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Journal of Food Engineering 179 (2016) 28e35
Contents lists available at ScienceDirect
Journal of Food Engineering
journal homepage: www.elsevier.com/locate/jfoodeng
An investigation on the application of ohmic heating of cold water
shrimp and brine mixtures
Søren Juhl Pedersen*, Aberham Hailu Feyissa, Sissel Therese Brøkner Kavli, Stina Frosch
Food Production Engineering, DTU Food, Technical University of Denmark, Søltofts Plads 227, DK-2800 Lyngby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2015
Received in revised form
17 January 2016
Accepted 24 January 2016
Available online 26 January 2016
Cooking is an important unit-operation in the production of cooked and peeled shrimps. The present
study explores the feasibility of using ohmic heating for cooking of shrimps. The focus is on investigating
the effects of different process parameters on heating time and quality of ohmic cooked shrimps (Pandalus Borelias). The shrimps were heated to a core temperature of 72 C in a brine solution using a small
batch ohmic heater. Three experiments were performed: 1) a comparative analyses of the temperature
development between different sizes of shrimps and thickness (head and tail region of the shrimp) over
varying salt concentrations (10 kg m 3 to 20 kg m 3) and electric field strengths (1150 V m 1 to
1725 V m 1) with the heating time as the response; 2) a 2 level factorial experiment for screening the
impact of processing conditions using electric field strengths of 1250 V m 1 and 1580 V m 1 and salt
concentrations of 13.75 kg m 3 and 25.75 kg m 3 and 3) evaluating the effect of pretreatment (maturation) of the shrimps before ohmic processing. The maturation experiment was performed with the
following maturation pre-treatments: normal tap water, a 21.25 kg m 3 brine solution and without
maturation. The measured responses for experiments 2 and 3 were: the heating time until the set
temperature of the shrimps was reached, weight loss, press juice and texture profile. It was possible to fit
main effects model relating process settings and the heating time, weight loss and press juice measurements. Furthermore, the results showed that over the tested process workspace no significant
changes were seen in the texture measurements of the shrimps and that the shrimp achieved a comparable quality compared to the conventional heating processes reported in the literature. The findings
show a promising utilization of ohmic heating as a unit operation for the shrimp processing industries.
© 2016 Elsevier Ltd. All rights reserved.
Keywords:
Ohmic heating
Voltage
Field Strength
Salt Concentration
Shrimp
TPA
1. Introduction
Optimal heating of solid foods using the conventional technologies can be challenging due to the fact that heat transfer is limited
by internal conduction. Heating of shrimps can be problematic due
to the size variation within a batch of shrimps. The size variation
within a batch induces over-processing of especially the smallest
shrimps in the batch. This means that in order to process according
to safety criteria and to meet product specification there is a certain
risk for deteriorated product quality and yield loss.
Ohmic heating is a technology, which potentially heats the
product volumetrically by passing an alternating electrical current
through a conductive food material (Sastry, 2008). The volumetric
heating can reduce or eliminate temperature gradients within the
* Corresponding author. Søltofts Plads, 2800, Kgs., Lyngby, Denmark.
E-mail address: sjpe@food.dtu.dk (S.J. Pedersen).
http://dx.doi.org/10.1016/j.jfoodeng.2016.01.022
0260-8774/© 2016 Elsevier Ltd. All rights reserved.
shrimps and thereby alleviate overcooking issues. However, this is
conditional on the electric conductivity of the product – this means
that – in the case of products with spatial variation in the electric
conductivity due to heterogeneous composition spatial differences
in the heating profiles can be induce. Ohmic heating is an old
application that has been under development for a long time and in
recent decades seen a rise in research and development. The
application of ohmic heating has been studied for various applications and food stuffs, and the research results have been extensively reviewed (Kaur and Singh, 2015; Knirsch et al., 2010; Sastry,
2008; Varghese et al., 2012). Salengke and Sastry (2007) investigated the temperature distribution when using ohmic heating of
solideliquid mixture in both static and mixed (agitated) conditions
for cases with a difference in electric conductivity between the
inclusion particle and the surrounding medium. The authors
observed that for the cases where the particle was more conductive
than the medium the cold spot was within the medium in zones
parallel to the particle (also referred to as shadow regions).
S.J. Pedersen et al. / Journal of Food Engineering 179 (2016) 28e35
Additionally, it was found that the process conditions (static and
agitation) and particle size had an impact on the temperature
development. This occurrence in solideliquid mixtures has been
observed and studied by several other authors (Davies et al., 1999;
de Alwis et al., 1989; Fryer et al., 1993). Such phenomenon is highly
important to understand and to be able to determine the slowest
heating location (cold spot) within the system to secure food safety.
Some possible benefits of ohmic heating include reduced
burning of the surface, better nutrient and vitamin retention,
increased energy efficiency and environmentally friendly systems
(Sensoy, 2012). The study on ohmic heating of meat and meat
products is a growing field (McKenna et al., 2006; Sarang et al.,
2008) although the application of ohmic heating on portions of
fish and crustaceans reported in the literature is sparse. The use of
ohmic heating for thawing of shrimps has been reported (Roberts
et al., 1996, 1998, 2002). The results were compared against water
immersion thawing and showed comparable findings in microbial
safety, quality and weight loss.
Several studies have reported on the influence of traditional
thermal processing of shrimps with heated or with boiling water,
and the resulting impact on product quality (Erdogdu et al., 2004;
Erdogdu & Balaban, 2000; Mizuta et al., 1999; Niamnuy et al.,
2007, 2008; Murakami, 1994). In these studies the quality assessments were on textural properties either with instrumental measurements or sensory evaluation, water loss and safety assessment
of microbial inactivation. Erdogdu & Balaban (2000) reported on
the change in texture of thermally processed shrimps and correlated the findings with sensory assessment showing a general
higher acceptability of minimally processed shrimp. Niamnuy et al.
(2007) studied the impact of boiling shrimps in a brine solution and
showed that time was the significant factor for the observed
changes in texture and shrinkage.
The scope of this study is to provide knowledge on ohmic
heating of shrimps, which could be either used as a pretreatment or
the main cooking operation. The experiments performed address
factors such as influence of shrimp size and ion (salt) concentration
in the brine, which in the literature has been identified as important variables in relation to ohmic heating (Sastry, 2008). The experiments were planned in a stepwise manner. The first step; is to
assess the effect of size variation and the effect of spatial variation
within the shrimp (head and tail) with respect to the temperature
profile. The second step; is to evaluate the effect of the ohmic
heating process variables (electric field strength and salt concentration) on the process time and quality of shrimps (weight loss and
texture). Finally, the third step; is to evaluate the effect of preprocessing (maturation step) on the process time and quality of
shrimps (weight loss, press juice and texture) when processing
with the ohmic heating. The maturation step is a common unit
operation used both as a buffer and pretreatment before heating of
shrimps in the industry to promote the peeling in the later stages.
The overall intention was to identify the possible process conditions for industrial implementation of ohmic heating and verification of the feasibility of the unit operation. The responses chosen
for the experiment were: the time until a core temperature of 72 C
was reached (standard processing conditions), press juice, weight
loss and texture of the cooked shrimp.
2. Materials and methods
2.1. Ohmic heater
The ohmic heater used in this study was built by BCH ltd.
(Lancashire, UK). The unit consists of a holding cell made of W500
grade polyethylene with variable size adjustment and mountings
for temperature loggers (K-type). The ohmic heater unit can
29
maximally supply a 230 voltage using alternating current (60 Hz,
sinusoidal). Titanium electrode was used which has high corrosion
resistance in chloride environments (Samaranayake and Sastry,
2005). The distance between the electrodes was set at 12 cm
apart; the width of the chamber was 9.5 cm and the liquid height
including shrimp was approximately 4.5e5 cm.
2.2. Raw materials
Raw frozen shrimps (Pandalus Borelias) were supplied by Royal
Greenland A/S (DK). The shrimps were kept in cold storage ( 18 C)
until testing at the Technical University of Denmark, Lyngby. The
individual shrimp weight varied from approximately 8-13 g.
2.3. Experimental procedure
The shrimps were matured in accordance with the following
procedure: first the shrimps were defrosted in tap water and then
placed in a specified brine solution for 24 h at refrigeration temperature of 0e5 C.
The concentration of the brine solution was prepared as weight
of salt to the total volume of water and salt (msalt/vwaterþsalt) with
the specific brine concentration shown for each of the experimental
conditions in the reported results tables. The water was normal tap
water at approximately 15e20 C. 20 shrimps were heated in each
experimental run, which approximately corresponds to a total
weight of 220 g shrimps. For the ohmic heating process a new brine
solution corresponding to the maturation brine concentration was
prepared, and together with the shrimps added to the ohmic
heater. The ratio of shrimp to brine used in the ohmic heater was
approximately 1:2 in the weight respectively. The placement of the
shrimps was in a parallel position of the body with the electrode
plates and the shrimp inserted with a thermocouple was placed
perpendicular to the bottom of the cell with the head pointing up
(the exact thermocouple placement within the shrimp for each
experiment is described in 2.6, 2.7 and 2.8). The shrimps and brine
were then heated with the ohmic heater until a measured core
temperature in the shrimp of 72 C was reached. The time, temperature and electrical current were recorded during ohmic heating of the shrimps. After the ohmic heating the shrimps were
cooled in excess ice water for five minutes. The weight of each batch
was recorded before the heat treatment and immediately after
cooling for the assessment of weight loss. The shrimps were then
placed in plastic bags. The samples were allowed to thermoset in
the water at room temperature (20e25 C) for an hour before
texture profile analysis (TPA) and press juice measurements (PJ)
were performed. The electrical current was used for calculating the
conductivity of the shrimp and brine mixture using Eq. (1).
s¼
I
AE
(1)
where the conductivity s (S m 1) is calculated from the measured
current I, the area of submerged electrode A and the electric field
strength E.
2.4. Texture profile analysis (TPA) and press juice (PJ)
measurements
Two shrimps from each experimental run were peeled before
the TPA and PJ measurements, respectively. The measurement
protocols are the same as used by Erdogdu & Balaban (2000) and
Niamnuy et al. (2007). For both the TPA and the press juice measurements a Texture Analyzer XT. Plus (Stable Micro Systems Ltd.
UK) was used with a cylindrical probe with Ø of 4 cm on plane
30
S.J. Pedersen et al. / Journal of Food Engineering 179 (2016) 28e35
surfaces. The compression was done at constant speed 1 mm/s to
60% deformation with 0.1 s pause between the compression readings. The PJ test was made with filter paper on top and underneath
the shrimp, which was then sustained a 30 s compression at 1 mm/
s constant speed and a 122.5N load cell. The shrimp and filter paper
were weighed before and after compression, and the response was
percentage of press juice of shrimp mass.
2.5. Data analysis
The statistical data analysis was performed using both Microsoft
Excel 2010 (USA) and R (R development core team, 2014).
2.6. Assessment of heat distribution in shrimp and effect of size
differences on the heating rate
Paired temperature measurements were performed. One thermocouple was placed in the head (Fig. 1, zone A) and one in the tail
part of the shrimp (Fig. 1 zone C), and the temperatures were
recorded to evaluate the difference in the heating rate according to
position. In the experiment for testing the effect of differences in
shrimp size paired temperature measurements were performed
where thermocouples were placed in the smallest and biggest of
the shrimps for each batch. The thermocouple was placed in the
thickest part of the shrimp between the 2nd and 3rd segment
(Fig. 1, zone B). The shrimps were then heated and the time to reach
a core temperature at 72 C was recorded for each shrimp. For
robustness considerations of the results both set of experiments
were replicated over varying voltages and salt concentrations. The
statistical tests performed were paired t-test against an alpha value
of 5%.
2.7. Effect of ohmic process conditions
The experimental design used was a replicated 2 level factorial
design fully randomized. The salt concentration and the electric
field strength were varied at 2 levels, respectively; salt concentrations of 13.75 kg m 3 and 25.75 kg m 3 and electric field strength of
1250 V m 1 and 1580 V m 1. The amount of water and shrimp were
held constant. Maturation of the shrimps before ohmic heating was
performed with a brine solution similar in the salt content to the
subsequent processing brine concentration. Each of the experimental runs constituted an individual brining and processing step
thus the replication refers both to maturation and heating. The
temperature was measured in one shrimp using a thermocouple
placed in zone B (Fig. 1). ANOVA and regression analysis was made
on the model containing main effects and interaction. The model
equation is given by Eq. (2).
Y ¼ ao þ a1 x1 þ a2 x2 þ a3 x1 x2
(2)
where the Y is the response, ao is the intercept, x1 is the electric field
strength, x2 is the salt concentration, and x1x2 is the interaction
term.
2.8. Effect of maturation
The effect of the maturation process before heating was
addressed by preparing samples of shrimps that were matured in
three different ways and subsequently heated under the same
settings in the ohmic heater at 1417 V m 1 and 21.25 kg m 3 brine
solution. The settings for the process conditions in the ohmic heater
are roughly at the center point of the 2 level factorial experiments
(section 2.7). The first maturation preparation method was to thaw
the shrimps as described in the standard procedure and mature in
normal tap water for 24 h at refrigeration temperature. The second
maturation method was maturation in 21.25 kg m 3 brine solution
for 24 h at refrigeration temperature. The third method was to
simply thaw the shrimps according to the standard procedure as
described in section 2.3 and immediately apply ohmic heating. Each
maturation method and the thawed shrimp preparation were
replicated twice. Core temperature profile of one shrimp was
recorded for each experimental run with the thermocouple placed
in zone B (Fig. 1). A one-way ANOVA model was used for the statistical test.
3. Results and discussion
3.1. Assessment of size variation and head and tail heating times
Fig. 1. Diagram of shrimp with numbering of segments. The letters mark zones for the
temperature measurements. The temperature measurements were made in the head
(A), the meat thickest part (B) and in tail meat thinnest part (C). The arrow indicates
the direction of the thermocouple placement.
Table 1 shows the results from the paired t-test performed on
the obtained heating times of the shrimps classified as the biggest
and smallest in each batch. The goal of the experiment was to
identify if volumetric heating is taken place in the shrimps. The
results showed no significant differences between the different
sizes of shrimps according to heating time. The results further
verify that this finding is irrespective of the voltage and salt concentration used. For the time difference between the head and the
tail part to reach 72 C we see a significant difference in heating
time over the varying process conditions depending on the position
of measurement (Table 2). The results show that the tail part of
shrimp is heated slower compared to the head part of shrimp. The
results also indicate that this difference disappears as salt concentration and voltage is increased which at the same time reduced
the heating time. These findings implied that the size of the
shrimps was not of concern for the following experiments,
screening the process conditions and assessing maturation (section
3.2 and 3.3). The evaluation concluded that the temperature measurements should be made in the tail meat part of the shrimps,
which would give the heating profile of the possible coldest spot.
S.J. Pedersen et al. / Journal of Food Engineering 179 (2016) 28e35
31
Table 1
The effect of the size of shrimp on the heating time e measured data and results from paired test.
Salt concertation (kg m 3)
10
Electric field strength (V m 1)
1150
1342
1533
1725
1150
1342
1533
1725
1150
1342
1533
1725
T-test (two-sided paired with H0: Mean diff ¼ 0)
15
20
Table 2
Measured data and results from paired t-test on the head and the tail part of shrimp
differences in heating time.
Salt (kg m 3)
Electric field strength (V m 1)
10
1150
58
1342
47
1533
33
1725
28
1150
45
1342
35
1533
28
1725
24
1150
36
1342
32
1533
24
1725
22
T-test (two-sided paired with H0:
Mean diff ¼ 0)
Time to reach
72 C (s)
Tail
15
20
Time to reach 72 C (s)
Difference
Head
52
42
33
28
39
32
25
24
36
32
24
22
P-value
6
5
0
0
6
3
3
0
0
0
0
0
0.0240
The results from the experiments on size variation and spatial
differences indicate a volumetric heating of the shrimps shown by
the small (0e2 s) to no differences in heating time either between
big and small shrimps as well as between head and tail part at
higher salt concentrations (20 kg m 3) and electric field strengths
(1533e1725 V m 1) (Tables 1 and 2). This was possibly due to the
shrimps and brine being at comparable electric conductivity at the
higher salt concentrations. Temperature zones have been observed
with ohmic heating of solideliquid mixture with differing electric
conductivities between the solid and the liquid (Salengke and
Sastry. 2007). This could be part of the explanation for the spatial
differences (head and tail meat) and the differences between sizes
observed at lower salt concentrations.
3.2. Effect of ohmic processing conditions on heating time and
quality
Table 3 presents the results from the factorial experiment performed on the impact of processing conditions on product quality.
The TPA and PJ measurements for the individual sampled units are
shown; however, the statistical models are based on the mean of
the two samples for each run. This means that the models fitted are
based on the replication of the process variables (electric field
strength and salt concentration) and an averaging over the shrimp
to shrimp variation.
Table 4 presents the results from the ANOVA and regression
models built on the results from the factorial experiment (see Eq.
Difference
Big
Small
54
47
37
27
47
38
26
23
36
28
25
17
49
43
37
27
51
33
26
23
36
38
23
16
P-value
5
4
0
0
4
5
0
0
0
4
2
1
0.8372
(2) and Table 3). The results from the analysis are based on the full
model fitted e this means that both factors and their interaction
term are fitted. Each model is tested against an alpha value of 5%.
The results show some interesting aspects as no significant changes
were seen for the texture profile between the assessed factor levels
whereas the weight loss and press juice showed significant effects.
For the press juice, the magnitudes of the effects are all under or
close to a 1% effect change between low and high levels (Table 4).
The low magnitude of effect change compared to the intercept indicates only marginal improvement attainable by raising the electric field strength and the salt concentration in regards to lowering
press juice. The weight loss is lowered when raising salt concentration and electric field strength. The findings show that the direction of lowering the amount of press juice and weight loss is in
the same direction as the faster heating. Comparing the results of
the press juice, weight loss and the TPA, the changes in water
holding capacity of the shrimps seem not to affect the texture. It
could be postulated that the shrimp to shrimp variation is bigger,
than the possible effect of changing the parameters of the ohmic
heating, with respect to TPA. This seems plausible considering the
short time needed to reach 72 C which is the factor found by others
(Niamnuy et al., 2007) as significantly impacting texture and that
the findings reported in here are for shrimps heated for relatively
short time periods for all experimental settings in comparison. The
results from the present experiment show that all the shrimps have
approximately the same texture profile. The results are comparable
to the findings made by others for texture measurements of shrimp
heated to a temperature of 75 C (Erdogdu & Balaban, 2000).
The model tested for the heating time (Table 4) show that the
salt concentration and electrical field strength effects were both
significant and with no interaction term. Both factors have the effect of lowering the heating time when set at the higher level
(28.75 kg m 3 and 1580 V m 1, respectively). Fig. 2 shows the
measured temperature profiles from the experiments and Fig. 3
shows the calculated electrical conductivity. Fig. 4 shows an interaction plot, which confirms the statistical result that no interaction
effects are present for the time temperature relation tested. In Fig. 2
this can be seen by the overlapping temperature profiles for experiments at 13.75 kg m 3 salt and 1580 V m 1 and 28.75 kg m 3
salt and 1250 V m 1. The temperature measurements also point
towards that raising electrical field strength and salt concentration
lower differences between replicated runs. This can be seen in the
difference between replicate temperature curves for the low setting
of electric field strength and the salt concentration compared to the
high settings (Fig. 2). This was also observed in the study on the
effect of shrimp size differences and spatial variation of the shrimp
on the heating time (3.1). A simple main effects linear model
32
S.J. Pedersen et al. / Journal of Food Engineering 179 (2016) 28e35
Table 3
Experimental settings and results from replicated 22 factorial design in the structured order.
Run
Exp. Field strength Salt
order No. (V m 1)
concentration
(kg m 3)
Time to
72 C (s)
Weight
loss (%)
Sample Press
Texture measurements
unit
juice (%)
Adhesiveness Hardness Springiness Cohesiveness Gumminess Chewiness
(N*mm)
(N)
(mm)
(N)
(N*mm)
1
1
1250
13.75
53
12.77
6
2
1580
13.75
40
8.35
8
3
1250
28.75
34
7.71
4
4
1580
28.75
22
2.63
5
5
1250
13.75
64
10.95
7
6
1580
13.75
37
7.92
3
7
1250
28.75
37
3.54
2
8
1580
28.75
25
3.35
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
13.57
16.91
11.92
16.57
17.53
13.37
13.00
12.36
16.77
14.02
12.56
20.53
17.66
16.57
13.20
19.42
0.03
0.02
0.00
0.00
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.00
0.01
0.01
0.75
3.37
3.45
2.74
4.79
1.67
4.57
2.35
3.64
3.25
1.17
10.85
5.71
2.67
7.66
4.85
3.20
3.89
3.52
5.78
4.96
4.33
4.84
3.89
4.46
3.96
5.34
4.52
4.27
3.20
4.90
5.02
0.70
0.76
0.88
0.86
0.82
0.86
0.85
0.84
0.80
0.86
0.85
0.81
0.86
0.77
0.75
0.83
0.48
2.41
2.94
2.28
3.72
1.40
3.71
1.90
2.73
2.69
0.97
8.34
4.73
1.93
5.40
3.81
1.54
9.38
10.33
13.20
18.44
6.04
17.95
7.39
12.18
10.63
5.16
37.70
20.18
6.18
26.44
19.12
Cohesiveness Gumminess
(N)
Chewiness
(N*mm)
Table 4
Results from ANOVA and regression on 22 designed experiment showing the effect estimates, model correlation and p value of the full model.
Statistical results
Intercept (a0)
Salt (a1)
Electric field strength
(a2)
Interaction (a3)
R2
P-value
Time to 72 C
(s)
Weight loss
(%)
Press juice
(%)
Texture measurements
39
9.5(**)
8(**)
7.154
2.85(**)
1.59(*)
15.38
0.70(*)
0.22
2
0.94
0.0056
0.27
0.88
0.0223
0.98(*)
0.87
0.0296
Adhesiveness
(N*mm)
Hardness
(N)
Springiness
(mm)
0.009
0.002
0.003
3.969
0.316
0.737
4.379
0.046
0.347
0.820
0.004
0.015
3.089
0.234
0.579
13.867
1.350
3.296
0.003
0.44
0.4572
0.165
0.36
0.5785
0.111
0.56
0.3041
0.019
0.38
0.5446
0.198
0.42
0.4952
0.788
0.48
0.4098
* Indicates significant effect term, with * being 5% and ** 1%.
Fig. 2. Measured temperature profiles of shrimps at varying salt concentration (13.75 kg m 3, 28.75 kg m 3) and electric field strength (1250 V m 1, 1580 V m 1). The experimental
setup is shown in Table 3. The small r and filled-in symbol indicates the replicate run.
seemed to explain the relation over the chosen design space on
heating time which is shown in Eq. (3) (with coded factors) and in
Eq. (4) (with transformed factors). All coefficients of the model are
shown with the appropriate standard error in the parenthesis as
shown below.
tsec: ¼ 134:52 0:05*E 1:27*C
ð14:79Þ
ð0:01Þ
ð0:22Þ
(4)
3.3. Maturation effect
tsec: ¼ 39 8×1 9:5×2
ð1:6Þ
ð1:6Þ
ð1:6Þ
(3)
Table 5 shows the results from the impact of the maturation step
on the heating time and quality (press juice, mass loss and TPA).
The results from the three treatments chosen were tested in a one-
S.J. Pedersen et al. / Journal of Food Engineering 179 (2016) 28e35
33
Fig. 3. Calculated electrical conductivity (S m 1) profiles of shrimp and brine mixtures at varying salt concentration (13.75 kg m 3, 28.75 kg m 3) and electric field strength
(1250 V m 1, 1580 V m 1). The experimental setup is shown in Table 3. The small r and filled-in symbol indicates the replicate run.
Table 6
Results from the ANOVA analysis on the maturation experiment.
P-value
Time to 72
Press Juice
Weight
Adhesiveness
Hardness
Springiness
Cohesiveness
Gumminess
Chewiness
Texture Measurements
0.25
0.58
0.038(*)
0.35
0.16
0.29
0.97
0.15
0.46
* Indicates significant effect term, with * being 5% and ** 1%.
Fig. 4. Interaction plot of the results from the factorial experiment of the heating
times.
heating and the samples that matured in brine. In Fig. 5 the temperature measurements of the shrimps are shown. The heating
times recorded until a temperature of 72 C is reached are within
the range observed for the other experiments. Fig. 6 shows the
calculated electrical conductivity as a function of time. The shrimps
Table 5
Results from the maturation experiments.
Time to 72 C Weight loss Sampled
(s)
(%)
unit
Exp.
No
Maturation
1
non
45
0.27
2
non
37
1.69
3
water
52
8.98
4
water
45
5.27
5
21.25 kg m-3
brine
21.25 kg m-3
brine
41
2.26
32
0.46
6
1
2
1
2
1
2
1
2
1
2
1
2
Press juice Texture measurements
(%)
Adhesiveness
Hardness
(N*mm)
(N)
16.56
15.92
19.79
22.94
14.44
18.67
16.76
19.57
10.24
18.99
16.58
18.18
0.01
0.00
0.01
0.01
0.00
0.02
0.01
0.00
0.01
0.01
0.01
0.03
way ANOVA and no significant differences were found on the
outcome except for the weight loss (Table 6). The weight loss was
expected because of the inclusion of the experimental setting using
tap water where some osmosis would be expected to occur over the
24 h time span. The results showed that no differences were found
between the samples that were thawed immediately before ohmic
6.15
1.81
1.76
5.00
2.73
3.03
1.53
2.70
2.48
3.59
3.94
3.23
Springiness
(mm)
Cohesiveness Gumminess Chewiness
(N)
(N*mm)
5.27
3.60
4.09
4.95
0.00
4.31
3.19
4.55
2.49
4.33
4.10
4.13
0.89
0.92
0.81
0.85
0.88
0.82
0.90
0.85
1.15
0.81
0.79
0.77
5.32
1.64
1.34
4.10
2.32
2.33
1.35
2.22
2.96
2.75
2.92
2.33
28.07
5.92
5.48
20.32
e
10.06
4.31
10.08
7.35
11.92
11.98
9.64
matured in water and without maturation indicate different trends
towards the end of the cooking compared to the shrimps matured
in the salt solution. The shrimps matured in a brine show the same
trend in rising electrical conductivity for the shrimp brine mixture
as seen in Fig. 3. However, for the water matured shrimps and those
without maturation, the electrical conductivity increases at the
34
S.J. Pedersen et al. / Journal of Food Engineering 179 (2016) 28e35
Fig. 5. Measured temperature profiles of shrimps during the ohmic heating (electric field strength of 1417 V m 1 and salt concentration of 21.25 kg m 3) with three pretreatment
methods (maturation): maturation in water, maturation in 21.25 kg m 3 salt brine and with no maturation just thawing. The small r and filled-in symbol indicates the replicate run.
Fig. 6. Calculated electrical conductivity (S m 1) profiles of shrimp and brine mixtures which have been applied 1417 V m 1 electric field strength after one of three pretreatment
methods (maturation): maturation in water, maturation in 21.25 kg m 3 salt brine and with no maturation just thawing. The small r and filled-in symbol indicates the replicate run.
beginning of the OH heating and towards the end of the heating
period it reaches the peak thereafter decreasing slightly. No difference was observed for the press juice test or the TPA results. That
there is a difference in weight loss and not in press juice between
the different maturation methods could be that there is osmosis
into the layer between shell and meat e.g. the sub cuticle layer
especially for the shrimps matured in water. This could maybe also
explain the tapering of the electrical conductivity for water
matured shrimps in Fig. 6.
3.4. General discussion
The results from this study are the first to show ohmic heating
for thermal processing of shrimps. The results seem to confirm
instances of volumetric heating, and indicated a main effects relation of the process variables (electric field strength and salt concentration) and the heating time with no interaction between
variables. The heating time decreases with increasing salt concentration and electric field strength level. Additionally, the results
show that over the chosen experimental settings no significant
difference occurred to product quality defined by the TPA. This has
major implications for the design and implementation of ohmic
heating industrially, which could imply a faster implementation of
the technology if the relations persist when up-scaling the process.
Further studies could be made on if or how the processing with
ohmic heating influences the naturally batch-to-batch variation in
raw material quality. Additionally, comparison with traditional
steam heated or boiled shrimps from an industrial production scale
on selected quality parameters (e.g. color, texture, peelability and
microbial stability) is needed.
The results show a rise in difference between replicate cooking
times when electric field strength and salt concentration is lowered
as seen in the paired t-test (Section 3.1) and the factorial experiment (Section 3.2). This would be a relevant investigation point
screening whether it was due to differences in electrical conductivity inducing shadows or whether it is a result from shrimp to
shrimp variations. The results are although opposite of the findings
by Salengke and Sastry (2007), where smaller temperature differences within the ohmic heater was shown when using lower
electric field strength. This was reported for the situation when the
solid had lower electric conductivity than the liquid medium. The
difference seen in the results from those reported for shrimp was
that higher electric field strength gave lower differences between
replicated temperature profiles, are hard to discern. An important
distinction between the experiments performed by Salengke and
Sastry (2007) and the experiments reported here is that the temperature differences are within the ohmic heater and between
experimental runs, respectively. This could cause a confounding by
the mentioned shrimp to shrimp biological variation. It could also
point towards that in between the range of tested electric field
strengths and salt concentrations reported in this paper and those
used by Salengke and Sastry (2007) an unknown transitional
phenomenon takes place for the physics concerning electric
conduction.
S.J. Pedersen et al. / Journal of Food Engineering 179 (2016) 28e35
For the results of the weight loss and press juice measurements,
when assessing both the 2 level factorial experiments and the
maturation experiment together, interesting relations are
observed. Over the span of salt concentrations, the combined results seem to indicate an optimum i.e. lower weight loss seen for
the brine matured shrimps in the maturation experiment. The
reason could be that the relationship between the ionic strength,
voltage and the solubility of the proteins in the shrimps over the
tested experimental range has been at levels approximately at
when both salting-in effects (at lowest salt concentration of
13.75 kg m 3) and salting-out (at lowest salt concentration of
28.75 kg m 3) effects are predominant (Tables 3 and 5). The electric
field strength and salt concentration used in the maturation
experiment is an approximate center point setting when compared
to the 2 level factorial experiments. Similar findings are also seen
when boiling shrimps (Niamnuy et al., 2007, 2008).
It should be noted that a tentative assessment of the ease of
peeling the shrimps was performed for the experimental trails. The
results are not given here since the scale used was not calibrated to
validate the results. However, the tentative findings did point towards an easier peeling of shrimps when higher electric field
strength and salt concentrations were used and also that the
maturation, be it either in brine or water, had a positive effect
compared to no maturation on the ease of peeling.
The results showed that using a set temperature gave a robust
operating criterion with respect to impact on the quality, which
was invariant over the chosen process settings. Due to limitations in
the experimental setup the application of a holding cell was not
possible. This would have allowed for a more extensive study into
thermal inactivation and a proper safety assessment of the technology. The set point temperature used is an industrial specified for
existing conventional processing.
Future studies are ongoing concerning the possible utilization of
ohmic heating in improving the efficiency in de-shelling (peeling)
operations. It would have been pertinent also to address the
question of the impact of water to shrimp ratio. Some preliminary
work has been done but the experimental design is not resolved in
a straight forward manner. It would be interesting to try other
species of commercially sold shrimps in order to assess whether the
findings are similar.
4. Conclusion
The findings exhibited a linear relationship between the heating
time (the time to reach 72 C) and process variables (electric field
strength and the salt concentration) over the experimental range
tested. The press juice was also influenced by the process variables
for the ohmic heating step but not the maturation method. The
variation in shrimp size had no impact on the heating time. For the
temperature history within the shrimp a difference was observed at
lower levels of salt and electric field strength. The difference was
that the head was heated faster than the tail part of shrimp. No
impact was observed on the quality measures from the texture
profile due to the processing conditions. The results showed a
comparable texture of the shrimp to that of conventionally cooked
shrimp reported in other studies. The findings show a promising
application and future possibilities in utilizing ohmic heating for
35
thermal processing of shrimp.
Acknowledgment
This work was performed within the research platform inSPIRe
and within the research project e innovation consortium project
Optimized heating and cooling of meat, seafood and cheese products.
The financing of the work by The Danish Agency for Science,
Technology and Innovation and all collaborators in this project are
gratefully acknowledged. This work was performed in collaboration
with Royal Greenland A/S and BCH ltd.
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