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Effects of konjac oligo-glucomannan on the physicochemical properties of frozen surimi from red gurnard (Aspitrigla cuculus)
Accepted Manuscript Effects of konjac oligo-glucomannan on the physicochemical properties of frozen surimi from red gurnard (Aspitrigla cuculus)
Jianhua Liu, Chunhua Fang, Yahong Luo, Yuting Ding, Shulai Liu PII:
S0268-005X(18)31701-6
DOI:
10.1016/j.foodhyd.2018.10.056
Reference:
FOOHYD 4737
To appear in:
Food Hydrocolloids
Received Date:
30 August 2018
Accepted Date:
29 October 2018
Please cite this article as: Jianhua Liu, Chunhua Fang, Yahong Luo, Yuting Ding, Shulai Liu, Effects of konjac oligo-glucomannan on the physicochemical properties of frozen surimi from red gurnard (Aspitrigla cuculus), Food Hydrocolloids (2018), doi: 10.1016/j.foodhyd.2018.10.056
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ACCEPTED MANUSCRIPT
1
Effects of konjac oligo-glucomannan on the physicochemical
2
properties of frozen surimi from red gurnard (Aspitrigla
3
cuculus)
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Jianhua Liu, Chunhua Fang, Yahong Luo, Yuting Ding, Shulai Liu*
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Department of Food Science and Engineering, Ocean College, Zhejiang University of
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Technology, Hangzhou 310014, P. R. China
7 8
Corresponding author: Dr. Shulai Liu (S. Liu). Phone/fax: + 86 571 88320237, E-
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mail: [email protected]
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Abstract：In this study, the effects of konjac oligo-glucomannan (KOG) on the
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physicochemical properties of frozen surimi from red gurnard (Aspitrigla cuculus)
13
during frozen storage at -18 ºC for 50 days were investigated. An aliquot of 0.5%
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(w/w) of KOG was added into the surimi and subjected to frozen storage (KOG
15
group), and 8% of a conventional cryoprotectant (4% sorbitol and 4% sucrose, w/w)
16
was used as a positive control (S group). The salt soluble protein content, the Ca2+-
17
ATPase activity, total sulfhydryl (SH) content, gel strength, thiobarbituric acid (TBA)
18
value, and whiteness of frozen surimi were determined. After storage for 50 days at -
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18 ºC, the contents of salt soluble protein of KOG group and S group were 14.81%
20
and 21.27% higher than the control, respectively, and the total SH contents of KOG
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group and S group were 6.90% and 8.46% higher than the control, respectively, and
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the gel strengths of KOG group and S group were 10.85% and 11.89% higher than the
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control, respectively. Although the activities of Ca2+-ATPase of KOG group and S
24
group reached to 0.10 ± 0.02 μmol Pi/mg•min-1 and 0.11 ± 0.01 μmol Pi/mg•min-1
25
after 50 days, respectively, they are significantly different from the control (0.04 ±
26
0.02) μmol Pi/mg•min-1, which indicated an strong cryoprotective effect of KOG.
27
KOG exhibited more powerful antioxidant activity with lower TBA values than S
28
group. It can be concluded that the addition of conventional cryoprotectant and KOG
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can inhibit protein denaturation and reduce the decrease of gel strength, but KOG
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displayed a stronger antioxidant activity with a small addition amount of only 0.5%,
31
despite having no effect to improve the whiteness. These make KOG a promising
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cryoprotectant candidate using in frozen surimi industry.
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Key words: konjac oligo-glucomannan, physicochemical properties, frozen storage
34 35
1. Introduction
36
Aquatic products taste delicious and are rich in nutrition. However, aquatic
37
products keep a higher moisture content and less natural immune material, and
38
contain unsaturated fatty acids which are easily oxidized. Therefore, aquatic products
39
undergo easier deterioration than general animal meat. At present, low-temperature
40
frozen storage is a widely used method for long-term storage of aquatic products (Liu,
41
Wang, & Ding, 2013). The frozen storage can effectively inhibit the microbiological
42
activity and reduce the rate of biochemical reaction in the internal of the protein. Thus
43
the shelf life of the aquatic products can be extended. However, during the frozen
44
storage the proteins of aquatic products might be denatured because of the structural
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changes of the proteins, especially fish myofibrillar protein. Proteins denaturation
46
would seriously reduce the quality of the aquatic products (Liu, Zhao, Zhang, Xu,
47
Ding, & Liu, 2017a). It is reported that, storing fish in a freezer for extended periods
48
of time can negatively affect the gel-forming ability of muscle proteins (Kobayashi &
49
Park, 2017).
50
Frozen surimi is an intermediate product for making other products, which is
51
stable by myofibrillar forming gel network and can be mixed with cryoprotectant to
52
achieve long-term storage. During the processing and frozen-storage of surimi, the
53
changes of water molecule motion, the formation and growth of ice crystals, and the
54
enrichment of solute will promote the protein aggregation of surimi. This would
55
accelerate the protein denaturation of frozen surimi, as well as oxidation would cause
56
myofibril proteins denaturation of frozen surimi (Cao, Zhou, Wang, Sun, & Pan,
57
2018; Du, Sun, Pan, Wang, Ou, & Cao, 2018a). In order to suppress protein
58
denaturation, the cryoprotectant is usually added when making the surimi. The
59
mechanism that the cryoprotectant can protect during the cryopreservation of minced
60
fish is through the interaction and integration of the functional groups and protein
61
molecules on the surface of the minced fish (Parvathy & George, 2014).
62
Polysaccharides have been widely used in the manufacture of meat products to
63
provide the emulsifying, viscous and gelation properties of these products (Chen,
64
Ferng, Chen, Sun, & Lee, 2005). Sucrose is the most traditional polysaccharide,
65
which could effectively protect proteins from denaturation during frozen storage.
66
However, sucrose poses a high sweetness, which not only destroys the taste of surimi,
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but also restrict the usage among certain people with diabetes.
68
Konjac glucomannan (KGM), a high-molecular weight plant polysaccharide
69
extracted from the tubers of Amorphophallus Konjac C. Koch, is composed of glucose
70
and mannose at 1:1.5-1:1.6 molar ratio with 5-10% acetyl substitution (Jian, Wu, Wu,
71
Wu, Jia, Peng, & Sun, 2016). KGM offers great potential for applications in food
72
technology because of its good water absorptivity, gel-forming ability, stability,
73
emulsifiability, thickenability, film-forming properties and it has been approved in
74
Europe and the FDA as a kind of food additive (Wang, Chen, Zhou, Nirasawa,
75
Tatsumi, Li, & Cheng, 2017). KGM could not be hydrolyzed by digestive enzymes in
76
the human upper gastrointestinal tract and is therefore considered as non-calorie
77
indigestible dietary fiber (Xiong, Wei, Ye, Du, Zhou, Lin, Geng, Chen, Corke, & Cai,
78
2009). As a healthy food, the most important benefits of KGM are reducing
79
cholesterol, normalizing triglyceride content and improving blood sugar levels, and
80
promoting intestinal activity and immune function in human beings (Behera & Ray,
81
2016). Besides the remarkable gelation properties and healthy dietary fiber, KGM
82
hydrolysates shows its excellent cryoprotective effect on glass carp myofibrillar
83
proteins and significantly mitigate the decrease in solubility, Ca2+-ATPase activity
84
and total and reactive sulfhydryl (SH) content during frozen storage (Wang, Xiong,
85
Peng, Wu, Li, Wang, Qiao, Liao, & Ding, 2014). Several researchers have reported
86
that KGM can increase breaking force, deformation and water-holding properties of
87
grass carp surimi gels and at the concentration of 1% showed the same good
88
cryoprotective as a conventional cryoprotectant (10% sucrose-sorbiitol, 1:1, w/w)
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(Xiong et al., 2009). konjac oligo-glucomannan (KOG) is a degradation products of
90
KGM, a new type of oligosaccharide with abundant hydrophilic groups and a small
91
amount of branches (Liu, Xu, Zhang, Zhao, & Ding, 2016; Liu, Luo, Gu, Xu, Zhang,
92
Zhao, et al., 2017; Liu, Fang, Xu, Su, Zhao, Ding, 2018).
93
In our previous study, we prepared KOG from KG successfully, and KOG is
94
proved to be a low molecular weight (average DP 5.2), with branched chains and
95
acetyl groups, and more importantly, KOG exhibited a strong antioxidant capacity
96
(Liu, Xu, Zhang, Zhou, Lyu, Zhao, & Ding, 2015). In the present study, the effects of
97
KOG on the physicochemical properties of frozen surimi from red gurnard (Aspitrigla
98
cuculus) were investigated, for the purpose of discovering cryoprotective effects of
99
KOG.
100
2. Materials and methods
101
2.1. Materials
102
Gurnard surimi was obtained from Xingye Group Co., Ltd. (Zhoushan, Zhejiang,
103
China). Konjac glucomannan flour, with purity 98%, was obtained from Hubei
104
Johnson Konjac Co., Ltd. (Hubei, China). Additionally, β-mannanase with activity of
105
50000 U/g, was purchased from Beijing Challenge Bio-Technology Co., Ltd.
106
(Beijing, China). Tris, sodium dodecyl sulfate (SDS), bovine serum albumin (BSA)
107
were purchased from Sigma Company (St. Louis, MO, USA). All other chemicals and
108
reagents were purchased from Beijing Dingguo Changsheng Bio-Technology Co.,
109
Ltd. (Beijing, China).
110
2.2. Preparation of KOG
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KOG was prepared according to our previous research with slight modifications
112
(Liu et al., 2015). An aliquot of 0.1 g β-mannanase (200 U/g) was added to 150 mL
113
HAc-NaAc buffer solution (0.2 M, pH 6.0) and stirred at 50 ºC for 10 min. Then 5%
114
konjac glucomannan was added and stirred at 50 ºC for 2 h. After enzymolysis, the
115
solution was heated to 100 ºC and maintained for 10 min to make the inactivation of
116
enzyme. The enzymatic hydrolysate was filtered through cotton gauze, and
117
concentrated with a vacuum rotary evaporator (RE-2000A, Yarong Co. Ltd.,
118
Shanghai, China), and then mixed with 95% ethyl alcohol. The precipitated KOG was
119
collected by centrifugation at 8000 rpm for 10 min and then resuspended in 95% ethyl
120
alcohol. This step was repeated five times. After being re-dissolved in distilled water,
121
the KOG was ultra-filtrated using a polymer membrane (MW cut-off limit = 8 kDa,
122
Advantec Toyo Co., Ltd. Tokyo, Japan) to remove undegraded KGM and then
123
lyophilized using a freeze-dryer (FD-1-50, Bo Yikang Co. Ltd., Beijing, China). The
124
average degree of polymerization of KOG was approximately equal to 5.2 according
125
to our previous report (Liu et al., 2015).
126
2.3. Preparation of sample and surimi gel
127
An addition of 0.5% of KOG (KOG group) and 8% of a conventional
128
cryoprotectant (4% sorbitol and 4% sucrose, w/w) (S group) was mixed with surimi,
129
respectively. Surimi without adding any cryoprotectants was used as a control (CK
130
group). The surimi was frozen at -18 ºC and stored for 50 days. The indexes were
131
measured every 10 days.
132
The surimi gel was prepared according to the following method. Frozen surimi
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was thawed at 4 ºC and chopped for 5 min. Then the surimi was mixed with 2.5%
134
edible salt (sodium chloride) and chopped for 5 min. The surimi was stuffed into
135
polyvinylidene casing with a diameter of 3 cm and both ends of casing were sealed
136
tightly, then the surimi sol was subjected to setting at 40 ºC for 60 min before heating
137
at 90 ºC for 30 min. The gels were cooled in iced water and stored for 24 h at 4 ºC
138
prior to analyses.
139
2.4. Preparation of myofibrillar protein (Mf)
140
Mf was prepared according to the method of Jiang, Zhang, Cai, Hara, Su and
141
Cao (2006). Red gurnard frozen surimi (5 g) was minced and homogenized with four
142
volumes of ice-cold 20 mM phosphate buffer (pH 7.5) using a homogenizer at the
143
speed indicator of 15, and the operation was carried out for 2 times with each time of
144
30 s and an interval of 1 min. The resulting homogenate was centrifuged at 8000 rpm,
145
4 °C for 10 min. The supernatant was discarded while the pellet collected and
146
resuspended in fourfold of ice-cold phosphate buffer. After three repeating cycles of
147
homogenization and centrifugation, the resulting pellet was suspended in 20 mM
148
phosphate buffer and further homogenized. Finally, after centrifugation at 8000
149
rpm for 10 min, the pellet was resuspended in 20 mM phosphate buffer (pH 8.0)
150
containing 0.5 M NaCl and this suspension was regarded as red gurnard Mf. Mf was
151
immediately used for experiment or stored at a -80 °C freezer for further use.
152
2.5. Salt soluble protein content of surimi
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An aliquot of 45 mL of 0.6 M of KCl (pH 7.0, precooling) was added to red
154
gurnard frozen surimi (5 g). The surimi was homogenized 90 s at 10000 rpm with
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each time of 15 s and an interval of 15 s. The homogenate was then mixed 20 s in an
156
ice water bath, and centrifuged at 8000 rpm, 0 °C for 30 min. The supernatant was
157
taken and filtered with a cloth. The filtrate was mixed with 3 volumes of deionized
158
water at 4 °C, and centrifuged at 5000 rpm, 0 °C for 20 min. The precipitate was
159
added with 20 mL of 0.6 M KCl, and centrifuged at 5000 rpm, 0 °C for 20 min. The
160
protein content of the supernatant was then determined by the Biuret method (Gornall,
161
Bardawill, & David, 1949).
162
2.6. Ca2+-ATPase activity
163
Ca2+-ATPase activity was determined according to our previous report (Liu,
164
Zhao, Zhang, Xu, Ding, & Liu, 2017a). The Ca2+-ATPase assay was performed at
165
37 °C in a solution containing 0.5 M NaCl, 5 mM CaCl2, 1 mM ATP, 25 mM Tris-
166
maleate (pH 7.0), and 6 mg/mL of protein for 10 min. The reaction was stopped by
167
adding HClO4 to a final concentration of 5%. Then the mixture was centrifuged at
168
6000 rpm for 10 min to obtain supernatant. The inorganic phosphate liberated was
169
measured using phosphomolybdate method. The myofibrillar Ca2+-ATPase specific
170
activity was expressed as µmol of Pi liberation hour-1 (mg of protein)-1.
171
2.7. TBA value
172
TBA value was determined according to the method of Cheng, Sun, Pu, Wang
173
and Chen (2015). Five grams of red gurnard frozen surimi was thawed and minced,
174
and then mixed with 25 mL of trichloroacetic acid (20%) and 20 mL of distilled water
175
for centrifuging for 10 min with the revolving speed of 8000 rpm. The filtrate was
176
diluted with ultra-pure water to 50 mL. The mixture of 10 mL of diluent and 10 mL of
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thiobarbituric acid solution was heated in a boiling water bath (95-100 °C) for 15 min
178
to develop a pink color, and then cooled with running tap water for 5 min. The
179
absorbance of the cooled supernatant was measured at 532 nm. A standard curve was
180
prepared using 1,1,3,3-tetrameth-oxypropane at a concentration ranging from 0 to
181
10 ppm, and the TBA values were expressed as mg of MDA/kg sample.
182
2.8. Total sulfhydryl (SH) content
183
Total SH content was determined according to the method of Ingadottir and
184
Kristinsson (2010). The surimi was diluted to give a protein concentration between 1
185
and 2 mg/mL. A 0.25 mL sample of the protein solution was added to 2.5 mL of 8 M
186
urea, 2% sodium dodecylsulphate (SDS) and 10 mM EDTA in 0.2 M Tris-HCl buffer
187
at pH 7.1. To this solution 50 μL of 10 mM Ellman’s reagent (10 mM 5,5′-
188
dithiobis(2-nitrobenzoic acid) was added, mixed and heated in a water bath at 40 °C
189
for 15 min. After the reaction, the absorbance of the solution was measured at 420 nm
190
and the total SH content was calculated using a molar extinction coefficient of
191
13,600 mol/cm.
192
2.9. Gel strength
193
Gel strength was measured using a TA. XT. plus Texture analyzer (SMS, Surrey,
194
UK) according to method of Santana, Huda and Yang (2015). Surimi gels were cut
195
into 2.5 cm thick slices. A slice was placed horizontally on the platform and then was
196
penetrated by a spherical probe (type P/0.25) at a constant 1 mm/s rate until 11 mm
197
depth was reached. The trigger force used was 5 g, with 1 mm/s of pre-test speed and
198
10 mm/s of post-test speed. The load cell capacity of the texture analyzer was 5 kg,
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and the return distance was 35 mm. Gel strength (g•cm) was calculated by multiplying
200
the penetration force (g) by with distance of the penetration (cm).
201
2.10. Whiteness of surimi gel
202
The surimi gels color was determined according to the method of
203
Ruttanapornvareesakul, Somjit, Otsuka, Hara, Osatomi, Osako, Kongpun and Nozaki
204
(2006) with a color difference meter (HunterLab Color Q, HunterLab, Ltd., Reston,
205
VA) by measuring the L* (lightness), a* (redness/ greenness) and b*
206
(yellowness/blueness) value. The whiteness was calculated by the following equation:
207 208
Whiteness = 100 − [(100 − L*)2 + a*2 + b*2 ] 1/2 2.11. Statistical Analysis
209
Data were analyzed using Excel 2010, and significant differences were analyzed
210
using SPSS Statistics 17.0. Multiple comparisons were performed using the Duncan
211
method (P <0.05 for significant differences) plotted with Origin 8.5.
212
3 Results and discussion
213
3.1 Effects of KOG on salt soluble protein in surimi
214
Mf protein is a major functional component of surimi products and is very
215
important for the gelation properties of surimi (Zhou, Jiang, Zhao, Zhang, Gu, Pan
216
and Ding (2017). Mf is a salt soluble protein. During the frozen storage, the
217
intermolecular hydrogen bonds, hydrophobic bonds, disulfide bonds and salt bonds
218
can be formed, which can lead to the decrease of salt solubility (Benjakul,
219
Visessanguan, Thongkaew, & Tanaka, 2003; Lv, Wang, Pan, Cao, Zhang, Sun, et al.,
220
2017; Zhou, Pan, Sun, Li, Xu, Cao, et al., 2018). As seen in Figure 1, the soluble
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protein content of all frozen surimi decreased. The CK group significantly (P<0.05)
222
showed a lower value than KOG and S group. After frozen stored 50 days, the salt
223
soluble protein contents in the CK group, KOG group, and S group were 56.38, 64.73
224
and 68.37 mg/g, respectively. The soluble protein contents of CK decreased by
225
37.76%, which was significantly (P<0.05) higher than the KOG and S groups, which
226
were 28.77% and 24.31%, respectively.
227
The decrease in protein solubility is the main indicator for the denaturation of
228
protein during frozen storage. The sharp decline of Mf solubility in frozen surimi
229
indicated that Mf underwent the denaturation caused by frozen storage. It was found
230
in the present study that the salt soluble contents of the frozen surimi mixed with
231
KOG and conventional cryoprotectant were significantly (P<0.05) higher than that of
232
the CK group, although KOG group showed significantly lower salt soluble content
233
than the S group. This demonstrated that with the addition of KOG, the denaturation
234
of Mf can be suppressed to some extent. An addition of 0.5% of KOG nearly had the
235
same effect with 8% of conventional cryoprotectant in maintaining the salt soluble
236
protein content in frozen surimi, however, KOG lowered the sweetness in surimi
237
products and created no potential crisis to the people with diabetes and obesity.
238
3.2 Effects of KOG on Ca2+-ATPase activity in surimi
239
In the present study, Ca2+-ATPase activity was expressed in the amount of
240
inorganic phosphorus per minute of protein per minute. Ca2+-ATPase activity is an
241
important indicator for the structural integrity and denaturation degree of Mf in frozen
242
surimi. As shown in Figure 2, the Ca2+-ATPase activity of CK, KOG, and S group
ACCEPTED MANUSCRIPT 243
was 0.30, 0.29 and 0.29 μmol Pi/mg•min-1, respectively. During the frozen storage,
244
the Ca2+-ATPase activity of frozen surimi of CK group dropped with the highest
245
speed. The Ca2+-ATPase activity of frozen surimi of CK, KOG and S group dropped
246
dramatically by 30%, 20.69% and 6.90% within the first 10 days. After frozen stored
247
40 days, the Ca2+-ATPase activity of CK, KOG and S group decreased to 0.11, 0.13
248
and 0.18 μmol Pi/mg•min-1, respectively. After frozen stored 50 days, the Ca2+-
249
ATPase activity of frozen surimi of CK, KOG and S group were 0.04, 0.10, and 0.11
250
μmol Pi/mg•min-1, respectively. The Ca2+-ATPase activity of KOG and S group is
251
significantly (P<0.05) higher than that of the CK group.
252
The activity of Ca2+-ATPase is controlled by the globular head of myosin in Mf,
253
so Ca2+-ATPase activity can be used as an indicator of myosin molecular integrity
254
(Zhou, Benjakul, Pan, Gong, & Liu, 2006). The decrease of enzyme activity in long-
255
term frozen storage showed the denaturation of Mf. It can be concluded that KOG can
256
maintain the activity of Ca2+-ATPase to some extent by protecting the integrity of Mf,
257
indicating that KOG has a certain cryoprotective effect during the period of frozen
258
storage of surimi. However, an addition of 0.5% of KOG had the weaker effect than
259
that of 8% of conventional cryoprotectant. Dey and Dora (2011) studied the
260
cryoprotective effects of chitosan on the frozen surimi, and the result showed that 1%
261
of chitosan had the ability to retard the decrease in myosin Ca2+-ATPase activity. In
262
addition, chitosan showed cryoprotective effect similar to commercial cryoprotectants
263
in preventing the decrease in Ca2+-ATPase activity .
264
3.3 Effects of KOG on TBA value of surimi
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The principle of TBA method is based on one of the degradation products of
266
lipid peroxidation, malondialdehyde (MDA), which can be reacted with thiobarbituric
267
acid to form color. The relative content of MDA was determined by the colorimetry at
268
520 nm to understand the peroxidation of lipids in frozen surimi. Figure 3 shows the
269
changes of TBA values within 50-day frozen storage at -18 °C. The TBA values of all
270
frozen surimi increased. However, the KOG group showed the significantly (P<0.05)
271
lower TBA values than the CK and S group within 50 days. At day 0, The TBA
272
values of frozen surimi of CK, KOG and S groups were 0.41, 0.37, and 0.38 mg/kg,
273
respectively. At day 50, The TBA values of frozen surimi of CK, KOG and S groups
274
increased to 0.62, 0.52, and 0.60 mg/kg, respectively. According to Zhang, Wang,
275
Pan, Cao, Shao, Chen, et al. (2016), TBA values ranging from 0.202 to 0.664 could be
276
recognized as fresh meat. The results suggested that KOG had the excellent
277
antioxidant activity in inhibiting the lipids oxidation in frozen surimi during storage,
278
which is supported by our previous research (Liu et al., 2015).
279
3.4 Effects of KOG on total sulfhydryl (SH) in surimi
280
Sulfhydryl group is the most active group in fish protein, which can be oxidized
281
into disulfide group and the content of SH group is reduced during frozen storage. The
282
amount of SH content reflects the protein denaturation degree. The lower the total SH
283
content is, the greater the protein denatures. Figure 4 shows the changes of the total
284
SH content of the frozen surimi during storage. The total SH content of all frozen
285
surimi decreased. The CK group showed the significantly (P<0.05) lower total SH
286
content within 50 days. The total SH content of frozen surimi of CK dropped
ACCEPTED MANUSCRIPT 287
dramatically within the first 20 days, while the KOG and S groups dropped more
288
slowly. After frozen stored 50 days, the total SH content of frozen surimi of CK,
289
KOG and S groups dropped by 39.72%, 36.63%, and 35.62%, respectively.
290
The decrease of the total SH content showed that more disulfide bonds were
291
formed during the 50-day cryopreservation, and the activity of reactive SH content
292
decreased significantly with the decrease of Ca2+-ATPase activity. The head of
293
sulfhydryl (SH1 and SH2) plays a key role in the activity of ATPase. Benjakul,
294
Seymour, Morrissey and An (2010) found that the oxidative parts of SH group,
295
especially the head, played an important role in the activity of ATPase, which also
296
caused Ca2+ATPase inactivation of myosin, resulting in the decrease of Ca2+-ATPase
297
activity. It can be speculated that myosin, especially the head, underwent the
298
conformational changes in the 50 days. These changes gave rise to the exposure of the
299
SH group, making it prone to oxidation or disulfide exchange. In the present study,
300
changes in the total SH content showed that KOG had a better effect in preventing
301
protein denaturation.
302
3.5 Effects of KOG on gel strength of surimi
303
The denaturation of the proteins during the frozen storage can directly affect the
304
gel strength of the surimi. Figure 5 shows the changes of the gel strength of the surimi
305
during 50-day frozen storage. As can be seen in Figure 5, the gel strength of frozen
306
surimi of KOG and S group was higher than that of the CK group during 50-day
307
frozen storage. After 50 days, the gel strength of frozen surimi of CK, KOG and S
308
groups dropped to 252.35, 279.74, and 282.36 g•cm, respectively. However, KOG
ACCEPTED MANUSCRIPT 309
group showed lower gel strength than S group, which is possibly attributed to the
310
relative lower KOG addition amount to surimi. The decrease in gel strength indicated
311
that the protein denaturation occurred. As can be seen in the changes of TBA values
312
of frozen surimi discussed in section 3.3, the increase of TBA values would induce
313
Mf oxidation, which also lead to Mf denaturation (Du, Sun, Pan, Wang, Ou, & Cao,
314
2018b). Mf aggregation and denaturation make it possible to form a weak gel
315
network, resulting in a significant reduction in gel strength. In the present study, the
316
KOG group significantly (P<0.05) retarded the decrease in gel strength of surimi, also
317
suggesting KOG could inhibit the Mf denaturation during frozen storage.
318
3.6 Effects of KOG on whiteness of surimi gel
319
Whiteness is an important indicator of the sensory evaluation of surimi, which
320
directly reflects the quality of the surimi products. Figure 6 shows the changes of
321
whiteness of surimi gel during the frozen storage. It can be found that the whiteness of
322
frozen surimi of KOG group remained unchanged during 50-day storage. The
323
whiteness of frozen surimi of CK group was higher than the KOG and S group in the
324
first 30 days. However, after 30 days, the whiteness of frozen surimi of CK group
325
decreased, and showed lower whiteness value than the other two groups. The results
326
showed that KOG had no significantly (P<0.05) effect on the whiteness of surimi gel.
327
Whiteness is one of the most important properties for surimi. By inhibiting lipids and
328
proteins oxidation, KOG might prevent the significant decline in whiteness of surimi.
329
4. Conclusion
330
In this study, the effect of KOG and conventional cryoprotectant on the
ACCEPTED MANUSCRIPT 331
physicochemical properties of frozen surimi from red gurnard was studied. After 50-
332
day frozen storage, the CK, KOG and S group displayed the decrease in Ca2+-ATPase
333
activity, salt soluble protein content, total SH content and gel strength, and the
334
increase in TBA values. The whiteness values remained nearly unchanged. However,
335
KOG and S group showed that the decrease and increase of these indices were
336
retarded, indicating that the Mf denaturation was inhibited by KOG and conventional
337
cryoprotectant during frozen storage of surimi. In some indices, KOG group showed
338
lower ability to inhibit the Mf denaturation, which was possibly owing to low addition
339
amount of only 0.5% KOG. However, KOG displayed a stronger antioxidant activity
340
in lipids oxidation, and meanwhile keeping the whiteness of surimi gel unchanged. On
341
the other hand, KOG lowered the sweetness in surimi products and created no
342
potential crisis to the people with diabetes and obesity, as compared to 8% addition of
343
conventional cryoprotectant (4% sorbitol and 4% sucrose, w/w). The present study
344
suggested that KOG could be a promising cryoprotectant candidate applying for
345
frozen surimi industry.
346
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571.
456 457
Fig. 1. Effects of KOG on the salt soluble protein content of surimi from red gurnard
458
during frozen storage.
459
Fig. 2. Effects of KOG on the Ca2+-ATPase activity of surimi from red gurnard during
460
frozen storage.
461
Fig. 3. Effects of KOG on the TBA values of surimi from red gurnard during frozen
462
storage.
ACCEPTED MANUSCRIPT 463
Fig. 4. Effects of KOG on the total SH content of surimi from red gurnard during
464
frozen storage.
465
Fig. 5. Effects of KOG on the gel strength of surimi from red gurnard during frozen
466
storage.
467
Fig. 6. Effect of KOG on the whiteness of surimi from red gurnard during frozen
468
storage.
ACCEPTED MANUSCRIPT
Salt soluble protein content (mg/g)
100
CK KOG S 80
60
40
0
10
20
30
40
50
60
Storage time (d)
Fig. 1. Effects of KOG on the salt soluble protein content of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
CK KOG S
0.3
0.2
0.1
2+
Ca -ATPase activity (mol pi/mg.min)
0.4
0.0
0
10
20
30
40
50
60
Storage time (d)
Fig. 2. Effects of KOG on the Ca2+-ATPase activity of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
0.7
CK KOG S
TBA values (mg/kg)
0.6
0.5
0.4
0.3
0.2
0
10
20
30
40
50
60
Storage time (d)
Fig. 3. Effects of KOG on the TBA values of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
7
6
-5
Total SH content (10 mol/g)
CK KOG S
5
4
3
0
10
20
30
40
50
60
Storage time (d)
Fig. 4. Effects of KOG on the total SH content of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
340
CK KOG S
Gel strength (g.cm)
320
300
280
260
240
220
0
10
20
30
40
50
60
Storage time (d)
Fig. 5. Effects of KOG on the gel strength of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
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CK KOG S
Whiteness
72
70
68
66
64
0
10
20
30
40
50
60
Storage time (d)
Fig. 6. Effects of KOG on the whiteness of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT Highlights ► KOG can inhibit surimi Mf denaturation during frozen storage. ► 0.5% KOG had the similar effect with the 8% conventional cryoprotectant. ► KOG would be a promising cryoprotectant candidate using in frozen surimi industry.
Jianhua Liu, Chunhua Fang, Yahong Luo, Yuting Ding, Shulai Liu PII:
S0268-005X(18)31701-6
DOI:
10.1016/j.foodhyd.2018.10.056
Reference:
FOOHYD 4737
To appear in:
Food Hydrocolloids
Received Date:
30 August 2018
Accepted Date:
29 October 2018
Please cite this article as: Jianhua Liu, Chunhua Fang, Yahong Luo, Yuting Ding, Shulai Liu, Effects of konjac oligo-glucomannan on the physicochemical properties of frozen surimi from red gurnard (Aspitrigla cuculus), Food Hydrocolloids (2018), doi: 10.1016/j.foodhyd.2018.10.056
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ACCEPTED MANUSCRIPT
1
Effects of konjac oligo-glucomannan on the physicochemical
2
properties of frozen surimi from red gurnard (Aspitrigla
3
cuculus)
4
Jianhua Liu, Chunhua Fang, Yahong Luo, Yuting Ding, Shulai Liu*
5
Department of Food Science and Engineering, Ocean College, Zhejiang University of
6
Technology, Hangzhou 310014, P. R. China
7 8
Corresponding author: Dr. Shulai Liu (S. Liu). Phone/fax: + 86 571 88320237, E-
9
mail: [email protected]
10 11
Abstract：In this study, the effects of konjac oligo-glucomannan (KOG) on the
12
physicochemical properties of frozen surimi from red gurnard (Aspitrigla cuculus)
13
during frozen storage at -18 ºC for 50 days were investigated. An aliquot of 0.5%
14
(w/w) of KOG was added into the surimi and subjected to frozen storage (KOG
15
group), and 8% of a conventional cryoprotectant (4% sorbitol and 4% sucrose, w/w)
16
was used as a positive control (S group). The salt soluble protein content, the Ca2+-
17
ATPase activity, total sulfhydryl (SH) content, gel strength, thiobarbituric acid (TBA)
18
value, and whiteness of frozen surimi were determined. After storage for 50 days at -
19
18 ºC, the contents of salt soluble protein of KOG group and S group were 14.81%
20
and 21.27% higher than the control, respectively, and the total SH contents of KOG
21
group and S group were 6.90% and 8.46% higher than the control, respectively, and
22
the gel strengths of KOG group and S group were 10.85% and 11.89% higher than the
ACCEPTED MANUSCRIPT 23
control, respectively. Although the activities of Ca2+-ATPase of KOG group and S
24
group reached to 0.10 ± 0.02 μmol Pi/mg•min-1 and 0.11 ± 0.01 μmol Pi/mg•min-1
25
after 50 days, respectively, they are significantly different from the control (0.04 ±
26
0.02) μmol Pi/mg•min-1, which indicated an strong cryoprotective effect of KOG.
27
KOG exhibited more powerful antioxidant activity with lower TBA values than S
28
group. It can be concluded that the addition of conventional cryoprotectant and KOG
29
can inhibit protein denaturation and reduce the decrease of gel strength, but KOG
30
displayed a stronger antioxidant activity with a small addition amount of only 0.5%,
31
despite having no effect to improve the whiteness. These make KOG a promising
32
cryoprotectant candidate using in frozen surimi industry.
33
Key words: konjac oligo-glucomannan, physicochemical properties, frozen storage
34 35
1. Introduction
36
Aquatic products taste delicious and are rich in nutrition. However, aquatic
37
products keep a higher moisture content and less natural immune material, and
38
contain unsaturated fatty acids which are easily oxidized. Therefore, aquatic products
39
undergo easier deterioration than general animal meat. At present, low-temperature
40
frozen storage is a widely used method for long-term storage of aquatic products (Liu,
41
Wang, & Ding, 2013). The frozen storage can effectively inhibit the microbiological
42
activity and reduce the rate of biochemical reaction in the internal of the protein. Thus
43
the shelf life of the aquatic products can be extended. However, during the frozen
44
storage the proteins of aquatic products might be denatured because of the structural
ACCEPTED MANUSCRIPT 45
changes of the proteins, especially fish myofibrillar protein. Proteins denaturation
46
would seriously reduce the quality of the aquatic products (Liu, Zhao, Zhang, Xu,
47
Ding, & Liu, 2017a). It is reported that, storing fish in a freezer for extended periods
48
of time can negatively affect the gel-forming ability of muscle proteins (Kobayashi &
49
Park, 2017).
50
Frozen surimi is an intermediate product for making other products, which is
51
stable by myofibrillar forming gel network and can be mixed with cryoprotectant to
52
achieve long-term storage. During the processing and frozen-storage of surimi, the
53
changes of water molecule motion, the formation and growth of ice crystals, and the
54
enrichment of solute will promote the protein aggregation of surimi. This would
55
accelerate the protein denaturation of frozen surimi, as well as oxidation would cause
56
myofibril proteins denaturation of frozen surimi (Cao, Zhou, Wang, Sun, & Pan,
57
2018; Du, Sun, Pan, Wang, Ou, & Cao, 2018a). In order to suppress protein
58
denaturation, the cryoprotectant is usually added when making the surimi. The
59
mechanism that the cryoprotectant can protect during the cryopreservation of minced
60
fish is through the interaction and integration of the functional groups and protein
61
molecules on the surface of the minced fish (Parvathy & George, 2014).
62
Polysaccharides have been widely used in the manufacture of meat products to
63
provide the emulsifying, viscous and gelation properties of these products (Chen,
64
Ferng, Chen, Sun, & Lee, 2005). Sucrose is the most traditional polysaccharide,
65
which could effectively protect proteins from denaturation during frozen storage.
66
However, sucrose poses a high sweetness, which not only destroys the taste of surimi,
ACCEPTED MANUSCRIPT 67
but also restrict the usage among certain people with diabetes.
68
Konjac glucomannan (KGM), a high-molecular weight plant polysaccharide
69
extracted from the tubers of Amorphophallus Konjac C. Koch, is composed of glucose
70
and mannose at 1:1.5-1:1.6 molar ratio with 5-10% acetyl substitution (Jian, Wu, Wu,
71
Wu, Jia, Peng, & Sun, 2016). KGM offers great potential for applications in food
72
technology because of its good water absorptivity, gel-forming ability, stability,
73
emulsifiability, thickenability, film-forming properties and it has been approved in
74
Europe and the FDA as a kind of food additive (Wang, Chen, Zhou, Nirasawa,
75
Tatsumi, Li, & Cheng, 2017). KGM could not be hydrolyzed by digestive enzymes in
76
the human upper gastrointestinal tract and is therefore considered as non-calorie
77
indigestible dietary fiber (Xiong, Wei, Ye, Du, Zhou, Lin, Geng, Chen, Corke, & Cai,
78
2009). As a healthy food, the most important benefits of KGM are reducing
79
cholesterol, normalizing triglyceride content and improving blood sugar levels, and
80
promoting intestinal activity and immune function in human beings (Behera & Ray,
81
2016). Besides the remarkable gelation properties and healthy dietary fiber, KGM
82
hydrolysates shows its excellent cryoprotective effect on glass carp myofibrillar
83
proteins and significantly mitigate the decrease in solubility, Ca2+-ATPase activity
84
and total and reactive sulfhydryl (SH) content during frozen storage (Wang, Xiong,
85
Peng, Wu, Li, Wang, Qiao, Liao, & Ding, 2014). Several researchers have reported
86
that KGM can increase breaking force, deformation and water-holding properties of
87
grass carp surimi gels and at the concentration of 1% showed the same good
88
cryoprotective as a conventional cryoprotectant (10% sucrose-sorbiitol, 1:1, w/w)
ACCEPTED MANUSCRIPT 89
(Xiong et al., 2009). konjac oligo-glucomannan (KOG) is a degradation products of
90
KGM, a new type of oligosaccharide with abundant hydrophilic groups and a small
91
amount of branches (Liu, Xu, Zhang, Zhao, & Ding, 2016; Liu, Luo, Gu, Xu, Zhang,
92
Zhao, et al., 2017; Liu, Fang, Xu, Su, Zhao, Ding, 2018).
93
In our previous study, we prepared KOG from KG successfully, and KOG is
94
proved to be a low molecular weight (average DP 5.2), with branched chains and
95
acetyl groups, and more importantly, KOG exhibited a strong antioxidant capacity
96
(Liu, Xu, Zhang, Zhou, Lyu, Zhao, & Ding, 2015). In the present study, the effects of
97
KOG on the physicochemical properties of frozen surimi from red gurnard (Aspitrigla
98
cuculus) were investigated, for the purpose of discovering cryoprotective effects of
99
KOG.
100
2. Materials and methods
101
2.1. Materials
102
Gurnard surimi was obtained from Xingye Group Co., Ltd. (Zhoushan, Zhejiang,
103
China). Konjac glucomannan flour, with purity 98%, was obtained from Hubei
104
Johnson Konjac Co., Ltd. (Hubei, China). Additionally, β-mannanase with activity of
105
50000 U/g, was purchased from Beijing Challenge Bio-Technology Co., Ltd.
106
(Beijing, China). Tris, sodium dodecyl sulfate (SDS), bovine serum albumin (BSA)
107
were purchased from Sigma Company (St. Louis, MO, USA). All other chemicals and
108
reagents were purchased from Beijing Dingguo Changsheng Bio-Technology Co.,
109
Ltd. (Beijing, China).
110
2.2. Preparation of KOG
ACCEPTED MANUSCRIPT 111
KOG was prepared according to our previous research with slight modifications
112
(Liu et al., 2015). An aliquot of 0.1 g β-mannanase (200 U/g) was added to 150 mL
113
HAc-NaAc buffer solution (0.2 M, pH 6.0) and stirred at 50 ºC for 10 min. Then 5%
114
konjac glucomannan was added and stirred at 50 ºC for 2 h. After enzymolysis, the
115
solution was heated to 100 ºC and maintained for 10 min to make the inactivation of
116
enzyme. The enzymatic hydrolysate was filtered through cotton gauze, and
117
concentrated with a vacuum rotary evaporator (RE-2000A, Yarong Co. Ltd.,
118
Shanghai, China), and then mixed with 95% ethyl alcohol. The precipitated KOG was
119
collected by centrifugation at 8000 rpm for 10 min and then resuspended in 95% ethyl
120
alcohol. This step was repeated five times. After being re-dissolved in distilled water,
121
the KOG was ultra-filtrated using a polymer membrane (MW cut-off limit = 8 kDa,
122
Advantec Toyo Co., Ltd. Tokyo, Japan) to remove undegraded KGM and then
123
lyophilized using a freeze-dryer (FD-1-50, Bo Yikang Co. Ltd., Beijing, China). The
124
average degree of polymerization of KOG was approximately equal to 5.2 according
125
to our previous report (Liu et al., 2015).
126
2.3. Preparation of sample and surimi gel
127
An addition of 0.5% of KOG (KOG group) and 8% of a conventional
128
cryoprotectant (4% sorbitol and 4% sucrose, w/w) (S group) was mixed with surimi,
129
respectively. Surimi without adding any cryoprotectants was used as a control (CK
130
group). The surimi was frozen at -18 ºC and stored for 50 days. The indexes were
131
measured every 10 days.
132
The surimi gel was prepared according to the following method. Frozen surimi
ACCEPTED MANUSCRIPT 133
was thawed at 4 ºC and chopped for 5 min. Then the surimi was mixed with 2.5%
134
edible salt (sodium chloride) and chopped for 5 min. The surimi was stuffed into
135
polyvinylidene casing with a diameter of 3 cm and both ends of casing were sealed
136
tightly, then the surimi sol was subjected to setting at 40 ºC for 60 min before heating
137
at 90 ºC for 30 min. The gels were cooled in iced water and stored for 24 h at 4 ºC
138
prior to analyses.
139
2.4. Preparation of myofibrillar protein (Mf)
140
Mf was prepared according to the method of Jiang, Zhang, Cai, Hara, Su and
141
Cao (2006). Red gurnard frozen surimi (5 g) was minced and homogenized with four
142
volumes of ice-cold 20 mM phosphate buffer (pH 7.5) using a homogenizer at the
143
speed indicator of 15, and the operation was carried out for 2 times with each time of
144
30 s and an interval of 1 min. The resulting homogenate was centrifuged at 8000 rpm,
145
4 °C for 10 min. The supernatant was discarded while the pellet collected and
146
resuspended in fourfold of ice-cold phosphate buffer. After three repeating cycles of
147
homogenization and centrifugation, the resulting pellet was suspended in 20 mM
148
phosphate buffer and further homogenized. Finally, after centrifugation at 8000
149
rpm for 10 min, the pellet was resuspended in 20 mM phosphate buffer (pH 8.0)
150
containing 0.5 M NaCl and this suspension was regarded as red gurnard Mf. Mf was
151
immediately used for experiment or stored at a -80 °C freezer for further use.
152
2.5. Salt soluble protein content of surimi
153
An aliquot of 45 mL of 0.6 M of KCl (pH 7.0, precooling) was added to red
154
gurnard frozen surimi (5 g). The surimi was homogenized 90 s at 10000 rpm with
ACCEPTED MANUSCRIPT 155
each time of 15 s and an interval of 15 s. The homogenate was then mixed 20 s in an
156
ice water bath, and centrifuged at 8000 rpm, 0 °C for 30 min. The supernatant was
157
taken and filtered with a cloth. The filtrate was mixed with 3 volumes of deionized
158
water at 4 °C, and centrifuged at 5000 rpm, 0 °C for 20 min. The precipitate was
159
added with 20 mL of 0.6 M KCl, and centrifuged at 5000 rpm, 0 °C for 20 min. The
160
protein content of the supernatant was then determined by the Biuret method (Gornall,
161
Bardawill, & David, 1949).
162
2.6. Ca2+-ATPase activity
163
Ca2+-ATPase activity was determined according to our previous report (Liu,
164
Zhao, Zhang, Xu, Ding, & Liu, 2017a). The Ca2+-ATPase assay was performed at
165
37 °C in a solution containing 0.5 M NaCl, 5 mM CaCl2, 1 mM ATP, 25 mM Tris-
166
maleate (pH 7.0), and 6 mg/mL of protein for 10 min. The reaction was stopped by
167
adding HClO4 to a final concentration of 5%. Then the mixture was centrifuged at
168
6000 rpm for 10 min to obtain supernatant. The inorganic phosphate liberated was
169
measured using phosphomolybdate method. The myofibrillar Ca2+-ATPase specific
170
activity was expressed as µmol of Pi liberation hour-1 (mg of protein)-1.
171
2.7. TBA value
172
TBA value was determined according to the method of Cheng, Sun, Pu, Wang
173
and Chen (2015). Five grams of red gurnard frozen surimi was thawed and minced,
174
and then mixed with 25 mL of trichloroacetic acid (20%) and 20 mL of distilled water
175
for centrifuging for 10 min with the revolving speed of 8000 rpm. The filtrate was
176
diluted with ultra-pure water to 50 mL. The mixture of 10 mL of diluent and 10 mL of
ACCEPTED MANUSCRIPT 177
thiobarbituric acid solution was heated in a boiling water bath (95-100 °C) for 15 min
178
to develop a pink color, and then cooled with running tap water for 5 min. The
179
absorbance of the cooled supernatant was measured at 532 nm. A standard curve was
180
prepared using 1,1,3,3-tetrameth-oxypropane at a concentration ranging from 0 to
181
10 ppm, and the TBA values were expressed as mg of MDA/kg sample.
182
2.8. Total sulfhydryl (SH) content
183
Total SH content was determined according to the method of Ingadottir and
184
Kristinsson (2010). The surimi was diluted to give a protein concentration between 1
185
and 2 mg/mL. A 0.25 mL sample of the protein solution was added to 2.5 mL of 8 M
186
urea, 2% sodium dodecylsulphate (SDS) and 10 mM EDTA in 0.2 M Tris-HCl buffer
187
at pH 7.1. To this solution 50 μL of 10 mM Ellman’s reagent (10 mM 5,5′-
188
dithiobis(2-nitrobenzoic acid) was added, mixed and heated in a water bath at 40 °C
189
for 15 min. After the reaction, the absorbance of the solution was measured at 420 nm
190
and the total SH content was calculated using a molar extinction coefficient of
191
13,600 mol/cm.
192
2.9. Gel strength
193
Gel strength was measured using a TA. XT. plus Texture analyzer (SMS, Surrey,
194
UK) according to method of Santana, Huda and Yang (2015). Surimi gels were cut
195
into 2.5 cm thick slices. A slice was placed horizontally on the platform and then was
196
penetrated by a spherical probe (type P/0.25) at a constant 1 mm/s rate until 11 mm
197
depth was reached. The trigger force used was 5 g, with 1 mm/s of pre-test speed and
198
10 mm/s of post-test speed. The load cell capacity of the texture analyzer was 5 kg,
ACCEPTED MANUSCRIPT 199
and the return distance was 35 mm. Gel strength (g•cm) was calculated by multiplying
200
the penetration force (g) by with distance of the penetration (cm).
201
2.10. Whiteness of surimi gel
202
The surimi gels color was determined according to the method of
203
Ruttanapornvareesakul, Somjit, Otsuka, Hara, Osatomi, Osako, Kongpun and Nozaki
204
(2006) with a color difference meter (HunterLab Color Q, HunterLab, Ltd., Reston,
205
VA) by measuring the L* (lightness), a* (redness/ greenness) and b*
206
(yellowness/blueness) value. The whiteness was calculated by the following equation:
207 208
Whiteness = 100 − [(100 − L*)2 + a*2 + b*2 ] 1/2 2.11. Statistical Analysis
209
Data were analyzed using Excel 2010, and significant differences were analyzed
210
using SPSS Statistics 17.0. Multiple comparisons were performed using the Duncan
211
method (P <0.05 for significant differences) plotted with Origin 8.5.
212
3 Results and discussion
213
3.1 Effects of KOG on salt soluble protein in surimi
214
Mf protein is a major functional component of surimi products and is very
215
important for the gelation properties of surimi (Zhou, Jiang, Zhao, Zhang, Gu, Pan
216
and Ding (2017). Mf is a salt soluble protein. During the frozen storage, the
217
intermolecular hydrogen bonds, hydrophobic bonds, disulfide bonds and salt bonds
218
can be formed, which can lead to the decrease of salt solubility (Benjakul,
219
Visessanguan, Thongkaew, & Tanaka, 2003; Lv, Wang, Pan, Cao, Zhang, Sun, et al.,
220
2017; Zhou, Pan, Sun, Li, Xu, Cao, et al., 2018). As seen in Figure 1, the soluble
ACCEPTED MANUSCRIPT 221
protein content of all frozen surimi decreased. The CK group significantly (P<0.05)
222
showed a lower value than KOG and S group. After frozen stored 50 days, the salt
223
soluble protein contents in the CK group, KOG group, and S group were 56.38, 64.73
224
and 68.37 mg/g, respectively. The soluble protein contents of CK decreased by
225
37.76%, which was significantly (P<0.05) higher than the KOG and S groups, which
226
were 28.77% and 24.31%, respectively.
227
The decrease in protein solubility is the main indicator for the denaturation of
228
protein during frozen storage. The sharp decline of Mf solubility in frozen surimi
229
indicated that Mf underwent the denaturation caused by frozen storage. It was found
230
in the present study that the salt soluble contents of the frozen surimi mixed with
231
KOG and conventional cryoprotectant were significantly (P<0.05) higher than that of
232
the CK group, although KOG group showed significantly lower salt soluble content
233
than the S group. This demonstrated that with the addition of KOG, the denaturation
234
of Mf can be suppressed to some extent. An addition of 0.5% of KOG nearly had the
235
same effect with 8% of conventional cryoprotectant in maintaining the salt soluble
236
protein content in frozen surimi, however, KOG lowered the sweetness in surimi
237
products and created no potential crisis to the people with diabetes and obesity.
238
3.2 Effects of KOG on Ca2+-ATPase activity in surimi
239
In the present study, Ca2+-ATPase activity was expressed in the amount of
240
inorganic phosphorus per minute of protein per minute. Ca2+-ATPase activity is an
241
important indicator for the structural integrity and denaturation degree of Mf in frozen
242
surimi. As shown in Figure 2, the Ca2+-ATPase activity of CK, KOG, and S group
ACCEPTED MANUSCRIPT 243
was 0.30, 0.29 and 0.29 μmol Pi/mg•min-1, respectively. During the frozen storage,
244
the Ca2+-ATPase activity of frozen surimi of CK group dropped with the highest
245
speed. The Ca2+-ATPase activity of frozen surimi of CK, KOG and S group dropped
246
dramatically by 30%, 20.69% and 6.90% within the first 10 days. After frozen stored
247
40 days, the Ca2+-ATPase activity of CK, KOG and S group decreased to 0.11, 0.13
248
and 0.18 μmol Pi/mg•min-1, respectively. After frozen stored 50 days, the Ca2+-
249
ATPase activity of frozen surimi of CK, KOG and S group were 0.04, 0.10, and 0.11
250
μmol Pi/mg•min-1, respectively. The Ca2+-ATPase activity of KOG and S group is
251
significantly (P<0.05) higher than that of the CK group.
252
The activity of Ca2+-ATPase is controlled by the globular head of myosin in Mf,
253
so Ca2+-ATPase activity can be used as an indicator of myosin molecular integrity
254
(Zhou, Benjakul, Pan, Gong, & Liu, 2006). The decrease of enzyme activity in long-
255
term frozen storage showed the denaturation of Mf. It can be concluded that KOG can
256
maintain the activity of Ca2+-ATPase to some extent by protecting the integrity of Mf,
257
indicating that KOG has a certain cryoprotective effect during the period of frozen
258
storage of surimi. However, an addition of 0.5% of KOG had the weaker effect than
259
that of 8% of conventional cryoprotectant. Dey and Dora (2011) studied the
260
cryoprotective effects of chitosan on the frozen surimi, and the result showed that 1%
261
of chitosan had the ability to retard the decrease in myosin Ca2+-ATPase activity. In
262
addition, chitosan showed cryoprotective effect similar to commercial cryoprotectants
263
in preventing the decrease in Ca2+-ATPase activity .
264
3.3 Effects of KOG on TBA value of surimi
ACCEPTED MANUSCRIPT 265
The principle of TBA method is based on one of the degradation products of
266
lipid peroxidation, malondialdehyde (MDA), which can be reacted with thiobarbituric
267
acid to form color. The relative content of MDA was determined by the colorimetry at
268
520 nm to understand the peroxidation of lipids in frozen surimi. Figure 3 shows the
269
changes of TBA values within 50-day frozen storage at -18 °C. The TBA values of all
270
frozen surimi increased. However, the KOG group showed the significantly (P<0.05)
271
lower TBA values than the CK and S group within 50 days. At day 0, The TBA
272
values of frozen surimi of CK, KOG and S groups were 0.41, 0.37, and 0.38 mg/kg,
273
respectively. At day 50, The TBA values of frozen surimi of CK, KOG and S groups
274
increased to 0.62, 0.52, and 0.60 mg/kg, respectively. According to Zhang, Wang,
275
Pan, Cao, Shao, Chen, et al. (2016), TBA values ranging from 0.202 to 0.664 could be
276
recognized as fresh meat. The results suggested that KOG had the excellent
277
antioxidant activity in inhibiting the lipids oxidation in frozen surimi during storage,
278
which is supported by our previous research (Liu et al., 2015).
279
3.4 Effects of KOG on total sulfhydryl (SH) in surimi
280
Sulfhydryl group is the most active group in fish protein, which can be oxidized
281
into disulfide group and the content of SH group is reduced during frozen storage. The
282
amount of SH content reflects the protein denaturation degree. The lower the total SH
283
content is, the greater the protein denatures. Figure 4 shows the changes of the total
284
SH content of the frozen surimi during storage. The total SH content of all frozen
285
surimi decreased. The CK group showed the significantly (P<0.05) lower total SH
286
content within 50 days. The total SH content of frozen surimi of CK dropped
ACCEPTED MANUSCRIPT 287
dramatically within the first 20 days, while the KOG and S groups dropped more
288
slowly. After frozen stored 50 days, the total SH content of frozen surimi of CK,
289
KOG and S groups dropped by 39.72%, 36.63%, and 35.62%, respectively.
290
The decrease of the total SH content showed that more disulfide bonds were
291
formed during the 50-day cryopreservation, and the activity of reactive SH content
292
decreased significantly with the decrease of Ca2+-ATPase activity. The head of
293
sulfhydryl (SH1 and SH2) plays a key role in the activity of ATPase. Benjakul,
294
Seymour, Morrissey and An (2010) found that the oxidative parts of SH group,
295
especially the head, played an important role in the activity of ATPase, which also
296
caused Ca2+ATPase inactivation of myosin, resulting in the decrease of Ca2+-ATPase
297
activity. It can be speculated that myosin, especially the head, underwent the
298
conformational changes in the 50 days. These changes gave rise to the exposure of the
299
SH group, making it prone to oxidation or disulfide exchange. In the present study,
300
changes in the total SH content showed that KOG had a better effect in preventing
301
protein denaturation.
302
3.5 Effects of KOG on gel strength of surimi
303
The denaturation of the proteins during the frozen storage can directly affect the
304
gel strength of the surimi. Figure 5 shows the changes of the gel strength of the surimi
305
during 50-day frozen storage. As can be seen in Figure 5, the gel strength of frozen
306
surimi of KOG and S group was higher than that of the CK group during 50-day
307
frozen storage. After 50 days, the gel strength of frozen surimi of CK, KOG and S
308
groups dropped to 252.35, 279.74, and 282.36 g•cm, respectively. However, KOG
ACCEPTED MANUSCRIPT 309
group showed lower gel strength than S group, which is possibly attributed to the
310
relative lower KOG addition amount to surimi. The decrease in gel strength indicated
311
that the protein denaturation occurred. As can be seen in the changes of TBA values
312
of frozen surimi discussed in section 3.3, the increase of TBA values would induce
313
Mf oxidation, which also lead to Mf denaturation (Du, Sun, Pan, Wang, Ou, & Cao,
314
2018b). Mf aggregation and denaturation make it possible to form a weak gel
315
network, resulting in a significant reduction in gel strength. In the present study, the
316
KOG group significantly (P<0.05) retarded the decrease in gel strength of surimi, also
317
suggesting KOG could inhibit the Mf denaturation during frozen storage.
318
3.6 Effects of KOG on whiteness of surimi gel
319
Whiteness is an important indicator of the sensory evaluation of surimi, which
320
directly reflects the quality of the surimi products. Figure 6 shows the changes of
321
whiteness of surimi gel during the frozen storage. It can be found that the whiteness of
322
frozen surimi of KOG group remained unchanged during 50-day storage. The
323
whiteness of frozen surimi of CK group was higher than the KOG and S group in the
324
first 30 days. However, after 30 days, the whiteness of frozen surimi of CK group
325
decreased, and showed lower whiteness value than the other two groups. The results
326
showed that KOG had no significantly (P<0.05) effect on the whiteness of surimi gel.
327
Whiteness is one of the most important properties for surimi. By inhibiting lipids and
328
proteins oxidation, KOG might prevent the significant decline in whiteness of surimi.
329
4. Conclusion
330
In this study, the effect of KOG and conventional cryoprotectant on the
ACCEPTED MANUSCRIPT 331
physicochemical properties of frozen surimi from red gurnard was studied. After 50-
332
day frozen storage, the CK, KOG and S group displayed the decrease in Ca2+-ATPase
333
activity, salt soluble protein content, total SH content and gel strength, and the
334
increase in TBA values. The whiteness values remained nearly unchanged. However,
335
KOG and S group showed that the decrease and increase of these indices were
336
retarded, indicating that the Mf denaturation was inhibited by KOG and conventional
337
cryoprotectant during frozen storage of surimi. In some indices, KOG group showed
338
lower ability to inhibit the Mf denaturation, which was possibly owing to low addition
339
amount of only 0.5% KOG. However, KOG displayed a stronger antioxidant activity
340
in lipids oxidation, and meanwhile keeping the whiteness of surimi gel unchanged. On
341
the other hand, KOG lowered the sweetness in surimi products and created no
342
potential crisis to the people with diabetes and obesity, as compared to 8% addition of
343
conventional cryoprotectant (4% sorbitol and 4% sucrose, w/w). The present study
344
suggested that KOG could be a promising cryoprotectant candidate applying for
345
frozen surimi industry.
346
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456 457
Fig. 1. Effects of KOG on the salt soluble protein content of surimi from red gurnard
458
during frozen storage.
459
Fig. 2. Effects of KOG on the Ca2+-ATPase activity of surimi from red gurnard during
460
frozen storage.
461
Fig. 3. Effects of KOG on the TBA values of surimi from red gurnard during frozen
462
storage.
ACCEPTED MANUSCRIPT 463
Fig. 4. Effects of KOG on the total SH content of surimi from red gurnard during
464
frozen storage.
465
Fig. 5. Effects of KOG on the gel strength of surimi from red gurnard during frozen
466
storage.
467
Fig. 6. Effect of KOG on the whiteness of surimi from red gurnard during frozen
468
storage.
ACCEPTED MANUSCRIPT
Salt soluble protein content (mg/g)
100
CK KOG S 80
60
40
0
10
20
30
40
50
60
Storage time (d)
Fig. 1. Effects of KOG on the salt soluble protein content of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
CK KOG S
0.3
0.2
0.1
2+
Ca -ATPase activity (mol pi/mg.min)
0.4
0.0
0
10
20
30
40
50
60
Storage time (d)
Fig. 2. Effects of KOG on the Ca2+-ATPase activity of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
0.7
CK KOG S
TBA values (mg/kg)
0.6
0.5
0.4
0.3
0.2
0
10
20
30
40
50
60
Storage time (d)
Fig. 3. Effects of KOG on the TBA values of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
7
6
-5
Total SH content (10 mol/g)
CK KOG S
5
4
3
0
10
20
30
40
50
60
Storage time (d)
Fig. 4. Effects of KOG on the total SH content of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
340
CK KOG S
Gel strength (g.cm)
320
300
280
260
240
220
0
10
20
30
40
50
60
Storage time (d)
Fig. 5. Effects of KOG on the gel strength of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT
74
CK KOG S
Whiteness
72
70
68
66
64
0
10
20
30
40
50
60
Storage time (d)
Fig. 6. Effects of KOG on the whiteness of surimi from red gurnard during frozen storage.
ACCEPTED MANUSCRIPT Highlights ► KOG can inhibit surimi Mf denaturation during frozen storage. ► 0.5% KOG had the similar effect with the 8% conventional cryoprotectant. ► KOG would be a promising cryoprotectant candidate using in frozen surimi industry.