The problem of taint in pork-II. The influence of skatole, androstenone and indole, Presented individually and in combination in a model lipid base, on odour perception

Responses to varying concentrations of androstenone and skatole in a neutral lipid base were studied using a trained 10-member panel. Concentrations for skatole and androstenone ranged between 0-1.32 and 0-6.0 μg g(-1), respectively. Odour, androstenone and skatole intensities were assessed by the line scaling method. Triangular tests showed that the panel significantly distinguished indole from androstenone, p ≤ 0.01, but not from skatole nor mixtures of androstenone and skatole. Regression analyses showed linear relationships between skatole and androstenone concentrations and odour intensity but with a significant (p ≤ 0.01) androstenone-assessor interaction. Correlation coefficients with odour intensity were 0.64 and 0.63 for androstenone and skatole concentrations, respectively. Response surface curves showed that odour intensity depended on the linear terms of skatole, androstenone and on the quadratic term of androstenone. Androstenone and skatole intensity ratings revealed that the effect of skatole was enhanced by the presence of androstenone, but not the reverse. It was concluded that whilst both androstenone and skatole may be important for the sorting (classification) of pork, the importance of androstenone lies in its detection threshold whilst for skatole, the acceptability threshold may be a suitable criterion for quality control purposes.     

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.     

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.     

An international study on the importance of androstenone and skatole for boar taint: IV. Simulation studies on consumer dissatisfaction with entire male pork and the effect of sorting carcasses on the slaughter line, main conclusions and recommendations

An international study has been conducted in order to determine the respective contributions of androstenone and skatole to boar taint and their possible variations according to production systems and consumer populations. The presentation of the study and the main results concerning skatole and androstenone levels and data from sensory evaluation or consumer surveys are reported in companion papers. The present paper summarises the main conclusions of the study and gives tentative recommendations. A simulation study was conducted, based on the skatole and androstenone levels currently observed in European populations of entire male pigs and on the results of the consumer surveys. The first part of the simulation study demonstrated that, overall, 6.5% (odour) and 3.0% (flavour) more consumers were dissatisfied with entire male than with gilt pork. The differences were, however, very variable according to countries. Consumer dissatisfaction for the odour of entire male pork was mostly associated with high skatole levels, while androstenone had little influence on it. On the other hand, androstenone and skatole had similar contributions to the level of dissatisfaction for flavour. From the present study it is not possible to determine clear cut-off levels for androstenone/skatole. The regression equations presented in [Matthews, K. R., Homer, D. B., Punter, P., Béague, M. P., Gispert, M., Siret, F., Leask, H., Fonti i Furnols, M., & Bonneau, M. (2000). An international study on the importance of androstenone, skatole for boar taint: III. Consumer survey in seven European countries. Meat Science, 54, 271-283] provide a basis for decision making. However, due to methodological limitations, the results may underestimate consumer reaction to entire male pork. The second part of the simulation study demonstrated that sorting carcasses on the basis of androstenone/skatole would reduce, but not eliminate, differences in consumer dissatisfaction between entire male and gilt pork. For odour, taking androstenone into account did not improve the efficiency obtained from sorting using skatole only. For flavour, sorting using both compounds was more efficient than sorting using skatole only. Sorting out 15% of the entire males, on the basis of skatole only, would result in a difference in the proportion of dissatisfied consumers of 4.2% (odour) or 2.0% (flavour) between entire male and gilt pork. The results of the last part of the simulation study demonstrated that decreasing skatole in entire male pig populations, to levels as low as 0.10 ppm, would still result in a difference in the proportion of dissatisfied consumers of 3.2% (odour) or 1.6% (flavour). To reduce this difference further, the levels of both compounds would have to be reduced still further. The lowest difference that can be achieved is 2.3% (odour) or 0.4% (flavour). The conclusions of the present study may differ according to whether immediate commercial applications or long-term goals are considered. On the basis of the skatole and androstenone levels currently observed in entire male pig populations, sorting out procedures based on skatole is the easiest way to rapidly achieve a significant decrease in consumer dissatisfaction with entire male pork. In most countries, however, this will not be sufficient to obtain the same level of acceptability as with gilts. In the long term, a sharp reduction in both skatole and androstenone would have to be achieved in entire male pig populations to obtain low differences in acceptability between entire male and gilt pork.     

An international study on the importance of androstenone and skatole for boar taint: III. Consumer survey in seven European countries

There has long been debate over the relative importance for consumer acceptability of androstenone and skatole. The objective of this study was to investigate consumer reaction to pig meat of varying levels of androstenone and skatole in seven member states of the European Union. Over 4000 entire male pigs (and 233 gilts as a control) were produced in six European countries, and a sub-sample of 420 were selected to represent a range of androstenone and skatole levels. Loin samples from these pigs were distributed to the seven participating countries. Samples were prepared and cooked according to standardised procedures in each of the countries and presented to pork consumers. Each consumer rated samples from five pigs for liking of flavour and liking of odour. Although there were significant differences between country panels in their liking of meat from entire males generally, there was a greater degree of dislike as both androstenone and skatole increased. This reaction was stronger for skatole than androstenone and consumers reacted more negatively in terms of odour than flavour.     

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.     

Temporal aspects related to the perception of skatole and androstenone, the major boar odour compounds

Pork fat samples from 50 boars slaughtered at a commercial abattoir were analysed for skatole and androstenone concentrations and grouped within a 3×3 matrix representing low, medium and high levels of the odour compounds. A 10 member sensory panel, screened and trained to recognise and quantify skatole and androstenone odour intensities, was used to verify the human perception of boar odour in these pork fat samples immediately after heating (±65°C) and following a cooling period of ten minutes (±25°C). Principal component analysis revealed that the sensory panel differentiated the pork fat samples first and foremost on the basis of the presence or absence of androstenone and/or skatole odours and secondly on the character of the androstenone or skatole odour. Evidence of adaptation towards the odour of skatole was shown by a decreased sensitivity over replicates. Sensory perception of boar odour seems to have a temporal character which can be explained by differences in volatilisation (involving both odourant release and retention) of skatole and androstenone, odour synergism and possibly differences in the properties of the fat matrix of different samples. The results suggest that factors influencing volatilisation of skatole and androstenone should also be considered when predicting sensory responses of boar 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.     

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.     

Acceptability of entire male pork with various levels of androstenone and skatole by consumers according to their sensitivity to androstenone

Consumer acceptability of entire male pork at eating was assessed in three experiments. The 140 consumers involved in each experiment were classified as insensitive (INSENS) to the odor of pure androstenone or sensitive perceiving it as pleasant (SENS-PLEA) or unpleasant (SENS-UNPL). Entire male pork with very low skatole and androstenone levels (LS-LA) was as well accepted as gilt pork, whatever the consumer category. Entire male pork with elevated levels in both skatole and androstenone (HS-HA) was clearly differentiated from LS-LA pork by SENS-UNPL, but not by SENS-PLEA or INSENS consumers. Whatever the consumer category, entire male pork with elevated levels of androstenone and very low levels of skatole (LS-HA and LS-HHA) were not significantly differentiated from LS-LA pork. The results suggest that, in the conditions of the present experiment, androstenone and skatole totally explain boar taint at eating and that the acceptability threshold for androstenone, in the absence of skatole, is in the range of 2-3 μg/g liquid fat.     

The effects of castration on the eating quality of dry-cured ham

The influence of the castration of entire male pigs on the eating quality of dry-cured ham was evaluated. Forty-eight dry-cured hams (435-day aged) were studied from entire and castrated males of two different crossbreeds. The proximate composition of the meat and the androstenone, indole and skatole fat content were determined. The entire hams were classified according to the androstenone and skatole content. Sensory analysis was carried out by a trained panel, evaluating marbling, juiciness, saltiness, graininess, toughness, overall flavour, boar odour and boar flavour. Also carried out was a preference and acceptability paired test by consumers. Castration increased meat fattening and reduced the androstenone and skatole levels of the fat. Significant differences (P<0.05) were found between entires and castrates for the average values given in all the sensory attributes studied. The dry-cured ham from castrates was scored as more flavoured, more marbled and softer. It was also perceived as less grainy, less salty and having less boar odour and flavour. The sensory perception of boar odour was more intense than that of flavour in dry-cured ham and appears to be related to the level of androstenone and skatole in fat. Dry-cured ham from castrated males was also more accepted and more preferred by consumers, especially women and habitual consumers. Castration of male pigs contributes to improve the quality of dry-cured ham. The rejection caused by boar odour and flavour is reduced, improving the overall flavour, texture and juiciness. In addition, the saltiness is less pronounced in ham from castrates. For this reason, the production of high quality dry-cured ham will have to shoulder the extra costs associated with processing castrated pigs.     

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.     

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.     

Reaction to boar odour by different South African consumer groups

A consumer sensory study of South African pork consumers (n=300) including equal numbers of males and females and consumers from three ethnic groupings (black, white and coloured consumers) was conducted to determine the effects of gender and ethnicity on consumer reactions towards boar odour compounds. Samples consisted of boar fat with varying concentrations of skatole and androstenone. Each consumer evaluated the odour of seven fat samples. Consumers’ liking of pork meat in general had a significant effect on the hedonic rating of the boar odour. The majority of consumers represented in this study would be dissatisfied with pork meat exhibiting detectable levels of skatole. In general, more females compared with males, will also respond more negatively towards samples with detectable levels of androstenone. An apparent liking for samples with medium levels of androstenone (0.5–1 μg/g) was found for some consumers, especially males, and can be partly attributed to the inability of some consumers to smell this compound at these levels or a genuine liking for the odour of androstenone. Significant differences in the sensitivity of consumers from different ethnic groups were found with white females responding more negatively than white males and blacks. Although it was not possible to compare responses directly with the white and black groups, it was found that a higher percentage of coloureds responded negatively to boar odour compounds. Coloured males responded particularly negatively towards samples with detectable skatole, while black males, in general, were found to be more critical than black females. Based on these results it is predicted that the majority of consumers would be less willing to consume pork meat exhibiting detectable levels of boar odour.     

Effects of raw potato starch and live weight on fat and plasma skatole,indole and androstenone levels measured by different methods in entire male pigs

A total of 96 entire male pigs were used to evaluate the effects of feeding raw potato starch (RPS) and live weight (LW; 90, 100 or 115 kg) on skatole, indole and androstenone levels in fat and plasma. Different methods to analyse the compounds were compared. Fat levels of skatole analysed by HPLC and colorimetric methods, and androstenone analysed by HPLC and ELISA, were highly correlated. However, androstenone values obtained by ELISA were overestimated. Feeding RPS reduced skatole levels in plasma and fat but did not affect indole levels. Androstenone levels in plasma measured by direct ELISA were not altered by diet, whereas the levels measured by ELISA with extraction were lower in the pigs fed RPS. Feeding RPS did not reduce androstenone levels in fat. Androstenone levels in fat were higher at 115 than at 90 kg, whereas skatole and indole levels were not affected by LW.     

Acceptability of boar meat by consumers depending on their age, gender, culinary habits, and sensitivity and appreciation of androstenone odour

Sensitivity of the consumers to androstenone, evaluated as the degree of smell (strong/weak) perceived by smelling the pure compound, is important in determining their acceptability of pork with different levels of androstenone and skatole. However, 8% (3.3% women and 16.2% men) of highly sensitive consumers like this odour, and 12.7% (9.1% women and 15.9% men) of mildly sensitive/insensitive consumers dislike it. The effect of the appreciation (like/dislike) of the smell in the acceptability of pork samples has not been reported previously. The aim of this paper is to ascertain if this liking for androstenone odour affects pork acceptability as well as the gender, age, and culinary habits of the consumers. Consumers evaluated the flavour and odour of five cooked and reheated samples, and recorded their acceptability on a 7-step scale. Results showed that acceptability of pork increases when the frequency of cooking and eating fresh pork are higher. The acceptability also differed depending on the sensitivity of the consumer within each age group. Appreciability of androstenone odour discriminates more than sensitivity in consumers’ acceptance of pork.     

Quantification of myosin heavy chain isoform in porcine muscle using an enzyme-linked immunosorbent assay

The aim of the present study (part of an EU AIR programme on boar taint) was to make objective the perception of boar taint in entire male pork, and to relate the perception to skatole and androstenone levels. Trained analytical sensory panels in seven European countries assessed pig meat with known levels of androstenone and skatole. The panels performed a sensory profiling using the attributes pig, urine, manure/stable, naphthalene/mothballs, rancid, sweet, sweat and abnormal, both for odour and flavour in separate sessions. It turned out to be difficult to harmonise sensory methodology for seven sensory panels throughout the EU, especially with respect to the exact level of training the panellists received. Sensory panels in general were able to differentiate between the two compounds and between different levels of the compounds, though substantial differences between the panels in the different countries existed. Androstenone was found to relate mostly to the urine attribute, while skatole related mostly to manure and, to a lesser extent, to naphthalene.