Potent Odorants in Boiled Cod as Affected by the Storage of Raw

Chapter 11

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 28, 2018 | https://pubs.acs.org Publication Date: July 1, 1997 | doi: 10.1021/bk-1997-0674.ch011

Potent Odorants in Boiled Cod as Affected by the Storage of Raw Material C. Milo and W. Grosch Deutsche Forschungsanstalt für Lebensmittelchemie, Lichtenbergstrasse 4, D-85748 Garching, Germany

The potent odorants of boiled cod, fresh and after prolonged storage at -13°C, were screended by two GC-sniffing methods in order to cover the whole range of volatility; these were aroma extract dilution analysis (AEDA) and gas chromatography/olfactometry of static headspace samples (GCO-H). Quantification of 19 odorants by means of an isotope dilution assay and calculation of odor activity values (ratio of the concentration to the threshold) revealed that methanthiol and dimethylsulfide followed by (E,Z)-2,6-nonadienal and 3-methylbutanal as being the most potent odorants for the nasal perception of fresh boiled cod, whereas (Z)-1,5-octadien-3-one and methional showed the highest retronasal odor activity values (OAV). After storage for 14 weeks at -13°C a drastic increase in the concentration, and therefore the OAVs, of3-methylbutanal and 2-methylbutanal as well as a decrease in the content of dimethylsulfide was noticed. Based on these results 3-methylbutanal was found to be primarily responsible for the malty odor defect in thefrozenstored cod.

The aroma of fish has been the subject of much research over the past decades. Several studies focused on raw fish and differences among species (1-3). So far more than 300 volatiles have been identified infreshand processed fish (4). However, the contribution of certain odorants to the aroma remains largely unknown, due to the absence of sufficient quantitative data. Furthermore, either extracts or headspace samples have been analyzed, thereby neglecting the impact of the highly volatile or low volatile compounds, respectively. The aroma of cooked fish has been studied (5-8), sometimes under drastic cooking conditions (5,6). Since the boiling affects the aroma, the cooking procedure should reflect the common preparation method in order to allow production of the odorants in a "ready to eat" product. A thorough study of the volatiles in cooked

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© 1997 American Chemical Society Shahidi and Cadwallader; Flavor and Lipid Chemistry of Seafoods ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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Effect of Storage on Odorants in Boiled Cod

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cod was performed by McGill et al. (8) who also followed quantitative changes in odorants depending on the time of frozen storage of raw cod. Fish and fish products are usually stored and handled frozen, a necessity that arises from the short shelf life of these products. Although oxidative susceptibility of fish lipid and enzymatic hydrolysis offish proteins in frozen products is well documented as being responsible for potential aroma defects (9), our understanding of the chemical basis of the off odors so produced is rather limited. In search for odorants that are mainly responsible for aroma changes in frozen stored cod, it was reported that freezing and thawing cycle per se does not affect the aroma (8). Gas chromatographic (GC) sniffing of extracts prepared, using a Likens-Nickerson apparatus, from frozen stored cod indicated three regions in the chromatogram that were associated with the odor defect described as the "cold storage flavor" (CSF). Odorants that were identified as the cause of this defect were (E)-2-heptenal, (E,E)-2,4-heptadienal and particularly (Z)-4-heptenal. The authors reported a good correlation between the sensory scores as reflected in CSF intensity and (Z)-4-heptenal concentration and concluded that this aldehyde is primarily responsible for the odor defect. (Z)-4-Heptenal was also associated with the fishy aroma quality of oxidized soybean oil (JO). (Z)-4-Heptenal, together with (E,Z,Z)-2,4,7-decatrienal at high concentration (> lppm) imparted a burnt-fishy aroma in deodorized oils (77). We have previously determined that certain lipid peroxidation products namely, (Z)-3-hexenal, (Z,Z)-3,6-nonadienal and also (Z)-4heptenal as being mainly responsible for the unpleasant fishy, oily aroma offrozenstored trout (72). This work examines the potent odorants of boiled cod as affected by frozen storage of the raw material. Prior to storage, skinned cod fillets were homogenized to ensure a uniform material. Furthermore, flavor changes of cod, a lean saltwater fish, as compared to frozen stored trout (72) was tested. The medium and higher boiling odorants were screened by aroma extract dilution alnalysis (AEDA) and the lower boiling volatiles were examined using gas chromatography/olfactometry of static headspace samples (GCO-H). After quantification of the odorants, according to the results of the two G C sniffing methods, odor activity values (OAV = ratio of concentration to odor threshold) were calculated. Based on these results branched aldehydes derivedfromamino acid, were found responsible for the odor defect of stored cod and not the lipid peroxidation products. Experimental Material. Skinned cod fillets (4 kg) were ground in a meat grinder. The raw mince was stored in polyethylene bags at -13°C for up to 26 weeks. The reference material was kept at - 60°C until analyzed and is referred to as fresh sample. The storage temperature of -13°C was chosen in order to suppress microbiological spoilage and to keep the storage time reasonably short. Propanal, methylpropanal, 2,3-pentanedione, dimethyltrisulfide and (Z,Z)-3,6-nonadienal were synthesized as deuterium labeled compounds. The other labeled odorants used in the isotope dilution assays were prepared as described elsewhere (73).

Shahidi and Cadwallader; Flavor and Lipid Chemistry of Seafoods ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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FLAVOR AND LIPID CHEMISTRY OF SEAFOODS

Isolation of volatiles. The poached fish (250 g in aluminum foil, 15 min at 80°C) was frozen in liquid nitrogen, ground to afinepowder, mixed with an equal amount (w/w) of anhydrous sodium sulfate, soaked in diethyl ether (1000 ml) and extracted, after 24 h, in a Soxhlet apparatus over a 7 h period. A fat free organic solvent extract was obtained by high vacuum distillation. After the separation into a neutraL^basic and an acidic fraction A E D A was performed as described earlier (14). The highly volatile odorants were analyzed by GC sniffing using a series of gradually decreasing volumes of static headspace samples (75). Identification and quantification of odorants was performed using gas chromatography - mass spectrometry (GC-MS) after an initial clean up by column chromatography on silica gel (72). Results and Discussion The aroma profile offreshboiled cod was described by a sensory panel as mild-fishy, vegetable-like with resemblance to cooked rice. This pleasant aroma changed to an unpleasant fishy, malty odor after 20 weeks of storage at -13°C. The final extract of the neutral volatiles in fresh, boiled cod analyzed by A E D A had a distinctfish-likeodor, but lacked the unique vegetable, cooked rice-like top note. As summarized in Table I, 15 odorants were detected by G C sniffing, 9 of which possessed a cod-like odor quality. To the best of our knowledge, 8 odorants, marked wth astrices in Table I, were identified for the first time in cod. High FD factors were determined for methional, (Z)-1,5-octadien-3-one, (E,Z)-2,6-nonadienal and l-octen-3one in bothfreshand stored samples. Greater differences in FD factors between fresh and stored cod samples were found for 2,3-pentandione, 2,4-nonadienal, dimethyltrisulfide, (Z)-l,5-octadien-3-one and (E,Z)-2,6-nonadienal. The aroma notes responsible for the uniquefreshcod aroma as well as the aroma changes, observed after improper frozen storage, could not be entirely explained by the A E D A results. Since highly volatile compounds may be lost during the preparation of extracts for A E D A and/or may be hidden under the solvent peak, and therefore not being perceived by GC sniffing, gas chromatography/olfactometry of static headspace samples was performed in order to complement the results of A E D A As summarized in Table II, in a volume of 10 ml headspacefroma cod sample, stored for 14 weeks at -13°C, 14 odorants were detected by GC-sniffing compared to 9 in the fresh sample. The additional odorants in the stored cod included methylpropanal, 3-methylbutanal and 2-methylbutanal, all of which exhibited a malty-like odor at the sniffing port. The importance of these branched aldehydes to the aromadefect of cod, boiled after prolonged storage, is reflected by a drastic decrease of headspace volumes sufficient for their perception in GCO-H. The compound 3-methylbutanal was the only odorant perceivable in a headspace volume of 0.5 ml. Both screening methods (i.e. A E D A and GCO-H) did not indicate any contribution from (Z)-4-heptenal to the odor defect in the stored cod used in these experiments. Accurate quantification of odorants was also performed in order to gain further insight into the actual contribution of individual compounds to the overall aroma. The method of choice for the determination of trace components and/or unstable compounds, as it is often the case with potent odorants, is the isotope dilution assay. Based on the

Shahidi and Cadwallader; Flavor and Lipid Chemistry of Seafoods ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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MILO & GROSCH

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Effect of Storage on Odorants in Boiled Cod

Table I. Potent odorants (FD > 8) in boiled cod* and influence of storage of raw material on the FD factors 1


FD-Factor* 6

b

No.

Compound

Odor description

6 16 17/18 19 10 20 11 12 13 21 22 15 23 24 25

diacetyl 2,3-pentandione* hexanal/(Z)-3 -hexenal (Z)-4-heptenal methional* 2-acetyl-1 -pyrrolin* dimethyl trisulfide* l-octen-3-one (Z)-1,5-octadien-3-one* (E,Z)-2,6-nonadienal 2,4-nonadienal dimethyl tetrasulfide* (E,E)-2,4-nonadienal* (E,E)-2,4-decadienal 4,5-epoxy-(E)-2-decenal*

buttery-like buttery-like green creamy-fatty boiled potato-like roasty cabbage-like mushroom-like geranium-like cucumber-like fatty cabbage-like, putrid fatty fatty metallic

6

c

RI on SE 54 595 700 800 900 908 925 957 981 985 1154 1193 1212 1216 1328 1384

A (fresh)

B (stored)

Potent Odorants in Boiled Cod as Affected by the Storage of Raw

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