The effect of different cooling techniques and temperature fluctuations on the storage life of cod fillets (Gadus morhua)

The effect of different cooling
techniques and temperature
fluctuations on the storage life
of cod fillets (Gadus morhua)

Hannes Magnússon
Hélène L. Lauzon
Kolbrún Sveinsdóttir
Björn Margeirsson
Eyjólfur Reynisson
Árni Rafn Rúnarsson
María Guðjónsdóttir
Kristín Anna Þórarinsdóttir
Sigurjón Arason
Emilía Martinsdóttir
Nýsköpun og neytendur

Skýrsla
Matís
23-09

Ágúst 2009

ISSN 1670-7192

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Titill / Title
The effect of different cooling techniques and temperature
fluctuations on the storage life of cod fillets (Gadus morhua)

Höfundar / Authors
Hannes Magnússon, Hélène L. Lauzon, Kolbrún Sveinsdóttir, Björn
Margeirsson, Eyjólfur Reynisson, Árni Rafn Rúnarsson, María
Guðjónsdóttir, Kristín Anna Þórarinsdóttir, Sigurjón Arason, Emilía
Martinsdóttir
Skýrsla / Report no.
23-09
Útgáfudagur / Date: August
2009
Verknr. / project no. 1682/1704

Styrktaraðilar / funding: AVS R&D Fund of Ministry of Fisheries in Iceland, the Technology
Development Fund at the Icelandic Centre for Research and EU
(contract FP6-016333-2)

Ágrip á íslensku:
Tilgangur tilraunanna var að kanna tvo kælimiðla um borð í veiðiskipi, að

nota mismunandi kælitækni við vinnslu, m.a. svonefnda CBC (combined

blast and contact) kælingu og kanna áhrif hitastigssveiflna við geymslu í

samanburði við stöðuga geymslu við -1 °C.
Lítill munur var á örveru- og efnamælingum hvort sem notaður var

plötuís eða vökvaís fyrir vinnslu en samkvæmt skynmati reyndist

hópurinn sem var kældur með vökvaís hafa eins dags lengri

ferskleikatíma og geymsluþol. Hitastig var yfirleitt aðeins hærra í
hópnum þar sem plötuís var notaður fyrir vinnslu yfir geymslutímann.

Samkvæmt skynmati, örverutalningum og efnamælingum reyndist CBC

kæling best til lengingar á ferskleikatíma og geymsluþoli. Hitastig

reyndist vera lægra í þeim hópum þar sem CBC kæling var notuð.
Örverufjöldi var svipaður í þeim tveimur hópum þar sem CBC kæling var

ekki notuð við vinnsluna (vökvakæling og engin kæling). Þessar

niðurstöður voru í samræmi við niðurstöður skynmats. TMA gildi voru

aðeins hærri á geymsludögum 12-19 í hópnum sem var vökvakældur.

Niðurstöður hitastigsmælinga yfir geymslutímann voru svipaðar.
Svipaður örverufjöldi reyndist vera í hópum sem geymdir voru við

stöðugt hitastig (um -1 °C) annars vegar og í hópum þar sem

hitastigssveiflum var beitt fyrri hluta geymslutímans hins vegar. Fyrstu 15

daga geymslunnar reyndust TVB-N og TMA gildi vera svipuð í
hópunum. Þeir hópar sem geymdir voru við stöðugt hitastig fóru ekki í

skynmat.

Örverumælingar sem gerðar voru með hinni fljótvirku aðferð qPCR voru

í góðu samræmi við ræktunaraðferðir m.t.t. til Pseudomonas spp. og
Photobacterium phosphoreum.

Lykilorð á íslensku:
Kælitækni, hitasveiflur, þorskflök, ferskleiki, geymsluþol

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Summary in English:
The purpose of this experiment was to examine two different cooling

methods on board fishing vessel, to apply different cooling techniques

during processing at fish plant including the CBC (combined blast and

contact) cooling and to compare storage of packed cod fillets kept either
at steady temperature (-1 °C) or under temperature fluctuations.

No marked difference was seen in microbial and chemical measurements

whether plate ice or liquid ice was used prior to filleting but according to

sensory analysis, the experimental group where liquid ice was used had
one day extension in freshness and shelf life compared to the group with

plate ice. Temperature was usually slightly higher in the plate ice group

than the liquid ice group during storage.

According to sensory, microbiological and chemical analysis, the CBC
cooling clearly resulted in longer freshness period and shelf life extension

in comparison to the two groups where this technique was not applied

during processing. Temperature was lower in these groups during the

storage period.

Similar microbial counts were found between the two experimental
groups where CBC was not applied during processing (liquid cooling and

no cooling). These results were in agreement with results from sensory

analysis. TMA values were however higher on storage days 12 to 19 in

the group with liquid cooling. Temperature measurements during storage
of these two groups were very similar.

No marked difference was seen in microbial counts between groups that

were stored at a constant temperature around -1 °C compared to groups

where temperature fluctuations were used during early phases of storage.
During the first 15 days of storage, TVB-N and TMA values were very

similar for these groups. Sensory analysis was not done on the two groups

kept at -1 °C.

The rapid qPCR analysis was generally in good agreement with the
cultivation methods for Pseudomonas spp. and Photobacterium

phosphoreum.

English keywords:
Cooling techniques, real temperature simulation, cod fillets, freshness,
shelf life
© Copyright
Matís ohf / Matis - Food Research, Innovation & Safety

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Table of Contents 
1 
INTRODUCTION ...................................................................................................... 1 
2 
MATERIAL AND METHODS .................................................................................. 2 
2.1 
Experimental design ............................................................................................. 2 
2.2 
Temperature measurements .................................................................................. 3 
2.3 
Sensory evaluation ............................................................................................... 4 
2.4 
Microbial measurements ...................................................................................... 6 
2.5 
Quantitative PCR measurements .......................................................................... 6 
2.6 
Chemical analysis ................................................................................................. 7 
2.6.1 
Total Volatile Base Nitrogen (TVB-N) and Trimethylamine (TMA) ............. 7 
2.6.2 
pH- measurements ........................................................................................ 7 
2.6.3 
Salt content and water content ...................................................................... 8 
2.7 
Water holding capacity (WHC) ............................................................................ 8 
2.8 
Drip measurements ............................................................................................... 8 
3 
RESULTS AND DISCUSSION ................................................................................. 9 
3.1 
Temperature measurements .................................................................................. 9 
3.1.1 
Ambient temperature from processor to Matis ............................................. 9 
3.1.2 
Steady temperature conditions ...................................................................... 9 
3.1.3 
Real temperature simulation ....................................................................... 11 
3.2 
Sensory evaluation ............................................................................................. 13 
3.3 
Microbial measurements .................................................................................... 19 
3.4 
Quantitative PCR analysis on spoilage microbes ............................................... 22 
3.5 
Chemical measurements ..................................................................................... 24 
3.5.1 
Total Volatile Base Nitrogen (TVB-N) and Trimethylamine (TMA) ........... 24 
3.5.2 
pH – measurements ..................................................................................... 26 
3.5.3 
Salt content and water content .................................................................... 26 
3.6 Water holding capacity (WHC) .......................................................................... 27 
3.7 Drip measurements .............................................................................................. 28 
4 
CONCLUSION ......................................................................................................... 30 
5 
ACKNOWLEDGEMENTS ...................................................................................... 31 
6 REFERENCES ........................................................................................................... 32 
APPENDIX 1 .................................................................................................................... 34 

 

 

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1 INTRODUCTION

Rapid cooling after catch and maintenance of low temperature throughout the whole
chain from catch to consumer is the prerequisite of high quality and long shelf life of fish
products.
During the last few years, Skaginn, Akranes has been developing and designing a new
method in the area of fish processing which is now patented. This process is called
Combined Blast and Contact (CBC) cooling. The technique involves superchilling of the
skin side of fillets by moving them through a freezer tunnel on a teflon coated aluminium
conveyor belt which has a temperature of approximately -8 °C and simultaneously
blasting cold air over the fillets. This rapid cooling process freezes the skin without
freezing the flesh. Before the CBC cooling, the fish goes through pre-cooler/fluid-ice
which contains approximately 2.5% salt and because of the salt content, the fillets can go
through the CBC process without freezing the flesh. This superchilling process facilitates
handling of the fillets, in particular skinning and effective cooling with a resulting fillet
temperature around -1 °C when packed.
The purpose of this experiment was to examine two different cooling methods on board
the fishing vessel, to apply different cooling techniques during processing at fish plant
including the CBC cooling and to compare storage of packed cod fillets kept either at
steady temperature (-1 °C) or under temperature fluctuations.

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2 MATERIAL AND METHODS
2.1 Experimental design
Cod used in the experiment was caught by a long liner SW-of Iceland (Jökuldýpi) on
Oct-8-2008. After bleeding and gutting the cod was washed in sea-water cooled with
liquid ice on deck. Then the fish was transported to the hold where it was on one hand
iced with plate ice and the other liquid ice in tubs.
The cod was landed on Oct-9-2008 and processed at a fish plant near Reykjavík Oct-10-
2008. The fish was filleted and fillets with skin-on got different treatment (see below)
prior to packaging in 5 kg Styrofoam boxes (8 fillets per box). After packaging the cod
was transported to Matís ohf were it was stored either at around -1 °C or under real
temperature simulation for up to 22 days from catch (20 days from packaging). The
purpose of the real temperature simulation is to simulate temperature fluctuations in the
cold chain for exported fresh fish that might take place during storage after processing
and during transport to airport, loading, flight, unloading and possible poor storage
abroad.

The experimental groups were as follows:

A. Plate ice pre-cooling on-board, liquid cooling and combined blast and contact
cooling at plant, storage at -1 °C at Matís.

B. Plate ice pre-cooling on-board, liquid cooling and combined blast and contact
cooling at plant, storage under real temperature simulation at Matís.

C. Plate ice pre-cooling on-board, liquid cooling at plant, storage under real
temperature simulation at Matís.

D. Plate ice pre-cooling on-board, no cooling (control) at plant, storage under real
temperature simulation at Matís.

E. Liquid ice pre-cooling on-board, liquid cooling and combined blast and contact
cooling at plant, storage at -1 °C at Matís.

F. Liquid ice pre-cooling on-board, liquid cooling and combined blast and contact
cooling at plant, storage under real temperature simulation at Matís.

2

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Following abbreviations of experimental groups will be used hereafter:

A. PI, LC-CBC, -1°C
B. PI, LC-CBC, RTS
C. PI, LC, RTS
D. PI, NC, RTS
E. LI, LC-CBC, -1°C
F. LI, LC-CBC, RTS



2.2 Temperature measurements
Temperature at the time of packaging was measured with a handheld thermometer, type
TFX410 (Ebro Electronic, Ingolstadt, GER), with a resolution of 0.1 °C and accuracy of
±0.3 °C. Two types of temperature loggers were used for the temperature measurements.
For measurements of the product temperature inside the wholesale fish boxes, loggers of
type iButton DS1922L with an accuracy of ±0.5 °C, a resolution of 0.0625 °C and an
operating range from -40 to +85 °C were used. Product temperature was measured close
to the centre of the fillet stack in each of the one to three boxes of each experimental
group, which were investigated. Product temperature was recorded at 5 - 10 minutes
intervals and read from the loggers at the end of the experiment.
In order to measure the ambient temperature, TidbiT v2 temperature loggers (Onset
Computer Corporation) were used. These have an accuracy of ±0.2 °C, a resolution of
0.02 °C and an operating range from -20 to +70 °C. Two ambient loggers were applied
right after packaging and thereby the temperature conditions from packaging during
transport to the air climate chambers at Matis were yielded. A total of three and six
ambient temperature loggers were used for the steady storage temperature groups and
dynamic storage temperature groups, respectively. In both cases the loggers were
distributed inside the chamber in order to grasp spatial temperature differences.
Temperature was recorded at 3 - 5 minutes intervals and read at the end of the
experiment.

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2.3 Sensory evaluation
Quantitative Descriptive Analysis (QDA), introduced by Stone and Sidel (2004), was
used to assess cooked samples (MA08sky055-56, 59-61, 63-65, 69) of cod. Eleven
panellists all trained according to international standards (ISO 1993); including detection
and recognition of tastes and odours, trained in the use of scales and in the development
and use of descriptors participated in the sensory evaluation. The members of the panel
were familiar with the QDA method and experienced in sensory analysis of cod. The
panel was trained in recognition of sensory characteristics of the samples and describing
the intensity of each attribute for a given sample using an unstructured scale (from 0 to
100%). Most of the attributes were defined and described by the sensory panel during
other projects (Sveinsdottir and others 2009). The sensory attributes were 30 and are
described in Table 2.
Samples weighing ca. 40 g were taken from the loin part of the fillets and placed in
aluminium boxes coded with three-digit random numbers. The samples were cooked for
6 minutes in a pre-warmed oven (Convotherm Elektrogeräte GmbH, Eglfing, Germany)
at 95-100°C with air circulation and steam, and then served to the panel. Each panellist
evaluated duplicates of each sample in a random order in nine sessions (four samples per
session). A computerized system (FIZZ, Version 2.0, 1994-2000, Biosystémes) was used
for data recording.
All groups evaluated with sensory evaluation were real temperature simulated (RTS). The
following experimental groups were evaluated with sensory evaluation:


B. PI, LC-CBC, RTS
C. PI, LC, RTS
D. PI, NC, RTS
F. LI, LC-CBC, RTS






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Table 1. Sensory vocabulary for cooked samples of cod (Gadus morhua)
Sensory attribute
Short name
Description of attribute
Odour
sweet
o-sweet
sweet odour
shellfish, algae
o-shellfish
shellfish, algae, characterict fresh odour
meaty
o-meat
meaty odour, reminds of boiled meat or halibut
vanilla, boiled milk o-vanilla
vanilla, sweet boiled milk
boiled potatoes
o-potatoes
odour reminds of whole, warm, boiled potatoes
frozen storage
o-frozen
reminds of odour found in refrigerator and/or freezing compartment
table cloth
o-cloth
reminds of damp, unclean cloth (left on kitchen table for 36 h)
TMA
o-TMA
TMA odour, reminds of dried salted fish, amine
sour
o-sour
sour odour, spoilage sour, acetic acid
sulphur
o-sulphur
sulphur, matchstick, boiled kale
Appearance
light/dark colour
a-dark
Left end: light, white colour. Right end: dark, yellowish, brownish, grey
homogenous/
a-heterog.
Left end: homogenous, even colour.
heterogeneous
Right end: discoloured, heterogeneous, stains
white precipitation
a-prec.
white precipitation in the broth or on the fish
Flavour
salt
f-salt
salt taste
metallic f-metallic
metallic
flavour
sweet
f-sweet
characteristic sweet flavour of very fresh (boiled) cod
meaty
f-meat
meaty flavour, reminds of boiled meat
frozen storage
f-frozen
reminds of food which has soaked in refrigerator/freezing odour
pungent
f-pungent
pungent flavour, bitter
sour taste
f-sour
sour taste, spoilage sour
TMA
f-TMA
TMA flavour, reminds of dried salted fish, amine
off flavour
f-off
strenght of off flavour (spoilage flavour/off-flavour)
Texture
flakiness
t-flakes
the fish portion slides into flakes when pressed with the fork
firm/soft
t-soft
Left end: firm. Right end: soft.
Evaluate how firm or soft the fish is during the first bite
dry/juicy
t-juicy
Left end: dry. Right end: Juicy.
Evaluated after chewing several times: dry - pulls juice from the mouth
tough/tender
t-tender
Left end: tough. Right end: tender. Evaluated after chewing several times
mushy t-mushy
mushy
texture
meaty
t-meaty
meaty texture, meaty mouth feel, grude muscle fibers
clammy
t-clammy
clammy texture, dry red wine, tannin
rubbery
t-rubbery
rubbery texture, springy



Data analysis. QDA data was corrected for level effects (effects caused by level
differences between assessors and replicates) by the method of Thybo and Martens
(2000). Principal Component Analysis (PCA) on significant mean level corrected values
of sensory attributes and samples was performed, using full cross validation. Analysis of
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variance (ANOVA) was carried out on QDA data corrected for level effects in the
statistical program NCSS 2000 (NCSS, Utah, USA). The program calculates multiple
comparisons using Duncan’s multiple comparison test. The significance level was set at
5%, if not stated elsewhere.

2.4 Microbial measurements
In all counts surface-plating was used. Total viable psychrotrophic counts (TVC) and
counts of H2S-producing bacteria were evaluated on iron agar (IA) as described by Gram
and others (1987) with the exception that 1% NaCl was used instead of 0.5% with no
overlay. TVC were also done on modified Long and Hammer´s agar (mLH) according to
van Spreekens (1974) with the exception that 1% NaCl was used instead of 0.5%. Plates
were incubated at 15-17 °C for 4-5 d. Bacteria forming black colonies on IA produce H2S
from sodium thiosulphate and/or cysteine. Cephaloridine Fucidin Cetrimide (CFC) agar
was modified according to Stanbridge and Board (1994) and used for enumeration of
presumptive pseudomonads. Pseudomonas Agar Base (Oxoid) with CFC Selective Agar
Supplement (Oxoid) was used. Plates were incubated at 22 °C for 3 d. Pseudomonas spp.
form pink colonies on this medium. Counts of Photobacterium phosphoreum were
estimated by using the PPDM-Malthus conductance method (Dalgaard and others 1996),
as described by Lauzon (2003).
In all experiments, cooled Maximum Recovery Diluent (MRD, Oxoid) was used for
dilutions. All samples were analysed in duplicate and results presented as an average.

2.5 Quantitative PCR measurements
One ml of the tenfold diluted fish samples in MRD buffer was frozen at -20 °C for later
DNA extraction. For the DNA extraction, the diluted samples were centrifuged at 11.000
x g for 7 min to form a pellet. The supernatant was discarded and DNA was recovered
from the pellet using the Promega Magnesil KF, Genomic system (MD1460) DNA
isolation kit (Promega Corporation, Madison, USA) in combination with KingFisher
magnetic beads automatic DNA isolation instrument (Thermo Labsystems, Waltham,
USA) according to the manufacturers´ recommendations.
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