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.     

Absence of an effect of dietary fibre or clinoptilolite on boar taint in entire male pigs fed practical diets

This study aimed to evaluate the possibility of reducing boar taint in boars (Piétrain × Hybrid) by addition of different feed ingredients (raw potato starch (RPS) 10%, raw potato starch 10% + wheat bran 5% (RPS + WB), lupins 10%, inulin 5%, clinoptilolite 1%) to a standard diet over a period of 4–6 weeks before slaughter. Control boars (CBOAR) as well as barrows were fed the standard diet. Efficacy of the different feed ingredients was evaluated by different boar taint detection methods: hot iron method, consumer panel, expert panel and laboratory analysis. According to all detection methods, clear differences were noticeable between boars and barrows. No differences in boar taint incidence were found between the boars on the different dietary treatments as assessed by consumers, experts, hot iron method or the concentration of skatole in fat. A significant effect on indole level was found, but no further differentiation could be made. The concentration of backfat androstenone was significantly higher for the inulin and control boar group compared to the lupin group.     

Predicting the likelihood of developing boar taint: Early physical indicators in entire male pigs

Three potential early-age predictors of which boars are likely to develop boar taint (testes volume, skin lesions and dirtiness) were measured on 102 boars every fortnight from 10weeks of age until slaughter. These predictors were correlated with the level of boar taint according to the hot iron method and the concentrations of skatole and androstenone as determined by chemical analysis. The chance of no/low boar taint according to the hot iron method decreased with higher testes volume (weeks 22 and 24) and increased with skin lesion score (weeks 12, 16 and 18). For the concentrations of androstenone and skatole, the strongest correlation was found with testes volume in week 12. Skin lesions in week 16 were negatively correlated with skatole levels. Dirtiness was negatively correlated with skatole concentrations (week 18) but positively correlated with androstenone concentrations (weeks 20 and 22). Testes volume has the greatest potential for predicting the likelihood of developing boar taint.     

Predicting the likelihood of developing boar taint: Early physical indicators in entire male pigs

Three potential early-age predictors of which boars are likely to develop boar taint (testes volume, skin lesions and dirtiness) were measured on 102 boars every fortnight from 10 weeks of age until slaughter. These predictors were correlated with the level of boar taint according to the hot iron method and the concentrations of skatole and androstenone as determined by chemical analysis. The chance of no/low boar taint according to the hot iron method decreased with higher testes volume (weeks 22 and 24) and increased with skin lesion score (weeks 12, 16 and 18). For the concentrations of androstenone and skatole, the strongest correlation was found with testes volume in week 12. Skin lesions in week 16 were negatively correlated with skatole levels. Dirtiness was negatively correlated with skatole concentrations (week 18) but positively correlated with androstenone concentrations (weeks 20 and 22). Testes volume has the greatest potential for predicting the likelihood of developing boar taint.     

Boar taint in pigs selected for components of efficient lean growth rate

Genetic and nutritional effects on the boar taint traits of androstenone, skatole and indole and the eating quality traits associated with boar taint were examined by testing animals from four selection lines and a control line on isoenergetic diets, which differed in ileal digestible lysine: digestible energy (0.40, 0.76 and 1.12 g lysine/MJ DE). The selected lines resulted from seven generations of selection for high daily food intake, lean food conversion ratio (LFC) and lean growth rate on ad libitum (LGA) or restricted (LGS) feeding regimes in a Large White population. During performance test, from 30 to 90 kg, boars were fed on either ad libitum or restricted (0.75 g/g ad libitum daily food intake) feeding regimes. A sensory panel assessed heated fat samples for androstenone odour, skatole odour and abnormal odour. There were no significant differences between the selection and control lines or diets for log transformed fat content of androstenone, skatole, indole. The significant diet with feeding regime interaction for log transformed fat content of skatole and indole were essentially due to significantly higher log transformed fat contents with ad libitum feeding of the high lysine diet compared to restricted feeding (skatole: -1.94 vs -3.06, s.e.d. 0.43; indole: -3.44 vs -4.22, s.e.d. 0.28), as differences between feeding regimes on diets A and C were not significantly different from zero. There were no significant differences between selection and control lines for sensory panel score for abnormal odour or androstenone odour, but the LFC and LGA selection lines had a significantly higher skatole odour score than the LGS selection line. Neither diet nor feeding regime had any significant effect on sensory panel assessment of odour. Log transformed fat content of androstenone and skatole were significantly correlated with sensory panel score for skatole odour (0.37 and 0.46, s.e. 0.12), but not with sensory panel score for androstenone odour (0.06 and 0.09), such that they would not be useful predictors of androstenone odour.     

Effect of feeding fermentable fibre-rich feedstuffs on meat quality with emphasis on chemical and sensory boar taint in entire male and female pigs

Skatole, androstenone and other compounds such as indole cause boar taint in entire male pork. However, female pigs also produce skatole and indole. The purpose of this experiment was to minimise boar taint and increase overall impression of sensory quality by feeding entire male and female pigs with fibre-rich feedstuffs. The pigs have been fed three organic diets for either 1 or 2 weeks prior to slaughter of which two diets contained different fermentable fibre-rich feedstuffs – 10–13.3% dried chicory roots or 25% blue lupines. These two treatments were compared with pigs fed with an organic control diet for either 1 or 2 weeks prior to slaughter. Lupines significantly reduced skatole in blood and backfat for both genders after 1 week. Moreover, lupines showed negative impact on growth rate and feed conversion whilst chicory showed no significant differences in this respect. However, the indole concentration was significantly lower in chicory than lupine fed pigs. From a sensory perspective, chicory and lupine feeding reduced boar taint since odour and flavour of manure related to skatole and urine associated to androstenone were minimised. The level of boar taint in the entire male pigs was most effectively reduced after 14 days by both fibre-rich feeds while lupine had the largest influence on “boar” taint reduction in female pigs.     

Investigation of factors responsible for the development of boar taint

The objective of this study was to evaluate the effect of 16-androstene steroids, skatole, size of accessory sexual glands and live weight on the development of boar taint in entire male pigs. In Trial I, 35 Yorkshire entire males were slaughtered at 116 ± 3.0 kg live weight. Levels of 16-androstene steroids in the salivary gland (STs), 16-androstene steroids in the fat (STf), skatole in the fat (SK), length of the bulbourethral gland (BG) and weight of the salivary gland (SG) were 44.2 ± 4.7 μg/g, 0.47 ± 0.06 μg/g, 0.20 ± 0.03 μg/g, 11.7 ± 2.2 cm and 63.50 ± 6.73 g, respectively. Samples of loin chops from the animals were evaluated by a trained sensory panel for the presence of boar odour and boar flavour. Sensory scores were correlated with all the investigated factors (P 0.05). Boar odour was particularly well correlated with SG (r = 0.61) and BG (r = 0.60); the correlation with STs was r = 0.53, with SK was r = 0.46 and with STf was r = 0.44. Similar results were obtained for boar flavour. Therefore, both skatole and 16-androstene steroids had significant effects on boar taint. The best explanation of boar taint was obtained by combining measurements of skatole levels in fat and bulbourethral gland length. Trial II involved 36 Yorkshire pigs, 12 each of entire males, gilts and castrates, slaughtered at 104.6 ± 1.1, 102.8 ± 1.8 and 101.0 ± 1.6 kg, respectively. Entire males were chosen which had low levels of 16-androstene steroids (in salivary gland: 13.4 ± 1.5 μg/g; in fat: 0.49 ± 0.09 μg/g) and low levels of skatole in the fat (0.10 ± 0.01 μg/g) but long bulbourethral glands (11.3 ± 0.2 cm). Sensory evaluation performed by a trained panel revealed stronger boar odour and boar flavour from meat from entire males than from other genders (P 0.05). The present results indicate that, although both 16-androstene steroids and skatole are important, they cannot completely account for the occurrence of boar taint detected by a trained sensory panel. Other factors, due to sexual maturity as indicated by the length of bulbourethral gland, are involved and should be considered when estimating boar taint in entire male pigs.     

The effect of age on distribution of skatole and indole levels in entire male pigs in four breeds: Yorkshire, Landrace, Hampshire and Duroc

High skatole and to a lesser degree indole levels in some entire male pigs result in occurrence of off-flavor in meat, called boar taint. In this study, plasma samples from 117 Yorkshire, 134 Landrace, 184 Hampshire and 75 Duroc entire male pigs collected at different ages were analyzed for skatole and slightly fewer samples for indole concentrations. For each breed, a distinct age-related distribution of plasma skatole levels was observed. Skatole levels were increased at approximately 180-200 days of age, reaching very high levels is some individuals. Afterwards, the levels decreased; in Yorkshire and Landrace at approximately 240-260 and in Hampshire and Duroc at 310-360 days of age. The decrease of skatole levels was also observed in samples collected from 15 Landrace boars at two different ages: 16.8 μg/L (SD=17.36) vs. 5.9 μg/L (SD=3.48, p=0.016) in samples taken at the mean age of 210 and 314 days, respectively. Indole levels showed similar age- and breed-related variations to skatole. At the time of increased skatole concentrations (age from 180 to 360 days, depending on breed) 25.5% of Yorkshire, 31.6% of Landrace, 20.3% of Hampshire and 61.1% of Duroc entire male pigs had skatole levels above 12.6 μg/L of plasma, which corresponds to the threshold level of 0.20 μg/g of fat that is used for selecting tainted carcasses. The results indicate that increased levels of skatole in entire male pigs are related to puberty and taking skatole measurements at that age could be advantageous when considering genetic selection to reduce boar taint level in entire male pigs. Breed differences should be also taken into account.     

Effect of feeding fermentable fibre-rich feedstuffs on meat quality with emphasis on chemical and sensory boar taint in entire male and female pigs

Skatole, androstenone and other compounds such as indole cause boar taint in entire male pork. However, female pigs also produce skatole and indole. The purpose of this experiment was to minimise boar taint and increase overall impression of sensory quality by feeding entire male and female pigs with fibre-rich feedstuffs. The pigs have been fed three organic diets for either 1 or 2 weeks prior to slaughter of which two diets contained different fermentable fibre-rich feedstuffs – 10–13.3% dried chicory roots or 25% blue lupines. These two treatments were compared with pigs fed with an organic control diet for either 1 or 2 weeks prior to slaughter. Lupines significantly reduced skatole in blood and backfat for both genders after 1 week. Moreover, lupines showed negative impact on growth rate and feed conversion whilst chicory showed no significant differences in this respect. However, the indole concentration was significantly lower in chicory than lupine fed pigs. From a sensory perspective, chicory and lupine feeding reduced boar taint since odour and flavour of manure related to skatole and urine associated to androstenone were minimised. The level of boar taint in the entire male pigs was most effectively reduced after 14 days by both fibre-rich feeds while lupine had the largest influence on “boar” taint reduction in female pigs.     

The measurement of the responses to different odour intensities of `boar taint' using a sensory panel and an electronic nose

This study explored the possibility of using an electronic nose (e-nose) with a 12-conducting-polymer sensor array combined with pattern recognition routines to discriminate between varying intensities of boar taint. A set of samples in a model system comprising a neutral lipid base containing varying combinations of androstenone and skatole were tested, as were pork fat samples. The e-nose responses for pork fat were calibrated against those given by a trained 10-member sensory panel for abnormal odour of the same samples from a total of 60 Large White cross-bred pigs. The e-nose responses related strongly to those of the sensory panel with a significant (p<0·01) canonical correlation of 0·78. The data set was used to develop a discriminant function for grouping pork samples into three `response classes': normal, doubtful and abnormal. Based on this, the e-nose identified all the abnormal samples correctly. However, 16% of the normal samples were also classified as abnormal. It was concluded that, in general, the electronic nose can discriminate between different levels of boar taint and that although a high specificity of sensors to androstenone and skatole may be desirable it may not be entirely important to the development and configuration of a boar taint sensor array.     

Factors affecting the level of boar taint in entire male pigs as assessed by consumer sensory panel

A sensory study was conducted to determine consumer responses to meat from entire male pigs in order to evaluate different factors involved in causing boar taint, including 16-androstene steroids, skatole, size of accessory sexual glands and live weight. Pork chops from 33 entire Yorkshire male pigs were divided into three groups according to the levels of 16-androstene steroids in salivary glands, high (H; 56-114 μg/g), medium (M; 35-55 μg/g) and low (L; 26-6 μg/g), and were distributed among 59 families. Pork chops from gilts were used as control (C). The families were asked to rate liking of cooking aroma, flavour and texture, and overall liking. Consumer ratings were best related to the size of bulbourethral gland and salivary gland, followed by the levels of 16-androstene steroids in salivary gland. Meat from the L group received similar ratings to the C group, except the ratings for texture and overall liking were better than the C group. M samples were liked similarly to C, with the exception of some samples that had high fat skatole levels and which were liked less. H samples were liked less than C. The samples evaluated at the beginning of the experiment were liked more than those evaluated afterwards indicating sensitization of consumers to boar taint. The results indicate that levels of 16-androstenes in salivary glands can be used to estimate boar taint due to 16-androstenes; however, skatole measurements also need to be included. In addition, consumers are sensitive to some other maturity-related factors, which need to be considered when estimating the levels of boar taint. The effect of sensitization of consumers to boar taint requires more study.     

Effect of dietary chicory on boar taint

Following preliminary screening and feeding trials on farms supplying a commercial abattoir, 360 entire male pigs were used to evaluate the effects of different percentages of chicory (Cichorium intybus L.) on levels of boar taint compounds and sensory aspects in backfat. Pigs were fed 0, 3, 6 or 9% chicory in the diet, 30 pigs being sampled at 3 different times: initially to measure basal levels of skatole and androstenone and after 1 and 2 weeks on the test diets. Cooked samples of backfat were presented to a trained sensory panel for "sniff" tests. Chicory fed at 9% for 2 weeks reduced skatole levels significantly (P<0.001), with 0.55 of pigs below 0.05 μg/g, typical of levels in castrated males. Abnormal odour scores were significantly lower for pigs in this group compared with 0% pigs (P<0.001), however, androstenone concentration was significantly higher in this group after the 2 week feeding period (P<0.005). Thus, feeding 9% chicory for 2 weeks was effective in reducing backfat skatole concentrations and abnormal odour scores of cooked fat but not androstenone concentration.     

Quantitative determination of the boar taint compounds androstenone,skatole, indole, 3α-androstenol and 3β-androstenol in wild boars (Sus scrofa) reveals extremely low levels of the tryptophan-related degradation products

The major boar taint compounds androstenone and skatole as well as the minor compounds indole, 3α-androstenol and 3β-androstenol were determined in back fat samples of 23 male wild boars by applying a recently published SIDA–HS-SPME–GC/MS method. The boar pheromones androstenone, 3α-androstenol and 3β-androstenol were found in extraordinary high concentrations, resulting in mean values of 3329 ng/g androstenone, 1273 ng/g 3α-androstenol and 545 ng/g 3β-androstenol. Interestingly, skatole was not detectable in about 50% of the boars and negligibly low in all other samples as expressed by a mean skatole value of only 14 ng/g. Indole was also found in every sample, but again in low concentrations with a mean value of 40 ng/g. Possible factors explaining this remarkably low skatole deposition in wild boars such as intestinal flora and anatomy, dietary composition, housing or genetic predisposition are discussed in this paper.     

Threshold Detection of Boar Taint Chemicals Using Parasitic Wasps

Abstract: Surgical castration has been long used to prevent consumers from experiencing taint in meat from male pigs, which is a large problem in the pig husbandry industry. Due to obvious animal welfare issues, the EU now wants an alternative for castration, suggesting an urgent need for novel methods of boar taint detection. As boar taint is only a problem when taint chemicals exceed a well‐defined threshold, detection methods should be concentration‐specific. The wasp, Microplitis croceipes’ ability to learn and respond to particular concentrations of the boar taint compounds, skatole, androstenone, and indole was tested. Also tested was the wasps’ ability to discriminate between known concentrations of indole, skatole, and androstenone in real boar fat samples at room temperature. Wasps were trained using associative learning by providing food‐deprived wasps with sucrose–water in the presence of specific odor concentrations. Trained wasps’ responses were tested to a range of concentrations of 3 compounds. Wasps showed unidirectional generalization of learned concentration responses, whereby the direction of concentration generalization was shown to be chemical‐dependent. Through both positive (sucrose) and negative feeding experiences (water only) with varying compound concentrations, the wasps can also be conditioned to respond to concentrations exceeding a defined threshold, and they were successful in reporting low, medium, and high concentrations of indole, skatole, and androstenone in boar fat at room temperature. The need for threshold detection rather than simple detection of absence/presence applies to many food quality issues, including the detection of spoilage or pest damage in crops or stored foods. Practical Application: An inexpensive and reliable means of detecting boar tainted pork at slaughter to avoid tainted meat on the market and dissatisfied consumers.     

Supercritical Carbon Dioxide Extraction of Androstenone and Skatole from Pork Fat

Boar taint, an unpleasant odor observed in pork from some mature intact male pigs, is attributed primarily to the presence of two compounds in boar fat, androstenone (Sα-androst-16-ene-3-one) and skatole (3-methyl indole). A rapid extraction method for the preparation of fat samples for androstenone/skatole screening assays would lead to more efficient use of boar carcasses for pork production. Supercritical carbon dioxide removed 97 ± 2% of the androstenone (extraction at 40°C; 5 min) and 65 ± 3% of the skatole (extraction at 40°C; 20 min) from 0.5 g of boar backfat. Supercritical CO₂ provides the basis for a fairly rapid extraction method to remove compounds associated with boar taint.     

Comparison of heating methods and the use of different tissues for sensory assessment of abnormal odours (boar taint) in pig meat

Five heating methods (microwave, hotwire, boiling at 25 °C and 75 °C and melting) were used to generate cooking odours from backfat of entire male pigs and a 'composite' sample consisting of fat and muscle from the head along with salivary glands. The methods elicited significantly different scores for odours from 4 groups of 10 samples differing in their concentrations and ratios of skatole and androstenone. The odours (pork odour, abnormal odour, skatole odour and androstenone odour) were assessed by 3 experienced assessors. Correlations between skatole and androstenone concentrations and abnormal odour score in backfat were higher for skatole, suggesting it is the more important boar taint compound. In the composite sample, androstenone concentration was much higher than in backfat and androstenone was a more important contributor to boar taint. The microwave, hotwire and boiling (75 °C) methods produced the clearest separation between samples and the microwave method was considered the most suitable for on-line use.     

Testosterone and the incidence of boar taint: Effects of testosterone or testosterone propionate on the incidence of boar taint in implanted barrows

Boars, barrows and barrows implanted with testosterone or testosterone propionate via polydimethylsiloxane (Silastic) capsules were placed on test in individual pens at 10 weeks of age. Each animal was slaughtered at 110 kg and evaluated for growth rate, efficiency of feed utilization, carcass merit and the incidence of objectionable odors (boar taint). Five capsules of testosterone or testosterone propionate were used in barrows since they substantially elevated concentrations of serum testosterone, decreased serum LH and stimulated weights of the accessory sex glands. Large variations within and between litters of pigs were found for performance and carcass traits; thus, the influences of castration and testosterone replacement therapy on these traits were inconclusive. In contrast, the effects of castration and hormone treatment on the incidence of boar taint were more definitive. The incidence of boar taint was relatively high in boars, according to a consumer taste panel. This characteristic odor was appreciably lower in barrows and was not reinstated with either testosterone or testosterone propionate implants. These results suggest that testosterone is not itself responsible for boar taint and that 5-androstenone, the pheromone most closely associated with boar taint, is not produced by peripheral metabolism of testosterone. Additional studies are warranted to provide insight into the regulation of testicular steroid secretion in the boar and the contribution of these steroids to boar taint and protein anabolism.     

A human nose scoring system for boar taint and its relationship with androstenone and skatole

A system for sensory evaluation of boar taint was used to evaluate boar taint in fat samples from 6574 entire males. The term "human nose scoring" has been used to describe this system. The samples from each boar were heated with a hot iron and three panelists assigned scores of 0 to 4. The reproducibility of HNS ranged from 0.19 to 0.32 reflecting natural variation in the ability of human beings to detect different odors. The correlations of HNS with androstenone ranged from 0.22 to 0.52, while those with skatole ranged from 0.31 to 0.89, suggesting that skatole is a better predictor of boar taint. Considering (1) the relationship of HNS with the boar taint compounds, (2) the ability of HNS to capture variation not accounted for by the boar taint compounds, (3) low estimation costs and (4) low time requirements, HNS can be used in large scale evaluations of boar taint.     

Boar taint detection using parasitoid biosensors

The off-flavor boar taint associated with the substances skatole, androstenone, and possibly indole represents a significant problem in the pig husbandry industry. Boar taint may occur in meat from uncastrated sexually mature male pigs; consumers commonly show a strong aversion to tainted meat. Consequently, there is a need for rapid methods to sort out and remove tainted carcasses at the slaughterline. We tested the ability of wasps, Microplitis croceipes to perceive and learn the 3 boar taint compounds both individually and in combination using classical conditioning paradigms. We also established the effectiveness and reliability of boar taint odor detection when wasps were used as biosensors in a contained system called the "wasp hound" using a cohort of trained wasps. We found that the wasps are able to successfully learn indole, skatole and to also detect them when presented a 1:1:1 mixture of all 3 compounds. This was shown for both a single hand-manipulated wasp bioassay and when using the "wasp hound" detector device. In contrast, the wasps showed a weak conditioned response to androstenone at the concentration tested. The estimated gas phase concentrations that the wasps perceived during training were in the range of 10 ± 0.4 pg/s for skatole and indole, and 2 ± 0.5 pg/s for androstenone. We conclude that use of these wasps as biosensors presents a promising method for boar taint detection and discuss future training paradigms that may improve their responses to compounds such as androstenone. Practical Application: The development of a perceptive, inexpensive, and reliable means of detecting boar taint before the product is presented to sensitive consumers.