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Fat oxidation in freeze-dried grouper during storage at different temperatures and moisture contents
Food Chemistry 114 (2009) 1257–1264
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Fat oxidation in freeze-dried grouper during storage at different temperatures and moisture contents Mohammad Shafiur Rahman *, Ruqaya Moosa Al-Belushi, Nejib Guizani, Ghalib Said Al-Saidi, Bassam Soussi 1,2 Department of Food Science and Nutrition, Sultan Qaboos University, P.O. Box 34, Al Khod 123, Muscat, Oman
a r t i c l e
i n f o
Article history: Received 11 May 2008 Received in revised form 3 November 2008 Accepted 3 November 2008
Keywords: Moisture isotherm Glass transition Differential scanning calorimetry (DSC) Plasticisation State diagram
a b s t r a c t The effects of moisture content levels and storage temperature on the lipid oxidation in freeze-dried grouper were studied. Three trends were observed in the changes of PV (peroxide value) depending on the moisture content and storage temperature. An exponential rise of PV was observed (p < 0.05) at lower temperatures (20 °C and 5 °C for samples at moisture 5 g/100 g; and 20 °C for samples at moisture 20 g/100 g). At temperatures 25 °C for samples at moisture 5 g /100 g, and 5 and 25 °C for samples at 20 g moisture/100 g peaks of PV were observed (p < 0.05). At higher temperatures (40 °C for both moisture content levels) an exponential decay instead of increase were observed (p < 0.05). Moisture content levels had no significant effect (p > 0.05) on PV stored at temperature of 40 °C. The monolayer moisture for freeze-dried grouper was estimated as 6.2 g/100 g dry solids using BET-isotherm. Differential scanning calorimetry (DSC and modulated DSC) thermogram line showed two shifts (i.e. glass transition) and two endothermic peaks, one for melting of oil and another for decomposition. The validity of the water activity and glass transition concepts were evaluated. It is concluded that glass transition concept may be used to explain the process more adequately. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fish constitutes an important part in animal protein consumption in many parts of the world. In addition fish, in particular fatty fish, is attracting a great attention because of the positive role of fish lipids on human nutrition and health (Lie, 2001; Marchioli, 2001, 2002; Simopoulos, 1997). Globally, around 100 million tons of fish are landed annually but only 70 million tons are utilised as human food (Huss, Reilly, & Ben Embarek, 2000). Only around 27% of this amount is consumed as fresh fish while the remainder is processed using different food preservation techniques, e.g. freezing, salting, drying, smoking, and canning (Huss et al., 2000). The high degree of unsaturation of x-3 fatty acids in fish and their close proximity to strong pro-oxidative systems predispose them to oxidation (Nawar, 1996). Lipid oxidation causes major quality problem in foods, such as off-flavour development (i.e. rancidity) (Hamilton, 1994; Nawar, 1996). The primary product of lipid oxidation is the hydroperoxide, measured as peroxide value (PV). Peroxides are unstable compounds and they break down to
* Corresponding author. Tel.: +968 2414 1273; fax: +968 24413 418. E-mail addresses: shafi[email protected], shafi[email protected] (M.S. Rahman). 1 UNESCO Chair, CAMS, Sultan Qaboos University, Oman. 2 Wallenberg Laboratory, University of Gothenburg, Sweden. 0308-8146/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2008.11.002
aldehydes, ketones, and alcohols which are the volatile products causing off-flavour in products. The PV and thiobarbituric acid (TBA) values are the major chemical indices of oxidative rancidity (Melton, 1983; Rossell, 1989). TBA value measures secondary products of lipid oxidation. TBA consists mainly of malonaldehydes as a representative of aldehydes. In order to extend the shelf-life, lipid oxidation in fish could be retarded by adding natural antioxidants (Decker, 1998; Frankel, 1995; Kelleher, Silva, Hultin, & Wilhelm, 1992), using vacuum and controlled atmosphere packaging with nitrogen or carbon dioxide, and using oxygen absorbers (Chin, Lia, Storkson, Ha, & Pariza, 1992). In dried fish, it has been proven that the rate of the oxidation process is strongly influenced by the presence of water, inorganic, and organic substances (Labuza & Chou, 1974). It has been found in lipid model systems that around the monolayer value, water has a protective effect against oxidation (Labuza, 1971). The concept of water activity has been used to provide a reliable assessment of the microbial growth, lipid oxidation, non-enzymatic and enzymatic activities, and the texture of foods (Rahman & Labuza, 2007). A food product is most stable at its monolayer moisture content, which vary with the chemical composition and structure (Rahman, 2006). Recently the limitations of water activity are pointed and alternatives are proposed (Rahman, 2006; Rahman & Labuza, 2007). The main limitations of water activity are: it
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is defined at equilibrium, whereas dried foods may not be in a state of equilibrium; the critical limits of water activity may also be shifted to higher or lower levels by other factors, such as nature of the solutes. Water activity does not indicate the mobility or reactivity of water and the nature of binding to the substrate. Thus, glass transition concept was put forward considering the limitations of water activity. The rules of glass transition concept are: (i) the food is most stable at and below its glass transition and (ii) higher the TTg or T/Tg (i.e. above glass transition), higher the deterioration or reaction rates (Levine & Slade, 1986; Rahman, 2006; Slade & Levine, 1988). Glass transition of date flesh (Rahman, 2004), date pits (Rahman, Kasapis, Al-Kharousi, Al-Marhubi, & Khan, 2007), abalone (Sablani, Kasapis, Rahman, Al-Jabri, & Al-Habsi 2004), tuna (Rahman, Kasapis, Guizani, & Al-Amri, 2003), king fish (Sablani et al. 2007), and fish skin gelatin (Rahman, Al-Saidi, & Guizani, 2008) were measured and presented in the literature. Little information available in the literature related to the oxidation of fats in fish as a function of moisture and wide variation of storage temperature. Most existing reports describe oxidation of oils in model food systems (Adachi, Ishiguro, & Matsuno, 1995; Khayat & Schwall, 1983). In this report, the orange spotted grouper (Epinephelus coioides), locally known as hamour, is a fatty fish found in Oman and is very little studied. The main objective of this research was to evaluate the storage stability of freeze-dried grouper in terms of fat oxidation. Samples were stored at two levels of moisture content (5 and 20 g/100 g sample), and five levels of temperature (40, 20, 5, 25, and 40 °C). Chemical composition (water, protein, fat, and ash), peroxide value (PV), water sorption isotherm and thermal characteristics were measured.
2. Materials and methods 2.1. Source of material The fresh whole grouper fish was purchased from the local market. Mass of the fish was 10 kg. The fish flesh was filleted after removing scales, fins, bones, viscera, gills, and head. Fillets were then cut into cubes and stored properly in ice and brought to the laboratory. 2.2. Freeze-drying Cube samples were placed in plastic bags into one layer and frozen in a chest freezer at 40 °C for at least 48 h. The samples were taken out from freezer and placed in an automatically controlled freeze drier (VirTis SP Industries Company, New York, USA). Freeze-drier was set at (20 °C) with a vacuum of 800 m Torr (108 Pa) and condensing plate temperature at 65 °C. After completion of 72 h freeze-drying, the fish sample was ground using electronic grinder (600 W Japan, Mumbai, India). The ground samples were placed in an airtight plastic container and stored at 40 °C until used for compositional analysis and storage stability experiments. 2.3. Chemical composition analysis The chemical composition was determined according to the Association of Official Analytical Chemists method (AOAC, 1990). The moisture content was measured by using oven drying at 105 °C for 24 h. Furnace oven method was used to find the percentage of ash by burning the sample at 530 °C for 24 h. Fat percentage was determined using ether extraction method (AOAC, 1990). Amount of protein was determined using Kjeldahl method (AOAC, 1990). Finally, crude carbohydrate was estimated by dif-
ference. Five replicates were used for chemical composition analysis. 2.4. Measurement of peroxide value (PV) Storage stability of grouper fish was conducted at two moisture contents (i.e. 5 and 20 g/100 g sample). The fish samples of the two moisture contents were placed in air tight plastic containers and stored at 40, 20, 5, 25, and 40 °C. At different time intervals 0.5 g samples were taken out and PVs were measured using the procedure developed by Egan, Krik, and Sawyer (1981). Freeze-dried grouper was ground into a powder with a grinder and 0.5 g of the powder was mixed with 25 ml solution of acetic acid and chloroform (ratio 3:2) and 1 ml of saturated potassium iodide. The mixture was stored in the dark for about 10 min prior to the addition of 30 ml of distilled water and 1 ml of freshly prepared 1% starch (w/v) solution. After shaking, the sample was titrated with 0.01 N sodium thiosulfate until the blue colour disappeared. The peroxide values were expressed as milliequivalents of peroxide oxygen per kg of sample (mEq/ kg) as used by Egan et al. (1981). In first trial PV values were measured weekly for stored samples, however samples stored at 25 and 40 °C indicated very fast reaction rate. Thus second trail was conducted for 3 weeks at 25 and 40 °C where samples were taken every day. The PVs were recorded as a function of storage time. Three replicates were considered for PV determination. 2.5. Water activity measurement Isopiestic method was used to determine the water activity of freeze-dried grouper (Spiess & Wolf, 1987). Freeze-dried grouper samples were placed in open weighing bottles and stored in airsealed glass jars while maintaining equilibrium relative humidity with saturated salt solutions by maintaining a layer of salt crystals at the bottom. The salts were: LiCl, CH3COOK, MgCl2, K2CO3, NaBr, SrCl2, and KCl. Relative humidity values of those solutions were obtained from the compilation of Spiess and Wolf (1987). The jars were placed at constant temperature of 20 °C and weight was recorded every alternate day until dried grouper samples reached to a constant mass. The equilibration time took around three weeks. The moisture content of equilibrated sample was measured by oven drying method at 105 °C for 24 h. 2.6. Moisture isotherm models The BET and GAB models were used for fitting the experimental data of the sorption isotherms of freeze-dried grouper. The BET model is according Eq. (1):
Mw ¼
Mb C b aw ð1 aw Þ½1 þ ðC b 1Þaw
ð1Þ
where Mb is the BET monolayer moisture content (dry basis, g water/g sample) and Cb is a constant related to the net heat of sorption. The BET-isotherm holds well between water activities of 0.05 and 0.45, an adequate range for the calculation of parameters Mb and Cb (Labuza, 1968). The GAB model (Eq. (2)) is (Rahman & Labuza, 2007):
Mw ¼
M g C g Kaw ½ð1 Kaw Þð1 Kaw þ C g Kaw Þ
ð2Þ
where Mg is the GAB monolayer moisture content (dry basis), Cg is a constant related to the monolayer heat of sorption and K is a factor related to the heat of sorption of the multilayer. The GAB model (Eq. (2)), an extension of the BET model takes into account the modified properties of the sorbate in the multilayer region and the bulk li-
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quid water properties through the introduction of a third constant K (Rahman & Labuza, 2007). 2.7. Glass transition measurement Differential scanning calorimetry (DSC) was used to characterise the phase transitions. The DSC is equipped with a mechanical cooling system. The freeze-dried samples are reconstituted by adding appropriate amount of water to obtain desired level of moisture content. The samples are sealed hermetically in aluminium pans. The instrument is first calibrated for heat flow and temperature using distilled water (m.p. 0 °C; DHm = 334 J/g) and indium (m.p. 156.5 °C; DHm = 28.5 J/g). Freeze-dried hamour samples (5–10 mg) containing water of 5 and 20 g/100 g sample were placed in aluminium pans (capacity 30 lL) and were cooled to 90 °C at 5 °C/min, and equilibrated for 10 min. They were then scanned from 90 to 130 °C at a rate of 10 °C/min using an empty aluminium pan as reference. Thermograms were obtained for the samples and they were analysed for the onset, mid, and end of glass transition. Three to five replicates were used for each sample. A different procedure was used for samples containing high water (30, 40, and 60 g/100 g sample) having freezable water. Samples of 5–10 mg of the powder in a sealed aluminium pan were cooled to 90 °C at 5 °C/min, and equilibrated for 10 min. The sample then scanned from 90 °C at 10 °C/min to 180 °C in order to determine freezing point and apparent maximal-freeze condition ðT 0m and T 000 g Þ. The initial or equilibrium freezing point was considered as the point of maximum slope of the endothermic peak. For the materials showing wide peak of ice melting on the DSC thermogram, the point of maximum slope corresponds well with the initial freezing point estimated from well established cooling curve method (Rahman, 2004). The latent heat of ice melting (or freezing) was estimated from the area of the melting endotherm. The average values of three replicates were obtained. 2.8. Reaction kinetics model In the case of consecutive or parallel reactions occurring in a food matrix with intermediate products that then react further, the following scheme can be written as showed in Eq. (3): k1
k2
A!B!C
ð3Þ
where A, B, and C are the fat, peroxide and degradation products from peroxide, respectively; and k1 and k2 are the reaction rate constants (day1), respectively. The analytical solution (Eq. (4)) for the differential rate equations when external conditions are assumed constant is (Giannakourou & Taoukis, 2007):
½B ¼ ½Bo expðk2 tÞ þ k1 ½Ao
fexpðk1 tÞgfexpðk2 tÞg k2 k1
Table 1 Chemical composition of grouper (g/100 g sample). Component
Ssample (g/100 g)
Water Protein Fat Ash
75.98 ± 0.30 16.20 ± 0.49 5.87 ± 0.04 1.33 ± 0.06
Note: average values ± standard deviations (n = 5 replicates).
3. Results and discussion 3.1. Chemical composition Chemical compositions of grouper fillet are shown in Table 1. Water and fat contents are 75.98 and 5.87 g/100 g sample, respectively. The moisture content was similar to that reported by Luo et al (2005) who reported values of 73.4–76.6 g/100 g grouper muscle. In contrast, the lipid content of the grouper used in this experiment was relatively higher compared to the values found by Luo et al. (2005), who reported values of 1.63–2.61 g/100 g grouper muscle. These differences could be attributed to differences in species, growing conditions and size of the groupers used in both studies. The grouper used in this work was about 10 kg which is much bigger fish compared to the 3.3 kg grouper used by Luo et al. (2005). 3.2. Moisture sorption isotherm The experimental adsorption isotherm data were plotted and fitted with the BET and GAB models (Fig. 1). As expected, the equilibrium moisture content increased with the increasing water activity. The values of Cb and Mb were found as 2.48 and 6.23 g dry solids, respectively. The BET monolayer is an effective method for estimating the amount of bound water to specific polar sites in dehydrated food systems. Results of BET monolayer values were in the range previously reported for freeze-dried abalone 2.1–6.8 g/ 100 g dry solids (Sablani et al., 2004), tuna 4.8–6.0 g/100 g dry solids (Rahman, Sablani, Al-Ruzeiqi, & Guizani, 2002) and sardines 4.94 g/100 g dry solids (Sablani, Myhara, Mahgoub, Al-Attabi, & Al-Mugheiry, 2001). Based on water activity concept BET monolayer is usually considered as the most stable condition for preserving dried food (Rahman, 2006; Rahman & Al-Belushi, 2006). The value of Cb close to 1 indicated that the binding of water with the solid matrix is very weak. The GAB parameters Cg, K, and Mg were found as 3.56, 0.921, and 5.95 g/100 g solids, respectively. It was recommended to use
ð4Þ
The measured PV (i.e. [B]) as a function of time can be fitted with the above equation and the rate constant k1 and k2 can be estimated. 2.9. Statistical Analysis The significance test between each data point during storage period was conducted by Duncan’s test in SAS GLM procedure at p value 0.05 (SAS, 2001). The significant difference of PV change as a function of storage period was determined by linear regression model using MS-Excel at p value 0.05. The kinetics model parameters of Eq. (4) were estimated by SAS nonlinear procedure (SAS, 2001).
Fig. 1. Moisture sorption isotherm of freeze-dried grouper at 20 °C: experimental data, BET model, GAB model.
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BET monolayer for stability determination for number of reasons as discussed by Rahman and Al-Belushi (2006). GAB model indicates the mathematical representation of the experimental data, whereas BET represents more physical meaning and explanation of the physico-chemical process. 3.3. Fat oxidation during storage Lipid deterioration is the main cause of low shelf-life of fatty fish due to progress in oxidation and enzymatic hydrolysis of unsaturated fatty acids in fish (Srikar & Hiremath, 1972). In the present study peroxide value was used to measure the extent of lipid oxidation as a function of moisture level and temperature of storage. Changes in PV of the grouper samples containing moisture 5 and 20 g/100 g sample stored at various temperatures are shown in Tables 2 and 3 respectively. Samples containing moisture 5 and 20 g/100 g stored at 40 °C showed no significant change in the PV during storage up to 80 d (p > 0.05), whereas significant change was observed in PV for all other storage temperatures (20, 5, 25, and 40 °C) (p < 0.05). Samples containing moisture 5 g/100 g showed an exponential increase of PV during storage at 20 and 5 °C (initially a higher rate followed with a slower rate), whereas at 25 °C a peak was observed. However samples stored at 40 °C did not show any peak instead an exponential decay of PV was observed. In Tables 2 and 3 was observed an exponential decay of PV at 25 and 40 °C. This could be due to the fact that the peroxides were produced at high rate at higher temperatures during the initial stages of lipid oxidation and these were then rapidly decomposed into secondary compounds, or may have interacted with other constituents during storage. Samples containing moisture 20 g/100 g stored at 20 °C showed an exponential increase up to 30 d followed by a slow increase up to 60 d. Samples stored at 5 °C and 25 °C showed an initial peak and then an exponential decay, whereas samples stored at
40 °C did not show any peak instead an exponential decay. It was interesting to observe that higher moisture content samples (Table 3) showed peak at lower temperature (i.e. 5 °C) and higher temperature (i.e. 25 °C), whereas lower moisture content sample (Table 2) showed peak only at higher temperature of storage (i.e. 25 °C). This indicated that high moisture content speeds up the formation of peroxide. The effect of temperature and moisture level on lipid oxidation was evident in the present study suggesting that these are shelflife limiting factors. Several researchers reported similar results indicating that storage temperature and time lead to differences in lipid oxidation. Aubourg (1999) studied the lipid oxidation in fresh blue whiting fillets during storage at 30 and 10 °C up to 1 y. In the case of the fillets stored at 30 °C, PV increased at a very slow rate from 3.1 to 9.32 mEq/kg lipids within 1 y. In case of fillets stored at 10 °C, PV showed a peak after 5 months (3.1–13.97 mEq/ kg lipids) and then decreased to 6.47 mEq/kg lipids after 1 y. The decrease of PV from the peak could be explained as a result of decomposition of hydroperoxides into smaller molecules (Cho, Endo, Fujimoto, & Kaneda, 1989; Melton, 1983). Miki, Nishimoto, Nishimoto, and Shindo (1994) also observed a sharp peak in PV after 4 d storage of sardine stored at 5 °C. Hwang and Regenstein (1996) studied the lipid oxidation in fresh mackerel mince during storage at 0 and 40 °C with or without air (vacuum). At both temperatures PV remained constant at around 1 mEq/kg under vacuum and then increased to 7 mEq/kg with a higher rate indicating that lipid oxidation occurred continually even at storage temperature 40 °C when the samples were exposed to air. Smith (1995) studied initial lipid oxidation in dried salted fish by measuring oxygen uptake during storage at 25–40 °C in exposure of light or dark. The initial oxygen uptake showed that lipid oxidation increased with the increase of storage temperature up to 40 °C and surprisingly temperature above 40 °C did not significantly increase the rate of oxidation further. Cho et al. (1989) studied the lipid oxidation in salted-dried sardine during storage at 5 °C. They reported
Table 2 Peroxide value of samples (water content: 5 g/100 g sample) stored at different temperatures. Time (days)
0 7 14 21 28 35 42 49 56 63 69 76
PV (mEq/kg)
Time (days)
40 °C
20 °C
5 °C
14.8 ± 0.6 ab 18.0 ± 1.1a 16.2 ± 2.3ab 13.7 ± 0.9b 14.2 ± 1.6b 14.8 ± 1.9ab 15.1 ± 0.8ab 14.2 ± 0.9b 13.9 ± 0.4b 13.4 ± 1.9b 14.6 ± 2.1b 16.7 ± 1.5ab
14.8 ± 0.6e 17.4 ± 1.4de 21.1 ± 1.2dc 17.9 ± 2.1de 22.3 ± 4.3bc 23.2 ± 2.6bc 24.2 ± 3.0bc 26.0 ± 3.1b 24.4 ± 2.0bc 23.8 ± 2.5bc 23.4 ± 1.9bc 29.7 ± 3.0a
14.8 ± 0.6c 25.6 ± 1.1b 27.3 ± 1.2ab 26.0 ± 0.8ab 27.1 ± 3.0ab 32.0 ± 4.6a 28.0 ± 1.4ab 27.5 ± 2.9ab 31.3 ± 2.5ab 28.6 ± 8.2ab 29.3 ± 1.7ab 28.9 ± 1.4ab
Note: average values ± standard deviations (n = 3 replicates). Values with same letter in a column are not significantly different (p < 0.05).
0 1 2 3 4 5 6 7 8 9 10 12 14 16 17 19 20 21 22 23 28 35 42 49 56 63 69 76
PV (mEq/kg) 25 °C
40 °C
14.8 ± 0.6f 5.5 ± 0.3fe 17.5 ± 0.3d 20.8 ± 0.9c 27.3 ± 1.1a 24.2 ± 0.3b 19.9 ± 1.6c 6.3 ± 0.9de 5.7 ± 0.2fe 5.3 ± 0.3fe 6.5 ± 0.8de 13.4 ± 0.4g 13.3 ± 0.1g 13.3 ± 0.5g 5.2 ± 1.7fe 13.2 ± 0.1g 12.4 ± 0.8g 12.4 ± 0.8g 12.5 ± 0.9g 11.0 ± 0.2h 10.7 ± 0.7h 10.7 ± 0.6h 8.6 ± 1.0i 8.2 ± 0.8i 7.5 ± 0.5i 7.3 ± 0.4i 7.7 ± 1.1i 7.2 ± 0.9i
14.8 ± 0.6a 13.1 ± 0.2b 11.1 ± 0.7c 8.6 ± 0.4d 8.2 ± 0.4e 9.2 ± 1.3d 8.2 ± 0.9e 8.9 ± 0.0d 6.9 ± 0.32f 6.7 ± 0.0fg 6.7 ± 0.0fg 6.5 ± 0.3fg 6.2 ± 0.3g 4.4 ± 0.1h 4.4 ± 0.2h 2.2 ± 0.0i 2.2 ± 0.0i 2.2 ± 0.0i – – 2.2 ± 0.0i 2.2 ± 0.0i 2.2 ± 0.0i 0.00j (0.00)
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0 7 14 21 28 35 42 49 56 63 69 76
PV (mEq/kg)
Time (days)
40 °C
20 °C
5 °C
14.2 ± 0.75d 16.6 ± 0.9cab 15.5 ± 1.7cdb 18.2 ± 0.9a 15.6 ± 0.9cdb 16.3 ± 1.4cadb 15.0 ± 0.5cd 15.7 ± 1.3cdb 14.7 ± 0.4cd 15.4 ± 0.9cdb 15.1 ± 0.5cdb 17.3 ± 2.3ab
14.2 ± 0.8g 18.6 ± 1.1f 22.8 ± 1.7e 25.2 ± 0.6de 26.6 ± 2.6dc 26.2 ± 1.2dc 26.4 ± 1.2dc 28.0 ± 1.3dbc 29.0 ± 1.6bc 27.3 ± 2.3dbc 30.1 ± 1.5b 33.7 ± 2.0a
14.2 ± 0.8e 29.8 ± 4.0cb 36.2 ± 3.6a 30.8 ± 5.6ab 32.5 ± 2.4ab 23.1 ± 3.8d 24.5 ± 3.2cd 19.7 ± 2.6ed 19.0 ± 3.2ed 20.2 ± 2.5d 19.4 ± 1.2ed 20.8 ± 3.2d
0 1 2 3 4 5 6 7 8 9 10 12 14 16 17 19 20 21 22 23
PV (mEq/kg) 25 °C
40 °C
14.2 ± 0.8c 26.2 ± 3.0a 17.2 ± 0.7b 15.1 ± 0.6c 8.8d ± 0.2e 9.6 ± 1.4d 8.6 ± 0.4e 8.7 ± 0.1de 8.2 ± 0.4e 8.1 ± 0.1e 6.6 ± 0.1f 6.5 ± 0.1f 6.5 ± 0.12f 5.5 ± 1.1fg 4.3 ± 0.2hg 4.4 ± 0.1hg 4.4 ± 0.0hg 3.3 ± 1.3hi 2.2 ± 0.0i 2.2 ± 0.0i
14.2 ± 0.8a 12.1 ± 0.6b 6.0 ± 0.5c 4.3 ± 0.2d 4.4 ± 0.0d 4.3 ± 0.2d 2.2 ± 0.0e 2.2 ± 0.1e 2.5 ± 0.5e 0.0 ± 0.0f
Note: average values ± standard deviations (n = 3 replicates). Values with same letter in a column are not significantly different (p < 0.05).
an increase in PV with an increase of storage time and a peak within 4–10 d was observed. Ozogul, Ozyurt, Ozogul, Kuley, and Polat (2005) studied the fat oxidation in fresh eel by measuring PV and thiobarbituric acid (TBA) during storage in ice and 3 °C. In ice storage PV increased from 5.19 to 19.7 mEq/kg muscle after 15 d and then decreased to a value 4.06 mEq/kg thus showing a peak. Tanaka, Xueyi, Nagashima, and Taguchi (1991) also studied the change of PV in fresh sardine meat during storage at 5 °C and found that it
increased from around 1 to 15 mEq/kg after 4 d of storage. They also found that washing and high pressure treatment could reduce the oxidation significantly. 3.4. Glass transition temperature Fig. 2 shows typical DSC and MDSC thermograms for sample of moisture content 5 and 20 g/100 g sample. At moisture 5 g/100 g
Fig. 2. DSC and MDSC heat flow thermograms. (a) DSC thermogram for sample containing 5 g/100 g sample moisture, (b) MDSC thermogram of total heat flow for sample containing 20 g/100 g sample moisture, (c) MDSC thermogram of reversible heat flow for sample containing 20 g/100 g sample moisture and (d) MDSC thermogram of nonreversible total heat flow for sample containing 20 g/100 g sample moisture.
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sample, Fig. 2a shows a shift A in the thermogram line, small endothermic peak B, another thermogram shift C, and endothermic peak D indicating decomposition (or melting). A shift in thermogram line at A (around 20 °C) could be due to the initiation of mobility in protein and/or fat molecules as suggested by Sablani et al. (2007) for king fish muscle. The endothermic peak B was due to the melting of oil as also noticed by Sablani et al. (2007). Another shift C in the thermogram line before the decomposition can be considered as glass–rubber transition. This shift was difficult to identify by DSC for the equilibrated samples in different water activity (moisture content: 9.4–27.6 g/100 sample). Sablani et al. (2007) also found that increasing water had little influence on the identification of the glass transition temperature range, i.e. plasticisation effects of water on the protein was very small. It is difficult to find a clear glass transition with thermal analysis since other changes interfere or complexity of the structure affects identifying the glass transition. In a recent study, Rahman et al. (2007) found similar complexity in the case of date-pits. They also could not trace the glass transition even though oil was separated from the date-pits. Acevedo, Schebor, and Buera (2006) also reported the difficulty in detecting the glass transition of chicken muscle in their DSC measurements. In addition, Green, Fan, and Angell (1994) concluded from their results that hydrated proteins indeed may be grouped among glass-forming systems, but due to their special structural features and to the disposition of the bound water, they showed great departures from thermo or rheological simplicity. Similar difficulty was also observed in the case of native starch due to the nature of structure in molecular and micro levels, which caused small step change or broadening in the heat capacity change at the transition (Biliaderis, Page, Maurice, & Juliano, 1986; Zeleznak & Hoseney, 1987). As mentioned earlier that the glass transition of sample containing moisture 20 g/100 g sample could not be traced by DSC, thus a more sensitive modulated DSC (MDSC) was used. Total heat flow thermogram shown in Fig. 2b indicates two base line shifts at
Fig. 3. DSC thermogram for sample containing freezable water (moisture content 30 g/100 g sample).
locations A and B. The second shift at B in the thermogram before the decomposition endotherm could be considered as the glass transition. However, the endothermic peak for fat could not be traced. Fig. 2c and d shows the reversible and non-reversible heat flow thermograms. The reversible heat flow could separate the specific heat change in the material (i.e. glass transition), whereas non-reversible heat flow indicates the kinetic change (i.e. melting or decomposition). The structural change indicated at the point A in Fig. 2c need to be studied further. Fig. 3 shows typical thermogram for the sample containing freezable water showing melting of ice at A and solids decomposition at D. The freezing point and maximal-freeze concentration conditions are shown in Table 4. The lowest measured values of T 0m and T 000 g are 21.9 and 26.6 °C, respectively. Similarly Sablani et al. (2007) observed T 0m and T 000 g as 18.4 and 21.4 °C for king fish muscle. The solids content at T 0m and T 000 g is 70 g/100 sample, indicating X 0s = 0.70 and un-freezable water equal to 30 g/100 g sample. The reactivity of the un-freezable water could be varied with the type and characteristics of solid matrix. Thus both samples containing 5 and 20 g/100 g did not contain any freezable water, and similar stability could be expected. 3.5. Storage stability based on the water activity and glass transition It would be interesting to check the water activity and glass transition concepts for determining the stability of the fish muscle in terms of fat oxidation observed by PV. Samples at 5 g/100 g moisture content were close to the BET monolayer value of 6.23 g/100 g dry solids (i.e. 5.9 g/100 g sample), thus these should be most stable. However, the results showed that these samples were stable only at 40 °C and higher temperature caused the formation of peroxide indicating molecular mobility increased at higher temperature although the sample remained at monolayer moisture. The PV data were fitted to the kinetic model Eq. (4) and the rate constants k1 and k2 were estimated (Table 5). In order to identify the validity of the water activity and glass transition concepts, reaction rate constant k2 are plotted as a function of T/ Tg and Xw/Xb (Figs. 4 and 5). If water activity concept is valid, there should be a change in slope for k2 when Xw/Xb is around 1.0. Similarly there should be change in slope at Tc/Tg (Tc is critical temperature, T in K) around 1, if the glass transition concept is valid. Fig. 4 shows a break in slope at Tc/Tg = 0.78 in the case of moisture content 5 g/100 g sample, whereas a break at 0.96 for moisture content 20 g/100 g sample. This indicated the validity of the glass transition concept to some extent although the break in slope was not observed exactly at 1.0 and moisture content affected the critical limit of Tc/Tg. The shift of the critical limit to higher level indicated that water may have a protective effect. The rate constant is plotted as function of Xw/Xb for different temperatures in Fig. 5. This plot showed that there are no breaks in slopes for temperatures 40, 20, 5, and 25 °C and moisture did not show any effect until Xw/Xb = 3.5, which was the maximum moisture content tested in this work. At 40 °C the rate constant increased when Xw/Xb was around 1.0 (Fig. 5). Further data (Xw/
Table 4 Glass transition temperature, freezing point and maximal-freeze concentration conditions as a function of water contents. Xw
Tgi (°C)
Tgp
Tge
T 0m
T 000 g
TFC (°C)
TFB (°C)
TFA (°C)
DH (kJ/kg)
0.05 0.20* 0.30 0.40 0.60
95 ± 4 31 ± 3 – – –
101 ± 3 33 ± 4 – – –
105 ± 5 35 ± 4 – – –
– – 21.9 ± 0.5 21.4 ± 1.7 12.4 ± 2.0
– – 26.6 ± 0.5 26.3 ± 2.5 18.9 ± 2.9
– – 21.8 ± 0.4 20.6 ± 2.4 5.4 ± 1.4
– – 17.5 ± 0.2 15.8 ± 2.7 1.8 ± 1.2
– – 12.9 ± 1.5 10.8 ± 2.9 2.9 ± 2.4
– – 8.1 14.3 78.9
Average values ± standard deviations (n = 3–5 replicates). MDSC.
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M.S. Rahman et al. / Food Chemistry 114 (2009) 1257–1264 Table 5 Chemical reactivity for PV as a function of temperature and moisture content. Xw
0.05
0.20
T (°C)
40 20 5 25 40 40 20 5 25 40
Rate constant k1 (day1)
k2 (day1)
0.0011 0.0208 0.0295 0.0317 0.0350 0.0110 0.0251 0.0279 0.0151 0.1217
0.0024 0.0203 0.0308 0.0731 0.1152 0.0020 0.0260 0.0267 0.0579 0.3488
1263
ple) and five storage temperatures (40, 20, 5, 25, and 40 °C), respectively. Water and fat contents of fresh grouper were found 75.98 and 5.87 g/100 g sample, respectively. At both moisture levels no significant changes (p > 0.05) in PV was observed when samples were stored at 40 °C. In general three trends were observed in the formation of PV depending on the moisture content and storage temperature. In this study, the validity of the water activity and glass transition concepts were evaluated. It is concluded that glass transition concepts could be used to explain the process more adequately than the water activity concept since glass transition indicates breaks in the slopes of rate constant. However, this does not mean that there is no effect of water activity in the fat oxidation. A more combined approach needs to be developed in the future. Acknowledgments This work was supported by a grant from His Majesty Sultan Qaboos Research Funds (Project number SR/AGR/FOOD/05/01), the Swedish Research Council and the Sahlgrenska Academy at the University of Gothenburg. References
Fig. 4. Rate constant (k2) as a function of T/Tg for sample containing 5 and 20 g/ 100 g sample.
Fig. 5. Rate constant (k2) as a function of Xw/Xb for different temperatures of storage (T1: 40 °C, T2: 25 °C, T3: 5 °C, T4: 20 °C, T5: 40 °C).
Xb < 1.0 and Xw/Xb > 3.5) needs to be generated for varied moisture content and temperature in order to conclusively support this observation. However, it is clear from the observation that the effect of moisture content on reaction depended on the temperature. Thus both the concepts may be necessary to explain the reaction rates. Future works need to be targeted to combine both concepts in order to develop more generic approach to explain the reactions in biological systems. 4. Conclusion Lipid oxidation in freeze-dried grouper was studied by measuring its PV considering two moisture levels (5 and 20 g/100 g sam-
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Luo, Z., Liu, Y., Mai, K., Tian, L., Yang, H., Tan, X., et al. (2005). Dietary L-methionine requirement of juvenile grouper Epinephelus coioides at a constant dietary cystine level. Aquaculture, 249, 409–418. Marchioli, R. (2001). Efficacy of n-3 polyunsaturated fatty acids after myocardial infarction: Results of GISSI-prevenzione Trial. Lipids, 36, S119–S126. Marchioli, R. (2002). Early protection against sudden death by n-3 polyunsaturated fatty acids after myocardial infarction: Time-course analysis of the results of GISSI-prevenzione. Circulation, 105(23), 1897–1903. Melton, S. (1983). Methodology for following lipid oxidation in muscle foods. Food Technology, 37(7), 105–116. Miki, H., Nishimoto, J., Nishimoto, M., & Shindo, J. (1994). Oxidation rate of lipid in fish muscles during storage at low temperature. Nippon Suisan Gakkaishi, 60(4), 509–513. Nawar, W. W. (1996). Lipids. In O. R. Fennema (Ed.), Food chemistry (pp. 226–314). New York: Marcel Dekker. Ozogul, Y., Ozyurt, G., Ozogul, F., Kuley, E., & Polat, A. (2005). Freshness assessment of European eel (Anguilla anguilla) by sensory, chemical and microbiological methods. Food Chemistry, 92, 745–751. Rahman, M. S. (2004). State diagram of date flesh using differential scanning calorimetry (DSC). International Journal of Food Properties, 7(3), 407–428. Rahman, M. S. (2006). State diagram of foods: Its potential use in food processing and product stability. Trends in Food Science and Technology, 17, 129–141. Rahman, M. S., & Al-Belushi, R. H. (2006). Dynamic isopiestic method (DIM): Measuring moisture sorption isotherm of freeze-dried garlic powder and other potential uses of DIM. International Journal of Food Properties, 9(3), 421–437. Rahman, M. S., Al-Saidi, G. S., & Guizani, N. (2008). Thermal characterisation of gelatin extracted from yellowfin tuna skin and commercial mammalian gelatin. Food Chemistry, 108, 472–481. Rahman, M. S., Kasapis, S., Al-Kharousi, N. S. Z., Al-Marhubi, I. M., & Khan, A. J. (2007). Composition characterisation and thermal transition of date pits powder. Journal of Food Engineering, 8(1), 1–10. Rahman, M. S., Kasapis, S., Guizani, N., & Al-Amri, O. S. (2003). State diagram of tuna meat: Freezing curve and glass transition. Journal of Food Engineering, 57(4), 321–326.
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Fat oxidation in freeze-dried grouper during storage at different temperatures and moisture contents Mohammad Shafiur Rahman *, Ruqaya Moosa Al-Belushi, Nejib Guizani, Ghalib Said Al-Saidi, Bassam Soussi 1,2 Department of Food Science and Nutrition, Sultan Qaboos University, P.O. Box 34, Al Khod 123, Muscat, Oman
a r t i c l e
i n f o
Article history: Received 11 May 2008 Received in revised form 3 November 2008 Accepted 3 November 2008
Keywords: Moisture isotherm Glass transition Differential scanning calorimetry (DSC) Plasticisation State diagram
a b s t r a c t The effects of moisture content levels and storage temperature on the lipid oxidation in freeze-dried grouper were studied. Three trends were observed in the changes of PV (peroxide value) depending on the moisture content and storage temperature. An exponential rise of PV was observed (p < 0.05) at lower temperatures (20 °C and 5 °C for samples at moisture 5 g/100 g; and 20 °C for samples at moisture 20 g/100 g). At temperatures 25 °C for samples at moisture 5 g /100 g, and 5 and 25 °C for samples at 20 g moisture/100 g peaks of PV were observed (p < 0.05). At higher temperatures (40 °C for both moisture content levels) an exponential decay instead of increase were observed (p < 0.05). Moisture content levels had no significant effect (p > 0.05) on PV stored at temperature of 40 °C. The monolayer moisture for freeze-dried grouper was estimated as 6.2 g/100 g dry solids using BET-isotherm. Differential scanning calorimetry (DSC and modulated DSC) thermogram line showed two shifts (i.e. glass transition) and two endothermic peaks, one for melting of oil and another for decomposition. The validity of the water activity and glass transition concepts were evaluated. It is concluded that glass transition concept may be used to explain the process more adequately. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fish constitutes an important part in animal protein consumption in many parts of the world. In addition fish, in particular fatty fish, is attracting a great attention because of the positive role of fish lipids on human nutrition and health (Lie, 2001; Marchioli, 2001, 2002; Simopoulos, 1997). Globally, around 100 million tons of fish are landed annually but only 70 million tons are utilised as human food (Huss, Reilly, & Ben Embarek, 2000). Only around 27% of this amount is consumed as fresh fish while the remainder is processed using different food preservation techniques, e.g. freezing, salting, drying, smoking, and canning (Huss et al., 2000). The high degree of unsaturation of x-3 fatty acids in fish and their close proximity to strong pro-oxidative systems predispose them to oxidation (Nawar, 1996). Lipid oxidation causes major quality problem in foods, such as off-flavour development (i.e. rancidity) (Hamilton, 1994; Nawar, 1996). The primary product of lipid oxidation is the hydroperoxide, measured as peroxide value (PV). Peroxides are unstable compounds and they break down to
* Corresponding author. Tel.: +968 2414 1273; fax: +968 24413 418. E-mail addresses: shafi[email protected], shafi[email protected] (M.S. Rahman). 1 UNESCO Chair, CAMS, Sultan Qaboos University, Oman. 2 Wallenberg Laboratory, University of Gothenburg, Sweden. 0308-8146/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2008.11.002
aldehydes, ketones, and alcohols which are the volatile products causing off-flavour in products. The PV and thiobarbituric acid (TBA) values are the major chemical indices of oxidative rancidity (Melton, 1983; Rossell, 1989). TBA value measures secondary products of lipid oxidation. TBA consists mainly of malonaldehydes as a representative of aldehydes. In order to extend the shelf-life, lipid oxidation in fish could be retarded by adding natural antioxidants (Decker, 1998; Frankel, 1995; Kelleher, Silva, Hultin, & Wilhelm, 1992), using vacuum and controlled atmosphere packaging with nitrogen or carbon dioxide, and using oxygen absorbers (Chin, Lia, Storkson, Ha, & Pariza, 1992). In dried fish, it has been proven that the rate of the oxidation process is strongly influenced by the presence of water, inorganic, and organic substances (Labuza & Chou, 1974). It has been found in lipid model systems that around the monolayer value, water has a protective effect against oxidation (Labuza, 1971). The concept of water activity has been used to provide a reliable assessment of the microbial growth, lipid oxidation, non-enzymatic and enzymatic activities, and the texture of foods (Rahman & Labuza, 2007). A food product is most stable at its monolayer moisture content, which vary with the chemical composition and structure (Rahman, 2006). Recently the limitations of water activity are pointed and alternatives are proposed (Rahman, 2006; Rahman & Labuza, 2007). The main limitations of water activity are: it
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is defined at equilibrium, whereas dried foods may not be in a state of equilibrium; the critical limits of water activity may also be shifted to higher or lower levels by other factors, such as nature of the solutes. Water activity does not indicate the mobility or reactivity of water and the nature of binding to the substrate. Thus, glass transition concept was put forward considering the limitations of water activity. The rules of glass transition concept are: (i) the food is most stable at and below its glass transition and (ii) higher the TTg or T/Tg (i.e. above glass transition), higher the deterioration or reaction rates (Levine & Slade, 1986; Rahman, 2006; Slade & Levine, 1988). Glass transition of date flesh (Rahman, 2004), date pits (Rahman, Kasapis, Al-Kharousi, Al-Marhubi, & Khan, 2007), abalone (Sablani, Kasapis, Rahman, Al-Jabri, & Al-Habsi 2004), tuna (Rahman, Kasapis, Guizani, & Al-Amri, 2003), king fish (Sablani et al. 2007), and fish skin gelatin (Rahman, Al-Saidi, & Guizani, 2008) were measured and presented in the literature. Little information available in the literature related to the oxidation of fats in fish as a function of moisture and wide variation of storage temperature. Most existing reports describe oxidation of oils in model food systems (Adachi, Ishiguro, & Matsuno, 1995; Khayat & Schwall, 1983). In this report, the orange spotted grouper (Epinephelus coioides), locally known as hamour, is a fatty fish found in Oman and is very little studied. The main objective of this research was to evaluate the storage stability of freeze-dried grouper in terms of fat oxidation. Samples were stored at two levels of moisture content (5 and 20 g/100 g sample), and five levels of temperature (40, 20, 5, 25, and 40 °C). Chemical composition (water, protein, fat, and ash), peroxide value (PV), water sorption isotherm and thermal characteristics were measured.
2. Materials and methods 2.1. Source of material The fresh whole grouper fish was purchased from the local market. Mass of the fish was 10 kg. The fish flesh was filleted after removing scales, fins, bones, viscera, gills, and head. Fillets were then cut into cubes and stored properly in ice and brought to the laboratory. 2.2. Freeze-drying Cube samples were placed in plastic bags into one layer and frozen in a chest freezer at 40 °C for at least 48 h. The samples were taken out from freezer and placed in an automatically controlled freeze drier (VirTis SP Industries Company, New York, USA). Freeze-drier was set at (20 °C) with a vacuum of 800 m Torr (108 Pa) and condensing plate temperature at 65 °C. After completion of 72 h freeze-drying, the fish sample was ground using electronic grinder (600 W Japan, Mumbai, India). The ground samples were placed in an airtight plastic container and stored at 40 °C until used for compositional analysis and storage stability experiments. 2.3. Chemical composition analysis The chemical composition was determined according to the Association of Official Analytical Chemists method (AOAC, 1990). The moisture content was measured by using oven drying at 105 °C for 24 h. Furnace oven method was used to find the percentage of ash by burning the sample at 530 °C for 24 h. Fat percentage was determined using ether extraction method (AOAC, 1990). Amount of protein was determined using Kjeldahl method (AOAC, 1990). Finally, crude carbohydrate was estimated by dif-
ference. Five replicates were used for chemical composition analysis. 2.4. Measurement of peroxide value (PV) Storage stability of grouper fish was conducted at two moisture contents (i.e. 5 and 20 g/100 g sample). The fish samples of the two moisture contents were placed in air tight plastic containers and stored at 40, 20, 5, 25, and 40 °C. At different time intervals 0.5 g samples were taken out and PVs were measured using the procedure developed by Egan, Krik, and Sawyer (1981). Freeze-dried grouper was ground into a powder with a grinder and 0.5 g of the powder was mixed with 25 ml solution of acetic acid and chloroform (ratio 3:2) and 1 ml of saturated potassium iodide. The mixture was stored in the dark for about 10 min prior to the addition of 30 ml of distilled water and 1 ml of freshly prepared 1% starch (w/v) solution. After shaking, the sample was titrated with 0.01 N sodium thiosulfate until the blue colour disappeared. The peroxide values were expressed as milliequivalents of peroxide oxygen per kg of sample (mEq/ kg) as used by Egan et al. (1981). In first trial PV values were measured weekly for stored samples, however samples stored at 25 and 40 °C indicated very fast reaction rate. Thus second trail was conducted for 3 weeks at 25 and 40 °C where samples were taken every day. The PVs were recorded as a function of storage time. Three replicates were considered for PV determination. 2.5. Water activity measurement Isopiestic method was used to determine the water activity of freeze-dried grouper (Spiess & Wolf, 1987). Freeze-dried grouper samples were placed in open weighing bottles and stored in airsealed glass jars while maintaining equilibrium relative humidity with saturated salt solutions by maintaining a layer of salt crystals at the bottom. The salts were: LiCl, CH3COOK, MgCl2, K2CO3, NaBr, SrCl2, and KCl. Relative humidity values of those solutions were obtained from the compilation of Spiess and Wolf (1987). The jars were placed at constant temperature of 20 °C and weight was recorded every alternate day until dried grouper samples reached to a constant mass. The equilibration time took around three weeks. The moisture content of equilibrated sample was measured by oven drying method at 105 °C for 24 h. 2.6. Moisture isotherm models The BET and GAB models were used for fitting the experimental data of the sorption isotherms of freeze-dried grouper. The BET model is according Eq. (1):
Mw ¼
Mb C b aw ð1 aw Þ½1 þ ðC b 1Þaw
ð1Þ
where Mb is the BET monolayer moisture content (dry basis, g water/g sample) and Cb is a constant related to the net heat of sorption. The BET-isotherm holds well between water activities of 0.05 and 0.45, an adequate range for the calculation of parameters Mb and Cb (Labuza, 1968). The GAB model (Eq. (2)) is (Rahman & Labuza, 2007):
Mw ¼
M g C g Kaw ½ð1 Kaw Þð1 Kaw þ C g Kaw Þ
ð2Þ
where Mg is the GAB monolayer moisture content (dry basis), Cg is a constant related to the monolayer heat of sorption and K is a factor related to the heat of sorption of the multilayer. The GAB model (Eq. (2)), an extension of the BET model takes into account the modified properties of the sorbate in the multilayer region and the bulk li-
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quid water properties through the introduction of a third constant K (Rahman & Labuza, 2007). 2.7. Glass transition measurement Differential scanning calorimetry (DSC) was used to characterise the phase transitions. The DSC is equipped with a mechanical cooling system. The freeze-dried samples are reconstituted by adding appropriate amount of water to obtain desired level of moisture content. The samples are sealed hermetically in aluminium pans. The instrument is first calibrated for heat flow and temperature using distilled water (m.p. 0 °C; DHm = 334 J/g) and indium (m.p. 156.5 °C; DHm = 28.5 J/g). Freeze-dried hamour samples (5–10 mg) containing water of 5 and 20 g/100 g sample were placed in aluminium pans (capacity 30 lL) and were cooled to 90 °C at 5 °C/min, and equilibrated for 10 min. They were then scanned from 90 to 130 °C at a rate of 10 °C/min using an empty aluminium pan as reference. Thermograms were obtained for the samples and they were analysed for the onset, mid, and end of glass transition. Three to five replicates were used for each sample. A different procedure was used for samples containing high water (30, 40, and 60 g/100 g sample) having freezable water. Samples of 5–10 mg of the powder in a sealed aluminium pan were cooled to 90 °C at 5 °C/min, and equilibrated for 10 min. The sample then scanned from 90 °C at 10 °C/min to 180 °C in order to determine freezing point and apparent maximal-freeze condition ðT 0m and T 000 g Þ. The initial or equilibrium freezing point was considered as the point of maximum slope of the endothermic peak. For the materials showing wide peak of ice melting on the DSC thermogram, the point of maximum slope corresponds well with the initial freezing point estimated from well established cooling curve method (Rahman, 2004). The latent heat of ice melting (or freezing) was estimated from the area of the melting endotherm. The average values of three replicates were obtained. 2.8. Reaction kinetics model In the case of consecutive or parallel reactions occurring in a food matrix with intermediate products that then react further, the following scheme can be written as showed in Eq. (3): k1
k2
A!B!C
ð3Þ
where A, B, and C are the fat, peroxide and degradation products from peroxide, respectively; and k1 and k2 are the reaction rate constants (day1), respectively. The analytical solution (Eq. (4)) for the differential rate equations when external conditions are assumed constant is (Giannakourou & Taoukis, 2007):
½B ¼ ½Bo expðk2 tÞ þ k1 ½Ao
fexpðk1 tÞgfexpðk2 tÞg k2 k1
Table 1 Chemical composition of grouper (g/100 g sample). Component
Ssample (g/100 g)
Water Protein Fat Ash
75.98 ± 0.30 16.20 ± 0.49 5.87 ± 0.04 1.33 ± 0.06
Note: average values ± standard deviations (n = 5 replicates).
3. Results and discussion 3.1. Chemical composition Chemical compositions of grouper fillet are shown in Table 1. Water and fat contents are 75.98 and 5.87 g/100 g sample, respectively. The moisture content was similar to that reported by Luo et al (2005) who reported values of 73.4–76.6 g/100 g grouper muscle. In contrast, the lipid content of the grouper used in this experiment was relatively higher compared to the values found by Luo et al. (2005), who reported values of 1.63–2.61 g/100 g grouper muscle. These differences could be attributed to differences in species, growing conditions and size of the groupers used in both studies. The grouper used in this work was about 10 kg which is much bigger fish compared to the 3.3 kg grouper used by Luo et al. (2005). 3.2. Moisture sorption isotherm The experimental adsorption isotherm data were plotted and fitted with the BET and GAB models (Fig. 1). As expected, the equilibrium moisture content increased with the increasing water activity. The values of Cb and Mb were found as 2.48 and 6.23 g dry solids, respectively. The BET monolayer is an effective method for estimating the amount of bound water to specific polar sites in dehydrated food systems. Results of BET monolayer values were in the range previously reported for freeze-dried abalone 2.1–6.8 g/ 100 g dry solids (Sablani et al., 2004), tuna 4.8–6.0 g/100 g dry solids (Rahman, Sablani, Al-Ruzeiqi, & Guizani, 2002) and sardines 4.94 g/100 g dry solids (Sablani, Myhara, Mahgoub, Al-Attabi, & Al-Mugheiry, 2001). Based on water activity concept BET monolayer is usually considered as the most stable condition for preserving dried food (Rahman, 2006; Rahman & Al-Belushi, 2006). The value of Cb close to 1 indicated that the binding of water with the solid matrix is very weak. The GAB parameters Cg, K, and Mg were found as 3.56, 0.921, and 5.95 g/100 g solids, respectively. It was recommended to use
ð4Þ
The measured PV (i.e. [B]) as a function of time can be fitted with the above equation and the rate constant k1 and k2 can be estimated. 2.9. Statistical Analysis The significance test between each data point during storage period was conducted by Duncan’s test in SAS GLM procedure at p value 0.05 (SAS, 2001). The significant difference of PV change as a function of storage period was determined by linear regression model using MS-Excel at p value 0.05. The kinetics model parameters of Eq. (4) were estimated by SAS nonlinear procedure (SAS, 2001).
Fig. 1. Moisture sorption isotherm of freeze-dried grouper at 20 °C: experimental data, BET model, GAB model.
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BET monolayer for stability determination for number of reasons as discussed by Rahman and Al-Belushi (2006). GAB model indicates the mathematical representation of the experimental data, whereas BET represents more physical meaning and explanation of the physico-chemical process. 3.3. Fat oxidation during storage Lipid deterioration is the main cause of low shelf-life of fatty fish due to progress in oxidation and enzymatic hydrolysis of unsaturated fatty acids in fish (Srikar & Hiremath, 1972). In the present study peroxide value was used to measure the extent of lipid oxidation as a function of moisture level and temperature of storage. Changes in PV of the grouper samples containing moisture 5 and 20 g/100 g sample stored at various temperatures are shown in Tables 2 and 3 respectively. Samples containing moisture 5 and 20 g/100 g stored at 40 °C showed no significant change in the PV during storage up to 80 d (p > 0.05), whereas significant change was observed in PV for all other storage temperatures (20, 5, 25, and 40 °C) (p < 0.05). Samples containing moisture 5 g/100 g showed an exponential increase of PV during storage at 20 and 5 °C (initially a higher rate followed with a slower rate), whereas at 25 °C a peak was observed. However samples stored at 40 °C did not show any peak instead an exponential decay of PV was observed. In Tables 2 and 3 was observed an exponential decay of PV at 25 and 40 °C. This could be due to the fact that the peroxides were produced at high rate at higher temperatures during the initial stages of lipid oxidation and these were then rapidly decomposed into secondary compounds, or may have interacted with other constituents during storage. Samples containing moisture 20 g/100 g stored at 20 °C showed an exponential increase up to 30 d followed by a slow increase up to 60 d. Samples stored at 5 °C and 25 °C showed an initial peak and then an exponential decay, whereas samples stored at
40 °C did not show any peak instead an exponential decay. It was interesting to observe that higher moisture content samples (Table 3) showed peak at lower temperature (i.e. 5 °C) and higher temperature (i.e. 25 °C), whereas lower moisture content sample (Table 2) showed peak only at higher temperature of storage (i.e. 25 °C). This indicated that high moisture content speeds up the formation of peroxide. The effect of temperature and moisture level on lipid oxidation was evident in the present study suggesting that these are shelflife limiting factors. Several researchers reported similar results indicating that storage temperature and time lead to differences in lipid oxidation. Aubourg (1999) studied the lipid oxidation in fresh blue whiting fillets during storage at 30 and 10 °C up to 1 y. In the case of the fillets stored at 30 °C, PV increased at a very slow rate from 3.1 to 9.32 mEq/kg lipids within 1 y. In case of fillets stored at 10 °C, PV showed a peak after 5 months (3.1–13.97 mEq/ kg lipids) and then decreased to 6.47 mEq/kg lipids after 1 y. The decrease of PV from the peak could be explained as a result of decomposition of hydroperoxides into smaller molecules (Cho, Endo, Fujimoto, & Kaneda, 1989; Melton, 1983). Miki, Nishimoto, Nishimoto, and Shindo (1994) also observed a sharp peak in PV after 4 d storage of sardine stored at 5 °C. Hwang and Regenstein (1996) studied the lipid oxidation in fresh mackerel mince during storage at 0 and 40 °C with or without air (vacuum). At both temperatures PV remained constant at around 1 mEq/kg under vacuum and then increased to 7 mEq/kg with a higher rate indicating that lipid oxidation occurred continually even at storage temperature 40 °C when the samples were exposed to air. Smith (1995) studied initial lipid oxidation in dried salted fish by measuring oxygen uptake during storage at 25–40 °C in exposure of light or dark. The initial oxygen uptake showed that lipid oxidation increased with the increase of storage temperature up to 40 °C and surprisingly temperature above 40 °C did not significantly increase the rate of oxidation further. Cho et al. (1989) studied the lipid oxidation in salted-dried sardine during storage at 5 °C. They reported
Table 2 Peroxide value of samples (water content: 5 g/100 g sample) stored at different temperatures. Time (days)
0 7 14 21 28 35 42 49 56 63 69 76
PV (mEq/kg)
Time (days)
40 °C
20 °C
5 °C
14.8 ± 0.6 ab 18.0 ± 1.1a 16.2 ± 2.3ab 13.7 ± 0.9b 14.2 ± 1.6b 14.8 ± 1.9ab 15.1 ± 0.8ab 14.2 ± 0.9b 13.9 ± 0.4b 13.4 ± 1.9b 14.6 ± 2.1b 16.7 ± 1.5ab
14.8 ± 0.6e 17.4 ± 1.4de 21.1 ± 1.2dc 17.9 ± 2.1de 22.3 ± 4.3bc 23.2 ± 2.6bc 24.2 ± 3.0bc 26.0 ± 3.1b 24.4 ± 2.0bc 23.8 ± 2.5bc 23.4 ± 1.9bc 29.7 ± 3.0a
14.8 ± 0.6c 25.6 ± 1.1b 27.3 ± 1.2ab 26.0 ± 0.8ab 27.1 ± 3.0ab 32.0 ± 4.6a 28.0 ± 1.4ab 27.5 ± 2.9ab 31.3 ± 2.5ab 28.6 ± 8.2ab 29.3 ± 1.7ab 28.9 ± 1.4ab
Note: average values ± standard deviations (n = 3 replicates). Values with same letter in a column are not significantly different (p < 0.05).
0 1 2 3 4 5 6 7 8 9 10 12 14 16 17 19 20 21 22 23 28 35 42 49 56 63 69 76
PV (mEq/kg) 25 °C
40 °C
14.8 ± 0.6f 5.5 ± 0.3fe 17.5 ± 0.3d 20.8 ± 0.9c 27.3 ± 1.1a 24.2 ± 0.3b 19.9 ± 1.6c 6.3 ± 0.9de 5.7 ± 0.2fe 5.3 ± 0.3fe 6.5 ± 0.8de 13.4 ± 0.4g 13.3 ± 0.1g 13.3 ± 0.5g 5.2 ± 1.7fe 13.2 ± 0.1g 12.4 ± 0.8g 12.4 ± 0.8g 12.5 ± 0.9g 11.0 ± 0.2h 10.7 ± 0.7h 10.7 ± 0.6h 8.6 ± 1.0i 8.2 ± 0.8i 7.5 ± 0.5i 7.3 ± 0.4i 7.7 ± 1.1i 7.2 ± 0.9i
14.8 ± 0.6a 13.1 ± 0.2b 11.1 ± 0.7c 8.6 ± 0.4d 8.2 ± 0.4e 9.2 ± 1.3d 8.2 ± 0.9e 8.9 ± 0.0d 6.9 ± 0.32f 6.7 ± 0.0fg 6.7 ± 0.0fg 6.5 ± 0.3fg 6.2 ± 0.3g 4.4 ± 0.1h 4.4 ± 0.2h 2.2 ± 0.0i 2.2 ± 0.0i 2.2 ± 0.0i – – 2.2 ± 0.0i 2.2 ± 0.0i 2.2 ± 0.0i 0.00j (0.00)
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0 7 14 21 28 35 42 49 56 63 69 76
PV (mEq/kg)
Time (days)
40 °C
20 °C
5 °C
14.2 ± 0.75d 16.6 ± 0.9cab 15.5 ± 1.7cdb 18.2 ± 0.9a 15.6 ± 0.9cdb 16.3 ± 1.4cadb 15.0 ± 0.5cd 15.7 ± 1.3cdb 14.7 ± 0.4cd 15.4 ± 0.9cdb 15.1 ± 0.5cdb 17.3 ± 2.3ab
14.2 ± 0.8g 18.6 ± 1.1f 22.8 ± 1.7e 25.2 ± 0.6de 26.6 ± 2.6dc 26.2 ± 1.2dc 26.4 ± 1.2dc 28.0 ± 1.3dbc 29.0 ± 1.6bc 27.3 ± 2.3dbc 30.1 ± 1.5b 33.7 ± 2.0a
14.2 ± 0.8e 29.8 ± 4.0cb 36.2 ± 3.6a 30.8 ± 5.6ab 32.5 ± 2.4ab 23.1 ± 3.8d 24.5 ± 3.2cd 19.7 ± 2.6ed 19.0 ± 3.2ed 20.2 ± 2.5d 19.4 ± 1.2ed 20.8 ± 3.2d
0 1 2 3 4 5 6 7 8 9 10 12 14 16 17 19 20 21 22 23
PV (mEq/kg) 25 °C
40 °C
14.2 ± 0.8c 26.2 ± 3.0a 17.2 ± 0.7b 15.1 ± 0.6c 8.8d ± 0.2e 9.6 ± 1.4d 8.6 ± 0.4e 8.7 ± 0.1de 8.2 ± 0.4e 8.1 ± 0.1e 6.6 ± 0.1f 6.5 ± 0.1f 6.5 ± 0.12f 5.5 ± 1.1fg 4.3 ± 0.2hg 4.4 ± 0.1hg 4.4 ± 0.0hg 3.3 ± 1.3hi 2.2 ± 0.0i 2.2 ± 0.0i
14.2 ± 0.8a 12.1 ± 0.6b 6.0 ± 0.5c 4.3 ± 0.2d 4.4 ± 0.0d 4.3 ± 0.2d 2.2 ± 0.0e 2.2 ± 0.1e 2.5 ± 0.5e 0.0 ± 0.0f
Note: average values ± standard deviations (n = 3 replicates). Values with same letter in a column are not significantly different (p < 0.05).
an increase in PV with an increase of storage time and a peak within 4–10 d was observed. Ozogul, Ozyurt, Ozogul, Kuley, and Polat (2005) studied the fat oxidation in fresh eel by measuring PV and thiobarbituric acid (TBA) during storage in ice and 3 °C. In ice storage PV increased from 5.19 to 19.7 mEq/kg muscle after 15 d and then decreased to a value 4.06 mEq/kg thus showing a peak. Tanaka, Xueyi, Nagashima, and Taguchi (1991) also studied the change of PV in fresh sardine meat during storage at 5 °C and found that it
increased from around 1 to 15 mEq/kg after 4 d of storage. They also found that washing and high pressure treatment could reduce the oxidation significantly. 3.4. Glass transition temperature Fig. 2 shows typical DSC and MDSC thermograms for sample of moisture content 5 and 20 g/100 g sample. At moisture 5 g/100 g
Fig. 2. DSC and MDSC heat flow thermograms. (a) DSC thermogram for sample containing 5 g/100 g sample moisture, (b) MDSC thermogram of total heat flow for sample containing 20 g/100 g sample moisture, (c) MDSC thermogram of reversible heat flow for sample containing 20 g/100 g sample moisture and (d) MDSC thermogram of nonreversible total heat flow for sample containing 20 g/100 g sample moisture.
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sample, Fig. 2a shows a shift A in the thermogram line, small endothermic peak B, another thermogram shift C, and endothermic peak D indicating decomposition (or melting). A shift in thermogram line at A (around 20 °C) could be due to the initiation of mobility in protein and/or fat molecules as suggested by Sablani et al. (2007) for king fish muscle. The endothermic peak B was due to the melting of oil as also noticed by Sablani et al. (2007). Another shift C in the thermogram line before the decomposition can be considered as glass–rubber transition. This shift was difficult to identify by DSC for the equilibrated samples in different water activity (moisture content: 9.4–27.6 g/100 sample). Sablani et al. (2007) also found that increasing water had little influence on the identification of the glass transition temperature range, i.e. plasticisation effects of water on the protein was very small. It is difficult to find a clear glass transition with thermal analysis since other changes interfere or complexity of the structure affects identifying the glass transition. In a recent study, Rahman et al. (2007) found similar complexity in the case of date-pits. They also could not trace the glass transition even though oil was separated from the date-pits. Acevedo, Schebor, and Buera (2006) also reported the difficulty in detecting the glass transition of chicken muscle in their DSC measurements. In addition, Green, Fan, and Angell (1994) concluded from their results that hydrated proteins indeed may be grouped among glass-forming systems, but due to their special structural features and to the disposition of the bound water, they showed great departures from thermo or rheological simplicity. Similar difficulty was also observed in the case of native starch due to the nature of structure in molecular and micro levels, which caused small step change or broadening in the heat capacity change at the transition (Biliaderis, Page, Maurice, & Juliano, 1986; Zeleznak & Hoseney, 1987). As mentioned earlier that the glass transition of sample containing moisture 20 g/100 g sample could not be traced by DSC, thus a more sensitive modulated DSC (MDSC) was used. Total heat flow thermogram shown in Fig. 2b indicates two base line shifts at
Fig. 3. DSC thermogram for sample containing freezable water (moisture content 30 g/100 g sample).
locations A and B. The second shift at B in the thermogram before the decomposition endotherm could be considered as the glass transition. However, the endothermic peak for fat could not be traced. Fig. 2c and d shows the reversible and non-reversible heat flow thermograms. The reversible heat flow could separate the specific heat change in the material (i.e. glass transition), whereas non-reversible heat flow indicates the kinetic change (i.e. melting or decomposition). The structural change indicated at the point A in Fig. 2c need to be studied further. Fig. 3 shows typical thermogram for the sample containing freezable water showing melting of ice at A and solids decomposition at D. The freezing point and maximal-freeze concentration conditions are shown in Table 4. The lowest measured values of T 0m and T 000 g are 21.9 and 26.6 °C, respectively. Similarly Sablani et al. (2007) observed T 0m and T 000 g as 18.4 and 21.4 °C for king fish muscle. The solids content at T 0m and T 000 g is 70 g/100 sample, indicating X 0s = 0.70 and un-freezable water equal to 30 g/100 g sample. The reactivity of the un-freezable water could be varied with the type and characteristics of solid matrix. Thus both samples containing 5 and 20 g/100 g did not contain any freezable water, and similar stability could be expected. 3.5. Storage stability based on the water activity and glass transition It would be interesting to check the water activity and glass transition concepts for determining the stability of the fish muscle in terms of fat oxidation observed by PV. Samples at 5 g/100 g moisture content were close to the BET monolayer value of 6.23 g/100 g dry solids (i.e. 5.9 g/100 g sample), thus these should be most stable. However, the results showed that these samples were stable only at 40 °C and higher temperature caused the formation of peroxide indicating molecular mobility increased at higher temperature although the sample remained at monolayer moisture. The PV data were fitted to the kinetic model Eq. (4) and the rate constants k1 and k2 were estimated (Table 5). In order to identify the validity of the water activity and glass transition concepts, reaction rate constant k2 are plotted as a function of T/ Tg and Xw/Xb (Figs. 4 and 5). If water activity concept is valid, there should be a change in slope for k2 when Xw/Xb is around 1.0. Similarly there should be change in slope at Tc/Tg (Tc is critical temperature, T in K) around 1, if the glass transition concept is valid. Fig. 4 shows a break in slope at Tc/Tg = 0.78 in the case of moisture content 5 g/100 g sample, whereas a break at 0.96 for moisture content 20 g/100 g sample. This indicated the validity of the glass transition concept to some extent although the break in slope was not observed exactly at 1.0 and moisture content affected the critical limit of Tc/Tg. The shift of the critical limit to higher level indicated that water may have a protective effect. The rate constant is plotted as function of Xw/Xb for different temperatures in Fig. 5. This plot showed that there are no breaks in slopes for temperatures 40, 20, 5, and 25 °C and moisture did not show any effect until Xw/Xb = 3.5, which was the maximum moisture content tested in this work. At 40 °C the rate constant increased when Xw/Xb was around 1.0 (Fig. 5). Further data (Xw/
Table 4 Glass transition temperature, freezing point and maximal-freeze concentration conditions as a function of water contents. Xw
Tgi (°C)
Tgp
Tge
T 0m
T 000 g
TFC (°C)
TFB (°C)
TFA (°C)
DH (kJ/kg)
0.05 0.20* 0.30 0.40 0.60
95 ± 4 31 ± 3 – – –
101 ± 3 33 ± 4 – – –
105 ± 5 35 ± 4 – – –
– – 21.9 ± 0.5 21.4 ± 1.7 12.4 ± 2.0
– – 26.6 ± 0.5 26.3 ± 2.5 18.9 ± 2.9
– – 21.8 ± 0.4 20.6 ± 2.4 5.4 ± 1.4
– – 17.5 ± 0.2 15.8 ± 2.7 1.8 ± 1.2
– – 12.9 ± 1.5 10.8 ± 2.9 2.9 ± 2.4
– – 8.1 14.3 78.9
Average values ± standard deviations (n = 3–5 replicates). MDSC.
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M.S. Rahman et al. / Food Chemistry 114 (2009) 1257–1264 Table 5 Chemical reactivity for PV as a function of temperature and moisture content. Xw
0.05
0.20
T (°C)
40 20 5 25 40 40 20 5 25 40
Rate constant k1 (day1)
k2 (day1)
0.0011 0.0208 0.0295 0.0317 0.0350 0.0110 0.0251 0.0279 0.0151 0.1217
0.0024 0.0203 0.0308 0.0731 0.1152 0.0020 0.0260 0.0267 0.0579 0.3488
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ple) and five storage temperatures (40, 20, 5, 25, and 40 °C), respectively. Water and fat contents of fresh grouper were found 75.98 and 5.87 g/100 g sample, respectively. At both moisture levels no significant changes (p > 0.05) in PV was observed when samples were stored at 40 °C. In general three trends were observed in the formation of PV depending on the moisture content and storage temperature. In this study, the validity of the water activity and glass transition concepts were evaluated. It is concluded that glass transition concepts could be used to explain the process more adequately than the water activity concept since glass transition indicates breaks in the slopes of rate constant. However, this does not mean that there is no effect of water activity in the fat oxidation. A more combined approach needs to be developed in the future. Acknowledgments This work was supported by a grant from His Majesty Sultan Qaboos Research Funds (Project number SR/AGR/FOOD/05/01), the Swedish Research Council and the Sahlgrenska Academy at the University of Gothenburg. References
Fig. 4. Rate constant (k2) as a function of T/Tg for sample containing 5 and 20 g/ 100 g sample.
Fig. 5. Rate constant (k2) as a function of Xw/Xb for different temperatures of storage (T1: 40 °C, T2: 25 °C, T3: 5 °C, T4: 20 °C, T5: 40 °C).
Xb < 1.0 and Xw/Xb > 3.5) needs to be generated for varied moisture content and temperature in order to conclusively support this observation. However, it is clear from the observation that the effect of moisture content on reaction depended on the temperature. Thus both the concepts may be necessary to explain the reaction rates. Future works need to be targeted to combine both concepts in order to develop more generic approach to explain the reactions in biological systems. 4. Conclusion Lipid oxidation in freeze-dried grouper was studied by measuring its PV considering two moisture levels (5 and 20 g/100 g sam-
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