Theofania N. Tsironi, Petros S. Taoukis

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1 Modelling the Effect of Osmotic Pre-treatment with Alternative Solutes on the Shelf Life of Gilthead Seabream Fillets during Refrigerated and Super-chilled Storage Theofania N. Tsironi, Petros S. Taoukis National Technical University of Athens, School of Chemical Engineering, Laboratory of Food Chemistry and Technology, Iroon Polytechniou, Athens, Greece, ABSTRACT The objective of the study was the kinetic modelling of the shelf life of osmotically pre-treated fish during super-chilled storage. Fresh gilthead seabream (Sparus aurata) fillets were pre-treated for to min at C, in osmotic solution % maltodextrin (DE) and % NaCl. Water loss, solid gain, salt content and water activity were monitored throughout pre-treatment. Slices, untreated (coded as Control) and pre-treated for min (coded as HDM), were aerobically packed and stored isothermally at -, -,, 2.,, and C. For samples stored at -, - and 2. C, two alternative solutions % HDM (DE ), % trehalose, % NaCl (coded as HDM-treh) and % HDM (DE ), % glucosamine, % NaCl (coded as HDM-gluc) were also used. Quality assessment was based on microbial growth (total viable count, Pseudomonas spp., Brochothrix thermosphacta, Enterobacteriaceae sp., H 2 S-producing bacteria, lactic acid bacteria, yeasts and moulds), total volatile nitrogen (TVB-N), trimethylamine nitrogen (TMA-N), lipid oxidation (TBARS) and sensory scoring. Quality indices were kinetically modelled and temperature dependence of quality loss rates was modelled by Arrhenius equation. Osmotic pre-treatment led to significant shelf-life extension of fillets, in terms of microbial growth and organoleptic deterioration. The pre-treatment with the alternative solutes led to depression of the freezing point (-., -2., -.2 and -. C for the untreated samples and the osmotically pre-treated with HDM, HDM-treh and HDM-gluc, respectively). TVB-N and TMA-N values were higher in untreated samples, followed by osmotically pre-treated fillets. Based on sensory evaluation, the shelf-life was, 2, and days at 2. C for untreated and pre-treated with HDM, HDM-treh and HDM-gluc, respectively, while the respective values at - C were 2,, and days. Keywords: Osmotic dehydration; fish; super-chilling; cryoprotective agents; kinetic modelling INTRODUCTION Gilthead seabream is a Mediterranean fish of high commercial value due to its desirable characteristics (aroma, taste, white flesh) and has high commercial potential if its shelf life can be extended through packaging or minimal processing. Although usually sold as whole fish, filleted products are of high interest but suffer from shorter shelf life. Considerable research therefore focuses on the control of microbial growth and biochemical reactions to improve gilthead seabream quality and extend shelf life. Partial dehydration of food products by an osmotic process has received increased attention as a pretreatment to further processing to improve nutritional, sensorial and functional properties of food. During the osmotic dehydration, water flows from the product into the osmotic solution, while osmotic solutes are transferred from the solution into the product. By reducing the water activity of the food matrix, microbial growth is reduced or inhibited []. Tsironi et al investigated the effect of osmotic solution concentration on the mass transfer and the potential of using osmotic pre-treatment to extend the shelf life of gilthead seabream fillets [2]. Combined processes with osmotic dehydration could synergistically further increase shelf life. Super-chilling is a technology that consists of storing food just above the initial freezing temperature and helps to inhibit most autolytic and microbial reactions. During super-chilling the temperature of the foodstuff is lowered often -2 C below the initial freezing point of the product []. Recent studies investigate the potential use of cryoprotective agents during frozen storage of fish products. Several studies evaluate the effect of superchilling storage (-. C to - C) on seafood products, including the use of flake or slurry ice. The objective of the study was to evaluate and model the combined effect of osmotic dehydration with alternative solutes and super-chilled storage on the shelf life of chilled fillets of marine cultured gilthead seabream.

2 MATERIALS & METHODS Fresh gilthead seabream (Sparus aurata) fillets directly obtained from the filleting line of a mariculture unit were cut into rectangular slices (xxcm, ±g) and pre-treated for to min at C, in osmotic solution % maltodextrin (DE) and % NaCl. Water loss, solid gain, salt content and water activity were monitored throughout pre-treatment. Slices, untreated (coded as Control) and pre-treated for min (coded as HDM), were aerobically packed and stored isothermally at -, -,, 2.,, and C. For samples stored at -, - and 2. C, two alternative solutions % HDM (DE ), % trehalose and % NaCl (coded as HDM-treh) and % HDM (DE ), % glucosamine and % NaCl (coded as HDM-gluc) were also used. Quality assessment was based on microbial growth (TVC, Pseudomonas spp., lactobacilli, Brochothrix thermosphacta, Enterobacteriaceae spp., H 2 S-producing bacteria, yeasts and moulds), total volatile basic nitrogen (TVB-N), trimethylamine nitrogen (TMA-N), lipid oxidation (TBARs) and sensory scoring. Values of the different measured indices were plotted vs time for all temperatures studied and the apparent order of quality loss was determined based on the least square statistical fit. Temperature-dependence of the deterioration rate constants, k, was modelled by the Arrhenius equation (eq. ), E a ln( k) ln( k () ref ) R T Tref where k ref is the rate constant of the degradation of the respective quality index at a reference temperature, T ref (in this study C), T is the temperature (K), E a is the activation energy of the studied action and R is the universal gas constant. The (E a ) values were estimated from the slope of Arrhenius plots of lnk vs (/T ref - /T), by linear regression []. In order to determine the freezing point of fish slices, raw samples were placed in a freezer (Whirlpool AFG M-B chest freezer, Italy) at temperature of - C. A type T thermocouple was inserted into the centre of the parallelepiped, temperature was constantly monitored and the cooling curves were developed. Analysis of variance (ANOVA) at a significance level of % was used for the analysis of quality degradation rates of untreated and osmotically pre-treated gilthead seabream fillets (STATISTICA., StatSoft Inc., Tulsa, USA). Significant differences were calculated according to Duncan s multiple range test (a=.). RESULTS & DISCUSSION The osmotic pre-treatment caused a significant moisture loss from the fish flesh. The water loss, solid gain, %NaCl and a w of gilthead seabream slices during osmotic pre-treatment are presented in Figures a-d. The water activity values decreased with time and averaged. after min of osmotic pre-treatment, leading to products without the significant quality and nutritional damage, observed using traditional drying methods. The use of the alternative solutes did not affect the mass transfer between the fish flesh and the osmotic solution. Processing time of min was selected as the reference pre-treatment used in the shelf life study. During this time, short enough for practical purposes, high rate of mass exchange is achieved, leading to a level of water loss-solid uptake that allows the product to retain optimum sensory characteristics. At the selected pre-treatment conditions (: ratio of product to solution of % HDM+% NaCl for min at ºC) the fish flesh has % moisture, 2.% solid gain and. water activity. The cooling curve is one of the most simple, accurate and widely used methods to define the freezing point of foods. The wide application of this method is due to its accuracy and simplicity. The cooling curves for the untreated and the osmotically pre-treated gilthead seabream slices are shown on Figure 2. The freezing point of untreated gilthead seabream was determined at -. C, being in agreement with the values reported in the literature for different fish species []. The pre-treatment with the alternative solutes led to depression of the freezing point (-2., -.2 and -. C for HDM, HDM-treh and HDM-gluc, respectively). Pre-treated samples were found to have improved quality stability during subsequent refrigerated storage, in terms of microbial growth, chemical and organoleptic deterioration. The exponential growth rates of total viable count, Pseudomonas spp. and lactic acid bacteria, in the various pre-treatments and storage conditions are presented in Figures a-c. Pseudomonas spp. dominated spoilage at 2. C, being in agreement with the results reported for aerobically stored fresh chilled fish [,].

3 Figure. (a) Water loss (WL: g of water/g initial dry matter), (b) solid gain (SG: g of total solids/g initial dry matter), (c) water activity (a w ) and (d) salt content (% NaCl) of gilthead seabream fillets during osmotic pre-treatment at C using different osmotic solutions: HDM, HDM-treh and HDM-gluc (Mean of three measurements in three different specimens±standard deviation) 2 Temperature ( o C) Time (min) Figure 2. Cooling curve of gilthead seabream slices: untreated (control) and osmotically pre-treated using different osmotic solutions HDM, HDM-treh, HDM-gluc ( incubator temperature).

4 ... (a) (b) (c).. k(d - ).2 k(d-). k(d - )....2 Control HDM HDM-treh HDM-gluc Control HDM HDM-treh HDM-gluc Control HDM HDM-treh HDM-gluc Figure. Exponential growth rates of total viable count, Pseudomonas spp. and lactic acid bacteria in gilthead seabream slices stored at (a) 2. C, (b) - C and (c) - C. (Mean values ± standard error based on the statistical variation of the kinetic parameters of the Baranyi growth model - regression analysis). Initial TBAR levels were.2 mg MDA/kg, being in agreement with the values reported for gilthead seabream and seabass [,]. TBAR values increased with storage time and temperature, with the untreated samples showing no significant differences with the osmotically pre-treated fish slices (. mg MDA/kg after 2 days at 2. C). TBAR values were lower at subzero temperatures and reached. mg MDA/kg after month at - C and 2 months at - C. TVB-N and TMA-N increased with storage time following apparent first order kinetics. TVB-N and TMA-N values were higher in untreated samples, followed by osmotically pre-treated fillets. Storage at subzero temperatures led to significantly lower rates of TVN-N and TMA-N production. A relatively slow production and the low levels of TMA-N in the present study can be attributed to the low initial concentration of trimethylamine oxide (TMAO) in gilthead seabream flesh. The sensory characteristics (appearance, odour, taste, texture) of untreated and osmotically pre-treated samples were evaluated and the results are given representatively for storage at - C in Figure. The sensory scores of untreated and osmotically pre-treated slices were modelled by apparent zero order lines. Osmotic pre-treatment and storage at subzero temperatures maintained the sensory attributes of fish slices, indicating freshness for longer times than the untreated samples. Osmotically pre-treated fish with the addition of trehalose and glucosamine exhibited the highest sensory scores in terms of odour, taste and overall acceptability, with trehalose and glucosamine showing no significant differences with each other. Appearance Taste (a) (c) 2 Odour Texture (b) (d) 2 Figure. Sensory characteristics (a) appearance, (b) odour, (c) taste and (d) texture of gilthead seabream slices stored aerobically at - C: HDM, HDM-treh and HDM-gluc (Mean scores of - panellists ± standard deviation)

5 Microbial growth was well correlated with the chemical and sensory indices during storage at 2. C. Due to the low rates of microbial growth and TVB-N and TMA-N production at - and - C, these were not considered as a good spoilage index at subzero temperatures. In this case quality degradation was attributed to dehydration due to reduced water holding capacity and texture changes (increased hardness) [,]. Trehalose and glucosamnine showed cryoprotective effect on fish slices during storage at subzero temperatures, maintaining freshness characteristics and leading to higher sensory scores. The mechanisms of action of cryoprotectants is not adequately known. Carpenter and Growe attributed the cryoprotective action of several compounds (glucose, sucrose, xylitol, sorbitol, mannitol etc.) to the fact that they are preferentially excluded from contact with the protein s surface [2]. It has also been reported that the chitin hydrolysates (such as glucosamine) increase the amount of unfrozen water in myofibrils and therefore suppresses the generation of ice crystals []. Quality indices were kinetically modelled and temperature dependence of quality loss rates was modelled by Arrhenius equation. The estimated E a values for microbial, chemical and sensory parameters ranged between and 2 kj/mol, indicating high temperature dependence for all the quality indices. The highest E a values were observed for the rates of chemical (TVB-N and TMA-N) changes. Based on the different quality indices the shelf life of gilthead seabream slices can be determined at any temperature conditions in the range studied (Figure ). 2 Shelf life (d) Figure. Shelf life of gilthead seabream slices (based on sensory scoring) at different storage temperatures: Control, HDM, HDM-treh, HDM-gluc. CONCLUSION The objective of the present study was to evaluate the effect of osmotic dehydration with alternative solutes and superchilled storage on the quality characteristics of gilthead seabream. The results of the study show the potential of using osmotic pret-reatment with trehalose or glycosamine to extend the shelf life of fish products. Super-chilled storage gave additional shelf life increase of pre-treated fillets. The proposed models can be a reliable tool for predicting the shelf life of minimally pre-treated gilthead seabream during superchilled storage. REFERENCES Temperature ( o C) [] Raoult-Wack A.L.. Recent advances in the osmotic dehydration of foods. Trends in Food Science & Technology,, 2-2. [2] Tsironi T.N., Salapa I, Taoukis P. 2. Shelf life modelling of osmotically treated chilled gilthead seabream fillets. Innovative Food Science and Emerging Technologies,, 2-. [] Huss H.H.. Quality and quality changes in fresh fish. In: Food and Agricultural organization of United Nations, (paper F.T., Ed.). FAO, Rome.

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