Heptadecanoic acid

Heptadecanoic acid as an indicator of BCFA content in sheep fat

Abstract

Branched chain fatty acids (BCFAs) are associated with ‘mutton flavour’ found in cooked sheep meat aroma. Three BCFAs, 4-methyloctanoic (MOA), 4-ethyloctanoic (EOA) and 4-methylnonanoic (MNA) acids, have been implicated as the main compounds responsible for ‘mutton flavour’. Animal age can also contribute to increasing BCFA concentration. Heptadecanoic acid (C17:0 FA) also increases with animal age in sheep. Using published data, a linear association has been found to exist between C17:0 FA with MOA and MNA in sheep fat, with C17:0 FA increasing proportionally with these two compounds. No association was found between C17:0 FA and EOA. As C17:0 FA is present in sheep fat in relatively higher proportions compared to BCFAs, it has potential to be used as a proXy for MOA and MOA as well as ‘mutton flavour’.

1. Introduction

Branched chain fatty acids (BCFAs) have been associated with the ‘mutton flavour’ found in cooked sheep meat aroma, particularly from older animals (Watkins et al., 2010). Their presence can result in lower consumer acceptance of the meat product (Watkins et al., 2014) with three BCFAs, 4-methyloctanoic (MOA), 4-ethyloctanoic (EOA) and 4-methylnonanoic (MNA) acids, implicated as the main compounds re- sponsible for ‘mutton flavour’ (Watkins et al., 2010). Animal age can also contribute to increasing BCFA concentration, particularly EOA (Watkins et al., 2010). Another fatty acid, heptadecanoic acid (C17:0 FA) also present in sheep fat, is reported to increase with animal age in
sheep (Oriani et al., 2005; Young et al., 2006). Typically, C17:0 FA is present in sheep fat in higher concentrations (%) compared to BCFAs (< 1 μg g−1), and less sensitive analytical methods are required to measure C17:0 FA compared to BCFAs. Thus, if the concentration of C17:0 FA is proportionally related to the BCFA content, a less sensitive and simpler analytical method could be used to measure this compound and act as a BCFA proXy. Therefore, we sought to determine if a re- lationship existed between C17:0 FA and BCFAs in sheep fat. It should be noted that most of the fatty acids present in sheep fat are bound, present as triacylglycerols. In this form, the fatty acids are not volatile and thus do not affect the odor. It is only when the fatty acids are re- leased into the free form that they become appreciably volatile and odor-active. 2. Materials and methods The data for this short communication has been published elsewhere in two separate studies. The first study was published in 2014 (Watkins et al., 2014), and was aimed at understanding the contribu- tion of the three BCFAs and compounds believed to be responsible for ‘pastoral flavour’ in sheep meat (3-methylindole and 4-methylphenol) on consumer sensory scores of grilled lamb meat. In this study, the free BCFAs were extracted molten sheep fat using sweep co-distillation and measured as trimethyl (TMS) esters using an ion-trap gas chromato- graph mass spectrometer (GC–MS). C17:0 FA was determined as relative ratios of the C17:0 FA TMS ester to the internal standard (C11:0 FA TMS ester). It should be noted that this method measured only BCFAs present in the free form. The second study, published in 2016 (Frank et al., 2016), sought to understand how forage brassicas (Bras- sica napus) and lucerne (alfalfa; Medicago sativa) used as ruminant feeds could impact on the acceptance of cooked meat by Chinese and non- Chinese Australian consumers. In this study, the sheep fat was directly transesterified using acidic methanol with the BCFA methyl esters (MEs) measured using a GC-quadrupole MS and the C17:0 FAME measured using GC with a flame ionization detector (GC-FID), assuming a relative response factor of 1. The FID was used with C17:0 FAME as, for this compound, the MS response was non-linear due to the higher concentration of this compound compared to the BCFAs. In the latter study, the total BCFA content (i.e. both free and in the bound form as triacylglycerols) were measured. Log10 transformation was used to normalize the data. Statistical analysis and data visualization was per- formed using R (R Core Team, 2018). 3. Results and discussion Fig. 1 shows the degree of association between each BCFA with C17.0 FA. For the 2014 dataset, there is a positive trend with both MOA and MNA with each increasing with increasing C17:0 FA content (R2 = 0.41 and 0.36, respectively, P < .001) while no apparent re- lationship between EOA and C17:0 (R2 = 0.0263, P > .05). The re- lationship between MOA and MNA with C17:0 FA is more evident in the second dataset with a higher degree of correlation between these fatty acids (R2 = 0.808 and 0.861, respectively, P < .001). Some degree of association is apparent between EOA and C17:0 as well (R2 = 0.66, P < .001). Fig. 1. Scatterplots and correlations for log10 transformed responses of heptadecanoic (C17:0), 4-ethyloctanoic (EOA), 4-methylnonanoic (MNA) and 4-methy- loctanoic (MOA) acids for data published in (a) 2014 and (b) 2016 (⁎⁎⁎ denotes statistical significance for correlation, P < .001). Increasing proportions of C17:0 FA (in sheep fat) with animal age has been reported to be consistent with increasing BCFA content. This was first noted by Young et al. (2006) who reported that C17:0 FA increased with age, and mentioned that this may be consistent with increasing BCFA content. The work of the late C.B. Johnson was cited by these authors, as a personal communication, which stated that branched C15:0 and C17:0 acids were good predictors for MOA. By extension, the latter compounds could also be good predictors for MNA, based on the correlation that exists between these two BCFAs. Else- where, Oriani et al. (2005) has reported that C17:0 FA increases with animal age. The relationship between the BCFAs and C17:0 FA with animal age was not investigated in this study but, based on previous work, it is feasible that they could be related to each other. This is, of course, speculative and further work would be needed to substantiate this. As noted in the Introduction, C17:0 FA is present in sheep fat in much higher concentrations (%) compared in BCFAs (< 1 μg g−1). Despite the order of magnitude differences in concentration, a linear response was measured, especially when comparing total BCFAs and total C17:0 FA. Analytically, the measurement of FAs in higher con- centrations is relatively easier than BCFAs as simpler techniques are required for the analysis (GC-FID vs GC–MS). In the cases where high sample throughput is required, using C17:0 FA as an indicator for BCFA content would be advantageous since higher sample throughput could be achieved with simpler analyses. It should be noted that the re- lationships between total and free BCFAs and C17:0 are not known. A better understanding of these relationships would be useful to predict the odor impacts in sheep meat. 4. Conclusion A linear association was found to exist between C17:0 FA with MOA and MNA in sheep fat, with C17:0 FA increasing proportionally with the two BCFAs. No positive association was found to exist between EOA and C17:0. The positive relationship between C17:0 FA with MOA and MNA suggests that this FA could be used as a marker of these two BCFAs. As C17:0 FA is present in sheep fat in relatively higher proportions to BCFAs, simpler extraction protocols and less sensitive analytical techniques could be employed, and C17:0 FA content could be used as a proXy for BCFAs (but not EOA) and ‘mutton’ flavour, particularly in high sample throughput studies.