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Vacuum frying process of gilthead sea bream (Sparus aurata) fillets
Innovative Food Science and Emerging Technologies 11 (2010) 630–636
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Innovative Food Science and Emerging Technologies j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i f s e t
Vacuum frying process of gilthead sea bream (Sparus aurata) fillets A. Andrés-Bello, P. García-Segovia, J. Martínez-Monzó ⁎ Food Technology Department, Polytechnic University of Valencia, Camino de Vera, s/n, 46022 Valencia, Spain
a r t i c l e Article history: Received 2 June 2009 Accepted 3 June 2010 Keywords: Vacuum frying Fish Oil content Shrinkage
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a b s t r a c t Vacuum frying was tested as an alternative technique to develop low oil content fried gilthead sea bream fillets. Three oil temperatures for vacuum frying (90, 100, and 110 °C) were considered. For each temperature the times investigated were 1, 2, 3, 4, 5, 6, 8 and 10 min. To compare the effect of vacuum frying to atmospheric frying on the characteristics of gilthead sea bream fillets and frying rate, treatment at 165 °C was considered. The effect of oil temperature and pressure conditions on the drying process and oil absorption of sea bream fillets was investigated. Other product attributes such as shrinkage and colour were analyzed. Atmospheric frying (165 °C) produces a greater decrease in the mass of the fillets than vacuum frying treatment. Compared with atmospheric frying, oil content of vacuum-fried fish fillets was lower. After ten minutes of treatment the values obtained for vacuum frying were 0.14 ± 0.01 goil/gdry solid, 0.18 ± 0.02 goil/gdry solid and 0.12 ± 0.01 goil/gdry solid for 90 °C, 100 °C and 110 °C respectively and 0.27 ± 0.01 goil/ gdry solid for traditional frying at 165 °C. Atmospheric frying was the treatment that produced the greatest shrinkage in the fillets reaching values of 36.8% with respect to the fresh sample after ten minutes of treatment. For vacuum treatments shrinkage values ranged between 23.7% and 33.0% at 100 °C and 110 °C respectively after 10 min of frying. According to the results, the vacuum treated samples became lighter, less greener and more yellow. Industrial relevance: Healthy products are a tendency in the international market. Vacuum frying offers some advantages that can include: the preservation of natural colour and flavours of the products due to low temperature and low oxygen content during the process (better than with conventional deep fat frying), and has less adverse effects on oil quality. The use of this technology on fish products can improve the problems of market saturation at the present time for some species like gilthead sea bream (Sparus aurata). © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Aquaculture in Mediterranean countries, including Spain has experienced a spectacular growth that could turn it into one of the main food producing industries in the near future (Lougovois, Kyranas, & Kyrana, 2003). The sector needs to diversify supply and to produce healthier food linked to the increasing demands of the consumer for quality food elaborated with natural and safe ingredients and long shelf-life (Goulas, Chouliara, Nessi, Kontominas, & Savvaidis, 2005). For these reasons, the study of the technologies that maintain the shelf-life of fish products (quality and safety) is more than justified, constituting a field of research of great interest responding to the demands of the sector and the consumer. Fish is a food that must be included in all healthy and balanced diets. The beneficial effect that fish consumption has for human health is based on its content in n-3 PUFA (polyunsaturated fatty acids). The influence of these fatty acids on the prevention and treatment of a multitude of diseases has been demonstrated (Das, 2000; Simpoulos,
⁎ Corresponding author. Tel.: + 34 963879364; fax: + 34 963877369. E-mail address: [email protected] (J. Martínez-Monzó). 1466-8564/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2010.06.002
2001). Since most fish species are consumed cooked, the nutritional value of the final cooked product is of major importance for human health. Culinary processes can significantly alter the content, composition and biological activity of fish lipids. Frying, which is commonly used for cooking fish, leads to an increase in the fat content of the fish fillet (Candela, Astiasaran, & Bello, 1997; Aro, Tahvonen, Mattila, Nurmi, Sivonen, & Kallio, 2000), extensive lipid exchanges between the fish and the frying medium (Sebedio, Ratnayake, Ackman, & Prevost, 1993) along with the production of oxidized and polymerized lipid products (Kubow, 1992; Skog, Johansson & Jagerstad, 1998). Deep-fat frying is the most common unit operation, involving the immersion and cooking of foods in hot oil, as evidenced by the worldwide annual production of more than 20 million tons of frying oil (Gertz, 2004). Vacuum frying is defined as the frying process that is carried out under pressures well below atmospheric levels, preferably below 50 Torr (6.65 kPa). Due to the low pressure, the boiling point of the water in the food is lowered. Vacuum frying offers some advantages that can include: (1) reduction of the oil content in the fried product, (2) preservation of natural colour and flavours (better than with conventional deep fat frying), and (3) reduction of adverse effects on oil quality (Garayo & Moreira, 2002; Shyu, Hau, & Hwang, 2005).
A. Andrés-Bello et al. / Innovative Food Science and Emerging Technologies 11 (2010) 630–636
In recent years, vacuum-fried studies have been oriented to fruits and vegetables. Da Silva and Moreira (2008) studied mango chips fried under atmospheric vs. vacuum fried conditions, showing more quality vacuum fried mango chips. Shyu and Hwang (2001), studied the effect of processing conditions on the quality of vacuum fried apple chips, concluded that oil content increased for increasing frying times and temperatures. Additionally, vacuum-fried products have higher retention of nutritional quality (phytochemicals), color is enhanced (less oxidation) (Shyu & Hwang, 2001; Fan, Zhang & Mujumdar, 2005a,b; Da Silva & Moreira, 2008; Pérez-Tinoco, Perez, Salgado-Cervantes, Reynes & Vaillant, 2008). Even though frying is an old worldwide process of manufacturing food products, most of the research found in literature is related to atmospheric frying. No studies are available for the effect of vacuum frying on the oil content, colour and nutritional value of fish products. The objective of the present study was to investigate the effect of the vacuum frying process and conditions on the characteristics (oil content, moisture, colour) of gilthead sea bream fillets.
2. Materials and methods 2.1. Sample preparation and storage conditions The cultured gilthead sea bream (Sparus aurata), used in this study were cultivated in net cages and harvested in GRAMASA (Gandía, Valencia, Spain). The sea bream were harvested in December 2005, covered in ice immediately and kept frozen (at −20 °C) until they were handfilleted. Before being hand-filleted, they were first defrosted in a refrigerator at 4 °C overnight. After filleting they were vacuum packaged in polyethylene bags and stored in a refrigerator at 2 °C. The weight of the sea bream before filleting was approximately 400 g (commercial size). The weight of the fillets was between 52 g and 35 g.
3. Vacuum frying For frying tests GASTROVAC® equipment was used (Fig. 1). This equipment allows control of the temperature in vacuum conditions or atmospheric pressure. The equipment was designed, constructed and patented at the Polytechnic University of Valencia (Martínez-Monzó et al., 2004). The electrical appliance consists of a pressure cooker vessel with an inner basket, a membrane vacuum pump (Model
631
24207, Selecta S.L., Barcelona, Spain) to provide vacuum to the vessel and a heating system controlled by a temperature probe (Fig. 1). Three levels of oil temperature for vacuum frying (90, 100, and 110 °C) were considered in this study. The vacuum pressures used in vacuum frying treatments for each temperature were 15 kPa for 90 °C, 20 kPa for 100 °C and 25 kPa for 110 °C respectively. For each temperature the times investigated were 1, 2, 3, 4, 5, 6, 8, 10 min. These times were selected on the basis of previous experiments. The vacuum vessel was set to the target temperature and allowed to operate for 30 min before frying started. The oil volume in the vessel was 2 L. Sunflower oil was used in all experiments and the oil was filtered by means a sieve (filter pore size 1 mm) after each experiment and changed for each temperature. The fillet/oil ratio was around 0.07 w/w. The quality of the oil was controlled, before and after the frying process, with a FOM-310 probe (Ebro Electronic GmbH γ Co. KG, Ingolstadt, Germany). The temperature in the centre of the fillet was controlled, before and after the frying process, with a Testo 925 probe (Testo GmbH y Co., Lenzkirch, Germany). Three sea bream fillets were fried each time. Once the oil temperature reached the target value the fillets (previously weighed) were placed into the basket, the lid was closed and the vessel was evacuated. Frying operation consisted of an initial depressurisation step with the product outside the oil (1 min), immersion of the product once the vacuum reached the target value ( ± 1 °C), the frying period, and pressurized time (1 min). Then, the lid of the vessel was opened and the sea bream fillets were removed from the basket and the temperature of the product was measured. The sea bream fillets were then allowed to cool to room temperature (25 °C) and dried with paper towels to remove the excess surface oil (Da Silva & Moreira, 2008). 4. Atmospheric frying For the atmospheric frying experiments, the same equipment and procedure were used but the vacuum pump was switched off. To compare the effects of atmospheric frying and vacuum frying on the quality and frying rate of gilthead sea bream fillets, an oil temperature of 165 °C was used, this temperature was selected because of it is a common temperature used in fish frying (Gladyshev, Sushchik, Gubanenko, Demirchieva & Kalachova, 2007). Times investigated were the same as those for vacuum treatments. The atmospheric fryer was set to the required frying temperature and left for 30 min to ensure that the oil temperature was constant. The
Fig. 1. Vacuum-frying system. (1) Main body, (2) pan, (3) basket, (4) heating element, (5) temperature probe, (6) temperature selector, (7) switch, (8) vacuum pump, (9) electric supply, (10) vacuum connector, (11) valve, (12) elevator, (13) manometer, (14) hermetic closing.
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sea bream fillets were then allowed to cool to room temperature (25 °C) and dried with paper towels to remove excess oil. 5. Proximate composition 5.1. Moisture content The initial moisture content of defrosted and uncooked gilthead sea bream and the moisture content after frying was measured for each time and temperature analyzed. Moisture content was determined by drying six homogeneous samples of 5 g of fish to a stable weight for 48 h at 110 °C (A.O.A.C, 1997). 5.2. Oil content Total fat content of six dried samples (5 g) was extracted with petroleum ether (BP 40–60 °C) during four hours in an extracting unit Soxtec System 2055 Tecator (FOSS, Hillerød, Denmark) and gravimetrically determined. 6. Characteristics of the fried product 6.1. Weight loss Weight loss was calculated as the percentage weight difference between the raw and fried samples relative to the weight of the raw fillets. The samples were dried with absorbent paper before being weighed in order to remove the surface water from the fresh fillets and the oil from the fried ones. The weight of the samples was measured with an analytical balance Mettler Toledo model PB 303-S (Mettler Toledo GmbH, Greinfensee, Switzerland). Three samples were used for each time and temperature. 6.2. Shrinkage Pictures of each fillet were taken before and after treatment with a digital camera (Sony model DSC-P7, Tokyo, Japan). The degree of surface shrinkage (S) was analyzed with a computer program (Image Tool version 3.00, University of Texas Health Science Center in San Antonio, Texas, U.S.A.), and was evaluated by Eq. (1): S = ðSo−Sðt ÞÞ = So4100
ð1Þ
where So is the original surface of the sample before frying (cm2) and S(t) is the surface of the sample at time t after frying (cm2). 6.3. Colour The colour of the gilthead sea bream samples was measured using a colorimeter Minolta CM3600d (Minolta Co. Ltd, Tokyo, Japan) (Illuminant D 65, 10° viewing angle). According to the CIE LAB system, Lightness (L⁎), green-red chromacity (a⁎), blue-yellow chromacity (b⁎), chroma (C⁎ab) and hue (h⁎ab) were measured. Measurements were taken in triplicate. The colorimeter was standardized using a white calibration plate.
Fig. 2. Temperature at the centre of gilthead sea bream fillets (Sparus aurata) after atmospheric pressure frying (AT) and under conditions of continuous vacuum frying (V).
8. Results and discussion 8.1. Temperature profile Fig. 2 shows the temperature at the centre of gilthead sea bream fillets (S. aurata) after atmospheric pressure frying and under conditions of continuous vacuum frying. The temperature reached in the fillet depends on the frying temperature. The higher values were achieved for treatments at 165 °C. Temperatures needed to boil water were reached approximately at 2 min for vacuum treatments at 90 °C (55.5 °C at 15 kPa), 2.5 min for vacuum treatments at 100 °C (62.3 °C at 20 kPa), 2 min for vacuum treatments at 110 °C (67.6 at 25 kPa) and 3 min for atmospheric treatments at 165 °C (100 °C at 101.3 kPa). 8.2. Weight loss Fig. 3 shows the weight loss experienced by gilthead sea bream fillets (S. aurata) after atmospheric pressure frying and under conditions of continuous vacuum frying. The weight variation experienced by the samples fried under atmospheric conditions was around 44 ± 2% with respect to initial weight while the mass variation experienced by fillets fried under vacuum conditions was 19 ± 1% at 90 °C, 22.1 ± 0.1% at 100 °C and 36 ± 2% at 110 °C after 10 min of treatment. As can be seen in Fig. 3, for both treatments greater water loss takes place with higher temperature and frying time. Therefore a high temperature (165 °C) combined with a long frying time (10 min) produces a drier product. The atmospheric frying treatment (165 °C) produces a greater decrease in the weight of the fillets than the vacuum frying treatment (90, 100, 110 °C). The greatest differences were found at 8 and 10 min as can be see in Fig. 3. Analysis of variance showed significant differences (p b 0.05) between both treatments (vacuum frying and atmospheric frying). The different temperatures for vacuum treatments were compared and results showed that there was a tendency to increase weight loss with temperature but
7. Statistical analysis The effect of temperature and vacuum pressure on the drying curve, the oil content, shrinkage, colour and weight loss of the gilthead sea bream fillets (S. aurata) was evaluated using a factorial design with four levels for temperature and eight levels for frying time. The statistical analysis of the data was conducted using Statgraphics statistical software package Statgraphics Plus 5.1. (Statistical Graphics Corporation, Herndon, U.S.A.). Statistical significance was expressed at the p b 0.05 level.
Fig. 3. Weight loss (%) experienced by gilthead sea bream fillets during frying process.
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Fig. 4. Moisture loss (d.b.) of gilthead sea bream fillets versus frying time for different treatments.
statistical analysis showed that these differences were not significant (p b 0.05). 9. Moisture loss Fig. 4 shows the moisture loss (d.b.) during the frying process for the different frying temperatures, times and treatments. At 165 °C (atmospheric frying) the moisture content was above 0.59 ± 0.03 (g water/g dry solid), 1.73 ± 0.03 (g water/g dry solid) at 90 °C (vacuum frying) and 1.2 ± 0.2 (g water/g dry solid) at 110 °C after 10 min of treatment. The moisture content of fried samples decreased significantly (p b 0.05) during frying. Oil temperature had a negative effect on the moisture content of fried fillets for all treatments. Moisture loss in vacuum-fried samples was lower (about 25–40% after ten minutes of frying) than the moisture loss in atmospheric fried samples (about 70% after ten minutes of frying) for all the times and temperatures studied. The moisture loss was significantly affected (p b 0.05) by frying treatment (vacuum frying or atmospheric frying) but no differences (p N 0.05) were found between frying temperatures in vacuum treatments. 10. Oil uptake Fig. 5 shows the effect of frying techniques on the oil content of the samples (d.b.). The initial fat content in fresh fillets was 0.15 ± 0.02 goil/gdry solid or 4.7 ± 0.7% in wet base. The maximum values obtained in this study ranged between 0.24 ± 0.02 goil/gdry solid at eight minutes of treatment at 90 °C and 0.30 ± 0.01 goil/gdry solid for the same time at 165 °C. In some cases, it appears that there was no oil intake, due to the variability in the initial fat content of the samples. Previous studies of vacuum frying were made using vegetables with no significant fat content (Moreira, Castell-Perez & Barrufet, 1999; Shyu & Hwang, 2001; Garayo & Moreira, 2002; Shyu, Hau & Hwang, 2005; Pérez-Tinoco et al., 2008; Da Silva & Moreira, 2008; Mariscal & Bouchon, 2008; Troncoso, Pedreschi & Zuniga, 2009).
Fig. 5. Oil content (d.b.) of gilthead sea bream fillets versus frying time for different treatments.
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The effect of temperature was significant (p b 0.05) in vacuum treatments. In vacuum frying treatments the increase in the frying temperature implies a decrease in oil absorption. The treatment at 110 °C was the one that produced the lowest oil content in fried fillets. Similar behaviour was observed by Garayo and Moreira (2002) in the vacuum frying of potato chips. They concluded that the faster the rate of water loss, the higher the oil adhesion on the surface of the chips surface, leading to greater oil absorption. In addition, as the percentage of free water is depleted in the product, less oil is absorbed. Several studies demonstrated that most of the oil does not penetrate the product during frying but during the cooling period, when the product is removed from the fryer (Moreira et al., 1999). When frying is completed, the food is removed from the fryer and the product starts to cool, leading to water vapour condensation and a subsequent decrease in internal pressure. Oil adhered to the food surface is sucked in due to the consequent ‘vacuum effect’. Therefore, oil uptake is a surface phenomenon, involving equilibrium between adhesion and drainage of oil as the food is removed from the oil bath (Ufheil, & Escher, 1996, Moreira, Sun & Chen, 1997; Moreira & Barrufet, 1998). Fig. 5 shows that the oil uptake in vacuum fried samples was lower than the oil uptake with atmospheric frying for all the times and temperatures studied. In this study and in agreement with other authors such as Garayo and Moreira (2002), it can be considered that the most important mass transfer mechanism in a continuous vacuum frying process is the pressurization period. The pressurization step plays an important role in reducing the oil absorption during vacuum frying. It can increase or decrease oil absorption in the product depending on the amount of surface oil and free water presented in the product (Garayo & Moreira, 2002). During pressurization from vacuum to atmospheric conditions, as the vessel is vented, the pressure in the pores of the samples rapidly increases to atmospheric levels. Air and surface oil are carried into the empty pore spaces until the pressure reaches atmospheric levels. However, because of the low pressure, gas diffuses much faster into the pore space thus obstructing the passage of the oil into the pores. In the cooling period, since less oil adhered to the surface of the samples during vacuum frying, less oil is absorbed on cooling (Garayo, & Moreira, 2002). In these experiments once the products were fried, the basket was raised and centrifuged for two minutes before vessel pressurization in order to reduce oil absorption. In order to analyze the frying process, Pinthus, Weinberg and Saguy (1993) introduced the criterion UR, which expresses the weight ratio between the amount of oil uptake and water removed. They defined the “uptake ratio”, UR, as: UR = Oil uptakeðgÞ = Water removedðgÞ
ð2Þ
The value of UR is derived from Eq. (2) for any specific point in the frying process by considering the initial and final moisture and oil content (Pinthus et al., 1993). In this study they assumed that for an equal volume exchange the theoretical value of UR should be 0.9 for an ideal food system in which oil uptake is governed by the water
Fig. 6. Shrinkage (%) of gilthead sea bream fillets as function of frying time and temperatures.
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replacement mechanism. Pinthus et al. (1993) concluded that when a system displayed significantly different values of UR which were below and above the expected theoretical value many factors other than “simple” water replacement are involved. The values obtained in this study are lower than 0.9 for all the temperatures studied, 0.08 was the value obtained at 90 °C, 0.03 for 110 °C and 0.4 at 165 °C after ten minutes of frying. This fact could suggest that in a vacuum frying process there is another important mechanism, such as the pressurization period that governs the process of water replacement by oil. Reduced values of the UR parameter for all the temperatures tested are related to low water loss and low oil uptake in the process. 11. Shrinkage Fig. 6 presents the degree of shrinkage of gilthead sea bream fillets cooked with atmospheric and vacuum frying as a function of temperature and time. Atmospheric frying was the treatment that produced the greatest shrinkage in the fillets reaching values of 36.8% with respect to the fresh sample after ten minutes of treatment. For vacuum treatments shrinkage values ranged between 23.7% and 33.0% for 100 °C and 110 °C respectively after 10 min of frying. The statistical analysis
showed significant differences (p b 0.05) in the percentage of shrinkage between atmospheric frying and vacuum frying, but no differences (p N 0.05) were found between the different treatment temperatures in vacuum frying. 11.1. Colour Fig. 7a shows the L⁎ evolution as a function of time and temperature. Statistical analysis showed that there were significant differences (p b 0.05) for Lightness (L⁎), green-red chromacity (a⁎), blue-yellow chromacity (b⁎) and chroma (C⁎) of gilthead sea bream fillets as a function of frying treatment (deep fat frying or vacuum frying). Statistical analysis indicated significant differences (p b 0.05) between atmospheric frying and vacuum frying for lightness. L⁎ values were higher for the vacuum frying treatment. The a⁎ values of fried fillets increased with the frying temperature and frying time (Fig. 7b). The a⁎ values of fried fillets became positive after frying at 165 °C for six minutes, indicating that dark red fried fillets were produced. This may be caused by a non-enzymatic browning reaction during heating (Richardson & Hyslop, 1985). However the significant differences for a⁎ values were between 165 °C (atmospheric pressure) and the vacuum frying temperatures. The blue-yellow chromatically
Fig. 7. Changes in CIE L⁎a⁎b⁎ values as affected by different frying times and temperatures. (a) Lightness (L⁎), (b) green-red chromaticity (a⁎), (c) blue-yellow chromaticity (b⁎), (d) Hue (h⁎ab) and (e) Chroma (C⁎ab).
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Fig. 8. Visual appearance of fried gilthead sea bream fillets under vacuum and atmospheric conditions after 10 min of treatment: (a) 90 °C-vacuum, (b) 100 °C-vacuum, (c) 110 °Cvacuum and (d) 165 °C-atmospheric pressure.
(b⁎) values were also significantly higher (p b 0.05) for fillets fried at atmospheric pressure than for vacuum fried fillets (Fig. 7c). The values calculated for chroma (C⁎ab) showed significant differences (p b 0.05) between atmospheric and vacuum treatments (Fig. 7e). Differences between vacuum treatments also appear; at 90 °C chroma values are lower. Hue values (h⁎ab) ranged from 186° for fresh samples to 81°for fillets fried at 165 °C (atmospheric pressure) and no significant differences between atmospheric and vacuum treatments (p N 0.05) were found for this parameter (Fig. 7d). Visual observation confirmed the results obtained with the colorimeter, as the fillets fried under deep-fat frying conditions were darker and redder than the others fried by the vacuum frying method (Fig. 8). Fish fillet retained better their original colour when fried under vacuum. The vacuum frying method clearly reduced colour degradation due to the absence of Maillard reactions and oxidation during the process. A sensory evaluation is needed to evaluate consumer preferences. 12. Conclusions Compared with traditional frying, the oil content of vacuum-fried fish fillets was lower. After ten minutes of treatment the values obtained for vacuum frying were 0.14 ± 0.01 goil/gdry solid, 0.18 ± 0.02 goil/gdry solid and 0.12 ± 0.01 goil/gdry solid for 90 °C, 100 °C and 110 °C respectively and 0.27 ± 0.01 goil/gdry solid for traditional frying at 165 °C. Deep-fat frying (165 °C) produces a greater decrease in the weight of the fillets than the vacuum frying treatment. Moisture loss in vacuum fried samples was lower than the moisture loss in atmospheric frying for all the times and temperatures studied producing a moister product. For vacuum treatments shrinkage values ranged between 23.7% and 33.0%. Atmospheric frying was the treatment that produced greatest shrinkage in the fillets reaching values of 36.8% with respect to the fresh sample. A reduction in colour attribute changes was observed for vacuum treatments. The process of frying under vacuum conditions may be an alternative to atmospheric frying for fish fillets to obtain convenience foods with appropriate composition attributes. Acknowledgements The authors would like to acknowledge the support of the INNOVA programme of the Polytechnic University of Valencia in the financing for this study.
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Innovative Food Science and Emerging Technologies j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i f s e t
Vacuum frying process of gilthead sea bream (Sparus aurata) fillets A. Andrés-Bello, P. García-Segovia, J. Martínez-Monzó ⁎ Food Technology Department, Polytechnic University of Valencia, Camino de Vera, s/n, 46022 Valencia, Spain
a r t i c l e Article history: Received 2 June 2009 Accepted 3 June 2010 Keywords: Vacuum frying Fish Oil content Shrinkage
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a b s t r a c t Vacuum frying was tested as an alternative technique to develop low oil content fried gilthead sea bream fillets. Three oil temperatures for vacuum frying (90, 100, and 110 °C) were considered. For each temperature the times investigated were 1, 2, 3, 4, 5, 6, 8 and 10 min. To compare the effect of vacuum frying to atmospheric frying on the characteristics of gilthead sea bream fillets and frying rate, treatment at 165 °C was considered. The effect of oil temperature and pressure conditions on the drying process and oil absorption of sea bream fillets was investigated. Other product attributes such as shrinkage and colour were analyzed. Atmospheric frying (165 °C) produces a greater decrease in the mass of the fillets than vacuum frying treatment. Compared with atmospheric frying, oil content of vacuum-fried fish fillets was lower. After ten minutes of treatment the values obtained for vacuum frying were 0.14 ± 0.01 goil/gdry solid, 0.18 ± 0.02 goil/gdry solid and 0.12 ± 0.01 goil/gdry solid for 90 °C, 100 °C and 110 °C respectively and 0.27 ± 0.01 goil/ gdry solid for traditional frying at 165 °C. Atmospheric frying was the treatment that produced the greatest shrinkage in the fillets reaching values of 36.8% with respect to the fresh sample after ten minutes of treatment. For vacuum treatments shrinkage values ranged between 23.7% and 33.0% at 100 °C and 110 °C respectively after 10 min of frying. According to the results, the vacuum treated samples became lighter, less greener and more yellow. Industrial relevance: Healthy products are a tendency in the international market. Vacuum frying offers some advantages that can include: the preservation of natural colour and flavours of the products due to low temperature and low oxygen content during the process (better than with conventional deep fat frying), and has less adverse effects on oil quality. The use of this technology on fish products can improve the problems of market saturation at the present time for some species like gilthead sea bream (Sparus aurata). © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Aquaculture in Mediterranean countries, including Spain has experienced a spectacular growth that could turn it into one of the main food producing industries in the near future (Lougovois, Kyranas, & Kyrana, 2003). The sector needs to diversify supply and to produce healthier food linked to the increasing demands of the consumer for quality food elaborated with natural and safe ingredients and long shelf-life (Goulas, Chouliara, Nessi, Kontominas, & Savvaidis, 2005). For these reasons, the study of the technologies that maintain the shelf-life of fish products (quality and safety) is more than justified, constituting a field of research of great interest responding to the demands of the sector and the consumer. Fish is a food that must be included in all healthy and balanced diets. The beneficial effect that fish consumption has for human health is based on its content in n-3 PUFA (polyunsaturated fatty acids). The influence of these fatty acids on the prevention and treatment of a multitude of diseases has been demonstrated (Das, 2000; Simpoulos,
⁎ Corresponding author. Tel.: + 34 963879364; fax: + 34 963877369. E-mail address: [email protected] (J. Martínez-Monzó). 1466-8564/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2010.06.002
2001). Since most fish species are consumed cooked, the nutritional value of the final cooked product is of major importance for human health. Culinary processes can significantly alter the content, composition and biological activity of fish lipids. Frying, which is commonly used for cooking fish, leads to an increase in the fat content of the fish fillet (Candela, Astiasaran, & Bello, 1997; Aro, Tahvonen, Mattila, Nurmi, Sivonen, & Kallio, 2000), extensive lipid exchanges between the fish and the frying medium (Sebedio, Ratnayake, Ackman, & Prevost, 1993) along with the production of oxidized and polymerized lipid products (Kubow, 1992; Skog, Johansson & Jagerstad, 1998). Deep-fat frying is the most common unit operation, involving the immersion and cooking of foods in hot oil, as evidenced by the worldwide annual production of more than 20 million tons of frying oil (Gertz, 2004). Vacuum frying is defined as the frying process that is carried out under pressures well below atmospheric levels, preferably below 50 Torr (6.65 kPa). Due to the low pressure, the boiling point of the water in the food is lowered. Vacuum frying offers some advantages that can include: (1) reduction of the oil content in the fried product, (2) preservation of natural colour and flavours (better than with conventional deep fat frying), and (3) reduction of adverse effects on oil quality (Garayo & Moreira, 2002; Shyu, Hau, & Hwang, 2005).
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In recent years, vacuum-fried studies have been oriented to fruits and vegetables. Da Silva and Moreira (2008) studied mango chips fried under atmospheric vs. vacuum fried conditions, showing more quality vacuum fried mango chips. Shyu and Hwang (2001), studied the effect of processing conditions on the quality of vacuum fried apple chips, concluded that oil content increased for increasing frying times and temperatures. Additionally, vacuum-fried products have higher retention of nutritional quality (phytochemicals), color is enhanced (less oxidation) (Shyu & Hwang, 2001; Fan, Zhang & Mujumdar, 2005a,b; Da Silva & Moreira, 2008; Pérez-Tinoco, Perez, Salgado-Cervantes, Reynes & Vaillant, 2008). Even though frying is an old worldwide process of manufacturing food products, most of the research found in literature is related to atmospheric frying. No studies are available for the effect of vacuum frying on the oil content, colour and nutritional value of fish products. The objective of the present study was to investigate the effect of the vacuum frying process and conditions on the characteristics (oil content, moisture, colour) of gilthead sea bream fillets.
2. Materials and methods 2.1. Sample preparation and storage conditions The cultured gilthead sea bream (Sparus aurata), used in this study were cultivated in net cages and harvested in GRAMASA (Gandía, Valencia, Spain). The sea bream were harvested in December 2005, covered in ice immediately and kept frozen (at −20 °C) until they were handfilleted. Before being hand-filleted, they were first defrosted in a refrigerator at 4 °C overnight. After filleting they were vacuum packaged in polyethylene bags and stored in a refrigerator at 2 °C. The weight of the sea bream before filleting was approximately 400 g (commercial size). The weight of the fillets was between 52 g and 35 g.
3. Vacuum frying For frying tests GASTROVAC® equipment was used (Fig. 1). This equipment allows control of the temperature in vacuum conditions or atmospheric pressure. The equipment was designed, constructed and patented at the Polytechnic University of Valencia (Martínez-Monzó et al., 2004). The electrical appliance consists of a pressure cooker vessel with an inner basket, a membrane vacuum pump (Model
631
24207, Selecta S.L., Barcelona, Spain) to provide vacuum to the vessel and a heating system controlled by a temperature probe (Fig. 1). Three levels of oil temperature for vacuum frying (90, 100, and 110 °C) were considered in this study. The vacuum pressures used in vacuum frying treatments for each temperature were 15 kPa for 90 °C, 20 kPa for 100 °C and 25 kPa for 110 °C respectively. For each temperature the times investigated were 1, 2, 3, 4, 5, 6, 8, 10 min. These times were selected on the basis of previous experiments. The vacuum vessel was set to the target temperature and allowed to operate for 30 min before frying started. The oil volume in the vessel was 2 L. Sunflower oil was used in all experiments and the oil was filtered by means a sieve (filter pore size 1 mm) after each experiment and changed for each temperature. The fillet/oil ratio was around 0.07 w/w. The quality of the oil was controlled, before and after the frying process, with a FOM-310 probe (Ebro Electronic GmbH γ Co. KG, Ingolstadt, Germany). The temperature in the centre of the fillet was controlled, before and after the frying process, with a Testo 925 probe (Testo GmbH y Co., Lenzkirch, Germany). Three sea bream fillets were fried each time. Once the oil temperature reached the target value the fillets (previously weighed) were placed into the basket, the lid was closed and the vessel was evacuated. Frying operation consisted of an initial depressurisation step with the product outside the oil (1 min), immersion of the product once the vacuum reached the target value ( ± 1 °C), the frying period, and pressurized time (1 min). Then, the lid of the vessel was opened and the sea bream fillets were removed from the basket and the temperature of the product was measured. The sea bream fillets were then allowed to cool to room temperature (25 °C) and dried with paper towels to remove the excess surface oil (Da Silva & Moreira, 2008). 4. Atmospheric frying For the atmospheric frying experiments, the same equipment and procedure were used but the vacuum pump was switched off. To compare the effects of atmospheric frying and vacuum frying on the quality and frying rate of gilthead sea bream fillets, an oil temperature of 165 °C was used, this temperature was selected because of it is a common temperature used in fish frying (Gladyshev, Sushchik, Gubanenko, Demirchieva & Kalachova, 2007). Times investigated were the same as those for vacuum treatments. The atmospheric fryer was set to the required frying temperature and left for 30 min to ensure that the oil temperature was constant. The
Fig. 1. Vacuum-frying system. (1) Main body, (2) pan, (3) basket, (4) heating element, (5) temperature probe, (6) temperature selector, (7) switch, (8) vacuum pump, (9) electric supply, (10) vacuum connector, (11) valve, (12) elevator, (13) manometer, (14) hermetic closing.
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sea bream fillets were then allowed to cool to room temperature (25 °C) and dried with paper towels to remove excess oil. 5. Proximate composition 5.1. Moisture content The initial moisture content of defrosted and uncooked gilthead sea bream and the moisture content after frying was measured for each time and temperature analyzed. Moisture content was determined by drying six homogeneous samples of 5 g of fish to a stable weight for 48 h at 110 °C (A.O.A.C, 1997). 5.2. Oil content Total fat content of six dried samples (5 g) was extracted with petroleum ether (BP 40–60 °C) during four hours in an extracting unit Soxtec System 2055 Tecator (FOSS, Hillerød, Denmark) and gravimetrically determined. 6. Characteristics of the fried product 6.1. Weight loss Weight loss was calculated as the percentage weight difference between the raw and fried samples relative to the weight of the raw fillets. The samples were dried with absorbent paper before being weighed in order to remove the surface water from the fresh fillets and the oil from the fried ones. The weight of the samples was measured with an analytical balance Mettler Toledo model PB 303-S (Mettler Toledo GmbH, Greinfensee, Switzerland). Three samples were used for each time and temperature. 6.2. Shrinkage Pictures of each fillet were taken before and after treatment with a digital camera (Sony model DSC-P7, Tokyo, Japan). The degree of surface shrinkage (S) was analyzed with a computer program (Image Tool version 3.00, University of Texas Health Science Center in San Antonio, Texas, U.S.A.), and was evaluated by Eq. (1): S = ðSo−Sðt ÞÞ = So4100
ð1Þ
where So is the original surface of the sample before frying (cm2) and S(t) is the surface of the sample at time t after frying (cm2). 6.3. Colour The colour of the gilthead sea bream samples was measured using a colorimeter Minolta CM3600d (Minolta Co. Ltd, Tokyo, Japan) (Illuminant D 65, 10° viewing angle). According to the CIE LAB system, Lightness (L⁎), green-red chromacity (a⁎), blue-yellow chromacity (b⁎), chroma (C⁎ab) and hue (h⁎ab) were measured. Measurements were taken in triplicate. The colorimeter was standardized using a white calibration plate.
Fig. 2. Temperature at the centre of gilthead sea bream fillets (Sparus aurata) after atmospheric pressure frying (AT) and under conditions of continuous vacuum frying (V).
8. Results and discussion 8.1. Temperature profile Fig. 2 shows the temperature at the centre of gilthead sea bream fillets (S. aurata) after atmospheric pressure frying and under conditions of continuous vacuum frying. The temperature reached in the fillet depends on the frying temperature. The higher values were achieved for treatments at 165 °C. Temperatures needed to boil water were reached approximately at 2 min for vacuum treatments at 90 °C (55.5 °C at 15 kPa), 2.5 min for vacuum treatments at 100 °C (62.3 °C at 20 kPa), 2 min for vacuum treatments at 110 °C (67.6 at 25 kPa) and 3 min for atmospheric treatments at 165 °C (100 °C at 101.3 kPa). 8.2. Weight loss Fig. 3 shows the weight loss experienced by gilthead sea bream fillets (S. aurata) after atmospheric pressure frying and under conditions of continuous vacuum frying. The weight variation experienced by the samples fried under atmospheric conditions was around 44 ± 2% with respect to initial weight while the mass variation experienced by fillets fried under vacuum conditions was 19 ± 1% at 90 °C, 22.1 ± 0.1% at 100 °C and 36 ± 2% at 110 °C after 10 min of treatment. As can be seen in Fig. 3, for both treatments greater water loss takes place with higher temperature and frying time. Therefore a high temperature (165 °C) combined with a long frying time (10 min) produces a drier product. The atmospheric frying treatment (165 °C) produces a greater decrease in the weight of the fillets than the vacuum frying treatment (90, 100, 110 °C). The greatest differences were found at 8 and 10 min as can be see in Fig. 3. Analysis of variance showed significant differences (p b 0.05) between both treatments (vacuum frying and atmospheric frying). The different temperatures for vacuum treatments were compared and results showed that there was a tendency to increase weight loss with temperature but
7. Statistical analysis The effect of temperature and vacuum pressure on the drying curve, the oil content, shrinkage, colour and weight loss of the gilthead sea bream fillets (S. aurata) was evaluated using a factorial design with four levels for temperature and eight levels for frying time. The statistical analysis of the data was conducted using Statgraphics statistical software package Statgraphics Plus 5.1. (Statistical Graphics Corporation, Herndon, U.S.A.). Statistical significance was expressed at the p b 0.05 level.
Fig. 3. Weight loss (%) experienced by gilthead sea bream fillets during frying process.
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Fig. 4. Moisture loss (d.b.) of gilthead sea bream fillets versus frying time for different treatments.
statistical analysis showed that these differences were not significant (p b 0.05). 9. Moisture loss Fig. 4 shows the moisture loss (d.b.) during the frying process for the different frying temperatures, times and treatments. At 165 °C (atmospheric frying) the moisture content was above 0.59 ± 0.03 (g water/g dry solid), 1.73 ± 0.03 (g water/g dry solid) at 90 °C (vacuum frying) and 1.2 ± 0.2 (g water/g dry solid) at 110 °C after 10 min of treatment. The moisture content of fried samples decreased significantly (p b 0.05) during frying. Oil temperature had a negative effect on the moisture content of fried fillets for all treatments. Moisture loss in vacuum-fried samples was lower (about 25–40% after ten minutes of frying) than the moisture loss in atmospheric fried samples (about 70% after ten minutes of frying) for all the times and temperatures studied. The moisture loss was significantly affected (p b 0.05) by frying treatment (vacuum frying or atmospheric frying) but no differences (p N 0.05) were found between frying temperatures in vacuum treatments. 10. Oil uptake Fig. 5 shows the effect of frying techniques on the oil content of the samples (d.b.). The initial fat content in fresh fillets was 0.15 ± 0.02 goil/gdry solid or 4.7 ± 0.7% in wet base. The maximum values obtained in this study ranged between 0.24 ± 0.02 goil/gdry solid at eight minutes of treatment at 90 °C and 0.30 ± 0.01 goil/gdry solid for the same time at 165 °C. In some cases, it appears that there was no oil intake, due to the variability in the initial fat content of the samples. Previous studies of vacuum frying were made using vegetables with no significant fat content (Moreira, Castell-Perez & Barrufet, 1999; Shyu & Hwang, 2001; Garayo & Moreira, 2002; Shyu, Hau & Hwang, 2005; Pérez-Tinoco et al., 2008; Da Silva & Moreira, 2008; Mariscal & Bouchon, 2008; Troncoso, Pedreschi & Zuniga, 2009).
Fig. 5. Oil content (d.b.) of gilthead sea bream fillets versus frying time for different treatments.
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The effect of temperature was significant (p b 0.05) in vacuum treatments. In vacuum frying treatments the increase in the frying temperature implies a decrease in oil absorption. The treatment at 110 °C was the one that produced the lowest oil content in fried fillets. Similar behaviour was observed by Garayo and Moreira (2002) in the vacuum frying of potato chips. They concluded that the faster the rate of water loss, the higher the oil adhesion on the surface of the chips surface, leading to greater oil absorption. In addition, as the percentage of free water is depleted in the product, less oil is absorbed. Several studies demonstrated that most of the oil does not penetrate the product during frying but during the cooling period, when the product is removed from the fryer (Moreira et al., 1999). When frying is completed, the food is removed from the fryer and the product starts to cool, leading to water vapour condensation and a subsequent decrease in internal pressure. Oil adhered to the food surface is sucked in due to the consequent ‘vacuum effect’. Therefore, oil uptake is a surface phenomenon, involving equilibrium between adhesion and drainage of oil as the food is removed from the oil bath (Ufheil, & Escher, 1996, Moreira, Sun & Chen, 1997; Moreira & Barrufet, 1998). Fig. 5 shows that the oil uptake in vacuum fried samples was lower than the oil uptake with atmospheric frying for all the times and temperatures studied. In this study and in agreement with other authors such as Garayo and Moreira (2002), it can be considered that the most important mass transfer mechanism in a continuous vacuum frying process is the pressurization period. The pressurization step plays an important role in reducing the oil absorption during vacuum frying. It can increase or decrease oil absorption in the product depending on the amount of surface oil and free water presented in the product (Garayo & Moreira, 2002). During pressurization from vacuum to atmospheric conditions, as the vessel is vented, the pressure in the pores of the samples rapidly increases to atmospheric levels. Air and surface oil are carried into the empty pore spaces until the pressure reaches atmospheric levels. However, because of the low pressure, gas diffuses much faster into the pore space thus obstructing the passage of the oil into the pores. In the cooling period, since less oil adhered to the surface of the samples during vacuum frying, less oil is absorbed on cooling (Garayo, & Moreira, 2002). In these experiments once the products were fried, the basket was raised and centrifuged for two minutes before vessel pressurization in order to reduce oil absorption. In order to analyze the frying process, Pinthus, Weinberg and Saguy (1993) introduced the criterion UR, which expresses the weight ratio between the amount of oil uptake and water removed. They defined the “uptake ratio”, UR, as: UR = Oil uptakeðgÞ = Water removedðgÞ
ð2Þ
The value of UR is derived from Eq. (2) for any specific point in the frying process by considering the initial and final moisture and oil content (Pinthus et al., 1993). In this study they assumed that for an equal volume exchange the theoretical value of UR should be 0.9 for an ideal food system in which oil uptake is governed by the water
Fig. 6. Shrinkage (%) of gilthead sea bream fillets as function of frying time and temperatures.
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replacement mechanism. Pinthus et al. (1993) concluded that when a system displayed significantly different values of UR which were below and above the expected theoretical value many factors other than “simple” water replacement are involved. The values obtained in this study are lower than 0.9 for all the temperatures studied, 0.08 was the value obtained at 90 °C, 0.03 for 110 °C and 0.4 at 165 °C after ten minutes of frying. This fact could suggest that in a vacuum frying process there is another important mechanism, such as the pressurization period that governs the process of water replacement by oil. Reduced values of the UR parameter for all the temperatures tested are related to low water loss and low oil uptake in the process. 11. Shrinkage Fig. 6 presents the degree of shrinkage of gilthead sea bream fillets cooked with atmospheric and vacuum frying as a function of temperature and time. Atmospheric frying was the treatment that produced the greatest shrinkage in the fillets reaching values of 36.8% with respect to the fresh sample after ten minutes of treatment. For vacuum treatments shrinkage values ranged between 23.7% and 33.0% for 100 °C and 110 °C respectively after 10 min of frying. The statistical analysis
showed significant differences (p b 0.05) in the percentage of shrinkage between atmospheric frying and vacuum frying, but no differences (p N 0.05) were found between the different treatment temperatures in vacuum frying. 11.1. Colour Fig. 7a shows the L⁎ evolution as a function of time and temperature. Statistical analysis showed that there were significant differences (p b 0.05) for Lightness (L⁎), green-red chromacity (a⁎), blue-yellow chromacity (b⁎) and chroma (C⁎) of gilthead sea bream fillets as a function of frying treatment (deep fat frying or vacuum frying). Statistical analysis indicated significant differences (p b 0.05) between atmospheric frying and vacuum frying for lightness. L⁎ values were higher for the vacuum frying treatment. The a⁎ values of fried fillets increased with the frying temperature and frying time (Fig. 7b). The a⁎ values of fried fillets became positive after frying at 165 °C for six minutes, indicating that dark red fried fillets were produced. This may be caused by a non-enzymatic browning reaction during heating (Richardson & Hyslop, 1985). However the significant differences for a⁎ values were between 165 °C (atmospheric pressure) and the vacuum frying temperatures. The blue-yellow chromatically
Fig. 7. Changes in CIE L⁎a⁎b⁎ values as affected by different frying times and temperatures. (a) Lightness (L⁎), (b) green-red chromaticity (a⁎), (c) blue-yellow chromaticity (b⁎), (d) Hue (h⁎ab) and (e) Chroma (C⁎ab).
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Fig. 8. Visual appearance of fried gilthead sea bream fillets under vacuum and atmospheric conditions after 10 min of treatment: (a) 90 °C-vacuum, (b) 100 °C-vacuum, (c) 110 °Cvacuum and (d) 165 °C-atmospheric pressure.
(b⁎) values were also significantly higher (p b 0.05) for fillets fried at atmospheric pressure than for vacuum fried fillets (Fig. 7c). The values calculated for chroma (C⁎ab) showed significant differences (p b 0.05) between atmospheric and vacuum treatments (Fig. 7e). Differences between vacuum treatments also appear; at 90 °C chroma values are lower. Hue values (h⁎ab) ranged from 186° for fresh samples to 81°for fillets fried at 165 °C (atmospheric pressure) and no significant differences between atmospheric and vacuum treatments (p N 0.05) were found for this parameter (Fig. 7d). Visual observation confirmed the results obtained with the colorimeter, as the fillets fried under deep-fat frying conditions were darker and redder than the others fried by the vacuum frying method (Fig. 8). Fish fillet retained better their original colour when fried under vacuum. The vacuum frying method clearly reduced colour degradation due to the absence of Maillard reactions and oxidation during the process. A sensory evaluation is needed to evaluate consumer preferences. 12. Conclusions Compared with traditional frying, the oil content of vacuum-fried fish fillets was lower. After ten minutes of treatment the values obtained for vacuum frying were 0.14 ± 0.01 goil/gdry solid, 0.18 ± 0.02 goil/gdry solid and 0.12 ± 0.01 goil/gdry solid for 90 °C, 100 °C and 110 °C respectively and 0.27 ± 0.01 goil/gdry solid for traditional frying at 165 °C. Deep-fat frying (165 °C) produces a greater decrease in the weight of the fillets than the vacuum frying treatment. Moisture loss in vacuum fried samples was lower than the moisture loss in atmospheric frying for all the times and temperatures studied producing a moister product. For vacuum treatments shrinkage values ranged between 23.7% and 33.0%. Atmospheric frying was the treatment that produced greatest shrinkage in the fillets reaching values of 36.8% with respect to the fresh sample. A reduction in colour attribute changes was observed for vacuum treatments. The process of frying under vacuum conditions may be an alternative to atmospheric frying for fish fillets to obtain convenience foods with appropriate composition attributes. Acknowledgements The authors would like to acknowledge the support of the INNOVA programme of the Polytechnic University of Valencia in the financing for this study.
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