By Dr Darryl D’Souza Australian Pork Limited
The last twenty years, in keeping with increased consumer health awareness, has seen the overall fat content of Australian pork cuts such as loin chops reduced by 70% (Muller et al, 2009). The production of leaner and heavier pigs has had a detrimental effect on intramuscular fat (IMF) levels and the perception is that Australian pork is now tougher, less moist and has reduced flavour.
Channon et al., (2001) found that the average IMF content of Australian pork sourced from five large abattoirs was 0.98 ± 0.50%, with 74% of pork loins having IMF levels ranging from 0.5 – 1.4%. So did the decline in IMF or marbling have a detrimental effect on pork quality or has the decline in pork quality been due to other factors?
Higher levels of IMF or marbling in pork have been shown to positively influence the juiciness, tenderness and flavour of pork (Wood, 1993, Table 1). For the most part the effect of IMF on sensory pork quality has been as a consequence of a breed effect. Fast growing “white” European pig breeds (Large White, Landrace, Yorkshire) have lower levels of marbling compared to the darker skinned breeds, such as Duroc (Wood, 1993; NPPC, 1995) and Berkshire (NPPC, 1995). The Meat and Livestock Commission, UK, evaluated the Duroc breed and found that tenderness of pork was improved in pigs with Duroc gene proportions above 50%, while juiciness increased when the proportion of Duroc genes was increased to 75% (MLC, 1992).
Similar improvements in juiciness, tenderness and flavour were also reported by the National Genetic Evaluation Programme (NPPC, 1995). D’Souza and Mullan (2001), reported that loins from Large White x Landrace x Duroc crossbred pigs with a high proportion of Duroc genes (50%) had higher IMF levels and better juiciness, tenderness and flavour compared to pork from Large White x Landrace x Duroc crossbred pigs with a low proportion of Duroc genes (<25%).
A number of studies have also investigated the effect of diet on IMF and subsequent pork quality. Alonso et al., (2010) conducted a study to determine the effect of protein content of the diet (either 17% and 14.9% crude protein) on meat quality attributes of pork from entire male pigs. Although the diets resulted in different percentages of intramuscular fat (1.76 vs. 2.63%, respectively) in loin muscles, pork from pigs fed the lower protein diet produced pork that was more tender, less fibrous and juicier than pigs fed the high protein diet, which may reflect the higher intramuscular fat content.
Teye et al.,(2006) showed that feeding a 18% protein diet to pigs increased total lipid to 2.8% in the longissimus muscle compared to 1.7% in pigs fed a 20% protein diet and tenderness and juiciness scores were also improved. D’Souza et al, (2008) also reported that feeding pigs a 15% reduced lysine:energy diet significantly increased IMF levels by 1.4% compared to pigs on the control diet and improved the sensory quality of pork.
Intramuscular fat typically constitutes 0.5 to 2.5% of muscle wet weight in the pork longissimus muscle and is related to carcass fat content. Devol et al., (1988) suggested a threshold value for IMF content of 2.5 – 3.0% for optimum tenderness and reported that of all the carcass and muscle characteristics evaluated (including shear force, tenderness, flavour, juiciness, firmness, colour, IMF and connective tissue content), IMF content had the best relationship with tenderness. However, Fernandez et al., (1999) stated that IMF levels above 3.5% were associated with a high rejection score by consumers due to negative effects of texture, taste and visual appearance.
Positive effects on sensory attributes of pork of IMF content in excess of 2% have been reported (Bejerholm and Barton-Gade, 1986; Touraille et al, 1989; Fernandez et al, 1999). Similarly, the minimum requirement for IMF in order to achieve acceptable consumer satisfaction for grilling “red meat” cuts, such as beef and lamb, has been quoted at
3 – 4% by Savell and Cross (1986) and at 5% for sheepmeat (Hopkins et al. 2006).
The positive relationship reported above is by no means clear-cut, with many studies reporting no effect of IMF on the sensory quality of pork (Goransson et al., 1992).
Rincker et al. (2008) reported that the percentage of extractable lipid did not correlate strongly with perceived tenderness, juiciness or pork flavour (as assessed by trained sensory panellists) for pork loins from a similar genetic background and management. Results from the consumer portion of the study indicated that IMF content had limited effects on perceived tenderness, juiciness, pork flavour, and oiliness; some significant differences (P < 0.05) were detectable, but they were numerically small.
Rincker et al (2008) proposed that many of the studies yielding positive results between IMF and palatability have used mixed-genotype populations in which the Duroc breed was potentially present. Consequently, any study including this breed along with other genotypes is more likely to find a favourable relationship between IMF and palatability. Studies involving mixed-breed populations risk confounding the effects of marbling with a number of other factors that may influence sensory properties. Similarly, van Laack et al (2001) reported that the relationship between objective tenderness and IMF was dependent on genetic type and hence increasing IMF levels did not necessarily improve pork tenderness.
Rincker et al. (2008), raised a salient point where other factors may confound the relationship between IMF and sensory pork quality. However, a number of studies have reported positive effects of IMF on sensory quality in cross bred pigs where the Duroc content was extremely low <12.5% or less (D’Souza et al., 2008) as indicated by the IMF content, and where there were no confounding factors such as incidence of PSE. In the case of Australia, where IMF levels of loins have been shown to be <1%, any increase in IMF has resulted in significant sensory quality improvements.
Perhaps the relationship between IMF and sensory quality is more pronounced at lower levels rather than where IMF levels are >2%. Fortin et al. (2005) compared the effect of various levels of intramuscular fat (IMF: <1%, 1.0–1.49%, 1.5–1.99%, 2.0–2.49% and 2.5–3.0%) produced from Canadian pigs on the eating attributes of pork loin and reported that increasing the level of IMF past 1.5% did not change (P < 0.05) the panellists scores for softness and initial tenderness.
Average shear force was higher (P < 0.05) when IMF% levels were less than 1%, but did not change past 1.0% IMF (P > 0.05). Fortin et al. (2005) proposed that the threshold level of IMF that will ensure a pleasing eating experience is 1.5% IMF.
In the case of beef, whilst marbling is generally an integral part of any grading scheme, the literature suggests that it has only a minor association with palatability. Dikeman (1987) concluded that marbling accounted for only 10 – 15% of the variance in palatability.
Thompson (2002) reported that in the case of the MSA model, the contribution of marbling to palatability (for beef cooked to a medium degree of doneness) was not high, but the effect of IMF tended to be additive to other chiller assessment, processing and production effects. This small increase in palatability score was sufficient to lift the product into a higher quality grade.
Channon et al (2012) conducted an extensive literature review (as part of her ongoing PhD studies) that included compilation of a data file comprising 320 scientific references and a total of 2 597 data rows to determine the key pathway parameters influencing the sensory quality attributes of pork. A random effects meta-regression, weighted to account for both within-study variances of treat- ment effects and the residual between study heterogeneity, was conducted to determine whether different pathway factors influenced tenderness, juiciness and flavour of pork. The resulting analyses indicate that there was a significant effect of IMF on tenderness and juiciness when the IMF data was analysed as actual values reported. However, when the data was analysed as IMF category (1.0-1.49%, 1.5-1.99%, 2.0-2.49%, 2.5-2.99%, >3%) there was a significant effect of IMF category on juiciness only.
So in asking the question “does IMF improve the sensory quality particularly tenderness of pork”, the answer is yes but with some qualifications. Given, that the relationship between IMF and sensory quality appears to vary across studies, it is clear that the role of IMF (for which marbling is a visual estimation) in the sensory quality of pork is far from understood. In reviewing the literature, it would appear that the impact of IMF from “leaner pigs” on sensory quality of pork is more pronounced. Intramuscular fat may also be additive to other animal, pre- and post-slaughter factors which can be sufficient to lift the palatability score for pork into a higher quality grade. So, in conclusion, it is suggested that IMF thresholds of pork loin of 2.5% to 3% may be too high and unlikely to significantly increase sensory quality scores, and an IMF target of 1.5% may be more appropriate.
References available upon request.
IMS Newsletter, 31 May 2013