosity when compared to control (US0), which
396
might be related to the structural changes in kiwifruit juice during ultrasound processing.
397
Similar results were found in mango juice and peach juice, ultrasound processing can
398
improve the viscosity of juice under optimized conditions (Huang et al., 2018; Rojas, Leite,
399
Cristianini, Alvim, & Augusto, 2016).
400
The yield stress increased when treated with ultrasound from 0 to 16 min in both the models,
401
while the flow behavior (n) of kiwifruit juice decreased after processing. The apparent
402
viscosity showed a significant increase after ultrasound processing, especially in US16. These
403
changes in the flow behavior of kiwifruit juice are influenced by a wide range of factors
404
including the disruption of cells structures and breakdown of large molecules under
405
high-intensity ultrasound treatment (Huang et al., 2018; Rojas, Leite, Cristianini, Alvim, &
406
Augusto, 2016). Furthermore, studies have reported that the particle size and particle size
407
distribution of fruit juice decreased after processing and the smaller particle size in the fruit
408
juice can provide a higher total surface area, which can explain the rise in yield stress and
409
apparent viscosity of the juice (Augusto, Ibarz, & Cristianini, 2012). In the study,
410
observations showed high-intensity ultrasound processing, especially US16 broke down the
411
cell structures of kiwifruit tissues into small size (Fig.1), resulting in obvious reductions in
412
the particle size and distribution of juice (S-Table 1), which agrees with this inference
413
mentioned above. However, there are a very limited number of studies that evaluated the
414
effects of ultrasound processing on the flow behavior of kiwifruit juice and further studies are 19
415
recommended.
416
3.8.2. Dynamic rheological characteristics of kiwifruit juice
417
As shown in Fig. 5c-d, the behavior of storage modulus (G′) and loss modulus (G″) was
418
determined at the frequency ranged from 0.1 to 10 Hz using the frequency sweeps model. In
419
comparison to the untreated samples (US0), the kiwifruit samples in other treatments showed
420
an increasing trend in the values of G′ and G″ with the rise of frequency. A significant
421
increase was observed in ultrasound treated kiwifruit juice compared to the untreated samples
422
at the same frequency. The highest G′ and G″ were observed in US16, followed by US12,
423
US8, US4, and US0. However, the differences in G′ and G″ found between US4 and US8
424
were not significant. The other significant increase in the values of G′ and G″ in kiwifruit
425
juice was similar to the flow behavior mentioned above. Furthermore, the results reported
426
that the value of G′ is higher than G″ at the frequency range of 0.1-10 Hz, which are similar
427
with the results obtained in mango juice treated with ultrasound at 20 kHz, 400W for 0-40
428
min (Huang et al., 2018).
429
4. Conclusions
430
In conclusion, high-intensity ultrasound processing significantly improved the color attributes
431
(a*, b*, and YI) and stability of kiwifruit juice compared to the untreated samples. In addition,
432
the yield of pectin, cloudiness, and carbohydrates (fructose and glucose) of kiwifruit samples
433
were obviously enhanced by the increased disruption of cell structures in kiwifruit tissues,
434
especially in US16. These changes mentioned above together resulted in the improvement in
435
rheological characteristics (flow and viscoelastic behavior) of kiwifruit juice. Further,
436
previous studies have reported that high-intensity ultrasound can significantly improve total 20
437
phenolics (e.g., catechin, gallic acid), flavonoids, and antioxidant capacity of fruit juice
438
compared to the untreated samples. Therefore, ultrasound processing can be considered as a
439
potential novel processing technique for improving the quality of kiwifruit juice.
440
Conflict of interest
441
All the authors declared that no conflicts of interest are reported for this work.
442
Acknowledgment
443
The study was supported by China Scholarship Council (CSC) [201506300009] and Natural
444
Sciences and Engineering Research Council of Canada (NSERC) [RGPIN-2014-04190] for
445
supporting this work. Authors also would like to thank Dr. Zhiming Qi in Department of
446
Bioresource Engineering, McGill University for access of dynamic light scattering
447
instrument.
448 449 450
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451
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563 564
Figure captions
565
Fig. 1. Optical microstructure ( × 10) of kiwifruit juice: untreated sample (US0) and those treated by
566
ultrasound for 4 min (US4), 8 min (US8), 12 min (US12) and 16 min (US16).
567
Fig. 2. Nanostructure changes of water-soluble pectin in ultrasound treated kiwifruit samples.
568
Fig. 3. FTIR (a) and cloudiness (b-c) analysis of ultrasound processed kiwifruit samples. Note: values with
569
different letters in various columns are significantly different (p < 0.05) from each other. Note: values with
570
different letters in various columns are significantly different (p < 0.05) from each other. 26
571
Fig. 4. Carbohydrates characteristics of kiwifruit juice under ultrasound processing: 1H NMR spectra of
572
untreated sample (a), signal intensity of sucrose (b), fructose (c), alpha-glucose (d), and beta-glucose (e).
573
Fig. 5. Rheological characteristics of ultrasound processed kiwifruit juice: (a) flow curves; (b) flow
574
viscosity; (c) storage modulus; (d) loss modulus.
575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 27
594 595 596 US0
200 µm
US8
US4
200 µm
US16
US12
597
200µm
200 µm
200 µm
598
Fig. 1. Optical microstructure (×10) of kiwifruit juice: untreated sample (US0) and those treated by
599 600 601 602 603 604 605 606 607 608 609 610 611 612 613
ultrasound for 4 min (US4), 8 min (US8), 12 min (US12) and 16 min (US16).
28
614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657
US0
1
US4
2
3
4
5 (μm)
US12
1
1
US8
2
3
4
5 (μm)
1
2
3
4
US16
2
3
4
5 (μm)
1
2
3
4
5 (μm)
Fig. 2. Nanostructure changes of water-soluble pectin in ultrasound treated kiwifruit samples.
29
5 (μm)
658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683
1026 3318
a
1721
1407 15990
1234
2926
a
c
b US0
US4
US8
US12
a
a
US16
b
684 685
c
686 687 688 689 690 691 692 693
Fig. 3. FTIR (a) and cloudiness (b-c) analysis of ultrasound processed kiwifruit samples. 30
694 695 696 697 698 699 700 701 702 703 704 705
4.10 4.09 4.06 4.05 4.05 4.04 4.01 4.00 3.99 3.99 3.99 3.98 3.98 3.97 3.96 3.95 3.94 3.94 3.93 3.92 3.92 3.92 3.91
4.53 4.52 4.50
5.11 5.11
kiwi-ck2_PRESAT_01 kiwi-ck2 5.30 5.29
707
4.80
706
708 709
β-Glucose
710
Fructose
711
α-Glucose
712
Sucrose
Sucrose
713 714
Residual water
(a) 5.4
715 (b)
5.3
5.2
5.1
5.0
4.9
4.8
4.7
4.6 4.5 f1 (ppm)
(c) 31
4.4
4.3
4.2
4.1
4.0
3.9
3.8 ppm
716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 32
738 739 740 741 742 743
Fig. 4. Carbohydrates characteristics of kiwifruit juice under ultrasound processing: 1H NMR spectra of
744
untreated sample (a), signal intensity of sucrose (b), fructose (c), alpha-glucose (d), and beta-glucose (e).
745 746 747 748 749 750 751
33
b
a
d
c
752 753
Fig. 5. Rheological characteristics of ultrasound processed kiwifruit juice: (a) flow curves; (b) flow
754
viscosity; (c) storage modulus; (d) loss modulus.
755 756 757 758 759 760 761
Table 1. Color attributes changes in ultrasound treated kiwifruit juice. Note: values with different letters in the same column are significantly different (p < 0.05) from each other.
Treatment
L*
a*
b*
∆E
C
YI
h°
US0
107.10 ± 3.32a
-5.22 ± 0.89d
4.99 ± 0.90d
Na
7.22 ± 0.87d
6.44 ± 0.77d
44.66 ± 2.44e
US4
110.10 ± 3.85a
-7.27 ± 0.25ab
10.04 ± 1.05c
6.22 ± 0.97c
12.39 ± 0.62b
11.91 ± 0.45c
51.63 ± 0.50d
US8
107.41 ± 1.92a
-5.94 ± 0.29cd
8.50 ± 1.09c
3.59 ± 0.88d
10.36 ± 0.44c
12.55 ± 1.20c
55.78 ± 0.84c
34
US12
106.50 ± 3.18a
-7.64 ± 0.53a
12.63 ± 0.93b
8.04 ± 1.10b
14.76 ± 0.59ab
17.08 ± 0.67b
60.60 ± 1.33b
US16
110.30 ± 2.84a
-6.45 ± 0.79bc
15.13 ± 1.20a
10.70 ± 2.23a
16.45 ± 0.80a
19.46 ± 1.81a
66.45 ± 2.08a
762 763 764 765 766 767 768 769 770 771 772 773 774 775
Credit Author Statement
776
777
778
The related contributions of each author are described as follows:
779
Jin
780
Writing-Original draft preparation.
781
Jun Wang: Investigation, Data analysis, Software, Writing- Reviewing and Editing.
782
Sai Kranthi Vanga: Writing- Reviewing and Editing.
Wang: Experimental
design
and
operation,
35
Methodology,
Data
analysis,
783
Vijaya Raghavan: Supervision, Reviewing and Editing
784 785
Highlights
786
• High-intensity ultrasound changed nanostructure of pectin into short-straight chains.
787
• US16 significantly increased the yield of pectin by 19%.
788
• Signal intensity of fructose and β-glucose was improved after pretreatment.
789
• Rheological characteristics were enhanced after ultrasound processing.
790
• Microstructure rupture explained why ultrasound improved physical properties.
791 792 793 794 795
Declaration of interests
796 797
☒
The authors declare that they have no known competing financial interests or personal
798
relationships that could have appeared to influence the work reported in this paper.
799 800
☐The authors declare the following financial interests/personal relationships which may be
801
considered as potential competing interests:
36
802 803 804 805 806 807
37