- Email: [email protected]
The effects of thyme (Thymus vulgaris L.) oil concentration on liquid-smoked vacuum-packed rainbow trout (Oncorhynchus mykiss Walbaum, 1792) fillets during chilled storage
Food Chemistry 128 (2011) 683–688
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
The effects of thyme (Thymus vulgaris L.) oil concentration on liquid-smoked vacuum-packed rainbow trout (Oncorhynchus mykiss Walbaum, 1792) fillets during chilled storage Zayde Alçiçek ⇑ Firat University Fisheries Faculty, Department of Fisheries Techniques and Fish Processing Technology, 23119, Elazig, Turkey
a r t i c l e
i n f o
Article history: Received 1 November 2010 Received in revised form 4 March 2011 Accepted 21 March 2011 Available online 26 March 2011 Keywords: Thyme oil Liquid smoke Polycyclic aromatic hydrocarbons Rainbow trout Sensory quality Spices
a b s t r a c t This study examined the effects of thyme oil on chemical properties, microbiological changes, and sensory quality in vacuum-packed liquid-smoked rainbow trout fillets under chilled storage. After 150 days of storage, the total volatile basic nitrogen (TVB-N) values were 30.52, 27.5 and 39.43 mg/100 g, and total viable counts (TVC) were 5.34, 4.96 and 5.53, for thyme oil additions of 10 (T10), 50 (T50) and 0 (control; T0) mL/L brine, respectively. The highest acceptable TVB-N value was adopted as 30 mg /100 g, corresponding to shelf lives of 135, 150, and 105 days for T10, T50, and T0, respectively. T50 addition of thyme oil to liquid-smoked rainbow fillets extended shelf life T10 and T50, gave acceptable sensory quality and limited microbiological growth during chilled storage. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Seafood has been preserved for centuries by smoking, one of the oldest meat and seafood preservation methods, which imparts antimicrobial and antioxidant characteristics to the food (Che-Man & Ramadas, 1998; Goulas & Kontominas, 2005; Hattula, Elfving, Mroueh, & Luoma, 2001; Kolsarıcı & Özkaya, 1998; Martinez, Salmero’n, Guille’n, & Casas, 2007; Muratore, Mazzaglia, Lanza, & Licciardello, 2007). Two smoking methods are commonly utilised: hot and cold smoking. Over the past three decades, liquid smoke flavours have been increasingly used as an alternative to smoking (Alçiçek, Zencir, Çelik, Çakırog˘ulları, & Atar, 2010). Liquid smoke flavours have some advantages over traditional smoking methods, such as easy application, lower cost, and environmental friendliness. It is also easier to control smoke contaminants like polycyclic aromatic hydrocarbons (PAHs), which are considered carcinogenic, and mutagenic molecules produced during pyrolysis of wood (Alçiçek et al., 2010; Dimitriadou, Zotos, Petridis, & Taylor, 2008; Siskos, Zotos, & Taylor, 2005; Varlet et al., 2007). Spices, which have been used as food additives for centuries and are called bio-preservatives or green chemicals by Rahman (2009), are now being viewed as potential alternatives to chemical preser-
vatives. Thyme (Thymus vulgaris L.), a plant species native to the western Mediterranean area, is now widely cultivated as a spice throughout temperate climes. Although the antimicrobial activity of some spices has been studied against pathogenic bacteria (Arora & Kaur, 1999), no information is yet available in the literature about preservative effects of thyme on liquid-smoked seafood. Thyme is active against many spoilage and pathogenic microorganisms and also improves sensory acceptability (Dimitrijevic, Mihajlovski, Antonovic, Milanovic-Stevanovic, & Mijin, 2007; Özkan, Sag˘dıç, & Özcan, 2003; Solomakos, Zotos, Melidou, & Tsikritzi, 2007). According to Solomakos et al. (2008), the thyme plant contains a number of important compounds, such as the phenols thymol (44–60%) and carvacrol (2.2–4.2%). The aim of this study was to determine the effects of thyme oil on the shelf life of vacuum-packed liquid-smoked rainbow trout fillets under chilled storage at 4 ± 1 °C. A number of chemical parameters, total viable count (TVC), psychrotrophic viable count (PVC), lactic acid bacteria (LAB), sensory analysis and PAH content were measured.
2. Materials and methods 2.1. Raw materials
⇑ Tel.: +90 5455614347; fax: +90 4242386287. E-mail address: [email protected] 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.03.087
A total of 450 fresh rainbow trout (Oncorhynchus mykiss), each weighing 220 ± 20 g, were obtained from ALBA Fish Co.
684
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
aquaculture farm located in Kemer Dam Lake, geographically located at Aydin in Western Turkey. After harvesting, the fish were transferred within 1 h to the fish processing plant at ALBA Fish Co., Bozdogan, Aydin, Turkey, in sealed polystyrene foam boxes containing ice. On the day of processing, the fish were eviscerated and washed with clean water. After brining, addition of thyme oil and liquid smoking, the fillets were vacuum packed and stored at 4 ± 1 °C until analysis on days 15, 30, 45, 60, 75, 90, 105, 120, 135 and 150. Three replicates were performed and each replicate per treatment contained 30 fish.
2005). After brining, addition of thyme oil and liquid smoking, all samples were air-dried at 17 °C for 20 min. The samples were then heated in a fan oven at 120–130 °C for 20 min. After cooling (at 20 °C for 30 min), the smoked products were filleted, vacuum packed (high barrier nylon polyethylene bags under vacuum (residual pressure 0.1 bar)), and stored at 4 ± 1 °C. This type of refrigerated storage is the most common preservative method for this type of seafood product (Fig. 1).
2.2. Thyme oil and liquid smoke flavouring
Prior to analyses, edible parts of the fish samples were homogenised. Chemical composition of fish samples was measured by standard methods, following protocols AOAC (1995). Dry matter content was analysed by oven drying 3 g of liquid-smoked rainbow trout at 105 °C until a constant weight was reached. A 10-g sample of fish was mixed with 50 mL of distilled water and the pH was measured using a pH metre. TVB-N analysis was performed using the method of Goulas and Kontominas (2005). Total fat content was analysed by hexane extraction. Percent protein (Kjeldahl N 6.25) was determined from a 1-g sample for each treatment (Alçiçek et al., 2010). NaCl content in fish samples was determined by the Mohr method (Treadwell & Hall, 1928). All analyses were carried out in duplicate.
Liquid smoke flavouring (Red Arrow, SmokEz-M-10, mesquite wood-based smoke) and water-soluble thyme oil were purchased from a food additives company (GMT Food Ingredients Co., Istanbul, Turkey). The thyme oil was added in the brine solution at two different concentrations (10 and 50 ppm), along with the liquid smoke flavouring. The pH of the thyme oil used was 5.78 (1% solution). Volatile oil content was 9.52%. 2.3. Brining, addition of thyme oil, liquid smoking, drying and heating The fish were separated into three groups; The first group (T0): control group, without added thyme oil. The second group (T10): 10 mL/L thyme oil was added in the brine solution. The third group (T50): 50 mL/L thyme oil was added in the brine solution. All groups were smoked with liquid smoke flavouring (100 mL in 1 L of brine solution) and were brined in 18% sodium chloride (NaCl) solution for 12 h. The brine solution was stirred every 2 h. The temperature of the brine solution was kept low (4 ± 1 °C), to minimise microbial growth (Dimitriadou et al., 2008; Siskos et al.,
2.4. Chemical analyses
2.5. Microbiological analyses Total viable counts (TVCs), psychrotrophic viable counts (PVCs), and lactic acid bacteria (LAB) were determined. Ten grams of each sample homogenate were diluted to 100 mL with peptone (1% water) buffered at pH 7.5. Rainbow trout samples were homogenised in Tryptone water for 30 s in a stomacher. Serial dilutions were prepared until 10 7 g/ml samples were obtained. One millilitre of each dilution was placed in a Petri dish and 15 mL of plate count agar were added for TVC/PVC and 15 mL de Man Rogosa Sharpe (MRS) agar double layered for LAB. TVCs were
Receiving and transport Raw fish were stored on ice in sealed foamed polystyrene boxes and transferred to the fish processing plant
Filleting Fish gutted and filleted
Brining, Addition thyme oil and Liquid Smoke Flavour (T10) Fillet immersed in liquid brine solution 18% for 4 hours at 4 ºC
Brining, Addition thyme oil and Liquid Smoke Flavour (T50) Fillet immersed in liquid brine solution 18% for 4 hours at 4 ºC
Brining and Liquid Smoke Flavour (T0) Fillet immersed in liquid brine solution 18% for 4 hours at 4 ºC
Drying Drying at 17 ºC for 20 min
Heating 120−130 ºC in a fan oven
Cooling Cool to 20 ºC for 30 min Fig. 1. Flow diagram of the production of liquid smoked samples either with or without thyme oil.
685
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
performed on pour plates with plate-count agar (Merck KGaA, Darmstadt, Germany), after 48 h at 37 °C. The same plates were used for the determination of PVCs; plates were incubated for 7 days at 4 °C. De Man Rogosa Sharpe (MRS) agar was used for LAB (Dimitriadou et al., 2008). Petri dishes were inverted and placed in an oven at 30 °C for 36 h. After incubation, TVC, PVC and LAB were counted, following the rules recommended by Gilliland, Busta, Brinda, and Campbell (1976). 2.6. PAH analysis Benzo(a)pyrene compounds in fish samples were determined using the method of Moret and Conte (2002). The limit of detection was 1 ppb. For quality assurance, spiked samples were studied together with the experimental samples. Blank fish samples were spiked with benzo(a)pyrene standard at ½ MRL (maximum residue level) and MRL levels, each in duplicate. The mean recovery was 90 ± 0.91% for the levels studied. 2.7. Sensory analyses Five experienced panellists, who were members of academic staff and trained in sensory descriptors for smoked trout, were used to evaluate the quality of rainbow trout fillets during storage. Rainbow trout fillets were assessed on the basis of appearance, odour, taste and texture characteristics using a nine point descriptive scale. A score of 7–9 indicated ‘‘very good’’ quality, a score of 4.0–6.9 ‘‘good’’ quality, a score of 1.0–3.9 denoted as ‘‘spoiled’’. All samples were stored at +4 °C until sensory analysis was performed. 2.8. Statistical analyses The significance of effects of thyme oil concentrations was determined by one-way analysis of variance and Duncan’s multiple range test, using SPSS for Windows, Version 16.0 (SPSS Inc., Chicago, IL) at p 6 0.05. 3. Results and discussion Tables 1 and 2 show the moisture, ash, protein, lipid, TVB-N, TVC, PVC and LAB content of raw and liquid-smoked rainbow trout. The approximate composition data for raw rainbow trout agreed with those reported by other researchers (Alçiçek, 2010; Kolsarıcı & Özkaya, 1998). The percentages of total ash, protein, and lipid of liquid-smoked rainbow trout increased due to moisture loss during processing, although TVC, PVC and LAB decreased because of liquid smoke flavouring content (e.g., phenols, etc.). Similar findings were reported by Kolsarıcı and Özkaya (1998) for smoked rainbow trout.
was due to the liquid-smoking process, as discussed in recent studies (Dimitriadou et al., 2008; Siskos, Zotos, Melidou, & Tsikritzi, 2007). TVC was found to be equal to 0.95 log cfu/g in T10 and T50 until the 30th storage day. The control, T0, showed values of 1.04 ± 0.05 log cfu/g and 2.1 ± 0.02 log cfu/g, respectively, for the 1st and 15th storage dayT0, higher than the other groupsT50T10. The TVC values in the T50 samples were still low even after 135 days at 4 ± 1 °C. This result can be explained by thyme oil effects on total viable counts (Arora & Kaur, 1999; Dimitrijevic et al., 2007; Özkan et al., 2003). Differences between the TVC values were statistically significant after the 30th day of storage time (p < 0.05). Maximum microbiological growth had already been reached in T10 and control T0 samples by the 135th day; while T10 samples were still found acceptable by the panellists, the T0 samples were unacceptable for all sensory scores. Similar maximum TVC values were also observed in other studies (Dimitriadou et al., 2008; Muratore et al., 2007). Psychotropic pathogens can grow in refrigerated foods and may cause some foodborne illnesses. A number of researchers have reported that these pathogens can be frequently isolated from smoked products (Suñen, Fernandez-Galian, & Aristimuno, 2001). PVC at the beginning of storage was measured in all samples as 0.95 log cfu/g. This did not change until the 30th day of storage time. A slight increase in PVC values to 1.38 ± 0.09 log cfu/g and 1.36 log cfu/g was found in the T10 and T0 samples, respectively, with no change in the T50 samples. On the 45th day, the PVCs for the T50 samples were still only 1.83 ± 0.04 log cfu/g, indicating that the higher concentration of thyme oil in the T50 samples reduced the initial PVC. Statistically significant differences were found between the PVC values according to the groups during storage time at the 1st, 15th and 105th days (p < 0.05). In vacuum-packed smoked seafood, LAB are a problem as they produce typical sour odours and flavours (Kolsarıcı & Özkaya, 1998; Leroi, Joffraud, Chevalier, & Cardinal, 1998). The LAB count at the beginning of storage was 0.95 log cfu/g and that level was maintained until the 30th day in all samples. Statistically significant differences were found between the PVCLAB values according to the groups after the 30th day of storage. At the end of shelf life, after 150 days, 2.26, 2.18 and 2.68 ± 0.02 log cfu/g were reached for the T10, T50 and T0 samples, respectively. These differences may explain the effects of thyme oil on the liquid-smoked rainbow trout fillets. The results of microbiological analyses confirmed that thyme oil used in liquid-smoked processing leads to a reduction in microbial contamination during long storage time, probably due to the presence of carvacrol and thymol in the thyme essential oil (Solomakos, Govaris, Koidis, & Botsoglou, 2008). 3.2. PAH analysis No detectable amounts of benzo(a)pyrene, which is an indicator for PAHs (Dimitriadou et al., 2008), were found in any samples.
3.1. Microbiological analysis 3.3. Chemical determinations Microbiological counts for TVC, PVC and LAB are shown in Table 2. The average TVC of fresh samples was 4.5 log cfu/g. After processing, a TVC reduction was observed in all groups. This reduction
Table 1 shows the moisture, protein, lipid, and salt contents of raw and T10, T50, and T0 samples after processing. The percentage of total protein and lipid increased, due to water loss during the
Table 1 Proximate composition of raw and liquid smoked rainbow trout either with or without thyme oil. Sample component
Raw rainbow trout
T10 (after process)
T50 (after process)
T0 (after process)
Dry matter (%) Protein (%) Lipid (%) Salt (%)
31.4 20.26 9.01 0.10
48.7 27.8 12.9 4.02
48.6 27.9 12.8 4.03
48.5 27.9 12.9 4.03
0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.07 ± 0.1a 1.74 ± 0.03c 1.81 ± 0b 1.87 ± 0c 1.98 ± 0.02b 2.29 ± 0.01c 2.47 ± 0.01c 2.68 ± 0.02c 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.33 ± 0.01a 1.54 ± 0.07a 1.56 ± 0.02a 1.83 ± 0.02a 1.99 ± 0a 2.03 ± 0.02a 2.18 ± 0a 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.16 ± 0.16a 1.48 ± 0b 1.54 ± 0.02a 1.72 ± 0b 1.92 ± 0.01b 2.16 ± 0b 2.16 ± 0b 2.26 ± 0b 0.95 ± 0a 0.95 ± 0a 1.39 ± 0b 2.26 ± 0b 2.41 ± 0.01b 2.93 ± 0.02c 2.98 ± 0b 3.29 ± 0a 3.93 ± 0c 4.7 ± 0c 5.2 ± 0c 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.83 ± 0.04a 2.09 ± 0a 2.64 ± 0.01a 2.79 ± 0a 2.97 ± 0a 3.46 ± 0a 3.91 ± 0a 4 ± 0a 0.95 ± 0a 0.95 ± 0a 1.38 ± 0.09b 2.24 ± 0.02b 2.55 ± 0c 2.83 ± 0b 2.93 ± 0.04b 3.19 ± 0a 3.58 ± 0b 4.04 ± 0b 4.54 ± 0b 1.04 ± 0.05a 2.1 ± 0.02b 2.41 ± 0c 2.47 ± 0b 2.74 ± 0b 3.42 ± 0c 3.7 ± 0b 3.99 ± 0a 4.66 ± 0c 4.99 ± 0b 5.53 ± 0c 0.95 ± 0a 0.95 ± 0a 1.87 ± 0.01a 2.16 ± 0.02c 2.36 ± 0c 3.16 ± 0a 3.53 ± 0a 3.75 ± 0a 3.95 ± 0a 4.11 ± 0.16a 4.96 ± 0.02a 0.95 ± 0a 0.95 ± 0a 2.22 ± 0.04b 2.4 ± 0.01a 2.93 ± 0.01a 3.31 ± 0.01b 3.75 ± 0.01c 3.95 ± 0a 4.15 ± 0b 4.4 ± 0.03a 5.34 ± 0b 18.31 ± 0b 18.32 ± 0b 18.35 ± 0a 18.42 ± 0a 18.44 ± 0a 18.59 ± 0a 19.16 ± 0a 20.54 ± 0a 21.78 ± 0.02a 24.85 ± 0.07a 27.5 ± 0a 6.25 ± 0b 6.25 ± 0a 6.24 ± 0ab 6.22 ± 0a 6.24 ± 0a 6.22 ± 0c 6.21 ± 0b 6.20 ± 0b 6.20 ± 0b 6.18 ± 0b 6.13 ± 0b
T10
6.27 ± 0a 6.27 ± 0a 6.26 ± 0a 6.25 ± 0a 6.23 ± 0a 6.20 ± 0b 6.18 ± 0a 6.20 ± 0b 6.18 ± 0b 6.17 ± 0b 6.14 ± 0.01b
Groups Days
1 15 30 45 60 75 90 105 120 135 150
6.30 ± 0c 6.30 ± 0a 6.27 ± 0.01b 6.25 ± 0.02a 6.20 ± 0a 6.18 ± 0a 6.17 ± 0a 6.11 ± 0a 6.05 ± 0.07a 5.81 ± 0.07a 5.30 ± 0.09a
18.46 ± 0.04a 18.51 ± 0.07a 18.53 ± 0b 18.88 ± 0.02b 19.47 ± 0b 19.92 ± 0.01b 20.17 ± 0.01b 23.54 ± 0.01b 28.73 ± 0.02c 29.44 ± 0.01b 30.52 ± 0.02b
18.50 ± 0a 18.51 ± 0a 18.54 ± 0b 19.51 ± 0c 22.4 ± 0c 23.10 ± 0c 24.5 ± 0c 25.33 ± 0c 28.53 ± 0b 34.57 ± 0c 39.43 ± 0c
T0 T50 T10 T0 T50
Psychrotrophic viable count (log10 cfu/g)
T10 T0
Total viable count (log10 cfu/g)
T50 T50 T10 T50
pH Properties
T0
TVB-N(mg/100 g)
T0
T10
Lactic acid bacteria (log10 cfu/g)
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
Table 2 Chemical properties and microbiological analyses of liquid smoked rainbow trout either with or without thyme oil. Values in rows followed by different superscripts are significantly different (p < 0.05).
686
liquid smoking process (Table 1). Similar findings were reported by other researchers (Dimitriadou et al., 2008; Goulas & Kontominas, 2005; Kolsarıcı & Özkaya, 1998; Muratore et al., 2007; Siskos et al., 2005, 2007). These results show that the addition of thyme oil had no effect on moisture, protein, and lipid content of liquid-smoked rainbow trout. The initial pH of the raw rainbow trout was 6.48 ± 0.02. After processing, the pH values were 6.27, 6.25 and 6.30, for T10, T50 and T0, respectively. Significant statistical differences were found between the samples (p < 0.05). The pH values of the samples were significantly influenced by the different processing conditions (Table 2) and the pH of each treated sample was significantly lower than the raw material. Recent studies show that liquid smoke condensates cause a reduction in the pH of the fillets (Alçiçek, 2010; Alçiçek et al., 2010; Dimitriadou et al., 2008). After processing, T50 samples had a lower pH value than T10 and T0 until the 45th day, which indicated that a high concentration of thyme oil led to a reduction of the pH value. However, the pH was lower in the control T0 samples than in the T10 and T50 samples after the 60th day of storage. This can be explained by high LAB growth which causes a decrease in pH value. Total volatile basic nitrogen (TVB-N) values for rainbow trout are presented in Table 2. Freshwater fish muscle has 10–20 mg/ 100 g TVB-N after harvesting. TVB-N amounted to 17.67 mg/ 100 g in the raw fish, and increased in the treated samples, to 18.46 ± 0.04, 18.31 and 18.50 for T10 T50 and T0 samples, respectively. These increases can be explained by proteolysis, due to enzymatic and microbial activities in the samples during processing. Similar TVB-N values were reported by Kolsarıcı and Özkaya (1998). The TVB-N levels increased gradually with storage time for liquid-smoked samples either with or without thyme oil. Significant statistical differences were found between the samples during storage time (p < 0.05). According to Leroi (1998), when TVB-N concentration of samples reaches 30 mg/100 g, depending on level of lactobacilli, panellists would consider that the samples were spoiled. Bugueno, Escriche, Martinez-Navarrete, Mar Camacho, and Chiralt (2003) also reported a positive correlation between pH, microbiological growth and TVB-N. These findings strongly agreed with our results. According to López-Caballero, PérezMateos, Montero, and Borderías (2000) an acceptable TVB-N level is 25–30 mg N/100 g for fresh fish. At the end of storage time, the TVB-N values of the samples in the present study reached maximum levels of 30.52, 27.5 and 39.43 for T10, T50 and T0, respectively. Thus, thyme oil lowered the TVB-N value of samples and provided a longer shelf life for liquid-smoked rainbow trout. 3.4. Sensory analyses The Table 3 shows the changes in sensory analysis scores during storage time. Sensory scores of samples were determined ‘‘good quality’’ after processing. However, the scores of T0 were especially decreased by the end of the storage period. This can be explained by the increase in microbial growth and TVB-N values. Similar results have been reported in other recent studies (Birkeland, Rora, Skara, & Bjerkeng, 2004; Cardinal et al., 2001; Martinez, Salmero’n, Guille’n, & Casas, 2007). The panellists found the textural quality very good until the 90th day for T10 and until the 135th day for T50. According to Dondero, Cisternas, and Carvajal (2004), autolytic enzymes can have a major effect on the textural quality of salmon. Addition of thyme oil provided very good textural quality of liquid-smoked rainbow trout over a long storage period. Smoke and the most intense odour of thyme (T50) were scored by the panellists as very good odour quality (9 points). The odour remained nearly constant until the 90th, 150th and 60th days, for T10, T50 and T0, respectively.
8.0 ± 0.70a 7.6 ± 0.54b 7.2 ± 0.44b 7.0 ± 0.70b 6.8 ± 0.83b 6.4 ± 0.54c 6.2 ± 0.44a 5.2 ± 0.83c 5.2 ± 0.83a – – 8.8 ± 0.44a 8.6 ± 0.54a 8.2 ± 0.44a 8.2 ± 0.44a 8.0 ± 0a 7.8 ± 0.44b 7.6 ± 0.54b 7.2 ± 0.44b 7.2 ± 0.44b 6.8 ± 0.44b 6.8 ± 0.44b 8.6 ± 0.54a 8.4 ± 0.54a 8.2 ± 0.44a 7.6 ± 0.54a 7.4 ± 0.54a 7.0 ± 0a 6.8 ± 0.54a 6.4 ± 0.54a 5.4 ± 0.54a 4.2 ± 0.44a – 8.2 ± 0.44a 7.2 ± 0.44b 7.2 ± 0.44a 7.0 ± 1.00b 6.8 ± 0.83b 6.4 ± 0.54a 6.2 ± 0.44a 5.8 ± 0.44a 5.2 ± 0.83a – – 9.00 ± 0b 8.4 ± 0.54a 8.4 ± 0.54b 8.4 ± 0.54a 8.2 ± 0.44a 8.0 ± 0.70b 7.8 ± 0.44b 7.6 ± 0.54b 7.4 ± 0.54b 7.0 ± 0.70b 6.2 ± 0.44b 8.4 ± 0.54a 8.2 ± 0.44a 7.6 ± 0.54a 7.6 ± 0.54a 7.4 ± 0.54a 7.0 ± 0a 6.6 ± 0.54a 6.4 ± 0.54a 5.4 ± 0.54a 4.2 ± 0.44a – 8.8 ± 0.44a 8.6 ± 0.54a 8.2 ± 0.83a 8.2 ± 0.44a 7.8 ± 0.44a 7.8 ± 0.44b 7.8 ± 0.44b 7.4 ± 0.54b 7.2 ± 0.44b 6.6 ± 0.54b 6.4 ± 0.54b 8.4 ± 0.54a 8.4 ± 0.44a 7.8 ± 0.44a 7.6 ± 0.54a 7.0 ± 0.70a 6.8 ± 0.44a 6.6 ± 0.54a 6.4 ± 0.54a 5.4 ± 0.54a 4.0 ± 0.70a – 1 15 30 45 60 75 90 105 120 135 150
7.8 ± 0.44b 7.6 ± 0.54b 7.2 ± 0.70b 6.8 ± 0.83b 6.6 ± 0.89b 6.5 ± 0.83a 6.4 ± 0.70a 5.6 ± 0.89a 5.0 ± 1.14a – –
8.4 ± 0.54a 8.2 ± 0.44a 8.0 ± 0a 7.6 ± 0.54a 7.4 ± 0.54a 7.0 ± 0a 6.8 ± 0.44a 6.4 ± 0.54a 5.4 ± 0.54a – –
8.8 ± 0.44b 8.6 ± 0.54a 8.4 ± 0.54a 8.2 ± 0.44a 8.0 ± 0a 7.8 ± 0.44b 7.6 ± 0.54b 7.4 ± 0.54b 7.2 ± 0.44b 6.8 ± 0.44b 6.4 ± 0.54b
7.8 ± 0.44a 7.6 ± 0.54b 7.2 ± 0.44b 7.0 ± 0.70b 6.8 ± 0.83b 6.4 ± 0.54c 6.2 ± 0.44a 5.8 ± 0.44a 5.2 ± 0.83a – –
T0 T50
Texture Odour
T10 T0 T50
Taste
T10 Groups Days
T0 T50 Appearance
T10
Sensory appearances
Table 3 Sensory scores of raw and liquid smoked rainbow trout either with or without thyme oil. Values in rows followed by different superscripts are significantly different (p < 0.05).
T10
T50
T0
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
687
Notably, T50 did not release any off-odours even after the end of the storage period. The taste of T10 and T0 were scored as ‘‘spoiled’’ by the panellists after the 135th day, while T50 continued to be scored as ‘‘good quality’’. Quattara, Sabato, and Lacroix (2001) reported that addition of thyme essential oils at 0.9% exerted no negative effects on the flavour and appearance of cooked shrimp. No samples were rejected by the panellists after processing. T10 (appearance 8.4 ± 0.54, taste 8.4 ± 0,54, odour 8.4 ± 0.54, texture 8.6 ± 0.54) and T50 (appearance 8.8 ± 0.44, taste 8.8 ± 0.44, odour 9.00 ± 0, texture 8.8 ± 0.44) samples were acceptable to the panellists. There was a significant difference (p < 0.05) between samples with and without thyme oil (Table 3), while sT10T50ignificant differences were found between T10 and T50 only after the 75th day (p < 0.05). These results showed that samples with high thyme oil have acceptable overall scores, due to the limiting effect of thyme on microbiological activity and TVB-N value and also the positive attributes of thyme flavour. T50 samples were assessed as the most acceptable products by the panellists. 4. Conclusion The present experiment demonstrates that addition of thyme oil to vacuum-packed liquid-smoked rainbow trout fillets has a profound effect on sensory quality, TVB-N value and microbiological growth. T10 samples had a shorter shelf life than T50 samples, but both had longer shelf life than the control samples without thyme oil. Thyme oil can therefore be viewed as a natural preservative for liquid-smoked rainbow trout. References Alçiçek, Z. (2010). The comparative investigate of using hot smoking and liquid smoking techniques of fillets of rainbow trout (Oncorhynchus mykiss) during vacuum packed and chilled storage with different salting techniques. Ph. Doctorate thesis. Ankara, Turkey: Ankara University, Graduate School of Natural and Applied Sciences Department of Fisheries and Aquaculture. Alçiçek, Z., Zencir, Ö., Çelik Çakırog˘ulları, G., & Atar, H. H. (2010). The Effect of Liquid Smoking of Anchovy (Engraulis encrasicolus, L. 1758) Fillets on Sensory, Meat Yield, Polycyclic Aromatic Hydrocarbons (PAHs) Content and Chemical Changes. Journal of Aquatic Food Product Technology, 19(3), 264–273. AOAC (1995). Patricia Cunniff (Ed.), Official methods of analysis of the association of official analytical chemists (16th ed.). Virginia, USA. Arora, D. S., & Kaur, J. (1999). Antimicrobial activity of spices. International Journal of Antimicrobial Agents, 12, 257–262. Birkeland, S., Rora, B. M. A., Skara, T., & Bjerkeng, B. (2004). Effects of cold smoking procedures and raw material characteristics on product yield and quality parameters of cold smoked Atlantic Salmon (Salmo salar L.) fillets. Food Research International, 37, 273–286. Bugueno, G., Escriche, I., Martinez-Navarrete, N., Mar Camacho, M., & Chiralt, A. (2003). Influence of storage conditions on some physical and chemical properties of smoked salmon (Salmo salar) processed by vacuum impregnation techniques. Food Chemistry, 81, 85–90. Cardinal, M., Knockaert, C., Torrissen, O., Sigurgisladottir, S., Morkore, T., Thomassen, T., et al. (2001). Relation of smoking parameters to the yield, color and sensory quality of smoked Atlantic salmon (Salmo salar). Food Research International, 34, 537–550. Che-Man, Y. B., & Ramadas, J. (1998). Effected of packaging environment on quality changes of smoked spanish mackarel under refrigeration. Journal of Food Quality, 21, 167–174. Dimitriadou, D., Zotos, A., Petridis, D., & Taylor, A. K. D. (2008). Improvement in the production of smoked trout fillets steamed with liquid smoke. Food Science Techniques International, 14(1), 67–77. Dimitrijevic, S. I., Mihajlovski, R. K., Antonovic, D. G., Milanovic-Stevanovic, M. R., & Mijin, D. Z. (2007). A study of synergistic antilisterial effects of a sub-lethal dose of lactic acid and essential oils from Thymus vulgaris L., Rosmarinus officinalis L. and Origanum vulgare L. Food Chemistry, 104, 774–782. Dondero, M., Cisternas, F., & Carvajal, L. (2004). Changes in quality of vacuumpacked cold smoked salmon (Salmo salar) as a function of storage temperature. Food Chemistry, 87, 543–550. Gilliland, S. E., Busta, F. F., Brinda, J. J., & Campbell, J. E. (1976). Aerobic Plate Count. In M. L. Speck (Ed.), Compendium of methods for the microbiological examination of foods (pp. 107–131). Washington: American Public Health Association, Inc. Goulas, A. E., & Kontominas, M. G. (2005). Effect of salting and smoking method on the keeping quality of chub mackerel (Scomber japonicus): Biochemical and sensory attributes. Food Chemistry, 93, 511–520.
688
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
Hattula, T., Elfving, K., Mroueh, U. M., & Luoma, T. (2001). Use of liquid smoke flavouring as an alternative to traditional flue gas smoking of rainbow trout fillets (Oncorhynchus mykiss). Lebensmittel-Wissenschaft und-Technologie, 34(8), 521–525. Kolsarıcı, N., & Özkaya, Ö. (1998). Effect of smoking methods on shelf-life of rainbow trout (Salmo gairdneri). Turkish Journal of Veterinary and Animal Sciences, 22, 273–284. Leroi, F., Joffraud, J. J., Chevalier, F., & Cardinal, M. (1998). Study of the microbial ecology of cold-smoked salmon during storage at 8 °C. International Journal of Food Microbiology, 39, 1–2. López-Caballero, M. E., Pérez-Mateos, M., Montero, P., & Borderías, A. J. (2000). Oyster preservation by high-pressure treatment. Journal of Food Protection, 63, 196–201. Martinez, O., Salmero’n, J., Guille’n, M. D., & Casas, C. (2007). Textural and physicochemical changes in salmon (Salmo salar) treated with commercial liquid smoke flavourings. Food Chemistry, 100, 498–503. Moret, S., & Conte, L. S. (2002). A rapid method for polycyclic aromatic hydrocarbon determination in vegetable oil. Journal of Separation Science, 25, 96–100. Muratore, G., Mazzaglia, A., Lanza, C. M., & Licciardello, F. (2007). Effect of process variables on the quality of swordfish fillets flavored with smoke condensate. Journal of Food Processing and Preservation., 31, 167–177. Özkan, G., Sag˘dıç, O., & Özcan, M. (2003). Note: Inhibition of pathogenic bacteria by essential oils at different concentrations. Food Technology International, 9(2). 0085-4.
Quattara, B., Sabato, S. F., & Lacroix, M. (2001). Combined effect of antimicrobial coating and gamma irradiation on shelf life extension of pre-cooked shrimp (Penaus spp.). International Journal of Food Microbiology, 68, 1–9. Rahman, S. (2009). Handbook of food preservation. New York: Marckel Dekler. pp. 299. Siskos, I., Zotos, A., Melidou, S., & Tsikritzi, R. (2007). The effect of liquid smoking of fillets of trout (Salmo gairdnerii) on sensory, microbiological and chemical changes during chilled storage. Food Chemistry, 101, 458–464. Siskos, I., Zotos, A., & Taylor, K. A. (2005). The effect of drying, pressure and processing time on the quality of liquid-smoked trout (Salmo gairdnerii) fillets. Journal of the Science of Food and Agriculture, 85, 2054–2060. Solomakos, N., Govaris, A., Koidis, P., & Botsoglou, N. (2008). The antimicrobial effect of thyme essential oil, nisin and their combination against Escherichia coli O157:H7 in minced beef during refrigerated storage. Meat Science, 80, 159–166. Suñen, E., Fernandez-Galian, B., & Aristimuno, C. (2001). Antibacterial activity of smokewood condensates against Aeromonas hydrophila, Yersinia enterocolitica and Listeriamonocytogenes at low temperature. Food Microbiology, 18, 387–393. Treadwell, F. P., & Hall, W. T. (1928). Analytical Chemistry (7th ed.). John Wiley and Sons Inc. Varlet, V., Serot, T., Knockaert, C., Cornet, J., Cardinal, M., Monteau, F., et al. (2007). Organoleptic characterization and PAH content of salmon (Salmo salar) fillets smoked according to four industrial smoking techniques. Journal of the Science of Food and Agriculture, 87, 847–854.
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
The effects of thyme (Thymus vulgaris L.) oil concentration on liquid-smoked vacuum-packed rainbow trout (Oncorhynchus mykiss Walbaum, 1792) fillets during chilled storage Zayde Alçiçek ⇑ Firat University Fisheries Faculty, Department of Fisheries Techniques and Fish Processing Technology, 23119, Elazig, Turkey
a r t i c l e
i n f o
Article history: Received 1 November 2010 Received in revised form 4 March 2011 Accepted 21 March 2011 Available online 26 March 2011 Keywords: Thyme oil Liquid smoke Polycyclic aromatic hydrocarbons Rainbow trout Sensory quality Spices
a b s t r a c t This study examined the effects of thyme oil on chemical properties, microbiological changes, and sensory quality in vacuum-packed liquid-smoked rainbow trout fillets under chilled storage. After 150 days of storage, the total volatile basic nitrogen (TVB-N) values were 30.52, 27.5 and 39.43 mg/100 g, and total viable counts (TVC) were 5.34, 4.96 and 5.53, for thyme oil additions of 10 (T10), 50 (T50) and 0 (control; T0) mL/L brine, respectively. The highest acceptable TVB-N value was adopted as 30 mg /100 g, corresponding to shelf lives of 135, 150, and 105 days for T10, T50, and T0, respectively. T50 addition of thyme oil to liquid-smoked rainbow fillets extended shelf life T10 and T50, gave acceptable sensory quality and limited microbiological growth during chilled storage. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Seafood has been preserved for centuries by smoking, one of the oldest meat and seafood preservation methods, which imparts antimicrobial and antioxidant characteristics to the food (Che-Man & Ramadas, 1998; Goulas & Kontominas, 2005; Hattula, Elfving, Mroueh, & Luoma, 2001; Kolsarıcı & Özkaya, 1998; Martinez, Salmero’n, Guille’n, & Casas, 2007; Muratore, Mazzaglia, Lanza, & Licciardello, 2007). Two smoking methods are commonly utilised: hot and cold smoking. Over the past three decades, liquid smoke flavours have been increasingly used as an alternative to smoking (Alçiçek, Zencir, Çelik, Çakırog˘ulları, & Atar, 2010). Liquid smoke flavours have some advantages over traditional smoking methods, such as easy application, lower cost, and environmental friendliness. It is also easier to control smoke contaminants like polycyclic aromatic hydrocarbons (PAHs), which are considered carcinogenic, and mutagenic molecules produced during pyrolysis of wood (Alçiçek et al., 2010; Dimitriadou, Zotos, Petridis, & Taylor, 2008; Siskos, Zotos, & Taylor, 2005; Varlet et al., 2007). Spices, which have been used as food additives for centuries and are called bio-preservatives or green chemicals by Rahman (2009), are now being viewed as potential alternatives to chemical preser-
vatives. Thyme (Thymus vulgaris L.), a plant species native to the western Mediterranean area, is now widely cultivated as a spice throughout temperate climes. Although the antimicrobial activity of some spices has been studied against pathogenic bacteria (Arora & Kaur, 1999), no information is yet available in the literature about preservative effects of thyme on liquid-smoked seafood. Thyme is active against many spoilage and pathogenic microorganisms and also improves sensory acceptability (Dimitrijevic, Mihajlovski, Antonovic, Milanovic-Stevanovic, & Mijin, 2007; Özkan, Sag˘dıç, & Özcan, 2003; Solomakos, Zotos, Melidou, & Tsikritzi, 2007). According to Solomakos et al. (2008), the thyme plant contains a number of important compounds, such as the phenols thymol (44–60%) and carvacrol (2.2–4.2%). The aim of this study was to determine the effects of thyme oil on the shelf life of vacuum-packed liquid-smoked rainbow trout fillets under chilled storage at 4 ± 1 °C. A number of chemical parameters, total viable count (TVC), psychrotrophic viable count (PVC), lactic acid bacteria (LAB), sensory analysis and PAH content were measured.
2. Materials and methods 2.1. Raw materials
⇑ Tel.: +90 5455614347; fax: +90 4242386287. E-mail address: [email protected] 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.03.087
A total of 450 fresh rainbow trout (Oncorhynchus mykiss), each weighing 220 ± 20 g, were obtained from ALBA Fish Co.
684
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
aquaculture farm located in Kemer Dam Lake, geographically located at Aydin in Western Turkey. After harvesting, the fish were transferred within 1 h to the fish processing plant at ALBA Fish Co., Bozdogan, Aydin, Turkey, in sealed polystyrene foam boxes containing ice. On the day of processing, the fish were eviscerated and washed with clean water. After brining, addition of thyme oil and liquid smoking, the fillets were vacuum packed and stored at 4 ± 1 °C until analysis on days 15, 30, 45, 60, 75, 90, 105, 120, 135 and 150. Three replicates were performed and each replicate per treatment contained 30 fish.
2005). After brining, addition of thyme oil and liquid smoking, all samples were air-dried at 17 °C for 20 min. The samples were then heated in a fan oven at 120–130 °C for 20 min. After cooling (at 20 °C for 30 min), the smoked products were filleted, vacuum packed (high barrier nylon polyethylene bags under vacuum (residual pressure 0.1 bar)), and stored at 4 ± 1 °C. This type of refrigerated storage is the most common preservative method for this type of seafood product (Fig. 1).
2.2. Thyme oil and liquid smoke flavouring
Prior to analyses, edible parts of the fish samples were homogenised. Chemical composition of fish samples was measured by standard methods, following protocols AOAC (1995). Dry matter content was analysed by oven drying 3 g of liquid-smoked rainbow trout at 105 °C until a constant weight was reached. A 10-g sample of fish was mixed with 50 mL of distilled water and the pH was measured using a pH metre. TVB-N analysis was performed using the method of Goulas and Kontominas (2005). Total fat content was analysed by hexane extraction. Percent protein (Kjeldahl N 6.25) was determined from a 1-g sample for each treatment (Alçiçek et al., 2010). NaCl content in fish samples was determined by the Mohr method (Treadwell & Hall, 1928). All analyses were carried out in duplicate.
Liquid smoke flavouring (Red Arrow, SmokEz-M-10, mesquite wood-based smoke) and water-soluble thyme oil were purchased from a food additives company (GMT Food Ingredients Co., Istanbul, Turkey). The thyme oil was added in the brine solution at two different concentrations (10 and 50 ppm), along with the liquid smoke flavouring. The pH of the thyme oil used was 5.78 (1% solution). Volatile oil content was 9.52%. 2.3. Brining, addition of thyme oil, liquid smoking, drying and heating The fish were separated into three groups; The first group (T0): control group, without added thyme oil. The second group (T10): 10 mL/L thyme oil was added in the brine solution. The third group (T50): 50 mL/L thyme oil was added in the brine solution. All groups were smoked with liquid smoke flavouring (100 mL in 1 L of brine solution) and were brined in 18% sodium chloride (NaCl) solution for 12 h. The brine solution was stirred every 2 h. The temperature of the brine solution was kept low (4 ± 1 °C), to minimise microbial growth (Dimitriadou et al., 2008; Siskos et al.,
2.4. Chemical analyses
2.5. Microbiological analyses Total viable counts (TVCs), psychrotrophic viable counts (PVCs), and lactic acid bacteria (LAB) were determined. Ten grams of each sample homogenate were diluted to 100 mL with peptone (1% water) buffered at pH 7.5. Rainbow trout samples were homogenised in Tryptone water for 30 s in a stomacher. Serial dilutions were prepared until 10 7 g/ml samples were obtained. One millilitre of each dilution was placed in a Petri dish and 15 mL of plate count agar were added for TVC/PVC and 15 mL de Man Rogosa Sharpe (MRS) agar double layered for LAB. TVCs were
Receiving and transport Raw fish were stored on ice in sealed foamed polystyrene boxes and transferred to the fish processing plant
Filleting Fish gutted and filleted
Brining, Addition thyme oil and Liquid Smoke Flavour (T10) Fillet immersed in liquid brine solution 18% for 4 hours at 4 ºC
Brining, Addition thyme oil and Liquid Smoke Flavour (T50) Fillet immersed in liquid brine solution 18% for 4 hours at 4 ºC
Brining and Liquid Smoke Flavour (T0) Fillet immersed in liquid brine solution 18% for 4 hours at 4 ºC
Drying Drying at 17 ºC for 20 min
Heating 120−130 ºC in a fan oven
Cooling Cool to 20 ºC for 30 min Fig. 1. Flow diagram of the production of liquid smoked samples either with or without thyme oil.
685
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
performed on pour plates with plate-count agar (Merck KGaA, Darmstadt, Germany), after 48 h at 37 °C. The same plates were used for the determination of PVCs; plates were incubated for 7 days at 4 °C. De Man Rogosa Sharpe (MRS) agar was used for LAB (Dimitriadou et al., 2008). Petri dishes were inverted and placed in an oven at 30 °C for 36 h. After incubation, TVC, PVC and LAB were counted, following the rules recommended by Gilliland, Busta, Brinda, and Campbell (1976). 2.6. PAH analysis Benzo(a)pyrene compounds in fish samples were determined using the method of Moret and Conte (2002). The limit of detection was 1 ppb. For quality assurance, spiked samples were studied together with the experimental samples. Blank fish samples were spiked with benzo(a)pyrene standard at ½ MRL (maximum residue level) and MRL levels, each in duplicate. The mean recovery was 90 ± 0.91% for the levels studied. 2.7. Sensory analyses Five experienced panellists, who were members of academic staff and trained in sensory descriptors for smoked trout, were used to evaluate the quality of rainbow trout fillets during storage. Rainbow trout fillets were assessed on the basis of appearance, odour, taste and texture characteristics using a nine point descriptive scale. A score of 7–9 indicated ‘‘very good’’ quality, a score of 4.0–6.9 ‘‘good’’ quality, a score of 1.0–3.9 denoted as ‘‘spoiled’’. All samples were stored at +4 °C until sensory analysis was performed. 2.8. Statistical analyses The significance of effects of thyme oil concentrations was determined by one-way analysis of variance and Duncan’s multiple range test, using SPSS for Windows, Version 16.0 (SPSS Inc., Chicago, IL) at p 6 0.05. 3. Results and discussion Tables 1 and 2 show the moisture, ash, protein, lipid, TVB-N, TVC, PVC and LAB content of raw and liquid-smoked rainbow trout. The approximate composition data for raw rainbow trout agreed with those reported by other researchers (Alçiçek, 2010; Kolsarıcı & Özkaya, 1998). The percentages of total ash, protein, and lipid of liquid-smoked rainbow trout increased due to moisture loss during processing, although TVC, PVC and LAB decreased because of liquid smoke flavouring content (e.g., phenols, etc.). Similar findings were reported by Kolsarıcı and Özkaya (1998) for smoked rainbow trout.
was due to the liquid-smoking process, as discussed in recent studies (Dimitriadou et al., 2008; Siskos, Zotos, Melidou, & Tsikritzi, 2007). TVC was found to be equal to 0.95 log cfu/g in T10 and T50 until the 30th storage day. The control, T0, showed values of 1.04 ± 0.05 log cfu/g and 2.1 ± 0.02 log cfu/g, respectively, for the 1st and 15th storage dayT0, higher than the other groupsT50T10. The TVC values in the T50 samples were still low even after 135 days at 4 ± 1 °C. This result can be explained by thyme oil effects on total viable counts (Arora & Kaur, 1999; Dimitrijevic et al., 2007; Özkan et al., 2003). Differences between the TVC values were statistically significant after the 30th day of storage time (p < 0.05). Maximum microbiological growth had already been reached in T10 and control T0 samples by the 135th day; while T10 samples were still found acceptable by the panellists, the T0 samples were unacceptable for all sensory scores. Similar maximum TVC values were also observed in other studies (Dimitriadou et al., 2008; Muratore et al., 2007). Psychotropic pathogens can grow in refrigerated foods and may cause some foodborne illnesses. A number of researchers have reported that these pathogens can be frequently isolated from smoked products (Suñen, Fernandez-Galian, & Aristimuno, 2001). PVC at the beginning of storage was measured in all samples as 0.95 log cfu/g. This did not change until the 30th day of storage time. A slight increase in PVC values to 1.38 ± 0.09 log cfu/g and 1.36 log cfu/g was found in the T10 and T0 samples, respectively, with no change in the T50 samples. On the 45th day, the PVCs for the T50 samples were still only 1.83 ± 0.04 log cfu/g, indicating that the higher concentration of thyme oil in the T50 samples reduced the initial PVC. Statistically significant differences were found between the PVC values according to the groups during storage time at the 1st, 15th and 105th days (p < 0.05). In vacuum-packed smoked seafood, LAB are a problem as they produce typical sour odours and flavours (Kolsarıcı & Özkaya, 1998; Leroi, Joffraud, Chevalier, & Cardinal, 1998). The LAB count at the beginning of storage was 0.95 log cfu/g and that level was maintained until the 30th day in all samples. Statistically significant differences were found between the PVCLAB values according to the groups after the 30th day of storage. At the end of shelf life, after 150 days, 2.26, 2.18 and 2.68 ± 0.02 log cfu/g were reached for the T10, T50 and T0 samples, respectively. These differences may explain the effects of thyme oil on the liquid-smoked rainbow trout fillets. The results of microbiological analyses confirmed that thyme oil used in liquid-smoked processing leads to a reduction in microbial contamination during long storage time, probably due to the presence of carvacrol and thymol in the thyme essential oil (Solomakos, Govaris, Koidis, & Botsoglou, 2008). 3.2. PAH analysis No detectable amounts of benzo(a)pyrene, which is an indicator for PAHs (Dimitriadou et al., 2008), were found in any samples.
3.1. Microbiological analysis 3.3. Chemical determinations Microbiological counts for TVC, PVC and LAB are shown in Table 2. The average TVC of fresh samples was 4.5 log cfu/g. After processing, a TVC reduction was observed in all groups. This reduction
Table 1 shows the moisture, protein, lipid, and salt contents of raw and T10, T50, and T0 samples after processing. The percentage of total protein and lipid increased, due to water loss during the
Table 1 Proximate composition of raw and liquid smoked rainbow trout either with or without thyme oil. Sample component
Raw rainbow trout
T10 (after process)
T50 (after process)
T0 (after process)
Dry matter (%) Protein (%) Lipid (%) Salt (%)
31.4 20.26 9.01 0.10
48.7 27.8 12.9 4.02
48.6 27.9 12.8 4.03
48.5 27.9 12.9 4.03
0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.07 ± 0.1a 1.74 ± 0.03c 1.81 ± 0b 1.87 ± 0c 1.98 ± 0.02b 2.29 ± 0.01c 2.47 ± 0.01c 2.68 ± 0.02c 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.33 ± 0.01a 1.54 ± 0.07a 1.56 ± 0.02a 1.83 ± 0.02a 1.99 ± 0a 2.03 ± 0.02a 2.18 ± 0a 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.16 ± 0.16a 1.48 ± 0b 1.54 ± 0.02a 1.72 ± 0b 1.92 ± 0.01b 2.16 ± 0b 2.16 ± 0b 2.26 ± 0b 0.95 ± 0a 0.95 ± 0a 1.39 ± 0b 2.26 ± 0b 2.41 ± 0.01b 2.93 ± 0.02c 2.98 ± 0b 3.29 ± 0a 3.93 ± 0c 4.7 ± 0c 5.2 ± 0c 0.95 ± 0a 0.95 ± 0a 0.95 ± 0a 1.83 ± 0.04a 2.09 ± 0a 2.64 ± 0.01a 2.79 ± 0a 2.97 ± 0a 3.46 ± 0a 3.91 ± 0a 4 ± 0a 0.95 ± 0a 0.95 ± 0a 1.38 ± 0.09b 2.24 ± 0.02b 2.55 ± 0c 2.83 ± 0b 2.93 ± 0.04b 3.19 ± 0a 3.58 ± 0b 4.04 ± 0b 4.54 ± 0b 1.04 ± 0.05a 2.1 ± 0.02b 2.41 ± 0c 2.47 ± 0b 2.74 ± 0b 3.42 ± 0c 3.7 ± 0b 3.99 ± 0a 4.66 ± 0c 4.99 ± 0b 5.53 ± 0c 0.95 ± 0a 0.95 ± 0a 1.87 ± 0.01a 2.16 ± 0.02c 2.36 ± 0c 3.16 ± 0a 3.53 ± 0a 3.75 ± 0a 3.95 ± 0a 4.11 ± 0.16a 4.96 ± 0.02a 0.95 ± 0a 0.95 ± 0a 2.22 ± 0.04b 2.4 ± 0.01a 2.93 ± 0.01a 3.31 ± 0.01b 3.75 ± 0.01c 3.95 ± 0a 4.15 ± 0b 4.4 ± 0.03a 5.34 ± 0b 18.31 ± 0b 18.32 ± 0b 18.35 ± 0a 18.42 ± 0a 18.44 ± 0a 18.59 ± 0a 19.16 ± 0a 20.54 ± 0a 21.78 ± 0.02a 24.85 ± 0.07a 27.5 ± 0a 6.25 ± 0b 6.25 ± 0a 6.24 ± 0ab 6.22 ± 0a 6.24 ± 0a 6.22 ± 0c 6.21 ± 0b 6.20 ± 0b 6.20 ± 0b 6.18 ± 0b 6.13 ± 0b
T10
6.27 ± 0a 6.27 ± 0a 6.26 ± 0a 6.25 ± 0a 6.23 ± 0a 6.20 ± 0b 6.18 ± 0a 6.20 ± 0b 6.18 ± 0b 6.17 ± 0b 6.14 ± 0.01b
Groups Days
1 15 30 45 60 75 90 105 120 135 150
6.30 ± 0c 6.30 ± 0a 6.27 ± 0.01b 6.25 ± 0.02a 6.20 ± 0a 6.18 ± 0a 6.17 ± 0a 6.11 ± 0a 6.05 ± 0.07a 5.81 ± 0.07a 5.30 ± 0.09a
18.46 ± 0.04a 18.51 ± 0.07a 18.53 ± 0b 18.88 ± 0.02b 19.47 ± 0b 19.92 ± 0.01b 20.17 ± 0.01b 23.54 ± 0.01b 28.73 ± 0.02c 29.44 ± 0.01b 30.52 ± 0.02b
18.50 ± 0a 18.51 ± 0a 18.54 ± 0b 19.51 ± 0c 22.4 ± 0c 23.10 ± 0c 24.5 ± 0c 25.33 ± 0c 28.53 ± 0b 34.57 ± 0c 39.43 ± 0c
T0 T50 T10 T0 T50
Psychrotrophic viable count (log10 cfu/g)
T10 T0
Total viable count (log10 cfu/g)
T50 T50 T10 T50
pH Properties
T0
TVB-N(mg/100 g)
T0
T10
Lactic acid bacteria (log10 cfu/g)
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
Table 2 Chemical properties and microbiological analyses of liquid smoked rainbow trout either with or without thyme oil. Values in rows followed by different superscripts are significantly different (p < 0.05).
686
liquid smoking process (Table 1). Similar findings were reported by other researchers (Dimitriadou et al., 2008; Goulas & Kontominas, 2005; Kolsarıcı & Özkaya, 1998; Muratore et al., 2007; Siskos et al., 2005, 2007). These results show that the addition of thyme oil had no effect on moisture, protein, and lipid content of liquid-smoked rainbow trout. The initial pH of the raw rainbow trout was 6.48 ± 0.02. After processing, the pH values were 6.27, 6.25 and 6.30, for T10, T50 and T0, respectively. Significant statistical differences were found between the samples (p < 0.05). The pH values of the samples were significantly influenced by the different processing conditions (Table 2) and the pH of each treated sample was significantly lower than the raw material. Recent studies show that liquid smoke condensates cause a reduction in the pH of the fillets (Alçiçek, 2010; Alçiçek et al., 2010; Dimitriadou et al., 2008). After processing, T50 samples had a lower pH value than T10 and T0 until the 45th day, which indicated that a high concentration of thyme oil led to a reduction of the pH value. However, the pH was lower in the control T0 samples than in the T10 and T50 samples after the 60th day of storage. This can be explained by high LAB growth which causes a decrease in pH value. Total volatile basic nitrogen (TVB-N) values for rainbow trout are presented in Table 2. Freshwater fish muscle has 10–20 mg/ 100 g TVB-N after harvesting. TVB-N amounted to 17.67 mg/ 100 g in the raw fish, and increased in the treated samples, to 18.46 ± 0.04, 18.31 and 18.50 for T10 T50 and T0 samples, respectively. These increases can be explained by proteolysis, due to enzymatic and microbial activities in the samples during processing. Similar TVB-N values were reported by Kolsarıcı and Özkaya (1998). The TVB-N levels increased gradually with storage time for liquid-smoked samples either with or without thyme oil. Significant statistical differences were found between the samples during storage time (p < 0.05). According to Leroi (1998), when TVB-N concentration of samples reaches 30 mg/100 g, depending on level of lactobacilli, panellists would consider that the samples were spoiled. Bugueno, Escriche, Martinez-Navarrete, Mar Camacho, and Chiralt (2003) also reported a positive correlation between pH, microbiological growth and TVB-N. These findings strongly agreed with our results. According to López-Caballero, PérezMateos, Montero, and Borderías (2000) an acceptable TVB-N level is 25–30 mg N/100 g for fresh fish. At the end of storage time, the TVB-N values of the samples in the present study reached maximum levels of 30.52, 27.5 and 39.43 for T10, T50 and T0, respectively. Thus, thyme oil lowered the TVB-N value of samples and provided a longer shelf life for liquid-smoked rainbow trout. 3.4. Sensory analyses The Table 3 shows the changes in sensory analysis scores during storage time. Sensory scores of samples were determined ‘‘good quality’’ after processing. However, the scores of T0 were especially decreased by the end of the storage period. This can be explained by the increase in microbial growth and TVB-N values. Similar results have been reported in other recent studies (Birkeland, Rora, Skara, & Bjerkeng, 2004; Cardinal et al., 2001; Martinez, Salmero’n, Guille’n, & Casas, 2007). The panellists found the textural quality very good until the 90th day for T10 and until the 135th day for T50. According to Dondero, Cisternas, and Carvajal (2004), autolytic enzymes can have a major effect on the textural quality of salmon. Addition of thyme oil provided very good textural quality of liquid-smoked rainbow trout over a long storage period. Smoke and the most intense odour of thyme (T50) were scored by the panellists as very good odour quality (9 points). The odour remained nearly constant until the 90th, 150th and 60th days, for T10, T50 and T0, respectively.
8.0 ± 0.70a 7.6 ± 0.54b 7.2 ± 0.44b 7.0 ± 0.70b 6.8 ± 0.83b 6.4 ± 0.54c 6.2 ± 0.44a 5.2 ± 0.83c 5.2 ± 0.83a – – 8.8 ± 0.44a 8.6 ± 0.54a 8.2 ± 0.44a 8.2 ± 0.44a 8.0 ± 0a 7.8 ± 0.44b 7.6 ± 0.54b 7.2 ± 0.44b 7.2 ± 0.44b 6.8 ± 0.44b 6.8 ± 0.44b 8.6 ± 0.54a 8.4 ± 0.54a 8.2 ± 0.44a 7.6 ± 0.54a 7.4 ± 0.54a 7.0 ± 0a 6.8 ± 0.54a 6.4 ± 0.54a 5.4 ± 0.54a 4.2 ± 0.44a – 8.2 ± 0.44a 7.2 ± 0.44b 7.2 ± 0.44a 7.0 ± 1.00b 6.8 ± 0.83b 6.4 ± 0.54a 6.2 ± 0.44a 5.8 ± 0.44a 5.2 ± 0.83a – – 9.00 ± 0b 8.4 ± 0.54a 8.4 ± 0.54b 8.4 ± 0.54a 8.2 ± 0.44a 8.0 ± 0.70b 7.8 ± 0.44b 7.6 ± 0.54b 7.4 ± 0.54b 7.0 ± 0.70b 6.2 ± 0.44b 8.4 ± 0.54a 8.2 ± 0.44a 7.6 ± 0.54a 7.6 ± 0.54a 7.4 ± 0.54a 7.0 ± 0a 6.6 ± 0.54a 6.4 ± 0.54a 5.4 ± 0.54a 4.2 ± 0.44a – 8.8 ± 0.44a 8.6 ± 0.54a 8.2 ± 0.83a 8.2 ± 0.44a 7.8 ± 0.44a 7.8 ± 0.44b 7.8 ± 0.44b 7.4 ± 0.54b 7.2 ± 0.44b 6.6 ± 0.54b 6.4 ± 0.54b 8.4 ± 0.54a 8.4 ± 0.44a 7.8 ± 0.44a 7.6 ± 0.54a 7.0 ± 0.70a 6.8 ± 0.44a 6.6 ± 0.54a 6.4 ± 0.54a 5.4 ± 0.54a 4.0 ± 0.70a – 1 15 30 45 60 75 90 105 120 135 150
7.8 ± 0.44b 7.6 ± 0.54b 7.2 ± 0.70b 6.8 ± 0.83b 6.6 ± 0.89b 6.5 ± 0.83a 6.4 ± 0.70a 5.6 ± 0.89a 5.0 ± 1.14a – –
8.4 ± 0.54a 8.2 ± 0.44a 8.0 ± 0a 7.6 ± 0.54a 7.4 ± 0.54a 7.0 ± 0a 6.8 ± 0.44a 6.4 ± 0.54a 5.4 ± 0.54a – –
8.8 ± 0.44b 8.6 ± 0.54a 8.4 ± 0.54a 8.2 ± 0.44a 8.0 ± 0a 7.8 ± 0.44b 7.6 ± 0.54b 7.4 ± 0.54b 7.2 ± 0.44b 6.8 ± 0.44b 6.4 ± 0.54b
7.8 ± 0.44a 7.6 ± 0.54b 7.2 ± 0.44b 7.0 ± 0.70b 6.8 ± 0.83b 6.4 ± 0.54c 6.2 ± 0.44a 5.8 ± 0.44a 5.2 ± 0.83a – –
T0 T50
Texture Odour
T10 T0 T50
Taste
T10 Groups Days
T0 T50 Appearance
T10
Sensory appearances
Table 3 Sensory scores of raw and liquid smoked rainbow trout either with or without thyme oil. Values in rows followed by different superscripts are significantly different (p < 0.05).
T10
T50
T0
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
687
Notably, T50 did not release any off-odours even after the end of the storage period. The taste of T10 and T0 were scored as ‘‘spoiled’’ by the panellists after the 135th day, while T50 continued to be scored as ‘‘good quality’’. Quattara, Sabato, and Lacroix (2001) reported that addition of thyme essential oils at 0.9% exerted no negative effects on the flavour and appearance of cooked shrimp. No samples were rejected by the panellists after processing. T10 (appearance 8.4 ± 0.54, taste 8.4 ± 0,54, odour 8.4 ± 0.54, texture 8.6 ± 0.54) and T50 (appearance 8.8 ± 0.44, taste 8.8 ± 0.44, odour 9.00 ± 0, texture 8.8 ± 0.44) samples were acceptable to the panellists. There was a significant difference (p < 0.05) between samples with and without thyme oil (Table 3), while sT10T50ignificant differences were found between T10 and T50 only after the 75th day (p < 0.05). These results showed that samples with high thyme oil have acceptable overall scores, due to the limiting effect of thyme on microbiological activity and TVB-N value and also the positive attributes of thyme flavour. T50 samples were assessed as the most acceptable products by the panellists. 4. Conclusion The present experiment demonstrates that addition of thyme oil to vacuum-packed liquid-smoked rainbow trout fillets has a profound effect on sensory quality, TVB-N value and microbiological growth. T10 samples had a shorter shelf life than T50 samples, but both had longer shelf life than the control samples without thyme oil. Thyme oil can therefore be viewed as a natural preservative for liquid-smoked rainbow trout. References Alçiçek, Z. (2010). The comparative investigate of using hot smoking and liquid smoking techniques of fillets of rainbow trout (Oncorhynchus mykiss) during vacuum packed and chilled storage with different salting techniques. Ph. Doctorate thesis. Ankara, Turkey: Ankara University, Graduate School of Natural and Applied Sciences Department of Fisheries and Aquaculture. Alçiçek, Z., Zencir, Ö., Çelik Çakırog˘ulları, G., & Atar, H. H. (2010). The Effect of Liquid Smoking of Anchovy (Engraulis encrasicolus, L. 1758) Fillets on Sensory, Meat Yield, Polycyclic Aromatic Hydrocarbons (PAHs) Content and Chemical Changes. Journal of Aquatic Food Product Technology, 19(3), 264–273. AOAC (1995). Patricia Cunniff (Ed.), Official methods of analysis of the association of official analytical chemists (16th ed.). Virginia, USA. Arora, D. S., & Kaur, J. (1999). Antimicrobial activity of spices. International Journal of Antimicrobial Agents, 12, 257–262. Birkeland, S., Rora, B. M. A., Skara, T., & Bjerkeng, B. (2004). Effects of cold smoking procedures and raw material characteristics on product yield and quality parameters of cold smoked Atlantic Salmon (Salmo salar L.) fillets. Food Research International, 37, 273–286. Bugueno, G., Escriche, I., Martinez-Navarrete, N., Mar Camacho, M., & Chiralt, A. (2003). Influence of storage conditions on some physical and chemical properties of smoked salmon (Salmo salar) processed by vacuum impregnation techniques. Food Chemistry, 81, 85–90. Cardinal, M., Knockaert, C., Torrissen, O., Sigurgisladottir, S., Morkore, T., Thomassen, T., et al. (2001). Relation of smoking parameters to the yield, color and sensory quality of smoked Atlantic salmon (Salmo salar). Food Research International, 34, 537–550. Che-Man, Y. B., & Ramadas, J. (1998). Effected of packaging environment on quality changes of smoked spanish mackarel under refrigeration. Journal of Food Quality, 21, 167–174. Dimitriadou, D., Zotos, A., Petridis, D., & Taylor, A. K. D. (2008). Improvement in the production of smoked trout fillets steamed with liquid smoke. Food Science Techniques International, 14(1), 67–77. Dimitrijevic, S. I., Mihajlovski, R. K., Antonovic, D. G., Milanovic-Stevanovic, M. R., & Mijin, D. Z. (2007). A study of synergistic antilisterial effects of a sub-lethal dose of lactic acid and essential oils from Thymus vulgaris L., Rosmarinus officinalis L. and Origanum vulgare L. Food Chemistry, 104, 774–782. Dondero, M., Cisternas, F., & Carvajal, L. (2004). Changes in quality of vacuumpacked cold smoked salmon (Salmo salar) as a function of storage temperature. Food Chemistry, 87, 543–550. Gilliland, S. E., Busta, F. F., Brinda, J. J., & Campbell, J. E. (1976). Aerobic Plate Count. In M. L. Speck (Ed.), Compendium of methods for the microbiological examination of foods (pp. 107–131). Washington: American Public Health Association, Inc. Goulas, A. E., & Kontominas, M. G. (2005). Effect of salting and smoking method on the keeping quality of chub mackerel (Scomber japonicus): Biochemical and sensory attributes. Food Chemistry, 93, 511–520.
688
Z. Alçiçek / Food Chemistry 128 (2011) 683–688
Hattula, T., Elfving, K., Mroueh, U. M., & Luoma, T. (2001). Use of liquid smoke flavouring as an alternative to traditional flue gas smoking of rainbow trout fillets (Oncorhynchus mykiss). Lebensmittel-Wissenschaft und-Technologie, 34(8), 521–525. Kolsarıcı, N., & Özkaya, Ö. (1998). Effect of smoking methods on shelf-life of rainbow trout (Salmo gairdneri). Turkish Journal of Veterinary and Animal Sciences, 22, 273–284. Leroi, F., Joffraud, J. J., Chevalier, F., & Cardinal, M. (1998). Study of the microbial ecology of cold-smoked salmon during storage at 8 °C. International Journal of Food Microbiology, 39, 1–2. López-Caballero, M. E., Pérez-Mateos, M., Montero, P., & Borderías, A. J. (2000). Oyster preservation by high-pressure treatment. Journal of Food Protection, 63, 196–201. Martinez, O., Salmero’n, J., Guille’n, M. D., & Casas, C. (2007). Textural and physicochemical changes in salmon (Salmo salar) treated with commercial liquid smoke flavourings. Food Chemistry, 100, 498–503. Moret, S., & Conte, L. S. (2002). A rapid method for polycyclic aromatic hydrocarbon determination in vegetable oil. Journal of Separation Science, 25, 96–100. Muratore, G., Mazzaglia, A., Lanza, C. M., & Licciardello, F. (2007). Effect of process variables on the quality of swordfish fillets flavored with smoke condensate. Journal of Food Processing and Preservation., 31, 167–177. Özkan, G., Sag˘dıç, O., & Özcan, M. (2003). Note: Inhibition of pathogenic bacteria by essential oils at different concentrations. Food Technology International, 9(2). 0085-4.
Quattara, B., Sabato, S. F., & Lacroix, M. (2001). Combined effect of antimicrobial coating and gamma irradiation on shelf life extension of pre-cooked shrimp (Penaus spp.). International Journal of Food Microbiology, 68, 1–9. Rahman, S. (2009). Handbook of food preservation. New York: Marckel Dekler. pp. 299. Siskos, I., Zotos, A., Melidou, S., & Tsikritzi, R. (2007). The effect of liquid smoking of fillets of trout (Salmo gairdnerii) on sensory, microbiological and chemical changes during chilled storage. Food Chemistry, 101, 458–464. Siskos, I., Zotos, A., & Taylor, K. A. (2005). The effect of drying, pressure and processing time on the quality of liquid-smoked trout (Salmo gairdnerii) fillets. Journal of the Science of Food and Agriculture, 85, 2054–2060. Solomakos, N., Govaris, A., Koidis, P., & Botsoglou, N. (2008). The antimicrobial effect of thyme essential oil, nisin and their combination against Escherichia coli O157:H7 in minced beef during refrigerated storage. Meat Science, 80, 159–166. Suñen, E., Fernandez-Galian, B., & Aristimuno, C. (2001). Antibacterial activity of smokewood condensates against Aeromonas hydrophila, Yersinia enterocolitica and Listeriamonocytogenes at low temperature. Food Microbiology, 18, 387–393. Treadwell, F. P., & Hall, W. T. (1928). Analytical Chemistry (7th ed.). John Wiley and Sons Inc. Varlet, V., Serot, T., Knockaert, C., Cornet, J., Cardinal, M., Monteau, F., et al. (2007). Organoleptic characterization and PAH content of salmon (Salmo salar) fillets smoked according to four industrial smoking techniques. Journal of the Science of Food and Agriculture, 87, 847–854.