Introduction

The newborn piglet has a low intestinal capacity to digest and absorb solid feeds, especially those of vegetable origin, so its digestive system must mature quickly to ensure its survival^1,2. Enteral nutrition (colostrum and milk) plays a fundamental role in the maturity of the piglet¹; however, milk soon ceases to cover the nutritional demand of the piglet and it begins a gradual consumption of other feeds, allowing the gradual maturity of the nervous, immune and digestive systems. This maturation process of the digestive tract is stimulated by the colonization of different bacterial genera^(3.4), which use some nutrients and produce enzymes^2,5 and, through competitive exclusion, prevent the adhesion of pathogens⁴.

Under natural conditions, piglet weaning occurs between weeks 10 to 22 of life^6,7. However, under commercial conditions, weaning is carried out between 21 and 28 d of age in order to allow the maximum productive efficiency of the female; higher number of births and piglets per sow per year, reduce the cost of facilities, etc.⁶. Commercial weaning, unlike natural weaning, is not gradual but an abrupt and sudden event, which is extremely stressful for the still immature piglet. This fact is characterized by the separation of the mother, the environmental change and from a dairy diet to a solid one (mainly composed of cereals). All this, added to the presence of new pathogens, causes neuroendocrine, immunological and digestive complications^2,7, with piglets showing in the first 24 to 48 h after weaning a low, and even zero, feed consumption, weight loss, atrophy of the intestinal structure and with it of the digestive and absorption capacity, as well as an increase in the incidence of post-weaning diarrheas^6,8.

The presence of post-weaning diarrheas is widely related to the sudden change in diet, as well as gastrointestinal infections. Both factors promote rapid dysbiosis, that is, an imbalance in the composition of bacterial populations, with an increase in E. coli, which contributes to the loss of intestinal structure⁹, as well as an abrupt reduction of Lactobacillus spp. The incidence of these post-weaning digestive disorders generates great economic losses¹⁰. To combat these gastrointestinal complications, antibiotics have been used as growth promoters (GPAs) in diets in recent years, since their use in animal feed favors the growth rate and reduces the incidence of diseases and mortality^11,12. These molecules decrease the number of pathogens and with it the atrophy of the intestinal villi and hypertrophy of the crypts are prevented. This leads to a greater and early consumption of feed, adequate digestive capacity and better feed efficiency, thus increasing the retention of nitrogen and energy from the diet^13,14. However, in recent years, the inclusion of GPAs in animal diets has been questioned, and even banned in some countries, since they represent a serious problem for public health, due to the development of bacterial resistance to antibiotics, which can potentially reduce the treatment of diseases in animals and possibly in humans. Therefore, the search for substitutes or alternatives to antibiotics is of great relevance for pig farming^15,16.

With the growing human population, the demand for safe food resources by the pig industry has increased dramatically in recent years¹⁷. Due to its nutritional and possibly therapeutic characteristics, potato protein concentrate (PPC), obtained after starch extraction, seems to be a good choice as a protein source. Therefore, the present work aims to review some characteristics of this protein ingredient, which place it as a nutritional alternative with potential to improve the intestinal health of newly weaned piglets.

Alternatives to growth-promoting antibiotics (GPAs)

The crisis of bacterial resistance to antibiotics shows no signs of a solution in the short term and the lack of new antimicrobial drugs, as well as the few companies that invest in this area, threatens the ability to treat and prevent infections. One reason for the shortage of new antibiotics is that typical points of action, such as cell wall and protein synthesis, as well as that of DNA/RNA, have perhaps been overexploited. Currently, thanks to access to complete bacterial genomes, strategies based on new molecular targets are sought; however, this approach has not been fully developed¹⁸.

However, the antibiotic crisis demands measures that contemplate a new approach and different therapeutic strategies¹⁸. There are many literature reviews on the different alternatives to GPAs. Among the most studied alternatives the following stand out: plant extracts, chicken egg antibodies, organic acids and enzymes¹², as well as essential oils¹⁰, probiotics¹⁹, prebiotics²⁰, minerals such as copper and zinc¹¹, as well as animal plasma²¹ and plant proteins such as potato protein concentrate²².

Among the most successful alternatives for the control of post-weaning digestive disorders, animal proteins stand out, such as animal plasma and fishmeal, which, due to their high digestibility and amino acid profile, favor feed consumption and growth rate^23,24. However, ingredients of animal origin are expensive and inadequate handling during storage could favor the transmission of some pathogens. Fishmeal is the essential protein in piglet feeds, but marine overfishing has caused a vertiginous increase in prices, reducing its availability.

The results of including these ingredients in piglet diets can sometimes be inconsistent and hardly able to match the effect of antibiotics in productive terms, however, benefits to intestinal health are reported, which should be considered when using antibiotic-free diets. Plant-based antimicrobial peptides (AMPs) are an alternative to GPAs that have shown potential use¹².

Potato protein concentrate in the feed of weaned piglets

There are about 5,000 varieties of potato, originating mainly from the Andes, with variants in size, shape, color, texture, as well as nutritional profile. About 10 varieties are cultivated, and the most cultivated variety worldwide is the species Solanum tuberosum. China is responsible for 80 % of the world’s production of this variety. The chemical composition of the potato is modulated by different factors such as geographical area and cultivation practices; its protein content in the fresh state varies between 0.49 and 2.7 %^42,43.

In addition to consumption in fresh form, the potato is also used to obtain starch, fiber and juice. In the European Union alone, 8 million tonnes of potatoes are processed annually. PPC is a coproduct of the starch industry, by recovering the liquid fraction that remains after its extraction⁴⁴. The process is based on thermal coagulation followed by protein separation and drying^27,45. Potato proteins are typically classified according to their molecular mass and electrophoretic separation into three large groups: patatins, protease inhibitors and other proteins³⁵. Patatins represent about half of potato proteins and are glycoproteins with molecular weights of 39 to 43 kDa, with enzymatic activities (hydrolases, phospholipases and glucanases)³⁵.

The potato has some antinutritional factors such as glycoalkaloids of solanines (927 - 2,632 mg/kg) and trypsin inhibitors (0.97 - 3.70 mg/kg), where the concentration can be variable according to the conditions of processing⁴⁴. Thermal processing (100°/15 min) before drying is capable of inactivating up to 48 % of protease inhibitors and up to 89 % of glycoalkaloids⁴⁶.

PPC is an ingredient that contains adequate amounts of essential amino acids, which can replace animal protein in piglet diets⁴⁷, as it has been characterized by a highly balanced amino acid profile and is especially rich in lysine, methionine, threonine, tryptophan and valine. In piglets that received a diet with PPC with the same level of inclusion as fishmeal, an improvement in the growth rate of piglets was observed⁴⁸, which can be attributed to the quality of its amino acid profile. Thus, the nutritional value of PPC is related to the concentration and availability of amino acids since it has a profile similar to that of soybean⁴⁹ and some animal proteins⁵⁰. The proportion of eight essential amino acids (threonine, valine, methionine, isoleucine, leucine, phenylalanine, lysine and tryptophan) corresponds to 40.7 % of the PPC protein⁴². The amino acid profile of potato protein completes or exceeds the ideal protein profile, except for tryptophan and lysine with 64.5 and 89.75 % of the requirement, respectively.

In 2008 and 2009, three articles were published ^28,32,33 in which a refined potato protein (RPP or PP) purified at laboratory level from a special variety of potato (Solanum tuberosum L. cv. Golden valley) was used. This protein fraction showed an inhibitory effect on the in vitro growth of pathogenic bacteria, so they studied the incorporation of RPP or PP at different levels in the diet of newly weaned piglets and compared it with a control diet with antibiotics. Results varied between studies, and an advantage of using the diet with antibiotics was observed in the three studies.

When the authors³³ used diets with 0, 200, 400 and 600 ppm of RPP, it was reported that the increase in levels of inclusion in the diets linearly improved the productive performance and reduced the populations of total bacteria, coliforms and Staphylococcus spp. in the contents of the colon and rectum and in feces. The apparent fecal digestibility of dry matter and crude protein, as well as apparent ileal digestibility of amino acids, did not differ between pigs fed the control diet (0 ppm) with antibiotics and diets with RPP³³.

With higher levels of inclusion (0.0, 2.5, 5.00 and 7.50 g PP/kg of diet), researchers²⁸ also observed a linear improvement in feed efficiency during the 28 experimental days and an increase in the apparent fecal digestibility of dry matter in phase II (0 to 14 days post-weaning) with the increase in the level of inclusion of PP. A linear decrease in fecal bacteria was also observed on days 21 and 28 with the increase in the level of inclusion of PP. Piglets that consumed the diets with PP had a linear reduction of total bacteria, coliforms and Staphylococcus spp. in the cecum and rectum. The apparent ileal digestibility of amino acids and the morphology of intestinal villi and crypts were not affected by the consumption of experimental diets²⁸.

In another study³² with piglets fed diets with 0.0, 0.25, 0.50 or 0.75 % potato protein (PP) during phase I (0 to 14 d post-weaning) and phase II (14 to 28 days post-weaning), the authors observed that increasing levels of PP linearly improved daily weight gain, daily feed consumption and feed efficiency in both phases (I and II); in addition to increasing the digestibility of the dry matter in phase II. Consumption of diets with increasing levels of PP linearly reduced bacterial populations in feces and contents of the cecum, colon and rectum. The height of the villi and the depth of the intestinal crypts did not vary with the increase in the level of PP in the diet. These three studies^28,32,33 opened an area of opportunity for the use of potato protein concentrate generated in the starch industry in animal feed.

The use of PPC of commercial origin²² in growing pigs (25 kg live weight) showed a high ileal digestibility of nitrogen, both standardized (93.0 %) and apparent (85.8 %). The apparent ileal digestibility and the digestibility of most AAs were similar to the digestibility of soybean protein concentrate and isolate, with the highest standardized and apparent ileal digestibility of leucine (96.3 and 94.7 %, respectively) and threonine (94.7 % and 86.9 %, respectively) standing out in PPC²².

In another study⁴⁷ with growing pigs (21 kg live weight), the inclusion in the diet of 17.5 % PPC with a low concentration of glycoalkaloids and low trypsin inhibitory activity did not affect daily weight gain, daily feed consumption or the relative weight of stomach, duodenum and jejunum, as well as other aspects of intestinal morphology. Protein digestibility was lower in pigs fed the diet with PPC than with the diet with casein, however, the apparent ileal digestibility of fat was better in piglets with PPC. The authors conclude that the diet with a high level of PPC was well used by growing pigs⁴⁷.

The good results to the consumption of PPC observed in growing pigs are probably related to its nutritional characteristics; however, due to the reported antimicrobial properties, the use of PPC is more recommended for its inclusion in diets for newly weaned piglets.

In 2005, it had already been observed⁴⁸ that piglets fed diets with 6 % PPC had a greater daily weight gain in the first 1-21 and 21-50 d post-weaning than piglets fed diets in which they included fishmeal, sunflower meal and gluten meal in similar amounts. Mortality in the period was low (about 4 %) and the severity of diarrheas was slight (1.6 points) in all piglets⁴⁸.

Some authors⁵¹, using PPC with a low level of glycoalkaloids (PPCLG) in diets of newly weaned pigs, found no differences in the productive behavior with respect to diets with animal plasma. The authors observed a quadratic response in weight gain and feed consumption in weaned piglets when using increasing levels of inclusion of PPCLG, replacing 25, 50, 75 and 100 % of animal plasma in the diet. Feed efficiency improved linearly with the inclusion of PPCLG. It was concluded that PPCLG may be an effective substitute for a part of the pig plasma in diets for weaned piglets⁵¹.

During the first week after weaning⁵², it was observed that daily weight gain and feed efficiency were also not different between piglets that consumed dehydrated porcine plasma or PPC in their diets. During the second week post-weaning and in the total experimental period, the daily feed consumption was similar among all animals. The apparent ileal digestibility of crude protein was higher in piglets fed antibiotic and PPC. The apparent total digestibility of dry matter and energy was higher for piglets fed PPC than the other diets. The severity index of diarrheas in piglets fed the PPC diet was similar among piglets fed the control diet with antibiotic⁵². These results demonstrate a potential use of PPC for newly weaned piglets.

Conclusions

In recent years, the search for alternatives to the use of antibiotics in feeds as promoters of growth and intestinal health in newly weaned piglets has increased. The potato is some food rich in antimicrobial peptides (AMPs) and other proteins that are involved in its defense system, which have already been purified at the laboratory level. The industry of starch extraction from the potato generates a large amount of a protein concentrate, which probably preserves these AMPs. In this context, potato protein concentrate could exert a positive effect on the productive development of pigs due to its nutritional value, in addition to the possible benefits of its AMPs on intestinal health. However, for it to be considered as an alternative to the use of antibiotics in starter diets, it is necessary to have more bibliographic evidence on the presence of these peptides in commercial products based on potato protein concentrate available on the market, and if their beneficial effects persist at the gastrointestinal level, decreasing post-weaning diarrheas.