Muscle fibre characteristics of a native pig breed longissimus lumborum muscle

Abstract

The aim of this study was to investigate the histochemical parameters of muscle fibre in a Spanish autochthonous pig breed. Due to its fatty meat and high prolificacy, this pork is revered among local consumers. However, the aforementioned breed has become endangered. A total of 11 native pigs (Black Canary Pig) were evaluated. Longissimus lumborum muscle samples were collected in order to evaluate contractile and metabolic characteristics of the varying fibre types by histochemical staining techniques (nicotinamide adenine dinucleotide tetrazolium reductase [NADH-TR] and myosin adenosine triphosphatase activity [mATPase]). Cross-sectional area, diameter and percentage of the fibres were calculated. The oxidative fibre percentage was elevated (33.63%) which in turn can improve the colour, tenderness and juiciness of the meat, hence the glycolytic fibre percentage being moderate (66.37%). The size of three fibre types was higher than the values provided by other pig breeds. A higher percentage of IIB fibres is a disadvantage because these fibres cannot store a high percentage of intrafibrilar fat which could be linked to myopathic process. In conclusion, this autochthonous pig breed could potentially produce red meat and high quality, dry-cured pork products thanks to its remarkably high percentage of oxidative fibres, which naturally possess a greater capacity to store intramuscular fat.

Keywords:

• fibre
• pig
• autochthonous breed
• meat quality

1. Introduction

Currently many autochthonous pig breeds are endangered despite the high demand for their meat among consumers. In recent years, numerous conservation and recovery programs have led to an increase in the number of animals within some of the breeds in question. The meat from these native, unselected breeds has a greater storage capacity of intrafibrilar fat and myoglobin content than commercial meats, which is attributed to a noteworthy increase in the juiciness and colour of the meat (Essén-Gustavsson and Lindholm 1984; Karlsson 1993; Klont et al. 1998; Lee et al. 2012). Furthermore, this heightened capacity could be linked to the high percentage of oxidative fibres, present in autochthonous breeds in comparison to crossbreed pigs that have a considerable number of larger glycolitic fibres because of the intensive genetic selection, aimed principally at improving productive parameters while decreasing meat quality as a consequence (Larzul et al. 1997; Ruusunen and Puolanne 2004). An additional factor associated with poor meat quality is the presence of anomalous fibres (Fiedler et al. 2004; Sobczak et al. 2010), specifically giant fibres. These are characterised as being much larger than normal fibres, oval and circular in shape in a transversal cross-section, and they are located primarily in the periphery of a bundle (Cassens et al. 1969; Karlsson 1993). A higher frequency of these fibres is exhibited by stress-susceptible pig breeds (Klosowska et al. 1984; Fiedler et al. 1999). Giant fibres show a higher staining intensity compared with surrounding normal fibres and the capillary density around these giant fibres is lower when compared with the areas occupied by normal myofibres (Schubert-Schoppmeyer et al. 2008).

One of the endangered European autochthonous pig breeds is the Black Canary pig; a rustic and well-adapted animal reared on traditional farms for personal consumption (García and Capote 1982). The carcass of these pigs has high levels of intramuscular fat and red colour. The purpose of this study was to examine the histochemical and morphometric characterisation of muscle fibres and evaluate the presence of giant fibres in the longissimus lumborum muscle of the Black Canary Pig.

2. Materials and methods

Eleven healthy native pigs from local farms located in the Canary Islands (Spain) were slaughtered at 16.5 months of age, weighing 129 kg on average. Muscle samples were taken from the longissimus lumborum at level of the last rib. Samples were cut and promptly frozen. Serial transverse muscle sections, 5 µm thick, were obtained from each sample using a cryostate at −20°C. Myosin adenosine triphosphatase activity (mATPase) after acid preincubation (Brooke and Kaiser 1970) and metabolic activity by nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) of muscle fibres were detected. Stained samples were photographed and morphometric analysis was carried out using an interactive image analysis system (Image Pro Plus®) where both the cross-sectional area (CSA) and the diameter were measured. Fibre number percentage was obtained from the ratio of the number of each fibre type to the total number of fibres counted. A comparative analysis of the percentage and size of different fibre types was made by Scheffe's test. Analyses were carried out using SPSS® 17.0 computer program.

3. Results and discussion

The histochemical profile of muscle fibres from the non-selected pig breed is similar to other pig breeds (Essén-Gustavsson and Lindholm 1984; Karlsson 1993; Larzul et al. 1997; Serra et al. 1998; Ruusunen and Puolanne 2004; Ryu and Kim 2005). Using mATPase activity after acid preincubation at pH 4.6 (Figure 1A), muscle fibres were named according to Lee et al. (2012) classification as: type I, type IIA and type IIB. NADH-TR reaction (Figure 1B) allowed observe for three fibre types: SO (Slow Oxidative), FOG (Fast Oxidative Glycolitic) and FG (Fast Glycolitic) (Peter et al. 1972). Comparing serial muscle sections stained with both techniques (Figure 1) showed that type I and IIA fibres had oxidative metabolism (SO and FOG) whereas type IIB (FG) fibre was non-oxidative glycolitic. However, 6.15% of IIB fibres had moderate oxidative activity similar to that observed in other studies (Larzul et al. 1997).

Figure 1.  Serial transverse sections of longissimus lumborum muscle of the Black Canary Pig stained for mATPase after acid preincubation (A) and for NADH-TR techniques (B).

Table 1 shows the relative percentage and average size of each fibre type in the longissimus lumborum muscle of the Black Canary Pig. Concerning relative percentage, the number of IIB fibres (72.53%) in this breed is less than other Spanish autochthonous pigs (Serra et al. 1998; Peinado et al. 2009; Vicente 2011) and commercial breeds with a fast growth rate (Karlsson 1993; Larzul et al. 1997; Gil et al. 2008). Interestingly, IIB-type fibre cannot store a high quantity of intrafibrilar fat which could have a considerable adverse impact on the quality of the juiciness of the meat and is related with myopathic processes (Fazarinc et al. 2002; Lefaucheur et al. 2002). The total percentage of oxidative fibres (33.63%) in Black Canary Pig was higher than values found in other autochthonous breeds and crossbreed pigs (Karlsson 1993; Larzul et al. 1997; Serra et al. 1998; Gil et al. 2008; Peinado et al. 2009; Vicente 2011). This is a positive result because oxidative fibres have more capillaries and intrafibrilar fat content than glycolitic fibres, thus improving both the juiciness and the colour of the meat (Cameron et al. 1990). With regards to size, all three fibre types in this autochthonous breed had a larger CSA compared with other breeds (Karlsson 1993; Larzul et al. 1997; Serra et al. 1998; Gil et al. 2008; Peinado et al. 2009; Vicente 2011). A larger size of type IIB has a negative effect on meat tenderness (Maltin et al. 1997). However, this could be offset by the high number and the large size of oxidative fibres.

Table 1. Morphometric parameters of longissimus lumborum muscle fibres.

Variables	Mean	SD	Maximum	Minimum
CSA (µm^2)
Type I	4301.96	1233.34	6180.24	2762.36
Type IIA	3344.17	1152.56	5667.65	1919.34
Type IIB	6096.30	1586.45	8354.6	3889.04
Minimun diameter (µm)
Type I	54.91	10.00	70.22	38.93
Type IIA	45.56	10.68	63.16	30.45
Type IIB	60.77	10.49	77.38	47.69
Relative percentage
Type I	15.46	4.75	24.92	10.02
Type IIA	11.56	3.73	19.02	5.95
Type IIB	72.53	4.41	77.74	65.86

Concerning anomalous fibre, giant fibres were observed in a few analyzed animals (25%) in a low proportion (<3%), therefore its presence should not be linked to poor meat quality. Other authors observed these fibres in a higher percentage (85%) of analysed animals (Handel and Stickland 1987; Fiedler et al. 2004; Vicente 2011).

In conclusion, regarding to the type and size of muscle fibres, the Black Canary Pig is a breed which could produce meat with optimum qualities for which consumers are willing to pay a high price. The results derived from this study will prove useful as a preliminary study for future research on the relationship between fibre types and meat quality parameters.

Acknowledgements

The authors are grateful to Mrs. Elizabeth Fitzpatrick for reviewing the English grammar for this paper.