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Review Article
 

Manipulation of Pork Production and Quality via Nutrition



C.A. Silva, A.M. Bridi, A. Passos, C.C.S. Martins and C.A. Lozano-Poveda
 
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ABSTRACT

The characterization of pork quality is complex, as it can vary according to the interest of the elements that participate in this chain. The producer, the industry and the final consumer have different perceptions about this qualification, which includes several parameters, such as the meat/fat ratio, colour, tenderness, pH, marbling, oxidation, water loss, etc. Additionally, meat quality can be linked to other issues, such as food safety (free of microbial contamination and/or chemical and hormone residues), sustainability (it was produced without damaging the environment, the people who participated in its production, etc.) and animal welfare. However, the first perception that meat has high quality lies in sensory attributes. The actions to optimize these characteristics involve processes linked to animal genetics, zootechnical management, nutrition, transport and slaughter procedures and meat processing. In this set, nutritional actions and the use of feed additives are one of the main tools for the modulation and enhancement of these parameters. The objective of the present review is to describe the nutritional resources more widely available as well as the additives and dietary concepts that are more effective when incorporated to minimize the damage related to the slaughter process, to modulate the transformations inherent to the post mortem or for the enrichment of meat quality meat, focusing on the development of functional food, thus meeting the main objective of this chain, the demanding final consumer.

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C.A. Silva, A.M. Bridi, A. Passos, C.C.S. Martins and C.A. Lozano-Poveda, 2022. Manipulation of Pork Production and Quality via Nutrition. Asian Journal of Animal and Veterinary Advances, 17: 73-90.

DOI: 10.3923/ajava.2022.73.90

URL: https://scialert.net/abstract/?doi=ajava.2022.73.90
 

INTRODUCTION

The concept of meat quality is multifactorial and includes animal welfare concepts (pre-slaughter management), food safety (absence of physical risks, chemical and biological residues), sensory quality (colour, texture, juiciness, flavour, odour and tenderness), nutritional value (amount of fat, fatty acid profile, protein, minerals and vitamins levels) and technological attributes (pH and water retention capacity), involving processes that start from conception, through pregnancy, post-birth to the preparation of food for fresh consumption or processed meat products1.

Of the parameters related to the prediction of meat quality, colour and marbling are the most easily and readily identified by consumers, being the first aspects observed when purchasing the product2. However, parameters such as pH and water holding capacity are extremely important2, as they directly influence the colour, tenderness, shelf life, yield and quality of meat and processed meat products. Also, it is relevant to associate that in the foreground, meat quality is linked to the quantitative muscle/fat ratio in the carcass, an attribute valued by the industry, which prioritizes the former over adipose tissue.

Admittedly, several factors are involved in the deposition of these tissues and the meat quality, highlighting genetics, muscle structure, chemical composition, microorganisms, pre-slaughter handling, processing and storage. Therefore, it is clear that many parameters linked to meat quality can be manipulated. Among all the factors related to meat quality, nutrition effectively comprises the broadest resource of opportunities to meet this premise. Thus, the objective of this review is to discuss the main pre- and post-parturition stages related to muscle composition and the changes this tissue undergoes, which express the final quality of the meat and also to evaluate the most important nutritional actions to improve these parameters, thus meeting the interests of the industry and the consumer.

MATERIALS AND METHODS

The study was carried out at the Department of Animal Science, Londrina State University, Londrina, Brazil and at the Department of Innovation and Applied Science, DSM Nutritional Products Ltd., Jaguaré, São Paulo, Brazil, from January-September 2020.

Role of energy, protein and amino acids: For lean meat deposition, the pig requirement is divided into two very distinct phases: One dependent on energy and the other one on protein. The first, food intake is the major limiting factor, being attributed that voluntarily the animal is unable to meet the fullness of its development, with emphasis on the maximum deposition of lean meat. In the protein-dependent phase, on the other hand, voluntary feed intake supports the animals’ requirements for protein deposition up to a maximum point3, meaning that any feed intake beyond that required for maximum protein deposition will result in increased fat deposition4.

In the energy-dependent phase, diets must be formulated based on the lysine: Energy ratio, as the increase in feed intake, will increase energy consumption and, consequently, a higher level of amino acids will be required for this energy ratio, resulting in additional protein deposition dependent on the higher energy being offered5.

In the protein-dependent phase, when the energy intake is higher than required for maximum protein deposition, diets must be formulated to meet the amino acid requirements in grams per day, as established in the nutrition tables6,7. Considering the performance curves, feed intake/energy and the concentration of dietary amino acids, it is clear that at this stage there is a limited correspondence between the deposition of body protein and the additional energy provided by the diet7-9. Adversely, the increase in the energy concentration of the diet has a well-known relation to intake reduction. In diets where the lysine: Energy ratio has not been corrected, this can lead to a reduction of lysine intake and all its consequences on carcass quality10.

Subject to the effects of the described correct ratio between energy and protein and/or amino acids, the potential of lipids and carbohydrates in a fat deposition is obvious11,12. In summary, the higher energy intake increases the rate of lipid deposition, with the largest fraction (50%) occurring in subcutaneous tissue13, a behaviour described in many papers14.

On the other hand, the nature of the lipid used as an ingredient in the diet of pigs, thus disregarding the amount, reduces the de novo endogenous synthesis of fats15, which means that the nature of the lipid source in the diet is essential to ensure the production of meat with the lipid profile16-19.

Another aspect related to dietary energy involves the differences that various energy sources have on muscle glycogen storage. Li et al.20 found that the reduction of muscle fibres glycolytic potential can be improved in diets where composition values fibre more than starch, with better consequences of the former on meat quality.

In skeletal muscle there is an anabolic and catabolic dynamics represented by a constant flow of amino acid deposition, parallel to synthesis, leading to constant protein degradation. With the absorption of amino acids from the diet, however, the rate of synthesis outpaces the rate of degradation, resulting in a net increase in muscle protein, which is supported by insulin21. In a situation of protein imbalance, for example, the energy that is required for the excretion of excess nitrogen is therefore not available for muscle growth and can affect the animal's performance and carcass characteristics22,23.

In this sense, offering optimal amino acid levels in diets with low total protein values, meeting the specific nutritional requirements for each age, animal category, sex and genetic pattern6,7 is a positive strategy to optimize the ratio of protein and fat deposition in the carcass. Along these lines, there is a positive effect of diets with low crude protein levels on the muscle: Fat ratio in the carcass.

Conversely, feeding diets with levels below the amino acid requirements impair protein deposition and increase fat deposition22. In this particular, supported by the recognition of the growth phase of the animal as being protein-dependent, thus the amino acid level of the diet at the end of finishing has a greater impact on lean meat deposition on the carcass.

Main et al.22 observed that there was no difference in the percentage of lean meat between pigs that received, at the beginning of the growing-finishing phase, diets with dietary lysine below or within the levels meeting the requirements for the category. However, when the requirements of this amino acid were not met at the end of the finishing period, the muscle deposition in the carcass was worse than in the group that was correctly supplemented. In general, the deficiencies of lysine, methionine, threonine, amino acids do not have a great impact on feed intake, but result in increased fat deposition in the carcass, in contrast to tryptophan, valine, isoleucine that have a greater influence on feed intake when they are deficient24-26.

The effectiveness of reducing the protein level in the diets, however, can be contradictory if the energy adjustments and the correct amino acid balance are not met, with worsening (increase) of the fat thickness and reduction of the lumbar muscle area27-30. This derives from the high deposition of surplus energy and from amino acid catabolism, which leads to the use of carbon skeleton for gluconeogenesis and lipogenesis and from the quality of the diet, often consisting of a high starch proportion that has a higher ability to serve as a substrate for fat deposition than amino acids31. This evidence, however, requires further studies23.

Considering the role of protein as a whole on aspects specifically linked to meat quality, Wood et al.32 and Teye et al.33 found that diets with reduced protein content and low lysine/energy ratios resulted in meat with more juiciness and tenderness. Also, diets with amino acids levels lower than requirements tend to increase intramuscular fat34. Working with amino acid levels that meet the requirements of castrated male pigs with high genetic potential for medium or high performance, Bertol et al.35 observed a higher marbling score in the loin of animals that were fed diets with lower amino acid content (1.66%±0.12 vs. 1.92%±0.11, p<0.05). Additionally, the authors did not find any losses in the performance and backfat thickness of these animals.

Tang et al.36 reiterated the effects of reducing the nutritional levels of the diet (14.19% crude protein and 13.81 MJ DE kg1 vs. 11.08% crude protein and 12.55 MJ DE kg1) on the intramuscular fat, added to the improvement in shear force and increase in the mRNA expression of μ-calpain (p<0.05) in skeletal muscle. However, the authors did not observe any effect of these diets on the gene expression of calpastatin.

Concerning the intrinsic quality aspects of pork, most studies do not indicate that 24 hrs post-slaughter pH and water-holding capacity are significantly influenced by dietary protein level, but the variations in meat colour are significantly increased When the diets have higher protein levels34,37.

At the same time, meeting the requirements for digestible lysine (the reference amino acid) and the other amino acids is based on their relationship with the metabolizable energy of the feed. Maximum energy intake for optimal muscle development implies adequate levels of this amino acid, which in turn must be supported by the correct intake of dietary energy recommended by the nutritional requirement tables6,7.

In this line, the practice of rationalizing the nutritional levels of diets, that is, without nutrients excess, is well served by the current use of enzymes that optimize the nutrients of diets according to their specificities. In this sense and within a more recent concept of use, it has been shown that levels of phytase in the diet between 2000 FYT (phytase units per kg of feed) and 3000 FYT meet the full performance of animals in fattening stages, with no influence on muscle and fat deposition in the carcass38. In addition to the availability of dietary phosphorus, there is also an extra-phosphoric effect, with an improvement in the utilization of protein, amino acids, calcium and dietary energy39-41. When high doses of phytase are used, these effects are also related to the increase of Myo-inositol in body tissues, its action is to mimic the role of insulin, with effects similar to those of GLUT 4 when blood glucose concentration increases, optimizing body protein deposition at the expense of adipose tissue.

Role of additives
Carnitine: Although it is not an amino acid, carnitine (3-hydroxy-4-N-trimethylamine-butyrate) comes from the synthesis of lysine and methionine in muscle and liver tissue. In the biochemical processes, carnitine forms long-chain fatty acid esters catalyzed by the carnitine palmitoyltransferase I enzyme42, that can penetrate the mitochondrial membrane43,44, being cleaved again by the action of carnitine palmitoyltransferase II and released into the mitochondria to be used for energy production43. This suggests that supplementing the diet with L-carnitine optimizes the use and transport of fatty acids, thereby increasing the production of energy used in animal development. In this context, L-carnitine supplementation decreases body fat levels and on the other hand, increases protein deposition43,45. Ringseis et al.46 observed positive results in the meat quality of growing and finishing pigs when they were supplemented with doses between 49 and 64 mg carnitine kg1 feed.

Ractopamine: Ractopamine is a β-adrenergic agonist is a β-adrenergic agonist that acts as a nutrient repartitioning agent, favouring protein synthesis (retaining nitrogen and thus increasing protein synthesis) over adipose tissue deposition (inhibiting lipogenesis and stimulating lipolysis). These actions determine an increase in muscle deposition by hypertrophy of the muscle fibre diameter, more specifically of the fibres of the white and intermediate fiber47,48, improving weight gain, feed conversion, lean meat deposition on the carcass and backfat thickness48-51. The effects of ractopamine are classically dose-dependent52 for performance parameters and may show this same behaviour for lean meat deposition on the carcass, as observed by Apple et al.53 and Carr et al.54, when evaluating the principle under different levels of inclusion in the feed (0, 5, 10, 15 and 20 ppm).

Conjugated linoleic acid (CLA): The term CLA is used to designate a specific group of fatty acids, geometric and positional isomers of linoleic acid (C18:2). Although there are several isomer combinations, the trans-10, cis-12 isomer has a special interest in animal production because it affects lipid metabolism, being responsible for reducing carcass fat and increasing lean tissue deposition in pigs55-60. The possible mechanisms attributed to the reduction of body fat deposition associated with CLA intake are increased energy expenditure, modulation of adipocyte metabolism and action of adipokines and cytokines and increased beta-oxidation of fatty acids in adipose tissue61.

In a meta-analytic study on the influence of meat and carcass quality modifying additives, Dunshea et al.62 concluded that dietary supplementation with CLA, at doses ranging from 0.12 to 5%, decreased backfat thickness by about 6% (1.2 mm) When compared to the control treatment, but increased the intramuscular fat percentage and the degree of marbling by 7 and 11%, respectively. Similar results were found by Jiang et al.60. According to the authors, the amount of subcutaneous and intramuscular fat always changes in the same direction, thus, the use of CLA becomes attractive because it promotes a reduction in the backfat thickness without influencing the amount of intramuscular fat and consequently, the sensory quality of the meat.

In another meta-analysis, where the average inclusion level of CLA was 1.49% (ranging from 0-10% of diets), Andretta et al.63 identified a positive correlation (0.59) between CLA content in the diet and the amount of lean meat and a negative correlation with fat thickness in pig carcasses. For each milligram of CLA consumed during the growing and finishing phases there was a 0.06 mm reduction in subcutaneous fat thickness. On average, CLA increased lean meat content by 9% and reduced-fat thickness in the carcass by 6%.

Betaine: Trimethylglycine, known as betaine, is considered to be a methyl radical donor64,65 and is involved in the synthesis of metabolites such as creatine and carnitine. Its inclusion in the diet can decrease the requirement for other methyls radical donors, such as methionine and choline. As methionine is one of the most limiting amino acids, plays a key role in the synthesis of body proteins, the use of betaine aims to spare it from the methyl donor function66. Also, after its complete demethylation, betaine will result in the formation of glycine. The reduction in fat deposition is due to increased activity of the hormone-sensitive lipase activity67 and reduced available substrate (acetyl coenzyme A) for lipid synthesis and deposition68.

To summarize, betaine decreases daily feed intake, increases weight gain e reduces the percentage of fat in the carcass and backfat thickness and, as a consequence, increases the lean meat percentage69,70.

Chromium: Dietary chromium supplementation for pigs is associated with increased lean meat deposition in the carcass71,72. This effect is attributed to its ability to increase glucose uptake by target cells, preventing excess plasma glucose from converting to fat73. The mineral also acts on protein metabolism, promoting greater use of amino acids, thus increasing protein synthesis73,74 and lean meat deposition. Along these lines, when using 200 μg chromium propionate kg1 feed during the growing and finishing phases of pigs, Jackson et al.75 observed lower backfat thickness on the tenth rib and a higher percentage of muscle in the carcass. And Untea et al.76 found that supplementation of 200 ppb chromium picolinate for 30 days before slaughter increased protein deposition in the loin and ham and decreased the rate of fat deposition in the belly and ham.

Reviewing 31 publications in a meta-analytic study on the subject, Sales and Sales77 concluded that dietary supplementation of chromium for finishing pigs was decisive in reducing backfat thickness at the tenth rib height and increasing muscle deposition in the carcass.

Chromium can also modulate the fatty acid profile of fat in pigs, impacting its quality. Supplementation of this mineral increases insulin sensitivity78,79, increasing the efficiency of glucose utilization and the availability of more energy from the diet, causing the higher glucose concentration to promote de novo synthesis of fatty acids, characterized by a higher saturation than the fatty acids from the diet that are generally plant-derived and therefore more unsaturated80. In the same line of studies, Jackson et al.75 found that growing and finishing pigs supplemented with 200 μg of chromium picolinate kg1 feed had a lower iodine content in belly fat, that is, a more saturated and firm fat, which results in bacon of better quality and sliceability.

Vitamins: The relationship of fat-soluble vitamins with characteristics linked to the development of muscle tissue is particularly clear for vitamins D and E. According to Starkey81, vitamin D plays a significant role in postnatal skeletal muscle maturation and hypertrophic growth and postnatal dietary supplementation is really necessary to optimize muscle protein deposition and ultimately increase muscle yield. High levels of vitamin E supplementation can improve the carcass characteristics of pigs slaughtered at high weight. A dietary vitamin E supplementation at the levels 100, 200 and 400 mg kg1 increased linearly the percentage and the kilogram of lean meat in the carcass of pigs slaughtered at 118 kg when compared to pigs fed diets containing 11 mg kg1 vitamin E82 and was also observed an increase in the loin eye area when they supplemented the pigs’ diet with 400 mg vitamin E kg1 feed during 28 days before slaughter83.

However, the use of vitamin E at doses higher than 10 or 20 times the established nutritional requirements6 did not alter the carcass characteristics when the slaughter weight was around 100 kg23,84. These results indicate that vitamin E protects the cell membrane, maintaining cell integrity, preventing swelling and lipoperoxidation of the cells. For this reason, it is important to maintain high levels of vitamin E in the diet during the period of rapid growth rate and cellular hypertrophy in pigs.

As for the isolated supplementation of some water-soluble vitamins (niacin, pantothenic acid and vitamin B12), Real et al.85 Groesbeck et al.86 and Tuffo et al.87, did not identify positive effects on carcass characteristics of pigs with slaughter weight between 90 and 120 kg. Minelli et al.88, however, observed that the addition of 110 mg pantothenic acid kg1 to diets of pigs between 107 and 168 kg b.wt., increased the percentage of lean meat and decreased backfat thickness when compared to pigs fed diets containing 10 mg pantothenic acid kg1.

Radcliffe et al.89 and Fiego et al.90 found that increasing the pantothenic acid levels in the pigs’ diet (13.2 and 30 mg kg1 and 10, 60 and 110 mg kg1, respectively) led to an increase in the percentage of carcass meat and lean cuts percentages. Böhmer and Roth-Maier91, working with 1.2, 36 and 72 mg of pantothenic acid per kilogram of diet, observed an increase of this vitamin level in blood, liver and muscle, which has a relationship with the pigs’ performance and carcass characteristics. The action of pantothenic acid seems to be most evident when there are high rates of de novo lipogenic synthesis, resulting from the modulation of mRNA expression for the synthesis of lipo synthetic enzymes92.

As for pyridoxine supplementation, Castilha et al.93 reported a positive effect on hot and cold carcass yield when it was added at a dose of 5 mg kg1 of feed.

Studies evaluating the effects of high supplementation of several vitamins simultaneously on carcass characteristics are less common. However, the results of vitamin supplementation above the levels established by nutritional tables6,7, when performed specifically for some vitamins, showed positive effects on some carcass parameters. In this regard, working with dietary supplementation of five B-complex vitamins (niacin, riboflavin, folacin, pantothenic acid and cobalamin), with doses equivalent to 70, 170 and 270% above the NRC6 recommendations, for pigs slaughtered at 105 kg live weight, Cho et al.94 observed an increase in loin depth, loin eye area and muscle tissue gain.

Resources to minimize damages resulting from slaughter: The deleterious effects of pre-slaughter stress can be reversed with the use of vitamin E. However, its best level of inclusion for this purpose has not been effectively defined as it also depends on the dose of the vitamin that was used and the administration time. Providing 500 mg vitamin E kg–1 feed for 30 days before slaughter, a value higher than the established nutritional requirements (ranging from 36 mg kg1 feed for growing pigs to 15 mg kg1 feed for finishing pigs, according to Rostagno et al.7), was verified a tendency to reduce the drip loss compared with a control group95, whose action is attributed to the decrease in plasma Ca+ release. Lower Ca+ levels reduce the glycolysis rate and the activity of the Phospholipase A2 enzyme.

Phospholipase A2 hydrolyzes the cell membranes phospholipids, releasing unsaturated fatty acids, which are subsequently elongated and desaturated, promoting cell membrane destabilization. This determines a loss in the water holding capacity of the meat and an increase in PSE meat. As vitamin E prevents lipid oxidation, especially of cell membrane phospholipids, which are rich in polyunsaturated fatty acids, the membrane integrity is preserved, improving the meat water retention capacity96,97.

In comparison with animals in the control treatment group, Peeters et al.98 found a decrease in plasma cortisol levels of pigs submitted to transportation when they were supplemented with vitamin E (150 mg kg1 feed for 21 days). The increase in plasma cortisol raises the metabolism and as a consequence, the intracellular levels of calcium which, in turn, activates phospholipases that start an oxidative process in the plasma membrane.

The use of magnesium in the pig diet can also alleviate the effects of slaughter stress, increasing the meat quality. The mineral prevents the sharp drop in pH after slaughter, preserving the colour and reducing water loss from the meat. Magnesium exerts a relaxing action on skeletal muscle and has an antagonistic effect on calcium, reducing the secretion of neurotransmitters (cortisol, adrenaline, noradrenaline and dopamine) and consequently, leading to a decrease in neuromuscular stimulation62. Supplementing pigs with magnesium acetate or sulfate heptahydrate for 5 days before slaughter, Chen et al.99 found lower concentrations of creatine kinase and serum lactate. Alonso et al.100 found a reduction in water loss and an improvement in meat quality in pigs supplemented with magnesium aspartate and Frederick et al.101, a reduction in the percentage of PSE meat. When supplementing pig diets for 7 days before slaughter with magnesium oxide at doses between 0 and 0.6%, Tarsitano et al.102 found a decrease in serum cortisol levels and meat water loss in those receiving the mineral when compared to the control group.

Dietary creatine supplementation for pigs, in the pre-slaughter period, also increases the muscle concentration of creatine and phosphocreatine, decreasing the rate of post-mortem glycolysis and promoting an improvement in meat quality. When creatine enters the muscle cells it is converted into phosphocreatine by the action of the creatine kinase enzyme. Phosphocreatine is involved in the mitochondrial oxidative production of ATP (aerobic energy production pathway). The higher concentration of creatine in the muscle soon after slaughter ensures the initial oxidative production of ATP, delaying the conversion of glycogen into lactic acid+H+ (glycolytic pathway) and, consequently, delaying production of lactic acid and pH drop in the meat103. Li et al.104 found an increase in initial pH values and a decrease in drip water loss in meat from Longissimus dorsi and Semitendinosus muscles of pigs that were fed diets supplemented with creatine monohydrate (0.8%) for 15 days before slaughter. Similar results were reported by Lindahl et al.103 and Lahucky et al.105.

The effects of pre-slaughter stress on meat quality can also be mitigated with the dietary use of tryptophan. The amino acid is a serotonin precursor, a neurotransmitter that has a sedative effect, acting to reduce hunger sensation, pain and expression of sexual and aggressive behaviours. When supplementing pigs with tryptophan-containing diets, corresponding to treatment with high tryptophan to amino acids isoleucine, leucine, valine, phenylalanine and tyrosine ratio (13.2%) and comparing to a control diet (only 3.4% of this ratio), Koopmans et al.106 found lower basal levels of cortisol, noradrenaline and adrenaline after a stress condition.

Pigs supplemented with 0.5% tryptophan five days before slaughter or with a bolus containing 0.1 g tryptophan kg1 b.wt., 2 hrs before slaughter had lower plasma cortisol and lactate levels and, consequently, higher pH values 45 min after slaughter and less drip water loss in the meats107.

Additionally, dietary supplementation of CLA can also exert positive effects on this process of muscle transformation into meat. CLA can influence the composition of the muscle fibre type20,108, as addressed by Men et al.109. These authors observed its role in the expression of the myosin heavy chain ATPase (MyHC) enzyme, increasing the expression of slow oxidation MyHC-I, which led to reduced damage from the sharp drop in muscle pH after slaughter.

Prevention of lipid, protein and meat pigments oxidation: In an equilibrium condition in vivo, cells can defend themselves against the damaging effects of oxygen radicals utilizing antioxidant mechanisms, including enzymatic (glutathione peroxidase, catalase and superoxide dismutase) and non-enzymatic (vitamins A, C, E) antioxidant systems110,111. Glutathione peroxidase and catalases are essential antioxidant enzymes that can break down hydrogen peroxide into water112. On the other hand, superoxide dismutase rapidly converts superoxide anion into hydrogen peroxide, which is less harmful. Glutathione peroxidase has selenium in the catalytic portion of its molecule and the enzyme superoxide dismutase can contain minerals such as copper/zinc or manganese. Thus, several micronutrients are involved in this antioxidant enzyme system, which signals the essentiality of their adequate supplementation in the diet.

During the meat storage period, cells show a gradual decrease in the ability to maintain their antioxidant defence system113. In this sense, the use of natural antioxidants in the diet of pigs has proven to be a high impact alternative for the preservation of its quality and to meet the anticipated expectations of its storage, mainly by promoting greater lipid and protein stability

Minerals as selenium114 and chromium115,116, vitamins C117,118 and E114,119,120 and conjugated linoleic acid (CLA)121-123, in addition to other substances found in plants, such as phytic acid present in grains124,125, are examples of products/principles are examples of products/principles with proven antioxidant action that can be incorporated into swine diets (or naturally be conveyed by the feed ingredients themselves) to stabilize oxidative processes.

Selenium is considered an antioxidant mineral because it is part of the active centre of the glutathione peroxidase enzyme, acting as a cofactor of thioredoxin reductase126. These enzymes are responsible for removing peroxides from cells, preventing oxidation of membrane lipids. Maintaining the cell membrane integrity is essential to prevent water loss and preserve the colour of the meat. According to Jiang et al.126, chelated selenium supplementation for pigs was able to reduce water loss and increase the tenderness of meats stored for six days at 4°C.

Dietary chromium supplementation for pigs can also improve meat quality by decreasing lipid oxidation and increasing its storage time. Chromium is related to the reduction of the formation of free radicals72, which come from AGEs (Advanced Glycation End-products) during glycation, a reaction in which the sugar molecule that is present in excess binds to a protein molecule (collagen, elastin, among others)127,128. Chromium reduces the action of circulating insulin and decreases the excess of circulating glucose129, determining a reduction of AGEs, which damage biological structures, leading to lipid oxidation of these meats. Peres et al.115 observed that chromium-methionine supplementation (200 ppm) for finishing pigs (15 days before slaughter) was effective in reducing meat lipid oxidation. According to Tian et al.116, supplementation of this mineral in chelated form (chromium-methionine) for pigs for 97 days before slaughter increased the final pH of the meat and decreased drip water loss, however, resulted in increased meat shear force.

When added to feed, vitamin E, one of the important non-enzymatic antioxidants, is deposited on the cell membranes and subcellular fractions of muscle and fat tissues, acting to prevent the formation of lipid hydroperoxides, degradation products that cause the deterioration of odour and flavour, which are associated with rancidity. The chromanol ring of α-tocopherol (α-T) is located between the phospholipid’s polar groups and the phytol side chain interacts with the unsaturated fatty acids towards the inside of the membrane. This specific localization of α-T in the membrane and the lateral mobility of the molecule allows it to function effectively to protect oxidizable PUFA from peroxidation by reactive oxygen species130,131. Dietary vitamin E supplementation, above the required levels (100-250 mg kg1 feed), is one of the most effective resources to promote improvement in meat quality, reducing its oxidation and its products96,97,105,114,117,120. By interrupting the chain of lipid oxidation in cell membranes, vitamin E preserves their integrity, which results in the increased water-holding capacity of the meat. In addition, by reacting with the free radical precursors of oxidation, it promotes their inhibition or acts in the sequestration of oxygen molecules, thus preserving the meat colour by keeping the iron in reduced form. Thus, there is a clear relationship between lipid oxidation and the oxidation of pigments responsible for meat colour132.

Regarding meat colour, myoglobin can be found in three main states in fresh meat: Oxymyoglobin (O2Mb), deoxy myoglobin (Mb+) and metmyoglobin (MMb). Oxymyoglobin (an oxygenated form of myoglobin) is responsible for the bright red colour that is highly desirable in meats, while the oxidized form (metmyoglobin) is responsible for the darker colour. The darkening or discolouration of meat results from the oxidation of Fe+2-Fe+3, forming metmyoglobin, whose greater or lesser presence is dependent on several factors, such as the rate at which the pH drops, the final pH value (the lower the pH, the higher the formation of metmyoglobin), temperature and the partial oxygen pressure133. With exposure to oxygen, the iron in myoglobin is oxidized, converting it to metmyoglobin, which limits its shelf life.

Vitamin C (ascorbic acid) is the main antioxidant in the extracellular fluid, intercepting oxidizing agents in the aqueous phase before they attack and cause oxidative damage to lipids. Silva et al.118 found that the use of vitamin C in meat preserved the pigmentation and lipid stability of the product. When supplementing diets of finishing pigs (60-112 kg b.wt.) with 250 mg vitamin C kg1 feed, Hanczakowska et al.117 found lower TBARS values in meat kept under freezing temperature for 2 weeks. However, after 6 months of freezing, TBARS values were equal to those of meat from the control pigs (which did not receive vitamin C supplementation).

Vitamin C is a precursor of oxalic acid and sodium oxalate134, which are potent inhibitors of pyruvate kinase enzyme135, that plays an important role in the anaerobic metabolism of carbohydrates. At the same time, vitamin C is also involved in the formation of lactic acid, which is mainly responsible for the pH drop after slaughter.

In this sense and working vitamin C supplementation (300 mg kg1 of feed per 21 days), Peeters et al.136 found more red and less pale pork loins compared with the control group.

Vitamin C, however, also promotes the reduction of Fe+3-Fe+2 and Cu+2-Cu+, increasing the pro-oxidant activity of these metals and generating excited oxygen, H2O2 and HO. Thus, vitamin C can be both a pro-oxidant and an antioxidant. In general, under low concentrations, it tends to be more of a pro-oxidant and an antioxidant at high concentrations137. Thus, the dietary supplementation of vitamin C as a meat antioxidant is still questionable.

On the other hand, according to Eichenberger et al.138, vitamin C associated with vitamin E interact synergistically to metabolically inhibit lipid peroxidation of the muscle fibre membrane and to protect the DNA. Vitamin C improves the antioxidant activity of vitamin E by reducing the tocopherol radicals to the active form of vitamin E139 or potentiating the vitamin E activity95. When subjecting finishing pigs to a diet supplemented with 0.4 mg of vitamin E and 0.1 mg of vitamin C kg1 of feed, Silva et al.118 observed a significant reduction in the lipid oxidation of the meat when compared to pigs that consumed a supplementation free feed.

According to Su et al.122, CLA also has an antioxidant effect in meat due to its conjugated structure, which is more resistant to the attack of free radicals and through its ability to incorporate into the membrane phospholipid fraction, which extends its antioxidant effect on these structures as well. Another aspect pointed out by Hur et al.140, is that CLA supplementation promotes a reduction in the content of arachidonic, linoleic and oleic acids in meat fat and, as a result, the fatty acid profile shows more saturated values. For this reason, meat from animals consuming CLA can be less susceptible to lipid oxidation and its consequences.

Effects of the lipid oxidation-reduction by CLA were reported by Joo et al.121, working with four levels of dietary supplementation of this compound (0, 1, 2.5 and 5%) and evaluated the quality of the samples throughout seven days of storage. Pinelli-Saaverdra et al.123, when supplementing pigs with 1% CLA for 30 days before slaughter, also reported that the intramuscular fat of these animals showed a higher percentage of saturated fatty acids and a lower percentage of monounsaturated fatty acids, resulting in lower TBARS indices, that is, less oxidation.

As to the role of betaine in preventing meat oxidation, Alirezaei et al.141 found that its inclusion in chicken increased the activity of glutathione peroxidase, catalase and superoxide dismutase enzymes and, as a consequence, reduced TBARS concentrations in meat. Betaine is a methylating agent and spares methionine, which contributes to an increased supply of substrate required for glutathione peroxidase synthesis142. Adversely, betaine can trap and subsequently dissolve the unpaired electron present in the free hydroxyl radical.

Meat tenderness: The final tenderness of the meat is dependent on several factors, such as the final sarcomere length, pH, collagen and intramuscular fat content, water retention capacity and action of proteolytic enzymes. Thus, ante and post-mortem factors are involved. Post-mortem proteolysis of structural proteins in muscle cells is the primary factor in the development of tenderness during the conversion of muscle into meat, with the calpain/calpastatin system being the most important family of proteases. Activation of these enzymes requires calcium and therefore they are called calcium-dependent proteases143. Thus, due to its action linked to calcium, vitamin D can improve the tenderness of the meat. The mechanism of action in this process comes from the effect of vitamin D on intestinal absorption and bone resorption of calcium, raising the levels of this mineral in the plasma and consequently, in the intracellular medium. Calcium, in turn, is important to activate calpains144.

Rowe et al.145 and Warner et al.146 have shown that vitamin E can also improve meat tenderness by preventing the oxidation of sarcoplasmic proteins. The main enzymes responsible for resolving rigour Mortis are μ-calpain and m-calpain, which have a cysteine residue that can be oxidized, making them less active. By preventing the oxidation of calpains, vitamin E contributes to increasing proteolysis during meat maturation, making it more tender.

Conversely, the inclusion of ractopamine in the diet of pigs can cause an increase in the force required to shear the meat, that is, making it tougher. When 7.5 mg ractopamine kg1 feed were given 28 days before slaughter, Rocha et al.147 found an increase in meat hardness measured in the Longissimus and Semimembranosus muscles. The authors concluded that this result was since ractopamine decreased the action of proteolytic enzymes responsible for the resolution of rigour mortis, determined by the low rate of myofibrillar fragmentation found in these animals. Another explanation for the impairment of the meat tenderness when the pigs were supplemented with ractopamine would be the increase in larger diameter fibres. Grijalva et al.148 reported an increase of almost 5% in the percentage of 2B fibres in the muscles of pigs that received 20 ppm ractopamine. These fibres generally have a larger diameter than other fibre types. Another hand, the diameter has a positive correlation with the meat shear force.

Minimizing meat odour defects: Sexual odour in pork is a worldwide problem, characterized by the sensory repulsion of meat by the consumer, who identifies the presence above acceptable limits of androsterone (testicular steroid) and scatole (product of hepatic degradation of the amino acid tryptophan), lipophilic substances released when meat is exposed to heat during processing, generating unpleasant taste and odour149.

Scatole (3-methyl indole) comes from the anaerobic degradation of dietary or endogenous tryptophan by the gut microbiota and therefore its concentration in the meat is dependent on the availability of tryptophan in the feed and the profile of the gut microbiota150. Thus, its presence in adipose tissue is common to both males and females and is responsible for the characteristic faecal odour and taste151.

Skatole comes from several stages of tryptophan degradation by microbial activity, mainly established in the large intestine of pigs, starting with the deamination of tryptophan, generating intermediate products, indole or indole-3-acetic acid. In the subsequent step, less than 0.01% of the total intestinal flora promotes the degradation of indole acetic acid, which is decarboxylated to skatole152,153.

Androsterone, on the other hand, is responsible for the taste/odour of male urine in the meat, which is also evident when the meat undergoes any thermal processing. Although androsterone and scatole act synergistically in expressing this "sensory defect", nutritional actions to minimize this damage have little impact on the first one, as its synthesis is directly related to sexual maturity and genetic profile issues154.

Conversely, the endogenous synthesis of scatole is mainly influenced by the animal’s diet, more specifically by its amino acid proportion, but can also be influenced by productive management and genetic factors.

To the dietary resources to minimize scatole production, there are several additives, ingredients and formulations that demonstrate satisfactory results. In their review, Wesoly and Weiler155 report that, in summary, these resources act on the modulation of the intestinal microbiota or directly on the gut mucosa, decreasing the process of apoptosis, which is involved in the reduction of scatole.

As to the dietary ingredient levels, the increased participation of fermentable carbohydrate sources leads to a reduced skatole formation and its accumulation in adipose tissue, as addressed in several studies. Feeds with high energy density or high levels of fermentable carbohydrates can alter intraluminal pH in the colon, which presumably influences the microbial ecosystem of the organ. They are the preferred energy sources for intestinal bacteria, leading to reduced protein catabolism and they increase the rate of cell proliferation in the mucosa and neutralize intestinal cell apoptosis, which leads to a decrease in scatole formation in the intestine155,156.

In addition, Claus et al.157 state that nutrition can also exert some influence on androsterone levels, concomitant with scatole. Feeds with a high nutrient: Energy ratio can indirectly increase androsterone and scatole levels, as they cause an acceleration in the sexual precocity of male pigs, a negative condition.

Nutritional enrichment of meat: Improvement of the nutritional/functional quality of pork through the diet provided to the animal is a relatively new concept, which consists in developing strategies to modulate or enrich the meat nutritional characteristics, or to implement bioactive substances in this food that are important for various issues related to human health.

The fat present in meats promotes flavour and aroma, affects their nutritional quality, storage time (oxidative stability) and technological quality158. The consumption of long-chain polyunsaturated fatty acids, from the omega-3 family, is known to promote benefits for human health, especially eicosapentaenoic (EPA) and docosahexaenoic (DHA) fatty acids159. The EPA and especially DHA play an important role in the prevention of some chronic diseases due to their actions, which involve the minimization of thrombus formation, reduction of inflammatory and immune suppression processes and cardiovascular problems. Although these compounds are most commonly found in cold-water fish, pork has low levels of omega-3 fatty acids, including DHA acid, which is particularly important for humans as it comprises approximately 14% of the cortex160.

To increase the nutritional content of omega-3 in pork, research has been conducted with supplementation of products with high levels of α-linolenic acids, such as flaxseed161-163 and canola161,162. However, as in humans, α-linolenic acid is poorly converted to DHA in pigs161. In this regard, the best responses to enrich pork with DHA were seen when fish oils164 or Schizochytrium sp., microalgae biomass162-166 were included in their diets. However, fish oils or co-products are commonly associated with adverse odour in meat and are a finite and expensive source164. In contrast, the use of algae for this purpose does not promote odour alterations in either fresh or processed pork.

The increase of DHA in pork is dose-dependent, as has been observed in different evaluations that have included its determination in various muscles167, loin162,168, subcutaneous fat168 and bacon169.

However, when the levels of polyunsaturated fatty acids in meat are raised, it becomes more susceptible to lipid oxidation158,163. It is therefore recommended to associate the antioxidant supplementation (such as vitamin E) in pig feeds to work around this problem.

Vitamin D deficiency is acknowledged as a public health issue, especially in countries where sunshine is limiting for its synthesis and food sources have low concentrations. Vitamin D is important in regulating bone mineral metabolism, but there is also growing evidence linking low vitamin D to a variety of non-skeletal health conditions, such as cardiovascular disease, diabetes, depression and various cancers170.

The term vitamin D encompasses precursors and metabolites, comprising vitamin D2 (ergocalciferol), vitamin D3 (cholecalciferol), 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 and the degradation products, which may still retain some metabolic activity.

Meat is among the few foods that contain natural vitamin D, making it an excellent food for biofortification170. In addition, meat contains the 25-hydroxyvitamin D (25-OH-D) metabolite, which has been shown to have a faster absorption and more effective in increasing serum 25-OH-D in humans. Dietary supplementation of pigs with 25-OH-D3 is effective in raising D3 and 25-OH-D3 levels in meat171. Duffy et al.172 found that pigs that were fed diets with 50 μg D3 or 25-OH-D3 kg1 feed for 55 days had higher total serum 25-OH-D3 and consequently, higher vitamin D3 activity in the Longissimus thoracis muscle.

As for the enrichment of pork with carotenoids via the diet, scientific information about it is limited and a larger volume of studies and use reports are related to egg, chicken and some fish production chains. Carotenoids are natural fat-soluble pigments found in vegetables, included in a group of more than 500 pigments, of which just over 40 are vitamin A precursors. β-carotene is considered one of the most bioactive carotenoids173. In human nutrition, β-carotenoids are associated with a decrease in the incidence of oxidation-induced diseases174-176. Their antioxidant function is based on their ability to suppress the formation of singlet oxygen, which is responsible for starting an oxidative process177.

The few studies on meat enrichment with carotenoids via feed have divergent results. Working with the supplementation of 200 mg of β-carotene kg1 of feed, Böhm et al.174 did not find any positive influence of this intake on the level of vitamin A in the meat when compared to a control feed. On the other hand, Dugan et al.178 indirectly observed that the inclusion of 0.5% conjugated linoleic acid in the diet of pigs significantly elevated hepatic retinol concentrations and retinol-binding protein mRNA expression, however, they did not estimate the presence of retinol in the muscles.

Regarding mineral modulation in pork via diet, some results of work with selenium, a mineral that participates in several biological functions and is therefore considered essential in human nutrition, stand out. Its deficiency has been associated with heart and cellular diseases. Research shows that selenium supplementation in humans can prevent some forms of cancer and improve immune functions62.

Of great relevance, selenium can also be included in pork by increasing its supplementation in the diet of finishing pigs. In this regard, it has been shown that, when complexed with an organic molecule and delivered through the feed to the pig, the selenium has a more effective presence in the body tissues of the animal114,126,179. Jiang et al.126 found that the inclusion of 0.5 ppm organic selenium in the diet of finishing pigs resulted in a 54% increase in muscle selenium or 0.2 mg per kilo of pork, which would be sufficient to provide 70% of the daily intake recommendation for a human adult consuming 200 g of pork.

CONCLUSION

Nutritional resources, feed management, adjustment of some minerals and vitamins levels and the use of some feed additives are tools that, in a specific way, can effectively modulate a series of pork sensory and technological characteristics. The identification of these opportunities is fundamental to meet the growing demands of the main elements of this important chain, the final consumer.

SIGNIFICANCE STATEMENT

This study concentrates information on several food and nutritional resources that have been scientifically proven recently to promote changes in the qualitative parameters of pork. This study allows researchers to identify and search in more depth, through new research, the effective application of actions that, in this line, will meet the demands of the industry and the modern consumer.

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