J Sci Food Agric 1992, 59, 37-43

Physical, Chemical and Sensory Analysis of Sardine (Sardina pilchardus) Stored in Ice Maria L Nunes, Irineu Batista and Regina Morlio de Campos Instituto Nacional de InvestigaGgo das Pescas, Departamento de Tecnologia dos Produtos Aquaticos, Av. Brasilia, 1400 Lisboa, Portugal

(Received 6 June 1991 ; revised version received 17 October 1991 ; accepted 30 December 1991)

Abstract: The shelf-life of iced sardine (Sardina pilchardus) was studied. The main changes which take place in raw fish were investigated by means of organoleptic assessments, chemical analyses (total volatile nitrogen (TVB-N), trimethylamine (TMA-N), trimethylamine oxide (OTMA-N) and hypoxanthine) and physical measurements (GR Torrymeter readings and pH). The influence of both seasonal changes and fat deterioration were also considered. The results obtained indicate that TVB-N and TMA-N parameters are not good freshness indicators for this species, but Torrymeter readings and hypoxanthine values can be used as indicators of freshness. However, they must always be confirmed by a sensory evaluation. From the different combinations tried, the most highly significant degree of correlation was obtained between sensory evaluation and Torrymeter readings.

Key words : sardine, Sardina pilchardus, seasonal variation, freshness indices.

INTRODUCTION

The distribution of sardine (Sardina pilchardus) is quite wide, it is predominant from the southern coast of Norway through to the Mauritanian coast, being also present in the Mediterranean together with Surdinella aurita. In Europe, this fish is particularly important off the coast of Portugal and in the Atlantic coastal waters of Spain, where it is one of the most important raw materials for the canning industry.

Sardines off the Portuguese coast are caught by purse- seiners which do not have the equipment on board for the preservation of sardines. It is transported in bulk on the deck without ice which leads to a decrease in quality and, hence, price at auction also decreases.

This species, like other small pelagic fish, is quite fragile and spoils quickly, however, its shelf-life can be extended by using ice on board. An alternative to this practice would be the utilisation of ice just after landing and storage in a refrigerated plant. In order to understand the main changes taking place in sardine under ice storage conditions in different seasons the evolution of different physical and chemical parameters was studied and correlations with sensory evaluations were also established.

MATERIALS AND METHODS

Raw materials

The sardines used in these experiments were caught by purse-seiners. After landing they were iced and trans- ported in isothermic boxes to the laboratory and placed in plastic boxes with sufficient crushed ice in a cold store at about 0°C.

Physical analyses

pH measurements were taken from a blend of fish : dis- tilled water (1 : lo), homogenised for 1 min (Lima Dos Santos et a1 I98 1). Torrymeter readings were obtained by using the GR Torrymeter instrument.

Chemical analyses

Moisture, protein (total N x 6.25), fat and ash contents were determined according to AOAC procedures (AOAC 1975). Thiobarbituric acid (TBA) determination was performed using Vyncke’s method (1970). Peroxide value and free fatty acids were determined on the lipids extracted by the method of Bligh and Dyer (1959)

37 J Sci Food Agric 0022-5142/92/$05.00 0 1992 SCI. Printed in Great Britain

38 M L Nunes, I Batista, R Moriio de Campos

according to AOAC methods (AOAC 1975). Total volatile bases (TVB-N), trimethylamine nitrogen (TMA- N) and trimethylamine oxide nitrogen (TMAO-N) were determined as described by Cobb et a1 (1 973).

Each chemical analysis and pH measurement was done in duplicate. The Torrymeter values are a mean of measurements done on 16 sardines.

Sensory evaluation

A trained panel, 10 persons on average, scored each sample using a 6-point descriptive scale. A score of 0 indicated the highest quality and was defined such that appearance was desirable, texture firm, and overall acceptability excellent. A score of 6 indicated an unacceptable quality.

RESULTS AND DISCUSSION

Seasonal changes

The chemical composition of sardine changes during the year and this influences, in a number of important ways, its shelf-life and also the technological applications. Hence, these characteristics were monitored over a period of 3 years. In addition, because the chemical composition of sardine depends also on the fishing ground, the study was done on samples caught as nearly as possible in the same fishing area in the centre of the Portuguese coast (Peniche). The range of proximate composition found in whole sardine can be seen in Table 1.

As can be seen, the protein content range is relatively narrow, about 25%. With regard to ash content, the generally high values obtained result from the presence of the bones. On the other hand, fat and moisture contents have significant ranges of variation and, as shown in Fig 1, fat has a periodic change with a maximum in August-October and a minimum in February-March.

TABLE 1 Variation range of main sardine constituents found in whole

sardine

Constituent Amount (%)

Moisture 58.2-78.6 Fat 16-22.4

Ash 26-3.9 Protein 1 5 9 5 - 1 8.4

Between fat and moisture content there is a negative relationship (Fig 2), which was referred to by Culley (1971) for this species and by Sat0 el a1 (1978) for Sardinops melanosticta.

Physical analyses

In general, Torrymeter values decreased throughout iced storage, however, the distribution pattern was not the same every month. Thus, in some months the values fit linearly but in other months the values fit a curve better with an initial higher slope (Fig 3) which could be related to higher bacterial activity.

With pH values there was a gradual increase during storage (Fig 4) and higher values were observed when the fat content was lower. This happens after the spawning period when the glycogen reserves of the sardine are at their lowest levels. However, the pH changes depend on a variety of factors such as fishing ground, feeding of the fish, etc., which do not permit an easy interpretation of the results. The initial drop in pH, due to formation of acid compounds normally associated with the early post-mortem changes in fish, was not observed be- cause measurements were performed a few hours after catching.

The evolution of both physical parameters suggests that lean sardine spoils faster than fatty sardine. In fact, for the same storage time, Torrymeter readings were always higher in fatty sardines than in lean fish, whereas the corresponding pH values present an inverse pattern.

Chemical analyses

During iced storage the moisture content of the edible part increased by about 2 % due to absorption of water from the melting ice and this variation seems to be higher when sardine is lean.

With TMA-N and TVB-N contents, it was found that they were both low until the seventh day of storage, and increasing amounts were formed only when the fish had deteriorated completely and was therefore unacceptable for consumption. The evolution pattern of both par- ameters in the flesh was quite similar, as can be seen, for example, for February sardine in Fig 5 . This pattern was not observed by El Marrakchi el a1 (1990) who found that TMA-N and TVB-N were very accurate in the prediction of the spoilage process and considered these indices suitable for the evaluation of sardine quality. Furthermore, dark muscle had higher levels of these amines than light muscle (c 2 4 %), but the evolution was similar. Figure 5 shows the TMAO-N concentrations in the edible part during the storage period. The higher rate of decomposition remained until the seventh day after which it decreased in a more controlled manner. The amount of TMAO-N present in light muscle was almost constant (about 26 mg per 100 g) until the fifth day,

Physical, chemical and sensory analysis of sardine 39

FAT CONTENT (lb) 25

. 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ' 1

J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

1987 MONTH

1088

Fig 1. Seasonal variation of fat.

1989

FAT CONTENT (%) 20

Fat (%I = 80.65 - 1.03 x water (%I

62 64 66 68 70 72 74 76 78 80

Fig 2. Inverse correlation of fat and moisture contents. MOISTURE CONTENT (%I

whereas in the dark muscle the initial value was about 34 mg per 100 g which decreased rapidly attaining 17 mg per 100 g after the seventh day. An identical behaviour has been described for other species (Martin et a1 1981).

increased linearly until the twelfth day when it attained its maximum level. The general pattern of hypoxanthine

development in sardine, illustrated in Fig 6, is similar to that observed in whiting (Martin el a1 1981) during iced storage, and, as can be seen, the levels of this nucleotide were similar in lean and fatty sardine. The decrease in

Hypoxanthine, a breakdown product from ATP, hypoxanthine content after 12 days in ice could be related to its reduction to uric acid by bacterial action.

In view of the fatty nature of the sardine some of the

40

7.5

7 -

6.5

6 -

5.5

M L Nunes, I Batista, R Mora'o de Campos

- FEBRUARY SEPTEMBER

,

-

' I I 1 I 1

lORRYYCTER R I A D I N 0 8 16 - JANUARY -k FEBRUARY SEPTEMBER

i

4 ' I 1 I I I

0 2 4 6 8 10 12 DAY8 IN ICE

Fig 3. Evolution of Torrymeter values.

main changes in the oil were examined (Fig 7). In The slight decrease of TBA values after 9 days could general, all three parameters studied showed, as was result from an interaction between malonaldehyde and expected, a regular increase during iced storage. The high proteins. Both TBA and peroxide values showed a higher peroxide value obtained in sardine caught in February formation rate when sardine was fattier. Although after 7 days of iced storage, could be due to an sardine oil had a relatively high peroxide content in the experimental error. In fact, in experiments conducted in first few days, this was not detected by sensory other months no decrease until 12 days was observed. evaluation. A rancid flavour only appeared when the.

Physical, chemical and sensory analysis of sardine 41

n 16 60

-TMA-N 4- TVB-N +G TMAO-N cn 14 -

I- 0

0 2 4 6 8 10 12 14 16 18 Days in ice

Fig 5. Evolution of TMA-N, TVB-N and TMAO-N contents in February sardine.

10

- February + September

0 5 10 15 20 Days in ice

Fig 6. Evolution of hypoxanthine.

sardine was unacceptable, probably due to the presence of more oxidised compounds. The presence of a maximum in TBA evolution can result from the reaction of malonaldehyde with proteins, and it is also possible that some compounds which give a red colour with TBA reagent are leached out during iced storage.

Sensory evaluation

The organoleptic assessment of raw sardine showed a monthly profile similar in evolution to those represented on Fig 8 for February and September.

After every catch there are significant changes in the

42

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5 -

M L Nunes, I Batista, R Mora'o de Campos

45 - 40

- .- - TBA - Feb.

0

rn &

Q, 35 0- 30 E

25 2 v

Q,

a > TJ

20 Q,

10 2 15 -8

c

5

0 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4

Days in ice Fig 7. Evolution of free fatty acid content, TBA and peroxide values.

- FEBRUARY -k SEPTEMBER

0 ' 1 I I I

0 2 4 6 0 10 12 DAY8 IN ICE

Fig 8. Evolution of sensory evaluation.

colour of sardine. Initially, it was a greenish-blue colour which progressively changed to a green-yellow tone, and after a few days in ice became grey and toneless. The eyes suffer some modifications as well, becoming progressively less crystalline, with the appearance of blood spots after 5 days. Flesh, which was initially firm, became slightly soft after a few days, and at the end of the storage period was pasty and easily separated from the bone.

On the basis of these assessments three levels of quality

can be proposed: (i) high-quality level (0-2 days); (ii) acceptable level (2-5 days) ; (iii) unacceptable level (more than 5 days).

CONCLUSIONS

Torrymeter readings can be used as a freshness index for sardines but it is always necessary to compare them with the results of an organoleptic evaluation.

Physical, chemical and sensory analysis of sardine 43

eeweonv ewunion 6

8eN8ORY EUILUATION 7.70 - 0.60 TORRYUCTCR R @ A D I W O ~

( r -0.877) .

3-

2 -

1 - 8

8 .

I

4 6 8 10 12 14 16

Fig 9. Correlation between sensory and Torrymeter readings. TORRYUCTER ACAOlNOb

Despite the widespread use of TVB-N and TBA in fish quality assessment, for this species they do not seem to be good freshness indicators because their contents are almost constant until the ninth day when the quality of the sardine is already unacceptable.

On the other hand, hypoxanthine can be used as a reliable freshness index on account of its linear increase during storage and, as previously stated, its evolution does not seem to be influenced by fat content.

Lastly, the tests used to follow oil deterioration can also contribute to a better characterisation of sardine quality.

Between the sensory evaluation and Torrymeter readings a significant linear correlation ( P < 0.001) was established (Fig 9), but no such correlation was found between organoleptic assessment and TVB-N and TMA- N values.

The results obtained indicate that objective deter- minations alone are not enough as freshness indices and must always be supported by organoleptic evaluation.

In general, from the results of the different analyses it can be said that this species well-iced after landing maintains a high degree of quality during the initial 2 days.

REFERENCES

AOAC 1975 Oficial Methodr of Analysis of the Association of Oficial Analytical Chemists, 12th edn. Washington DC, USA.

Bligh E G, Dyer W J 1959 A rapid method of total lipid extraction and purification. Can J Biochem Physiol 37

Cobb B F, Alaniz I, Thompson Jr C A 1973 Biochemical and microbial studies on shrimp: volatile nitrogen and amino nitrogen analysis. J Food Sci 38 431436.

Culley M 1971 The Pilchard. Biology and Exploration. Pergamon Press Inc., Oxford, UK.

El Marrakchi A, Bennour M, Bouchriti N, Hamama A, Tagafait H 1990 Sensory, chemical, and microbiological assessments of Moroccan sardines (Sardina pilchardus) stored in ice. J Food Protection 53 600-605.

Martin R, Flick G, Hebard C, Ward D 1981 Chemistry and Biochemistry of Marine Food Products. AVI Publishing Company Inc., CT, USA.

Lima Dos Santos C, James D, Teutscher F 1981 Guidelines for chilled fish storage experiments. FA0 Fish. Tech. Pap., 210p.

Sat0 B, Sasaki Y, Abe S, 1978 Developing Technology of Utilization of Small Pelagic Fish, Fisheries Agency, Japan,

Vyncke W 1970 Direct determination of TBA value in extracts of fish as a measure of oxidative rancidity. Fette Seifen Anstrichm 12 1084-1087.

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105-1 16.