Detection & Characterisation of Unwanted Off-Flavors Caused by Rancidity

Detection & Characterisation of Unwanted Off-Flavors caused by Rancidity

DETECTION & CHARACTERISATION OF UNWANTED OFF-FLAVORS CAUSED BY RANCIDITY TROUGH ANALYSIS OF VOLATILES

Håkan Palm

E-mail:

[Presented at the OFIC 2000 Conference, Sept. 4, 2000, Kuala Lumpur]

Abstract:The aim of this study was to find an analytical method, which could serve as a tool for both quantitative and qualitative flavor comparisons between different refining techniques and flavor reversion. The headspace solid-phase micro extraction technique was developed into the ART method. ART is based on detection and characterization of unwanted off-flavors caused by various reactions and absorption of non-process matter. Through sampling with headspace solid phase micro extraction at elevated temperatures and analysis with GC-MS the needed degree of sensitivity and repetition is being reached. ART comparisons made between traditional TD (Tray deodorizers) and SC (SoftColumn deodorizers), working with the same type of oils, soybean & rapeseed oil, and under similar conditions, show clearly up differences between the two refining techniques. The SC with its VHE-Economizer gives RBD oil, which have a zero or minimum content of volatiles. The TD RBD oils tested had a distinctly higher content of initial total volatile flavor compounds and in general scored lower panel ratings. The method shows good correlation with taste panel results and the results presented as chromatograms can be used for “scanning” changes in volatile content of RBD oils so that a taste panel can be put to evaluate only the cases that show abnormal results.

[Key Words: ART, volatiles, flavour, vegetable oil, rancidity]

Malaysian Oil Science and Technology 2002 Vol. 11 No. 11

Detection & Characterisation of Unwanted Off-Flavors caused by Rancidity

Introduction

Off-flavours in vegetable oils & other foods are becoming increasingly important for three major reasons. There is a commercial pressure for manufacturers to produce vegetable oils and fats at a smaller number of centralized locations to provide economies of scale. This in turn necessitates longer distribution runs, often via regional warehouses and thus requires longer shelf lives. In addition public awareness of nutritional issues is creating a demand for oils with higher levels of polyunsaturated fatty acids, and reduced levels of food additives such as synthetic antioxidants. On top of it all there is an increasing part of the market that demands vegetable oils with a completely neutral or bland taste for use in frying and baking.

When considering off-flavors in vegetable oils there are at least four types of rancid off-flavors that need to be taken into consideration, namely hydrolytic-, oxidative-, enzymatic- and ketonic- rancidity. Apart from different pathways to degradation of fat molecules rancid flavors can also come from absorption or contamination of the oil since a lot of rather strong tasting subjects have good solubility in oil.

Volatile Products and Their Flavor Significance

In general many lipids are important precursors for pleasant flavours in agricultural and animal food products. Plant lipoxygenase is important in the production of hydroperoxides, which breaks down into volatile secondary products with a variety of flavours. These volatiles are for instance responsible for the taste of bananas, grape, apple and mushrooms. One example is 1-octen-3-ol, with linolenic acid as precursor, which is believed to be main responsible for the taste of mushrooms1. Important components in tomato flavour are cis-3-hexanol, cis-3-hexenal and trans-2-hexanol which all have linolenic acid as precursor. It is no exaggeration to say that our daily lives would be a lot poorer without the flavour contribution of volatile products. Apart from all these wanted flavours, already mentioned, many of the volatiles serve as signal substances warning us from eating rancid food. Everyone that for instance has tried to taste highly oxidised vegetable oil, will think once or twice before doing it again. The types of flavour defects caused by volatile compounds depend on complex interactions, their concentrations and the medium in which they are tested. It is well known that the human palate is very discriminating. When as few as 1 in a 1000 double bonds in vegetable oil has reacted with oxygen it is already too late. Table 1 gives approximate threshold values for some common classes of volatile compounds.

Flavour Evaluations

The natural and most frequently used method to determine rancidity is to taste or sniff the oil. Even if trained taste panels are used the method is highly subjective, and another panel in another part of the world will not likely give a certain sample the same rating. Differences in food culture, air humidity etc play a vital role and are in high grade affecting the rating of the sample. The subjectivity in tasting has given rise to development of ART which is meant to determine the “flavour quality” instead of concentrating on the actual flavour experienced by a trained taste panel. One of the major benefits with a non-subjective flavour analysis such as ART is that it can be used for global comparisons between different refining plants and refining techniques through characterisation and quantification of volatile components. It can also be used

Malaysian Oil Science and Technology 2002 Vol. 11 No. 11

Detection & Characterisation of Unwanted Off-Flavors caused by Rancidity

Triglycerides

FFA and Hydroperoxides

Secondary and Tertiary Reaction Products

Methyl ketones, lactones, esters, etc.

Secondary and Tertiary Reaction Products

Saturated, unsaturated, di- and epoxy-aldehydes, ketones, lactones,

furans, mono- and di-basic, oxy- and hydroxy- acids,

saturated- and unsaturated hydrocarbons etc.

Figure 1. Overview of Reaction Pathways to Off-flavours

Malaysian Oil Science and Technology 2002 Vol. 11 No. 11

Detection & Characterisation of Unwanted Off-Flavors caused by Rancidity

for determining the cause of rancidity since the actual flavour components can be identified.

Table 1. Threshold Values of Different Volatile

Compound Classes2

Experimental

Materials

Various commercially available low erucic acid rape- and soybean oils were tested. All oils were stored at 2oC in 250 ml clear glass bottles under nitrogen blanketing until sensory and ART analyses were performed. The oils were aged at 60oC by a modification of the Schaal oven test3, using 10-ml clear glass vials filled 2/3 with oil. It was previously estimated that 4 days of storage at 60oC was equivalent to 4 months at room temperature4.

Sample Preparation

Before sampling the SPME needle was heated to 200 oC for 10 min in the injector of a specially prepared GC with helium as carrier gas. The heating of the needle was made to desorb all material attached to the fibre and thereby prevent carry-over. Between runs the septum’s and vials were washed in 99.5 % ethanol and were left to dry in an oven at 110 oC. 1 ml of the oil to be tested was injected to a 4–ml vial fitted with a magnetic stirrer. The vial was immediately sealed with a Varian GC-Septum and heated under stirring to 50 oC in a special device. The sample was then left to reach equilibrium for 30 min. In all analyses a CW/PDMS 65 m SPME fibre was used.

Headspace Solid Phase Micro Extraction Sampling

After desorption the SPME needle was injected to the headspace of the vial and was left to reach equilibrium for 45 min. A special holding device was used to make sure that the position of the needle inside the vial was the same at all samplings. The holding device thereby insured that the needle never got in contact with the oil sample.

GC-MS Volatile Analysis

The GC-MS apparatus used was a Varian 1200 gas chromatograph, equipped with a 400 m SE-30 (glass capillary column 0.6 mm id., 6 mg/cc) linked to an MS-30 mass spectrometer (A.E.I, Manchester) via a membrane separator. Operating conditions for the GC-MS coupling were: temperature programming from 25 – 100oC at 2o/min and 100 – 200oC at 5o/min and hold 10 min at 200oC; 6 ml He/min; interconnecting lines 200oC; ion source pressure 10-6 mm Hg; ion source temperature 200oC; filament voltage 70 V; trap current 300 A; scan speed 3 sec per decade. The volatiles were analysed by direct injection of the SPME needle into the inlet liner of the GC injector.

Sensory Evaluation

A 6-member well-trained and experienced oil panel at Karlshamns Oils & Fats AB evaluated the oils for flavour on the basis of a 1-7 scoring scale with 7 as bland and 1 as very strong off-flavour. All tests were performed in Karlshamns panel testing room and according to their normal standard procedure.

Figure 2. Head Space Sampling in 10 ml Vial

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Detection & Characterisation of Unwanted Off-Flavors caused by Rancidity

Table 2. Detected Volatile Flavour Compounds and Their Threshold Values

UFA precursor

Flavour description

UFA = unsaturated fatty acid. References are private communications: a Badings (1970); b Meijboom (1964); cStark and Forss (1962); d Swoboda and Peers (1977); e Keppler (1977); f Meijboom and Jongenotter (1981);

g Evans et. al. (1971); h Ho et. al. (1978)

Table 3. Detected Volatile Flavour Compounds and Their Threshold Values

UFA Precursor

References are private communications: a Badings (1970); b Meijboom (1964); c Stark and Forss (1962); d Swoboda and Peers (1977); e Keppler (1977); f Meijboom and Jongenotter (1981); g Evans et. al. (1971); h Ho et. al. (1978)

Table 4. Flavour Scoresa of Initial and Aged Soybean Oils

a Scores based on 1-7 scale with 7 as bland and 1 as very strong.

Figure 3. ART Analyses of Total Volatiles in Initial and Aged SBO

Table 5. Comparison of Initial total Volatile Content Between TD and SC Oils

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Detection & Characterisation of Unwanted Off-Flavors caused by Rancidity

Results and Discussion

Detected and identified volatile flavour compounds from soybean and rapeseed oils are shown in table 2 and 3. They are all identified by comparison with MS library. All of the found compounds are typical oxidation products of triglycerides, which can be expected to be present.

Flavour evaluation of fresh and aged SBO samples showed distinctive patterns of flavour deterioration (Table 4). Evaluation of integrator counts for both total volatiles and individual MS peaks (Fig. 3) showed good correlation with panel scores.

Fig. 3 shows on an increased rate of oxidation after a induction period of approximately 9 days which shows good correlation with results from direct GC and GC headspace analyses reported in literature5.

A comparison was made between commercially available TD and SC deodorised oils (Table 5). The SC with its VHE-Economizer gives RBD oils, which have a non-or minimum content of volatiles. The TD RBD oils tested had a distinctly higher initial content of volatile flavor compounds and in general scored lower panel ratings. The reason for these differences can probably be found in the gentle and highly efficient SC treatment of the oil together with low air leakage built plants.

This study shows that ART can estimate the flavour quality readily and reliably. It also points out that it can be used for measurement of induction periods for vegetable oils and thereby may be a tool for comparisons between relative differences in flavour stability. As it produces analytical fingerprints in terms of chromatograms it can be used for surveying RBD oil production. One of the largest benefits of the method is that it can be used for field sampling.

Acknowledgment

Anders Hillstrom has helped and supported in all ways trough the hole period of the study. Anna Karin Borg Karlsson has helped with interpretation of mass spectra. Margareta Emilsson has arranged all panel evaluations.

References

1. Buttery GR, Flavor Chemistry Of Lipid Foods, ed. Min BD and Smouse HT (eds), AOCS Honored Scientist Series, The Stephen S. Chang Symposium, Ch. 8.
2. Emilsson M, Fettoxidation, Karlshamns Oil & Fats AB, pp. 34, Margareta Emilsson, private communication.
3. Joyner NT and McIntyre JE (1938). Oil and Soap 15: 184.
4. Evans CD, List GR, Moser HA and Cowan JC (1973). JAOCS 50: 218.
5. Warner K and Frankel EN (1985). JAOCS 62(1): 101.

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