Research Article Received: 23 June 2014

Revised: 19 September 2014

Accepted article published: 7 November 2014

Published online in Wiley Online Library: 11 December 2014

(wileyonlinelibrary.com) DOI 10.1002/jsfa.6995

Quality changes during cod (Gadus morhua) desalting at different temperatures Helena Oliveira,a,b,c,d* Amparo Gonçalves,a Maria L Nunes,a Paulo Vaz-Piresb,c and Rui Costad Abstract BACKGROUND: To advise consumers and manufacturers regarding decreasing the time needed to desalt salted cod (a time-consuming process), there is a need to develop knowledge about quality changes at different desalting temperatures. The objective of this work was to evaluate the physico-chemical, microbiological and sensory quality changes and their causes during cod desalting at 5, 10 and 15 ∘ C, using a cod/water ratio of 1:9 without water changes. The influence of slices with different thickness and different desalting times was also evaluated. RESULTS: Desalting promoted a decrease in the levels of total volatile basic nitrogen, thiobarbituric acid reactive substances and free amino acids nitrogen (FAA-N). The highest FAA-N values were found in ‘thicker’ samples desalted at 15 ∘ C due to the higher proteolytic bacteria number observed in these samples, which activity compensated the leaching of soluble components to the desalting solution. The water uptake and the salt leaching out of the muscle found during the processes created conditions for the bacterial growth, contributing to the spoilage at 15 ∘ C. Based on fresh odour and ‘off’-flavours results, ‘thicker’ samples desalted at 15 ∘ C after 72 h were close to spoilage and on microbial levels were spoiled. CONCLUSION: In order to extend the shelf life and safety of cod desalted products, desalting at temperatures above 10 ∘ C is not advisable. © 2014 Society of Chemical Industry Keywords: cod; Gadus morhua; quality changes; desalting; temperature

INTRODUCTION

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Atlantic cod (Gadus morhua) is highly appreciated as a salted product in Mediterranean countries and Latin America due to its nutritional value and specific sensory properties (colour, texture, aroma, and a characteristic taste) imparted by salt curing.1 These important sensory changes remain during desalting and cooking.2 Due to the high salt concentration in the fish muscle (approximately 160–200 g kg−1 ), salt-cured cod must be desalted before consumption.3 Very often desalting involves the fish soaking in tap water at room temperature or under refrigeration with several water changes, resulting in water uptake and salt leaching out of the muscle and, consequently, in a decrease of the muscle firmness obtained by salting.1,4 The process generally takes about 2 days although it depends on the thickness of the fish pieces.1 After rehydration, the fish may be either consumed immediately, stored chilled for a number of days, or stored frozen and then used for the preparation of various cod dishes.5 Desalting is a time-consuming and tedious process that is usually carried out at home by the final consumer.3,4,6 However, increasing consumer demand for ‘easy’ or ‘ready-to-use’ products and the current trend in low-sodium diets have increased the need to include the desalting step among the industrial operations. This has been done by several manufacturers, mainly to produce frozen desalted cod,4,6 in a way similar to that performed by consumers at home.1,6 J Sci Food Agric 2015; 95: 2632–2640

The optimisation of cod desalting on an industrial scale involves the analysis of many process variables, such as process temperature, sample size, the fish muscle zone (since the thickness can affect the desalted product characteristics), cod/water ratio and contact time.4 Temperature control is extremely important due to the fast microbial growth in cod once it is desalted, which might contribute to the desalted cod spoilage.7,8 This happens because, after rehydration, the bacterial growth conditions become very favourable, due to high water content (approximately 800 g kg−1 ) and low salt concentration (20–40 g kg−1 ) found in the product.7 Hence, some new desalted cod products found in the market



Correspondence to: Helena Oliveira, CIIMAR/CIMAR, Interdisciplinary Center of Marine and Environmental Research, University of Porto, Rua dos Bragas 289, 4050-123 Porto, Portugal. E-mail: [email protected]

a IPMA, I.P., Portuguese Institute for the Sea and Atmosphere, I.P., Division of Aquaculture and Upgrading, Avenida de Brasília, 1449-006 Lisboa, Portugal b ICBAS-UP, Abel Salazar Institute for the Biomedical Sciences, University of Porto, Rua Jorge Viterbo Ferreira 228, 4050-313 Porto, Portugal c CIIMAR/CIMAR, Interdisciplinary Center of Marine and Environmental Research, University of Porto, Rua dos Bragas 289, 4050-123 Porto, Portugal d CERNAS, College of Agriculture of the Polytechnic Institute of Coimbra, Bencanta, 3045-601 Coimbra, Portugal

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Quality changes during cod desalting

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Table 1. Analyses made to evaluate the cod quality during 72 h of desalting at 5, 10 and 15 ∘ C in ‘thicker’ samples (TK), ‘thinner’ samples (TN) and desalting water (DW) Analyses Desalting time

Chemical

Physical

Microbiological

0, 72 h

TK, TN: Moisturea, NaCla, proteina, fata, FAA-N, TVB-Na, TBARsa

TK: Colour; TK, TN: Texture; DW: Turbidity

12, 24, 36, 48 h

TK, TN: FAA-N

TK: Colour; TK, TN: (only after 36 h): Texture; DW: Turbidity (TK)

TK: TVCb, proteolytic bacteriab, enterobacteriab, H2 S-producing bacteria; TN, DW: TVCb TK: TVCb, proteolytic bacteriab, enterobacteriab, H2 S-producing bacteria; TN, DW: TVCb

Sensory TK: After 72 h colour, odour, flavour, juiciness, firmness –

a Analyses were performed for a better chemical characterisation of the salted cod and the final desalted products. b Mesophilic and psychrotrophic.

FAA-N, free amino acid nitrogen; TBARs, thiobarbituric acid reactive substances; TVB-N, total volatile basic nitrogen; TVC, total viable counts.

present microbiological quality problems that are probably related to the desalting method.9 Before desalting, salted cod is usually cut into pieces of different size and shape (loins, fins, tail, and pieces corresponding to muscle with mass differences depending on the fish part) that should be commercially acceptable. This is a vital practical aspect for the cod industry because the time needed for desalting will depend largely on the muscle zone and piece thickness.10 Based on the literature, it is concluded that the existing scientific knowledge of the Atlantic cod desalting process is not yet considered enough. No study on the influence of desalting water temperature and thickness of cod slices was published. Therefore, the effects and limits of desalting temperature must be defined. The main objective of this work was to evaluate the physico-chemical, microbiological and sensory quality changes and their causes during cod desalting for up to 72 h at three temperatures (5, 10 and 15 ∘ C). The thickness influence was also assessed.

MATERIALS AND METHODS

J Sci Food Agric 2015; 95: 2632–2640

Methods During the processes, quality was regularly evaluated by physico-chemical, microbiological and sensory analyses made in cod samples and by microbiological and physical analyses in desalting water (Table 1). Samples were taken before desalting and after 12, 24, 36, 48 and 72 h of desalting. For each set of conditions, quality was analysed at least in two cod cubes and two cod parallelepipeds. Chemical analyses Moisture content was determined by oven drying over 24 h at 105 ± 1 ∘ C until constant weight was reached (Method 950.46).11 Measurement of salt content was done by titration according to Mohr’s method (AOAC Method 937.09).12 Determinations were done in triplicate. Protein content was quantified by the Kjeldahl method as total nitrogen content × 6.25 (AOAC Method 981.10).11 Free fat was extracted with diethyl ether solvent in a Soxhlet apparatus (Behr Labor–Technik, Düsseldorf, Germany) (AOAC Method 991.36) during 7 h.11 Fat was determined by weight after drying to constant weight in a 105 ± 1 ∘ C air oven. The protein and free fat analyses of each sample were run in duplicate or triplicate. Results were expressed as g kg−1 of sample. Free amino acids nitrogen (FAA-N) content was determined in duplicate in trichloroacetic acid (20%) extracts of the homogenised samples. The extracts were neutralised with sodium hydroxide (NaOH, 15%) and formaldehyde (40%) was added prior to the titration with 0.1 mol L−1 NaOH. Total volatile basic nitrogen (TVB-N) was determined in duplicate in trichloroacetic acid (5%) extracts of the homogenised samples by the microdiffusion Conway method.13 Results of FAA-N and TVB-N contents were expressed as mg N kg−1 of sample. Lipid oxidation was followed by determining the thiobarbituric acid reactive substances (TBARs) according to Vyncke.14 These substances include the malonaldehyde (MDA) which is an oxidative secondary compound. TBARs were determined in duplicate in trichloroacetic acid (7.5%) extracts of the homogenised samples by the spectrophotometric method. The extract volume analysed was 5 mL. Results were expressed as mg MDA kg−1 of sample and calculated using a standard curve prepared from 1,1,3,3-tetraethoxypropane.

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Cod desalting experiments One salted cod batch (about 20 kg; 10 cod) was produced in a local factory under commercial conditions: thawed fish was pickle-salted (0.5 kg marine salt with 94.5% of sodium chloride kg−1 fish) for approximately 2 weeks and then salt-cured by kenching in stacks (skin side down) over 21 days, at around 4 ∘ C. After that, salted cod was cut into slices with approximately 200 g of unit weight. The slices were transported at 5 ± 1 ∘ C to the laboratory and were cut in similar boneless parallelepipeds and cube-shaped pieces, with 1.0 ± 0.1 cm and 3.5 ± 0.2 cm of thickness, respectively, to reduce variability. A total of 78 cubes with an average weight per unit of 54.1 ± 6.4 g were cut from the salted cod loins. A total of 78 parallelepipeds (each with minimum perpendicular area of 10 × 10 cm2 ) with an average weight per unit of 36.4 ± 6.9 g were taken from the belly flaps. The different-shaped cod pieces were then desalted separately in distilled water throughout 72 h at different temperatures (5, 10 and 15 ∘ C ±1 ∘ C), using a cod/water ratio of 1:9 (w/w) without water changes. All samples were soaked with the skin side up, without overlapping and without stirring. The choice of the temperatures of 5 ∘ C and 10 ∘ C was justified to reproduce industrial conditions and the higher temperature (15 ∘ C) to reproduce eventual temperature abuse which can happen in industry and also to simulate domestic conditions. The

cod/water mass ratio of 1:9 was considered adequate to carry out the process according to previous experiments.4,6,10

www.soci.org Colour measurements The colour of the muscle surface was measured instrumentally using a tri-stimulus colorimeter (Minolta Chroma Meter, model CR-200b; Minolta Camera Co. Ltd, Osaka, Japan), in the L*a*b* system, after calibration with a white standard (L* = 98.0; a* = −0.3; b* = 2.4). In this system, L* denotes lightness on a scale of 0 (black) to 100 (white); the a* value describes the intensity in green colour (negative) and in red colour (positive); and the b* value in blue colour (negative) and in yellow colour (positive). Whiteness of the muscle surface was determined by the equation: Whiteness = L* −3b*.15 The colour measurements were only evaluated in the ‘thicker’ pieces, in all the sides of the cod cubes, except in the side with skin (five points taken in each cube). The final value was presented as an average. Texture measurements Hardness of cod samples was measured using a texture analyser TA.XT Express Enhanced (Stable Micro Systems, Surrey, UK), equipped with a load cell of 5 kg. The instrument was controlled by a computer using the TA.XT Express V 1.1.9.0 software. For each time/temperature combination analysed, two cubes and two (occasionally four) parallelepipeds were analysed. To obtain the height desired, the side with the skin was cut in the ‘thicker’ pieces. All samples were placed with the myotomes oriented perpendicular to the probe. A cylinder probe with 5 mm of diameter was used and the probe was moving downwards 21.6 mm and 5 mm, respectively in each cube and parallelepiped, at a constant speed of 5 mm s−1 during the test. Hardness results were expressed in newtons (N). Turbidity Cell concentration was determined during desalting of ‘thicker’ pieces by measuring the turbidity. The turbidity of the desalting water samples (10 mL) was measured using a portable turbidity meter HI 83749 (Póvoa do Varzim, Portugal), after calibration with different turbidity standards (

Quality changes during cod (Gadus morhua) desalting at different temperatures.

To advise consumers and manufacturers regarding decreasing the time needed to desalt salted cod (a time-consuming process), there is a need to develop...
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