Quality Index Method developed for Frigate tuna (Auris thazarA

An investigation was carried out to develop and evaluate a Quality Index Method (QIM) scheme for Frigate tuna (Auxis thazard) for the prediction of past and remaining storage time in ice. The quality index method (QIM) provides weighted evaluation of the key parameters in deterioration of individual species, assigning demerit points according to the importance of each parameter. For the development and evaluation of QIM scheme, Frigate tuna were stored in ice and assessed raw and cooked. The QIM developed for raw Frigate tuna comprised of 11 parameters (appearance of skin and stiffness: cornea, form and colour of eyes; colour, smell and mucus of gills; condition of viscera; colour of blood and fillets) covering attributes, which gave a total of 25 demerit points. Sensory analysis of cooked Frigate tuna using Torry scheme were carried out in parallel to determine the shelf life. In order to obtain more information about the quality of Frigate tuna, Total viable counts (TVC), hydrogen sulphide (H2S) producing bacteria, trimethylamine (TMA), total volatile basic nitrogen (TVN) content and pH value were determined. The sensory evaluation was carried out by a panel of 6-7 judges and the rejection level was found to be 22 days. The QIM scheme developed for Frigate tuna showed a linear relationship between QIM scores and storage time in ice, (r 2 = 0.9378) with slope of 0.755. The TVC varied from 10 2 cfu g -1 to 10 6 cfu g -1 within 7 to 22 days and H 2 S producing bacteria at 10 2 cfu g -1 within 22 days of storage in ice. The TMA and TVN amounts increased with time and the amounts ranged from 1.2 mg/100g to 2.5 mg/100g and 17.2 mg/100g to 34.9 mg/100g respectively. The variation of pH ranged from 5.4 to 5.9 within the storage. The QIM calibration curve obtained for Frigate tuna indicates it’s applicability to determine the storage life of fish in ice. Further, it will facilitate the requirements of buyers and sellers while fulfilling the demands of inspection authorities and the consumers.

Ar investigation was carried out to develop and evaluate a Quality Index Method (QItrO scheme for Frigate tuna (Auxis tha4ard) for llre prediction of past and rcmaining siorage time in ice.The quality index method (QIM) provides weighted evaluation of the key pmameters in deterioration of individual species, assigning demerit points according to the importance of each parameter.For the development and evaluatiol of QIM scheme, Frigate tuna were stored in ice and assessed raw and cooked.The QM developed for mw Frigate tuna comprised of l l pammeters (appearance of skin aDd stiffness: comea, form and colour of eyes; colour, smell and mucus of gills; condition-of viscera; colou of blood and fillets) covering attdbutes, wlDch gave a total of 25 demerit poin6.Sensory analysis of cooked Frigate tuna using Tolly scheme were caded out in parallel to detemine the shelf life.In order to obtain more infomation about the quality of Frigate tuna, Total viable counts (TVC), hydrogen sulphide (HrS) prcducing bacte a, trimethylamiDe (TMA), total volatile basic nitrogeD (TVN) conteDt and pH value were detemined.
The sensory evaluation was carried out by a panel of 6-7 judges and the rejection level was fouDd to be 22 days.The QIM scheme developed for Frigate tuna showed a fnear relationship between QM scores atd s^torage time in ice, (l = 0.937E) with slope of 0.755.The TVC varied ftom l0'?cfr.r/g to 106cfir/g within 7 to 22 days and HrS producing bacteria at 10'z ctu/g within 22 days of storage in ice.The TMA aDd TVN amounts increased with time and the amounts ranged fiom 1.2 lng/l008 to 2.5 mE/lUJB ar]j 17.2 mg/t0og to 34.9 my100g respectively.The variation of pH ranged ftom 5.4 to 5.9 within the storase.
The QM calibration curve obtained for Frigate tuna indicates its applicability to detennine the storage life of flsh in ice Further, it will facilitate lhe requirements of buyers and sellers while fulfiIling the demands of inspection authorities and the consumers.

Inhoduction
Improving ihe quality and the availability of flsh for human consumption is an important arca for management action llandling and preservation of catch on board need to be improved, by using ice, chilled water, refrigeratior/ fteezing systems, etc.The fish that is landed must be kept clean and stored in ice, in refrigerated rooms ot bansported in ice, in refiigerated trDcks depending oD the duration of stotage or transpoftation.
The fteshness of fish detedofttes \r.ith time.Most of the changes resulting from loss of freshness can be evaluated by sight, touch, smell and taste which detemine consumer acceptance.It is therefore extremely useful to have methods for describing the sensory prope ies of fish as a means of ascertaining their iDitial sensory characteristics and any changes undergone by the product in the colrrse of storage.
The quality index method (QItr4) is a very effective tool and a mpid and simple desciptive sensory method to determine freshness and quality (Bremner 1985;Brcmrer et al. 1985).QIM essentially evaluates those sensory parameters and attributes tlat charge most significandy in each species duriDg storage period ( Huss I995,.By using QIM.rhe fish inspector can give estimation of the past and remaidng stotage time.In addition, the krowledge of shelf life of fish species would allow fishermen, processors and rctailers better confol of their markets. Regulations in European Union require fteshness grading of most fish to be marketed within the union.Futher, the fish inspection department has to perform quality inspection of fish prior to expofi in order to issue health cefiificate in accordance with EU regllations (EU Directive l99l).Fisheries and Aquatic Resources Act of 1996 also descdbes the fteshness category of selected fish species (Fish Products (Export) Regulation 1998).
The aim of this study was to develop a quality index specific for raw fish of Frigate tttlla (Arxir thazatd) and predict the shelf life during ice storage and to compare the sensory analysis with midobial counts (total viable counts and H2S producing bacteda) and chemical measuements.

Materials atrd Methods
A total of 70 fresh Frigate tui (Auds thazaftl) caught in Matam, rhe south cost of Sri Lanka werc used in this experimenl The fish which were Quali, index for Frigate tlola kept in jce 18 hrs after captue were brought to NARA laboratory.The first batch of Frigate tuna (30) was used for haining of assessors, development of quality index and for photography.A batch of 40 fish was storcd at 0'C to 2oC in iced boxes until analysis, which was used for test measurcments and sensory evaluation.
The sensory evaluation of raw and cooked Frigate tuna were caried out in parallel on each sampling day (no. 1,no. 7,no. 12,no. 16,no. l9 and Do. 22 days after capture) along with measuements of microbiological, chemical and pH Each sampling day,5 Frigate tuna were used.Three fish were analyzed whole by QIM (Table 1).Pieces ftom one fish werc steam cooked and evaluated with the Torry scheme (Table 2) for odour and flavour.Sensory assessments of cooked fish were carried out in parallel.Rest of the fish were used for microbiological analysis for TVC and H2S producing bacteria (Cram et al. 1989), chemical analysis for TVN and TMA (Horwits 1980) and pH measurements.A panel of5-7 fained judges evaluated the fish individually for each quality pfiameter in tle scheme.Selected pictues were used to develop the QIM scheme for raw fish.

Results
The results ftom sensory evaluation, chemical measutements, microbial counts and pH are presented in relation to storage time of Frigate tuna in ice The sum of scores evaluated ranging from 0 for the best qualily fish to 25 for the poorcst quality f1sh.According to the QIM scheme (Table l) quality is presented as Quality Index (QD.The QI was liDearly related to storage period (R'= 0.937) (Fig. 1).
The scores for all quality attdbutes of the QM increased with storage time in ice (Figure 2).On the first day appeararce of flsh was score.d0-1, as the skin of fish was bright.On the 22"d day fish were scoring 2 as the appearance of skin was dull.The skin did not become very soft after 22days of stomge in ice.The scorcs of the quality aftributes of gills increased with storage time.The colour approached maximum after 22 days in ice.The smell of gills did not rcach the maximum of scorc of 3, the average scorc of which was however above 1 after 16 days.The scorcs for gill odour appeared to have the same trend as gill mucus, but the average score reached around 2 at the end of storage time, The scores for yiscera inueased with storage time in ice.constantlv.Using Torry scheme (Table 2) cooked Frigate tuna was evaluated in parallel to the quality index method.High score (10) gives for the best quality and the low (3) score for the poorcst quality.Scores for Frigate tuna stored 22 days in ice were found to be significantly different ftom ihose storcd in ice for 1 to 7 days.The positive odour attributes, seaweed odour dedeased with storage in ice and were hardly detectable after 12 days.The sour flavorl decrcased to unacceptable level of Frigate tuna for human consumption after 22 days in ice. Figure 3 shows the result of assessment of cooked fish using Torry scale dudng storage days in ice.  Figure 4. Hydrogen Sulphide (}I S) producing bacteria (spoilage bacteda) and roral viable counts (TVC) during storage in ice A similar trend was evident in flavour as for odour' Positlve attdbutes such as seaweed, sweet, metallic, oily flavour decleased with storage days il ice, while the Degative attributes rancid, sour flavours increased with storage days-Especially the sour flavour was dominant alter 22 days of stomge, indicating that the Fri8ate tuna was of marginal acceptability.Some panglists were unable to detect some of the attributes

Microbial counts
Total viable counts (TVC) hcleased with storage time st-arting ftom 7 ilays of storage (Fig. 4).When the {ish reached the limits of acceptability (sensory score of 4 in Torry scheme), TVC reached level of l0o cfu,/g.
Counts of HrS producing bacteda could be detected in fish after 16 days of storage.Figue 4 shows that they werc absent at the beginning and at the end of shelf life the couDts werc reached above 10'cftlE.

Chemical analysis
During stomge in ice the conceDtrations of the Total Volatile Nirogen (IVN) in tuna inqeased with time while Tri Methyl Amine (TMA) content remained more or less constant (Fig. 6).At the rcjection time, TVN has rcached the concentration of around 35 mg/1009 and TMA amount was 2.5n9/1009.The TVN level is highly conelated to QI score €ig.7).
Photographs were used as guides fot assessors to assess Frigate tuna by the QIM scheme on the basis of colour/appeannce of whole fish, mucus and colour of gills and comea, form and colour ofpupil.The photo$aphs of Frigate tuDa a{ter I, and 22 days in ice are shown itr FiSure 9.The increase in discolomtion of skin of Frigate tuna during tle period of stomge in ice can perhaps be connected to ho\\, carotenoids are bound in the flesh.The carotinoid-binding proteins or lipoproteins are known to break down in the deterioratjon process of autolysis and bacterial activity (Tonissen et al. 1989).
The stiffness of fish was scored 0 at day 1 as the Frigate tuna was at rigor.The resolution of rigor causes the muscle to telax again and through storage in ice, the flesh becomes soft due to autolysis influenced by both fish muscle enzyme and microbial enzymes, (Nielsen 1995;Gill 1995).
The scores for the quality attdbutes of eyes increased constantly throughout the storage time in ice.At the end of shelf life comea become concave.Nevertheless enough information on form and colour of pupil could not be obtained.At the beginning of the stomge time when the Frigate tuna was very liesh, the odour of gills was described as fresh sea weedy or neutral This is probably because Dewly caught fish contains low levels of Qualit, inderfor FriSate t na volatile compounds (Olafsdottir and Fleurence 1997).The score for colour of blood was close to 0 Dntil 7 days jn ice.The blood in the abdomen was blood rcd at first.but later on it became brownish.When the fish meat exists in the folm of oxymyoglobin, this compound is oxidized to meta myoglobil, which is brown in colour.The increase in these negative odour attributes is caused by activity of spoilage bacteria producing substances of bad smell and oxidation of fat (Gram 1995;Jorgensen 1995). Milo andGrosh (1996) analyzed various odourants in salmon of different fteshness levels They found propionadehyde and (Z)-1,5-octadien-3-one as tle most potent high volatile odorants in cooked fresh salmon samples.The odour of these compounds is described as sweet and metallic (respectively).Various odourants detected ftom cooked salmoD (Milo and Grosh 1996) may also be responsible for the flavour and odour of cooked Frigate tuna.The odour and flavour were attributed to acetaldehyde (sweet), hexanal and (2, Z)-3,6' nonaldienal.The mncid odour inc.eased in the last days of storage.The rancid flavour is known to be caused by fomation of volatile oxidation products such as aldehydes and ketones (Milo and Grosh,1996).At thl beginning, micmbial counts in flesh of Fdgate tuna were very low.It implles that flesh oihealthy,live or newly caught fish is sterile as the immune system of the fish prevents the bacteria from growing in the flesh.W}en fish dies, the immune system collapses and during stomge, bacteria invade lhe flesh (Gram 1995).Similar results were noted by Lande and Rora (1999).Magnusson (1987) found TVC and HrS producing bacteda in farmed salmon, whose the total viable cormt reached 10'cfu/g in flesh after the stotage time of 22 days.In ice stored Nile perch the total counts were at l0' cfi/g on the day before the fish was rejected (Gftm et al. 1989).
For the T\rN concentration in muscle tissue of ce ain fish species a legal limit has been flxed by the Ec-cornmission at 25mgl1009.TVN is produced mainly by HrS producing bacteria.In lhe present study, that is detected after 16 days in ice.TMA is a pungent volatile amine o,{ten associated with the typical "fishy" odour ofspoiling seafood.TNLA.and m IJ other aml)es become volatile at elevated pH.
The lorv value of pH in first day of storage could be explaincC 'oy 'bost mortem" glycolysis that results in the accumulation of lactic acjd, which in tum lower the pH of muscle.Huss (1995) has rcported pH Crops from 6.8 to 6.1-6.5 in cod and 6.8 to 5.4-5.6 in tuna and halibut.The increase jn pH with storage time is probably caused by lorrnetion of basic compounds (Huss 1988).
The changes occuffing in the outer appearance of Frigate tuDa with storage days in ice are presented in Figue 9.The appearance of the ski changes from being pearl-shhy to dull, the eyes change from being black and convex to grey and sunken.The gills change fiom rcd with clear mucus to

Figure 2 .Figure 2 .
Figure 1.Relationship between Quality Index of Frigate tuna and stomge

Figure 2 .
Figure 2. Avemge scores of each quality atfiibute assessed with QIM scheme for FrigaF tuna stored in ice against days itr ice (Continued).

Figure 8 .
Figure 8. Changes in pH during storage of ungutted Frigate hrna in ice.

Table 2 .
The score sheet for Torry scheme of fteshness for cookeil Frigate t[IJ' (A xis thazard).