The Quality Traits of Pork Belly and Impact Factors of Quality

Animal sex affects various carcass properties such as weight, lean meat yield, and fat content, which can change the quality of pork belly. Several studies have investigated the effects of sex on pork belly quality (Table 1). Male pigs are generally castrated to prevent boar taint caused by androstenone and skatole and to reduce aggressive and sexual behavior, thereby improving growth performance (Prunier et al., 2006). Barrows (castrated male pig) are generally heavier than gilts, increasing the proportion of pork belly in the carcass (Bahelka et al., 2011; Correa et al., 2008; Duziński et al., 2015; Lee et al., 2013; Overholt et al., 2016; Stupka et al., 2004). Barrows also tend to have a higher fat deposition in the pork belly, whereas gilts have a higher proportion of lean meat in the belly (Bahelka et al., 2011; Stupka et al., 2004). These results were reported because barrows require less energy to deposition lean tissue than gilts and excess energy is accumulated as fat (Overholt et al., 2016). Additionally, the fatty acid composition of belly fat can be affected by sex (Correa et al., 2008; Lee et al., 2013). Correa et al. (2008) found that compared to barrows, gilts had lower SFA content and higher linoleic acid and PUFA contents in belly fat. Lee et al. (2013) reported similar results, with barrows having a higher palmitic acid content, the SFA, and lower linoleic acid, the PUFA in belly fat than that in gilts. Overall, compared to barrows, gilts have a higher degree of belly fat unsaturation and a higher IV. Therefore, barrows produce firmer pork bellies with the higher proportion of SFA than the pork bellies of gilts.

Immunocastration is emerging as an alternative to traditional surgical castration, and the effects of different castration methods on pork belly quality have been widely studied (Costa e silva et al., 2017; Font-i-Furnols et al., 2023; Jeong et al., 2011; Kyle et al., 2014; Lowe et al., 2016; Tavárez et al., 2014). Most studies indicate that immunocastration produces softer pork belly with lower fat content, higher PUFA content, and higher IV than pork belly obtained from surgically castrated pigs (Costa e silva et al., 2017; Font-i-Furnols et al., 2023; Kyle et al., 2014; Lowe et al., 2016). Jeong et al. (2011) reported the sensory properties of pork belly based on different castration methods. They reported that pork belly obtained from immunocastrated pigs had higher visual evaluation traits than the pork belly obtained from surgically castrated pig, although the former did not significantly affect taste, tenderness, and overall acceptability (Jeong et al., 2011). In addition, there have been studies on examining the effects of supplementing ractopamine hydrochloride (RAC) in combination with immunological castration (Costa e silva et al., 2017; Kyle et al., 2014; Lowe et al., 2016). RAC, a β-adrenergic agonist, is known to improve feed efficiency and increase lean meat content (Leick et al., 2010). Kyle et al. (2014) reported that immunologically castrated barrows fed a diet supplemented with RAC produced a wider and softer pork belly with a significantly higher percentage of PUFA in belly fat. In contrast, Costa e silva et al. (2017) and Lowe et al. (2016) found no significant effects of RAC supplementation on pork belly quality in immunocastrated barrows. Harris et al. (2018) and Tavárez et al. (2014) reported changes in pork belly quality based on time interval between administration of the second dose of Improvest (GnRF analog diphtheria toxoid conjugate) for immunocastration and slaughter. In a study by Harris et al. (2018), the thickness of pork belly increased as the time interval increased, and the PUFA percentage and IV increased as the time interval decreased. Therefore, additional studies are necessary to determine the optimal combination of immunocastration with other treatments.

Genetic factors, including genotype and breed, are key determinants of meat quality. Understanding genetic factors is important to improve the quality of pork belly effectively. Studies have investigated the heritability and genetic correlations between pork belly quality traits (Hermesch, 2008; Kang et al., 2015; Lee et al., 2023a). In their study, the heritability of traits such as belly weight, dimensions, fat content, and muscle area of pork belly had a moderate heritability ranging from 0.2 to 0.5 (Hermesch, 2008; Kang et al., 2015; Lee et al., 2023a). In particular, studies by Kang et al. (2015) and Lee et al. (2023a) identified genetic parameters of several individual muscles of pork belly. These estimated genetic parameters suggest that pork belly traits can be improved through genetic selection. Additionally, Lee et al. (2023b) predicted key genes associated with pork belly traits, including transcription factors. They determined the traits related to pork belly yield and three muscle areas (cutaneous trunci muscle, rectus abdominis muscle, and external abdominal oblique muscle) in pork belly slices, and identified related genetic factors. The results confirmed that adipogenesis-associated transcription factors affected pork belly composition.

Various studies have been conducted on genotypes associated with pork quality to improve pig genetics (Table 1). Halothane is a well-known gene that influences pork quality traits. Halothane gene is associated with a pale, soft, and exudative (PSE) meat. Pigs carrying the halothane gene have increased lean meat content, but have significant negative effects on water-holding capacity and the color of meat (Swan et al., 2001). Swan et al. (2001) investigated pork belly quality based on genotype by comparing pigs without the halothane gene and pigs heterozygous or homozygous recessive for the halothane gene. Consistent with the known effects of the halothane gene, pigs lacking the halothane gene showed increased fat accumulation and pork belly firmness. The IGF2 (insulin like growth factor 2) is a gene involved in myogenesis. The A/G mutation at position 3072 within intron 3 of IGF2 affects up to 30% of the variation in muscle mass and up to 20% of backfat thickness (Clark et al., 2014). The quality of pork belly obtained from pigs heterozygous (AG) or homozygous (AA) for IGF2 mutation has been investigated (Clark et al., 2014). Thicker and firmer pork bellies were obtained from pigs that were heterozygous (AG) for the IGF2 mutation. The CRTC family regulates mitochondrial metabolic activity, and of the genes of this family, CRTC3 has been reported to play an important role in controlling obesity development and energy metabolism (Lee et al., 2018). Lee et al. (2018) genotyped 360 Yorkshire pigs and identified the p.V515F mutation in exon 16 of 40 single-nucleotide polymorphisms. The p.V515F mutation in CRTC3 gene significantly affected intermuscular fat thickness, total muscle area, and total fat percentage in the belly.

Pig breeds have continuously improved production capacity and meat quality. Commercial breeds of pigs include many different breeds such as Duroc, Yorkshire, Hampshire, and Landrace. Commercial purebred pig breeds include many different breeds, including the Duroc, Yorkshire, Hampshire, and Landrace. Duroc has excellent growth and muscle quality attributes and is used as a terminal sire (National Swine Registry [NSR], 2015). Yorkshire and Landrace have excellent litter size and birth and weaning weight and are used as parent-stock females (NSR, 2015). The difference in pig breed traits can ultimately affect the meat quality. Studies have been conducted on the influence of pig breeds on meat quality, investigating the differences between breeds, such as single breeds, crossbreeds, or novel breeds (Bahelka et al., 2011; Hoa et al., 2023; Lim et al., 2013; Lim et al., 2014; Lowell et al., 2019). Lim et al. (2013) and Lim et al. (2014) investigated the differences in the quality traits of pork belly from two- and three-way crossbreeds of Yorkshire, Berkshire, Chester White, Landrace, and Duroc pigs, which are widely used Korean commercial pigs. Yorkshire×Berkshire pigs showed the lowest moisture content and cooking loss. Yorkshire×Landrace pigs have a high MUFA composition, whereas Yorkshire×Chester White pigs have a high PUFA composition and high sensory evaluation results (Lim et al., 2013). On the other hand, in three-way crossbred pigs, there were no significant differences in most quality traits except for high moisture content in Yorkshire×Chester White×Yorkshire pigs and high sensory evaluation results in Yorkshire×Landrace×Duroc (Lim et al., 2014). Lowell et al. (2019) investigated the effects of breed type (Pietrain or Duroc) on pork quality traits by controlling inherent and environmental factors. These results confirmed that thicker and firmer pork bellies were obtained from Duroc sired pigs. This was consistent with the expectations that the Duroc breed had fast growth rate and higher intramuscular fat content and that the Pietrain breed had lean meat yield. Studies on crossbreeding between local and commercial breeds to improve meat quality have also been reported. The difference in pork belly quality was confirmed between a novel breed (Woori Heukdon, WHD) which crossbreeding between Duroc sow with Korean native black pig sire and a commercial breed (Landrace× Yorkshire×Duroc, LYD; Hoa et al., 2023). The fat content and cooking loss increased in the WHD group. Additionally, WHD belly had a higher volatile aroma associated with a fatty odor, whereas LYD belly had a higher compound with a roasty odor (Hoa et al., 2023). There are various studies on crossbreeding with local breeds, but most studies deal with growth performance or overall carcass traits rather than pork belly quality (Pugliese and Sirtori, 2012). To enhance pork belly quality, further studies are needed on pork belly traits across various breeds, including commercial breeds, crossbreeds, and novel breeds. Saikia et al. (2024) reported the effectiveness of genetic improvement based on the estimated feed conversion ratio breeding value. The effects of the genetic line (sire or dam) and feed efficiency groups (low, intermediate, or high) on breeding value were investigated. Belly weight and belly thickness was highest in the high-efficiency group of the sire line. Pork belly quality is influenced by various genetic parameters. Therefore, further research is needed to identify the genetic factors that can improve quality to meet the needs of producers and consumers.

Many studies have reported that the growth performance of pigs, such as carcass weight and fat content, is significantly related to the quality of the pork belly (Correa et al., 2008; Harsh et al., 2017; Hoa et al., 2021; Lee et al., 2023a; Vališ et al., 2005). In a study by Lee et al. (2023a), carcass weight had a strong positive genetic correlation with belly weight, total belly volume, and several muscle areas of the pork belly. Similar results have been reported by Correa et al. (2008) and Harsh et al. (2017). They reported that a thicker and firmer pork belly was obtained from heavier carcasses, and a higher proportion of SFA was observed in the group with a faster growth rate.

Albano-Gaglio et al. (2024) and Hoa et al. (2021) studied the relationship between fat content and pork belly quality. The high fat content of the carcass increased the yield of pork belly, reduces cooking loss, and decreased the proportion of PUFAs in the pork belly fat (Hoa et al., 2021). Additionally, the content of oleic acid-derived compounds associated with fatty and oily flavors increases, which can improve the sensory properties. Albano-Gaglio et al. (2024) reported similar results for fat content. However, these effects may differ depending on the breed, even if the fat content is similar. In that study (Albano-Gaglio et al., 2024), although the groups had similar amount of fat content, Iberian×Duroc barrows produced lower width and firmer bellies compared to Duroc pigs. In conclusion, heavier pigs have a higher pork belly yield. However, increased carcass weight may increase fat accumulation in the pork belly, which should be considered because it affects consumer preferences.

Dietary components can be readily transferred from feed to the muscle and fat tissues of pigs, thereby affecting pork quality (Soladoye et al., 2015). Changes in pork belly quality according to diet are summarized in Table 2.

Numerous studies have been conducted to improve the quality of pork belly fat and fatty acid composition by supplementing it with dietary fat. The fat sources used varied from vegetable oils, such as corn, flaxseed, and sunflower oil, to animal fats, such as poultry fat and beef tallow. Many studies have confirmed that the supply of fat significantly affects the fatty acid composition of pork bellies (Apple et al., 2007; Eggert et al., 2001; Gatlin et al., 2003; Kellner et al., 2014). Supplementing the diet with conjugated linoleic acid oil (CLA) increased the total CLA and SFA proportions in pork belly fat, decreased IV, and resulted in a firmer pork belly (Eggert et al., 2001). Eggert et al. (2001) noted that CLA functions as an anticarcinogen and antiatherogen in animals and can improve the fat properties of pork belly without significantly affecting lean meat properties. Varying the IV of pig feed changed the physical characteristics and fatty acid composition of the pork belly (Gatlin et al., 2003). In a study by Gatlin et al. (2003), the thickness of the pork belly decreased and its length increased with an increase in IV in pig feed. Additionally, the linoleic acid content in pork belly fat increased, while palmitic acid and stearic acid content decreased with variations in IV levels in pig feed. Supplying an animal fat source to pig feed can increase the SFA proportion in pork belly, decrease IV levels, and produce a firmer pork belly (Apple et al., 2007; Kellner et al., 2014). Kellner et al. (2015) investigated whether feeding unsaturated fat followed by a withdrawal period could prevent quality deterioration in pork belly but found that the withdrawal period did not lead to improvement in the quality of pork belly. On the other hand, in some studies, the supply of dietary fat did not have a clear effect on the quality of pork belly fat (Engel et al., 2001; Huang et al., 2019; Swan et al., 2001). This may result from differences in genetic factors, the energy state of animals, or experimental conditions. Further studies under various conditions are needed to clarify the effect of dietary fat sources on pork belly quality.

Many studies have investigated the effects of feed supplementation with dried distiller grains with solubles (DDGS). DDGS is a by-product of ethanol production from grains, and extensive research has been conducted on its feeding value (Stein and Shurson, 2009). Researchers have conducted studies on DDGS, investigating the effects of treatments such as DDGS dosage (Overholt et al., 2016; Whitney et al., 2006), supplementation duration (Harris et al., 2018; Tavárez et al., 2014; Xu et al., 2010), or combination with other dietary sources (Browne et al., 2013; Davis et al., 2015; Gaffield et al., 2022; Lee et al., 2013; Villela et al., 2017), on pork belly quality. DDGS contains many unsaturated fatty acids, especially linoleic acid. Thus, feeding pigs a diet containing DDGS generally increases the IV levels (Stein and Shurson, 2009). Overholt et al. (2016) and Whitney et al. (2006) reported similar results, that state increasing the DDGS content in pig diets increased the IV of pork belly fat, resulting in a thin and softer pork belly. The authors concluded that the optimal DDGS content in grower-finisher pig diets, which were formulated based on the total amino acid content, was less than 20% (Whitney et al., 2006). Overholt et al. (2016) also investigated the effect of a diet supplemented with DDGS and found that pellet-fed pigs had heavier bellies with higher IV. Several studies have considered feeding strategies that included DDGS, followed by a withdrawal period or a gradual reduction in DDGS (Harris et al., 2018; Tavárez et al., 2014; Xu et al., 2010). The increased IV and tenderness found in pork belly supplemented with DDGS were significantly reduced by including a withdrawal period for DDGS (Harris et al., 2018; Tavárez et al., 2014; Xu et al., 2010). Lee et al. (2012) investigated the characteristics of pigs fed DDGS and corn germ and found that the pork belly firmness in these pigs decreased regardless of the DDGS supply. Several studies have been conducted to reduce the negative effects of DDGS by supplying additional fat sources (beef tallow, palm kernel oil, glycerol, cottonseed oil, or yellow grease) to pig diets. However, most dietary fat sources did not reduce the decrease in pork belly firmness (Browne et al., 2013; Davis et al., 2015; Lee et al., 2013; Villela et al., 2017). Meanwhile, in a study where researchers addedhigh-oleic soybean oil (HOSO) to diets with DDGS, it was found that the HOSO supplementation increased the proportion of MUFAs in pork belly fat and improved the physical properties and firmness (Gaffield et al., 2022).

Other dietary treatments for pigs have also been considered. In a study by Apple et al. (2011), the addition of carnitinewith a fat source changed the fatty acid composition but did not significantly affect the dimensions or firmness of pork belly. Zhu et al. (2021) found that adding camelina press cake to pig diets decreased the thickness of pork belly but had no significant effect on firmness. The effect of supplying antioxidants to pig diets on pork belly quality has been previously investigated (An et al., 2019; Lu et al., 2014). Lu et al. (2014) investigated the effects of supplying antioxidants to a high-oxidant diet. Pigs fed oxidized diets had softer pork belly, and the addition of antioxidants tended to slightly improve firmness; however, the effect was not significant. An et al. (2019) reported that effects of supplying lycopene and tomato paste as antioxidants to pigs. There was no significant difference in the belly yield and lipid properties; however, the malondialdehyde content was reduced, which improved the oxidative stability. With the ban on the use of antibiotics in livestock diets, Lowell et al. (2018) investigated the effect of antibiotic use on pork belly quality, and found no significant differences in pork belly quality between pigs fed antibiotic-free, natural antimicrobials, or antibiotics. Methionine (Met), an essential amino acid, is commonly added as a supplement to growing-finishing pig diets because it is the second most limiting amino acid in pigs. Remole et al. (2024) found that differences in Met source did not significantly affect pork belly quality. Therefore, diets with various ingredients and treatments can significantly affect pork belly quality, and further research should be conducted to produce high-quality pork belly.

Pork belly quality is influenced by factors other than feed intake. Bryan et al. (2020) reported that infection with porcine reproductive and respiratory syndrome viruses (PRRSV) reduced pork belly firmness. Zomeño et al. (2024) investigated the effect of the boning processing method (hot or cold) on pork belly quality. They reported that the hot-boned belly (cut immediately postmortem) was shorter, wider, thicker, and firmer than the cold-boned belly (cut at 24 h postmortem) due to intense shortening and hardening. These results confirm that disease and carcass handling can affect pork belly quality. However, since studies addressing pork belly quality are insufficient, further research on various influencing factors is necessary.