(OSI)
DEFINITION
All oils and fats have a resistance to oxidation which depends on the degree of saturation, natural or added antioxidants, prooxidants or prior abuse. Oxidation is slow until this resistance is overcome, at which point oxidation accelerates and becomes very rapid. The length of time before this rapid accel-eration of oxidation is the measure of the resistance to oxidation and is commonly referred to as the
“induction period.”
In this method for determining the induction period, a stream of purified air is passed through a sample of oil or fat which is held in a thermostated bath. The effluent air from the oil or fat sample is then bubbled through a vessel containing deionized water. The conductivity of the water is continual-ly monitored. The effluent air contains volatile organic acids, swept from the oxidizing oil, that increase the conductivity of the water as oxidation proceeds. Formic acid is the predominant organic acid formed (see References, 1). The conductivity of the water is monitored by a computer or strip chart recorder. The Oil Stability Index (OSI) is defined as the point of maximum change of the rate of oxidation, or mathematically as the maximum of the second derivative of the conductivity with respect to time (see Fig. 1). This time-based end point may be determined by a computer that can cal-culate the maximum of the second derivative with respect to time, or by a slope-change algorithm, which is similar to detecting the onset of peaks for integration of GLC chromatograms. The end point may be approximated by using other methods. One commonly used approximation is a graphic method in which tangents are drawn manually (see Fig. 2). The OSI may be run at temperatures of 100, 110, 120, 130 and 140°C. Because by its nature this analysis has this temperature flexibility, all OSI results should specify the OSI time, with the analysis temperature reported immediately after (for example, “OSI 11.7 hours at 110°C”).
SCOPE
This method is applicable in general to all fats and oils and has been subjected to a collaborative study (see References, 2) covering a broad range of sample types. It can be used to analyze crude oils, or other oils that are prone to foaming, if one drop of silicone antifoam is added prior to analy-sis. It may be used for other types of oils outside the range of samples tested in the collaborative study (see Notes, 1 and References, 3). This analysis is an automated replacement for the Active Oxygen Method (AOM) for fat stability, AOCS Official Method Cd 12-57.
GENERAL PRECAUTIONS
1. Trace-metal contamination of the glassware will cause accelerated oxidation. Because of the difficulty in removing the final traces of chromate from a glass sur-face, chromate cleaning solutions should not be used.
Only detergents without surface-active agents should be used for cleaning. Traces of surface-active agents
remain adhered to a glass surface even after the most thorough rinsing with deionized water. Water used for rinsing should be checked as a potential source of trace-metal contamination.
2. Improper temperature control is the most likely source of error. The temperature must be calibrated by check-ing the actual temperature of a sample in the bath. The
Raw data
2nd Derivative 1st Derivative
Time
Figure 1. OSI determined by first and second derivatives.
Intersection of tangents
Time
Figure 2. OSI determined by tangential method.
temperature must be maintained within at least
± 0.1°C. The temperature of the water in the eff l u e n t trap must not exceed 25°C, which minimizes the loss of formic acid.
APPARATUS
1. Two commercial instruments embody the equipment needed for this analysis:
(a) Oxidative stability instrument—from Omnion Inc., Maple and Plain Streets, Rockland, MA, 02370, USA (manufactured under license from Archer Daniels Midland Co., Decatur, IL, USA); and (b) Rancimat—model 617 from Brinkmann
Instru-ments, Inc., subsidiary of Sybron Corporation, Cantiague Road, Westbury, NY, 11590, USA.
2. Constant temperature bath—to maintain all samples at the desired temperature to within ± 0.1°C.
3. Air distributing manifold—constructed of stainless steel, plastic, aluminum or glass. Airflow between sample channels should be calibrated to permit the same airflow to within 10%. This may be accom-plished by using either matched capillary tubes or nee-dle valves and flow meters.
4. Source of low pressure, clean, oil-free, low CO2, com-pressed dry air (see Notes, 2)—5.5 pounds per square inch (PSI) is sufficient to supply air to 24 sample tubes.
The air may be purified with the system as follows:
(a) A i r i n l e t t u b e f r o m c o m p r e s s e d a i r s o u r c e equipped with stainless steel needle valve or pres-sure regulator.
(b) A Balston Model 75-45 Purge Gas Dryer for FTIR and a Balston A963-000 Trace Hydrocarbon remover. Balston, Inc., Naperville, IL 60563, or its equivalent.
(c) Bottled air may be used, provided that it has low CO2levels (<2 ppm CO2).
5. Thermometer—accurately calibrated, able to indicate the temperature to within ± 0.1°C. For OSI operation, a precision thermometer calibrated against an NIST traceable thermometer is required. Brooklyn Thermometer, Farmingdale, NY provides such a ther-mometer with a temperature range of 95–150°C in 0.1°C finest readable interval, 76 mm immersion. The stock # is 3551 RM. Brooklyn Thermometer will pro-vide NIST traceable calibration for an additional c h a rge. For temperature calibration of the Rancimat Analyzer, an NIST traceable calibrated platinum resis-tance (RTD) digital thermometer is required with cali-bration points at 100, 110, 120, 130 and 140°C. Omega Engineering, Inc., Stanford, CT, provides such a ther-mometer. A custom 3 wire 2 mm ×7 mm sensor probe is required for calibration of sample temperature with air flowing through the sample.
6. Gas flow meter—either a rotameter type or a soap bubble type is satisfactory. Airflow should be measured at each tube under the conditions of the test and should be adjust-ed to 2.5 ± 0.1 mL/sec. Alltech provides a digital flow m e t e r, Model 7080, suitable for air flow measurement.
7. Sample reaction tubes—soda glass 25 mm ×200 mm.
Each tube requires a two-hole stopper and aeration
tube. An all-glass assembly, either Martin M 7920, or Brinkmann EA1087, or equivalent, is satisfactory.
8. Conductivity measurement tube and probe—The tube should be capable of holding 50 mL water such that a depth of 5 cm or more covers the tip of the aeration tube to provide contact time with the water. This is necessary for the volatile organic acids to dissolve. The conductivity probe should supply a full-scale sensitivi-ty of 300–500 uS-cm−1.
9. Instrumentation for detecting conductivity and means for recording and evaluating the end point—A computer or multichannel strip chart recorder may be used to monitor the conductivity of each probe in the instrument.
10. Tube-cleaning bath—consisting of a stainless steel pan or sink, heated with either steam coils (note: do not use copper tubing) or gas, and of sufficient area to permit laying the required number of aeration and test tubes on the bottom.
11. Test tube brush—fan tip, nylon.
12. Tongs—stainless steel or nickel plated, suitable for handling tubes in hot detergent solution.
13. Hood.
REAGENTS
1 . Water—deionized or distilled (conductivity < 5 µS - c m−1) . 2. 1 , 1 , 1 - Trichloroethane, purified grade (see Notes,
Caution).
3. Acetone—ACS Grade (see Notes, Caution).
4. D e t e rgent—for cleaning glassware without leaving contaminating residue. Contrad 70, Liquinox, Citranox RBS (Fluka Ltd., Buchs, Switzerland) and Micro are satisfactory.
5. Silicone antifoam—Dow Corning A. One drop is added to oils that have a tendency to foam.
CLEANING OF SAMPLE REACTION TUBES
The following procedures are either for instruments that do not utilize disposable glassware, or for precautionary clean-ing of disposable glassware.
1. Initial cleaning—The initial cleaning should be per-formed immediately after the preceding determination.
Melt and drain off as much of the fat from sample tube as possible. Wash off the remaining fat with a suitable solvent. 1,1,1-Trichloroethane is a satisfactory fat and oil solvent.
2. Tube cleaning—Prepare a 1% solution of deterg e n t (Reagents, 4) and heat almost to boiling in the cleaning bath. Rinse each tube with the hot detergent solution, brushing briefly with the nylon brush (Apparatus, 11).
Then place the tubes in the hot detergent solution con-tained in the cleaning bath (Apparatus, 10) in such a manner that no air bubbles are trapped within. Boil vigorously for 30 min. Brush each tube vigorously with a nylon brush, and rinse in the hot detergent solu-tion. Rinse thoroughly with tap water, followed by deionized water, and place upright in a test tube rack.
Fill with deionized water and soak at least 1 hr. At the end of the soaking period, rinse the tubes once again with fresh deionized water, drain on clean filter paper and dry. Store in a dust-free location.
SAMPLING AND ANALYSIS OF COMMERCIAL FATS AND OILS
Cd 12b-92 • Oil Stability Index
Tube cleaning, alternative procedure—Add excess potassium hydroxide (KOH) to an isopropyl alcohol/water solution (90:10, v/v) and mix until satu-ration has occurred. Half fill the tubes with this solu-tion, and heat in a boiling water bath for several hours with occasional shaking. Finally, rinse with deionized w a t e r. If traces of oil remain, the saponification must be repeated. When the tubes have been cleaned in this manner, they should be immersed in a 1% solution of detergent at >80°C for 1 hr. Finally, rinse all glassware with deionized water and dry.
3. Conductivity tube and probe cleaning—To clean the conductivity tubes and probes, first rinse the reaction products with denatured ethanol. The hot deterg e n t solution should then be used as a soak or, with the aid of a brush, to scrub conductivity probes that have the rigidity to withstand such treatment. Rinse repeatedly with deionized water, and soak the tubes and probes in deionized water for at least 1 hr. When ready to reuse, fill the tubes with deionized water and connect the probe to the conductivity meter. After 30 min, if the water conductivity is under 25 µS - c m−1 and is not changing, then the conductivity tube is ready for reuse.
If the water conductivity in the tube is too high (i.e.,
>25 µS-cm−1), the probe must be recleaned.
SAMPLING
1. Because this procedure is a measure of oil stability, anything that might detrimentally affect the stability of the sample must be avoided. Samples should be kept cool and in the dark. Where packaged fats are involved, the sample should consist of an unopened package, if possible. If this is not possible, samples must be removed from large containers or processing equipment with clean sampling devices of stainless steel, aluminum, nickel or glass. Samples of solid fat should be taken at least 5 cm from the walls of large containers and 2.5 cm from the walls of small contain-ers. If liquid oil is poured from a container, the pouring spout or lip should first be cleaned thoroughly using a clean cloth moistened with acetone. After removal from packages or processing equipment, samples should be transported and stored only in clean glass or plastic containers. Samples should be protected from contact with heat, light and air as much as possible.
Samples should be stored with little or no headspace; if headspace is unavoidable, purge the headspace with nitrogen.
TEMPERATURE CALIBRATION
Note—Because variations in temperature are potentially the most serious source of error, temperature calibration of the instrument is critical.
1. The oil sample should be added to the reaction tube to the level equivalent to 5 g for OSI and Rancimat instruments without an insert, and 2.5 g for Rancimat instruments fitted with disposable inserts. The airflow is then adjusted as directed (Apparatus, 6), and a ther-mometer (Apparatus, 5) is suspended in the oil such that it does not touch the walls of the reaction tube.
2. Allow the oil temperature to equilibrate for 15 min before reading the thermometer. When noting the tem-perature, do not move the thermometer up or down.
3. Adjust the block temperature controller to compensate for any differences. Block temperature must be within
± 0.1°C of the target temperature (e.g., for 110°C in the range of 109.9–110.1°C; for 130°C in the range of 129.9–130.1°C).
4. After adjustments have been made, wait until the con-troller is showing the new temperature, and then allow the oil to equilibrate for 15 min. Record the tempera-ture and repeat calibration steps 2–4 until the targ e t temperature is reached.
5. Check the temperature periodically to ensure accuracy.
PROCEDURE
1. Fill the conductivity tubes with 50 mL deionized water and attach the probes. Verify that the water conductivi-ty in the tube is 25 µS-cm−1or less and that the con-ductivity is constant.
2. Unless already completely liquid, the sample should be melted at a temperature not more than 10°C above its melting point. The sample size is 5.0 ± 0.2 g for OSI and Rancimat instruments without an insert, or 2.5 ± 0.2 g for Rancimat instruments fitted with disposable inserts. Carefully place the sample directly into the bottom of the reaction tube. It has been found that bet-ter precision is obtained if the sample is not allowed to coat the side of the tube. Avoid, as noted in the sam-pling procedure, contaminating the sample during transfer.
3. In the collaborative studies, all determinations were performed at 110 and 130°C. The temperature should be selected so as not to permit the OSI time to be less than 4 hr or more than 15 hours. Times less than 4 hr result in a wider variation of end point determination.
Sample temperature should be checked periodically to ensure the temperature controllers have not drifted.
4. Connect the tubing from the air manifold to the con-ductivity measurement tube, and adjust the aeration tubes to within 5 mm of the bottom of both the reac-tion and the conductivity tubes. Measure the airflow, adjusting to 2.5 ± 0.2 mL/sec (see Note, 3). An exhaust hood is beneficial for the removal of obnoxious volatiles resulting from oxidation reactions.
5. A computer or multichannel strip chart recorder should be used to monitor the conductivity of each probe in the instrument. A plot of water conductivity vs. time obtained from the recorder is then analyzed, and the OSI inflection point is determined either by a micro-processor-computed slope/change algorithm or a maxi-mum of the second derivative, or by the tangential method (see Fig. 2), etc.
PRECISION
1. A collaborative study, in which the stability of rape-seed oil and palm oil was determined at 100°C by 11 laboratories in Norway and the UK, was reported by Wo e s t e n b u rg and Zaalberg (References, 4) to have given an interlaboratory reproducibility coefficient of
SAMPLING AND ANALYSIS OF COMMERCIAL FATS AND OILS
Cd 12b-92 • Oil Stability Index
variation (RSDR) of 9.1% and an instrumental repeata-bility coefficient of variation of 3.3%.
2. The OSI method was subjected to a collaborative study among 15 laboratories, using all currently available commercial instruments noted in this method. The overall average coefficient of variation was 10.2% for samples having from 7 to 80 hr of stability at 110°C.
Summaries of the statistical analysis of the results of the collaborative study carried out by the AOCS Active Oxygen Method (AOM) Alternatives Committee are shown in Table 1 for 110°C and in Table 2 for 130°C. Generally, a larger variation was noted for lower temperatures and longer induction periods.
3. Repeatability limit—The absolute difference between two independent single test results, obtained with the same method on identical test material in the same lab-oratory by the same operator using the same equipment within short intervals of time, should not be greater than the repeatability limit (r) as calculated from the formulas in Tables 1 and 2.
4. Reproducibility limit—The absolute dif f e r e n c e between two single test results, obtained with the same method on identical test material in different laborato-ries with different operators using different equipment, should not be greater than the reproducibility limit (R) as calculated from the formulas in Tables 1 and 2.
5. Table 1 contains the statistical analysis of the interlab-oratory study completed by the AOCS AOM Alternatives Committee in 1991, in which 15 laborato-ries participated, each obtaining four test results for each sample analyzed at 110°C [statistical results eval-uated in accordance with International Org a n i z a t i o n for Standardization (ISO) 5725-1986].
6. Table 2 contains the statistical analysis of the interlab-oratory study carried out by the AOCS A O M Alternatives Committee in 1991, in which 14 laborato-ries participated, each obtaining four test results for each sample analyzed at 130°C (statistical results eval-uated in accordance with ISO 5725-1986).
NOTES Caution
Trichloroethane is an irritant to eyes and skin. The TLV is 350 ppm in air.
Acetone is highly flammable and forms explosive perox-ides with oxidizing agents. Use effective fume-removal device. Do not mix with chloroform. The explosive limits in air are 2.6–12.8%. The TLV is 750 ppm. It is narcotic in high concentrations and moderately toxic by ingestion and inhala-t i o n .
NUMBERED NOTES
1 . The OSI analysis may be used to analyze many other types of oils and fats; however, some of these were not included in the collaborative study. Extremely stable fats may be analyzed by accelerating the rate of oxidation by raising the temperature at which the oil is oxidized (e.g., to 140°C). The analysis of these high-stability oils and fats can be performed if the conductivity cell water loss is kept to a minimum by cooling, or is kept at constant volume by the periodic addition of fresh water to make up for any loss. The stability of free fatty acids has been found to be too low to permit analysis at 110°C, but the analysis could be performed at a lower temperature. Fish oils and linseed oils have been successfully analyzed at 80°C, but in general have a long, sloping oxidation
SAMPLING AND ANALYSIS OF COMMERCIAL FATS AND OILS
Cd 12b-92 • Oil Stability Index
Table 1
Statistical results of interlaboratory study in which samples were analyzed at 110°C.
Samplea
A B C D E F I J L
Number of labs after removal of
outliers 15 14 15 15 13 14 13 12 13
n 58 55 56 57 48 52 49 45 51
Outliers 0 3 1 1 8 4 0 4 0
Mean, hoursb 10.1 7.7 17.9 23.7 13.9 46.5 68.1 69.7 18.6
Repeatability, Sr 0.6 0.4 0.8 0.9 0.3 1.2 4.2 3.5 1.8
RSDr 5.96 4.80 4.34 4.00 2.11 2.51 6.22 5.03 9.62
r (2.8 ×Sr) 1.68 1.12 2.24 2.52 0.8 3.36 11.76 9.80 5.04
Reproducibility, SR 1.3 0.9 2.0 1.9 1.14 4.1 13.5 9.0 4.4
RSDR 12.33 11.23 11.09 8.13 8.18 8.92 19.88 12.92 23.93
R (2.8 ×SR) 3.64 2.52 5.60 5.32 3.19 11.48 37.91 25.23 12.32
aKey to samples: A, sunflower/soybean oil blend; B, nonhydrogenated soybean oil; C, liquid/hydrogenated soybean oil blend; D, hydro-genated soybean oil/cottonseed oil blend; E, liquid/hydrohydro-genated corn oil blend; F, hydrohydro-genated corn oil/cottonseed oil blend; I, jojoba oil; J, high-stability oil; L, crude corn oil.
bInduction period in hours.
curve rather than a sharply defined rapid rise.
M a rgarines and mayonnaises may be tested after sep-aration and drying of the oil with anhydrous magne-sium sulfate (MgSO4) prior to analysis.
2. The presence or absence of moisture in the air may have an effect on the oxidative stability of certain antioxidants (References, 5). As noted in Reference 5, when analyzing fats containing moisture-susceptible antioxidants, it is advisable to place a drying tower containing a suitable drying agent in the air train.
3. Precision decreases if the oil coats the side of the reaction tube. If the required airflow causes splattering of the
sam-ple onto the walls of the reaction tube, it may be helpful to decrease the size of the bubbler orifice (References, 6).
REFERENCES
1. deMan, J.M., F. Tie and L. deMan, J. Am. Oil Chem.
Soc. 64:993 (1987).
2. Jebe, T., M.T. Matlock and R.T. Sleeter, J. Am. Oil Chem. 70:1055 (1993).
3. Matlock, M.G., T. Jebe and R.T. Sleeter, U n p u b l i s h e d . 4. Wo e s t e n b u rg, W.J., and J. Zaalberg, Fette Seifen
Anstrichm. 88:53 (1986).
5. M e h l e n b a c h e r, V.C., The Analysis of Fats and Oils,
SAMPLING AND ANALYSIS OF COMMERCIAL FATS AND OILS
Cd 12b-92 • Oil Stability Index
Table 2
Statistical results of interlaboratory study in which samples were analyzed at 130°C.
Samplea
A C D E F I J
Number of labs after removal of
outliers 14 14 14 14 14 11 14
n 51 51 53 54 50 41 49
Outliers 1 0 2 2 3 4 4
Mean, hoursb 2.6 4.4 5.8 3.4 10.9 11.5 16.1
Repeatability, Sr 0.13 0.18 0.18 0.13 0.46 0.39 0.60
RSDr 4.98 4.14 3.13 3.85 4.24 3.44 3.70
r (2.8 ×Sr) 0.36 0.50 0.50 0.36 1.29 1.09 1.01
Reproducibility, SR 0.32 0.49 0.54 0.39 1.09 1.74 1.70
RSDR 12.26 11.04 9.27 11.37 10.01 15.17 10.51
R (2.8 ×SR) 0.90 1.37 1.51 1.09 3.05 4.87 4.76
aKey to samples: A, sunflower/soybean oil blend; C, liquid/hydrogenated soybean oil blend; D, hydrogenated soybean oil/cottonseed oil blend; E, liquid/hydrogenated corn oil blend; F, hydrogenated corn oil/cottonseed oil blend; I, jojoba oil; J, high-stability oil.
bInduction period in hours.
SAMPLING AND ANALYSIS OF COMMERCIAL FATS AND OILS
SAMPLING AND ANALYSIS OF COMMERCIAL FATS AND OILS