1+ - Fisheries and Oceans · PDF file- 3 - ( specific and reproducible than the AOCS...
Transcript of 1+ - Fisheries and Oceans · PDF file- 3 - ( specific and reproducible than the AOCS...
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1+ ENVIRONMENT CANADA FISHERIES AND MARINE SERVICE
TECHNOLOGY BRANCH HALIFAX~ NOVA SCOTIA
E.G. BLIGH~ DIRECTOR
NEW SERIES CIRCULAR ~m. 61 NOVEMBER~ 1976
AN IMPROVED TITRIMETRIC METHOD FOR DETERMINATION OF FREE FATTY ACID IN FISH OILS
- P.J. Ke, A.D. Woyewoda, L.W. Regier, and R.G. Ackman -
The free fatty acid (FFA) content, a continuing concern of workers
assessing the quality of oils and fats, has been widely used as a routine
quality index for fresh fish and related frozen and processed products.
The American Oil Chemists' Society (AOCS) method (1) for determination
of FFA has often proven unsatisfactory for fish oils which ·are yellow or
red due to carotenoids, or brownish due to poor quality. Furthermore,
the slight turbidity in the marine oils and the ethanol-aqueous NaOH
titration system combined with endpoint fading, often masks the endpoint
color of the phenolphthalein indicator. C~spite special precautions,
such as multiple determinations, great difficulty may still be experienced
in obtaining consistent and accurate values for the FFA content of some
fish oil samples when using the AOCS or similar methods (2,3). A revised
titrimetric method has been developed for the determination of FFA content ~ ,
of oils which gives precise and accurate resul.ts ~ven when oils are highly
colored or rancid. The method is based upon the use of a mixed solvent
and meta cresole purple (MCP) as the indicator.
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EXP ERIMEN'l'AL
A. Reagents:
Solvent Mixture: Chloroform, methanol and isopropanol (ACS grade)
mixed in a volume ratio of 2:1:2, is used for
dissolving the oil sample in. the titration.
Indicator: Meta Cresole Purple (MCP), obtained from Fisher
Sci. Co. and dissolved in distilled water at 0.5%
concentration, is used as the indicator.
Titrant: 0.05N NaOH in aqueous solution is used.
B. Procedure:
About 1 g of fish oil is accurately weighed <± 0.005 g) into a 125
ml Erlenmeyer flask. To this is added solvent mixture (75 ml) and 4
drops of MCP indicator solution. While the dissolved sample is being
mixed on a magnetic stirrer, the free fatty acids are titrated to the
purple end point with 0.05N NaOH using a 10 ml burette. A blank titrac .(.1sJl/
tion should be about 0.1 ml of ~ NaOH and should run daily.
C. Calculation:
The percentage of free fatty acids in most fish oils is calculated
as oleic acid from the following formula:
(T-B) x 1. 41
FFA, % in oleic S
{l.d.N where T and B denote the volume (ml) of ~ NaOH used to titrate the
sample and the blank, respectively, and S the weight (g) of sample.
RESULTS AND DISCUSSION
A comparison of the determination of FFA in three kinds of fish oils
by the AOCS and the MCP method described above is given in Table I. The
recommended multiple solvent system and MCP as indicator is clearly more
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( specific and reproducible than the AOCS titrimetric method procedure for
determining the content of FFA in fish oils. At concentrations of FFA
up to 13%, the results (Table I) obtained by the recommended MCP method
are slightly higher than the data from AOCS, but with a much lower standard
deviation of 1.5% or less. Inexperienced operators had no difficulty in
using the MCP method where in with the AOCS method there was inevitably
extra time spent on titrations due to the indecipherable color change of
phenolphthalein. The method described has been applied rapidly and
successfully to estimate the FFA contents of the various types of fish
oils shown in Table II. These samples included highly pigmented salmon
and redfish oils.
The multiple solvent system chosen for the recommended titrimetric
method for the FFA determination was a mixture of chloroform/methanol/
o isopropanolol ratio of 2:1:2 by volume. This solvent mixture will
completely mix with more than 12 ml of aqueous NaOH titrant before
becoming turbid. Furthermore, this solvent mixture can be simply prepared
by adding isopropanol and methanol to the chloroform extractant from
Bligh and Dyer's method of lipid recovery (4). A direct method for
estimating the content of FFA in a fish meat sample without removing or
changing the solvent from the extractant is being developed from the
new system of solvents and indicator.
The MCP endpoint, from yellow to purple at pH 12, was found to be
the most suitable in the newly developed titrimetric method. Other
indicators such as phenolphthalein (colorless to pink) and alkaline blue
(blue to orange-red), are subject to various degrees of undesirable
, o color interference from the fish oil color interference at their endpoints.
Since the titration problems caused by color interference and turbidity
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were eliminated, as little as one gram of oil (about 1/10 that used in
AOCS procedure) gave more reliable FFA results in various fish oils
(Table II).
Since the MCP titrimetric method for FFA analysis has advantages
over the AOCS procedure such as a greater reproducibility, a sharper
endpoint, a smaller sample size and superior miscibility of sol~ents,
it can be employed by various fish processing industries for the
determination of the FFA content in fish oils, particularly in highly
colored and oxidized samples.
REFERENCES
1. AMERICAN OIL CHEMISTS' SOCIETY. 1974. Official and Tentative
Methods of AOCS, 3rd edition, ca. 5a-40.
2. NERING, B. and MODZELEWSKA, K. 1975. Factor affecting the results
of fatty acid determination in various oils. Tluszcze Jadalne
19 : 129-136.
3. HORWITZ, W. 1976. Methods of analysis approved by the Codex
Alimentarius Commission. I. Acid Value. J. Assoc. Off.
Anal. Chern. 59 : 658-661.
4. BLIGH, E.G. and DYER, W.J. 1959. A rapid method of total lipid
extraction and purification. Can. J. Biochem. Physiol. 37 : 911.
Table I. Comparison of the determination of FFA in fish oils by the AOCS
and the recommended MCP method.
Sample
Herring oil
Oxidized herring oil
Redfish oil
\ FFA
added
0.59
10.10
5.10
10.22
2.90
12.49
* AOCS Method
\ FFA±SD
1.58 ± .06
2.05 ± .09
11. 67·± .25
2.58 ± .14
7 •. 30± .27
12.38 ± .25
2.87 ± .11
12.33 ± .36
\ RSD
3.8
4.4
2.2
5.4
3.7
2.0
3.8
2.9
* All data were calculated from 7 determinations.
* MCP Method
\ FFA±SD \ RSD
1.60
2.13
11.64
2.63
7.60
12.85
2.92
12.52
· .02 1.2
.02 0.9
.06 0.5
.04 1. 5
.08 1.1
.10 0.8
.02 0.7
.04 0.4
S.D. and \ RSD are the standard deviation and percentage of relative
standard deviation, respectively.
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Table II. The content of" FFA in several fish oils determined by the
recommended MCP method.
Fish Oils % FFA % RSD
Cod liver 3.34 ± .01 0.3 6.40 ± .02 0.3
Seal 2.35 ± .02 0.8 5.26 ± .03 0.6
Cod frame 1.98 ± .01 1.0
Salmon 2.56 ± .03 1.2 5.54 ± .04 , 0.7
Herring 4.61 ± 0.1 0.2
0 Redfish 5.41 ± .03 0.7
Mackerel Frame 3.20 ± .05 1.5 8.42 ± .11 1.3
Mackerel skin 3.57 ± .01 0.3
Mackerel meats 2.70 ± .03 1.1
Oxidized herring 5.67 ± .06 1.1
Oxidized redfish 3.92 ± .06 1.5
Oxidized mackerel frame 4.81 ± .07 1.4
All data were calculated from 7 determinations.