Evaluation of the contribution of skatole, indole, androstenone and androstenols to boar-taint in back fat of pigs by HPLC and capillary gas chromatography (CGC)

Preliminary Test of the Reduction Capacity for the Intestinal Adsorption of Skatole and Indole in Weaning Piglets by Pure and Coated Charcoal

To reduce the risk of boar taint, intact male piglets are immuno- or surgically castrated. One alternative is reducing skatole by adding skatole reducing or adsorbing substances to the boars' diet. Charcoal with a high capacity for adsorbing skatole and indole in vitro (tested before, data not shown) was fed to the boars to test the hypothesis that a fat coating prevents the unspecific adsorption of charcoal before entry into the large intestine while increasing skatole adsorption. Twelve male and six female weaning piglets with initial body weights of 7.74 ± 0.75 kg were fed for 18 (or 19) days with either 2% pure (untreated) charcoal or 4% coated (50% charcoal + 50% fat-coating) charcoal or no charcoal. After euthanasia, skatole and indole were quantified in caecum and colon chyme. Skatole and indole contents in caecum chyme were significantly lower (p < 0.05) in the group fed with coated charcoal (33 ± 4.2, 7 ± 2.8 µg/gDM, respectively) than in the group fed with pure charcoal (51 ± 7.3, 14 ± 3.0 µg/gDM) or with no charcoal (73 ± 12.6, 15 ± 1.7 µg/gDM). Similar effects were obvious for colon chyme. The results indicate that a fat coating of charcoal might prevent unspecific adsorption in the small intestine and might consequently lead to a higher adsorption capacity for skatole and indole in the large intestine, as skatole and indole concentrations in the chyme of caecum and colon were approximately 50% lower in the piglets who received coated charcoal.

Evaluation of Coated Biochar as an Intestinal Binding Agent for Skatole and Indole in Male Intact Finishing Pigs

The ban on piglet castration without anaesthesia poses a challenge for the meat industry since alternatives ensuring the production of flawless pork have to be established. The aim of this study was to evaluate the effects of biochar on skatole and indole concentration in faeces and plasma on a small scale in finishing boars to prove whether biochar was suitable for use in commercial pork production. Moreover, it was investigated whether biochar affects faecal properties or the performance. For a four-week trial period, 54 boars (bodyweight 97.2 ± 6.88 kg) were divided into three groups. The control (BC0) received no dietary biochar, one group received a diet containing 4% coated biochar (corresponding to 2% pure biochar) for the final two experimental weeks (BC2), and another group for the entire four weeks (BC4), respectively, prior to slaughter. Skatole and indole concentrations were measured in faeces and plasma at the beginning, in the middle and at the end of the trial. Mean skatole concentrations did not differ between groups, but in BC2 faecal skatole was significantly decreased at day 26, whereas in BC4 initial and final faecal skatole levels did not differ. At day 15 and 26, the faecal dry matter content was significantly higher in pigs fed the biochar diet (p < 0.05).

Combined Effect of Dietary Protein, Ractopamine, and Immunocastration on Boar Taint Compounds, and Using Testicle Parameters as an Indicator of Success

This study investigates the combined effect of immunocastration, dietary protein level (low, medium or high) and ractopamine hydrochloride supplementation (0 or 10 mg/kg) on the adipose concentrations of androstenone, skatole and indole in pigs, and explores whether body mass, carcass fatness or testicular parameters may be indicators of boar taint in these carcasses. Immunocastration was successful in decreasing testicle functioning, and adipose androstenone and skatole concentrations, in all individuals. Immunocastration decreased testicle weight and length, seminiferous tubule circumference and epithelium thickness. Testicle tissue from immunocastrates was also paler, and less red in color, in comparison to non-castrated controls. Dietary protein level and ractopamine hydrochloride supplementation had no influence on the adipose concentration of androstenone, skatole and indole. Testicle size and color were moderate to strong indicators of androstenone and skatole concentrations in the carcasses, and thus vaccination success. Immunocastration together with the adjustment of dietary protein and ractopamine hydrochloride supplementation, is successful in preventing boar taint while maintaining growth performance.

Feasibility of on/at Line Methods to Determine Boar Taint and Boar Taint Compounds: An Overview

Simple Summary

Due to welfare issues, the physical castration of male pigs is decreasing, and the entire male pig production is increasing. Fattening entire male pigs requires control due to the possibility of accumulating off odour/flavour called boar taint, which is mainly due to two compounds - skatole and androstenone. If carcasses with boar taint reach the market, it can cause a negative consumer reaction which may have economic consequences for the whole meat chain. Thus, it is necessary to sort out carcasses at the slaughter line. Today, a sensory quality control (human nose method) is used in some slaughter plants for this purpose. Detection by physical or chemical methods is also envisaged. A colorimetric method to determine skatole has been used in Danish abattoirs for decades, but it is foreseen that it will soon be replaced by the laser diode thermal desorption ion source coupled with a mass spectrometry equipment that allows a fully automated classification based on skatole and androstenone levels at speed line, with a delay of less than 40 min. Other potential methods such as the electrochemical biosensors, rapid evaporative ionization mass spectroscopy and Raman spectroscopy, still need further development and validation for an application at abattoir level.

Abstract

Classification of carcasses at the slaughter line allows an optimisation of its processing and differentiated payment to producers. Boar taint is a quality characteristic that is evaluated in some slaughter plants. This odour and flavour is mostly present in entire males and perceived generally by sensitive consumers as unpleasant. In the present work, the methodologies currently used in slaughter plants for boar taint classification (colorimetric method and sensory quality control-human nose) and the methodologies that have the potential to be implemented on/at the slaughter line (mass spectrometry, Raman and biosensors) have been summarized. Their main characteristics are presented and an analysis of strengths, weaknesses, opportunities and threats (SWOT) has been carried out. From this, we can conclude that, apart from human nose, the technology that arises as very promising and available on the market, and that will probably become a substitute for the colorimetric method, is the tandem between the laser diode thermal desorption ion source and the mass spectrometry (LDTD-MS/MS) with automation of the sampling and sample pre-treatment, because it is able to work at the slaughter line, is fast and robust, and measures both androstenone and skatole.

Identification and importance of mitochondrial citrate carriers and ATP citrate lyase for glycolipid production in Starmerella bombicola

Starmerella bombicola is a non-conventional yeast commercially used as a microbial cell factory for sophorolipid production. Sophorolipids are glycolipid biosurfactants composed of a glucose disaccharide sophorose and a fatty acid. In de novo sophorolipid synthesis, the fatty acid moiety is derived from the fatty acid synthesis (FAS) complex; therefore, the yeast's lipid metabolism plays a crucial role in sophorolipid biosynthesis. As a fatty acid precursor, citric acid is a key primary metabolite that connects carbohydrate and lipid metabolism, and in S. bombicola, it also has a regulatory effect on sophorolipid composition and productivity. We aimed to identify the mitochondrial transporters involved in citrate shuttling and the ATP citrate lyase (Acl), the enzyme that converts citric acid into acetyl-CoA. Subsequently, we studied their role in the citric acid shuttle and glycolipid synthesis and the potential of citrate metabolism as a genetic manipulation target for increased glycolipid synthesis. Bioinformatics analyses predicted 32 mitochondrial carriers of which two were identified as citrate transporters, named SbCtp1 and SbYhm2. Deletion of these mitochondrial carriers led to a lesser sophorolipid yield and a shift in the lactonic/acidic sophorolipid ratio. However, only the knockout of SbYhm2 caused a decrease of citric and an increase of malic acid extracellular concentrations. Additionally, deletion of SbAcl1 had a negative effect on S. bombicola's specific growth rate and sophorolipid synthesis and contributed to extra- and intracellular citric acid accumulation. Unexpectedly, SbAcl1 overexpression also decreased glycolipid production.Key Points• Starmerella bombicola is an industrially relevant microbial cell factory for biosurfactant production.• There are 32 predicted mitochondrial carriers in S. bombicola.• Citrate mitochondrial carriers SbYhm2 and SbCtp1 are essential for glycolipid synthesis in S. bombicola.• Deletion of SbAcl1 negatively affects growth and sophorolipid production in S. bombicola. Graphical abstract.

Review: Production and consumption of pork meat with different levels of boar taint / Revisión: Producción y consumo de carne de cerdo con diferentes niveles de olor sexual

Boar taint or sexual odor is a sensorial defect (a disagreeable smell) that occurs in some meat from entire male pigs. The main compounds responsible are androstenone and skatole stored in the fat of the animal. The main technique for reducing the androstenone level in the meat is by castration, either surgically or by immunocastration. Recent studies in the field of immunocastration appear to provide a solution to the androstenone problem by reducing the levels of this substance while taking advantage of the yield and carcass quality of entire male pigs. The quantity of skatole is mainly affected by the diet and handling of the pigs. The consumer decides whether to accept or reject the meat from male pigs according to the presence or absence of boar taint. However, variations in the consumer response depending on country, preparation and type of product consumed, as well as the individual's sensitivity to sexual odor, can be observed by looking at different works. According to some of these studies, UK, Irish, Canadian and US consumers will more readily accept meat from entire male pigs than consumers from France, Sweden, the Netherlands and Spain.

Effect of the use of entire male fat in the production of reduced salt fermented sausages

The effect of the use of entire male fat and salt reduction in dry fermented sausages was evaluated. Four different sausage formulations were manufactured with back fat from gilt or entire male and two different salt contents. The physicochemical parameters, sensory characteristics, texture, lipid composition, volatile compounds and boar taint compounds were analysed. The use of entire male fat produced the highest weight losses producing high hardness and chewiness while salt reduction produced a decrease in hardness. Entire male sausages had the lowest oxidation values due to the low content of C18:2n6 while salt reduction did not affect the oxidation process. Boar taint odour was due to the presence of androstenone and skatole but entire male fat sausages had different generations of volatile compounds. The presence of androstenone was perceived by consumers as abnormal odours but also other sausage characteristics such as texture (high hardness) and oxidation were detected due to the different chemical compositions of entire males versus gilts.

Genome-wide association analyses for boar taint components and testicular traits revealed regions having pleiotropic effects

Background

The aim of this study was to perform a genome-wide association analyses (GWAS) for androstenone, skatole and indole in different Pietrain sire lines and compare the results with previous findings in purebred populations. Furthermore, the genetic relationship of androstenone and skatole were investigated with respect to pleiotropy. In order to characterize the performance of intact boars, crossbred progenies of 136 Pietrain boars mated to crossbred sows from three different breeding companies were tested on four test stations. A total of 598 boars were performance tested according to the rules of stationary performance testing in Germany. Beside common fattening and carcass composition traits, the concentrations of the boar taint components and testicular size parameters were recorded. All boars were genotyped with the PorcineSNP60 Illumina BeadChip. The GWAS were performed using the whole data set as well as in sub groups according to the line of origin. Besides an univariate GWAS approach, principal component (PC) techniques were applied to identify common expression pattern affecting the biosynthesis and the metabolism of androstenone.

Results

In total, 33 SNPs were significantly associated with at least one of the boar taint components. Only one SNP was identified being significant in both subgroups. The analyses of the testes size parameters revealed 31 significant associations. The numbers of significant SNPs within the genetic groups evidenced the strong population specific effects. A multivariate approach using PC revealed 33 significant associations for five different PC.

Conclusions

Based on Pietrain sired cross bred boars, the mayor objective of our study was to identify QTL for boar taint components and to detect pleiotropy among boar taint and testes traits. The high number of identified QTL revealed that boar taint traits are influenced by a large number of loci. Analyzing pleiotropy allowed identifying a QTL affecting androstenone and the gonasomatic index. In this region, QTL for ovulation rate and age at puberty of sows have been described in literature. This supports the physiological findings that the androstenone level of boars and reproduction performance of sows might be linked by an antagonistic relationship.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0194-z) contains supplementary material, which is available to authorized users.

Evaluating the performance of sensory quality control: the case of boar taint

Detection of malodours referred to as 'boar taint' in entire male pigs is essential for quality control when refraining piglet castration. This study analysed the sensitivity and specificity of sensory evaluation by trained assessors (n=18) compared to chemical analysis of two marker compounds (androstenone, skatole) in backfat (n=794). Taking the measurement uncertainty into consideration, several cut-off thresholds for chemical analysis were exemplarily evaluated. Using the panel average score, sensitivity and specificity of sensory analysis ranged from 61 to 69% and 77 to 85%, respectively. Performance of individual assessors varied highly (sensitivity: 47 to 86%; specificity: 45 to 88%) and correlated to olfactory acuity to the compounds. According to receiver operating characteristic-curves, the average panel performed better than single assessors regardless of the sensory criterion shift. Agreement plots illustrate that high skatole levels are better detected than high androstenone levels (useful for assessor feedback). The agreement between sensory and chemical analyses was moderate. Assessors need to be selected carefully.

Biochemical, nutritional and genetic effects on boar taint in entire male pigs

Pork odour is to a great extent affected by the presence of malodorous compounds, mainly androstenone and skatole. The present review outlines the current state of knowledge about factors involved in the regulation of androstenone and skatole in entire male pigs. Androstenone is a pheromonal steroid synthesised in the testes and metabolised in the liver. Part of androstenone accumulates in adipose tissue causing a urine-like odour. Skatole is produced in the large intestine by bacterial degradation of tryptophan and metabolised by hepatic cytochrome P450 enzymes and sulphotransferase. The un-metabolised part accumulates in adipose tissue, causing faecal-like odour. Androstenone levels are mostly determined by genetic factors and stage of puberty, whereas skatole levels in addition to genetic background and hormonal status of the pigs are also controlled by nutritional and environmental factors. To reduce the risk of tainted carcasses entering the market, male pigs are surgically castrated in many countries. However, entire males compared to castrates have superior production characteristics: higher growth rate, better feed efficiency and leaner carcasses. Additionally, animal welfare aspects are currently of particular importance in light of increasing consumers' concerns. Nutrition, hormonal status, genetic influence on boar taint compounds and the methods to develop genetic markers are discussed. Boar taint due to high levels of skatole and androstenone is moderately heritable and not all market weight entire males have boar taint; it should thus be possible to select for pigs that do not have boar taint. In these studies, it is critical to assess the steroidogenic potential of the pigs in order to separate late-maturing pigs from those with a low genetic potential for boar taint. A number of candidate genes for boar taint have been identified and work is continuing to develop genetic markers for low boar taint. More research is needed to clarify the factors involved in the development of boar taint and to develop additional methods to prevent the accumulation of high concentrations of skatole and androstenone in fat. This review proposes those areas requiring further research.

Review of analytical methods to measure boar taint compounds in porcine adipose tissue: the need for harmonised methods

This comprehensive review describes the analytical methods developed for quantification of the boar taint compounds skatole and androstenone in porcine adipose tissue. The following parts are considered; sampling, sample preparation, calibration and instrumentation. Additionally, method performance characteristics and level of validation of the existing methodology are discussed. It is concluded that there is a need for further validation of existing methods and need for standardisation of methodology to quantify boar taint compounds. Facing a possible near future ban of castration of male piglets would enforce further method harmonisation in this field.

Prevalence and relationships of sensory taint, 5α-androstenone and skatole in fat and lean tissue from the loin (Longissimus dorsi) of barrows, gilts, sows, and boars from selected abattoirs in the United States

This study assessed prevalence of boar taint in backfat and lean of barrows, gilts, sows, and boars, collected from abattoirs, without knowledge of the farms of origin, in different regions in the United States. Concentrations of 5α-androstenone (liquid chromatography/mass spectroscopy) and skatole (liquid chromatography with fluorescent detection) in backfat were measured. A trained panel evaluated boar taint aroma in heated samples. Mean 5α-androstenone and skatole levels were low among barrows, gilts, and sows, whereas 55.8% of boars scored above a 1.0 μg/g threshold for 5α-androstenone concentrations and 34.2% were above a 0.2 μg/g threshold for skatole concentrations. Mean aroma scores for backfat and lean from barrows, gilts, and sows were low. In comparison, 59.2% of boars had elevated mean aroma scores from fat samples and 31.7% from lean. Importantly, boar taint aroma was detectable by the trained panel in at least some animals in each of the sex classes.

Gene expression of 3beta-hydroxysteroid dehydrogenase and 17beta-hydroxysteroid dehydrogenase in relation to androstenone, testosterone, and estrone sulphate in gonadally intact male and castrated pigs

Androstenone is one of the main compounds responsible for boar taint, and 3beta-hydroxysteroid dehydrogenase (3betaHSD) might be involved in its metabolism. In this study, the gene expression of 3betaHSD and 17beta-hydroxysteroid dehydrogenase (17betaHSD) were determined by real-time PCR analysis and related to the concentrations of androstenone, testosterone, and estrone sulphate (E1S). The experiments were performed on gonadally intact male pigs classified based on high or low fat androstenone concentrations, as predetermined by HPLC, as well as on immunocastrated and surgically castrated male pigs. The male pigs with high androstenone concentrations in fat had low 3betaHSD gene expression in liver and testis. Moreover, the 17betaHSD gene expression in liver, but not in testis, varied negatively with fat androstenone concentrations. Immunocastrated and surgically castrated male pigs had nondetectable concentrations of fat androstenone and plasma testosterone and E1S, and the castration procedure induced a significant increase of 3betaHSD and 17betaHSD gene expression. The mRNA expression was generally much greater from the 3betaHSD than from the 17betaHSD gene. Furthermore, fat androstenone was negatively correlated with liver 3betaHSD gene expression (Pearson correlation, r = -0.69; P < 0.05), and the 17betaHSD gene expression in liver was negatively correlated with plasma E1S (r = -0.95; P < 0.001), indicating an important role of liver 17betaHSD in the estrogen metabolism of gonadally intact male pigs. Another strong correlation was found between 3betaHSD and 17betaHSD gene expression in liver of the gonadally intact male pigs (r = 0.86; P < 0.01), possibly reflecting similar regulation mechanisms of these genes.

Flavor improvement in pork from barrows and gilts via inhibition of intestinal skatole formation with resistant potato starch

Skatole originates from microbial processing of tryptophan in the large intestine of pigs and accumulates in adipose tissue. Formation may be inhibited by the anti-apoptotic function of butyrate formed out of raw potato starch. Two groups of pigs (each consisting of gilts and barrows) were fed from 30 to 110 kg life weight either a conventional diet (controls; n = 35) or an isocaloric diet containing 300 g of raw potato starch/kg of body weight (RS; n = 34). Skatole concentrations were measured in colon content, blood, and adipose tissue. Odor of cooked meat samples was evaluated by a test panel. RS reduced concentrations in colon content and blood plasma (P < 0.001). Back fat concentrations were decreased significantly from 25 to 1.40 ng/g (barrows; P < 0.001) and from 40 to 9 ng/g (gilts; P < 0.001). Odor rating (scale of 1-5 from very unpleasant to very pleasant) was 3.07 for low skatole concentrations and 2.66 for both medium and high skatole concentrations (P < 0.05).

Does the Mind Reflect the Mouth? Sensory Profiling and the Future

Descriptive Sensory Profiling methods have been used for over half-a-century. As the application of these methods obviously does not occur in a vacuum, we sketch the contextual basis of these methods. A unifying model, illustrating the tasks required from a sensory panellist, is presented with the aim of illuminating some major differences between methods. The history and main developments in the field of descriptive sensory profiling are outlined in a basic pedigree of methods. The authors suggest an approach to sensory profiling where taylor-made, problem oriented, methods are devised and used.

Determination of androstenone in pig fat using packed column supercritical fluid chromatography-mass spectrometry

Packed column supercritical fluid chromatography (SFC) in combination with atmospheric pressure chemical ionisation mass spectrometry was applied to the analysis of androstenone in pig fat samples. Liquefied fat samples were dissolved in dichloromethane and analysed directly by SFC without any sample purification. Chromatographic separation was achieved with a density/pressure gradient using pure carbon dioxide as the mobile phase and the analysis resulted in a quantitation limit of 0.25 microg/g with 1 microl injection volume. Good agreement was found between the SFC method and time-resolved fluoroimmunoassay by the analysis of 15 boar back fat samples.

Rapid determination of skatole and indole in pig back fat by normal-phase liquid chromatography

A rapid normal-phase high-performance liquid chromatographic method for the quantitative determination of indole and skatole in pig back fat samples has been developed. The compounds were extracted by shaking the samples at ambient temperature in hexane-2-propanol (92:8, v/v). The sample preparation procedure was simple because it was not necessary to remove the fat from the samples. The compounds were separated on a 250 x 4.6 mm I.D., 5 micron Hypersil aminopropylsilica column. Fluorescence (excitation at 280 nm and emission at 360 nm) was used for selective detection. Recoveries for skatole and indole, relative to the internal standard, were 10.3 +/- 0.9% and 99.6 +/- 4.4%, respectively. Linearity determined in fat samples was in the range of 0.05-1 microgram/g and the correlations observed were R2 = 0.9914 for the indole and R2 = 0.9916 for skatole.

Rapid high-performance liquid chromatographic method for simultaneous determination of androstenone, skatole and indole in back fat from pigs

A rapid high-performance reversed-phase liquid chromatographic method for the simultaneous quantitative determination of the main boar taint compounds androstenone, skatole (3-methylindole) and indole, in back fat from pigs has been developed. The compounds are extracted by a simple homogenisation of adipose tissue in methanol; interfering lipids are removed by precipitation after cooling and centrifugation. Androstenone is derivatized pre-column with dansylhydrazine (5 min at ambient temperature) using BF3 as catalyst. The compounds are separated on a 60 x 4.6 mm I.D., 3 microns Hypersil ODS column (Hewlett-Packard) using a step-gradient; total time for the separation is 15 min. Fluorescence is used for selective detection. The limit of quantitation for indole and skatole is 30 ng/g and for androstenone 200 ng/g back fat. The results for skatole obtained by the present method were compared with those of colorimetric method, while androstenone determinations were compared with two GC-MS methods and a RIA method. The correlations observed were in the range of 0.946-0.993. The average contents of androstenone and skatole were 640 +/- 700 and 78 +/- 113 ng/g (n = 1162 male pigs), respectively.

Gas chromatographic determination of indole and 3-methylindole (skatole) in bacterial culture media, intestinal contents and faeces

A simple, rapid and inexpensive gas chromatographic method was developed for the determination of indole and 3-methylindole (skatole) in faeces, intestinal contents and bacterial cultures. It involves a simple homogenization and extraction with chloroform. The extract is injected onto a gas chromatograph equipped with a 12.5-m fused-silica capillary column coated with BP20 and a film thickness of 0.5 micron. To simplify the chromatograms and to get a higher sensitivity a nitrogen-phosphorus-sensitive detector is applied. The detection limit for indole and 3-methylindole under the conditions employed is 20 micrograms/kg, which is well below the values typically found in intestinal contents (up to 100 mg/kg). Recovery for both compounds was close to 100%, and the mean coefficients of variation were 3.5% for indole and 3.0% for 3-methylindole. The method has demonstrated its practical value in the analysis of more than 50,000 samples in our laboratory. More than 100 samples can be analyzed per day.

High-performance liquid chromatographic method for the determination of 3-methylindole (skatole) and indole in adipose tissue of pigs

A rapid method for the determination of skatole (3-methylindole) and indole in adipose tissue of pigs by reversed-phase high-performance liquid chromatography has been developed. Tissue samples were melted in a microwave oven, and 100 microliters of the liquid fat were dissolved in 1 ml of n-hexane and extracted with acetonitrile-water (75:25, v/v). Portions of 100 microliters of the solution were used for chromatographic analysis. Elution was performed on a reversed-phase column with a mobile phase composed of acetic acid and isopropanol (70:30, v/v). A fluorescence detector was used for quantification. The detection limit was 4 ng/g fat. The mean recoveries of added amounts of skatole and indole were 98.9 and 93.8%, respectively. The mean coefficients of variation were: inter-assay, 6.6% (skatole) and 8.8% (indole); intra-assay, 4.2% (skatole) and 2.9% (indole). Mean skatole concentrations in fat samples from boars (40 ng/g; n = 349) were not significantly higher than those from barrows (24 ng/g; n = 98).