Male piglet castration is a practice that has been carried out historically to avoid a sensory defect (smell and taste) in pig meat known as boar taint, reduce aggression and provide the appropriate amount of fat for the production of certain products. In pork, odour can be adversely affected by accumulation of high levels of androstenone and/or skatole. Androstenone is a testicular steroid (with no anabolic effects) and is described as having urine or sweat-like odour, although only some humans (from 30% to 60% depending on the country) are able to smell it. It is produced by testicular Leydig cells of sexually mature males, so their levels depend fundamentally on the state of sexual maturity of the animal at the time of slaughter and genetics variations. Once in the blood, the androstenone can visit three places, the salivary glands, the liver and adipose tissue. When the androstenone arrives to the salivary glands, becomes 3-alfa-androstenol and 3-beta-androstenol, being both pheromones that can also be found in humans (this proximity is the reason why some people smell the precursor). Three-beta-androstenol for instance contributes to axillary odour in humans and derivates of androstenol are found in black truffles, what is used as explanation of how pigs locate them deep in the ground.
As mentioned above, the androstenone can also finish in the liver, where is destroyed, or alternatively, can be storage in the adipose tissue due to its lipophilic character. In fact, it can be found in higher concentrations in fat than other steroid hormones. This is interesting for pigs, because a reserve to produce pheromones without a continuous androstenone testicular production is very useful. The problem is then for humans consuming this product if they are sensible to the odour, as specially when the fat gets warm, the intensity of the smell can be unbearable and the consumer can be lost forever.
Skatole, for its part, comes from the degradation of the amino acid tryptophan in the distal part of the large intestine, and its presence depends to a large extent on the food received during fattening and the environmental conditions of temperature and hygiene in the pen. As dirtier of faeces the pig, for instance, higher the risk of abortion of skatole through the skin. Skatole, if present, is detected by practically all consumers, and is associated with fecal or, to a lesser extent, naphthalene odour. Functions of skatole are not clear and it is probably related with gut and mental health, as tryptophan is in the gut precursor of important neurotransmitters and compounds that contribute to the health and welfare of the animal. When absorbed in the blood (through the gut or skin), should be metabolised in the liver, but its hepatic metabolism is inhibited by steroid hormones (including androstenone). Therefore, it is clear that it has a role in the sexual communication between pigs. In fact, as previously, due to its lipophilic nature, accumulates in the adipose tissue. So an increased concentration of androstenone in fat will be correlated with high levels of skatole. Consumers detect boar taint in ranges from 0.5 to 1.0 ppm for androstenone and 0.2-0.25 ppm for skatole in fat. As mentioned previously, the castration of pigs seeks to eliminate the presence of androstenone and other steroid hormones that favour their metabolism in the liver, in the case of skatole, being the main reason why is performed. However, castration has been strongly contested by animal welfare organizations in several European countries. With some variation between studies, 40 to 50% of the consumers indicates awareness of castration of male pigs, while only 14 to 21% indicates that they are well aware of this practice. In fact, most of the citizens take for granted that when performed, the castration does not differ dramatically from the one performed in other domestic species, such as dogs (where anaesthesia and analgesia is ensured at all ages). Citizens and pork production chain as well as other societal stakeholders have demanded a ban on surgical castration without pain relief, and the European Commission set a deadline of 2018. However, while alternatives are applied in a number of countries, others have not adhered to this deadline. Finally, for some citizens of the EU, and this movement increases every year, castration and other so called “mutilations”, such as tail docking and teeth clipping, are seen as violations of the animals’ integrity and “naturalness”.