Different methods to chemically evaluate lipid oxidation are described in Chapter 8 and this chapter will only discuss different methods to evaluate the impact of lipid oxidation on the sensory properties of foods. However, correlations between data from selected chemical and instrumental methods and sensory analysis will also be touched upon.
6.2.1 Sensory methods
Sensory evaluation of an oil or other food stuff is the most reliable method to determine whether the product is acceptable for consumption, at least when it concerns evaluation of off-flavour formation due to lipid oxidation. Therefore, the results obtained from chemical/instrumental methods should always be compared to sensory data at some stage. Sensory evaluations are time con-suming and costly and sometimes the results obtained are not reproducible.
Therefore, there is a need to develop methodologies that can replace sensory evaluations. Today, the current recommendation is to use sensitive chemical and instrumental methods to complement the sensory analyses (Frankel 2005).
Sensory evaluations can be performed by different types of sensory panels: the expert panel, the analytical panel and the consumer panel (Meilgaard et al.
1991). The expert panel consists of a relatively low number of trained panel members (3±5) and is mainly used for routine control (Meilgaard et al. 1991).
The analytical panel consists of a larger number of assessors (at least five persons), which are trained to perform different types of tests such as difference tests (e.g., triangle tests) and quantitative descriptive tests such as flavour profiling. The latter method includes a rating of the different sensory attributes evaluated by the panel. This kind of test is often used in product development before consumer tests are performed. In lipid oxidation studies, the quantitative descriptive test can be used to correlate certain important off-flavours with chemical data as will be discussed later. Sensory attributes such as bland, green, cucumber, rancid and painty are used to describe the progress of oxidation in vegetable oils or products thereof, whereas attributes such as train oil, fishy and metallic are often used in foods rich in long chain omega-3 lipids. The intensity of a given attribute is ranked on a continuous intensity scale, e.g. from 0 to 15. It is generally not recommended to use the analytical panel to evaluate the Understanding and reducing oxidative flavour deterioration in foods 123
acceptability of the product as the assessors are trained and use their senses in a different way than the normal consumer. The consumer panel consists of a large number of untrained consumers, who are believed to be potential customers of a product. This type of test is used to evaluate the preference and acceptance of the consumers for a product or a range of products.
Traditionally, sensory data have been calculated by determining the overall mean scores for intensity or quality by dividing the total points by the number of panelists for each session. Subsequently, the significance of the overall mean scores have been calculated statistically by two-way analysis of variance (ANOVA). This methodology does not, however, take into account the differ-ences in the sensory score levels between the different assessors that will always exist no matter how much the panel is trained. This is particularly a problem when sensory evaluations on the same product are carried out in a storage experiment over several weeks. In this case, it will inevitable happen that one or more assessors are not present at all the sensory evaluations. If these assessors are inclined to use, e.g. the higher end of the intensity scale, the absence of the assessors will significantly reduce the overall mean score for all samples on that day. Recent developments in multivariate data analytical techniques make it possible to project away these differences between the assessor score levels and thereby only the difference between samples are evaluated (Martens and Martens 1986). The projection is done by making a so-called ANOVA partial least squares regression using the assessor design variables as X-data and sensory data as the Y-data. The residuals from this model can then be used for further calculations.
6.2.2 Accelerated methods versus traditional storage experiments Evaluation of the oxidative flavour deterioration of neat lipids can either be carried out at ambient temperatures or at elevated temperatures to accelerate oxidation. When it comes to emulsified foods such as dressing and mayonnaise it is often difficult to increase temperature much above 30 ëC as the emulsion may break at higher temperatures. For other food products such as raw meat and fish products it may be difficult to increase temperature to higher than 5 ëC as higher temperatures would lead to microbial spoilage. For neat oils and emulsions storage experiments at ambient temperatures may be accelerated by light or by the addition of metals such as iron or copper, whereas for fish muscle models oxidation is accelerated by addition of heme iron (Jacobsen et al. 2008). Storage experiments accelerated by light should only be carried out if the commercial product under real circumstances is stored under light. This is due to the fact that light induced oxidation will give rise to other types of secondary oxidation products and thereby other off-flavours than those produced by autoxidation.
6.2.3 Correlations between sensory data and data from chemical or instrumental measurements of lipid oxidation
Measurement of lipid oxidation can be carried out by a wide range of methods such as peroxide value (PV), anisidine value (AV), thiobarbituric acids reacting 124 Oxidation in foods and beverages and antioxidant applications
substances (TBARS) as well as instrumental methods such as HPLC, GC-MS, NIR, FTIR and DSC. In lipid oxidation studies on meat products there seems to be a fairly good correlation between TBARS and sensory data, although in many cases no attempts have been made to statistically evaluate the correlations (Eckert et al. 1997; Murano et al. 1998; Winne and Dirinck 1997). However, newer data indicate that even in meat, TBARS may be an unreliable method (see further discussion in Section 6.3.6 below; Summo et al. 2010).
PV and AV are often used to evaluate the lipid oxidation in oils and the author's experience is that AV, at least to some extent, correlates with sensory data in oxidized oils. In contrast, findings from fish oil enriched spreads clearly showed that the correlation between sensory and PV and AV data was very poor (Jacobsen 1999b) (Fig. 6.1(a) and (b)). It is not surprising that PV did not correlate with sensory data since lipid hydroperoxides are not responsible for off-flavour formation. Moreover, since lipid hydroperoxides are broken down to secondary oxidation products during the later stages of oxidation a low PV does not necessarily indicate that the lipids are not oxidized. However, it was surprising that AV in some samples was high despite a high flavour acceptability score. This finding could indicate that the sensitivity and specificity of this method is too low to detect the changes in concentrations of volatiles responsible for the off-flavour formation. Therefore, AV cannot be recommended for measurement of lipid oxidation in complex food systems.
Good correlations between sensory data and data from headspace GC analysis on a range of different oxidized products such as boiled potatoes (Blanda et al.
2010), fish (Milo and Grosch 1995), fish oil enriched milk (Venkateshwarlu et al.
2004a), mayonnaise and dressing (Hartvigsen et al. 2000; Let et al. 2007) have been extensively reported in the literature and headspace GC analysis can therefore be recommended as one of the best methods to chemically assess
Fig. 6.1 (a) Peroxide values, and (b) Anisidine values versus overall flavour acceptability obtained from storage experiment with fish oil enriched spreads. Reprinted from Jacobsen, C. (1999b). Sensory impact of lipid oxidation in complex food systems.
Fett/Lipid 12, 484±492 with permission from Wiley.
Understanding and reducing oxidative flavour deterioration in foods 125
oxidative flavour deterioration. Different headspace methods are available for collection of the volatile oxidation products including static headspace, dynamic headspace and solid phase microextraction. It is beyond the scope of this chapter to discuss these methods in further details, but the reader may refer to Chapter 8 for more information.