J Sci Food Agric 82:1764–1771 (online: 2002) DOI: 10.1002/jsfa.1261 Journal of the Science of Food and Agriculture Effect of previous chilled storage on rancidity development in frozen horse mackerel (Trachurus trachurus) Santiago P Aubourg,1* Ines Lehmann2 and Jose´ M Gallardo1 Instituto de Investigaciones Marinas (CSIC), Eduardo Cabello 6, E-36208 Vigo, Spain Bundesforschunganstalt fu¨r Fischerei, Institut fu¨r Fischereitechnik und Fischqualita¨t, Palmaille 9, D-22767 Hamburg, Germany Abstract: Rancidity development during frozen storage (À20 °C) of an underutilised medium-fatcontent fish species, horse mackerel (Trachurus trachurus), was studied Special attention was given to the effect of previous chilled storage (0, 1, and days) on the quality of the frozen fish For this, chemical (free fatty acid and conjugated diene contents; peroxide value, PV; thiobarbituric acid index, TBA-i; fluorescent compound formation) and sensory (rancid odour and taste) analyses were carried out Hydrolytic rancidity showed an increase with frozen storage time; however, no effect of previous chilling time was observed on the frozen product Oxidative rancidity measured by chemical (PV, TBA-i and fluorescence) and sensory (odour and taste) indices increased with frozen storage time and also with previous chilling time Satisfactory quality was maintained up to months of frozen storage of horse mackerel provided that a short chilling time (not longer than days) was employed # 2002 Society of Chemical Industry Keywords: underutilised fish; chilling; frozen storage; chemical and sensory analyses; rancidity; shelf-life INTRODUCTION Fish and other marine species give rise to products of great economic importance in many countries The fish industry is actually suffering from dwindling stocks of traditional species as a result of drastic changes in their availability Thus fish technologists and the fish trade have turned their attention to some unconventional sources of raw material.1,2 One such species is horse mackerel (Trachurus trachurus), a medium-fat-content fish abundant in the Northeast Atlantic.3,4 Efforts have been made to utilise it in the manufacture of several smoked,5 canned,6 frozen7,8 and restructured9 fish products During processing and storage, fish quality may decline as a result of several factors One of the most important is oxidation of highly unsaturated lipids,10 which is directly related to the production of offflavours and odours.11,12 Frozen storage has been widely employed to maintain fish properties before consumption or use in other technological processes.13,14 However, during frozen storage of fish, lipid hydrolysis and oxidation have been shown to occur and to influence fish acceptance.15–17 Before the freezing step is accomplished, adequate storage techniques of fish material should be employed to reduce post-capture losses Among the different onboard handling systems that efficiently cool fish, the most common one employed is chilling.18,19 During chilled storage of fish, important changes take place in the lipid fraction, leading to significant losses of sensory and nutritional values.20,21 The present study is related to the commercialisation of frozen horse mackerel and aims to study the effect of previous chilled storage on the stability of horse mackerel lipids during frozen storage Chemical and sensory lipid damage indices were studied to assess rancidity development and, accordingly, to evaluate the shelf-life of this species under such conditions MATERIALS AND METHODS Raw fish, sampling and processing Fresh horse mackerel (T trachurus) were obtained 10 h after catching in June 2000 The length of the fish was in the range 18–24 cm; the weight was in the range 250–280 g Upon arrival in the laboratory the fish were stored on ice in an isothermal room (0–2 °C) Individual fish (120 pieces) were randomly distributed into three batches that were studied independently Horse mackerel from each batch (40 individual pieces) were taken for freezing after 0, 1, and days of chilled storage Freezing was carried out at À80 °C for 24 h After * Correspondence to: Santiago P Aubourg, Instituto de Investigaciones Marinas (CSIC), Eduardo Cabello 6, E-36208 Vigo, Spain E-mail: saubourg@iim.csic.es Contract/grant sponsor: Co-operation Program (1999–2000) Germany–Spain (MCyT–INIA) of Agricultural Research Contract/grant sponsor: Comisio´n Interministerial de Ciencia y Tecnologı´a; contract/grant number: ALI 99-0869 (Received 31 October 2001; revised version received 14 May 2002; accepted August 2002) # 2002 Society of Chemical Industry J Sci Food Agric 0022–5142/2002/$30.00 1764 Rancidity development in frozen horse mackerel that time, all fish samples were stored at À20 °C Analysis of fish samples was undertaken on the white muscle of fish material stored for 0, 1, 3, and months at À20 °C Composition analyses Chemicals employed (solvents and reactants) were reagent grade (E Merck, Darmstadt, Germany) Water content was determined by weight difference between the fresh homogenised muscle (1–2 g) and after heating for 24 h at 105 °C Results were calculated as g water per 100 g muscle Lipids were extracted by the Bligh and Dyer22 method Results are expressed as g lipid per 100 g wet muscle Chemical lipid damage measurements Free fatty acid (FFA) content was determined by the Lowry and Tinsley23 method based on complex formation with cupric acetate/pyridine Results are expressed as g FFA per 100 g lipid Conjugated diene (CD) formation was measured at 233 nm.24 Results are expressed according to the formula CD = BV/w, where B is the absorbance reading at 233 nm, V is the volume (ml) and w is the mass (mg) of the lipid extract measured The peroxide value (PV), expressed as meq oxygen kgÀ1 lipid, was determined by the ferric thiocyanate method.25 The thiobarbituric acid index (TBA-i) was determined according to Vyncke.26 Results are expressed as mg malondialdehyde kgÀ1 fish sample Interaction compound formation Interaction compounds formed between lipid oxidation products and protein-like compounds were investigated by measuring fluorescence formation (Perkin-Elmer LS 3B) at 327/415 and 393/463 nm as described in previous studies.27,28 The relative fluorescence was calculated as RF = F/Fst, where F is the fluorescence measured at each excitation/emission pair and Fst is the fluorescence intensity of a quinine sulphate solution (1 mg ml À1 in 0.05 M H2SO4) at the corresponding wavelength The fluorescence ratio calculated as FR = RF393/463 nm/RF327/415 nm was studied in the aqueous phase resulting from lipid extraction.22 Sensory analysis Sensory analysis was conducted by a trained tasting panel consisting of six to nine experienced judges For each sample analysis, fillets of five fish were cooked in plastic bags in a water bath Rancid odour and taste were then evaluated on a scale from (stage of no rancidity at all) to 100 (stage where no increase in rancidity is possible) Three categories were ranked: good quality (0–29), fair quality (30–59) and rejectable quality (60–100) Scores among panellists were averaged J Sci Food Agric 82:1764–1771 (online: 2002) Statistical analyses Data from the different chemical and sensory quality measurements were subjected to one-way ANOVA and correlation analysis (p < 0.05);29 comparison of means was performed using a least squares difference (LSD) method RESULTS AND DISCUSSION Water content ranged between 750 and 790 g kgÀ1 Lipid content ranged between 6.0 and 20.0 g kgÀ1 Differences in both constituents may be explained as a result of individual fish variation, and not arising from chilled or frozen storage; it is recommended to employ several individual fish for each sample to minimise this source of variation Comparison of the present results with previous research showed a higher water content for horse mackerel than for fattier fish species27 and a lower water content for horse mackerel than for leaner fish species,30,31 in accordance with an inverse ratio between water and lipid matter.32 Lipid hydrolysis In the present study the FFA content of the raw material (7.9 g kgÀ1) was similar to that of fatty fish species (tuna, sardine)27,33 and lower than that of lean fish species (blue whiting, haddock, cod).30,31 As a result of frozen storage, significant (p < 0.05) differences were only observed at month 3, where a sharp increase in the FFA content of all samples was found (Fig 1) For each frozen storage time, few significant (p < 0.05) differences could be observed among the four different chilling times studied, so that an effect of previous chilling time on hydrolysis during frozen storage could not be concluded Lipid oxidation Oxidised compounds as measured by CD formation (Fig 2) did not show a clear trend as a result of frozen storage, nor as a result of previous chilled storage Increases were found at month in samples previously chilled for and day and also at month in samples previously chilled for and days In the present study, CD detection did not prove to be sensitive for following changes arising from the chilling and freezing treatments employed; this index has been shown to be more accurate in the case of initial oxidation and model systems.34,35 Peroxide formation (Fig 3) showed a big increase at month of frozen storage in all samples; thereafter, few significant increases were observed A strong effect of chilled storage time on peroxide formation during frozen storage was observed During the whole frozen storage period, samples previously chilled for and days showed a higher PV than those previously chilled for and day, indicating a negative effect of the previous chilling treatment Secondary oxidation as measured by TBA-i showed a gradual general increase during frozen storage for all samples (Fig 4) A marked effect on carbonyl com1765 SP Aubourg, I Lehmann, JM Gallardo Figure Free fatty acid (FFA) content in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) pound formation during frozen storage was observed for samples previously chilled for and days, resulting in higher TBA-i values than those for samples previously chilled for and day Interaction compound formation Few significant differences in fluorescence development (FR) were observed among samples during the first months of frozen storage (Fig 5); thereafter a significant (p < 0.05) increase was detected, which was especially sharp at month in samples previously chilled for days and at month in samples previously chilled for and days Increasing the length of chilled storage time prior to freezing led to a slight increase in the formation of fluorescent compounds Formation of fluorescent products as a result of interactions between lipid oxidation compounds and nucleophilic molecules (proteins, peptides, etc) has been reported to be dependent on the formation of lipid oxidation products (peroxides and carbonyls) and also on temperature.36,37 In the present experiment, fluorescence formation developed faster in the later stages (5 and months) of frozen storage, in accordance with increases in PV and TBA-i values (Figs and 4).38,39 Sensory evaluation40 Rancid odour showed an increase in all samples at month (Fig 6) Thereafter a significant increase could only be assessed at month in samples previ- Figure Conjugated diene (CD) formation in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) 1766 J Sci Food Agric 82:1764–1771 (online: 2002) Rancidity development in frozen horse mackerel Figure Peroxide (PV) formation in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) ously chilled for and days, indicating a negative effect of previous chilling time on rancid odour According to the categories assessed by the trained panel, samples previously chilled for and days were judged to be of fair quality at month of frozen storage; however, those previously chilled for and day were judged to be of good quality at month Rancid taste increased at month of frozen storage (Fig 7) and continued to increase with frozen storage time for most samples However, compared with most samples, those previously chilled for days scored higher values After months of frozen storage, samples without chilling pre-treatment were found to be of good quality, those previously chilled for and days were of fair quality and those previously chilled for days were rejected Correlation analyses The different chemical and sensory analyses were tested for correlation with the previous chilled storage time at each frozen storage time (Table 1) According to the results shown in Figs 1–5, PV and TBA-i were the only chemical indices that showed significant (p < 0.05) linear correlation values with the chilled storage time at all the frozen storage times tested PV showed, in all cases, correlation values higher than 0.84 Sensory analyses (odour and taste) also showed satisfactory correlations in most cases, Figure Thiobarbituric acid (TBA-i) values obtained in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) J Sci Food Agric 82:1764–1771 (online: 2002) 1767 SP Aubourg, I Lehmann, JM Gallardo Figure Fluorescence (RF) formation in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) according to Figs and In all cases, exponential and logarithmic correlations were also tested; however, the best results were obtained if linear correlations were used Correlation between the most reliable chemical indices (PV and TBA-i) and the sensory assessments was also carried out (Table 2) According to Table 1, satisfactory correlation values were obtained for raw samples and also in cases of advanced oxidation degree (months and 7) Figure Changes in rancid odour value in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) 1768 J Sci Food Agric 82:1764–1771 (online: 2002) Rancidity development in frozen horse mackerel Figure Changes in rancid taste value in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) CONCLUSIONS In accordance with previous research on lean8,31 and fatty15,27 fish species, the present experiment showed important hydrolytic and oxidative rancidity development during frozen storage of horse mackerel This lipid damage was assessed satisfactorily by traditional chemical indices (FFA, PV, TBA-i and FR) and by sensory analysis (rancid odour and taste) Indeed, some chemical analyses (PV and TBA-i) showed satisfactory correlations with sensory values and proved to be reliable methods for assessing quality changes (Tables and 2) Chilled storage time prior to frozen storage did not provide a significant effect on hydrolytic rancidity Table Linear correlation values between lipid indices (chemical and sensory)a and previous chilling time calculated for each frozen storage time development (FFA formation) in the frozen product However, a negative effect on oxidative rancidity, according to chemical (PV, TBA-i and FR) and sensory (odour and taste) parameters, was observed for fish chilled for 3–5 days prior to frozen storage, leading to a significant quality loss in the frozen product These results concur with previous studies on fattier species (frozen herring41–43 and canned sardine28,44) indicating a faster increase in oxidation product formation when pre-freezing chilled storage was extended In the present study on the medium-fat-content species horse mackerel, satisfactory quality was maintained up to months of frozen storage provided that onboard fish handling was not longer than days In cases where longer chilling pre-treatments are needed Frozen storage time (months) Lipid index FFA CD PV TBA-i FR Odour Taste 0.34 À0.76* 0.92* 0.58* 0.60* 0.89* 0.89* 0.37 À0.32 0.85* 0.84* 0.30 0.06 0.77* 0.20 À0.13 0.93* 0.67* 0.27 0.30 0.43 0.33 À0.63* 0.91* 0.75* 0.57 0.72* 0.69* 0.21 À0.28 0.85* 0.77* 0.72* 0.96* 0.96* a Abbreviations: FFA, free fatty acids; CD, conjugated dienes; PV, peroxide value; TBA-i, thiobarbituric acid index; FR, fluorescence ratio * Significant at p < 0.05 J Sci Food Agric 82:1764–1771 (online: 2002) Table Correlation values between sensory (odour and taste) and chemical (PV and TBA-i)a indices calculated for each frozen storage time Frozen storage time (months) Odour (PV/TBA-i) Taste (PV/TBA-i) 0.87*/0.99* 0.06/À0.03 0.13/0.01 0.78*/0.78* 0.96*/0.92* 0.82*/0.56* 0.61*/0.73* 0.23/0.11 0.77*/0.73* 0.85*/0.90* a Abbreviations as specified in Table * Significant at p < 0.05 1769 SP Aubourg, I Lehmann, JM Gallardo for further frozen horse mackerel commercialisation, employment of chilling or frozen storage in conjunction with appropriate antioxidant treatment45,46 is recommended to guarantee a longer shelf-life when consumed as a frozen product 16 17 ACKNOWLEDGEMENTS The authors thank Mr Marcos Trigo, Mrs 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(online: 2002) Rancidity development in frozen horse mackerel Figure Peroxide (PV) formation in frozen (0, 1, 3, and months) horse mackerel that was previously chilled (0, 1, and days) ously chilled. .. month in samples previously chilled for days and at month in samples previously chilled for and days Increasing the length of chilled storage time prior to freezing led to a slight increase in. .. a result of frozen storage, nor as a result of previous chilled storage Increases were found at month in samples previously chilled for and day and also at month in samples previously chilled