Stability of oils and fats

K. Warner and N. A. M. EsMn, Methods to Assess Quality and Stability of Oils and Fat-Containing Foods, AOCS Press, Champaign, USA, 1995. 

Although there are numerous publications on the effect of natural and synthetic antioxidants on the stability of oils and fats used as food and feed, until recently relatively little publicly available information was available on the effect of antioxidants on the oxidative stability of biodiesel. One of the earliest studies reporting of the effects of antioxidants on biodiesel was that of Du Plessis et aL (1985), which examined storage stability of sunflower oil methyl esters (SFME) at various temperatures for 90 d. Effects of air temperature, presence of light, addition of TBHQ (see Figure 1.1) and contact with steel were evaluated by analysis of free fatty acid content, PV, kinematic viscosity, anisidine value, and induction period. Addition of TBHQ delayed oxidation of samples stored at moderate temperatures (<30°C). In contrast, under unfavorable (50°C) conditions, TBHQ was ineffective. 

Gas Chromatographic Methods. Gas chromatographic methods may be used for measuring volatile oxidation products. Static headspace, dynamic headspace, or direct injection methods may be employed. Specific aldehydes may be measured as indicators for oxidative stability of oils and fats. Thus, propanal is an and as indicator for stability of omega-3 fatty acids, whereas hexanal is best for following the oxidative stability of omega-6 fatty acids. 

Several temperature-catalyzed stability tests are used in evaluating the oxidative stability of oils and fats. The oldest method is the Schaal oven test (39). It is inexpensive but subjective, because it uses organoleptic and odor intensities in the procedure and still requires days to obtain the result. This approach has been standardized into a recommended practice (AOCS method Cg 5-97). In the active oxygen method (AOM) (39), the development of peroxide is measured with time. As the formation and decomposition of peroxides are dynamic processes, the results obtained by this method do not correlate well to the actual stability of the oils and fats observed under practical application conditions. Other methods that have been based on oxygen absorption are the gravimetric (59) and the headspace oxygen concentration measurement (60, 61). 

AOM or Swift Stability Test. This is a method for measuring the oxidative stability of oils and fats. The induction period is determined from a plot of peroxide value against time when air is bubbled through at 98+0.2°C and is based on the time required to reach a specific peroxide value (e.g. 100). Standard methods are described by AOCS [Cd 12 57(89)]. 

The oxidative stability of oils and fats is an important parameter for the analysis of their quality. It is performed using the Rancimat test equipment or the Omnlon Instrument to measure the changes in the electrical conductivity in water caused by the formation and release of volatile oxidation compounds [13]. 

The methods for testing the oxidative stability of fats and oils in foodstuffs and the assessment of the oxidation state of oils and fats" have been reviewed. A critical review appeared on the FIA methods for edible oils, including various acidity indices . ... 

A widespread method for determining the induction period for autoxidation of oils and fats consists of passing a continuous stream of air through the heated sample and collecting the volatile acids evolved in a water trap, where they are determined on a real time basis. The time plot usually presents a flat appearance for a certain period and then takes off in an accelerated manner. This test is the basis of several national and international standards (e.g. AOCS Cd 12b-92—oil stability index" ISO 6886—accelerated oxidation test for oxidative stability of fats and oils ) and the design of the Rancimat equipment, where the end determination is based on conductivity measurements . In addition to oxidation stability as determined by the Rancimat method and POV, which negatively affects virgin olive oil stability, other nonstandard properties were proposed for better assessment of the quality of this oil, namely LC determination of Vitamin E (21), colorimetric determination of total polar phenols and UVD of total chlorophyll. ... 

Functions as an emulsifier of oils and fats in the manufacture of fluid or paste emulsions for industrial lubricants, consumer products, and textile lubricants and softeners. This waxy lipophilic surfactant is also used as a thickening agent and stabilizer for starch coatings on paper, and as a water dispersible paper size. EMEREST 2640 is also a good lubricant for channeling wire through conduit. 

Other physical properties such as the smoke, flash, and fire points of oils and fats are measures of their thermal stability when heated. The smoke point is important for the oils and fats used for deep-frying. The flash point and fire points are a measure of residual solvent in crude and refined oils and are also a safety requirement. 

Oxidation. Oxidation of oils and fats is due to prolonged exposure to air. By virtue of the low polyunsaturated fatty acid content, palm oil is relatively more stable to oxidative deterioration than the polyunsaturated vegetable oils. However, in the presence of trace metals such as iron and copper, excessive oxidation at the olefin bonds of the oleic and linoleic acids can occur, resulting in rancidity. Highly oxidized crude palm oil is known to have poor bleachability and thus requires more bleaching earth and more severe refining conditions, and the final product will likely be of poor stability (44, 45, 68). 

Peroxide value is the most widely used method (AOCS methods Cd S-53 and Cd 8b-90) to determine the quality of the oil. The primary oxidation products of oils and fats are the hydroperoxides. They can be quantitatively measured by determining the amount of iodine liberated by their reaction with potassium iodide. The peroxide content is expressed in terms of milliequivalents of iodine per kilogram of fat. However, when these hydroperoxides start breaking down to produce off-flavor compounds, correlation to the quality and stability of the oil will no longer be valid. Freshly deodorized oil should have zero peroxide value. In most cases, for the product to have acceptable storage stability, the peroxide value of oils used should be less than 1.0 meq/kg fat at the point of use. 

The modification processes which will be described are hydrogenation, fractionation and interesterification. These processes are used to widen the applicability of oils and fats and also, in the case of hydrogenation, to make products of acceptable oxidative and flavour stability from unstable or relatively unstable raw materials. 

Oils or fats and oil/fat-based foods are readily denatured during a long storage period, because the oil or fat absorbs oxygen from air and is then oxidized. An appraisal of the stability of oils or fats can be made according to its initial oxidation time f i at ambient temperature, which can easily be calculated using thermogravimetry [45]. 

In addition to having the required spedfidty, lipases employed as catalysts for modification of triglycerides must be stable and active under the reaction conditions used. Lipases are usually attached to supports (ie they are immobilised). Catalyst activity and stability depend, therefore, not only on the lipase, but also the support used for its immobilisation. Interesterification reactions are generally run at temperatures up to 70°C with low water availability. Fortunately many immobilised lipases are active and resistant to heat inactivation under conditions of low water availability, but they can be susceptible to inactivation by minor components in oils and fats. If possible, lipases resistant to this type of poisoning should be selected for commercial operations. 

Murray (4I) has recommended that, so far as film stability of larvicidal mixtures is concerned, the oil should have either a small or a very high aromatic content (it should not contain 50% of aromatics unless they are very high boiling—e.g., of lubricating base fraction) consist of a mixture of wide and overlapping cuts of oil and should not contain fats or fatty acids added as spread-aiders. If a spreading agent is desired, resins should be employed. 

---