The use of sodium benzoate in piglet diets

By: André Meeusen and Yvonne van der Horst, Kemira ChemSolutions, the Netherlands

Benzoic acid is one of the oldest chemical preservatives used in the food industry. Gabel (1921) was one of the first to demonstrate that benzoic acid was effective against bacteria. Similar results were reported for fungi and yeasts. The principle mechanism responsible for the antimicrobial activity is the uptake of the benzoic acid molecule by diffusion through the bacterial membrane of the un-dissociated form of the acid which is not charged and lipophilic.

Because of the low solubility, benzoic acid is slowly absorbed and because of a higher dissociation constant (pKa = 4.19) benzoic acid is able to exercise a good antimicrobial effect, not only in the acidic gastric environment but also in the more neutral intestinal environment of piglets. The kinetics of dissociation of benzoic acid is given in Table 1. Benzoic acid might also change the permeability of the microbial cell membranes and can also inhibit specific enzyme systems within cells. This makes benzoic acid effective against gram-negative as well as against gram-positive bacteria, as shown in Table 2 and 3.

A comparative study of six organic acids showed that the inhibiting effect of the acids was more pronounced in stomach content than in content of the small intestine and it appears that coliform bacteria, in contrast to lactic acid bacteria, were unable to grow in stomach content at pH 4.5. Benzoic acid had the highest growth inhibitory effects compared to the five other short-chain fatty acids (SCFAs).

Less ammonia
Benzoic acid or benzoate does not accumulate in the body. Once the acid is absorbed from the intestines, it will be metabolised in the liver and be transformed into hippuric acid (by reacting with glycine). Hippuric acid is excreted by the kidneys via the urine. As such, more nitrogen from the protein catabolism is excreted as hippuric acid instead of being excreted as urea. This results in an acidification of the urine and the urinary tract and leads to less ammonia being released from the slurry in the manure pit. Indeed, at a lower pH the urease activity, which transforms urea into ammonia, is inhibited: ammonia is mainly formed from urea in the urine, catalysed by the enzyme urease from faeces according to the formula:

urease
CO(NH₂)₂ + H₂0 2 NH₄ + CO₂

The use of benzoic acid in rearing piglets has gained a lot of interest, however, disadvantages of benzoic acid are the low water solubility, the pungent smell and it creates a dusty environment.

Benzoic acid in another form
Sodium benzoate was the first chemical preservative approved for use in foods by the US Food and Drug Administration (FDA). It is a naturally occurring substance and is found in cranberries, prunes, greengage plums, cinnamon, ripe cloves, apples and many more. The product is bacteriostatic and fungi static under acidic conditions. The FDA labels sodium benzoate as GRAS (generally recognised as safe) and it is authorised in the EU as a food additive: Council Directive No 95/2/EC, E No 211, Annex III: Conditionally permitted preservatives and antioxidants.

It has been reported as sweet, salty and bitter. Results have shown that there may be some differences in the palatability of different organic acid-supplemented diets. When allowed to choose, piglets preferred diets supplemented with sodium benzoate.

Although un-dissociated benzoic acid is the most effective antimicrobial agent for preservation purpose, sodium benzoate is widely used, because it is about 200 times more soluble than benzoic acid. Sodium benzoate converts to benzoic acid when it arrives in the acidic environment of the stomach.

Trials from the Animal Science Group at Wageningen University (The Netherlands, 2007) have shown that sodium benzoate outperformed all other essential oils or plant-derived additives in a piglet trial challenged with rotavirus and E.coli 0149K91+K88(ETEC). Feed intake was the highest in the benzoate group compared to the negative control, the carvacrol and the butyrate group. As a consequence, bodyweight gain after ETEC challenge was highest in the benzoate group compared to the negative control, the carvacrol, the butyrate and the allicin groups.

Kemira sodium benzoate
The weaning period of piglets is worldwide frequently associated with infectious diseases and post-weaning diarrhoea (PWD) or post-weaning enteric colibacillosis. Enterotoxigenic Escherichia coli (ETEC) is the most common cause of this disease and antibiotics have been used over decades as growth promoters in animal production as well as a therapeutic agent, but many bacteria are becoming resistant to antibiotics.

Protural is the sodium benzoate registered in the EU by Kemira Oyj as a zootechnical feed additive for piglets. In January 2011 the EFSA gave a scientific opinion on the safety and efficacy of Protural and said that sodium benzoate is a natural substance widely occurring in the environment and safe for the animal and the environment. It is not an irritant to skin and eyes and has limited exposure of the respiratory system.

Protural is highly soluble and converts easily into benzoic acid in the acidic environment of the stomach:

– solubilisation of sodium benzoate:
C₆H₅COONa C₆H₅COO-_(sol.) + Na⁺_(sol.)

– precipitation of benzoic acid at pH 4:
C₆H₅COO^– + H⁺ C₆H₅COOH_{(precipitation)}

In a laboratory trial, this precipitated benzoic acid showed to be very fine and dispersible compared to an industrial produced benzoic acid and thus could present a much higher active surface.

A meta-analysis from five piglet trials showed that the addition of Protural at 4 kg/tonne feed results in significant improvements in growth parameters of piglets as shown in Table 4. Daily growth and final weight are significantly higher in weaning piglets, daily feed intake is consistently increased and feed efficiency (FCR) improved. Moreover treated piglets that received Protural in their diets had more consistent faeces than the control group. The number of piglets treated with antibiotics for diarrhoea was lower in the Protural group than in the control group. Mortality, although statistically not significant, was reduced in all trials. It was mentioned that most of mortalities in the control group are due to Streptococcus suis infection. Faecal samples taken from piglets showed that sodium benzoate dietary supplementation reduced the number of total aerobes, total anaerobes, Enterobacteriaceae and Streptococci.

SCFA combinations
No antimicrobial is completely effective against all microorganisms present in the GI tract of animals. In theory, one should be able to combine various antimicrobials having different modes of action to compensate for this deficiency. It should then be possible to achieve a broader spectrum of action or an increased antimicrobial action– the eubiotic effect – by using such a combination, improving animal performances. Indeed, the practice of dietary acidification is one of the most consistent and economical alternatives to antibiotic growth promoters and especially in piglet diets. Over the past two decades, various acids and salts have been used for this purpose. Organic acids lower the stomach pH, improving digestion and increasing the barrier function against harmful microbes. The short-chain fatty acids (SCFA) have a bactericidal effect in an acidic environment, mostly against gram-negative bacteria. Formic acid is the smallest of the SCFAs, but has the highest acidic character and bactericidal effect in feed and animals. It is widely accepted that combinations of organic acids have a broader antimicrobial effect compared to single acids. Contrary to the SCFA, Protural works as well in the more neutral environment throughout the intestinal tract and is effective also against gram-positive pathogens.

Trials at the University of Leuven- Belgium (2004) with growing piglets showed that acid mixtures with sodium benzoate improve overall feed efficiency and daily growth significantly better than mixtures with benzoic acid or benzoic acid alone. Sodium benzoate mixtures consistently showed an increased feed intake. At Wageningen University, the Netherlands (2008), it was shown in weaned piglets that feeding sodium benzoate increased the external surface area of both jejunum and total small intestine. This effect was associated with increased daily feed intake since the external surface area of the gut showed a positive correlation with daily feed intake, comparing pigs of all dietary groups. The greater gut surface was not associated with any acute pathological symptoms such as the absence of mucus, gastric ulcers, local erosions or local haemorrhages. This was also confirmed in the experimental farm of Kasetsart University in Bangkok, Thailand in 2010. Piglets fed a combination of acids with Protural have shown a significant increase in villous height versus piglets receiving high inclusions of zinc oxide.

Moreover, this mix with Protural significantly increased feed intake and final weight of piglets, replacing zinc oxide. In many countries indeed zinc oxide is prescribed by veterinarians and used at therapeutic dosages (up to 3,000 ppm) to overcome diarrhoea problems, leading to an enormous environmental challenge.

Discussion
It may be concluded that sodium benzoate has a positive effect on the intestinal morphology in piglets, showing an increased surface area of jejunum which is associated with increased daily feed intake. These are very important observations as small intestinal development is a nutritional strategy to adapt piglets to solid feed during the weaning period. Moreover, trials confirmed that Protural can control the intestinal microflora, creating a healthy gastrointestinal tract. It can also be concluded that Protural or mixtures with Protural do not show any taste-aversion, on the contrary, they consistently increase daily feed intake, resulting in a significantly increased average daily gain. Feed efficiency is also significantly improved in the period immediately after weaning.

Further information: Kemira