A comparison of solid-phase microextraction (SPME) with simultaneous distillation–extraction (SDE) for the analysis of volatile compounds in heated beef and sheep fats
Highlights
► Techniques for extracting volatile compounds from sheep and beef fat have been compared. ► First comparison of SPME and SDE for volatile extraction from heated sheep fat. ► Approximately 100 volatile compounds found for both fat types using SPME & SDE. ► Differences in volatile profile from SPME & SDE, which are complementary to one other.
Introduction
Flavour is an important component of the eating quality of meat, and can be regarded as a combination of taste, the sensation perceived by the taste buds, and odour, the sensation perceived by the olfactory organ (Maarse, 1991). In its fresh uncooked state, meat has very little flavour and it is only as a result of cooking that meat develops a flavour, often characteristic of the product. During cooking, a complex series of thermally induced reactions occurs between the non-volatile components of lean and fat tissues which generate a large number of products (Mottram, 1998). While some compounds contribute to the meat's taste, it is mostly the volatile compounds formed from cooking that are responsible for the aroma and which typify the specific flavour associated with the meat. The major precursors of meat flavour are either lipids or water-soluble components that, during cooking, are subject to two sets of reactions: Maillard reactions between amino acids and reducing sugars, and thermal degradation of the lipid content. Mottram (1998) also notes that the lipid-derived volatiles are the compounds primarily responsible for explaining the differences between the volatile profiles of meat species, and are the main contributors to the species-specific flavour.
For sheep, two aromas are associated with the cooked meat of the animal. The first, ‘mutton’ flavour, is related to an animal's age while the second aroma, known as ‘pastoral’ flavour, is related to an animal's diet. Mutton flavour, regarded as the characteristic flavour associated with the cooked meat of older animals, becomes more pronounced as the meat is being cooked (Young & Braggins, 1998). A range of fatty acids in cooked mutton fat were reported to be responsible for this aroma (Wong, Nixon & Johnson, 1975), with focus been given to branched chain fatty acids (BCFAs) as the main contributors to the aroma (Young & Braggins, 1998). The presence of this particular note has been cited as one of the reasons historically that sheepmeat consumption has been low in some markets (Sink & Caporaso, 1977). ‘Pastoral’ flavour can be present in the cooked meat of pasture fed ruminants (Berry et al., 1980) and, for sheep meat, is linked to the presence of 3-methylindole and, to a lesser extent, p-cresol (4-methylphenol, Young, Lane, Priolo, & Fraser, 2003). The presence of a ‘pastoral’ flavour in sheepmeat may not be consequential to Australian consumers, who are unable to distinguish between grilled lamb from animals finished on either pasture or concentrate-based feeding systems (Pethick et al., 2005). However, the presence of this flavour note could cause the product to be less palatable to other lamb consumers, more accustomed to the meat from grain fed sheep (Prescott, Young, & O'Neill, 2001).
In order to characterise ‘pastoral’ flavour in sheep meat, simultaneous distillation and extraction (SDE) has been the principal technique for the extraction of 3-methylindole and p-cresol from sheep fat (Ha and Lindsay, 1990, Ha and Lindsay, 1991, Lane and Fraser, 1999, Osorio et al., 2008, Schreurs et al., 2007, Young et al., 2003) as it is a one-step isolation–concentration process using steam distillation to extract the analytes from the sample (Chaintreau, 2001). While it is a relatively simple extraction technique, it has also been regarded as lengthy and laborious (Prescott et al., 2001, Young and Braggins, 1998).
Recently, solid-phase microextraction (SPME) has become the method of choice for aroma analysis since it offers solvent-free, rapid sampling with low-cost, ease of operation and sensitivity (Sides, Robards, & Helliwell, 2000). SPME integrates several steps of the analytical process, and allows sample extraction and introduction to be performed as a simple process (Stashenko & Martinez, 2004). Due to its simplicity and ease of use, SPME has been widely applied to the measurement of aroma profiles of, and monitoring lipid oxidation in, meat and related products (e.g. ham (Garcia-Esteban, Ansorena, Astiasarán, Martín, & Ruiz, 2004), beef (Giuffrida et al., 2005, Machiels and Istasse, 2003, Moon et al., 2006, Moon and Li-Chan, 2004, Song et al., 2011, Watanabe et al., 2008) and goat (Madruga, Elmore, Dodson, & Mottram, 2009)). SPME has also been used to monitor the volatile profile of cooked lamb (Nieto, Bañón and Garrido, 2011, Nieto, Estrada, Jordán, Garrido and Bañón, 2011, Vasta et al., 2010) and lamb fat (Vasta et al., 2011) as well. The aim of this work was to evaluate the performance of SPME for measuring the volatile profile of heated sheep fat in comparison to that found with SDE. For comparison, we included beef fat in this study, reflecting the interest in the literature in SPME's application to the measurement of volatile compounds in beef and related products.
Section snippets
Materials
Divinylbenzene/Carboxen®/polydimethylsilicone (50/30 μm DVB/Car/PDMS) SPME fibres (Cat. no. 57329-U) were purchased from Supelco, Inc. (Sydney, Australia). The SPME fibre was pre-conditioned at 300 °C for 1 h as per the manufacturer's recommendation.
Fat samples
A commercial beef fat (“Allowrie Prime Beef Dripping”) was purchased from a local retail store. Subcutaneous fat samples, taken from forty 22-month old sheep, were combined to form an aggregate sample, representative of sheep fat. These samples were
Comparison of samples
A total of 100 compounds were detected in the commercially available rendered beef fat sample using both SPME and SDE with GC–MS (Table 1) while, for the sheep fat, a total of 97 compounds was detected using both techniques (Table 2). For the beef fat, 89 compounds were extracted with SPME while 55 compounds were extracted using SDE with 44 compounds common to both techniques. For the sheep fat, 74 and 67 compounds were extracted by SPME and SDE, respectively, with 44 compounds common to both
Conclusions
A comparison has been made between SPME and SDE for extracting volatile compounds from heated beef and sheep fats. As far as we are aware, this represents the first time that such a comparison of these two techniques has been made for measuring the volatile profile of sheep fat. Around 100 compounds (in relatively high abundance) were characterised in the volatile profiles using SDE and SPME. It was not possible to identify every compound by comparison to a commercial mass spectral library.
Acknowledgements
This work was funded by the Cooperative Research Centre for Sheep Industry Innovation, which is gratefully acknowledged. We are also grateful to Dr. C. Wijesundera and Mr. C. Ceccato of CSIRO Food and Nutritional Sciences, for use of the Agilent GC–MS system.
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