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1. Phytic acid and its anti-nutritional effect Brief introduction Phytate phosphorus is the main form of phosphorus in the seeds of cereals, beans and oil crops, accounting for 60% to 80% of total phosphorus in plant feed, content of 1% to 5%. Phytic acid is an anti-nutritional factor that reduces the bioavailability of feed minerals, proteins, amino acids, starch, and fats. Phytic acid has a strong complexing ability to Zn2+, Mg2+, Ca2+, Cu2+, Fe2+, Fe3+, Mn2+, etc. at pH 3 to 10 (in the gastrointestinal pH conditions of livestock and poultry), easily forming stable and insoluble compounds, thus reducing these The utilization of the elements; under acidic conditions, negatively charged phytates can combine with positively charged proteins and amino acid residues to form phytate-protein binary complexes, and in the neutral conditions they form phytate-metal ions. - Protein ternary complexes, phytic acid can also form complexes with protein degradation intermediates polypeptides and amino acids, reducing the digestibility of these substances; In addition, phytic acid can also be combined with amylase and lipase, thereby reducing starch and fat Digestibility.
2. Citric acid improves dietary phosphorus availability
2.1 Rats
In 1937, a clinical study at Harvard University showed that the addition of citric acid and sodium citrate in the low-phosphorus, low-calcium diets can prevent the occurrence of rickets in rats. However, the results of this study have not been taken seriously until 1956, Pileggi et al. further examined the effect of citric acid on dietary phosphorus availability in rats. Pileggi's study added 1:1 mixed citric acid and sodium citrate to diets containing phytate, resulting in a 61% increase in rat femur ash content and a reduction in fecal phosphorus (percentage of man-made phosphorus). 94% was reduced to 48%; an important finding by Pipeggi et al. was that adding citric acid only worked on phytate-containing diets but not on phytate-free diets, indicating that citric acid played a major role. Phosphorus in phytate.
2.2 Pigs
Pallauf et al. (1993) added 1.5% citric acid to growing pig diets, resulting in improved availability of phosphorus and other minerals; Han et al. (1998) showed that: in the presence of 10%/15% wheat secondary powder + 300U/ The addition of 1.5% citric acid to the basal diet of kg microbial phytase can completely increase or increase the inorganic phosphorus content by 0.2% or more. Therefore, they believe that citric acid can completely replace inorganic phosphorus. Radcliffe et al. (1998) added 1.5% and 3.0% citric acid to a 7.4kg piglet diet. The results significantly increased daily gain, feed efficiency, and calcium digestibility, but the diet and substance and phosphorus digestibility. There was no effect on skeletal shear force, bone ash, and ossification weight; another study found that 2.0% citric acid had no effect on the above indicators of 0.6 kg of piglets and that citric acid was added to diets containing no or 250 U/kg phytase. The phytase activity in the stomach had no adverse effect, but it reduced the recovery of phytase activity in the stomach containing 500 U/kg phytase, while citric acid had a synergistic effect with phytase on improving rib shear force. Boling et al. (2000) found that 1% and 2% were added to the diet of 10 to 11 kg of piglets. 3%, 6% citric acid increased feed efficiency, 2% added increased weight gain, but all additions did not improve ash ash content.
2.3 Poultry
Boling et al. (2000) studied the effect of citric acid addition on phytate phosphorus utilization in chick diets. The results showed that 0,1%,2%,4%,6% of citric acid/sodium citrate (1:1) was added to basal diets with Ca and P contents of 0.60% and 0.1% (effective phosphorus), respectively. The mixture linearly increased chick weight gain and ash ash content, 6% increased weight gain by 22%, tibia ash increased by 43%, and skeletal zinc content also increased significantly; dietary Ca:P on lemons The effect of acid/sodium citrate addition was influential. It was found that the effect of citric acid on the humeral ash content of chicks was in the diet Ca:P=4:1-6:1, and on the day of Ca:P=6:1. The addition of 6% citric acid to the diet and 6% citric acid/sodium citrate (1:1) are equally effective, and the addition of 1450 U/kg phytase to the above diet further increases the weight gain and tibia ash content. Boling et al.'s experiments also showed that the addition of citric acid in diets that did not contain phytate had no effect on performance and tibia ash content, and 6% additions showed negative effects. Although Boling et al.'s trials in chick diets were very significant, the addition of citric acid to the broiler trials did not improve the availability of phytate phosphorus in the diets of layer-bean meal.
3. The mechanism of citric acid to improve dietary phosphorus availability
3.1 Lowering pH
Han et al. (1998) believe that the mechanism of citric acid to improve dietary phosphorus availability is related to the fact that citric acid reduces the pH of the diet and digestive tract, because the lower pH increases the solubility of phosphorus on the one hand and decreases on the other hand. The emptying speed of the stomach prolongs the passage of digestive tract contents through the small intestine, thereby increasing the digestion and absorption of total phosphorus. The pH of the experimental diet is 5.9 (inorganic phosphorus) and 6.2 (phytase +10% Sub-powder group), 4.5 (15% sub-powder + phytase + 1.5% citric acid group), 4.3 (10% sub-powder + phytase + 1.5% citric acid). The results show that the citric acid-containing diet can be completely Instead of inorganic phosphorus additions; Radcliffe et al. (1998) found that the addition of 1.5% and 3.0% citric acid reduced the pH of the diet by 1.51 and 2.16 units, and significantly reduced the pH in the stomach. In addition, it is believed that lower pH values ​​can provide a suitable environment for phytase to function. Studies have found that phytase has two activity peaks, namely, pH 5.0-5.5 and pH 2.5, while calcium phosphate is typical. The pH value of the inorganic phosphorus-derived corn-soybean meal diet is around 6.0. Adding citric acid can reduce the pH in the stomach and increase the phytase activity.
3.2 Chelation Although the mechanism by which citric acid increases dietary phosphorus availability is not known, the mode of action of citric acid may be related to its strong Ca binding capacity (Hamilton & Dewar, 1937; Day, 1940; Pipeggi et al., 1956). Pileggi et al. (1956) reported that the anti-rickets effect of citric acid may be related to its ability to reduce Ca inhibition of phytic acid hydrolysis; phytic acid molecules can be combined with Ca and other mineral elements (Erdam, 1979), so citric acid may be a A strong Ca chelator can remove Ca or reduce the binding of phytic acid molecules to Ca, thereby reducing the stability of phytic acid or making it susceptible to decomposition by phytase in the body (Boling et al., 2000).
4. Factors affecting the action of citric acid
4.1 Differences in animal species and diets Boling et al. (2000) found that citric acid significantly improved the dietary phosphorus utilization rate of chicks, but the effect on the piglets was not as pronounced as the chicks. The reason for this may be related to the different species of the test animals. Biehl& Baker (1996, 1997a,b) found that the addition of lα-OH D3 in low-phosphorus corn-soybean meal diets containing higher levels of VD3 effectively increased the availability of phytate phosphorus in poultry, but not in pigs, but Pigs and poultry have positive reactions to the addition of phytase, so Boling et al. believe that pigs and poultry may have unknown reactions to additives that increase phytate phosphorus availability. However, studies by Paullauf et al. (1993), Radicliffe (1998), and Han et al. (1998) found that adding citric acid increased the utilization of phytate phosphorus in pig diets.
4.2 Dietary Calcium to Phosphorus Ratio
BOLING et al. (2000) found in chicks that when dietary Ca:P=2:1 (ie limiting Ca), the addition of citric acid did not increase ash ash content, but when Ca:P=4-6:1 (ie, When P was limited, 6% citric acid significantly increased the content of humeral ash, and the response of ash to citric acid was greater than the decrease of Ca:P from 6:1 to 1 and indicated that citric acid played a major role in this reaction. Phytic Phosphorus but not Ca, in the Ca-restricted diet, citric acid released P may further aggravate the adverse effects of Ca deficiency; Boling believes this may be associated with a higher Ca level in the diet of the layer (3.8%) Relevantly, when the Ca content is high, the added citric acid is first blended with the non-phytate, and there is still a large amount of calcium in the diet that can be combined with phytic acid, so that the citric acid cannot be combined with the calcium in the phytate. .
4.3 Dietary Phosphorus Forms
Pileggi et al. (1956) in rats. Studies by Boling et al. (2000) in chicks have shown that citric acid only acts on phytate-containing diets but is not effective on non-phytate and available phosphorus.
4.4 Phytase
Han et al. (1998) found that 10%/15% wheat sub-powder + 300U/kg phytase + 1.5% citric acid can completely replace the dietary inorganic phosphorus addition in growing pigs; Boling et al. (2000) found 6% citric acid with The combined use of 1450 U/kg phytase was superior to 6% citric acid alone in weight gain and humeral ash, but Rad-cliffe et al. (1998) found phytase (25/500 U/kg) and citric acid (2.0). %) was effective in increasing dietary phosphorus availability in weaned piglets, and no interaction was found between the two, and citric acid seemed to reduce the recovery of phytase activity. Therefore, the interaction between citric acid and phytase remains to be further studied.
5. Concluding remarks The use of citric acid to increase the utilization of phytate phosphorus in animal diets has rarely been reported in domestic studies, and foreign reports have been limited. It has been suggested that the effective combination of citric acid and phytase may increase the use of phytate phosphorus in practice. Rate, and effective measures to reduce phosphorus levels in animal faeces (Boling et al., 2000); but whether citric acid has a synergistic effect with phytase, the optimal amount of citric acid when added simultaneously, and its mechanism of action, and many influencing factors, etc. The data and information on the aspect still need to be further studied, verified and supplemented. The research in this area is of great significance to the deep development of acidifier products, the improvement of phytate phosphorus utilization and environmental protection.
Application of Citric Acid in Improving the Utilization of Dietary Phosphorus
Phosphorus availability in the form of phytic phosphorus in pigs and poultry is very low, but in order to meet the animal's growth and production needs of phosphorus, expensive inorganic phosphorus must be added to the feed, and a large amount of organic phosphorus discharged from the feces. It has caused serious pollution to the soil and water resources. Therefore, how to increase the utilization rate of phytate phosphorus, reduce the addition of inorganic phosphorus, reduce feed costs, and reduce environmental pollution have become hot spots in the world. The addition of phytase is currently an effective measure to increase dietary phosphorus utilization in monogastric animals. As an organic acidifier, citric acid has a wide range of applications in animal production. Some recent studies have shown that citric acid can also improve pigs. Phytate phosphorus utilization in poultry diets. This article gives a brief overview of the research in this area.