Productive variables and carcass characteristics

The variables were: productive performance (feed intake, FI; average daily gain, ADG; feed:gain ratio, FGR; fat free lean gain, FFLG; and final body weight, fBW; carcass characteristics (backfat thickness, BT; lean meat percentage, LMP (for second stage); Longissimus dorsi muscle area, LMA; and plasma urea nitrogen concentration, PUN).

The BT and LMA were measured at the lumbar region in the penultimate rib, using a real-time ultrasound (SonoVet 600, Medison, Inc., Cypress, California, USA) at the beginning and at the end of each stage. Based on this data -as well as the iBW and fBW data-, FFLG and LMP were calculated, using the ^{National Pork Producers Council (1991)} equation.

At the end of each experimental stage, blood samples were taken from the superior vena cava, using Vacutainer® tubes with herapin. The samples were put on ice, before they were centrifuged (SIGMA 2-16K, Germany), at 2500 g for 20 minutes, in order to separate the plasma from the cellular package. The plasma was transferred to polypropylene tubes and it was stored in a freezer (SANYO MDF-436, USA) at -20 °C, until it was analyzed, in order to determine plasma urea (^{Chaney and Marbach, 1962}).

Physicochemical characteristics of meat

The physicochemical characteristics of meat were evaluated at the end of the second experimental stage, when pigs reached 100 kg of live weight. Four pigs per treatment were slaughtered and samples were taken from their ham (Biceps femoris) and loin (Longissimus dorsi); color, pH, water holding capacity (WHC), and texture were measured. Part of the samples were frozen, in order to analyze their FA profile. The slaughter process was carried out at the farm’s slaughterhouse, complying with the Mexican Official Standard NOM-033-ZOO-1995 (^{Secretaría de Agricultura, Ganadería, Desarrollo Rural, Pesca y Alimentación, 1995}).

Color was determined 24 h post mortem using a portable colorimeter (Hunter Lab, Chroma meter CR-410, Konica Minolta Sensing, Inc., Japan). White color was calibrated in three different points on the surface of the pork ham and loin, in order to measure the level of brightness (L*), red (a*), and yellow (b*).

The pH was measured directly on the ham and loin muscles, 24 h post mortem, using a Hanna puncture portable potentiometer (pH 1100 model).

The WHC in pork ham and loin samples was measured 24 h post mortem, using the method proposed by ^{Guerrero et al. (2002)}: 2 g of finely ground meat were placed on a centrifuge tube, before being homogenized with 5 mL of a 0.6 M sodium chloride solution, shaken in a vortex during 1 min. Samples settled 30 min on a refrigerator, at 4 °C, before they were centrifuged during 15 min, at 3500 g. The supernatant was decanted and measured using a test tube. The volume of holding water is reported as the amount of water held in 100 g of meat.

Texture was determined 24 h post mortem, using pork ham and loin samples, obtained with a Warner-Bratzler blade, using a TA-XT2 texture analyzer (Textura Technologies Corp, Scarsdale, NY). Raw meat cubes (1X1X1 cm) were aligned with muscle fibers at an oblique angle with regard to the edge of the blade, and maximum cutting force and the known force were recorded (^{Guerrero et al., 2002}).

Fatty acid profile in ham and loin

Fatty acids profile was determined using an HP® chromatograph (6890 Model), methyl esters standard, Supelco 37 (Component FAME Mix, Catalog No. 47885-U), with a Supelco column (SP™-2660 FUSED SILICA Capillary Column, 100MX0.25mmX0.2 µm film thickness). The carrier gas was helium at 0.8 mL min^-1; a 1 µL was injected manually in Split mode 1:10; the initial ramp temperature was 140 °C per 1 °C min^-1, with a 3 °C min^-1 increase up to 210 °C, and a 0.7 °C min^-1 decrease down to 235 °C. The analysis of each sample took 60 minutes.

Statistical analysis

The experimental design was a completely randomized, with six treatments and eight replicates (a pig per replicate). Based on the data obtained, an ANOVA was carried out, using the GLM procedure of SAS, and the treatments means were compared using the Tukey test (p≤0.05) (^{SAS, 2010}). The initial body weight of the pigs was used as a covariate, because of the heterogeneity of the iBW (p≤0.05).

Productive performance, urea and carcass characteristics

Flaxseed concentration (4 % and higher amounts) on the diet of 50 to 75 kg pigs had an adverse effect on ADG, FGR, and fBW (p≤0.05); there was no difference for the FI (p>0.05) in any treatment (Table 3). For 75 to 100 kg pigs, the productive performance did not change when flaxseeds were added to the diet (Table 4).

Table 3 Productive performance, carcass characteristics, and plasma urea concentration of finishing pigs (50-75 kg PV) fed ground flaxseeds in the diet. 

abc: Means with different letters in a column are statistically different (p≤0.05). ^†SEM: standard error of the mean. ^ÞÞT: treatment. ^¶ADFI: average daily feed intake. ^§ADG: average daily gain. ^ÞFGR: feed:gain ratio. ^¤BW: body weight. ^††BT: backfat thickness. ^¶¶LMA: Longissimus dorsi muscle area. ^§§PUN: plasma urea nitrogen concentration.

Table 4 Productive performance of finishing pigs (75-100 kg PV) fed ground flaxseeds in the diet. 

abc: Means with different letters in a column are statistically different (p≤0.05). ^†SEM: standard error of the mean. ^ÞÞT: treatment. ^§ADFI: average daily feed intake. ^¶ADG: average daily gain. ^ÞFGR: feed:gain ratio. ^¤BW: body weight. ^††FFLG: fat free lean gain.

In the study carried out by ^{Eastwood et al. (2009)}, the flaxseed flour level had a quadratic effect (p≤0.08) on the weight gain and the feed conversion of 32 to 60 kg pigs, but they did not determine which one was the best; nevertheless, the level did not have an impact (p>0.1) on the productive variables in pigs with 60 to 85 kg of BW, which was similar to the results of this study. According to ^{Matthews et al. (2000)}, adding up to 1 % of flaxseeds to the diet of 30 to 85 kg pigs did not produce differences in FGR, ADG, and fBW. ^{Haak et al. (2008)} reported that adding ground flaxseeds (0 and 3 %) to the diets of finishing pigs (70 kg) did not result in differences in ADG and FI.

The above-mentioned studies included 3-15 % flaxseed flour to the diet of finishing pigs, but they did not make clear which level had adverse effects on productive variables. According to our study, the adequate percentage of ground flaxseed is the following: 2 % for 50-75 kg pigs; and 10 % for 75-100 kg pigs.

The evaluation of carcass characteristics of 50-75 kg pigs indicated that ADG decreased (p≤0.05) after 4 % or more flaxseeds were included in the diet; LMA was not affected by the inclusion of up to 10 % of flaxseeds (Table 3). For 75-100 kg pigs, LMA was not affected (p≤0.09), but pig’s LMP was modified (p≤0.04) when 4 % of flaxseed was used in the diet (Table 5). ^{Lisiak et al. (2013)} did not observe changes in ADG and LMA, in 25 to 60 and 60 to 105 kg pigs that were fed with up to 2.5 % flaxseed oil; meanwhile, in our study we used a maximum concentration of 10 % ground flaxseed, which is equivalent to 1.2 kg of flaxseed oil, depending on the extraction method (^{Eastwood et al., 2009}). ^{Wojtasik et al. (2012)} mentioned that 0.1 and 2.5 % levels of flaxseed oil did not change the BT and LMA variables in 25 to 60 and 60 to 105 kg pigs.

Table 5 Carcass characteristics and plasma urea nitrogen concentration of finishing pigs (75-100 kg PV) fed six levels of ground flaxseed added to the diet. 

abc: Means with different letters in a column are statistically different (p≤0.05). ^†SEM: standard error of the mean. ^ÞÞT: treatment. ^¶iBT: initial backfat thickness. ^§fBT: final backfat thickness. ^ÞiLMA: initial Longissimus dorsi muscle area. ^¤fLMA: final Longissimus dorsi muscle area. ^††iLMP: initial lean meat percentage. ^¶¶fLMP: final lean meat percentage. ^§§PUN: plasma urea nitrogen concentration.

Differences between our study and those conducted by ^{Lisiak et al. (2013)} and ^{Wojtasik et al. (2012)} can be caused by the increase of omega-3 fatty acids in the meat, favored by adding flaxseeds to the diet, that could increase the intramuscular fat content of meat, as a result of alterations in the gene expressions involved in the adipogenesis (^{Luo et al., 2009}). However, if omega-3 fatty acids concentration in meat is very high and it is not accompanied by antioxidant supplements, the oxidation of polyunsaturated fatty acids can increase (^{Cunnane et al., 1990}), which could alter the increase in the fat deposit.

Flaxseeds levels did not affect (p>0.05) the plasma urea nitrogen concentration (PUN; Tables 3 and 5) during the two experimental stages, possibly because the amino acid and metabolizable energy concentration was constant in all diets. ^{Abreu et al. (2007)} and ^{López et al. (2010)} mention that when diets are isoproteinic and isoenergetic do not modify the plasma urea concentration.

Physicochemical characteristics

The concentration of ground flaxseeds did not change the pH, water holding capacity (WHC), color (L* a* and b*), and texture of loin meat (p>0.05); the effect on ham meat was similar, except for texture (p≤0.05) (Tables 6 and 7). This agrees with ^{Lisiak et al. (2013)}, who reported that 1, 2.3 and 2.5 % of flaxseed oil did not change the physicochemical characteristics of loin and ham meat at 24 and 45 h post mortem in 65 kg BW pigs.

Table 6 Physicochemical characteristics in ham meat of finishing pigs fed six levels of ground flaxseed added to the diet. 

abc: Means with different letters in a column are statistically different (p≤0.05). ^†SEM: standard error of the mean. ^ÞÞT: treatment. L*: brightness. ^§a*: red index. ^Þb*: yellow index. WHC: water holding capacity.

Table 7 Physicochemical characteristics in loin meat of finishing pigs fed six levels of ground flaxseed added to the diet. 

abc: Means with different letters in a column are statistically different (p≤0.05). ^†SEM: standard error of the mean. ^ÞÞT: treatment. L*: brightness. ^§a*: red index. ^Þb*: yellow index. WHC: water holding capacity.

The treatments did not modify the pH and, therefore, the other variables did not change either, and this probably explains the results of this study. The pH ranges in ham (5.40-5.69) and loin (5.43-5.50) meat could be the result of the production of decomposition compounds, such as biogenic amines, aldehydes, ketones, and short-chain fatty acids. Decomposition of meat starts with values of pH lower than 5.4 and higher than 5.8, post rigor mortis (^{Guerrero, 2009}). The pH values of our study seem to be in the adequate pH ranges for meat (^{Flores et al., 1999}).

The degree of acidification is related with WHC; as the pH level increases, so does the water holding capacity. Some studies report 1.98-2.54 % WHC in ham meat and 3.82-5.15 % WHC in loin meat (^{Homsi and Francisco, 2003}). In our study, WHC had minimum values of 0.88 and 0.85 mL, and maximum values of 1.0 mL and 0.97 mL, in ham and loin meat, respectively. However, different levels of ground flaxseed did not change WHC, probably because the treatments did not modify the pH of the meat. This matched the findings of ^{Haak et al. (2008)}, which did not find important differences in color, pH, and WHC, neither when testing 0 and 3 % ground flaxseed levels, nor in diets with and without flaxseeds (^{Nuernberg et al., 2005}; ^{Mas et al., 2011}). Color is a variable that consumers relate with a fresh product; in our study, we did not find difference (p>0.05) in the L*, a*, and b* indexes of meat. In a meta-analysis, ^{Corino et al. (2014)} analyzed 24 studies about adding flaxseed to pig diets and did not find changes in pH and in meat color.

Including inadequate concentrations of flaxseed oil in animal diets can have adverse effects in the meat’s sensory and physicochemical characteristics, because it can cause an increase in the oxidation susceptibility, as a result of the higher deposition of Ω-3 FA (^{Grau et al., 2001}). ^{Ahn et al. (1996)} point out that a meat product’s FA content is its main oxidizing agent, because polyunsaturated fatty acids are more easily attacked by free radicals than monounsaturated and saturated FA. Likewise, this greater tendency is not the same for all polyunsaturated FA; rather, it increases along with the unsaturation level, because they are highly sensible to the attack of free radicals (^{Angelo, 1996}). Therefore, the oxidation processes of polyunsaturated fatty acids cause undesirable modifications to the meat’s organoleptic and physicochemical characteristics (^{Cherian et al., 1996}).

However, ^{Corino et al. (2008)} found that including ground flaxseed increases the meat’s linolenic acid content, without diminishing its oxidative stability, or affecting its color or flavor. Likewise, ^{Aguilar-Guggembuhl et al. (2014)} found that the odor and flavor of meat was not altered when docosahexaenoic, eicosapentaenoic, and conjugated linoleic FA were included; they considered that the staleness had diminished because these FA captured electrons (^{Huang and Ho, 2011}): Ω-3 fatty acids have a positive effect in the preservation of cell membrane and antioxidant activity (^{Valenzuela et al., 2011}). Therefore, including flaxseeds in our study -although it increased Ω-3 and Ω-6 fatty acids deposition- does not seem to increase the meat’s oxidative rate; consequently, the physicochemical characteristic remained unaltered, even with the highest flaxseed concentration.

The texture data of the ham meat in our research is different from the values reported by ^{Lisiak et al. (2013)}, who did not find different textures in the ham and loin meat when they added 1, 2.3, and 2.5 % flaxseed oil; these levels are higher than those resulting from the addition of 10 % ground flaxseeds. Up to 120 g of oil can be obtained from each kg of flaxseed (^{Eastwood et al., 2009}). Therefore, in our study, 10 % of ground flaxseeds provides approximately 1.2 % of oil, compared with 2.5 % of oil supplied to pigs by ^{Lisiak et al. (2013)}.

Fatty acid profile in ham and loin meat

No significant (p>0.05) differences were found in the saturated and monounsaturated FA profiles of ham meat between treatments (Table 8). The saturated and monounsaturated FA profile of loin meat had a similar response (p>0.05; Table 9), except for stearic and arachidic FA; these fatty acids showed differences (p≤0.05) as a result of the amount of ground flaxseeds added to the diet. This agrees with the study of ^{Eastwood et al. (2009)}, who used 15 % flaxseed and observed an increase in omega-3 FA and a reduction of palmitic and stearic fatty acids. According to ^{Haak et al. (2008)} and ^{Liziak et al. (2013)}, including up to 3 % of seeds or 2.5 % of flaxseed oil does not affect the saturated FA content; however, the concentration of monounsaturated FA (cis-11-eicosatrienoic, oleic and palmitoleic) increased when the flaxseed oil level in the diet increased. Increasing omega-3 FA in meat seems to reduce the content of other fatty acid(s), since -as our experiment proves- the body’s fat content does not necessarily increase; changes can also be the result and reflect the diet’s lipid profile (^{Kouba and Mourot, 1999}).

Table 8 Fatty acid profile of ham meat of finishing pigs fed flaxseeds supplemented diets. 

abcd: Means with different letters in a column are statistically different (p≤0.05). ^†SFA: saturated fatty acids. ^¶MUFA: monounsaturated fatty acids. ^§PUFA: polyunsaturated fatty acids. ^ÞSEM: standard error of the mean. ^ÞÞT: treatment.

Table 9 Fatty acid profile of loin meat of finishing pigs fed flaxseeds supplemented diets. 

abcd: Means with different letters in a column are statistically different (p≤0.05). ^†SFA: saturated fatty acids. ^¶MUFA: monounsaturated fatty acids. ^§PUFA: polyunsaturated fatty acids. ^ÞSEM: mean standard error. ^ÞÞT: treatment.

With regard to polyunsaturated FA, the content of linolenic (p≤0.0001) and cis 11, 14, 17-eicosatrienoic (p≤0.0004) fatty acids -from the Ω-3 family- increased in ham and loin meat (Tables 8 and 9), as the amount of flaxseed in the diet increased.

^{Liziak et al. (2013)}, ^{Corino et al. (2014)}, and ^{Skiba et al. (2015)} proved that adding flaxseed oil or flaxseeds increases the content of linoleic, arachidonic, cis-11, 14, 17-eicosatrienoic, and cis-11, 14-eicosadienoic unsaturated fatty acids. The increase of Ω-3 fatty acids in meat is proportional to the amount of flaxseeds added to the diet (^{Matthews et al., 2000}; ^{Nuernberg et al., 2005}). This has a beneficial effect because Ω-3 fatty acids prevent cardiovascular diseases and strengthen the immune system (^{Zhan et al., 2009}). Therefore, using flaxseeds in pig diets seems to be a good source of Ω-3 fatty acids that increases their content in meat, as a result of their high linolenic fatty acid content.